Gravity is not a force. But what does that mean?

[@SabineHossenfelder]

That was a tough one! The quiz for this video is here: quizwithit.com/start_thequiz/1702972458163x675901602454850000

[@smlanka4u]

Gravity is a force, and space doesn't bend due to gravity. The density of space increases near massive object due to the gravitational force. The existence of Dark Matter shows that space is material. General relativity is not a quantum theory, and it doesn't explain the high gravitational force that make Black Holes during the supernova explosion. Neutrinos can be the cause of gravity because stars emit neutrinos from their 99% of energy of the supernova explosion, making pressure to make a small Black Hole.

[@javahaxxor]

​@@smlanka4uinteresting statements care to explain that to us chimps?

[@_John_P]

You left out tidal forces, which break the premise in the video's title.

[@emifro]

Quizwithit asks for registration to see the correct answers :/

[@davidmaxwaterman]

You should have given the correct answers 😜

[@dbracer]

Took a shot for every time Sabine said "Gravity is not a force". Now I'm definitely in free fall.

[@solapowsj25]

Gravity is the force of acceleration on a mass. Gravity is the curved path of bodies in free fall in space. Einstein.

[@manticore5733]

Exactly what I was thinking watching this. Things get really tricky when you explain the notion of space-TIME curvature due to mass and why that means you're accelerating ... you really need a drink for that part!

[@gabbleratchet1890]

Well , she did say that acceleration is vodka.

[@im1who84u]

What would Einshstine say about that?

[@jimstenlund6017]

Listening to Tom Petty?

[@user-bi7nq4nj7q]

I tried to tell my wife this the other day... she just pretended to care and nodded her head in approval. The life of a physicist :-/

[@user-hk8yp7cw1v]

I tell this to both family and friends and they tend to do the same so don't feel alone 😅

[@nicklacelle]

That's just the life of a husband.

[@dtibor5903]

Well, don't try to explain this to regular people. For regular people and for practical purposes gravity is a force.

[@TransdermalCelebrate]

Very Funny, I’d wished to of been there 😄👍

[@josir1994]

She cared enough to pretend to care, that's a good start

[@alonamaloh]

Perhaps Sabine didn't want to introduce reference frames, and there are good reasons for that, but for some people it might help to think about this by talking about different types of reference frames. The whole thing can be summarized by saying that the usual reference frame, where the floor is not moving, is not inertial. The force of gravity is then a 'pseudo-force', an illusion that appears because we chose a non-inertial reference frame, similar to the centrifugal force or the Coriolis effect in a rotating reference frame. In general relativity, inertial reference frames follow geodesics of space-time, which implies that the origin must be in free fall.

[@Earthstein]

So much of a construct; right?

[@Matthew-by2xx]

@@kleinerprinz99 Statements that start with “It’s vert simple,” and then simply miss the nuances are always fun.

[@NewNecro]

This so much. I think it'd have been much more helpful to better explain the spacetime model with geodesics, worldline and gravity's role within it rather than vaguely affirm what gravity is not. For most layman Newtonian gravity is the standard which makes special and general relativity particularly unintuitive. The fundamental differences between inertial and non-inertial reference frames are very important distinctions to explain Fictitious Forces you mentioned.

[@ObjectsInMotion]

There’s no reason to not consider pseudo-forces to be as “real” as a “real” force. “Real” forces are mediated by virtual particles, which are themselves not “real”, so why do those forces get special consideration? They shouldn’t. A pseudo-vector is just as “real” as a normal vector. This entire video is just pedantry.

[@gramail2009]

I have a vague sense you might be able to explain this better than Sabine does. It makes no sense to me yet. Maybe it is just a matter of language. Seems to work quite well for me (and most of the world's scientists too!|) to think in terms of the 'force of gravity 'pulling me onto this chair! Will I really benefit by pretending there is no such force??! Or calling it something else. First I guess I will have to find out what people mean by an inertial frame of reference as opposed to any other kind...

[@peterwilson8039]

Whew!! Having spent a good part of my life trying to teach Newtonian Physics, I can only surmise that you have managed to hopelessly confused a great many people with this video. Fortunately I am now retired so none of those hopelessly confused people will end up in one of my classes, and for that I am very grateful.

[@richtheobald4390]

"9.8 m/s/s as you were probably taught in kindergarten" Maybe in Germany but I grew up in Canada and was still figuring out that plasticene wasn't a food group. I think you're right though: never too young to learn that thing that holds you down is not holding you down.

[@MrKotBonifacy]

"PLASTICINE", perhaps...? ;-)

[@hooked4215]

Say pleistocene better@@MrKotBonifacy

[@milanstevic8424]

@@MrKotBonifacy no he likely means Plasticene, as an informal "geological" epoch nomenclature, as the last part of the current age called Holocene, which is further subdivided to Anthropocene, an epoch in which all humans tend to be terminally guilty for existing. Needless to say these are all unofficial addendums, and are mostly there for rhetorical and socioeconomical purposes, of which Canada is a prime consumer.

[@AlexAnteroLammikko]

@@milanstevic8424 Wonderful, but definitely wrong. OP obviously meant Plasticine because thats putty and thats what children tend to eat, and its not a food group. So your Chat GPT/wikipedia blurb doesn't add much to that.

[@c.augustin]

Well, it was a joke. As Sabine likes to do. I can assure you that we don't have physics in Kindergarten here in Germany.

[@DruMusica]

The fact that you use several examples makes room for different brain wirings to link in. At each step in this video, I felt a little closer to getting this right. It was extremely satisfying and educative. Well done and thank you!

[@chrisstevens-xq2vb]

Pffft this is beyond stupid. If gravity wasn’t a force it wouldn’t do anything.

[@andrewjoliver82]

​@@chrisstevens-xq2vb just because you're incapable of understanding does not make a complex set of ideas stupid. The stupid is you 🤷

[@bartsanders1553]

​@@chrisstevens-xq2vbIt's just another lie from big globe. Stay strong, brother.

[@CSpottsGaming]

​@@chrisstevens-xq2vbIf you don't understand, you can say that instead of being rude.

[@thenonsequitur]

@@chrisstevens-xq2vb Gravity isn't "doing anything". Gravity is the natural fall of mass toward other mass due to the curvature of space-time. It's a description of the structure of space-time, not "doing something".

[@waynesaban2607]

The fact that Einstein married his first cousin Elsa, means even he didn’t understand relativity….

[@frequentflyer56]

😂😂😂

[@77kaczka77]

Lol! But it is “relative” easy to understand reading books of G. Gamow (Mr. Thompkins…) Btw: People, who say, they understand quantum theory, don’t understood it.

[@SlipKnotRicky]

Einstein was a RedNeck?

[@sistajoseph]

There are other kinds f relativity and some communities prefer to marry their cousins any way.

[@truthorpropaganda9001]

🤣🤣🤣

[@Zandaarl]

As a layperson in physics, I consider myself to be fairly educated. But this was a wild ride. I went from "Wait, what?!" to "That can't be right but Sabine wouldn't tell us something incorrect." to "Oh, now I get it!" to "I'm just slightly confused but I get it but I'm not trying to explain it to my friends." Thank you Sabine for expanding our understanding and knowledge with every video! 🎉

[@dmariehatch8825]

By C300

[@BlackistedGod]

Veritasium has a video on the same topic years ago, I think he did a pretty good explanation

[@rayRay-pw6gz]

As a Star Trek fan , this is very disturbing. How can we travel without gravity, our bodies were designed to work with gravity. I can not accept the ability to create artificial gravity. ✌️

[@MosheFeder]

@@rayRay-pw6gz Speaking as a professional SF editor, while artificial gravity has been a common feature in SF stories and novels for decades, the writers have fewer ideas about how it might work than they do about warp drives or FTL drives in general. As SF ideas go, it's certainly one of the most unlikely. But we keep using it anyway because it's so convenient. This kind of winking compromise with physics is why the genre is called “science _fiction_”!

[@rayRay-pw6gz]

@@MosheFeder 😀. Reality sucks ! Thanks . 👍✌️

[@Slitter_the_Dubstep]

Every time she says "Gravity is not a Force!" I feel like she got me. Its like a punchline that doesnt grownold and messes you up no matter how often you hear it, just because most of our lives weve been learning something different that we adapted into our Framework of reality

[@andrew3203]

Not something different, simply wrong. If you teach wrong things in school, you shouldn't be surprised when people say those things.

[@biopsiesbeanieboos55]

I agree. It’s like an unripe plum. No matter which direction you approach it from, it doesn’t become any more palatable.

[@robert-wr9xt]

Thankfully the phone didn’t ring.

[@Slitter_the_Dubstep]

@@robert-wr9xt huh :D

[@robert-wr9xt]

@@Slitter_the_Dubstep New to the channel? Sometimes she has a red phone on a desk. It rings and she answers it. Charlie Brown adult voice talks on other end. She makes comments and hangs up. You’ll laugh. Have a nice week.

[@thehadster7043]

I usually can at least grasp the content of your videos. But... I gotta say, this had my head spinning. I eventually got it, but it was difficult. Thanks for the mental gymnastics!

[@simonbowden8408]

But Sabine isn't acceleration also defined as rate of change of velocity (I know that velocity is relative to something)? Can one separate acceleration from force? If you're in a black box and it accelerates then you can't tell the difference between gravity & acceleration? Which means that gravity is equivalent to a force?

[@FensterwischerX]

There is the equivalence principle, yes. But Gravitation is Not a force, it's an emergent effect of the curvature of spacetime caused by mass. An accelerometer would Not detect any force in free fall. I don't think there is a contradiction.

[@chiralanomalous2904]

@@simonbowden8408 If you're in a black box, centrifugal force would also be indistinguishable from a force pushing you against a wall opposing the normal force (normal force is centripetal and exists. The opposing force is centrifugal and is imaginary. It's just inertia and centripetal force tricking your brain) But everyone knows there isn't actually any centrifugal force.

[@OvisMilitaris]

Sooooo, what you're saying is... Gravity is a force. Got it.

[@jonathandavid3298]

Fantastic video! Please do a video covering Mark Kasevich's experiment demonstrating the Aharonov Bohm effect for gravity. I don't know why this is never mentioned in physics when it seems to be one of the greatest findings in decades. Your take would help naive science hobbyists like me who don't know if this finding is significant or why nobody covers it.

