Newton vs. Mach: The Bucket Experiment

[@dialectphilosophy]

Hey KZbinrs, thanks for watching! Please note: there is an ERROR at 8:15 in our video. The equation KE = PE (kinetic energy equals potential energy) for the parabolic shape of the surface of the water is NOT a consequence of the conservation of energy as we wrongly state in the video (obviously energy is not conserved as the bucket must be spun by putting work into the system.) The equation KE = PE instead comes from setting the Lagrangian (KE - PE) equal to zero. Our many apologies for that mix-up! Next up -- if your initial response to this video is to ask, "why not just do such-and-such to the experiment (spin the masses, add more springs, throw a free-body, etc.)" to improve it -- STOP -- read this first. It may address your confusion. Due to the introductory nature of this video, we didn't go deeper in length into the essential argument Mach was making (we will follow up with it in future videos, not to worry!) about Newton's experiment. But we can lay it out a little more in-depth for the interested viewer here: Mach's argument was NOT about the lack of cleverness of Newton's experiment. He was not arguing that Newton's experiments were mechanistically insufficient for determining true motion, and could be fixed with some new addition that would detect motion in some way the old experiment might overlook. Rather, Mach's argument was about the essential limits of empirical knowledge. Many a philosopher have written books on this subject. There's Hume's "An Enquiry Concerning Human Understanding" -- or Kant's response to that book, "The Critique of Pure Reason". The conclusions of these works turn on essentially the same fact: that empirical knowledge is impossible without some type of aprioristic reasoning or prior knowledge of the system. Our goal with this video was to bring an example of that philosophical principle into the concrete realm of physics. Empirical knowledge hinges on measurement and observation; measurement and observation themselves hinge on a prior knowledge or a system of reasoning, from which more general conclusions may be extracted. In the stretched-spring example, the length of the spring is the "observation" or "measurement" which constitutes the raw empirical data. The understanding of what length the spring should take in an inertial system is the "apriori knowledge" or reasoning which allows the raw empirical data to be interpreted (i.e., via the logical statement: if spring length = inertial length, then frame = inertial; else frame = non-inertial) and a conclusion to be drawn. But if you don't already know the inertial length of the spring, the empirical knowledge of the length of the spring is in-itself meaningless -- an empty statement with no interpretation yet assigned to it. Thus, the knowledge of the length of the stretched spring can't be used to determined inertial-ness in a closed system, because assigning an interpretation to that length requires having access to a known inertial frame elsewhere. The key to understanding Mach's argument lies in then realizing that this problem extends to any measurement whatsoever. Without some prior (and as Kant would argue, "god-given" or "transcendent") knowledge, no raw data could have any meaning whatsoever. Thus, for every clever addition you can think up "just throw a free body off of yourself!" or "try spinning the globes!" or "add more springs!" you are in fact just importing additional a-priori systems of knowledge into the experiment, and then exploiting your prior knowledge of those additions to make a measurement of "absolute" motion -- and then claiming this measurement could still be made if no prior knowledge was ever had! But none of these additions would have any meaning without some knowledge of how those additions already work in inertial frames elsewhere. Another way to look at it: the argument for always being able to perform a test for absolute motion is based on the following inductive statement: "it works always elsewhere, so it must work here!" Since empiricism is essentially induction, this works fine as an empirical and scientific statement. But take away the "it works always elsewhere" (the rest of the universe) part of the logic, and what are you left with? A simple statement: "it works here" with no other justification. A very non-scientific statement, and why Mach would argue against it!

[@purplenanite]

I still don't understand what your points are. Are you saying that the physical laws inside a rotating frame are identical to that of an inertial frame? Than that is false, because of Coriolis forces. Or, are you saying that if you import no information into your system, you cannot tell whether it is inertial or not? Then that statement is true, but doesn't seem useful.

[@einszwei6847]

I rewatched the video with this new context and now I agree with you that without the knowledge about how something "should" behave in an inertial system (which is the outside information you were talking about) one could not determine with any experiment whether a system is inertial or not. In fact, an observer who does not have memory from "other systems" would a priori not even know that there could be something like an inertial system. But an observer could still collect purely empirical data in only their own system by doing experiments with e.g. the globes connected by a spring. One experiment such an observer might try is rotating the spring. Looking just at the empirical data generated by this experiment the observer would realize that the length of the spring changes. Now assume that the observer is practically immortal and has infinite time to do these experiments and think about the results. They might want to find out whether there is a "special" system, maybe after running through a thought experiment where they imagine that the spring is "at rest" and only the observer is spinning. The two simplest possibilities would be to look for a rotation that maximizes the spring length and one which minimizes it. Experiments looking for a maximal spring length would of course lead to nothing because the spring would either break or extend further and further. But the observer knows that the minimization looks a lot more promising, because the spring length cannot shrink indefinitely (it has to stay positive). Thus they could do several experiments and after a lot of data has been collected, they could conjecture that there is indeed a system where the spring length is minimal and that this system would in some way be "special" (even if it is only special because of the minimal spring length). The observer can of course not know that this is definitely true, but that is not possible with any physical theory (one could always be missing something). So the observer now continues experiments until they are sure to have found a rotation at least very close to this "special system" and would now be enticed to try other experiments in that system (for example shooting a projectile out of a tube that is rotating in this special way) and they would discover that this system is special in several other ways. They might not call it "absolute rest" or "inertial system" but they will realize that there is something special about it. Of course your point still stands, because you can see it as "cheating" that the observer rotates the spring, thus basically gaining knowledge about the behaviour of the spring in a rotating frame. Instead of remembering this knowledge from another reference system they would generate it themselves. But still the experiment would be local and easy to come up with, so even an observer that remembers nothing really from outside their own reference frame could deduce that there is some special reference frame.

[@-danR]

I believe the first video of yours I saw was the map analogue of the metric tensor, which was the best intro I've ever seen. Fortunately I didn't see this present one but started more or less at the beginning of twins paradox and worked my way through. So I was prepared by that progression for something other than the usual Veritassium/Science Asylum/Physics Girl "hey guys, this is what's really going on here" explanation. It has driven me over the past week into a deep-dive with Gedankenexperimenten, sometimes with my eyes closed for 20 minutes. I won't bore anyone with the details, but it's left me baffled with a gnawing question that won't go away: how does a rotating system* _know_ it is rotating? I can't cheat by reading Newton. Mach didn't cheat. Even Newton didn't cheat by reading Newton. They knew there was some kind of epistemological brick wall that needed deeper answers. _____ * edit: 'rotating' system . Because I can hear VSauce, with his quizzical eyebrow, popping up and skeptically asking "...or _is_ it?..."

[@usermlgbzzcnm]

at 8:22 why K.E.=P.E. here, how can it be derived from the conservation of energy?

[@einszwei6847]

@@usermlgbzzcnm Indeed you have to be a little careful here. The derivation is not the point of the video, so the decision to gloss over some details was reasonable. I try to explain what happens: Energy conservation seems to be not very useful here. As we know it is a condition that tells us when a change can occur, e.g. if a pendulum with a specific velocity at its lowest point can reach a specific height exactly when it has zero velocity - if the energy would not be conserved, the change cannot occur. In the case of the pendulum you would say the kinetic energy would be converted to potential energy, leading to the equation m/2v^2=mgh where v is the velocity at the initial point (where the height is 0) and h is the height at the highest point (where the velocity is 0). But this is not the case here because we instead want to find the condition for the surface not to change. In fact one would guess that energy conservation would give us m/2v^2=-mgh (+constant), so E=m/2v^2+mgh is zero (or at least constant). But that does not lead to the correct solution. The fact that only a sign is missing indicates that another approach would be more useful: The stationary action. The Lagrangian has a different sign between kinetic and potential term when compared to the energy. I will not do that here because - although it works perfectly - it might cause confusion and also it wouldn't answer the question. The reason everything looks like something is missing is because there is indeed something missing: The kinetic energy is not only m/2 r^2w^2 (I will use w instead of omega to make typing easier) - this is only the term corresponding to rotational movement - but it is m/2 (r^2w^2+v_r^2) where v_r is the radial velocity corresponding to radial movement. In our case this is zero, but we want to find the condition for it to stay zero. This now means that the term m/2 r^2w^2 is for the radial motion - which is of interest to us - something like an effective potential. Okay, but that does not solve our problem because v_r is supposed to be zero anyways. So we are still missing something. And that something is: The rotational frequency w is not constant. I mean in our case it is everywhere the same, but there is no law of motion to keep it that way. Instead the correct conserved quantity to use here would be the angular momentum L=mr^2w (or something like that). If we plug that back into our equation - i.e. replace the w^2 by L^2/(m^2r^4) - we have E=L^2/(2mr^2)+mgz. This now looks promising: Before we had the problem that both the kinetic and the potential term decrease when you decrease r, but this is not really the case anymore. Now we see that the kinetic term would in fact increase. If one now considers the force coming from the effective potential L^2/(2mr^2) it is exactly L^2/(mr^3), i.e. mrw^2 (computed by taking the gradient). That looks a little bit like it would come from an energy conservation equation where the kinetic term does have opposite sign, so something like mgz-m/2 r^2w^2=0. The key point to realize was that angular momentum is conserved, not angular frequency, so if the ring of water mentioned in the video would "move" (change its radius/height) it would not necessarily rotate with angular frequency w anymore. The only reason that does not happen is because we are already in some equilibrium. But to compute the condition necessary for this equilibrium we needed to ccount for the fact that the angular frequency could change, but L can't. An easier approach would be to use forces from the beginning. The water on the surface can only move tangentially along the surface, thus the projections of gravitational and centrifugal force tangential to the graph of z(r) have to cancel. This should also lead to the correct result.

[@RocinanteGold]

In round one, the concept of "bucket", itself, references and depends upon the external gravity field. Astronauts on the ISS do not drink from cups (little buckets, containers with open tops) because rhe water only stays contained in the "bucket at rest" due to gravity. The equivalent experiment for space would require a closed container half filled with water, with the remainder as air. Preferably transparent. Shaking the container would distribute globules of each within the volume of the other. Whenever like globules touched, surface tension would merge them, until there were two distinct volumes, one of water, one of air. The interface surface would assume a roughly spherical shape by surface tension. At "absolute rest", the interface spheroid could occupy any position. In linear accelleration, the water would accumulate opposite the direction of acceleration. Spinning the container would cause the water to form a cylindrical interface outside of the air volume.

[@medexamtoolscom]

Why use a container at all? It should just be a sphere of water. If it is not rotating, it will be a sphere, and if it is rotating, it will be an oblate spheroid which is larger at the equator, provided it is not spinning fast enough so that it is no longer able to hold itself together from its surface tension and self-gravity.

[@Tazhsalis]

@@medexamtoolscom Are you suggesting that the strong and/or weak force exist outside the realm of reletivity? HERACY!

[@hillaryclinton2415]

@Phumgwate Nagala and no one can hear you scream. The container is closed.

[@casey6104]

You’re forgetting to include interactive aspects such as capillary action, which would have a higher significance in a zero gravity environment- while also still being subject to earth’s gravitational fields, though at a much lower magnitude of effect. I actually came up with an idea for this experiment that minimized the openness of the system. Unfortunately, all I can remember is that it involved minimizing the gravitational sway of the experiment without leaving the surface of the earth. Thanks brain damage lol.

[@jimmcneal5292]

It really doesn't. Just cover it with a gel or something

[@Rastlov]

Try spinning the "stationary objects" connected by a spring. If they move closer together, then they were moving faster than now. Eventually you should be able to experiment until you minimize the length and find the absolute inertial rest state. If you spin them in one direction and they move apart, simply reverse the direction and repeat as above.

[@patricktherrien5310]

This seems like it would work to me.

[@fredbloke3218]

You are only allowed to observe the system.

[@awesomedavid2012]

​@@fredbloke3218that seems like a copout. So your eyes can absorb photons, but you can't touch the system? To observe it, you already are modifying the system. Look at the quantum world.

[@stephanieherman2861]

seems legit

[@deathorb]

you cant find it, in real universe spinning in any direction they will seperate. then you will conclude absolute rest to be your start rest state.

[@chili209]

I think switching the third experiment's rope to a spring is what creates the need for remembering the state at rest. With the rope, you can always just pull on the rope at any place and the force of the two objects pull can be measured.

[@tymmiara5967]

My thoughts, precisely! Replacing the cord with a spring is a straw-man. The chord can either be taught or loose. The spring does not have that property it can never be loose.

[@willo7734]

I dunno. It seems to me that you would still feel a force when pulling the cord even at rest and even in zero gravity due to the mass of the two globes. That still leaves you with no clear indication that the frame is inertial.

[@tymmiara5967]

@@willo7734 What force? If you have two balls which connected by a 1m cord and you release them 0.5m apart, then if you are inertial the cord will stay loose. If you see the cord gets taught, then you can conclude with certainty that you are non-inertial

[@matthewhall6288]

@@tymmiara5967 This rebuttal may seem a bit out there, but what if the system were between two massive objects such as planets? Since you can't observe the exterior of the system, you couldn't be certain that you weren't between gravitational fields. The frame may still be non-inertial, but the experiment is to show whether motion is relative or absolute.

[@BLenz-114]

@@willo7734 It's not what you learn by pulling the cord, it's what happens AFTER you pull the cord. If the spheres move away from each other, their motion is real. If the cord stays slack, then they are NOT spinning relative to the universe.

