Mach's Principle - Sixty Symbols

[@VeteranVandal]

Brady's really learning to be a scientist. His questions get better and better. And his answers and retorts too.

[@23PowerL]

He's a doctor now after all.

[@Quantiad]

Maybe a little bit patronising, given how long he's been immersed in the scientific community.

[@WizardAngst]

His hair has gotten worse and worse though

[@1ucasvb]

@WizardAngst: Physics. Not even once.

[@klausolekristiansen2960]

Sir Martyn's influence.

[@zedex1226]

I love watching the whole catalogue and witnessing Brady's "well hang on a minute" questions get increasingly insightful. I can see why teachers are so passionate about their work.

[@malik_alharb]

when you've been watching sixty symbols long enough to watch the professors go grey

[@krak9927]

even his beard is grey

[@pavphone2616]

Isaac Hatch I didn't notice CGP in this video.

[@rockets4kids]

That's a sign you'll be turning grey soon.

[@malik_alharb]

im in my 20's say it aint so!

[@frowningJoker]

That grey guy was A.Einstein.

[@LukePalmer]

"You could make the universe rotate if you wanted to" -- love his positive can-do attitude!

[@mihailazar2487]

go subscribe to Isaac Arthur if you wanna hear the absolute madlad casually talking about moving solar systems, peeling off continents and transferring them to other planets, etc

[@Mernom]

An expanded version of Archimedes' claim of 'give the the right pivot point, and I'll move the Earth.'

[@jursamaj]

@@mihailazar2487 No thanks. There are plenty of other videos available, and I just can't get past his speech impediment.

[@GiI11]

Really love how a rotating bucket with water in it is one of the most controversial experiments in the history of physics.

[@sansamman4619]

I remember learning about centripetal forces when I was 13 and I went to my garden with a bucket trying to show my sister that if you spin the water around it would go towards the centre due to the centripetal force and then, it went to the edges, I have been in love with physics ever since, trying to understand why both mathematically and physically and here I am now 4 years later discovering that the answers still are not as clear as I thought, perhaps our ancestors where not as smart as we think they were, or perhaps the universe has more mysteries than we could imagine :')

[@Zeegoku1007]

The water in the bucket is trying to go in straight line , the bucket is held by the rope it's providing the centripetal force to keep the bucket stationary. But there is no such force to keep water in place that is directed towards centre. And bucket is stopping the water going away in a straight line tangent to circle of rotation.

[@StreuB1]

Prof. Merrifield is one of the best. His talks on Gamma are some of my absolute favorites and you can add this to that list. His ability to tell the tale of these fundamental yet hard to understand concepts is second to none.

[@Nehmo]

He didn't even answer the question. How can this be "one of the best"?

[@Rando_Shyte]

I'm finally getting it. I'm getting it. I've been watching this, Fermiab and PBS spacetime for years and years, and my mind has been boggled soooooooooooo many times.. But I'm actually starting to really grasp a lot of the principals. Thank you guys you are all amazing

[@richardnicholas2957]

I am not getting it. Please help me out. I think the guy on rollerskates would be pushed up against the wall no matter if the universe around was rotating or not or if there was a University around it at all.

[@Captainspamo]

@@richardnicholas2957 if the universe rotates around the room, the room remains stationary, so the roller skates wont move. On the other hand, if the room rotates and the universe stays still, the skates will be flung outwards. Therefore there must be some difference between those two scenarios. The question then is how does the room know if it's moving or stationary?

[@MrStarman926]

@@Captainspamo is inertia then not just related to gravity, a local force? why is this surprising or controversial?

[@Jobobn1998]

THANK YOU!!! This weirdly paradoxical aspect of centripetal force popped into my head about five months ago, and it's been constantly gnawing at me ever since! Since there's no objective reference frame in Einsteinian physics, I was having an incredibly difficult time trying to figure out how you could have centripetal/centrifugal force even exist. The only thing I could come up with (Disclaimer: I'm a biologist, not a physicist, so this is gonna be amateur) is that, when an object is spinning, the rest of the mass in the universe is then moving around it at incredible speeds. From the reference frame of the spinning object, the entire universe that's zooming around it now has a gigantic relativistic mass. Which means that, perpendicular to the axis you're rotating on, you now have more mass generating more gravity, and thus can feel the "pull" outward. Assuming that would be a decent hypothesis, that creates some weird implications, such as that, in a universe with no other mass, an object could not experience centrifugal/centripedtal force. Also, that means that, if there is an edge to the universe and one were somewhat near it, one could detect it by the "wobble" of a spinning object, since there would be a mass-deficit in one direction, which would cause an uneven/inconsistent centrifugal force.

[@lcdvasrm]

Conclusion : a videos series about Mach's Principle is needed.

[@taitywaity1836]

I love how the silver play button is just propped up on a stack of binders in the back of the room

[@franklinbrown7389]

I've always wondered about this, and now I can still wonder about it! Thanks

[@logaandm]

If one had roller skates, that can go in all directions and were frictionless, the skater wouldn't move all, the room would rotate around the skater to an outside observer. This is what the Newtonian mechanics at the time of Mach would say and, more importantly, it is what is observed in this low mass situation. You can easily demonstrate this to yourself by quickly rotating a glass with an ice cube in it. One needs to have friction for the skater to move in the example. The skater analogy is a rehash of Newton's famous bucket, but it is the friction on which both examples depend to enlist our common experiences. One can, of course argue that gravitational forces between the room and the skater would ultimately cause the skater to move, but this was and is experimentally almost impossible to achieve. Bringing two unlikely ideals, frictionless and rotating the universe, into the thought experiment make it less useful. In other words, the example as stated is wrong and a poor start to understand Mach. In my view, while the philosophical implications of the Mach's thought experiment are thought provoking and useful, they are still not necessarily correct. Cracks in General Relativity are showing and 100 years from now our view of how space and time work could be as radically different from General Relativity as GR is from Newton. Often such thought experiments tell us more about the logic and mathematics of modeling and may speak little to the actual workings of the universe. I would also like to point out that while the Newtonian view of space and time and the Einstein view of space and time are massively different, both models compute almost exactly the same results. A cautionary tale for all would be philosophers or physicists. The universe really doesn't care what we think, it just does what it does.