[@ChaoticNeutralMatt]

Effect.. for gravity? I'm not familiar with that effect in that context.

[@dougjamesberwick2625]

Absolutely fascinating as always - most accessible explanation I've ever heard!

[@schweinehundbullshit9176]

As Absolutely fascinating as absolutely meaningless.

[@dr.danielmckeownastrophysics]

The equivalence principle only applies locally, its actually possible to see the difference between a person standing in a gravitational field and a person standing in a box with a rocket because when you look at the 2nd derivative and compare the fact that the person in a gravitational field will experience differing ("non-uniform") accelerations at their feet vs. their head while a person standing in a box with a rocket accelerating will experience uniform acceleration, you can see that the gravitational field can be distinguished. So while the two are close, they actually are very different and cannot be said to be physically the same. One could be treated as essentially a uniform field, while the other is non uniform when you compare it at different regions of spacetime.

[@random_nick_for_comments]

So if we can distinguish, following the lector logic - gravity is the force. I finally got it.

[@toargueortonotargue]

Nooo... That would ruin the Equivalence Principle and that implies all of General Relativity is wrong. Because the equivalence principle doesn't just tell you they're similar, but that acceleration and being on the surface of the earth is literally and physically congruent.

[@sshreddderr9409]

@@toargueortonotargue relativity is all wrong, something being mathematically identical does not AT ALL mean they are actually identical, just that they can be quantitatively described the same way. accelerating on a flat surface and a non accelerating car rolling down a hill is mathematically equivalent, but whats actually happening is NOT the same, only the structure to calculate the involves quantities are the same. in both cases, the acceleration has a completely different origin, one is caused by gravity, the other is caused by the engine accelerating, not at all the same thing, it just is experienced the same and can be described mathematically the same way, but it IS not the same thing. saying that gravity is because of space bending is a subjective way to give meaning to a formula that shows acceleration. mathematically, it is no different than any other cause for a force. So why is it that people buy this crap, and why is it taught as if it followed from the math, when the math just quantitatively describes an acceleration, and says nothing about its cause or mechanism? the cause is literally just made up by Einstein, it is highly irrational, has no proof, and there has never been any attempt to prove any causal claim, its just accepted alongside the functioning math. ALL of EInsteins verbal claims are complete bunk, and people buy them only because his formulas work and they mistakenly assume that he is somehow a genius and was just smarter, its all based on faith. gravity is a normal force, and how you describe or experience two interactions says nothing about what mechanism causes them. modern physics make zero attempt at causal relationships, its purely quantitative math when it get to a certain point, and then ridiculous explanations and notions are smuggled in with that math and presented as if they were valid or somehow proven by its descriptional math being quantitatively valid.

[@toargueortonotargue]

@@sshreddderr9409 All yap... General Relativity has been proven a LOT of times. Take the Global Positioning System, for example. It relies on the mathematical description of General Relativity. GR also describes the orbits of planets around the sun, even the moon of the Earth around it, and the precession of the orbit of Mercury around the sun, very accurately and precisely. GR is also able to describe things like gravitational lensing, which we've confirmed so far, with our images from the James Webb Telescope. Evidences for GR, from the first ones, up until now, suggest that General Relativity is an excellent descriptor in areas and dominions where it works. GR also describes the existence of black holes, which we too have confirmed, when in fact, early physicists cannot even and would not believe in it and discredit its existence as being a mathematical glitch in the mathematical description of GR. GR also predicts, and later we confirmed it, that ripples and disturbances in spacetime, gravitational waves, exists. GR matches observed data as well, when it predicts an expanding universe, see Edwin Hubble. Claiming that Gravitation is a force, is irrelevant, when the Law that describes it as a force is but a mere approximation to GR. General Relativity, is also rooted in Special Relativity... Now, Relativity cannot be wrong because it explains a lot of stuffs, from Electromagnetic Induction, the detection of high energy cosmic muons on the surface of the earth, and it too describes Dirac Particles when combined with Quantum Mechanics, (see. Dirac Equation, Relativistic Quantum Mechanics, Dirac Particles, Quantum Electrodynamics, Quantum Field Theory, and Quantum Chromodynamics), up until Gravitation. You're like Nikola Tesla, actually. You remind me of him... People like you yap around and complaining about Relativity when you don't understand it. Did you somehow attempt to study it, but found that you could not handle the complicated math? That shows no passion for the truth. You see, in science, what can and is considered to be "real" or "true" is an ingredient, (could be anything, an idea, a hypothesis, or a mathematical description), that is necessary for explaining what we can observe. And both SR and GR are very precise in their descriptions, and does not go in conflict with empirical data, they made predictions that has been so far confirmed. And if you got a problem with that, then that means you would make a bad scientist.

[@orthicon9]

That gravitational shear or tidal forces was a central gimmick in Larry Niven's 1966 short story "Neutron Star". Another thing about the Equivalence Principle that bothers me: Could you not also tell the difference by measuring the angle between two plumb lines? A windowless box accelerated by a rocket would result in the plumb lines being parallel. Gravity from a nearby Massive Object would have them converging toward the centre of gravity of the Massive Object. Make the Massive Object a small piece of neutronium, to exaggerate the angle between the plumb lines.

[@ionsilver557]

One of my favorite explanations of gravity is a quote from John Wheeler, which interestingly, doesn't include the word "gravity" at all: "Space-time tells matter how to move; matter tells space-time how to curve."

[@Nocholas]

yes, but maybe not.

[@samibraheem1579]

I think this ia why we haven't and will not see a subatomic particle for gravity since it's a force like nuclear and electromagnetic

[@juliavixen176]

The thing about General Relativity, is that this _is_ all that it says about gravity. It exactly describes how gravity works... but not _why_ Why does mass and energy curve space? Yeah, it just does, and we can calculate exactly how much and stuff... but what's the actual mechanism? Why should geodesic worldlines converge towards the largest pile of confined energy, and curve away from a vacuum. What is the mass (or vacuum) actually *doing* ? General Relativity just says that the spatial distance between two points shrinks as the time distance increases... that's it, that's all it says. It's not very satisfying. It really is just pure geometry.

[@magicmulder]

Nobody puts gravity in a corner! 😂

[@patinho5589]

So the matter matters. It makes a curvature within which lifeforms like us do our stuff. This mean planets matter.

[@cavesalamander6308]

1:00 These graphs only show that all three sensors are not calibrated to '0' (have offsets typical of electronics). Sorry, this is not theoretical physics, it's engineering.

[@declanwk1]

if you allowed the accelerometer sensors to freely fall, then they would read zero during the free fall, so they are calibrated.

[@Earwaxfire909]

It would be helpful to explain why charge interactions are driven by a force and the differences with gravity.

[@drgetwrekt869]

Maxwell equations are linear, and thats why they can be represented as a field of vector """particles""" (photons) that interacts with electrons and so on. Gravity apparently doesn't fit in this formalism because it is inherently non-linear and defines the same coordinates that are used for the calculations. Edit: actually even "non-linear" fields can be quantized without issues, for example Higgs or phi^4 terms. But as far as I know that's it ? Not sure tho

[@josefpharma4714]

@@drgetwrekt869 I'd say linearity (none-linearity) should not make any difference. But AFAIK: If gravitation is not a force, electro magnetic interactions are no force, too. (But this is a kind of definition only?)

[@dannydetonator]

@josefpharma57.. Difference is in the origins: electro/magnetic forces have quantised matter-energy as a direct cause for forces exerted. Gravity is causing forces, but itself it's just a constant of spacetime bending per general relativity. The latter have no particles or known fields carrying or causing the forces created. It's like acceleration without an engine doing the work, while still carrying the accumulated potential energy.

[@PrivateSi]

Spacetime is not a technically not a force, but gravity could be, and the cause/bits of space time could/SHOULD exert a force. Unless you believe space is empty or some nonsense like that..

[@Dom-Nom-Nom]

⁠​⁠@@dannydetonatorbut if EM forces require work, like an engine, why don't I quickly run out of energy from all the EM acceleration from sitting on top of the Earth?

[@spock2009]

Without gravity, the marble will not roll into the ball on the rubber sheet. I don't find this comparison helpful when trying to explain that gravity is not a force. It's a circular argument. The explanation that worked best for me is that everything moves at the speed of light all the time. When we rest, we move through time at the speed of light. In the vicinity of masses, time dilation takes effect and therefor we fly a bit through space. That is because we don't slow down but just change direction from time into space. Is this compatible with Einstein's theories?

[@juzoli]

Here is my favorite analogy which helped me understand the concept: Imagine you and your friend are standing at the equator, and start walking towards north, parallel to each other. But as you walk, you notice that you start to get closer to each other, and would collide by the time you reach north pole. Some mysterious “force” is pulling you together. You have to physically accelerate to keep your paths parallel. Is it a force pulling you together? Of course not. The Earth’s surface is curved.

[@Markielee72]

I like that. 👌🏻

[@audience2]

​@@harmless6813Lines that intersect are not parallel by definition.

[@acebulletman7389]

It seems that latitude lines are parallel, but longitude lines are not since they intersect.

[@TBJ1118]

Nah it's the force of love 'cause we gay for each other

[@ak74udieby]

@@acebulletman7389a latitude doesnt have a "line" besides the equator

[@thisuserhasaname]

Here's what I don't get: If the argument is that a spring in free fall does not experience acceleration because it doesn't change shape, then would the same not also be true if we swapped the gravitational field for a magnetic one? Since magnetism also works on the entire spring at once (rather than just on contact area), the observed effect would be the same: The spring keeps its shape and therefore is not accelerated. So therefore magnetism should also not be considered a force? Same with an electric field.

[@pasqgrasso]

@thisuserhasaname, yes your argument is valid. Electric and electromagnetic forces are recognised as forces, but due to a lack of understanding, gravity is not seen as a force by some (which it is of course, sorry Sabine). The wider community of Physicists STILL haven't got a clue what gravity is. They must discard Einstein's theory in order to move forward. He was very good at describing effects, but he was not good at identifying causes. This is a major issue with General Relativity and Quantum Mechanics - cause and effect are divorced, which has led to misunderstanding. We will never make leaps forward if we do not get past this paralysis.

[@187nemesis3]

Who says a spring has to be made of a material that can be affected by magentism?