[@mattcy6591]

As Stanley lifted his bucket, he felt a connection to all buckets everywhere. This adventure, he decided, was for all of them.

[@codehustler]

"The Stanley Parable"?

[@hrsmp]

@@codehustler they've added a section with a bucket in the deluxe version

[@ScienceClicEN]

To determine whether or not you're rotating or not, couldn't you just let go of an object, and see if it stays motionless or not ?

[@dialectphilosophy]

Great question -- you are referring to the "free body" test for inertia. The short answer is yes, you could. Except, like with the rod/spring experiment, there's a big "but..." That "but" is, how do you know that it is the object which is accelerating away, and not yourself, the observer? The answer is of course, that you are familiar with the materials with which you are working, and have understood and observed their behavior at times prior, and have a level of confidence that your object is a "free" one. This of course, requires referencing a greater collective of knowledge. That answer may seem a little pedantic and silly, but it goes to Mach's essential argument that all empirical knowledge must, by its very nature, be relative. If we import something that is "absolute" into our theoretical frameworks, it is not because it is a brute fact of nature, but rather because it is necessary for the understanding of our model, or because it is a useful approximation, like absolute space and time were for Newtonian physics.

[@GumbyTheGreen1]

@@dialectphilosophy If there's nothing but you and the object in space, then neither of you are fixed in place by any kind of axle, so you must be rotating around each other - i.e., around your shared center of mass - so when you let go, you will each fly (I wouldn't say "accelerate") away from that center of mass. Your speed will depend on 1) your mass compared to the object's and 2) the difference between the tension that existed in your arms before letting go and that which exists after. Correct?

[@ScienceClicEN]

@@dialectphilosophy Oh I see, thanks for the reply, it's pretty hard to imagine what one would infer without any knowledge of inertial frames!

[@gotbread2]

@@dialectphilosophy But one thing the observer could see is that the velocity at which the object drifts away depends on the location where it was let go. They would be able to determine that there is one position where no drift occures (on the axis), and that point would be special. Even linear motion of the observer would not prevent them from determining this "special point", hinting at some effect.

[@classicalmechanic8914]

When you let go object in space don't you create motion in opposite direction? Even if you are stationary, you are not stationary anymore when you let go the bucket.

[@joefromzohra]

One must distinguish between velocity and acceleration. Clearly, Newton had acceleration in mind. In the case of the bucket of water - when rotating the bucket, there are forces at work, hence the curve surface of the water, and the observer is in an inertial frame. But when the observer is rotating, the water surface is flat, but what is neglected in that case is that the rotating observer will experience a force, and is no longer in an inertial frame. And therefore, the rotating observer cannot claim that the bucket is at rest.

[@nolanliang]

Perhaps I'm missing something, but I don't think you'd need to ascertain the size of the spring at absolute rest. In fact, you wouldn't need a spring at all. Why not just cut the cord. If the end goes flying away, it wasn't at absolute rest. If it stays in place, it is. Edit: I just realized that you might object to this idea on the same grounds that you objected to the bucket test in space - that the bucket of water would need to appear at relative rest in both scenarios. But that feels a little pedantic to me. In both cases, the bucket itself looks to be at rest, it's just the water that's moving or eventually completely gone. But if you necessarily need to still be observing the water, what about encasing it in a glass sphere that may or may not be rotating? Once it gets to an equilibrium, the water would be in two different states depending on absolute rest or not, but would also display relative rest for both. Or heck, get rid of the container altogether - m.kzbin.info/www/bejne/eKncl5x4fKiqnrM

[@dialectphilosophy]

Hey Nolan, thanks for watching! We definitely understand your objections, and unfortunately due to the length of the video we weren't able to flesh out all the subtleties of our argument, but hopefully we can elucidate them a little further for you here: There are a number of additional "tests" for inertial-ness; the one you cite (cutting the cord) would be an example of the free-body test. On the surface it seems a good way to test for the presence of force; one certainly wonders why Newton never brought it up. Our understanding is that he seemed much more interested in situations of apparent rest (though possibly he also neglected it because the free-body test would have failed for the gravitational force, which Newton took to be real at the time). We feel this was likely because the free-body test involves multiple bodies in different states of motion relative to one another, so the question can easily be posed: who's to say that the end of the cut cord isn't flying away from you, but rather that you are flying away from it? And indeed, to be able to answer this latter question, you have to have certain prior knowledge about the nature and behavior of materials involved in your experiment. Unfortunately, this lands you in the same quagmire that the spring experiment does; such knowledge can only have originated in or been imported from frames that were already known to be inertial. We did not intended the spring experiment to be an encompassing look at tests for "inertial-ness", it was merely the simplest case we could come up with. Ultimately the issues boils down to a deeply epistemological one: our knowledge of reality -- whether it be knowledge of space, time, motion or anything else -- all rests on relative foundations. So to assert that something is "absolute" is equivalent to asserting that it is fundamentally unobservable. The question then becomes which un-observables you want to include in your descriptive framework of reality and whether you're comfortable with them being there.

[@GumbyTheGreen1]

@@dialectphilosophy Let's say that it's rotating clockwise right in front of your face. You cut the cord right when it's vertical. If the balls fly apart - one to the left, one to the right - and stay directly to your left and right (i.e., only their x values change from your perspective, not their y or z values), then you know that you aren't rotating and that the object was. What other conclusion could you draw?

[@-danR]

"If the end goes flying away, it wasn't at absolute rest" That's a standard conclusion derived from prior knowledge of Newton's Laws of Motion, and suitable for the usual Physics Girl, Veritassium, or Science Asylum presentation. If I understand Dialekt channel's viewpoint, it's not your standard "Here's the answer they didn't give in the back of the textbook", but a deeper dive to discover the primitives that underlie the behavior of nature. So we cut the cord. And a previously connected thing goes flying away. Or it doesn't. We have to uncover the mysterious factor that governs why things sometimes fly away and sometimes don't. The fact of that flight-motion and its connection with any purported prior motion has yet to be tied to some kind of ground principle.

[@-danR]

@@GumbyTheGreen1 One wouldn't immediately draw any particular conclusion without importing from a prior knowledge or experience with the behavior of bodies back on Earth. Keep in mind the strictures that the video imposed (blame Newton, because he started it all) repeatedly. Newton, Mach, and Einstein really _were_ experimenting under a particularly weird--to most people's way of thinking--set of mental constraints. Sensible people simply don't entertain notions of a bucket's water-surface developing a curvature because the entire universe is revolving around it. But once you set up such bizarre gedankenexperiment as a bucket of water in space in the first place, you have to consider all the variables and exclude none. So, perhaps a "force" (vaguely defined as yet) is generated with respect to the stationary, severed, system by my rotating around it. I know nothing of Kepler, Newton, or Einstein. I'm just a blind AI neural network, trying to empirically arrive at a self-consistent set of rules for this particular setup. And why not? In the real world doesn't something almost as strange happen through frame-dragging?

[@GumbyTheGreen1]

@@-danR That’s a lot of words and I’m not sure what they’re meant to convey with regards to my comment. The point is that when you cut the cord, if the balls fly away, they were absolutely rotating. If they don’t, you’re absolutely rotating. The fact that we might refer to the known laws of physics when drawing that conclusion is not a problem. Of course, virtually all knowledge builds on other knowledge - that doesn’t make it invalid.

[@jiminverness]

14:38 _"The question then becomes: _*_How does the observer know whether the measurement they make corresponds to the shortest possible length of the spring or not?_*_ For in rotational frames, only the shortest possible length of the spring or its natural resting length indicates absolute rest. Every other length indicates varying amounts of rotation."_ Use a spring who's coil has no space at rest (ie. each loop of the spiral is in contact with the previous loop and subsequent loop). Thus, any visible space indicates rotation.

[@kap1117]

You would still need to have prior knowledge that the springs natural resting state has no space between its coils tho. The experiment is not to observe the spring in one state and then observe it in its other state right after to be able to tell if its in motion or not, but to be able to determine if its in motion or not through only observing one state.

[@martinsoos]

The spring works for Newton if you can vary the speed of rotation from wone observation to the other. The shortest would have the least spin. To find zero spin in that direction you would need to get the spring as short as it gets before getting larger again. At its shortest point would be the zero spin for that plane.

[@frankyjayhay]

That seems the best idea, I'm surprised it hasn't been picked up on. Dialect did say in reply to a similar comment that you'd need a means to cause the rotation but surely it would be an easy task to conduct the experiment in the space station which either has or could be fitted with side propulsion to make it spin.

[@En_theo]

@@frankyjayhay Then what about this : "the eye" cuts the spring in the middle and sees if the blocks "fly" apart. If yes, there was a rotation, if not then there was no rotation.

[@travcollier]

No external frame is needed for the spring example, or Newton's marginally better tension example. Just measure the stress (force). But there really is no mystery here and Newton is just wrong. Angular momentum is a thing, and centripetal acceleration is also easily measurable. You can't have a rotating reference frame which is indistinguishable from a non-rotating reference frame without adding in some additional force. By our current understanding of physics, there is no absolute motion or absolute rest. I'm pretty sure that idea was discarded long before absolute time and space were. BTW: Netwon was very often wrong when he got into anything remotely philosophical. He was a fabulously brilliant mathematician and a crackpot at the same time ;)

[@tombackhouse9121]

@@travcollier Was newton wrong? Forgive me if I have the wrong end of the stick here, but it sounds to me like the assertion you made about centrifugal effects in a rotating reference frame are more consistent with Newton's view than Mach's, at least as presented in the video...

[@tombackhouse9121]

@@frankyjayhay Or one could simply rotate two copies of the mass and spring against each other in opposite directions, and observe the length change that way. No need to invoke spacecraft thrusters at all then.

[@liggerstuxin1]

Crazy that Newton was so long ago and we still struggle to understand his genius. To have a set of physics and an understanding that lasts for so long

[@Hypernefelos]

Just put a lid on the bucket. If it's spinning the water will take the shape of hollow cylinder in the absence of other acceleration vectors (such as gravity). Make the lid transparent and you'll be able to see inside. Point to Newton. That said, one _could_ have a gravitational source spinning around the bucket, which would produce the same effect. So point to Mach, but not because of the water spilling out to space.

[@akulkis]

If the gravitational force was orbiting the bucket along its equator, it would produce tides, not a cylinder of water.

[@davido.newell4566]

You are right, @@akulkis : the cylinder would experience some tidal effects, but not likely enough to notice, probably. What altitude, how many orbits/ day.

[@Pengochan]

The final argument is bogus. And yes, I'll address the "a priori knowledge" argument thoroughly below. The rotating observer could orient the spring/weight system in different directions (the weights could be constrained by letting them slide on some rods). In zero gravity one could even just observe a sphere of water held together by surface tension (or a water filled balloon). If it rotates it becomes an oblate ellipsoid, unlike a sphere when not rotating. The axis of rotation would emerge to be special I.e. he doesn't need a refernece frame outside his rotating system to know he is in a rotating system. Also in a rotating frame of reference there's not only a centrifugal force, but also a coriolis force, which could also be measured. In fact it is possible to determine earths rotation even in some laboratory without any view of or access to the outside world, or any a priori knowledge beyond F=m*a, given sufficient experimental equipment. Foucaults pendulum is a well known experiment. Even without *any* prior knowledge at all eventually (after taking a few centuries deriving the laws of motion) phsicists in a rotating reference frame would find out that there is a special direction (the axis of rotation), and be able to determine that they are in a rotating reference frame and the angular speed with which they rotate. At some point they'd be able to counter that rotation and generate nonrotating environments for experiments (analogous to microgravity environments). In fact that's exactly what happened when physicists experimentally demonstrated, that it's not the fix stars and the sun that somehow rotate around a still earth but instead the earth is rotating about its axis. Up to that point no physicist or in fact any human had ever left their rotating reference frame: earth, yet they were able to distinguish between these situations; a rotating earth vs. a still earth with everything else moving around it, based on experiments they did in that same reference frame. Your "clever" argument about a priori knowledge just demonstrates your lack of understanding the physics or even the history of physics. It is unhelpful to pick up a thought experiment from 300 years ago, stretch it beyond some limit irrelevant to the underlying question, and based on those limitations crown a "winner" especially when the creator of said thought experiment is long dead and can't respond. This is not how physics works, neither as a science, nor in nature. It's like arguing that eventually the buckets would rust and the water evaporate, so Newton must've been wrong. The true problem with "absolute motion", i.e. (linear) acceleration, is that it is indistinguishable from the forces experienced in a gravity field, something that Einstein based his theory of general relativity on. Newtons bucket experiment is about absolute rotation (i.e. a separate case from linear acceleration), and for that Einstein coined the term "Mach's Principle". To my understanding Einsteins conclusion was, that if the whole universe (all masses in it) were rotating at a fixed rate it would be indiscernible from a situation without rotation. The Lense-Thirring effect seems to support this. There are really better discussions and arguments to be found on the subject of Mach's Principle with minimal search. The sad thing is, that this is really an interesting subject and scientific discussion, but the arguments presented in the video are just silly and lead to nowhere, or worse, a misguided illusion of understanding. If this video is supposed to demonstrate dialectical reasoning I'm not impressed.