[@Reapernav]

I came to uni thinking I will one day meet these people, but 3 years of being a 'student' at Nottingham, all those dreams fell short, now I'm graduating in2 months and still havent met the professors I grew up watching from numberphile and sixty symbols. Mainly because I've been intimidated tbh, if I email them and say I want to meet them, I need to have a meaningful scientific conversation with them, that pressure's just too damn high!

[@yash1152]

> _"I came to uni thinking I will one day meet these people, but 3 years of being a 'student' at Nottingham, all those dreams fell short, now I'm graduating in2 months and still havent met the professors"_ thanks for sharing your experiences

[@lawlessjohn2840]

This always bothered me and I couldn't untangle it. Thanks. I never knew Mach figured it out. It's mysterious.

[@Pfhorrest]

Frame-dragging explains everything. Take a stationary room and spin the rest of the universe around it, and the rest of the universe's tremendous frame-dragging effects will pull things in the room around in the way you expect from a room rotating in a stationary universe, because those are equivalent states. Rotating a room in a stationary universe and rotating a universe around a stationary room are the same thing, and the reason why the person in the room feels it and the rest of the universe doesn't is because the rest of the universe is comoving with the massive objects doing the frame dragging (the rest of the universe itself), while the person in the room is not.

[@Pfhorrest]

And yes, as a consequence, you have the power to spin the entire universe around, by pushing on the Earth with your feet to make it spin underneath you and letting it drag the rest of the universe around with it. Try this in a field of soft grass on a warm clear starry summer night, it's great fun.

[@Gladdig]

So maybe the speed of light is constant in a vacuum only because the universe is spinning and we have only tried it on or near earth?

[@BENebuchadnezzar]

I once asked this in a mechanics class and was dismissed by the professor for asking silly questions. It's really bugged me ever since, am glad to have found this video at last.

[@taascheee]

Did anyone notice Wilhelms scream? 1:13

[@Daisho32]

It's used everywhere. It's not fun anymore.

[@wierdalien1]

That Annoying Redhead its got better again.

[@jonismack1]

But did anyone noticed he moved in the wrong direction?

[@xxxx85]

@Jonás S. Barjau - What do you mean wrong direction?

[@jonismack1]

When the cylinder starts spinning, the dude would stay stationary in the reference frame of the universe (because inertia and no friction), but for the frame of reference of the cylinder he would apear as "lagging" behind, not accelerating faster than the cylinder is spinning. I don't know if I'm explaining it right...

[@jackwhite3820]

This was beautifully animated, perfectly illustrating the point at any moment.

[@wiadroman]

All I understood is that if the whole Universe rotates crickets start chirping.

[@anonymoose3423]

The illustrations and the the Wilhelm scream are just great

[@ReneMalingre]

The fact of absolute rotation has been bugging me for years and years. I've asked physicists about it, and they could barely even understand what I was asking, had given it no thought, and certainly not explain it. Even the Q 'what is a planet rotating with respect to?' was not answered. I'm so happy to know this IS an unsolved problem, and has a name. 'The distant stars' is the usual reference frame for rotation. Fascinating.

[@ReneMalingre]

Nope. You can measure the rotation of the earth with respect to anything you like (including the Sun) but the Sun's gravitational field lines aren't the primary reference system for the rotation of the Earth; it is the whole Universe, with 'distant stars' being a convenient surrogate to determine what that absolute reference field is for rotations. Mind blowing.

[@JRush374]

Think of two rooms separated by some distance. We're in room a and someone else is in room b. If the universe is rotating around us that means that room b is rotating around us. If that's true, then the person in room b should experience a centrifugal force where the center of the circle is the center of room a. That doesn't happen, so doesn't that mean that room a itself is rotating?

[@Yakoable]

nah faulty logic (don't feel bad, it happens to everyone, a lot): your premise is: (X being "the universe rotates around us, in room A", and Y being "the person in room B gets stuck to the wall") when X is true, then Y is always true but Y is not true, so... Z (Z being "room A is rotating") and that's just wrong, the only thing you can deduce here is that X is not true.

[@macronencer]

Haha! Wow, I just posted almost exactly the same thing. Join the club :)

[@Surfurplex]

I believe the cosmic microwave background can serve as an adequate frame of reference as well.

[@KipIngram]

Great video, guys. This is an important concept that I don't think we've dug into as much as we should. If the local mass around me is "connected to" the remote mass of the universe, then it may be possible to make use of that connection. This is basically what the Mach effect thruster purports to do. We need to understand this completely so that we can optimally exploit it.

[@panulli4]

I have studied for few years now and I have always asked my self this exact same question (How do we know which frame is rotating?)! Thanks for this video! Although it didn't really explained much about why this is the case it is comforting to know that that's actually a valid question.

[@dylanparker130]

how good are the animations here - great work whoever did those!