[@DanielSamaniego-of5xl]

Gravity is the bending of spacetime in a 4th conceptual medium per Albert Einstein it's an effect not a force. (Pseudo math formula for a conceptual medium) This replaced Newton for mass does not attract mass i.e. 🎈 ☁ Not 1 single scientific (natural phenomenon independent variable and dependent variable) experiment has even been conducted to prove Gravity!

[@Bluemoon4415-s5v]

If you were to experience being pulled by a magnetic field (say you were wearing a suit of steel armour) you would feel the force, when in free fall you feel nothing. Oh and electric and magnetic fields are the same thing.

[@QuasiRandomViewer]

@@187nemesis3 Who says Dr. Hossenfelder's spring has to be made of a material that can be affected by gravitation? ;^)

[@BosqueProfundo]

I think I undestood pretty much everything Sabine said in this video, but I still don't get the most important part: The space is curved because of mass, but why would you follow the path of that curvature (towards the center of Earth) instead of remaining on the spot you are? Why follow that direction of the curve specifically? Is it because you have to assume a pre-existing movement of the object relative to (towards) the other bodies (eg. the Earth)?

[@streettrialsandstuff]

It's not the space alone that is curved, but the space-time. As time passes, you are moved in space in a direction of a nearby object with a large mass.

[@lorscarbonferrite6964]

It's because that apparently curved path is actually a straight line (or sort of one, the search term you want to look up is a geodesic) in 4d space. Imagine a 2d being walking around on a 3d curved object, like a sphere. If they plot their coordinates in a 2d grid and move around, they'll notice some really weird things about their movements. For instance, if they were to try to walk in an equiangular triangle by moving in a straight line for a fixed distance then turning 60 degrees (both measured according to their 2d grid) 3 times in a row, they won't end up where they started, as on a curved surface the angles of a triangle don't add up to 180. But to the being that only knows 2d space, there will appear to be something weird deflecting their path. Similarly, assume two of these beings standing at the equator of a sphere. They move in opposite directions along the equator at the same speed, and then, at the same time, both turn 90 degrees towards the north and starting moving north at the same speed. In flat 2d space, their lines are parallel, so they should never meet, and yet they both meet at the north pole. To them, it looks like something is dragging them towards the north pole.

[@markoszouganelis5755]

The best explanation for Gravity, so far! Thank you SO much Sabine!🦋

[@lovromedic2822]

The fact is that we are still not know what exatcly is gravity…

[@santyclause8034]

Newton was better at this stuff than I am.

[@markoszouganelis5755]

@@santyclause8034 Gravity is the love of matter (mother) to her children (us)!

[@jeremypearson9019]

The problem that people have with this is that they have a hard time accepting that there is positive net acceleration when there is no apparent movement. We're trained to think that if an object appears to be at rest, then all of the forces are balanced and there is no net acceleration. The key is to understand what Sabine is trying to explain is that gravity interacts in 4D SPACETIME, not just 3D space. In 3D space, gravity appears to be a force pulling massive objects together, but in the 4D spacetime equations the objects are simply at "rest" (no acceleration). In the 4D General Relativity equations, gravity never accelerates any object--they will always move at a constant "4D velocity" until they interact with an outside force. A rock that appears to be at rest on the 3D surface of the earth is actually accelerating in 4D spacetime. 🤯

[@onedaya_martian1238]

You lost me at " In the 4D General Relativity equations, gravity never accelerates any object--they will always move at a constant "4D velocity" ..... until they interact with an outside force." How do objects interact with an "outside force" ? The ball rolling around on a rubber sheet, "captured" by a mass sitting on the sheet is NOT interacting with an outside force but it is changing its relative velocity and is therefore being accelerated. Or is that the wrong way to understand this ?

[@jeremypearson9019]

@@onedaya_martian1238 A ball rolling on a rubber sheet is touching the rubber sheet. The atoms in the rubber and the atoms in the ball are repelling each other by the electromagnetic force. The ball travels in a circle because the sheet is pushing it that way. If the sheet (and the air) weren't there, then General Relativity would say that the ball would travel in a non-accelerating trajectory through spacetime, which is curved by the strong gravitational influence of the nearby Earth. To our perception, the ball would seem to accelerate because it increases its speed with respect to the dimension of altitude. But, in General Relativity, it's not accelerating when you analyze it in the spacetime equations. Space and spacetime are not the same thing. I had a hard time with this concept when I was younger. People would usually describe relativistic gravity by explaining that an object in orbit travels on a "straight line in curved spacetime". That kind of made sense to me, but what about if a metal ball were to fall straight down, starting at rest, from 1000 kilometers above the earth? That doesn't seem like a "constant spacetime curve". The ball starts at rest, then is accelerated to hundreds or even thousands of km/hour before it hits the atmosphere. Well, I had a breakthrough in understanding when I studied the General Relativity equations and realized that their definition of "non-accelerating" is in 4 dimensions. An object can accelerate in 3 spatial dimensions but be non-accelerating in the 4D spacetime equations. I get a little irritated when people use the rubber sheet analogy to explain Relativity. The only way to really understand it is in the 4D equations. Gravity doesn't curve space, it curves spacetime, which is a mathematical concept.

[@jeremypearson9019]

@@onedaya_martian1238 The rubber ball contacts the rubber sheet. The atoms in the rubber and the atoms in the ball repel each other with the electromagnetic force. The point I was trying to make is that General Relativity uses 4 dimensional math. People say that gravity "curves" space like a rubber sheet. It's much more than that. Gravity curves *spacetime* (there's a difference between spacetime and space). If gravity only curved 3D space, not 4D spacetime, then I think that it could explain how moving objects could orbit the planet, but I don't think that it would explain why stationary objects fall straight down. The fact that they are 4D equations enables gravity to actively morph 3D space over time. The altitude dimension of 3D space around a planet is constantly shrinking. According to General Relativity, a ball dropped from a tower doesn't fall because of gravitational "acceleration", it falls because the space underneath them is actively contracting. That morphing of space isn't considered to be acceleration. In spacetime coordinates, the object isn't moving. Once the ball contacts the ground, it does accelerate due to the contact force. After bouncing for a while, the ball settles on the ground. Gravity is contracting the space under the ball, but the earth is accelerating the ball upward. To us, the ball seems like it is at rest, but it's actually under constant upward acceleration that counteracts the shrinkage of the altitude dimension.

[@kylebelle246]

​@@jeremypearson9019Interesting. It kind of reminds me of a flat earth theory which states that gravity is just the earth moving/accelerating upwards at 9.8m/s/s. But what i wanted to actually ask was about the seemingly perceived acceleration of free fall, or rather in this case according to your explanation, the increase of rate contraction over time. Like it's possible I'm missing something really simple which explains it but i dont really see why it should be the case that we "accelerate" in free fall

[@jeremypearson9019]

The flat earthers seem to have borrowed the acceleration/gravitation equivalence to make their ideas seem more scientific. But Relativity actually matches with observation and is mathematically sound and the flat earth theory fails miserably. Your comment got me thinking: imagine you had two balls. You drop one of them from the top of the Tower of Pisa, then, when that ball reaches the middle of the tower, you drop the second one alongside it. The first ball will have a much higher velocity because it has already had time to accelerate, so it will speed past the second ball and strike the ground first. However, the two balls travel along the same path at the same time--straight down. If their motion is explained solely by the curvature of spacetime and not by acceleration due to gravity, then how could they move differently while occupying nearly the same space? Well, the short answer is that 1. the math is very complicated and 2. I don't actually have a sound enough understanding of this particular case to give a satisfactory explanation. It just goes to show that when we talk about Relativity in layman's terms, the analogies that we use don't adequately explain straight, vertical falling. It's been a long time since I studied it. The bottom line is that the 4D math is complicated and the analogies we use (like the ball rolling on the rubber sheet) don't really do it justice.

[@Thomas-gk42]

Fabulous explanation, you´re an extraordinary teacher. Peace and love for you.

[@SabineHossenfelder]

Thanks! Wish you happy holidays 🎄🎅

[@keithscott1957]

Yes, Sabine is quite a … um … force.

[@Thomas-gk42]

@@keithscott1957😉

[@VolodymyrLisivka]

She make me a lot of laugh this time..

[@danielstan2301]

Although a nice explanation I feel it is incomplete. For example it doesn't explain why gravity still accelerates mass while is not a force, what happens with the body once the whole earth suddenly disappears(will it continue moving towards where it was the center of the mass, stay still or will it go towards the direction where it was pushed by the force of the surface and why is that) and a few more questions that really makes gravity seem to behave like a force. On another note, can we consider gravity as a "force" that pushed against spacetime fabric causing its curvature? 😊

[@lrvogt1257]

One of the great things about the TV series "The Expanse" is how important acceleration, deceleration, and rotational simulated gravity are to the entire series. Spaceships are built like skyscrapers rather than ocean liners. They accelerate to keep everyone on the floor for half of a journey then flip the ship 180º around and decelerate for the second half so we see the rocket's engines firing towards the destination. Too rapid a change has obvious dire consequences. Spin gravity on larger ships usually provide 1/3 G. In one scenario people injured in a sudden deceleration had to get to the spin gravity ship so the simulated gravity would allow their wounds to heal. Very smart stuff.

[@sednahbellick2059]

This was an excellent video, as always! You explain difficult things so clearly in such a short time! Thank you! ❤

[@georgeholloway3981]

I think Sabine has either redefined what acceleration means, or she is explaining to us that the common use of the word "acceleration" is the wrong one. Either way, she should explain this directly at the start (or middle, or anywhere for that matter). She does not seem to do this, however.

[@JT-sv9bi]

Exactly. Thank you. It is arguably somewhat addressed near the end, but indeed one should lead with that.

[@rivergladesgardenrailroad8834]

Acceleration is relative, my friend.

[@woobilicious.]

Acceleration is a change in velocity, there's nothing different about how she explains it here. I'm not sure where your confusion is coming from, maybe it's because you're still assuming distances and time are constant (newtons model), but the reality is that the speed of light is the only constant, and acceleration is absolute, and distances and time are relative.

[@rukidding7588]

@@rivergladesgardenrailroad8834 I would love to have a cousin named Acceleration, so I could truly say Acceleration is relative.