[@boguslawszostak1784]

You assume unjustifiably that in the state of "zero gravity" there exists some distinguished absolute coordinate system in which the ball may rotate ... or not. What if the ball creates this coordinate system and it does not rotate in it? How do you make a ball spin when there is nothing else? Only the second object creates the distinguished direction, only many objects create the structure of space. We forget that "interactions" give structure to space. In real space, inertial systems do not exist, they are only a local approximation. In general relativity, motion and time are inseparable from mass, because it is the resting mass of an object that prevents it from moving "at the speed of light", the "infinite energy" needed to accelerate the "non-zero rest mass" (whatever it is) prevents to achieve this speed. An object with zero mass moves at the speed of light and time in its frame of reference does not flow (zeroing) and the distance that it travels is also zero.

[@Pengochan]

@@boguslawszostak1784 Did you even read what i wrote? The thing is: If that absolute frame of reference exists, or if it is the rest of the masses in the universe determining this absolute coordinate system is still an open discussion, although as far as i can see there is more experimental evidence supporting the latter. I already wrote that. The problem with the video is: It doesn't go there. Instead it makes a completely bogus argument that is more about mincing words based on the original formulation of some thought experiment, than about looking at the actual physics. The video demonstrates a completely wrong way of learning anything new about our universe. Instead of looking at experimental knowledge, and how that supports or disproves one hypothis or another it gets hung up on some very rigid interpretation of a centuries old formulation of a thought experiment. My point was: even without any prior knowledge or "looking outside" it is possible to determine if one is experiencing forces resulting from being in a rotating frame of reference. Discussing the state of mind of some hypothetical physicist in that thought experiment doesnt't tell us anything about how the rest of the masses in the universe may be what determines an absolute frame of reference.

[@igorkarlic2297]

Bingo. You could rotate faster or backward and determine minimum stretch. 🎉

[@CandidDate]

Round four: spin two spinning buckets of whiskey. The eyeball drinks a drop each time relativity is proved due to coriolis effects spilling the whiskey and gets so drunk, doesn't care about absolute space proof anymore and starts talking about the aliens it sees instead!

[@keith.anthony.infinity.h]

Haha 😂 This made my day. Thank you! Haha 😂

[@bushraj03]

Realllllllll🎉

[@dennisgunn468]

Regarding round 3 If the spinning object is a closed tube full of a compressible fluid then the observer would be able to observe the relative lack of pressure in the fluid at the center compared to the higher pressure at the ends of the tube and use that data to make some kind of deduction. This version would also work if there was air and water in the tube and the observer had to try and figure out why there was water at both ends of the tube and air in the middle. Also the limits of the tools used to observe the effects on the spinning object are relevant to the discussion. If the object was just a spring with no weights for instance the profile of tension in the spring would exist but it would look different.

[@donaldcharlong9586]

This comment regarding observing the pressure differential in the ends of a cylinder as compared to the center is correct. One might also place known different weights on the ends of a spring. An equation can then be found for the tension in the spring verses the rotational rate. Solving for the zero point would give the rest length of the spring. It would seem that motion IS absolute.

[@TheMorhaGroup]

​@@donaldcharlong9586How exactly do you believe that proves motion to be absolute? If something, other than motion, is required to deduce motion, then motion is relative to whatever you are using to deduce it

[@TheOneMaddin]

I would say there is a difference between being able to discern between a rotating and a resting frame, and being able to tell which is which. I always thought Mach's argument was about the first, not the second. As you say, for the second you need knowledge of the physics that you must have acquired in a system that you believed to be, say, resting in the first place. You don't need this for the first question. And I consider the first question much more meaningful. After all, you can just CALL a system resting if the spring is minimally stretched, and no one can prove you wrong because thats just your definition of rest.

[@miquelmarti6537]

To Newton: Moving and rotating are very different things. Centrifugal force doesn't mean that the movement is absolute. To Mach: Anyway I don't buy the "able to know" argument from Mach. For example, the Earth rotates and the equator expands despite of any axial reference.

[@Buddahabrot]

Wouldn't your ruler ALSO become distorted/stretched due to the rotation of Mother Earth?? That would interfere with an accurate measurement. BTW I upvoted your remark because I think 🤔 it is a very smart question!!

[@bautibunge737]

Hi Dialect, great video. I have to say that i don't agree that the case of the spring requires knowledge about a different distant system, you just need that de observer can actually make an experiment (interact with the system). You can locally define the restness of the system as follows: "If, when you change the state of rotation of the system in any direction, the spring increases its tension, then the system is at rest. If there is a direction such that, when you make the system rotate in that direction, the spring decreses its tension, then the system is rotating in the opposite direction."

[@timelyspirit]

Doing what you say would help determine rest vs motion, but requires modification of the system. The point made here is you couldn’t “wake up” atop one of the boxes connected to the spring and fundamentally know whether it was in a state of motion or rest from observation.

[@bautibunge737]

@@timelyspirit Well maybe, but restricting ourself to that case means eliminating the notion of experiment. In that case you can't get dynamical laws of any kind, as you are only able to know the present state of any system. Also, if we still belive that what we have observed on our actual universe still aplies in this hipotetical universe (not that the observer actually knows how the actual laws works, but that their effects are still present), the rotating system would loss its energy due to gravitational waves, tending towards equilibrium length. So in this case the definition of rest would be "conservation of string length". Althought I would understand if we want to restrict ourself to a fieldless universe in this thought experiments.

[@Alberto-mq7gw]

Yes, the example seems to ignore other laws and accepts perpetual motion. The fact that perpetual motion (defined as perpetual without outside energy contributing to it) doesn't exist in our universe already tells us that _in our universe_ motion is always relative to the universe itself and not to anything else. Therefore both arguments are partly right and partly wrong. Within the universe we have to take motion as absolute, since it does not depend on the observer or any other object in the universe. But ultimately, motion in the universe is always relative to the universe itself. This has never been understood by physics, even though it's a relatively simple fact.

[@bautibunge737]

@@Alberto-mq7gw Well, that would mean that you can talk about the state of motion of the universe itself (or of space itself, if I'm understanding correctly what you're saying). But to talk about this fells odd to me, as for example, you may be able to talk about the acceleration of the universe relative to something (you can define it as minus the proper acceleration of this object) but you won't be able to define the speed of space with respect to anything, so to associate motion to space itself is a strange kind of notion.

[@Alberto-mq7gw]

@@bautibunge737 Our universe is something finite and therefore you can imagine this simplistically as a room with walls, floor and ceiling. Inside that room anything can be defined as moving or at rest relative to anything else in the room. But ultimately what really defines real motion is the room itself. Anything that moves relative to the room is in motion and anything that doesn't move relative to the room is at rest (within our universe). It's a simple concept that is compatible for example with the laws of thermodynamics and related ones (conservation of energy, impossibility of perpetual movement,...) while standard definitions of movement in physics as being relative to the observer are incompatible with them.

[@michaelkaliski7651]

Well the simple answer has to be that all measurements are relative. In order to conduct a measurement, there needs to be a standard against which whatever is being measured can be gauged. While a self contained system is rotating or doing whatever in empty space, an observer will be taking measurements against their own imposed standard, or indeed relative to empty space, otherwise no information is obtained and the experiment fails. The main difference in the approaches taken by Newton and Mach is whether or not the observer is external to the experiment or is part of the experiment. Therein lies the discrepancy in their results and conclusions. Thus motion can only be conclusively demonstrated by comparison to and against some other object or standard and is therefore relative to that object or standard.

[@MadScientist267]

Fear not. It's only been a few thousand years...

[@awesomedavid2012]

Good argument but I find two flaws. If the object or standard being compared to is the universe itself, then motion is relative - relative to the universe. Which means it's absolute. Consider the original rope-mass system. If you allow the observer to cut the rope, a stationary system will do nothing. In a rotating system, the masses will spread apart because of newton's first laws. Is that measurement of the spreading of the masses merely due to gauging against a standard?

[@TheMorhaGroup]

​@@awesomedavid2012All your rebuttals require the knowledge of notion to exclusively come from comparison, yet you still claim motion to be absolute, how exactly does that make sense? 🤔

[@snakefl]

"Fire the sound engineer!" "The music is not loud enough."..I can still hear the very interesting lecture. Can think about the subject; But at least I can hear the music.

[@apolloniuspergus9295]

As I appreciate this channel a lot, let me play the Advocatus Diaboli: The tension in the rope or spring has an absolute observable: the point of rupture. And, if it is true that gravity generates observational differences in the bucket experiment, doesn't this mean that acceleration too would generate them, ultimately making it possible to tell an accelerating object apart from a stationary one? In addition to that, isn't the difference posited by a gravitational field only quantitative, that is, there's a difference in shape and behavior of the water, but not a difference in the ultimate fact that the water moves when the motion is on the bucket, but remains still when the motion is on you? I believe there's a long and fruitful discussion possible on this subject.

[@dialectphilosophy]

We enjoy our Advocatus' Diabolis! Indeed there are a great number of subtleties in this argument we didn't have time to fully flesh out. The commenter below has made the same point about the rupturing cord (the free-body test) and we recommend you read over our full response; but briefly the same point can be made about the free-body test as can be made about the spring test. Can we define what constitutes a "free body" without using information/knowledge gleaned from inertial frames elsewhere? For, if we have to important information from external frames about how bodies should behave once freed from one another, then we are stuck at the same place which the spring experiment leaves us stuck.

[@apolloniuspergus9295]

@@dialectphilosophy Thank you for responding. Although I spot the similarities between my comment and his, what I meant by "the point of rupture" was the precise moment when the objects passes from a single object to a broken object. We have no other way of explaining this other than by the action of a force. And while we sure can explain acceleration mathematically without the notion of "Force", not the same can be said about the converse proposition. We can't explain force well without acceleration. Meaning that there must have been an acceleration over the object. We can play Hume's epistemological game and state that there's no need to accept laws of causality, and we can say that the rope breaking was something that just so happened to coincide with the apparent rotation, but what good can come from this? I still think the Twin paradox is unsolved, though. You people from Dialect should also check out Bell's spaceship paradox. To my eyes, this paradox is taken less seriously than it should be.

[@dialectphilosophy]

@@apolloniuspergus9295 Bell's spaceship paradox actually interests us a lot, but we haven't been able to reason our way towards any conclusive understanding of it yet. Check back in later with us on that one! The spring experiment as shown in our video doesn't actually preclude the observance of a force; for instance, an observer who sees a stretching spring would know (given they know how springs work) that there is a force being applied within the system. They simply can't use that stretching to identify whether they were in an inertial frame or not to begin with. Similarly, the free-body might tell us there is a force at work somewhere, but it can't be used to identify "true motion" from "true rest". As for the association of "force" with "causation" that's a whole different can of worms. But you might want to consider the perspective of General Relativity, where you can have two bodies of mass that are the cause of attraction towards one another, but neither body experiences a force at any point.

[@-danR]

Outside a knowledge and memory of what constitutes "rupture"*, I don't find an absolute observable. I'm just an eyeball in space, and I'm back to an observation by Dialect in an earlier video: what is the physical meaning and implication of some kind of fire erupting from the back of my spaceship? Perhaps it's a phenomenon accompanying the presence of a gravitational field and that (helpfully) manages to keep me in a fixed position--how and why being issues to be resolved by further study. _____ *that may sound supercilious, but until the full nature of the chemical bond (something entirely lacking in Newton's day, and poorly understood even by Mach's contemporaries) is unveiled we are not outside the bounds set by the Master himself; Newton demanded an epistemologically context-free environment for his experiment with balls and cord.

[@airfriedquadsbw]

You can't have motion without a place to move to. That alone makes motion relative. If you put everything in the bucket. (A set that contains all) the bucket would have no place to move to. Only the thingsnin the bucket could move, and that motion would be relative to the other things, or relativebto the point it was at and the points it moves to. But the bucket would contain all points, it can't move because there is no other relative point. The bucket can not be rotated because you wouldnneed a start point or a 1° and then a 1.0001° but you dont have them, those points only exist in thw bucket. So you can't rotate the bucket without a relative starting point. Motion is therefore relative.

[@elymaranhao1418]

Well, you can use two systems of coils and masses, the masses made of the same material but with different volumes (so they don't have the same mass). If the systems have the same length they are at rest, if not they are rotating.

[@atmospheres11]

Get 3 weighted end springs, all parallel, all the same length in reference to 3 rigid rullers. Then once shown all to be equal, reposition each spring and ruler as 3 sets of each pair on the x, y, z axis. Measure all 3 and average the results to quantify motion (spin) in any direction and any observer can see the variation in average change per axis and overall to calculate direction even if observer was fixed to any of the axis or indifferent to all 3. I manufacture directional drill guidance systems and using gravity and magnetic variables we can reposition (relocate, move) drill heads up to 2 miles away in ground with an error of only a few inches in any direction so this stuff is part of my daily problem solving servicing of the equipment. We also use Time Of Flight 3 axis radar for open air positioning for tracking assets on the ground in heavy industry like workers walking around trucks etc on mining sites. I think though that there is a deeper yet simpler approach to this subject that isn't Mach nor Newton based. Experientially I personally believe conventional science is not on the correct path in either case. As I watched this well presented video I could see variables unaccounted for. It is a more appropriate case to say that every atom is affected by and affects every other atom in all the 'universe' in ways that can't be quantified yet is simple in its function. The problem we humans have is in using cut down concepts that are mentally manageable to try explain an infinite system no matter how simple. 'Define' (remove something) actually 'means' (averaging) taking away from the subjects full details and in doing so makes an explanation impossible. Eg, the only way to represent the universe correctly without error is to present the universe itself. Math is NOT the law of universe, math like time is a human construct, just as space is the absence of 'something'. Simply put, we can't reify space! Space ISNT! Time isn't! And the notion of spacetime is a nonsensical human construct that has no capacity to BE outside of thought experiments and should have no place in any teaching as if spacetime actually factually in any way 'exists'

[@Pukovnik141]

@@atmospheres11 If you happen to teleport to such system, how would you know what spring is at rest and what is rotating without previously knowing spring length at rest. What you explained is the same as explained in the video. You would need to know spring length prior to going to that reference frame and also have knowledge how springs behave.