[@eltimbalino]

Whoah, wind back to 6:40 for a moment. Does that mean if you made two spheres, one of thick lead, one of thin aluminium, and hung identical pendulums swinging in each. Then if you spun both your lead sphere, and your aluminium sphere, that your pendulum in the lead sphere would rotate more than the pendulum in your aluminium sphere? Is this a strong effect? Or are we talking something so tiny that it would take a century of rotations to effect it by 1 degree? And is this effect outside of other known effects like movement generating a magnetic field and so on?

[@nicoepsilon0]

yeah the effect surely would be extremely small, as are most of the general relativistic effects. for example, time on earth is slower than in deep space by about one part in a billion, and the advance of the perihelion of Mercury caused by space-time deformation is 0.01 degree per century (!!). Now compare the mass of earth or of the sun compared to the mass of your lead sphere and your effect size will be diminished by a comparable order of magnitude. So you are correct to suspect that, whatever effect we will observe in real life - should we ever try this experiment - will likely be explained by other reasons (magnetic, coriolis, thermal, mechanical, aerodynamic etc.)

[@lommjubbg]

Does anyone have a link to the einstein paper? Or the calculation?

[@nicoepsilon0]

couldn't find it

[@AlexKnauth]

Re: 6:40 What could this? So the Coriolis effect is a thing, but it's not an actual force, just a result of inertia within a rotating reference frame. The "centrifugal force" is just a special case of the Coriolis effect. I get that. But how does that mean that a sphere around you would influence that? Would the Coriolis effect be less at the center of the earth than outside at the north pole? What would the rotation of the earth do to make it seem more like the "zero" of rotation? Inertia still exists within an accelerating box (you can tell it's either accelerating or in a gravitational field, but same difference, equivalence principle) no matter what mass the box has, so why wouldn't inertia exist in the same way in a spinning box if the spinning box is massive enough to effect this? Why would that matter? Wouldn't you be able to tell that parts of it are accelerating, the same way you would normally, no matter what its mass is?

[@nicoepsilon0]

You need to look into the theory of general relativity to understand why, because this effect is not explained by classical mechanics. In the book ''Gravity: an introduction to Einstein's general relativity by James Hartle'' he covers rotational dragging of inertial frames, gyroscopes in curved spacetime and Geodetic precession only starting in chapter 14, meaning that it's pretty advanced stuff. And they only talk about what happens outside a rotating sphere, not inside. Basically, in GR, the curvature of spacetime is a function of the ''stress-energy tensor'' which contains a description of the energy/matter density, energy flux, momentum density and a pressure/stress tensor in each point in spacetime. So you can have weird gravity fields next to rapidly rotating objects, even some kind of twisting of space itself. This is too complicated for me anyway

[@MateusKahler]

I understood, then I got lost. I was comforted to learn that many are lost with me. Then I got lost again. I think my knowledge is spinning. This video was a great trip.

[@brendawilliams8062]

Triangulation to the rescue 🛟

[@ZukaroTravon]

I feel like this wasn't explained very well (that's not to say I particularly know what I'm talking about though). As far as I'm aware, the cause of centrifugal forces is the fact that mass will continue to move in its direction of travel, so when you rotate something the mass tries to fly out of the rotation and as such creates the centrifugal force. The only thing stopping it from flying apart is the material itself (or in the case of a room, the walls), which is why if you spin something fast enough it pulls itself apart as the material fails due to how great the forces are. But the centrifugal force is really just the momentum of the particles. I think saying that the universe is sending itself messages is very misleading as to what's actually going on.

[@DANGJOS]

Zukaro Travon But how do you know it is the particles that have the momentum and not the whole Universe? After all, the top pictures look the same. So somehow, the Universe knows to differentiate between the two. That's the point I believe

[@ZukaroTravon]

DANG JOS, If the entire universe was rotating around the room, then the particles would just stay in the same position as in that case they'd have no momentum.

[@AuroraNora3]

Anticonny But in your frame of reference you have no momentum so the sum of all forces acting on you has to be zero. Therefore there's an induced centrifugal force canceling out the centripetal force. But only in your frame of reference. To an outside observer there's only the centripetal force

[@ZukaroTravon]

MrKumbancha, I'm not sure what you mean? From the rotating point of view you're getting pushed away from the centre due to your own momentum (which pushes you in a straight line away from the centre) but the wall then keeps you from flying away (so pushes you back toward the centre). But this all depends entirely too on there being friction. Without friction there's nothing to get the object moving and so it'd stay in place relative to the spinning room. I don't think a frame of reference is relevant as if the entire universe is spinning but the room itself isn't then you'd stay put, but if the room itself is spinning (and there's friction) the room will accelerate you, which results in pushing you into the wall (which your own momentum then pushes again, with the wall pushing back, and that's the force we feel).

[@DANGJOS]

Zukaro Travon How are they staying in the same position if the Universe is rotating around them? Define position. In the frame of the rotating Universe, they would be rotating

[@Braeden123698745]

Annnnnnnnnnnddddddddddddd thats why this guys a great professor. That's the best explanation of why centripetal forces exist I've heard. 1:20

[@andrewleister-frazier3570]

So to answer Brady's question and this might seem crazy. That asking; "how the mass knows its relationship to other mass?" is like asking the question; "how does the number 2 know its relationship to other numbers?" How does the number 2 know its 3 away from 5? How does it know that? We'd answer that question by saying "numbers define the relationship between them". And the analogy is mass are like numbers and the inertial frame is the relationship between them and we would say "Mass defines the inertial frame" that's why I say that Mach's principle really is like the bridge between philosophy and science. To get a better understanding of this you really have to turn abstract thinking up to eleven and imagine how we would recreate phenomenon like motion in our minds. Of course a requirement of motion is a reference frame like an inertial frame, in order to have that you need more than one object of mass. The state of motion of an object with no inertial frame of reference is undefined, in order for motion to have meaning there has to be frame by which to gauge it. My intuition tells me from thinking about Mach's principle for a long time, that its like a Platonic basis for reality.