[@matsogren7143]

In general relativity there are only local inertial systems, that is, inertial systems that are (approximately) valid in the vicinity of a point in space-time. An inertial system is by definition a coordinate system in which Newton's laws of motion holds. Thus, these are the coordinate systems that do not accelerate. In Newtonian gravitation, there are inertial systems that cover the whole universe. For example, this means that an object in free fall towards the earth will have an acceleration with respect to such an inertial system. However it will not have have an acceleration with respect to a local inertial system that follows the falling object, and that is was is dealt with in general relativity. In Newtonian gravitation, a freely falling system will experience a cancellation of the gravitational force by a so called fictitious force that arises because the system is accelerating with respect to a global inertial system. For example, a local inertial system could be attached to a space station orbiting the earth, since the gravitational force is cancelled by a centrifugal force. An observer in the space station that does not look out, will not be aware of either force, though, and will not detect any acceleration or any gravitational force from external bodies; a fundament of general relativity is that gravitation and acceleration are equivalent. In Einstein's general relativity, both the Newtonian gravitational forces and the fictitious forces can be thought of as being absorbed into the space-time geometry. Still, the claim that gravity is not a force is rather pointless if you ask me, since you cannot describe gravitational interaction using only local inertial systems, but chacun à son goût.

[@alikifahfneich]

Thank you for making such great videos about Important and debated Topics!

[@domrogg4362]

Merry Christmas Sabine! 🎄🎅🌌🌟

[@SabineHossenfelder]

Merry Christmas to you too!

[@bluesque9687]

This is brilliantly explained! Very lucid; however, for a layman like me this is mind shattering!! I can appreciate that you have done your best to make it clear but I am just so confused now!! I will have to rework my ideas in my head and find some answers!! Thanks!! I can't believe the ease of access to the privilege of these things being explained by a physicist of your caliber!! Love you, and love KZbin!! ❤

[@Markielee72]

I feel the same. I am beyond grateful to people like Sabine, who attempt to convey complex physics to the layperson. But videos like this just remind me how little I know. 🤯

[@VolodymyrLisivka]

I like how gravitational force is used to demonstrate that gravitational force is not a force because of geomethry of nothing. It's like 1 apple and 1 bannana: 1 = 1.

[@BooksRebound]

Just wait until you realize that the reason things fall is because your head is moving throught time slightly (like 0.00001 nanoseconds or something ridiculously small) faster than your feet, which basically takes your flat horizontal floating line and starts curving it downward (falling) to the ground. Time passes at different speeds depending on the curvature of space time, so that's further away from the planet move through time slightly faster.

[@antonystringfellow5152]

I find it helps to think of space and time as part of the same thing... spacetime. After all, that's how causality works (faster through space = slower through time and vice versa). When you take time into account, everything travels at the same speed, the speed of causality (cause and effect). From there, understand that time passes slower nearer a massive object, such as the Earth. Therefore, in order to maintain the same speed through spacetime, your path must be in the direction of the slower time... towards the object (or down). An object in orbit is not travelling a curve, it is travelling a straight path through spacetime. The difficulty comes from starting off with simple analogies that are very different from the reality. At the heart of space, time, speed and the gravitational effect is one single thing; causality. It is constant everywhere and for eveything.

[@ChristopherCurtis]

I'm not a physicist and I've seen too many videos to recommend one, but a moment that "clicked" for me was the realization that if you see someone throw a basketball and watch it curve up and back down into a net, you are not observing gravity, but are watching the ball travel in a straight line through a curvature in time (mostly in time; space itself is "flat"). For more related videos/channels, check out PBS Spacetime, especially "Does time cause gravity". Sabine has another video titled "You move through time at the speed of light". Science Asylum has "The REAL source of Gravity may surprise you". And then, to confuse everything, Fermilab has "Is gravity a force?". Have fun!

[@OrpheusSonOfCalliope]

@SabineHossenfelder I am struggling with several statements in the video. 13:00 " *If you're staying in place in a gravitational field, so you aren't falling, then you are not at zero acceleration as any accelerometer will tell you.* " Q1. Excuse me if I resort back to Newtonian definitions, but the accelerometer does not physically move when you are holding it and I agree that we are applying a force to it, so doesn't there have to be an opposite force downwards on the accelerometer? It doesn't go flying up in the air due to the acceleration from our hand. 02:30 " *Gravity is a consequence of moving in a space-time that's been curved by the presence of masses.* " You later state that we " *shouldn't ask why the big marble bends the rubber* ... " Q2. What is curving space if there is no force?

[@singh2702]

Gravity is a force, don't listen to KZbin videos, they are all wrong.

[@George3-3-3]

The accelerometer doesn't move relative to the earth it moves relative to spacetime, cus the spacetime is moving like it does at 8:30

[@peterromero284]

This was great, Sabine. Another thing that would be interesting to address would be, why does curved space time cause objects to move?

[@soyosunset]

Same problem. The bowling ball on the trampoline illustration is used to explain the reality behind our naive notions of how gravity works. But the illustration makes sense to this naive person only because it implicitly shows a world with an up and a down and a bowling ball that goes down, just like our naive ideas about gravity say it should. This seems circular and evasive. I am very willing to accept that there is no way of explaining physics to ordinary naive people such as me. You can't teach even Aristotelian physics to dogs or goldfish -- why should we imagine that all people can understand Einstein? If something can't be explained, that's the end of it -- a pretense of explanation accomplishes nothing.

[@Gingnose]

Because mass also causes the time of curvature not only space curvature. Every object in this universe is moving with 'a speed of light' as GR says and that makes the object move towards mass as if there's a force but this is just a visual illusion. Since we can only visualize 3D space, we cannot recognize the axis of time dimension. But it is still there although we can't see. The Earth causes the time curvature and time moves slowly as you get closer to the Earth. Since we're all moving in the time dimension with a 'speed of light', the delay of time which is closer to the Earth side causes you to move towards Earth. Space curvature works likewise but it is only relevant when the two objects have the motion vector that is different from the axis between the two objects (if two objects aren't just free falling to each other but moving to other direction as well).

[@ValeriePallaoro]

it doesn't. I think is the answer. f=ma is the math to explain the movement, gravity is the explanation for how they move.

[@douginorlando6260]

Maybe the only thing moving is space, not the object …🤯

[@calinculianu]

Something something rotates you with respect to time but not space or something. The Science Asylum guy did a good video on this.

[@hu5116]

Great video Sabine! Two comments. First, I’m with you on the whole gravity is not a force. BUT, then there are really only 3 fundamental “forces” (interactions if that is the preferred term), and then there is no need to quantize gravity, because gravity is not a force. This would explain also why it has been so hard to do. Second comment, it would be very good to get your take on the time causes gravity (or visa versa) discussion in many KZbin videos. There have been counter videos on this as well, which is why I think you weighing in would be a great arbiter. Thanks!

[@S.L.S-407]

@hu5116-Sabine already did a video on does time cause gravity.

[@dhruvvikrant]

Does time cause gravity? Need a video on this pls!😅

[@hu5116]

@@S.L.S-407ok thanks! I guess missed that one so need to track it down.

[@Miggy19779]

Floatheadphysics channel has a video to help visualise this rather neatly, he uses paper cutouts to show how it's the bending of time that causes gravity

[@matseriksson8177]

6:40 Excuse me for not being Einstein, but Einstein says: "If you're in the box, you can't tell whether you're being accelerated, or whether you are sitting still on the surface of a planet." I say: "YES, WE CAN! Consider these two measurements:" Measurement 1. Get two accellerometers. Place one at the floor of the box. Put the other one at the ceiling of the box. If you are accelerated by a rocket the two accelerometers show the same value. If you are sitting on the surface of a planet the accelerometer in the ceiling shows a lower value than the one at the floor, since it is further away from the planet. Measurement 2. Get two accellerometers. Place both at different locations on the floor of the box. Check the directions of the accelerations measured. If you are accelerated by a rocket the two directions are parallell. If you are sitting on the surface of a planet the two directions are not parallell, but directed away from the center of the planet.

[@TheLevano22]

you would have to then think about the accuracy and the difference in said measurement which you are seeking to find. for your experiment the difference should be significantly large, if not not all accellerometers would work.

[@matseriksson8177]

@@TheLevano22 Many things don't always do what you expect from them, relatively speaking. :) kzbin.info/www/bejne/iHfZpaKAp7BkkMk

[@matseriksson8177]

@@TheLevano22 Accuracy is relative.

[@scottbreseke716]

Is Schrodinger's cat in that same box?

[@matseriksson8177]

@@scottbreseke716 Let me check... Oh... Turned out it just died when I opened the box. :( But fortunately it had all of its nine lives left. (Eight now. :) So I guess that means it's both dead and alive, again, or something.

[@billmcdonald180]

So... If someone 'falls' off a cliff and reaches the bottom, they don't die of "sudden deceleration syndrome" but rather die of "sudden re-acceleration syndrome"?

[@asdfoifhvjbkaos]

acceleration and deceleration are physically the same thing

[@DukeHastMich]

Sudden unequal acceleration.

[@dingusdingus2152]

The coyote doesn't die because he's a cartoon character 😅

[@StuHol-jb1hh]

more like sudden re-synchronization with the earth

[@mrzoinky5999]

@@StuHol-jb1hh Yes yes , you are experiencing a new frame of reference .... or something like that ... where's that Vodka?

[@geralddejong]

This is one of those shows about things I already know, but after watching I understand more than before. Thanks again, Sabine (and hidden team!).

[@__christopher__]

Gravitational force is a force in the same way that centrifugal force and Coriolis force are forces. All three appear because you are describing movement in an accelerated frame of reference.

[@jacksons1010]

We need to understand the word "force" has different meanings in context. For practical engineering, gravity is considered a force. The English language is full of such words with multiple definitions.

[@__christopher__]

@@viktorm3840 Yes, the (usually unspoken) assumption is that the box is small enough that all tidal forces/effects of curvature are too small to be measured. Otherwise you can just let two objects fall side by side and notice that they don't fall exactly parallel. And when falling into a black hole, the eventual spaghettification will quite violently tell you that you are not just floating in free, flat space.

[@DrDeuteron]

The reverse is true too. I.e., the Coriolis force is in fact gravitation.

[@DrDeuteron]

@@viktorm3840the vomit comet chooses a frame with no gravity. And no one cares about tidal forces, which is why the word local is used.