[@Pukovnik141]

What if both are rotating and spring is same length? And how would one observer that is on one system know min spring length or know that other systems parameters. It makes sense if you are outside of that reference frame, but if you are on one of the systems, how would you know. Similar frame is earth-moon > rotating or not? It is easy to see from solar probe outside of the system.

[@Pukovnik141]

If a rope and scale is used, one could measure tension and determine rotation/rest. Spring example is wrong way of displaying this problem, as explained in the video.

[@NeonGreenT]

@@atmospheres11 You're absolutely right, Whole-ism is the THE THING. I always say there are no real numbers to begin with - only fractions of the whole. There aren't two apples, there are just two fractions of an apple tree. energy density of matter tends to infinity. Wave function of electrons tends to infinity. likelyhood of locality of an electron in orbital model tends to infinity. magnetic volume tends to infinity. electric anti-volume tends to 0 (or a point). all those are just referencing the whole universe. Hamilton knew this with his quaternions.

[@u_t2347]

Simplify the experiment a little, have just a ball of water in space. If the shape is oblate spheroid, then it's spinning. If it's perfectly round, it's at rest. After all, a bucket of water in space would never be flat, ignoring the fact it would freeze instantly. I think the hardest part of this experiment is getting anything to absolute rest, even empty space is expanding.

[@jtws124]

wouldn't it boil away?

[@peterjansen7929]

@@jtws124 That just needs one more refinement: You could use mercury, because that will allow you to observe it at relative leisure before it partly evaporates and partly solidifies. Preferably, you would reduce the question to something much less complicated: If all the universe vanished, except the planet Jupiter and the observer, would Jupiter still be flattened or would it turn into a perfect sphere? The real solution, of course, involves the fact that the universe isn't going to vanish - that we are asking about the behaviour of a completely hypothetical, that is fictional, universe. A fictional universe will behave just as the author of the fiction wishes it to, just like anything else in fiction. Newton and Mach can write incompatible SF. Neither version has much to do with the world we actually occupy. (Materialism itself is untenable, anyway, but that is a different subject and need not concern us here.) Incidentally, the paragraph above implies that on this one point it was Newton who was more pragmatic, while Mach was more dogmatic! In Mach's SF, Jupiter would HAVE TO become spherical, as it would have nothing relative to which it COULD rotate. Newton, asked what Jupiter would do, would have claimed ignorance and stated that one would have to look and see!

[@Markoul11]

Acceleration is absolute because the observer will see the spring extending. Therefore, acceleration is always observable.

[@Vaquero4382]

"Tension" could be an independent measurement. The absence of tension is rest, any positive tension is motion.

[@MacShrike]

Thinking along the same 'line' (pun intended) yet isn't that Tork? Since you don't know the in-rest shape of the spring you cannot tell if it has expanded. But reading a Tork meter would expose the, indeed, tension on it. Anything greater than 0.0 would show rotation.

[@charlesbrouwers9377]

An observer in a spinning system will detect a centrifuge force, they might not be able to distinguish it from a gravitational filed but they will be able to determine one of the two need to exist.

[@kevinlevin7672]

Both thought experiments are limited in that they are attempting to observe only one type of movement while other types of movement are possible and the experiments cannot test for them. Are you moving or is it only me? Great vid!

[@pierre8235]

14:38 "The question then becomes: How does the observer know whether the measurement they make corresponds to the shortest possible length of the spring or not? For in rotational frames, only the shortest possible length of the spring or its natural resting length indicates absolute rest. Every other length indicates varying amounts of rotation." Well, just rotate the spring rod one way, and the other, relative to you. If you are truely at rest, with both rotations, the lengh if the spring will increase. If you were not, with one way of rotation, it will increase, and with the other, it will decrease. Hence, telling you if you were originally rotating or not.

[@pierre8235]

Or, just take water. If you want to see it in equilibrium, without altering it in any way, well just take water. If it is a perfect sphere, it is not rotating. If it is slightly bulged, rotating. There is no "interfering", or having something without equilibrium, so you can't argue that.

[@jaca2899]

15:53 Just move the spring around into various orientations to see if its length changes. Its length should be the minimal length whenever the spring is aligned with the axis of rotation

[@dialectphilosophy]

You wouldn't be able to move the spring around and apply your own force unless you had greater access/awareness of the total system, which would certainly violate Newton's desire that the knowledge of the motion be "locally" confined.

[@jaca2899]

@@dialectphilosophy What do you mean? The changing of orientations is a purely local thing to do. I'm not translating the spring onto another location in space. I'm simply changing its orientation while leaving the spring in the same location in space

[@dialectphilosophy]

@@jaca2899 We have to remember that this is an "abstract" thought experiment, and not a real experiment. The purpose is to remove as much as the external environment as possible, to demonstrate the reality of motion beneath. If we allow the observer to manipulate the spring using by applying various forces, we then have to ask a series of follow-up questions: how does the observer apply the force, i.e. through the use of what tools? How do they know if those tools work? Would these tools even make sense in an empty-universe environment? Ultimately, how do they know they are actually applying a force without the aid of passive measurements? If you think it's silly to try imagining experiments with spinning springs, rods and observers in an entirely empty universe, then you are arguing Mach's argument -- that none of these experiments makes sense when removed from the context of a greater environment.

[@khanmaxfield7974]

@@jaca2899 if you didn't know how fast it was rotating to begin with i would think it would be impossible to distinguish the effect you are describing from gyroscopic effects

[@jaca2899]

@@khanmaxfield7974 I don't think so. The spring would undeniably change its length depending on its orientation in space, as long as the rotational velocity is non-zero. You don't need to know the rotational velocity in order to observe the spring changing its length

[@cmvamerica9011]

There is no absolute motion or absolute rest; because motion and rest are defined by what matter is doing, relative to other matter. If only a single object existed in the universe, it would have no motion except it would feel any acceleration because the atoms in the object would be in motion relative to the other atoms in the object.

[@deepthoughtswithpaul604]

15:59 I don't think so- the observer could cut the connection between the two masses and measure whether they moved apart using the measuring stick. If the masses moved apart after cutting the string, the frame is not inertial. If they did not move after the severing of the spring, the frame is inertial. No calibration is needed to determine this.

[@TimJSwan]

I agree. Also, motion actually is relative but depends on the surrounding space-time which is affected by surrounding mass. I think if you were in the middle of a neutron star spinning fast around you, your rest spin would match closer to the spin of the star.

[@MadScientist267]

Holup. You still haven't proven if you yourself are rotating or if you are part of an entire rotating system. Centered on Earth's rotation for simplicity, "flying" at the equator... the ball/string/spring model cannot distinguish the rotation of the earth from its orbit around the sun. A real world experiment for this may indeed involve a spring: The solar noon side will experience more compression (want to move toward the ground) than the all else equal spring sitting at midnight, because of centrifugal force from Earth's orbit. So are you spinning with a partner or are you stopping a twirl? Either way your arms want to fly out 😉

[@NeonGreenT]

Yeah but an observer doesn't cut. thats the whole point, how would an observer determine it without modifying the experiment ? Or to make it simpler, lets say the Balls are two Planets and the string/rope/spring is actually gravity. how would you determine anything now, as just an OBSERVER ? only if the distance of those planets never shrinks you could say they must be orbiting. But this only holds if you just have masses, motion and gravity. But reality has more forces which could be at play that the observer doesnt know of and in return would draw wrong conclusions. I guess this will never be fully resolved as you cant abstract away the entire universe and then postulate that's how the universe works in the first place.

[@deepthoughtswithpaul604]

@@NeonGreenT In order to see something, light or some other particle has to be shot at it and then you can measure how the particles scatter off the object. Even this modifies the system in a small way. There is no way to observe a system without changing it- this is a basic principle of quantum mechanics.

[@deepthoughtswithpaul604]

@@NeonGreenT Also, if the objects were connected by gravity instead of a string, if you are in a rotating frame, there would be an asymmetry in how a third object would orbit if you launched it to the left or to the right. You wouldn't actually need two objects orbiting and your own third to measure this- you would only need one preexisting and a second that you launch into orbit with a known velocity from your own reference frame.

[@Matyanson]

1. You have 2 globes attached to a spring like this: ⬤|/\/\/|⬤ 2. Attach identical spring and globe to the globe on right: ⬤|/\/\/|⬤|/\/\/|⬤ 3. Repeat step 2: ⬤|/\/\/|⬤|/\/\/|⬤|/\/\/|⬤ Compare the length of each spring. Now apply force to the rightmost globe (perpendicular to springs). With no prior knowledge the observer could discover there is some 'force' affecting the springs. Apply force from one side: the springs stretch more, From the opposite side: the springs shrink, Continue pushing. The springs reach the same size eventually. (They could achieve that trough trial and error with no instructions or without knowing what is it they want to find out) The observer could distinguish between the system rotating more, less and not rotating at all. They would just have no idea of knowing that not rotating is the inertial frame. And that when the springs have the same length, that is the inertial length of the spring. But still, the centrifugal force has to be absolute. If there are these 2 systems (one rotating one not) and 2 observers (one rotating one not). Then they would observe the same thing! One system would have spring more stretched than the other. The only think that would be relative is if they decide to label the state of 1st system to be rotating or label that the other state that. What is important is they could differentiate between each state and all observers would be in agreement!

[@gastube22]

Very good and well-explained, but I think you've changed the "2 connected bodies" example by swapping the cord for a spring, and then reasoning about the measuring the length of the spring and what meaning can be attributed to that measurement. In the original version, where a cord was used, there was no suggestion of measuring the tension in the cord by measuring its length (with its consequential need for a rest-state length-reference). If a reference-free method of determining the presence (in motion) or absence (at rest) of tension in the cord is possible, then the arguments used about length are not applicable in that original version. It's a pity that some discussion about the direct determination of whether or not tension is present wasn't at least covered. Is reference-free detection of cord tension possible in therory - and in practice?

[@MrRolnicek]

Of course, there's plenty of methods. Simply putting the cord between your fingers and twisting with a known torque will tell you based on how much resistance there is from the cord the speed of "absolute" rotation (as long as you know the masses at the ends but you'd probably measure them by accelerating them and that would already tell you if there's tension from the cord). In the end if you had a LOOONG time and only wanted to know IF there's any rotation at all, you could just observe whether gravity pulls the two weights together or not. A cord resists tension but it will simply fold up if you "push" on it.

[@knurlgnar24]

Newton wins the 1st round. If the entire universe consisted of nothing but a bucket and a quantity of water, the water and bucket would attract eachother and you'd have a bucket with water vapor surrounding it. If the system was spinning you'd have a bucket with a disc of water vapor around it. The argument still works perfectly.

[@mmenjic]

15:26 observer can additionally rotate all that in random directions and if he finds one where spring is shorter than now or shortest at all then he found the natural state of the spring and also he found out that spring and masses were rotating before. Pretty simple !

[@simonruszczak5563]

Gravity can't go to zero, the bucket of water itself has its own small amount of gravity. Do the experiment in space with a very big bucket of water, the size of an asteroid or bigger. Round 1 to Newton.

[@einszwei6847]

One of the things I seem to not really understand: You said that in the experiment with the two globes and the spring the observer in the rotating frame could not know that the length of the spring he observes ist not the shortest one. But why doesn't he just try to rotate the two globes in his frame? If he would by accident cause a rotation that at least partially counteracts the rotation of his own frame the spring should shorten, or am I missing something? Of course the observer would not a priori know which axis of rotation he had to chose. Additionally he could accidentally let the globes spin "too fast", i.e. such that in the inertial frame both globes spin in the opposite direction than the observer, but faster, in which case the spring would extend. But the observer is not limited to one experiment. He can vary the axis of rotation and the angular velocity with which he spins the globes in his frame arbitrarily often (systematically or randomly like in a Monte-Carlo-type experiment). And by varying in smaller and smaller steps he would eventually reach the point where he observes that the spring shortens, i.e. he is in a rotating frame himself, or he would give up when he is sure that the angular velocity of his frame had to be so small that the error introduced by assuming that he is in an inertial frame is negligible compared to his error of measurement. So the observer could either determine in potentially infinite time whether he is in a rotating frame or not, or he could determine in finite time whether he is in a rotating frame or approximately in an inertial frame. And the "approximately" would also be the case with any other realistic experiment because of measurement error, so it should not be a problem, especially because the error could be reduced arbitrarily by just varying in smaller steps (i.e. more repetitions of the experiment).