[@zagyex]

Actually Newton had a more Platonic idea, the idea of the absolute reference frame in which all reality existed in. But Mach saw it disturbing because the gyro always seem to point towards the stars you pointed it - it is not independent from the stars that it could have been. The problem is not the relationship between the masses but the forces that act between them. You see, if the acting force was gravity then the gyro would be aligned with the large mass of the earth. But it is not. So what is the force acting there that connects the gyro to the stars?

[@hydraslair4723]

I honestly believe your comment has a lot of problems. The two answers are in no way similar, because in physics we know that masses communicate with other masses at all times through gravitational forces. Numbers, on the other hand, only have a natural order in our minds. Without an order relation between the numbers, which is given simply by our ability to iterate the same action again and again and remember the amount of times we did it (counting), the natural numbers are just like a fine dust throughout space. No order, no relation. It is only when you impose the rules by which you consider something to be an ordering, that there is an ordering. In fact, we could have all been wrong and 3 could have come before 2. From the point of view of order, all orders can be bijectively traced back to the "natural order", (key word being "bijectively") means that all orders are in fact perfectly equivalent as long as they follow a certain set of rules that we have decided.

[@Ragnarrage]

Thanks for a new 60 symbols video. I get excited when new ones come out. I watch your other channels but this one impacts me the most. Keep em coming Brady!

[@MrPomboskate]

interesting vid. but is that animation with the wheel and the rollerblade guy correct? wouldnt he just stay in the same place, or considering friction, wouldnt he just go around the circle going more and more to the outer of the circle because centrifugal force? i just got confused. but this is secondary, the actuall question made was damn interesting. there should be alot more regular uploads on this channel. for me it is the most interesting, after this one numberphile

[@Yora21]

If there is actually no friction at all and no kind of contact between the person and the surfaces of the wall, then he would indeed remain stationary. Something has to give him an initial push. I think the roller skates only add more confusing here than helping. They don't negate friction, just make it impossible to keep your feet on the ground at much smaller rotational speeds. However, even in a perfect vacuum and floating in space, the walls of the rooms still have mass and therefore gravity, so they would very very slightly pull you towards the nearest surface. But for the mass of a human and a wooden room of this size I wouldn't be surprised if it would take days or weeks to reach the wall with that tiny amount of gravity.

[@-receptor4803]

Duarte Pombo off coarse it is not so easy or mighbe difficult!

[@NikiHerl]

The idea is that the wheels are orientated along the (or rather "some") diameter and that they roll along that diamater totally frictionless, while they don't slip at all perpendicularly​ to that line

[@WhatKindOfNameNow]

William White Did you mean to say the centrifugal force is considered fictitious?

[@fabros9290]

its because you are looking at a non inertial frame of refrence

[@arturhellmann9138]

I love how you use the Wilhelm Scream in your videos... This is the second time I notice it :)

[@MrRolnicek]

Isn't it just the curvature of the space all along then? You even said it. So the communicating thing there would be the ever elusive graviton, wouldn't it?

[@IceMetalPunk]

Except general relativity only solves the problem for some specific cases, not the general case (I know, the name is ironic there :P ).

[@MrRolnicek]

I did not know that. I thought with the equivalence principle and all that, it would cover pretty much all bases (except of course where quantum mechanics is involved)

[@Chris.Davies]

A ball on a spinning turntable does NOT fall off the edge. The ball rolls along the same track on the record. And the ball stays in one place And there's no outward (or inward) force on the stylus of a turntable, either. So, I think the skate analogy is bad. There's only an outward force if you are anchored to the floor of the rotating room. If you aren't, there's not. Momentum is conserved nicely. It's the same as "falling" towards the floor of an O'Neil cylinder. You aren't falling. You're drifting in microgravity. Only when you "land" on the rotating floor can you be affected by centripetal/centrifugal forces. That title in this sport, by the way, will be known as "The Coriolis Cup". :)

[@TravisGarris]

Isn't the skater under constant acceleration? Shouldn't that be how one tells the room is rotating and not the universe? In another video, you mentioned the reason the twin paradox is resolved is because the traveling twin has to accelerate to change directions and in doing that that twin changes frames.

[@jasonslade6259]

They're effectively the same thing yes. The reason why the "twin paradox" is resolved is the same reason why the rotating room is fundamentally not an internal reference frame. They're both subject to acceleration. But this leads us into the same question of 'how do we know its not the entire universe accelerating around me?' which is what leads us into the conclusion that the very large scale structure of the universe must influence the physics of smaller scale objects.

[@taraspokalchuk7256]

but what if you find those twins apart alread after acceleration? then you don't know which one is actuall moving?

[@erikmihalj]

I think that here the rotation (of the universe) should be replaced with the linear acceleration of the universe in the direction skater->center of room. That way the skater really can't know what is accelerating/rotating.