[@DrDeuteron]

@@__christopher__ or you could but an earth mass black hole in your elevator and then even your though experiment doesn't work.

[@alexneigh7089]

4:57 Unless the spring itself has zero mass. It would be a clearer illustration if a weight is attached to the other end of the spring, and the spring's mass is assumed to be zero.

[@HughCStevenson1]

And for a non-massless spring the extension proportion (strain) of the spring is not uniform. The end that is attached extends more and the free end extends 0, proportionally, right at the end, because there is no mass attached to the end. It seems to be shown as uniform in the animation...

[@andrewm9425]

Why would you assume a massless spring for this discussion?

[@alexneigh7089]

@@HughCStevenson1 Yes. In effect, it measures (the manifestation of) weight, and it would be more intuitively clear if weight is attached to the spring rather than the non-uniformly distributed weight of the spring is measured with an additional complication of changing distribution when the spring extends/contracts. In simple terms, when you use scales, you do not determine the weight of the scales, but the weight of the item whose weight you try to determine.

[@viewpointzero1420]

Quite possibly. As it is, and I imagine a real spring tied to a real rocket, my intuition is that the spring would maybe extend a small bit, but on the whole the forces holding spring's atoms and molecules together would ensure that the spring simply goes after the rocket as a whole and that's it. 🤷‍♂️

[@GeorgeGeorge-k8j]

I think is like when you trow a big heavy ball in a pool of water, as it sinks it creates an empty space behind it ,so if you are in that space you practically fall in after the ball!! So if the planet would not rotate around the sun and would be stationary, this "gravitation" would not be possible !! I'm I right or wrong?

[@LuvHrtZ]

This concept is one that I still can't get my head around. As always, love your stuff, Sabine.

[@vibaj16]

Everything moves in a straight line when under no force. Since gravity is not a force, the Earth is under no force. So why does it orbit the Sun? That's not a straight line, right? Actually, it is. The sun's mass warps spacetime's geometry such that a straight line gets bent around the sun. Geometry itself is warped.

[@rockovahsacralonte570]

I can't get my head around 1+1=3, mainly because it's not true!

[@fewwiggle]

@@vibaj16 OK, but why does the floor push on me?

[@D1N02]

Think of it as deceleration instead of acceleration and the quarter will fall (decelerate :p)

[@D1N02]

@@fewwiggleit doesn't you push on it because you want to free fall the the center of the earth, but the floor is in your way. Your atoms do not want to be in the same spot as the floor atoms, so you are stuck in the cosmic water slide because a fat kid called "the floor" is blocking it.

[@phenanrithe]

In the video, the question about a = dv/dt is quickly discarded "because it's another referential", which doesn't help if you don't know about general relativity. The spatial position of a free-falling object doesn't change in freefall because it's not simply dv/dt = a in space. There's an additional term cancelling out the acceleration upward, which comes from spacetime distortion. That's what explains that Earth's surface is accelerating upward without Earth expanding. The geodesic equation shows that d²z/dt²=a - Γ (dz/dt)². If a, which is F/m, equals the gamma term, the position remains constant: the ground pushes the object upward but spacetime distortion compensates it. Anyway, it's only one theory, so saying gravity's not a force is only true in that theory. Don't try to give it any meaning.

[@kylelochlann5053]

Gravity is not a force by direct measurement, and has nothing necessarily to do with relativity.

[@Mobri]

I watched this 3 times. Absolutely fascinating! Thank you so much for explaining this!

[@klauswassermann8054]

This seems profound. Still wrapping my head around it. Great way to launch the New Year. Heartfelt thanks Sabine, brilliant food for thought as always :)

[@ivoryas1696]

klauswassermann8054 It is, and back in 1915 it was a _such_ a big deal for a reason. 🧠

[@RobertStCyr-zh1tw]

Is gravity a force? Now my answer will depend on why you want to know. Lol.

[@lfelype.azevedo]

Thanks for the awesome video about the matter (or the space-time curvature in this case). As much as we study it, having a graphical and very well done explanations is good to cement the ideas, and this one was a blast to watch.

[@undercoveragent9889]

If 'mass' does not exert a force on spacetime then why should spacetime experience any warping?

[@randelbrooks]

Wonderfully spoken and difficult to comprehend. Merry Christmas Sabina and everyone else!

[@bvrulez]

Liebe Sabine, ich bin so begeistert von deinem Content, deinem Humor und deiner Kritik! Und dass es von einer Deutschen kommt macht mich (irrational natürlich)unglaublich stolz! Wir haben so eine schöne Tradition überragender Mathematiker und Physiker, es ist eine Freude zu sehen, dass es mit Dir weiter geht! Ich genieße deine langen Videos sehr (auch wenn der Zeitgeist es kurz mag). Vielen Dank für deine Arbeit, deinen Mut, und viel Erfolg weiterhin!

[@MrClivesinger]

Nice video! I teach this material to undergraduate students, and must admit i sometimes use the wrong terminology in the moment. I would say that I think your black hole animation may confuse some viewers who have heard of spaghettification - though i understand why you might have wanted to avoid the complication that two ends of the same object can be travelling along diverging spacetime paths and get stretched. Perhaps that's another video for another time!

[@oddarneroll]

Good point, idk if i understand your point about spagettification, maby you can help me underastand this betterr? What about equliliance principle in a box resting on a massive small object, in my head you chould measure a change in direction of accelleration across your box? And what about the theorised graviton particle, can you have a graviton particle but no force? All suggest gravity being a force, no? If quantum gravity theorems look at gravity as a force, is not quantum theroy really more fundamental (and therefore, closser to truth) than general relativity? If gravity is a force in quantum gravity, is it correct to say we know gravity to not be a force? im confused.

[@KaiVieira-jj7di]

@@oddarneroll Gravity is not force by direct measurement and measurements of other parameters that determine if the gravitational field cannot/(can) exert a force (the Local Lorentz violating parameter, β, for example). To date all measurement confirms that absence of a gravitational force, and this is independent of any theory. The graviton further demonstrates that the gravitational field cannot produce a force as the massless spin-2 field we associate with it reproduces the Einstein-Hilbert action and the Einstein field equations in the appropriate limits. What the virtual graviton field does (assuming it exists for the moment) is communicate the curvature, the curvature over which particles would travel along their geodesic paths.

[@raymondlines5404]

What book do you use for undergraduates? And does it go into simple dynamics examples like Sabine did in this video?

[@MrClivesinger]

@@raymondlines5404 It's quite a surface level introduction in a UK University 2nd year module called "Fundamental Forces" (and yes I do stress the irony regarding gravity not being a force!), where the majority of the year long module is spent on electromagnetism and the nuclear fundamental interactions. On the quick intro to gravity I go through a number of thought experiments involving a little doll of Einstein in a little elevator flying through space. My favourite part is explaining gravitational redshift :)

[@darkpheonix77]

What is the definition of "force" then? Its never stated in the video. I was actually thinking about spaghettification, or conversely an object long enough to mesure the difference in "gravitational interaction" on each end. Would that not show an acceleration?

[@PeterMoore-q5k]

On the question of whether you're "accelerating" while in free fall vs. resting on the Earth's surface- Too many people say "space" is curved by gravity. That's wrong. If it were just space being curved it wouldn't take any more energy to move away from a gravitational field than to move into it - any more so than it requires more energy to move north on the earth than to move south. Nor would there be gravitational time dilation. Spacetime is what's curved by gravity in the GR model. The time part of that is what makes the model work. It therefore doesn't make sense to directly compare GR's four dimensional "spacetime" model of motion with Newtonian mechanics' 3D model where time is absolute and acceleration is *defined* as the second derivative of distance with respect to absolute time. In GR, the Newtonian definition of acceleration doesn't even make sense because the absolute magnitude of any object's 4D velocity vector is a constant (spoiler alert - it's always c); only the direction can change, which is of course not a constraint of 3D velocity vectors in classical mechanics. So any statement that you "are" or "are not" accelerating in GR has to be heavily qualified as to whether you're talking about a 4D velocity vector or a 3D classical velocity. When you are being acted upon by no non-gravitational influences, it is true that your 4D velocity vector doesn't change as you follow a 4D geodesic - because that vector is *defined* relative to a 4D geodesic! If it makes you happy, you can say you are not "accelerating in 4 dimensions." When you are being acted upon by a non-gravitational influence, on the other hand, your 4D velocity components DO change relative to a geodesic, for as long as that influence is acting on you. If it makes you happy, you can say that you are "accelerating in 4 dimensions". When you're standing on Earth's surface, the electromagnetic repulsion from the surface is pushing you away from the 4D geodesic you would otherwise be following, and therefore, if it makes you happy, you can likewise say you are "accelerating in 4 dimensions". But if you drop the "in 4 dimensions" part, then you're mixing apples and oranges - taking a statement that's true for a particular model and applying it to concepts from the prior model, which have no applicability in the new model, as if they prove the prior model wrong. The ugly truth is that all models are wrong, especially when it comes to spacetime. Some just make better predictions than others. No one has any clue what space or time even are. And the fact that GR doesn't work at the quantum level, and vice versa, ought to make us even more humble about making sweeping claims such as "gravity is not a force." The most common sin physicists commit in my opinion is confusing models for reality. This video, I think, is such an example.

[@OldZoZo]

"The most common sin physicists commit in my opinion is confusing models for reality." is probably the truest statement to be said about modern science.

[@1st2nd2]

You appear to know what you are talking about. I have a question: Suppose I am in an indestructible vessel that has made it to the center of my local gravity well (planet, star, etc.). Then I am being acted on by the pressure of the atoms/matter around me, but also following my (local) geodesic. I assume the pressure/forces of the matter (at least roughly) evens out and I would just continue on the geodesic at the center of the local gravity well. Let's go a bit more realistic from spherical cows. The Earth is not uniformly created (gravity varies on the surface as mentioned in the clip). Would I get pulled around and forced to rotate also? Additionally, how does the Earth's rotation fit into GR? Does each molecule/quanta of matter have to follow it's own geodesic "spiral" until acted on by the matter below, forcing it into a "circle?"