[@dialectphilosophy]

If you read other comments, you'll see there are a lot of "just try this" and "why not just do that?" responses. But these are all missing the main take-away from the video: it's not about how clever your experimental set-up is. Mach's argument is inherently about knowledge at its core. To measure absolute motion through whatever experiment, we always have to invoke some prior knowledge about our instrumentation tools and/or our system. This knowledge can only be relative to other observations and facts/data gathered in the past. This is what makes "absolute" motion impossible, not the lack of cleverness of one's measuring device.

[@GitBits]

@@dialectphilosophy eins zwei's suggestion does not seem to me to require any particular prior knowledge regarding the elements of his thought experiment... Unless you consider "knowledge of how a spring works" to be "prior knowledge about our instrumentation tools" ?? If that's the case, then no conclusion can possibly be drawn from any scientific experiment, because conclusions are always based on stated or unstated knowledge and premisses. Among others: that the experimenter has a minimum understanding science and deductive reasoning. But I would consider that to be philosophical mumbo-jumbo, that adds nothing interesting to the debate. Experiments are of course expected to be carried out by people with at least a minimum amount of knowledge and scientific training. This does not in my view make "absolute" motion impossible. In fact, I question the term "absolute" motion. What we are discussing here in the context of the Mach Principle, is the distinction between UNIFORM motion and ACCELERATED motion. Uniform motion is the basis of Einstein's Special Relativity, and he successfully proved that such motion is relative. Accelerated motion, however, is something else, and is dealt with in General Relativity. And that is what we are dealing with in the context of the Bucket experiment. Rotation induces a centrifugal force inside the bucket which causes an acceleration away from the center... Which CAN be felt by an observer sitting on the bucket floor. No need to look outside. The fact that you feel pressed against the bucket wall will tell you that the bucket is rotating. (or in a gravitational field... but this option can be eliminated if you start walking around... does the direction of the force change ? Or does it always go thru the center ?) But I wouldn't call that "absolute" motion; I would call it "acceleration" - which IS absolute according to General Relativity... But then, would knowledge about centrifugal forces and General Relativity fall into the category of "observations and facts/data gathered in the past" ?

[@sphaera2520]

@@GitBits I’m having a similar struggle with the point of the video’s/mach’s argument. It would seem the whole “a priori knowledge…” would disqualify any attempt because we would need to presume the observer knows anything or understands the need to carry out this experiment. If I’m understanding the response I’ve read thus far, anything we could imagine must include an observer who understands what matter, objects, physics, are and understand their relationship relative the experiment being conducted. What is a spring, what is a ruler, why must anything be measured, what even is a measurement, etc…

[@crabby7668]

Surely the rotating bucket in space, without gravity, is not in equilibrium as it is losing its contents. On the other hand the same bucket at rest would probably lose its contents also. I don't know what that does for the argument, but it does seem to change it.

[@eigenchris]

Could you use 3 identical copies of the mass-spring-mass device and orient them in 3 orthogonal directions? I'd figure that would allow you to determine which axis you're rotating about.

[@dialectphilosophy]

Hey Eigenchris! Thanks for watching and commenting. You probably don't know this, but we learned half of general relativity from watching your videos. We absolutely love your channel, and are quite flattered you stopped by ours! In terms of our inertia argument, an experiment such as what you propose is what you'd find a plane or something of the like and which can be used to constantly orient oneself to a "true" direction. However, the point we wanted to make in our video is that this orientation still happens with respect to a greater environment, and that once the environment is removed, that orientation is no longer truly definable. Newton's argument is that we can always define true motion/orientation even in the absence of the environment; Mach's is that we cannot. We side with Mach on this issue, simply because while "true" motion is measured relative to inertial frames, the definition of an inertial frame is somewhat ambiguous and seems in to invoke a larger collection of frames of reference (an environment) from which an average can be extracted.

[@eigenchris]

@@dialectphilosophy Ah, I didn't realize. I'm glad my videos are helping people. I really liked your previous video on the metric. The visualizations were great and must have taken a lot of work. Though I do notice a number of videos have a recurring them of rejecting the idea of an "objective" inertial frames, and this confuses me. My understanding is that the difference between inertial and non-inertial frames is objective and physically verifiable, and this resolves any twin paradoxes you can come up with. To check if you're in an inertial frame, you just need to carry an accelerometer with you (accelerometer begin a ball surrounded by springs suspended inside a rigid box). If all the springs are in their neutral position (i.e. all the same length), then you're in an inertial frame. This is consistent with what you'd see floating in deep space, or falling out of an airplane towards earth. If any of the springs stretch/compress, then you're in a non-inertial frame. This is what you see in an accelerating rocket in deep space, or standing on earth. This covers linear motion. For rotational motion, you could grab 3 rigid rods and tie them together so they are all perpendicular (like mini coordinate axes) then place an accelerometer at each of the 6 ends. Again, if all the springs are neutral/the same length, you're in an inertial frame. If you see the balls fly "outward" along the x and y directions, you know you must be rotating in xy plane, relative to an inertial frame. Technically only 3 accelerometers are needed (1 along each axes). I'm not sure if you consider this apparatus an "environment" but it seems like something that could be done on any smartphone (though it's done with silicon chips instead of a spring-ball system).

[@dialectphilosophy]

@@eigenchris There's a lot to be said on this subject! To some degree, the questions being posed by us and yourself go to the very depths of the ancient philosophical debate of "what is motion" and whether it can be regarded as real or not. But moreover, these questions are really about the various roles that knowledge, deduction, and observation play in our theories of reality. To us, first learning relativity was extremely frustrating. You get told all this is crazy stuff -- time is relative, space is relative, rest and velocity are relative, and just when you learn to accept these difficult, non-intuitive concepts, they hit you with the fact that, oh wait, good ole' acceleration is just the same absolute, concrete thing it was in Newtonian physics. And you're like, wait, acceleration, that thing who's definition is velocity over time? Velocity and time are both relative, meaning all the components that make up acceleration are relative... yet it somehow winds up magically coming out that acceleration is absolute? The answer to this confusion, of course, is that the accelerometer experiments (and their many variations which you describe above) do not actually measure "absolute" acceleration. They measure the 3-acceleration of the measuring device relative to the observer performing the measurement. This measurement of 3-acceleration is then coordinated to an "absolute" acceleration not via another measurement, but rather via an inference or deduction -- namely the inference that this 3-acceleration occurred in an inertial frame. To be able to determine absolute motion from observations and measurements alone, one would first have to establish that the frame from which they were making the measurement was inertial, using only measurements. But since this inertial quality would have to be established PRIOR to using the accelerometer, this means that no matter how clever our accelerometer is, it could never identify true motion is in the absence of a pre-established class of inertial reference frames. It is basically Kant's classical notion of apriori or transcendental intuition which he delineates in The Critique of Pure Reason, and without which he states that the manifold representations given in a sensuous intuition could not be conjoined into a unified conception of understanding. Another way to say this is that, all the experiments you describe require the observer to be familiar with the instrumentation that they are using, and to have a prior understanding of how that instrumentation would behave in inertial systems. This is the knowledge that allows them to infer, once the 3-acceleration measurement has been made, that such a measurement corresponds to a "real" or "true" motion. But this knowledge could not have been acquired if the observer had never inhabited an environment of inertial references frames to begin with. Thus, at its base, Mach's issue is an epistemological one; he was essentially trying to point out that all human knowledge is relative, and that since we construct our theories of reality based upon that knowledge, the introduction of "absolute" qualities like that of motion can never be entirely consistent.

[@zemm9003]

​​​@@dialectphilosophy the inertial quality is not defined before the accelerometer. You build a set of 3 equal springs. Then you start experimenting. If the length stays equal we say that the motion is inertial and otherwise there is acceleration. Pretty straightforward. We can create an entire theory postulating the existence of inertial frames and how the laws of Physics look like in these so-called inertial frames. Then we define how to experimentally decide if a reference frame is inertial or not. Then we make experiments and notice the results. If the theory deviates from the predictions then one of two things happened: a) you don't know the laws of Physics; b) either inertial reference frames don't exist or l your mechanical definition of how to tell if a reference frame is inertial is bad. Then you go back to the drawing board and try to modify the assumptions. Since all experiments that we have ever made confirm SR and GR we have to conclude that the a) premises of these theories work very well and b) our arbitrary choice of how to define an inertial reference frame in practice has proven to be accurate.

[@TerryBollinger]

Like inertial frames, rotating frames are energy excitations of space-enclosing particle sets. Also as with inertial frames, the only precondition for creating such an excitation is a partner object to keep the net angular (or linear) momentum of the system at zero. Thus if your entire universe consists of nothing but two bicycle wheels with water inside and a motor to spin them in opposite directions, the water in the wheels moves outward and stays there unless and until the angular momentum excitations are removed by locking the wheels back together. No stars or distant galaxies are involved. Their existence or nonexistence is, in fact, completely irrelevant. That is why the centrifugal effect begins the _instant_ angular momentum energy is applied. Making serious progress on questions like these requires moving away from over a century of reliance on the untestable and infinity-ridden assumption that space and time exist independently of matter. What we call space and time are real, but they are also an interpretation of deeper and less intuitive relationships between conserved quantities, some of which asymptotically present themselves as what we call particles. That's also why the launching point for the next phase of physics is not the generalization of special or general relativity _per se,_ but a new look and more thoughtful look at the most successful physics theory of all time: The Standard Model of particle physics. ---------- Terry Bollinger CC BY 4.0 2022-08-20.23.00 EDT Sat PDF: sarxiv.org/apa.2022-08-20.2300.pdf

[@Littleprinceleon]

What about the constructor theory approach?

[@nettewilson853]

Such a great video. It addresses the concerns I have had with understanding inertial frames.

[@paulpalmer864]

Bingo!

[@frankargenti]

take a book and read it .. dont watch fallacious KZbin videos

[@Zamicol]

"Round 3" is illogical. It does not rely on the presence of an external system. It simply requires a difference over time of the frame. The apparatus frame, including a ruler, does not need any reference to an external frame. "Shortest" is a useful point, but it's not needed. All that's needed is a difference, which simply requires acceleration.

[@dialectphilosophy]

In that case however, you still cannot differentiate acceleration from jolt, or jolt from the next derivative (all the way down the chain). So you still cannot claim it is "true" or "absolute" acceleration.

[@leodoms4682]

I find round 1 and 2 are no different really. If there were no friction between the bucket and the water. Wouldn't the water stay unmoving in the rotating bucket ? So one assumes a friction force between bucket and water. What if there were other forces, like capilary ones that suck water up at the edge ? So in order to evaluate all the forces that exist between bucket material and water, one would also need to be able to compare with the "unmoving" waterbucket setup. Just like the spring setup. I think...

[@dialectphilosophy]

That's an excellent point, thank you for bringing it up! You're entirely correct, you would have to have all that comparative information about the system on hand, a-priori. It's a different way to arrive at the same conclusion

[@Theusualprojects]

Why replace the rope with a spring? If you were to use a strain gauge, if the two masses were pulling apart, it would show on a strain gauge. Which would show that there is motion even if an observer is seeing the cord holding the two masses together not appearing to be in motion. I submit that said observer would actually draw an entirely different conclusion. If there is an observed strain on the cord, but no observable motion, it would be reasoned that any mass pushes away from another mass when a string is placed between the two. Probably not a good example of these theories.

[@richardtheweaver4891]

Place a thruster on one object perpendicular to the line through both objects. If nothing changes but the distance between the objects they're spinning, and the math should tell you how fast. (Would need a couple tests to determine the orientation)

[@krzysztofnapiontek8971]

detaching weights from the spring determines the "natural" length of the spring hence it is a way to make your calibration done in the absence of gravitation.