[@AuroraNora3]

Travis Garris You're correct. In his reference frame there would have to be some sort of mystical induced centrifugal force cancelling out the centripetal force to keep him stationary in his own reference frame. This mystical force of course seems silly, but it explains how gravitational time dilation works and is a valid perspective. To him this force really exists, but to an outside observer it does not. That's how you tell who's rotating

[@TravisGarris]

These are great replies, but look at it like this... The skater is initially set in motion by the movement of the floor beneath him. He accelerates to a constant speed. At that moment, no one can tell if he is moving, or if everything else is moving and he is stationary. He encounters the wall and is now acted on by the force of the wall. The resulting vector is his new direction. The problem is, he is constantly under acceleration. He is constantly having that vector adjusted so that it remains perpendicular to the radius between him and the center of the round floor. Because he is constantly under acceleration, then he, and all other observers, should know that he is moving. The rest of the universe may be moving, too, but it isn't moving around him. That is something they can prove because he is constantly being accelerated in a way that is not also experienced by the rest of the universe. Does that make sense?

[@nagcopaleen9078]

The quick mention at 6:46 of the rotating sphere "dragging" spacetime to move a pendulum was fascinating. In the scenario where the room is stationary and the universe is rotating around it, would there be a similar effect on the roller skater? Would this effect be distinguishable from the reverse situation?

[@ethanmye-rs]

Centripital vs centrifugal is all about your frame of reference. Both exist, just a different perspective

[@AuroraNora3]

Ethan M Finally someone understands

[@ronaldderooij1774]

Yes, well, halfway understands. The point is, that March principle says that the Centripetal force is the one that exists, and the Centrifugal force is deemed a secondary effect. There is not really a physical explanation found yet (hence only a principle, not a law), but it is applied logic. The point of the video was: How do the skates know in a rotating room that they want to go on in a straight line (and thus go into the direction of the wall producing centrifugal sensation)? For this, the universe must talk to the roller skates. Even Einstein struggled with it as it contradicts somehow the principle of relativity. We still don't know how that happens. That was the point.

[@Nehmo]

A principle rather than a law? I didn't know the definition of principle included an unexplained observation. I think Einstien use "principle" for the lack of a better word. It probably would have been better to say "Mach's observation".

[@99bits46]

True, it's like Gravity & Normal Reaction

[@YuvrajSingh-vm8lt]

Salman Memehood other than there is no gravity.

[@ronaldderooij1774]

Wow, this video is really advanced! How many people will understand how deep this is?

[@msironen]

I think the video should've been named after Mach's Principle. It's a lot cooler than some high school level physics concepts like centripetal force.

[@sixtysymbols]

I can always change it later. :)

[@macronencer]

Yes, I second this! It was the part about Mach's principle that really excited me.

[@MichaelFoleyPhotography]

I just read up on Mach's Principle:, I'm having an existential crisis. Physics is awesome.

[@HeavenlyTennyo]

Brilliant placement of your silver play button right in front of the prof's often questioned Atlas of Creation! Yes, yes, I know he keeps it just for the pretty pictures. Awesome animations in this vid!

[@WilliamDye-willdye]

Mach's principle gives me hope that we might be able to detect a larger multiverse.

[@zagyex]

why? It rather suggest the opposite

[@luchilinares]

You´re great! I love your enthusiasm explaining everything and how clearly you express your ideas!!

[@lithium820]

love the beard

[@lithium820]

and the topic as well

[@jiaming5269]

#nohomo

[@haintthatabitracialist2964]

#noAIDS

[@jasonpatterson8091]

I liked the little Wilhelm scream when the guy hit the wall. Nice touch.

[@JAN0L]

Could you really make the whole universe rotate in principle? No matter how low angular speed you choose if you get far enough away from the center of rotation objects would have to move faster than the speed of light to keep up.

[@hoola_amigos]

OmG.. u have a point..

[@OneWorldLikeItOrNot]

Right, you would need to give it a push from something with more resistance/inertia. Like? (Obvious 'your mother' joke ignored.)

[@Yora21]

That would be if the universe has a center. I believe current understanding is that it doesn't.

[@mrmelee8469]

Janol space expanding or in some bizarre world contracting isnt bound by the speed of light so if only the space holding everything were moving the entire universe could shift around a single point but not seem to move relative to itself. but tje only way to ever have that perspective would be to observe the universe from an outside perspective which is by definition impossible

[@Fly21SE]

Hmmm... Could dark energy causing the expansion of the universe be an effect of a rotation of the universe relative to a higher order object / brane?

[@Gooberpatrol66]

Yes... I've been waiting for this. Disappointed this principle isn't being studied much. It seems pretty fundamental to physics.

[@MephLeo]

Sorry for the stupid question, but couldn't we call cetripetal force "counteracted inertia"? Wouldn't it be more precise?

[@Yora21]

As a non-physicist, I believe it's an accurate description.

[@jasonslade6259]

Inertia is the tendency for an object to remain at a constant velocity, so if you like you can call any net-force acting on an object to be 'counteracted inertia', though its not really accurate because an accelerating object still has inertia. Its really just simpler to call it a force.

[@alexmanning600]

Not quite sure what you mean by 'counteracted inertia'. Inertia is the property that an object has to resist movement based on a force... a counteracted inertia would have to be a force that keeps an object in place, which we would call the 'centripetal force.' The inertia of the object isn't changing fundamentally, unless it's completely changing shape, but we aren't keeping the object from flying out of the circle because we're changing its inertia. We're keeping it from flying out of the circle because of an equal and opposite force keeping it in place.

[@nicoepsilon0]

what you call inertia, I call ''mass''. if you talk about Inertia as the specific property of a body to ''resist changes in velocity'', then this property has the same meaning as ''mass''. mass is a very different concept than force

[@freshrockpapa-e7799]

Mass isn't exactly the same thing as inertia though.