[@mydogbrian4814]

- I got more out of your explanation then *Sabine H's* unfortunately confusing gobbledybook explination. Which I have heard many times before. - Although your explination was also somewhat confusing and lengthy. The fault is not yours. - I myself found it difficult to formulate into words novel new mental constructs involving a hidden 4th spatial D into the mix, trying to hammer them into our familiar [3-D+1] geometric visual understanding of reallity. - However you mentioned 2 key points that caused me to have Eureka moments in visualization simplicity. - And being a firm believer in the *KISS* directive since my college days introduction to it. I am grateful to you for sharing your incite on the subject of gravity. { *KISS* = "Keep it simple, stupid!" }

[@rakhatthenut3815]

Tldr

[@santyclause8034]

Space-Time is Entangled: Diffusion of condensate matter transits state phases and particle density seperates out mass as the dispersal volume increases. There's more than one thing going on, for sure. *Edit* I would go so far as to say there is some Simultaneity here.

[@alexandergofen1771]

In my view, Dr. Hossenfelder here confused herself and the audience. The confusion begins with the vague term "accelerometer". Here is why this term is vague. 1) In practice, the device called "accelerometer" is based on springs and a mass, whose position is measured. This device in fact is akin to a spring scale. Springs and spring scales measure the force acting on the probe. 2) When such an accelerometer rests on the surface of earth, it measures the weight of the probe, i.e. the FORCE applied to the probe by the gravity of Earth. This probe DOES NOT experience g-acceleration, because its velocity does not change at all (except the effect of spinning of Earth). And vice versa: when such a spring accelerometer is installed into a satellite in space moving along say an ellipse, the vector of velocity will dramatically change, and acceleration will change respectively, but the spring accelerometers would show zero because it will be in a state or weightlessness. On the contrary, the true accelerometer measures the differences in velocity of the probe relative to "stars", and it would detect the accelerations of this probe moving by the Kepler law. Sure enough, the forces caused by acceleration are mutually canceled by the force of gravitation - that's why the state of weightlessness is reported by devices measuring weight. 3) The equivalence principle is the axiom stating that gravitational mass is proportional to the inertial mass. That is why gravitation may be viewed either as a force, or as a curvature of the space/time.

[@kurtn4819]

I missed 2 out of 12. I LOVE the quiz after the “lecture” because I often wonder how much I retained and this is a good way to gauge that. Thanks Sabine. Only one suggestion: We don’t know which ones we got wrong, or am I missing something?

[@kabongpope]

General Relativity was such a breakthrough, it's quite amazing after all these years

[@pholdway5801]

General Relativity is much more fun than Corporal Punishment

[@kabongpope]

@@pholdway5801 both can lead to Major Issues!

[@petergroves3153]

@@pholdway5801 Chacun à son goût.

[@silvergreylion]

None of the theories of relativity define an absolute rest frame. WIthout that, how do you apply the light speed limit to any inertial frame?

[@vonmagiernemcunddemkosmos2290]

Physicists still don't understand the difference between space and surface. A spherical surface is curved, but the space inside is not, nor is the space around it. Gravity and electrical charges are determined using the Heaviside torsion balance. The torsional force of a wire is used. Coulomb determined this power. Consequently, gravity and electric charge have the same origin in atoms.

[@njhoepner]

I just learned something I knew and didn't know at the same time. I knew that gravity is curvature of space-time and thus not a force. I hadn't realized the part about acceleration, even though I'd read Einstein's elevator thought experiment. Thanks Sabine for putting it all together for me!

[@scottmcshannon6821]

gravity is not a force, does not accelerate you but when you are on a planet you are accelerated. gravity is not a force, but you are always accelerated when in a gravity field.

[@aldobrezenti]

​@@scottmcshannon6821 When you are in a freefall within a gravitational field, you are not accelerating.

[@ivilivo]

I understand less now, even if my previous understanding was wrong.

[@ivilivo]

@davidmudry5622 Thanks, nice video you linked to. .. Guess I have to figure out what space time is. As that must be what an immoving object is constantly accelerating trough. (constant acceleration. Something somewhere must be going faster and faster) Edit: part of vid you linked: The 2 person's walking parallel on earth getting closer and closer to each other. Was beutifully easy to grasp. So somehow spacetime is similarly curved. But that doesn't explain acceleration.

[@HelloThere.....]

@@aldobrezenti Yes you are, you are accelerating relative to other reference frames. This is why time dilation happens.

[@BarryPiper]

I have to say that gravity is a force because I teach high school physics and not university-level relativistic physics. The same reason I tell middle schoolers that there are three phases of matter and that electrons orbit the nucleus in a nice, neat circle. We can't jump right into relativistic physics on day 1, so we have to use the best working equivalent that students might have a chance of wrapping their brains around.

[@Feroand]

But, this approach mess up with the minds of your students. İt's impossible to "unlearn" something unless you lose your memory. There should be a better approach.

[@BarryPiper]

@@Feroand No need to "unlearn" anything. Just add to current understanding. We teach three states of matter in early science education because children don't have the capacity to understand plasma and theoretical states of matter. We teach Newtonian physics because kids don't have the capacity or math education to learn relativistic physics. The omissions and "corrections" can come later, when students have the capacity to understand them.

[@xinpingdonohoe3978]

But the later you teach simplifications, the harder it is to undo. You teach that you can't subtract past 0, or divide a number by something bigger than it, then these are early concepts and get replaced easily. You teach that you can't take the square root of a negative number, and most people still think it's true because it comes later. The first definition of a function has "exactly one output", and this one is taught late and very hard to override when it comes to multivalued functions.

[@kazukawasaki97]

​@@Feroandeven Einstein was taught this,yet he did smth else

[@kazukawasaki97]

​@@BarryPiperIm at 9:29 ,theres something I would like to disagree,this is a cherry pick logical fallacy, Einstein specifically conducted a thought experiment that mimmiked the experience of standing of earth, let me add something else, lets attach tiny rockets on every unit area of our body and the spring and accelerete them the same as the big rocket accelerating the box, now what? isn't this mimicking free fall,maybe his equations work because his form of mathematics is more concrete but people using the equivalence principle to just tell that moving under gravity is not a consequence of a definite "force",mayne Einstein used this concept as an eye opener but not this exact thing to frame his everything

[@dougdupont6134]

As a programmer making a hard sci fi game and not a physicist, it's a little scary trying to advance a theory of gravity without knowing what I'm talking about. A character in the game says that if you only perceived in 2D but approached a 3D hill, you would experience it's effects as a mysterious pull (or push as the case may be). I was especially concerned that I was only moving the goal posts on this one. Nice to see I might not be so far off. Thanks for the great explanation!

[@juliavixen176]

Well in the case of a two (space-like) dimensional manifold with intrinsic curvature... or extrinsic curvature as a hill in a three dimensional embedding space (with no time-like coordinate) What happens to two 2D creatures walking in straight parallel lines a constant distance apart from each other, when they encounter the hill, is that even as they continue to walk straight, the distance between them will change. The 2D creatures might interpret this as a mysterious force that is moving them either closer or further away from each other... but there is no force... they are not actually accelerating... they are still on straight line inertial paths and feel no force... but the distance between them is changing because the space between them is curved. This is General Relativity... it's just like this except in a 4D Spacetime (so the time interval between events can also stretch and shrink, and it will look like things are mysteriously changing velocity without accelerating, but it's actually just spacetime curving).

[@dougdupont6134]

@@juliavixen176 Yeah, I'm a programmer and writer of fiction trained academically as a philosopher, so I want to write stories and craft games with a meaningful and accurate portrayal of science on characters that are digestible to regular people. My limited understanding of physics can be frustrating in that endeavor, especially since I know enough to know that I don't know anything (as Plato would say). It seems like what you wrote essentially confirms that my example might be a meaningful and accurate portrayal. I appreciate you taking the time to explain it better than I can. I hope you don't mind that I might borrow some of it.

[@MrGemaxos]

@@dougdupont6134 Dont be frustrated, if you go down the rabbithole its like a Hydra. Every answer makes a few new questions and in the end you are rarely understanding, but you are still just realizing that there is more and more that you dont understand. (youd still be in platos place) In my Opinion its a good thing, it leaves more room for the fiction :3 if not, wouldnt it be just science? I have read so many good books with physic that dont work out. But without the "wrong" physics you couldnt tell the story. Jules Verne for example. With correct physics as Dogma most of his storys dont work out and you would have a very hard time to find a possibility to tell a similar story.

[@eVill420]

That's really cool! So it can be imagined as falling into a Whirlpool and streching like spaghetti

[@voltydequa845]

@@juliavixen176 I liked you other comment (though I do not remember what it was about). you say «when they encounter the hill, is that even as they continue to walk straight,» It is a hill for you, looking from outside, from a superior dimension, their "walk straight" from from 3d pov is not "walk straight" from their 2d pov. Their "walk straight" would put them to walk with constant distance between them, but could present some other "irregularities", like the impossibility to maintain the same distance while walking at the same speed. I usually use the example of 2d to try to show that there's no way 2d's can imagine seeing them from a 3d, or that they should be that conformist to buy into an abstruse 3d model if they already have some another explanation that is simpler. The main point being "Man is a measure of all things". What "exists" is the representation of the "reality". While the abstruse and overcomplicated curvature of the "reality" should be left to parrots.

[@plumtree1846]

At 2:00, could you expand on what you mean by 'pressure'? Is the curve of space/time causing atoms to accumulate in one place. The atoms get close to each other to the point they push off of each other thus causing 'pressure'?

[@harmless6813]

The pressure is due to the forces holding atoms and molecules together.

[@awesomedavid2012]

Yes. The atoms inherently follow this path which brings them together. Then, the electromagnetic force takes over and electrons push eachother apart by exchanging photons. This creates that pressure. On a continued note, if you have a dying star that is massive enough, this pressure creates by electrons can break down until the electrons collapse into the protons and produce neutrons. This is how you get a neutron star. Then, the strong nuclear force keeps the neutrons apart by exchanging particles. If that is still overcome, then the neutrons collapse a black hole

[@AH-jt6wc]

you comparison between newtons law and how we applied it up to now and general relativity point of view is amazing. First time I understand this difference and I have seen many videos on that...

[@peterturner6497]

Yeah she certainly proved beyond doubt that Einstein was indeed a fraud and his "theory" is a worthless hunk of junk didn't she? Garbage is garbage no matter whey you try to spin it.