[@RainbowPreprimarySchoolS-qq7kz]

I hope this point is taken up by Dialect in their next video

[@sarahbell180]

One thing I want to quickly note is I very much disagree on the presentation of the history and sides (labeling absolutists dogmatists), muddling some history of science (some things with Newton that are more slight). More particularly, Ernst Mach didn't have an irreversible impact on physics in the sense of a paradigm shift. Essentially, Mach's principle while an initial motivating principle Einstein himself had later rejected once he had cleaned out some of the remnants of his theory. So what exact impact did it have? Now, I am somewhat confused about your depiction of the bucket argument and have some criticisms. In round 1, mention that Mach wins because Newton needs a gravitational field. But all this gravitational field does is show what it needs to be concave. Rotating along with the bucket in free space, the bucket will still appear to be at relative rest and the water goes everywhere, so this would ultimately be a win for Newton since it has demonstrated two clearly distinguishable cases of a bucket at relative rest. In round 2, I agree. In round 3, fair point but you can tweak the argument. Suppose you attach a spring around the string. Now, you cut the string at the ends. You will the observe a stretching of the spring that wasn't there, caused by tension in the string that held the spring there before. So, in this form, Newton wins. So, I agree with Newton on all counts. But I do not believe in an absolute rest frame and am a full on relativist. How is this possible? I once again believe that the framework general relativity holds all the keys and allows the essential observations of both points of view to be right and I will draw out a depiction. See, a core assumption in all of these arguments is made, and that is the inherent description from a 'frame'. But what if, we considered instead there to be no intrinsic notion of a 'frame' and that all 'motion relative to a frame' is itself based on motion of observers to comprise that frame? Considering motion to be the 'evolution of a physical object throughout space and time', the idea of motion then is an absolute one, and it is the relative nature between two kinds of motion (of an object and of that frame) that then allows one to characterize motion in this relative way that leads to this sense of observationally distinct characterizations of motion. Physics is the study of absolute physical laws (to a given degree). If laws of physics do not hold in other frames, those aren't a law of physics but a law of physics in that frame. So, all laws of physics (albeit in a very different, tensorial form) are expected to hold irrespective of the choice of frame and be absolute, although the form of characterization once put against a certain frame may be different. Historically, this meant given the a priori picture of euclidean space, for the need of an absolute rest frame. But, with invariance of laws under inertial frames, physics with respect to relatively chosen inertial frames was sensible, where the nature of such a frame was seen as implicit. But, the advent of Einstein's theories of relativity gradually took time to undermine this idea, with the idea of coordinates not having intrinsic metrical meaning to be pretty much Einstein's summary on the difficulty of general relativity. With the global inertial frame dismissed, the intrinsic meaning of a 'frame' was lost. Here is then the description: consider the manifold of space-time which is physically defined as the coincidence of space-time events. An object's motion is simply a world-line on this manifold. Now then, when I say in my frame an event occurs at (5m, 12s), what that really mans is that I take a form of a meter stick and place a clock at the marking on the stick for 5 meters, then that event will be coincident on that marking on the meter sick when the clock reads '12 seconds' on its face. (this is the point coincidence argument) There is no intrinsic measure of time beyond this. I can more absolutely and mathematically describe this as a world-line for a clock and a world-line for the observer (and a world-line of a light beam defining their synchronization) that intersect and this intersection is the measurement of 'time' for that observer [there is a better definition overall in terms of cauchy surfaces but this is the more clear physical depiction]. Moreover, given the lack of meaning behind an 'intrinsic measure of time', means it comes down to a certain test of clock synchronization that this works which needs observers on either end. Thus, a frame is ultimately not a single observer but a prearranged collection of observers undergoing some understood motion connected in some way. It is up to experiment to find how these relate together. (details of this type of reasoning is in the idea of world-line congruencies and frame fields) In your spring example, you would need two observers on either end of the ruler to read out such a measurement. But notice-for this to make sense, they need to have clocks synchronized to which they can then read out both parts. It is up to experiment to then synchronize and see how our different measurements of time compare. If you were to try to do synchronization in rotation, you'd find that it would in fact fail (reading up on Born coordinates and Rindler coordinates would be helpful) and this would tell you that your frame is not inertial. The stars can be entirely forgotten about if you allow yourself a global enough arrangement of observers to consider your relation to so that in an empty universe. More realistically, you would observe the nature of forces of these objects (the geodesic equation as the 'law of inertia' is what connects these two although thats a whole separate point) Thus, the 'relational interpretation' is NOT between you and the stars, but you and the rest of the world-lines around you. Notice then that inertial and rotational motion are clearly distinguished by their relation to the metric of space-time (one is geodesic, one isn't) but that this doesn't mean that there is an absolute 'state of motion' (that is, a characterization with respect to a frame, so the typical description of 'rotation' or 'linear' motion in the coordinate way). Rotational motion can be seen with respect to a rotating observer to be 'at rest' and still account for all the observations around them and yet for inertial observers to be no more fundamentally special. Now, Mach's principle is often demonstrated as 'matter elsewhere influences inertia here'. That is, you start rotating and notice your arms come out and the stars rotate (or, the bucket spews out water). You identify the force on your arms based on the large-scale observation of the stars rotating around you. So, do the stars influence the inertia here? No. The core aspect of Mach's principle in relativity is hidden in the form of Einstein-field equations. In this form, matter elsewhere influences the space-time there which dictates the inertial world-lines there that gives information on the space-time and world-lines here. In the case of far away stars, to an approximation, inertial world-lines correspond with motion with the stars themselves that act in a certain way with respect to yours. You undergo your own entirely distinct world-line. But, would you be spinning, now have a background against which to compare to known inertial world-lines and then conclude you are not inertial but rotating. Do you find this to be a satisfying resolution, I find this to be very satisfying and tying up pretty much all loose ends.

[@dialectphilosophy]

Hi Sarah, it is entirely correct to assert that Mach caused a paradigm shift, thought we meant this mostly in reference to special relativity, not general relativity. Einstein first read Mach's works while he was a university student. Mach heavily pushed the idea of relative time, space, and motion in these works, as well as the need to use operative empirical definitions for such quantities over the abstract ones Newton provided. It was exactly all this that Einstein did with Special Relativity, so in fact, Mach very much birthed special relativity; indeed we can say he is entirely responsible for our current understanding of modern physics. Einstein attempted to carry through some of Mach's ideas about inertia with General Relativity that were ultimately unsuccessful; it is likely these that you are thinking of. But to deny Mach's legacy because some of his ideas didn't carry through would be like asserting that we should deny Newton's legacy because he unsuccessfully practiced alchemy.

[@sarahbell180]

​@@dialectphilosophy If-as in the video-the 'paradigm shift' is in regarding criticisms of absolutism, then no. Relational thinking goes back to people like Leibniz who even debated with Newton on these same arguments. Mach's principle is his most powerful contribution then to this area but wasn't certainly any new scientific theory nor one that demonstrated a new paradigm (as mentioned in general relativity rejecting it). While Mach certainly influenced Einstein (probably most with his conception of the 'event'), the actual physical qualitites of relativity justified with such approachs-the true relativity of time given time dilation and relativity of simultaneity that were emblematic of the paradigm shift-were not of Machian foresight. Mach is certainly then not 'entirely responsible for our current understanding of modern physics', not even close. I wouldn't even say Einstein would be, I just don't view history of science that way. His main core ideas on relational space and time are based on his logically positivist, empiricist, viewing psychology sensory as the primal point of physics. These core aspects that defined his contribution to the idea of space and time are ones just not wholly held in modern physics since most in practice view physics as in some sense having a form of existence outside. *But that was really an aside to the main points that I really think should be considered.* Not demanding an answer either, but just think thinking about these would be really good. My first was practical: one can come up with a better measurement of Newton's rotating spheres argument-attach a spring around with ends attached to the continuous rope, then cut the rope at the end that spring connects on. Now, given a fixed measuring stick to compare against: the spring will expand if it is rotating: stay still if it is not. How is this resolved? My second was theoretical and most important: I believe if we are to go all in with this relational reasoning, it is important to mention there is then no sense of an intrinsically extended frame to an observer as shown in special and especially general relativity, but instead a frame has to priorly arranged by many observers and thus any idea description of relativistic thinking based on motion with respect to an intrinsic frame is then itself malformed-and in that case, does it even make sense to speak about true physical motion considered in such a way?

[@dialectphilosophy]

@@sarahbell180 You're incorrect here; we'd suggest reading Mach's work to better familiarize yourself. Yes we're aware of Leibniz (he's featured in the video) and that relational vs. absolutism arguments go back to the very beginning of philosophy (as we mentioned in the intro). But Mach's contribution is different and indispensable. Historically speaking, we know Einstein read Mach; many of the very ideas explicitly mentioned by Mach in his works (mainly needing to come up with an operative definition for space and time) are the ones Einstein implemented in his early special relativity papers; to say those ideas came from Leibniz is one-hundred percent false. In every sense of the meaning, yes, Mach is one of the founding fathers of modern physics, and he has had an immense lasting impact. Whether you like it or not, this is simply historical fact.

[@dialectphilosophy]

Sarah, we appreciate your time and interest in our channel. Unfortunately, we didn't get time to address the other points you raised. We didn't intend for this video to offer any ultimatums and what is right or what is wrong about the nature of motion, etc.; we felt it was more important to get people thinking about the topic and so we merely presented a few historical arguments alongside a few of our own. Concerning our own argument we do have many counterpoints and further arguments, and we appreciate the points you've raised as well, but it is not necessary to go further into discussion about it here (don't worry they will be addressed in future videos). Mach's legacy on the other hand we will defend to the death 😂!

[@sarahbell180]

@@dialectphilosophy Of course! I don't demand any answer, just found an emphasized response on one point that I viewed as more minor a bit odd, since in my perspective I really wanted to drive home a point on considering the very notion of a relatively globalized frame as nonintrinsic and how general relativity preserves these relativistic ideas as a result. I do look forward to these further points raised, although I am fairly convinced with this perspective. One thing is I suppose I misplaced what my issue was, and it wasn't on Mach's lack of importance but the precise identification, which is a lot harder to put in words. In the vid, it seemed remarking that Mach was responsible to be the one responsible for the collapse of absolute space and in that sense no, Mach's critique was a lot less physical and tied to his epistemic viewpoints without any sense of inherent physical ontology and taking into account this leaves a still fairly absolutist perspective. So it wasn't that per se, I would say Mach identified that all those intuitively physical aspects to us are operational, combined with the mechanics of relativity theory, experimental evidence against, and an overall fading away from theological metaphysical aspects tying fundamentally to the idea of absolute space, is what I say drove this shift away from absolute space.

[@henrikrolfsen584]

Circular motion, is not simple motion, because the objects are constantly changing their direction. This makes circular motion vastly different from straight line motion.

[@jelteduchene7845]

what about cutting the spring?

[@airfriedquadsbw]

There is nowhere to move to without a relative position. A rotating frame is only rotating if there is a nonrotating position. A set that contains everything, as a whole, can not be in motion, only the things in the set can be in motion relative to eachother, or relative to the boundary of the set. If a set that contains everything is rotated.... not rationale, the set would have to be in something to rotate it. But it contains everything so it is not in anything, it can not have motion.

[@Littleprinceleon]

Does the set containing everything also contain itself?

[@airfriedquadsbw]

@Littleprinceleon if you figure that out, let me know how the cat is. And the pile of sand that has been washing away. Is it still a pile? 😄

[@agentdarkboote]

Hollup, in the first round, what if you made the bottom of your bucket either hundreds of kilometers thick, or extremely dense, so that it had its own gravitational field that it could keep the water in the bucket even when the whole thing is spinning? I guess it depends on how local "local" is, perhaps a bucket is already too macroscopic. I'd be interested to know if there are any quantum effects that could be used to probe this question Edit: Well now after watching your other video on the twin paradox, I guess you could just define a new set of laws of physics by the principle of general covariance and then you'd be able to say that both are stationary solutions to your new laws, and who can say you're wrong if it all checks out. I can't tell if that feels like cheating or not...

[@Vielman_]

Wow, that’s is really a good point

[@hollander04]

The real problem I think lies in the nature of “thought” experiments as opposed to real world experiments: it is for instance impossible to do the bucket 🪣 experiment in gravity free space, as there the sentence “in the bucket” i.e. confined to the bucket has no meaning, the water would clump together and not exert pressure on the walls and thus friction would not drive the water ….. apart from the fact that water needs pressure to exist at all….. On the other hand, if no masses are attached to the spring, it has no reason to stretch even in a rotating system, so take the masses away or increase them and you’ll see a change in length … or not, and that decides and answers the question.

[@TheOneMaddin]

Aren't these discussions about the relativity of ACCELERATION instead of MOTION?

[@jeeeeeeeeeeezus]

Acceleration is just something you extract from motion.

[@edimbukvarevic90]

Yes, and acceleration is clearly absolute. Claiming it's relative to the distant stars is a laughable cop out.

[@TheOneMaddin]

@@edimbukvarevic90 Yeah, right? Ridiculous like the passage of time being relative, or like space being able to deform.

[@jeeeeeeeeeeezus]

@@edimbukvarevic90 Acceleration being relative to distant stars (or more accurately, "the rest of the universe") is the current stance within the scientific world. It's called general relativity.

[@jeeeeeeeeeeezus]

@@TheOneMaddin Sad thing is, I can't tell if you're being sarcastic or you really think so.

[@SkylerLinux]

There is no Absolute motion, because without Absolute Time and Absolute Space there simply is not Absolute Motion.

[@allenemersonn1227]

Dump the incessant music or turn it way way down. It contributes little if anything to your presentation.

[@Vielman_]

I acknowledged everything but the music

[@Domi2gud]

>observer can't know what an at-rest spring looks like OK then use a string and two weights instead of a spring. No foreknowledge needed.

[@Domi2gud]

You personally know when you're at rest vs when you're being accelerated because when you're at rest, your eyeballs aren't popping out of your head. Doesn't sound very relative to me.

[@gctl4313]

Brilliant exposition, thank you. I've been scratching my head about this since I first heard about it. In particular I wondered how an isolated star 'knows' that it's spinning and therefore 'needs' to be an oblate spheroid. I just thought I was dumb... I still think I'm dumb but it appears I have good company.

[@real-timelabel-freeimaging4653]

THis question I also like, and expand, to how do black holes know with their ergosphere....

[@kennethmarx6656]

Globaphelia

[@kennethmarx6656]

I'm coining the term globaphelia and globaphile. I created them as far as I know.