[@addeleven]

Thanks a lot for this video! I'd never heard of Mach's Principle. Small Latin thing: _fugo_ (infinitive fugare) means "I make sb. flee, I put sb. to rout". Latin for "I flee" is _fugio_ (infinitive fugere, both stressed on the u).

[@minecraftermad]

so basically... with pendulums we could find the center of EVERYTHING?

[@ruben307]

at least we could proof if there is a rotational center of the universe. Which I don't think there is or we would know.

[@minecraftermad]

No but iy doesn't even need to rotate

[@minecraftermad]

or more like it can't because there's nothing to refer it to except maybe higgs field

[@moiquiregardevideo]

A pendulum can reveal the absolute inertial frame of the entire universe. This is what the Foucault pendulum in Paris demonstrated : at the latitude it is located, the pendulum swing angle is not performing 360 degres per day, but roughly half of that. The same pendulum could reveal the 1 year rotation around the sun if it could swing for that long. It could reveal the rotation of the solar system around the spiral arm of the milky way, if it could swing freely for 250 million years. As the scale increase, the actual capacity of a pendulum to measure a rotating frame of reference becomes more impractical. This video clearly show that the force experienced by a rotating object has different name depending on the location of the observer, but it's effect is real anyway. Just saying "it is fictitious" doesn't make it disappear magically.

[@s.ananthkarthikeyan4560]

Beat me to it, I was going to tell him about the Foucault Pendulum :P

[@cornfall]

I really enjoyed your presentation! Very few consider Mach’s principle as a big theoretical question. But some do! Carver Mead, John Cramer, and several other physicists, scientists, engineers, astronomers, biologists are also studying and publishing on Marx principle and its relationship to GR and Einstein’s first static field pre-Riemannian tensor curved space work published in 1911 and 1912 which you can see at Princeton

[@Yutani_Crayven]

The centripetal force example seems like a bad choice. Isn't what stops you from moving through the wall the structural integrity of the wall rather than a force acting independently from the wall pushing you in?

[@mohammedmiyaji6799]

playgrrrr I'm leaning towards the fact the centripetal force doesn't exist, but then gravity of our solar system pops in my head as an example.

[@Yutani_Crayven]

But gravity is a thing of its own, no fancy centripetal force needed. Moreover, planets move in ellipses not circles (with respect to the Sun, they move very differently with respect to the galaxy). This feels like a needless concept, somewhat like when people cut up kinetic energy into all these sub categories of energy.

[@edskev7696]

playgrrrr the 'cenripedal force' is the name given to any force which makes you move in a circle, whether that force is generated by the structure of a wall, gravitational pull, or some other source

[@Yutani_Crayven]

+Ed Skev Well that's just redundant then, isn't it. I was lied to my whole life and never understood the concept because of this. -_- ;;

[@mohammedmiyaji6799]

Ed Skev as to what I was taught in school, -fugal is the force that throws you away from the centre and -petal is the force that pulls you towards it. No imagine if there wasn't a wall for miles and I'm being thrown of because of the rotating surface; there exists only centrifugal. However, if suddenly a wall pops up and I get squashed by it, isn't it just the normal reaction the wall gives me that stops my centrifugal motion. I've had this doubt for ages as to why would they, either rename a force just to make it sound like a counterpart, or make some new concept. BTW still hasn't cleared up. Oh and about gravity, even though it is an elliptical orbit that the planets rotate about, they are still being pulled towards a centre (focus in this case), does that mean gravity can be considered as a centripetal force?

[@frankharr9466]

It's a constant excelleration. And I love that Xkcd comic.

[@hansgrettle8240]

Under 301 club :)

[@taitywaity1836]

aw man I just missed out, I only made the 301+ club :(

[@macronencer]

Professor Merrifield, you have my eternal gratitude for laying to rest one of my demons! I noticed the paradox of rotation through my own thought experiments when I was young, and it has baffled me for decades. I asked many people to explain it, and some of them were even physicists, but I was never satisfied. Not one of them ever mentioned that the answer was still out there awaiting a detailed explanation! Now that I know this, I can sleep a lot more easily at night. :)

[@Chlorate299]

The revelation for me about centrifugal/petal force was when D'Alembert's principle was explained to me a dynamics lecturer. Effectively neither force is a real force, they're "fictitious" forces that appear as a result of inertia. The other thing to imagine is a hammer thrower, when the chain is released the ball travels tangentially and not radially - i.e. a force has to be applied to the ball for it to follow a circular path, else it will just carry on in a straight line. If there were a real force causing the ball to travel outwards, then this would remain when the chain was released.

[@trailogy]

Just found this and boy am I learning. Thank you for making these!!! I love your video on reynolds numbers

[@KafshakTashtak]

I remember reading something related to this experiment in the book: "Was Einstein right? ". In that book, it described the experiment as this: There is a bucket of water on a rotating table. Scenario 1, the table is rotating, therefore the water in the bucket gathers on the walls of the bucket (centripetal force). Scenario 2, the bucket is stationary, the rest of the universe is rotating around it (I don't know how stuff in the distance can go faster than the speed of light., Will it have the same effect as scenario one? That book says according to general relativity, the whole universe rotating around the bucket pulls the space around the bucket and causes the water to go toward the walls again.