[@zappababe8577]

Dr Rohin Francis demonstrates a very good point here - it would be extremely difficult to administer CPR to a patient in zero-G. Best not to take any risks whilst you're in zero-G, like doing flips or somersaults...oh, dear...

[@edwardlulofs444]

Yes, exactly. But after 300 years of Newton saying gravity is a force, and only 100 years of a deeper understanding from Einstein, it’s still difficult to understand and believe. But I know it’s true. This might be the best video you have made this year.

[@cherubin7th]

Also in school you learn Newton's gravity, not General Relativity.

[@DanielCheng]

I don't know, but it's Einstein redefining things without giving it a new name..

[@brothermine2292]

Also, just because acceleration can be measured (with an accelerometer), that doesn't imply it can't be deduced by observing its (relative) velocity and applying Newton's equation for acceleration ( a = dv/dt ) as we were all taught to do in high school.

[@pootthatbak2578]

Sabine shut me up..i cant seem to absorb any of this lesson

[@edwardlulofs444]

@@DanielCheng yes that does happen. Sometimes I wish life was more simple. But I want the truth.

[@michaelyaziji]

I find it most intuitive to START by thinking of a leaf in a stream. That is like us in a gravitational field. :) The stream (gravitational field), will have us float effortlessly downward. If we get stuck against the rock in the stream the rock will impede our “natural” flow, and push against us. Then, we’ll feel like we’re accelerating against the water flowing across us. If this analogy is helpful for anyone, give me a thumbs up, please. :)

[@forsakenquery]

The issue with this analogy is the issue with sabines video. It's just wrong to say you don't accelerate in free-fall. You keep accelerating (unlike the leaf, which will reach the speed of the water and stop getting faster) - as in, your relative velocity to objects resisting gravity continues to increase. To say you aren't accelerating at G is to redefine the terminology - the unit of measure of G is it itself metres per second per second - acceleration. It may be the accelerometer is seeing past the curtain of apparent acceleration but it requires us to redefine our terms so as to make them meaningless.

[@michaelyaziji]

@@forsakenquery Thanks for your note. I think what you are describing is exactly what the Einsteinian revolution is about; it does involve redefining terms. :)

[@forsakenquery]

@@michaelyaziji but...that's not a revolution. That's just semantics. Einstein offered a different view of reality. Either it's a flawed view, or these science communicators (of which Sabine is usually one of the better ones) are failing. Because you can't say "you aren't accelerating" while ignoring the acceleration we observe without explaining what you mean. Acceleration means "change in relative velocity with respect to time". It doesn't mean anything else. The idea that it is absolute while velocity is relative is circular nonsense.

[@soopergoof232]

>> "It's just wrong to say you don't accelerate in free-fall." If gravity is the accelerating flow *of "space" itself*, you are weightless in freefall, simply "going with the fllow".

[@forsakenquery]

@@soopergoof232 I'm not making an argument about weight. I'm saying your relative velocity changes.

[@randallnewcomb]

Our household always looks forward to the bits of humor that you embed in each video. Knowledge AND entertainment - a great combination.

[@TerribleShmeltingAccident]

Agree, she delivers with such eloquence too!

[@robertkiss5461]

I like your sense of humor. Really. On the other hand, I like the Newtonian explanation better.

[@scilencium7178]

Sabine, everyone is repeating that there is no way to distinguish free fall in a gravitational field from absence of any gravitational field. But recently I found some papers about the velocity dependence of the free fall "acceleration" (name it as you like, you know what I mean). This velocity dependence would allow experiments with high speed particles in a closed cabin which would definitely enable the observer to distinguish these two situations ...

[@harmless6813]

Name the paper(s).

[@__christopher__]

You mean like the Coriolis force is velocity dependent? So how would the experimenter determine that it's not just that the cabin is rotating?

[@mikenewey3949]

If two bodies are falling in a gravitational field won't the distance between them reduce as they get closer to the centre of the earth? That wouldn't be the same as an absence of gravitational force where the distance would remain constant.

[@AlmaBuru]

>distance between them reduce No, it won't.

[@__christopher__]

@@mikenewey3949 Yes, it would. But if the box is small enough, and the time of measurement is small enough, the relative movement will be too small to be measured.

[@noobsauce]

Thanks for drilling in the phrase "because gravity is not a force", it really does beg for repetition haha. I love this topic. I originally came across it while watching a simulation of the universe expanding, through the perspective of what our solar system looks like as its moving away from the center of the galaxy. Coincidentally, the planets' orbits in tandem with the whole system moving across space time simultaneously follow the shape of a 4D spring. Thought that was a fun little fractal coincidence when you used a spring as a measuring unit for acceleration.

[@silvercloud1641]

It's more like water then? A calm still ocean until something acts on it, then it can become something that has an effect on other things like a wave?

[@jamesconger8509]

Although this explanation is perfectly valid, I always have trouble visualizing how all of us standing on different points of a sphere feel a similar acceleration in different directions.

[@briancrowther3272]

From my understanding since retiring from teaching physics high school pre uni level, is this. Read a great book, why e equals m c squared by brian cox and a mate, I think that is where I got this. IN general relatively, the clocks run slower (ill need to check, faster or slower the point is the same though) as you move further away from a mass. With big ones like Earth. That gives the illusion of acceration as the relative velocity at the mass is different than away from it. So this is a bit like say travelling from the equator to the north pole (in my case south as I am in Australia), but you want to go in a straight line (no warped sapce time due to no mass), but space time is curved due to the mass and that bends the line to the pole, as on a globe. The effect is you are made to conintually change direction (or it feels like it) as your path is being constantly corrected or resisted by the curved line between the pole and the equator. This gives the effect of acceleration but is not a force, it is like centripetal or centrifugal force by analogy. This makes the clocks run at different speeds at different disatnces from the mass. SO cool. I really want to re read all that but cant find the book in a local bookstore ie Sydney CBD rats. Ill need to try harder, it has disappeared from my library.

[@heisag]

@@briancrowther3272 I guess one can use a clock inside "Einsteins elevator" to tell wether one is beeing at rest on a planetary surface or accelrated in space then? If it ticks differently when at the bottom compared to ceiling, one is at rest on a planet, and if there is no difference, one is beeing accelrated somewhere in space?

[@gcewing]

@@heisag You're right, if you can detect a non-uniformity in the acceleration then you must be in a gravitational field. This doesn't contradict the equivalence principle, because the equivalence principle is only talking about a uniform gravitational field. BTW, you don't need a clock to detect non-uniformity, just make measurements with the accelerometer in different places.

[@JerehmiaBoaz]

You could try imagining a rocket hovering 1 meter above earth's surface. It's rather obvious that the rocket needs 1g of thrust to keep hovering above the surface and avoid falling down. If the rocket needs 1g of acceleration to maintain the same distance to earth's surface then earth's surface must be experiencing exactly the same acceleration as the rocket does.

[@JT-sv9bi]

@@JerehmiaBoaz Okay, except the rocket isn't accelerating. It's hovering, like you said. Now I get that's what this entire video is about, but if we're going to change the definition of something so fundamental, we should really lead with that, and replace the old meaning with something else. If the earth was accelerating upwards in all directions in a Newtonian sense, it would constantly expand. That's why people, myself included, still don't understand this. Pressure -> force -> acceleration, sure, if there is movement. If I press my hand on the wall, I'm exerting a force, but nothing is moving. No kinetic energy, no motion, no acceleration.

[@digitalife8719]

It's meaningless to keep saying "it's not a force" without defining what is a force.

[@DukeHastMich]

A force is a coherent field of energy that directly contributes to an induced charge that induces an effect, whereas gravitation is an emergent property of the interaction of other forces and structures.

[@hi12235]

@@DukeHastMich??? Dk if this is just jargon I’ve never heard before or bs

[@harrykirk7415]

I used to say that gravity was a force, but that was back before I started describing everything in terms of curved space time coordinates. Before when I did something like building a wooden shed at my job I would say crazy stuff like " this shed must be built strong to resist the force of gravity acting on the building materials and potential occupants". It was so confusing!!! Now I just layout the whole building in curved space time coordinates, and all the confusion just disappears!!! All the workers on the job site can clearly see that the building is accelerating upwards and there are no gravitational forces at all. This is fantastic!!!! Thanks, Einstein and Sabine!!!

[@every1665]

If the shed collapses soon after being finished, most builders will gently point out that you accelerated it upwards too fast. Nothing to do with inferior materials or construction methods.

[@mikegale9757]

@@every1665 Not sure if that argument would stand up in court. Engineers are supposed to anticipate the unexpected and build in some safety margins. Nobody expects the Spanish Inquisition of course, but come on. Look at my shed. It's in ruins!

[@mikegale9757]

Quite. There's no force pushing you down, but that doesn't mean you're not going to fall when you remove the force which is pushing you up. It's not entirely incorrect to refer to the latter as the force of gravity. It's just semantics. The force due to gravity would be more accurate.

[@Krokodil986]

​@@every1665 that's what I tried telling them 😢 Just like that time when they accused me of punching that kid. Little do they know, atoms never touch So no I didn't punch him

[@Krokodil986]

​@@mikegale9757 it would be as correct as saying that the centrifugal effect is a force, which can simplify things a lot in certain cases

[@simba995]

Sabine…one of a kind. Another great tutorial.

[@Mcfreddo]

Another bloody good one, I have to say! Thank you.

[@Bob4golf1]

I've had a lot of exposure to Einstein's work but this particular one violates my physical experience and teachings. At 73 I've had a lot of experience with being in touch with mother earth and this view requires a significant adjustment to ones thinking. Thanks for this interesting lesson.

[@junaidsajid8867]

Another highly inspired video. Thank you for teaching us how to think scientifically :) peace and love

[@kylelochlann5053]

The gravitational field defines the paths of zero force. Free falling in gravity is following the zero-force path (geodesic).

[@DrDeuteron]

It’s not that simple. Say you fire some artillery. Gravity, duh, but where to you put the coriolis force?

[@MADHIKER777]

May the Force be with you, Sabine! Fantastic video. I love your humor!

[@ronaldderooij1774]

I don't, but then again, I am not here for humor.

[@AndrewBlucher]

Humors were a myth. Humour, on the other hand, is what keeps me alive.

[@peterdamen2161]

@@AndrewBlucher Humor is US English while humour is UK English.