[@PeteLaric]

This is my new favorite channel -- the production value eclipses that of anything I've seen on KZbin! Your videos are extremely well thought out, and equally well presented. With this particular video, I do think that something is being overlooked here. With centripetal acceleration being given as a=v^2/r, and force given as F=ma, we see that the centripetal force scales proportionately with the masses of the objects at the ends of the springs. Thinking about this in a purely intuitive way, two space shuttles orbiting each other on opposite ends of a tether are going to exert much greater centripetal force on that tether than two Apple watches on a similar tether with a similar rotational velocity. So, your difficulty in calibrating this hypothetical rotational measurement tool can be resolved by attaching different weights of known masses, then measuring the tension at the same observed rate of rotation. Assuming you are in the linear region of spring tension where Hook's law holds, it should be possible to deduce the zero mass tension by linear regression of known nonzero mass data points. Alternatively, you can detach the masses and measure the relaxed spring length, although this neglects the nonzero but probably negligible mass of the spring itself, and the possibility that it (and you) are spinning when measuring it. Hence the suggestion to use large, known masses that should dwarf any negligible mass the spring might have. When you find a frame of rotation in which the spring tension is constant regardless of the masses selected, and when this tension is equal to the spring's natural resting tension (as measured or as calculated), then you have achieved rotational stationarity. 🙂

[@HarryBalzak]

Thanks for this. Is it possible to fashion a spring that has uniform spacing when at "absolute rest" and nonuniform spacing when at "absolute motion"? That would solve the problem with Newton's hypothetical experiment(or would it?). I don't know how springs are made or their limitations.

[@lindumenzidludlu2213]

How would you determine that absolute rest you'd create that uniform spacing for? Through another spring?

[@sealteamsix1784]

what about plutonium springs that reach a critical mass and melt when they are at absolute rest and the coils are bunched together.

[@guilhermetorresj]

There's a flaw in your argument. You could make the spring change in length by spinning it. Even if it seems stationary to you, it would apparently compress when rotated in a reference frame that is rotating in the opposite direction.

[@leapdaniel8058]

Great video! I'm going to add to the "why not just do such-and-such" chorus that you mention in your pinned comment. I hope you don't mind. But suppose you gradually apply torque to the spring-mass system to get it to rotate at different speeds. You don't need to reference an inertial frame to do this, and, pausing at each speed, the system will be in equilibrium, with no parts moving relative to each other. Your goal as an observer will be to find the rotation speed at which the spring length is at a local minimum. A system which minimizes the spring length will be in an inertial frame. So you will have identified an inertial frame without prior reference to one, or so it seems to me.

[@dialectphilosophy]

Depending on what knowledge and information you import a-priori into your system, different experiments will identify inertial systems for you. In your case, you need some prior knowledge that would tell you the spring has a minimum length, as well as that the instrumental torques you are applying are "absolute ones". Our goal is to get viewers not to come up with savvier experiments, but to question where the source of such a-priori knowledge arises from in the first place.

[@leapdaniel8058]

@@dialectphilosophy I don't think either of those forms of a priori knowledge are needed in this case, actually. Re: knowledge of the spring's minimum length: You need to decide to look for a minimum length, true, but you don't need to know in advance that you'll find one. You just have to scan various rotation speeds until you do. Re: knowledge of the "absolute"ness of your applied torques: I think the applied torques could be fictitious and the experiment would still work. Really, the torque doesn't matter, all that matters is that angular acceleration occurs from your perspective, so that the speed of the spring's rotation as you measure it changes. I guess the reason so many commenters feel motivated to come up with "savvier experiments" as you say is that they are still doubting that a priori knowledge from an inertial frame is really necessary. And so, coming up with a sufficiently savvy experiment that doesn't require a priori knowledge from an inertial frame is directly linked to your claim about the necessity of prior knowledge.

[@kennethferland5579]

You would know that your "now" in motion after your first application of acceleration, but this won't tell you if the initial state was motionless or not.

[@Dadas0560]

How can we emprically test that hypothesis? It's impossible! In this universe we always have a reference point. Thus, there is no empirical way to test it in a "place" that has no reference points at all. Two objects tied by a rope, or spring, that's already two big reference points - the objects - and lots of reference points on the rope/spring. This is a paradox. It is trying to prove absolutes using relativistic experiments.

[@simonjamesodell]

This channel deserves way more attention than it currently has.

[@willoughby1888]

I did the bucket experiment at age 4 at the beach on Galveston Island back in 1961. I'd just been given my new plastic pail and shovel. While our mother was waist-deep in the water catching "Blue Claw" crabs with sticks, strings and soup bones, I was filling my pail with beach sand and swinging it around in fast circles high over my head, spewing it out like a tornado until it was empty or I got too dizzy to finish. It wasn't my fault my little sister kept getting in the way. I blame science! Mom didn't buy it though. She took away my gravity bucket but there were so many other scientific things to use for "little kid scientist" study.

[@Alexcoman51]

One thing I find interesting about absolute motion arguments is that they always rely on rotational motion, not translational. Could it be possible that spacetime/motion is only absolute in orientation but is relative for any linear translation?

[@travcollier]

No. But angular momentum is a real (as far as we know) thing. This really doesn't seem like any sort of grand mystery. To convert between a rotating and non-rotating reference frame you have to add in (or remove) a force. That force can be measured. But that doesn't make the acceleration "absolute motion"... it is just a force. I honestly don't know why Netwon got fixated on the idea of absolute rest and absolute motion. Probably some old philosophy or maybe Biblical thing he was trying to reconcile.

[@Alexcoman51]

@@travcollier bc you are able to definitively tell if a frame or reference is undergoing acceleration it must be absolute or else a force will appear to act on the object from nowhere. Therefore it cannot be relative because from the accelerates point of view it makes no sense to claim to be at rest or else forces will exist which you cannot explain

[@travcollier]

@@Alexcoman51 What? I really don't understand what you are trying to say. "At rest" just means that it isn't experiencing any forces, and is relative. In many cases, you can describe the same system with equal accuracy/"truth" from the POV of an accelerating reference frame which can make some forces apparently vanish or new ones appear. The maths is usually just harder so we don't most of the time. The particular example in this video seems like it might be why so many physicists are adamant about the centrifugal effect being a "fictional force"... But all forces are equally fictional (or equally real) if you really drill down. However, folks seem to be easily confused by rotation and angular momentum for some reason... So I understand why centrifugal gets special treatment.

[@Alexcoman51]

@@travcollier not really. If you are in a moving train and the train stops you will lurch forward suddenly due to deceleration. If you were truly at rest the entire time there would be no way to explain this sudden movement in a sensible way. The deceleration that you underwent is a certainty you cannot simply claim that the frame was at rest the entire time because you would have no way to explain that sudden lurch on any object in the frame.

[@mehill00]

@@Alexcoman51 A moving train that’s suddenly stopped makes the passengers lurch. A motionless train that is suddenly moved makes the passengers lurch. From inside how do you know if the train is stopping or starting to move?

[@ЗакировМарат-в5щ]

I do not uderstand round 3 argument. One can still imagine simple scheme with masses where spinning state has deformed structure at Dame time resting has non deformed structute

[@petercbrandon]

The “music” is competing with the dialogue. Too bad. I can’t listen to this. It’s so loud it’s not even background. Please Get rid of it.

[@rustworker]

I’m confused. Surely the effects of rotation are because rotation involves continuous acceleration - as the objects rotating change direction. All these experiments are detecting the acceleration not motion. You are seeing the effect of forces produced by acceleration. The motion is all relative, the change in motion - acceleration - is absolute. What am I missing?

[@marcelotemer]

If space and time are relative, motion cannot be absolute because change of places (in time) refers to 2 relative concepts.

[@mikegranville4174]

Trying to follow this, as a non-physicist. Takes steady concentration. What makes anybody think that stupid music helps with any of this?

[@RedeyedJedi00]

3rd round: All you have to do is use a spring that has a distinguishable straight line drawn across it while it is at rest. That way when you observe the experiment... 1. If all the marks on the spring create a straight line, you know the device is not spinning. 2. If all the marks do not line up, you know the spring has expanded and the device is in motion. As soon as the spring expands, each coil begins to rotate. The more it expands, the more each coil rotates and the more out of line the marks on each coil become. If you stretch a long enough spring far enough, the line will wrap all the way around, creating a 2d wave, or a 3d coil. (Obviously, if you compress the spring, it does the same but in the opposite direction.)

[@Nephelokokkygia1215]

For anyone researching this, it's Principia Book I, Definitions, Scholia, Part IV. The 'pillars' are actually four: time; space; position; motion. Important to note that Newton makes a distinction between 'true' time, space, position, and motion "temporis, spatis, loci, et motus veri" and 'absolute' time, space, position, and motion, "temporis, spatis, loci, et motus absoluti", making the issue a little more subtle than how it is usually regurgitated.

[@rxbracho]

I had the correct philosophical inference from the experiment without understanding why the experiment proved what I inferred. Thanks for the best explanation of the "bucket experiment". Bravo.

[@bernardedwards8461]

To decide whether the two space capsules connected by a spring are in motion, the spaceman only has to look out of the window. If the background stars are static, he is not in motion, but if they are apparently orbiting his spacecraft he will know he is in motion. So to make it a little bit harder, we will have a blind astronaut who cannot see the background stars. He decides to visit the other capsule at the opposite end of the spring. As he hauls himself along the spring he will notice the artificial gravity pulling him toward the capsule he has just left will get weaker, until at the middle of his journey he weighs nothing, but as he proceeds on his way he gets heavier until he reaches the space capsule at the far end, when he will be at his full starting weight.

[@bothieGMX]

Round 3: Just cut the connection between the two masses and the spring and look, whether it retracts. If it does, you're in rotation. If not, you're in rest. No outside information needed, you only have to bring scissors along ;)

[@peterfireflylund]

Or a small flame that will burn through, say, a rubber band. That should disturb the system less.

[@bothieGMX]

@@peterfireflylund Depends on how much impact will be caused by the smoke.

[@jonbold]

This is a beautiful thought experiment. Both Newton and Mach are correct in asserting that external information is necessary to make sense of this experiment which attempts to locate reality. The surprise is the fact that neither of them had any idea of the source or pathology of this external information. Even today, nobody understands what is absolute about rotational motion. We know, on rotating systems, that the speed and the angular acceleration are in lockstep. When the acc. is zero, the speed is zero and the system is stationary. This relation does not exist in linear motion. Anything slower than one percent of the speed of light is as good as stationary. If I ask you how fast you are moving right now, a question about linear motion, I would expect a common answer: Relative to what? the Earth, the Sun, Sag A*, or the local group? I suggest three of those answers must be wrong and nobody knows which it is. Here is another question nobody can answer: On the spinning spring experiment, what is it that is right here, right now, that is pulling on the spring? It is not gravity. It is not the Earth. It is not the Sun. It is the same thing that is influencing Foucault's Pendulum. What is it? I suggest that galaxies create their own inertial frame of reference. I use the term "Galactic Plasma" as a placeholder to label the massless plasma of the galaxy that is moving at c., functioning as a medium that is not stationary. Science has not found such a thing. I can explain many details of physics that are consistent with its existence, but I cannot prove it exists. Einstein believed there would have to be some kind of Aether. The Michelson-Morley experiment proved there is no stationary aether, but left us with no further explanation. Looking forward to the next video!

[@TheySuckFatLongDonkeyLogs]

Michelson-Morley did when you consider that "light is limited to a speed based upon the medium through which it is travelling.", and since the atmosphere was the medium that the light was traveling through, it should have been obvious that, relative to the atmosphere, there was an effective "aether", it was the atmosphere, so of course they would not measure any appreciable difference in direction because they were travelling on the earth with it. Because it was not done in a vacuum, it should have been apparent that there would be no appreciable difference in measurement because of the influence of the light travelling through the atmosphere. Edit: note that this does not answer the question they were trying to answer, the same experiment would have to be done in a perfect vacuum to test that, so no claim as to whether the overriding aether exists or not, just that the experiment they were conducting would not have answered that, and that the atmosphere was the positionally relative reference point at which all their measurements were taken.

[@colinellicott9737]

An excellent set of observations - thx. I tend to think it is space-time that is the fundamental inertial frame, and the impetus for the forces acting on Foucault's Pendulum and all mass - though proving it is beyond me and this thought experiment. I can picture these experiments either in a galaxy or in the voids between galaxies, yet the results being consistent. This may clear up the question of galaxies being primary ... but I'm not holding my breath for that result ; )

[@bosvos99]

I think it's easier to understand the limitations of the thought experiment if one imagines the observer as someone looking at a video feed from a camera attached to one of the balls. The observer cannot change anything in the system and would not feel any effects of gravity or rotational forces. Just a live image. Without knowing the properties of the spring it would be impossible to calibrate against a ruler even if a reference frame change occurs and the spring shortens. You will have no way say "this is the shortest it will ever go and therefore at rest, let me record the length now". This reminds me of the hosts in Westworld asking "is this now?" when they come back online...

[@Pengochan]

The question is: how does it help understanding to impose limitations on those observers in the thought experiment? In zero gravity we could instead watch a rotating drop of water held together by surface tension. If it rotates / in a rotating system, that'd be an oblate spheroid (without rotation it's a sphere). Even without any other knowledge it is clear that one direction (the axis of rotation) is somehow special, something that would not be observed in a nonrotating reference frame.