[@PinchOfLuck]

Amazingly well done. You can see a lot of effort went into animations etc. Keep it up. :)

[@mikecrapse5285]

I think that it's important to note that if you actually rotate the universe you need to also be rotating the Higgs field. in which case the path the bystander takes on the wheel is the same looking at the Higgs fields. In this scenario the interaction with the Higgs field is also the same. as the Higgs field interacts with mass, this is what will cause your apparent centripetal force. your "drag" against the Higgs field will cause you to attempt to remain in the same state(an object at rest stays at rest). if you are moving and not the universe this would be "drag" , if the universe is moving and not you, the Higgs field would exert what's effectively a "wind". in both cases the interaction is the same. the universe isn't " communicating" any differently than any other mass interaction with the Higgs field. Put another way, given a choice patch of space far from any gravitational, strong, or weak force's influence. do you still retain mass? do you still retain inertia? as the universe(which needs to include the Higgs field) moves , you mass will tend to stay in the same spot in relation to the field, as that is frome whence mass is derived.

[@mikecrapse5285]

put in "general" relativity terms. the much used fabric analogy, where the more heavy and dense an object is the further is sinks into the fabric causing a gravity well. think of it as, as you travel along that fabric you are continually working against your own well(your mass) to keep moving. if you are rotating you are traveling along that fabric, if the universe is rotating, that fabric is being pulled out from under you. the resulting forces are the same. equal and opposite reactions. this only happens because you have mass.

[@rpyrat]

As time goes on I get fewer and fewer mind blown moments. Well this was one...

[@PaulPaulPaulson]

I was thinking of the xkcd comic when i read the title, then it appeared immediately in video! Nice!

[@OblivionFalls]

6:45 Is the pendulum in a vacuum for this example? (So that we know that it isn't just the air dragging on the pendulum). If it is, then that raises some interesting questions for me. You say the only way for us to know if the universe was rotating would be if we could observe centripetal force in action, but has anyone actually looked at the universe to see if this is the case? Is the expansion of the universe consistent with some kind of space-time rotation? What if dark matter drives some kind of space-time rotation in a higher dimension, such that it appears to be expanding in all directions no matter what your reference point is. It's an interesting thought. I could be COMPLETELY missing some basic rule of physics that blows this idea out of the water, so please feel free to correct me on that.

[@Snakeyes244]

dark energy is just a rotation of the observable universe

[@SamuelHauptmannvanDam]

The pendulum part is one of the reasons I think scientists really appreciate Einstein even more then the general person. That there is so much more to gain of yield by something so simple.

[@DavidKennyNZL]

Thank you! I have always wondered why rotating frames of reference (e.g. space ship) would feel like gravity. The reason no one could tell me is they don't knows yet. I have a machine to rotate the rest of universe but my friends think I'm spinning around. Thank you again for explaining how rotating spacetime causes large objects "drap" small ones away. I want to learn more.

[@kinggeorgedental5766]

Love those Pete animations.

[@balazsmarcsek2949]

Imagine the following: Take the rotating reference frame Professor Merrifield mentioned, with walls of ice, let's say one meters thick. Now imegine a tempered object with higher temperature that of the wall, between the inner face of the wall and the center of motion. For the sake of simplicity, let's say the friction between the floor and the object is zero (spherical object should be close enough) and the objects temperature is constant (should be possible in practice too with electromagnetic waves and a metal object, or so one would imagine). My questions (of which i kinda have my own answers): - Would it drill through all the way to the outside? - How would the bore look like? - How would the drilling process look from outside the rotating reference frame? - How long path would the object travel during the drilling process?

[@nimrodlevy]

Why i didn't learn with this professor in high school! i finally understand physics!

[@BritishBeachcomber]

Roller skates are a bad example. Low rolling friction but high lateral friction and resistance to side slip. That's how you push. You need ball skates for your demo to work.

[@raphaelreichmannrolim25]

Bohmian mechanics!!! One single possible answer, but that answers so many questions!..

[@GrunOne]

Rotation is nothing but linear motion amongst particles that are bound together, constantly dragging each other in a circle. If molecular bonds all let go, each atom would fly in a straight line. The room is rotating and you feel a force, sure, but that's because everything wants to move in a straight line but is being acted upon by the walls, and the walls are held together by the bonds within the wood. The reason why spinning feels like a special case of relativity is because it's just constant acceleration, same as if you were being accelerated in a straight line, but constantly changing the direction of acceleration so it forms a circle. The fact that it's a constantly changing direction of acceleration forming what we know as 'spinning' doesn't require any communication from the wider universe. It's bound by the strength of the material keeping the spinning object from flying apart (at small scales). The only acceleration an 'unmoving' object naturally feels (barring collisions) is the normal force of an object its gravitationally bound to or the 'wall' of a spinning object it's within.

[@robstorms]

Hmm.... If mass in the universe began gradually disappearing ( getting down to half or less perhaps) would the person in the rotating room gradually feel a reduction in the " centrifugal" force he feels?

[@MotorGoblin]

Never thought about that. Very interesting.

[@BC3012]

best thumbnails on KZbin!

[@williamking9707]

Never knew Mach's legacy extended to more than 'just' revolutionising Aerodynamic theory. Interesting!

[@Kumaryoku]

I think this idea deserves another video

[@klapstoelpiloot]

I am mostly intrigued by the last example, the pendulum in the rotating shell. Let's say one does this in a vacuum. Does the pendulum change swinging direction because of some sort of gravitational drag? Has anyone ever attempted such experiment and maybe measured the effect?

[@Galakyllz]

Those animations were great!

[@Leeengold]

The issue is the believe in a rotation as a fundamental state of movement. The rotation of an object is made of mass points that at every point in time have a velocity in a given direction that determines their future path. But due to the boundaries between every point of mass they are consistently acting forces on each other that change the velocity vectors. You can't compare that example to the one for specia relativity where two spaceships fly by - there those ships are made of masses with the same momentum and therefore not interrupting themself. In case of our rotating "ship" it is the difference in momenta that exposes the rotating object.

[@beeble2003]

1:13 Wuh? If I'm stationary in a circular room that starts to rotate, and there's no friction between me and the floor, I'll stay in the same place, not hit the wall.

[@papaburgundy71]

So would humans then have a mechanism by which we subconsciously or instinctively observe and ostensibly measure the distribution of mass throughout the universe, and subsequently perceive depressions and distortions in space and time throughout the cosmos?

[@xxxx85]

I've always wondered around things like this. Like, how does particles and waves keep track of how fast and in what direction they are moving? We know that as they approach the speed of light, "so and so" happens. It becomes exponentially harder to gain further speed, time moves faster/slower and so on.. But approaches the speed of light, in reference to *what*? Imagine one spaceship standing still, and another spaceship is heading towards the first spaceship at .99% the speed of light. With no points of reference, it will seem as if both spaceships are approaching eachother. Still one timeframe will move faster than another. This tells me there has to be a universal point of reference somehow... which feels weird. Same as rotation.

[@icedthai]

It's bugged me for many years. I was grateful when I finally learned what's it's called but still remain flummoxed that there is no solution. In my example a person is inside a dark sphere that's deep in space. They have barbells. They lift the barbells to their sides and they can tell if they are holding still or spinning against the universe. It drives me crazy!

[@RedcoatsReturn]

Mach‘s principle relies on dimensions we don‘t realise are there, which connect everything, which is how quantum entanglement functions. These dimensions manifest in our 4D and baryonic experience throughout the whole universe as the “unexplained things“, dark energy, dark matter and so on. We learned to think quantum, so let us think multidimensional thus we shall begin to understand and find answers.

[@foleytrim]

Animations are wonderful.

[@pattern2054]

The centrifugal force isn't pushing you away from the center it just pushes you tangently left or right depending on the room's rotation, clock wise or counterclock wise.

[@ruben307]

well it depends if you are viewing from the rotating system or not.

[@AlanKey86]

2:22 - oops! There is not a balance of forces (as the Prof. states) because you _are_ being accelerated, away from the wall towards the centre of the room.

[@kylebeauchamp5345]

Just a high-school physics teacher here... I wondered about this in college. And what I decided was a little different than (Brady's?) guess.. I think that the universe's rotation would, indeed, not be noticeable in the way a merry-go-round is; but I think things (like roller skaters) would still be flung outwards (like 'centrifugal' force was acting on them) and I quietly (until now; I certainly don't teach my students this) have thought this is the reason the universe is expanding... Any thoughts, criticisms, explanations why I'm certainly wrong, etc?

[@cyrilio]

Great animation in this video!

[@michaelsnodgrass9415]

You can easily tell that the universe ...those things... the lights in the sky right above us are rotating and that we are on a motionless plane. Very easy to see and feel.

[@MagnusSkiptonLLC]

For some reason all I can think of is that carnival ride shaped like a ufo that spins around. It's run by a guy in the middle and I'm wondering how much centrifugal acceleration he's feeling, and whether he ends up getting sick after a while.

[@b43xoit]

For a bit, this was blowing my mind. But not after further thought. Rotation is absolute, not just relative. It can be measured by the Coriolis force. The eyes only pick up relative rotation, but just because they don't pick up absolute rotation doesn't mean it can't be observed with other instruments.

[@zagyex]

but if it is absolute how come the stars seem to stay in the same "absolute" position?

[@Zephon9]

The centrifugal 'force' is the effect of inertia when there is not enough force keeping an object toward the center of a rotating body. The centripetal force is simply the countering force to this effect of inertia, which is generated from something like a tether connecting you to the center (building tension with rotation speed), or a wall at the circumference of the rotating body (building Normal Force against your body with rotation speed).

[@freshrockpapa-e7799]

Uh... No. You clearly don't understand this. You need to say from which reference frame you are observing.

[@Zephon9]

I don't think the reference frame really matters. Explain how it would.

[@freshrockpapa-e7799]

In different reference frames there are different experiences with different causes. For instance, in a non inertial frame of reference, there are centrifugal and coriolis forces.

[@Zephon9]

You were supposed to demonstrate how a frame of reference itself could render what I said false, and not simply reiterate the same point that it depends on reference.

[@theultimatereductionist7592]

If there were NO other matter in the universe outside the rotating frame, would rotation still be absolute? How could one know? No way to test it!

[@Chase_Myles]

u could have posted this video when i had my Fluid Mechanics exam.....anyways gr8 video as always

[@davidb5205]

This made my head spin right round

[@von_nobody]

For me answer is simple, you look at local space time curve, if you rotating compared to it you will feel this force, this is similar to gravity if you move with it you will not feel anything but if you try resist you will feel full force of this.

[@mirakodus1]

I highly recommend reading Brian Greene's The Fabric of the Cosmos: chapter 2 - The Universe and the Bucket. It has some amazing insight into the Newton's view, Mach Principle and how Einstein changed it, first with his SR and then with GR.

[@tothespace2122]

6:32 Is so funny when you realize it's a ringtone you have for home telephone.

[@DanielRowe]

This has blown my mind, I have to know how the information is mediated.

[@Yora21]

That seems like a pretty big and important question in the physics of space and motion.

[@S1yFQX]

Can Sixty Symbols please do Newtons Bucket Argument?