[@simonbrissenden5878]

Dialect, you have produced an outstanding series of videos. The quality of analysis and presentation are exceptional, and the choice of subject is spot on. I believe understanding the Twin Paradox and Mach's Principle lie on the path to the next breakthrough in physics. The Twin Paradox illustrates that while time in Newtonian physics is one thing, time in relativistic physics are two quite distinct things. Two objects start at a common set of 4 coordinates, diverge in space, then meet up at a new common set of 4 coordinates. The start and finish points have common coordinate times, but the objects arrive at the finish with different (elapsed) proper time. Coordinate time and proper time are profoundly different. Coordinate time is a dimensional property, like space, that can exist without the presence of objects. Proper time is a scalar property of objects, like energy. Because we grow up seeing the world as Newtonian, I think most physicists don't appreciate how different they are. I postulate that coordinate time is actually a fourth spatial dimension, with baryonic matter arranged in the universe as a bubble, inflated by radiation pressure. Baryonic matter is held in place by minimal surface bubble physics, with gravity acting like surface tension in a soap bubble. Our 3D classical world is the bubble surface and the quantum world is 4D space. Baryonic matter therefore scales as R^3 and radiation scales as R^4. When baryonic objects move, they tug at the bubble surface, which is the origin of inertia, and explains why inertial and gravitational mass are equivalent. Minimal surface bubble physics is the mechanism of action for Mach's Principle, and provides the missing link between the distant stars and the motion of objects. Our place in the expanding bubble is the universal reference frame against which motion (including twins and buckets) can be assessed. These concepts lead to solutions for more than a dozen long-standing physics problems, including dark energy, dark matter, missing antimatter, the arrow of time etc. Details are given in three papers by Simon Brissenden on www.Researchers.one ; - "It's time to stop talking about time" - "Matching supernova redshifts with special relativity and no dark energy" - "Big Bubble Theory"

[@-danR]

random internet dude has a theory

[@simonbrissenden5878]

#danR - yes, indeed "random internet dude has a theory". Random internet dude also has a first class honours degree in physics. The paper "Matching supernova redshifts with special relativity and no dark energy" was published in the April 2020 Journal of the Royal Astronomical Society of Canada, and was recently reviewed and given the thumbs up by a US professor of Cosmology who has written textbooks on the subject.

[@dialectphilosophy]

@@simonbrissenden5878 We appreciate you watching and thank you for sharing your thoughts! We agree that coordinate time vs. proper time is something definitely worth looking into -- even if random internet trolls don't!

[@simonbrissenden5878]

@@dialectphilosophy I think a video about coordinate time vs. proper time would be fascinating. You could address the aspects of the Twin Paradox that other KZbin videos omit, like should NASA pay its astronauts for the months of coordinate time (expensive) or proper time (cheaper) that they are in flight? Should dating websites ask for a returning astronaut's age in coordinate time years or proper time years? What age should be on your driving licence? Once you start asking this type of practical question, you quickly realise that there really are two types of time, not one, which is something I've never seen adequately discussed in science books or videos. The Law and society at large are oblivious to this scientific fact. (Can you imagine how confusing science would be if it had the same technical word for space and energy? You'd probably end up with two major areas of physics that would appear to be mutually incompatible.... just saying.) If I were a physics journal editor, I wouldn't allow anyone to write a paper using the unmodified word 'time', but get them to always be specific about whether they mean proper time or coordinate time. Hence the title of my first paper, "It's time to stop talking about time" (researchers.one/articles/19.02.00002 ). I look forward to seeing what you come up with.

[@bmanlexie4875]

In the bucket experiment if you give the bucket wall infinite height and if they are parallel, the idea would work outside of a gravitational field. If r of the water equals R of the bucket wall you have motion if any of the water is less than r the water would be at rest. r=R motion, r

[@rupertchappelle5303]

Time is fluid and variable. The time in the center of the bucket is moving less because there is less motion, therefore it is faster (high pressure), while the mass at the edge of the bucket is moving faster, therefore causing time to move slower (low pressure) and the difference in time pressure produces "lift" and in this case.

[@mishaangelo926]

What if you used an isotropic glob of liquid matter? If the glob settles to a sphere, then might you say it is non-rotating without having to predetermine a rest state?

[@mishaangelo926]

@@narfwhals7843 That implies one would deduce different laws of physics to ours if experimenting within that system. The spherical rest state is deducible from basic anaysis of force and matter so if you live in a world with a fundamental assymettry you can figure that out. Humans have worked out the earth has polarities of spin and magnetic field and that the solar system and gallaxy also have spcial axes. If you have developed a universal description which requires a reference frame relative to a particular point in space, you must be thinking it's at the Vattican. Rotation is all about relative motion of the system components and nothing about "absolute motion" at all. So it seems to me this whole line of reasoning leads up a vacant tree.

[@OxyShmoxy]

Isn't spinning is indistinguishable from staying in a circular gravitational field that pulls around you with the constant force at every point? There is no way to tell spinning and staying in such a gravitational field apart without an external reference.

[@dannydewario1550]

They would be similar when just observing, but we can still tell the difference after cutting the string. Let's cover two different scenarios, both of which will include you as an observer isolated from seeing the outside, two cube masses connected by a string (and appearing to be at rest relative to yourself), and a massive ring acting as a gravitational force (with the enclosure you're in at its center). In scenario 1, you and everything in the enclosure are "truly at rest" and nothing is rotating. Once you cut the string, the two masses fly off in opposite directions, giving the illusion you were rotating the whole time. When in reality cutting the string released the two masses and gravity pulled them away from the center. Nothing crazy here. In scenario 2, you and everything in the enclosure are "truly rotating". Although the two masses on the string appear to be at rest, both centrifugal force and gravity are pulling on them from the center. When you cut the string, similar to scenario 1, they both fly off in opposite directions. But there will be a key difference. Both of the masses will begin spinning on their own axes and their separate strings will wrap around them. To understand why, let's focus on just one mass before you cut the string. One of its sides is connected to the string, and is rotating closest to the center (and will be called the inner-side). The side of the mass opposite of the string is rotating furthest from the center (and will be called the outer-side). This means the inner-side is travelling a shorter distance around the center than the outer-side, which also means it has to be travelling at a slower speed. So when you cut the string, the outer-side's faster speed will outpace the inner-side. If the two sides could freely travel on their own, the outer-side will travel away from the center faster than the inner-side. But because they are connected by being part of the same mass, the mass as a whole will rotate as it flies away from the center. Anyways, because you observe both masses rotate on their own axes as they fly away from the center where you cut the string, you must conclude you are in a rotating frame of reference.

[@nhansen197]

I don't believe it would be necessary to have outside information to determine rotational motion with a spring. It would seem to me that knowing the elasticity of the material would be an ideal place to start. Secondly, if we are allowed to change the orientation of the spring we would get feedback allowing us to know if it stretched in one orientation as opposed to the other. As for how the spring reacts in a known environment we must also take into consideration the gravitational pull of the environment. A spring at rest is only at rest relative to the environment. The spring will never truly be at rest if gravitational forces are pulling it.

[@jeeeeeeeeeeezus]

The observer can rotate the spring system relative to the observer at different angular velocities. At a certain relative rotation to the observer, you will reach a local minimum of spring tension... and thus absolute rest. Except the fact that you can't ever do this experiment in the absence of the rest of the universe, so extrapolating that centrifugal force still works without the rest of the universe is now the new assumption. (which is false if general relativity didn't deceive us)

[@tkimaginestudio]

First of all, you are talking about acceleration, not motion. Second, should one not see Coriolis effects whenever a system appears to be in rest but is actually rotating?

[@52flyingbicycles]

If you are sitting on a spring and suspect it to be spinning, it’s also possible it’s being held in place at one end and dangling in a gravitational field. That would also cause it to stretch out but it wouldn’t be “moving”

[@ItaiBasel]

the 3rd round is Newtowns. he suggested a string, where tension goes to zero in a measurable way... not crossing to negative tension. What gave the round away is you changing to spring, where the zero point is more subjective because you can cross over zero without noticing

[@QuicksilverSG]

In practice, it's easy to resolve the issue of the resting state of a spring connecting a pair of masses. Simply select a spring whose resting state is fully compressed, with each coil contacting the two coils on either side of it, but not exerting any pressure on those adjacent coils. Such a spring can be stretched, but cannot be compressed, making it an appropriate device for measuring the centrifugal force produced by rotation. When the experiment with attached masses is conducted, any measurable deflection of the spring will indicate rotation.

[@BartvandenDonk]

I learned a lot watching this very informative video. Thank you. Now my thoughts: 1. Gravity is depending on motion. If there is no motion there isn't any gravity. 2. In our universe everything moves. There is no point (mathematical coördinat) that doesn't move. 3. Everything is relative and related. All of us are never coming to the same spot (point) in time.

[@BartvandenDonk]

And another thought: It is of no interest how long the spring is in the first observation. If it shortens than you know you were in motion and are slowing down. If the spring is growing longer you know your motion is going faster. So the observer in the system can be aware of relative motion (not absolute motion). If the spring gets bend there's a force outside of the system that is attracting (most possibly a mass with gravity or a magnet if the spring is a metal that can be attracted by a magnet).

[@SpydersByte]

"If there is no motion there isn't any gravity"? What? Who says that? A steel ball floating in empty space would still produce it's own gravity, regardless of whether it was moving or had things moving around it would it not?

[@BartvandenDonk]

@@SpydersByte You describe a potential movement. Even light would be attracted to your steel ball. Light is motion...

[@SpydersByte]

@@BartvandenDonk Ok but potential movement isnt actual movement... that's like saying potential energy IS kinetic energy. Sure, it can be *turned into* kinetic energy but they are not the exact same thing or we wouldnt be calling them by different names.

[@BartvandenDonk]

@@SpydersByte Well lets investigate this: kzbin.info/www/bejne/qWeTeaZqbt2cjdE

[@mandolinmadness1307]

How does conservation of energy make the potential and rotational kinetic energy equal (8:51)? If initial energy state of the stationary bucket is zero, hasn't energy been added to raise both the amount of potential and kinetic energy from outside the system? Why would they increase equally? Sorry if this is a stupid misunderstanding, but I am genuinely confused about this step.

[@rockapedra1130]

In my experience, whenever an argument descends into the depths of philosophy, everything falls apart and nobody can explain anything. It's like asking "but why" continually to any response to a question. It always ends up in "because that's the way it is". That last bit of throwing memory in there just puts us in this spiral of confusion. You can destroy any argument like this! What is 1+1 ends up with questions about what is "1", what is "+", how do you know, blah blah. Philosophy can often be a trap.

[@iplanes1]

These are thought experiments which allow us to ignore practicalities. However, one practicality sets me thinking. In a universe that is totally empty except for the bucket of water or the masses connected by a spring, how do you get them to rotate? Conservation of momentum tells us that getting an object to move requires the transfer of momentum to it from some other moving object. In the empty universe there are no other objects. I have always assumed that the net sum of momentum in the universe is zero but if this is the case the bucket or masses in an empty universe can never be made to move as there is no other mass to transfer momentum from or to. I am not sure where this is going but, even if the whole scenario is purely in the imagination, if that scenario is itself not possible does this invalidate conclusions derived from that scenario? I need a coffee.

[@Littleprinceleon]

Isn't the net momentum conserved only in a finite universe? Or it's sufficient for it to be a closed system? Also the net momentum doesn't need to be zero, but conserved. Your body parts (in a life supporting suit) for example can move relative to each other even if you were in rest, can't they? What the hack is even that "absolute motion"?

[@peterclarke3020]

In the two ball connected by a spring experiment. An alternative would be if you have two identical springs, both with no masses attached. Then one spring has masses attached to it. If the assembly is not rotating, the two springs will still be the same length. If the assembly is rotating, the two springs will be of different lengths. In this case we only need a ‘local reference’.

[@Rampart.X]

Correction (?) - in a spinning spring experiment, an observer can apply equal forces to change the angular velocity of the spring in both directions and observe how much the spring expands or contracts for each application of force. The difference will reveal what angular velocity (with respect to the original observation) represents absolute rest.

[@arnesaknussemm2427]

The water does not get ‘flung’ outwards towards the walls. This ‘centrifugal’ force is simply the inertia of the water trying to continue in a straight line.

[@overunityinventor]

The music was good, but why was he talking along with the music?

[@matthewhall6288]

Since everything in reality is connected by virtue of being real, there is no such thing as a "completely isolated system".

[@codeawareness]

Timestamp 12:48 -- I would dare contradict that statement, in this way: in the absence of other masses we don't know the initial state of the system. We fall prey to the preconception that we can "setup" the experiment and "run" it, because space is such common sense to us. The constant "k" is not fundamental, it's something we measured here on Earth, in our space-time metric. Out there in a completely flat space-time metric, such as (potentially) the chasm between super-clusters, we would need to measure "k" again, locally. The act of measurements defines the rest state of the spring, and therefore you cannot deduce its rotating or not.

[@sciencefordreamers2115]

Very important topic! Kind of surprised that no Einstein is mentioned; after all General Relativity is all about relative accelerations, which is equivalent of gravity. GR means relativity of any motion.

[@carultch]

I don't entirely agree. The Coriolis effect can still determine absolute rotation. Acceleration in general, not so much, since gravity can give the illusion of acceleration and vice-versa. But you'd need a very unrealistic setup of a gravitational field to give the illusion of a Coriolis effect. You'd need gravitomagnetism to make for non-conservative gravitational fields, in order to set up this effect.

[@Electromechaniac]

Whoa... my brain has boiled up by this abstraction. It's mind blowing. I think I'll view this later again.