[@Gunni1972]

@@AndrewBlucher You mean Rumours. 🤣

[@AndrewBlucher]

@@peterdamen2161 No. In the Middle Ages it was believed that a person's health and disposition were the result of a balance or imbalance of four fluids in the body. These fluids were called "humors," from the Latin word humor ...

[@Martin-zj6qu]

So the universe brought me to Sabine today or perhaps it always pre-calculated/pre-determined this many and many galactical years ago. I am just so glad I found her, please, keep up the good work!

[@sRoGoRs]

I've been watching this channel for years. This is the best explanation of gravity ever. One of the most insightful videos I've watched. I'm not taking the quiz though 😊

[@paweljakubas7186]

Hi Sabine! Any chance you could make a video with book recommendations for beginners, medium and advanced physicist enthusiasts, amateur that want to follow your content with some fundamentals understood, that you found particularly worthy and didactic? Thanks in advance and best regards!

[@laestrella9727]

Seconded

[@kilroy1964]

Alternately maybe a spring is only a good accelerometer for external forces, but not for forces that act equally on all the particles of a body being accelerated.

[@slthbob]

it doesnt act equally friend... Spaghettification In astrophysics, spaghettification is the vertical stretching and horizontal compression of objects into long thin shapes in a very strong, non-homogeneous gravitational field. It is caused by extreme tidal forces. Wikipedia

[@kilroy1964]

Yes, my friend I'm familiar with spaghettification. It's a result of extreme tidal forces. And you're right, I was ignoring tidal forces. Guess who else was... Sabine. That's why I allowed myself to when replying to her.

[@slthbob]

@@kilroy1964 you mean a planned oops to make a point? an oversimplification for a lesser mind?... that falls apart when additional data is presented... but served its point in the presentation of the subject... kind of like she did and does? ;)

[@kilroy1964]

@@slthbob Sabine made a video in which she decided to neglect tidal forces (which is reasonable except for the falling into the black hole), I was responding to her on the terms she set. Now go back and read my first comment and see that it actually makes sense within that context.

[@slthbob]

@@kilroy1964 here... reread what I just said friend... should I link to the comment directly above... friend

[@iggydeveloper]

The title confused me a lot at first, since we even call gravity "weight force" in Dutch (zwaartekracht). Wonderfully explained, very intriguing video!

[@ericrose419]

Thanks for sharing this, Sabine. The discussion of the equivalence principle really helped me to understand why physicists talk about gravity as a force even though it is just an artifact of the structure of spacetime. What I still don't understand is why people keep trying to come up with a 'theory of everything' that unifies the electromagnetic, strong, and weak forces with gravity when gravity is not, in fact, a force like the other three.

[@paulhaynes8045]

Exactly! It really bugs me that physicists (of all people) labour under the idea that 'nature' is neat and tidy, when there is absolutely no evidence for this - for instance both pi and e are irrational numbers. I've always enjoyed imagining how frustrated the discoverers of pi must have become, when their increasingly accurate measurements of pi failed to prove that it was exactly 3! And here we are , thousands of years later, still expecting things to make sense to our primitive monkey brains...

[@AndrewBlucher]

​@@paulhaynes8045I enjoy imagining that you imagine that the discovererers (!) of pi imagined that it could be three. Or maybe I'm imagining that.

[@danielstan2301]

But there are a few more things that makes it look like a force , not just this principle. For example it doesn't explain why gravity still accelerates mass while is not a force, what happens with the body once the whole earth suddenly disappears(will it continue moving towards where it was the center of the mass, stay still or will it go towards the direction where it was pushed by the force of the surface and why is that) and a few more questions that really makes gravity seem to behave like a force. On another note, can we consider gravity as a "force" that pushed against spacetime fabric causing its curvature? 😊

[@undercoveragent9889]

It is nonsense to think that spacetime can be warped without the application of a force. Try bending or stretching any other 'object' without the application of a force.

[@AndrewBlucher]

@@undercoveragent9889 Nonsense!

[@ZennExile]

I cannot express to you in words how fully appreciated this video, and your humor is. I am shook.

[@sntxrrr]

As a layman with an interest in the sciences I sort-of know this but it is good to have it spelled out so clearly every now and then. You can put this video on repeat every couple of years to help drive it home.

[@AndrewBlucher]

You call that clear?

[@undercoveragent9889]

@@AndrewBlucher Incredible, isn't it? These are the same people who voted traitors into positions of power. 'Ooh! Mass does not exert a force but it does cause spacetime to become warped.' lol

[@enadegheeghaghe6369]

I thought it was clear. ​@@AndrewBlucher

[@jochenherrmann6000]

The usual example of gravity being indistinguishable from uniform acceleration is wrong. If an object is in a gravitational field then it is not a point but has an extension and with sufficiently good instruments one can see that the effects of gravity on this body are different in strength. Obvious example: spaghettification of a body in a black hole. However, this is not the case with a uniform acceleration, e.g. in a rocket. So this assumption is not tenable.

[@todddembsky8321]

Wonderful Channel, Incredible Host, Makes learning fun again. Thank you Sabine for a wonderful channel. Wishing you and yours a wonderful Holiday Season.

[@VcSaJen]

Accelerometers only work when force is external. When force is affecting everything equally, including the insides of the accelerometer, you can't distinguish between not accelerating or accelerating, but with some opposite force affecting you.

[@michaelaxton5005]

I'm confused. When you are falling above the earth, according to Einstein and an accelerometer, you are not being accelerated. But acceleration means change in velocity, which is definitely occurring relative to the earth. Also, I was looking for what would be different in our world if gravity _was_ a force. I don't think either were addressed in the video.

[@EdwinMartin]

Sabine explained why acceleration is absolute and not “relative to some other thing (earth)”.

[@forsakenquery]

@@EdwinMartin she did, and it made no sense. The definition of acceleration is "change of relative velocity with respect to time". If acceleration is absolute but the thing that defines it and it derives from (both conceptually and mathematically, acceleration is the literal derivative of velocity) isn't, then logic is broken. You can't just make words mean different things. Use a new word if you want for this idea of "acceleration without experiencing force".

[@EdwinMartin]

@@forsakenquery In our Newtonian world it doesn't make sense, in Einstein's spacetime it does make sense. It's fascinating! 🙂

[@forsakenquery]

@@EdwinMartin no, it doesn't make sense as the words don't have variable meanings in science.

[@michaelaxton5005]

@@forsakenqueryExactly. It makes sense to me only if acceleration is defined to be "what an accelerometer measures." Otherwise, if two objects leave the earth, moving with equal velocity and equal and positive increase in velocity with respect to (WRT) the earth then their acceleration WRT each other is 0 while their acceleration WRT the earth is positive. So is that what Einstein did? Redefine acceleration? Or is it more subtle than that?

[@RicksterX-92fs]

Excellent explanation!! Best I’ve ever heard or read on the fundamentals of gravity.

[@PatrickPoet]

I understood everything you said, it seemed to me, but at the end when you said that falling I wouldn't accelerate because gravity is not a force my brain still wanted to know why then, does my relative velocity toward the mass curving space time increase exponentially. I couldn't answer even though I got a 12/12 on the quiz!

[@smolpp420]

Let me try to clear this up with an analogy: Imagine spacetime as a river and earth's gravity as downstream. Now imagine you're a leaf stuck on a rock. Finally, imagine the speed of the water as the flow of our spacetime. If you suddenly dislodge from the rock, you will start moving more with the flow of spacetime, but you're not being accelerating because gravity is not a force so there is no force pushing you. What you are perceiving as acceleration is the diminishing force that use to be pushing against spacetime until you finally hit 0g. The leaf would only be accelerated if it had an engine (an actual force) pushing it faster than the river's natural flow back to normal.

[@stonecastle858]

@@smolpp420 why does spacetime "flow" towards the mass?

[@smolpp420]

@@stonecastle858 ​ I'm answering out of my depth here but this is how I understand it: Spacetime is a geometrical thing and mass causes a disturbance in its geometry. In 2D you can imagine spacetime line. If you introduce a point of mass, the line will "dip" into that point. As we travel through time, we feel this dip as gravity. So to answer your question, my "flow" analogy describes spacetime's disturbance caused by earth's mass, and the "stream" of spacetime flows towards earth because of general relativity. Us not moving towards the center of the earth is only possible if a force pushes against this flow, therefore falling is not an acceleration, but rather the rapidly diminishing force against spacetime that was preventing us from moving with this flow. Remember that gravity and acceleration are indistinguishable for a local observer (the equivalence principle) so we can only theoretically comprehend the difference rather than visualize it. It's like asking someone to identify C# or Db with no other context than the sound of those notes one after the other. As local observers, we will see the piano player pressing the same key over and over when in truth we are witnessing two different notes beyond our local comprehension.

[@frogboi4607]

@@stonecastle858it’s just an analogy. I’m reality, your not flowing, but following a geodesic in spacetime

[@stonecastle858]

@frogboi4607 why does a body follow that geodesic and why in one direction and not the opposite?

[@TheOneMaddin]

"How am I accelerating if I am standing still on the surface of earth". Your answer to that seems too simplistic and can be easily challenged. You say "All you do is not moving relative to the surface of earth, which itself is accelerating by the same amount." My complaint however is: I am also not moving relative to a chunk of earth 10m below the ground. This chunk however has a different acceleration than I have, and still, we stay at constant distance. I guess this comes down to the tricky definition of "not moving relative to each other" when spatially separated. I just want to emphasize that it cannot be brushed off as easily as done in the video.

[@Observ45er]

Good point. My views: There are some things we simply do not have a mental model for, so, therefore, we must make up some explanation that fits into our world view. Middle World is a term coined by evolutionary biologist Richard Dawkins. This is the realm generally experienced by humans that lies between the microscopic world of quarks and atoms and the cosmic world of stars and galaxies. We know how things 'work' in the realm, so we build models out of the things in that world. . For example, photons sometimes act like particles and sometimes waves. We understand particles by comparing them to balls. We understand balls This is the best analogy we are familiar with so we use it. On the other hand, in other circumstances they act like waves, so we have a mixed explanation to fit our 'middle world' view. . So, we come up with the rubber sheet model .. HOWEVER, please note that she totally avoids the question of WHY mass curves space-time. . . She also says that a planet surface changes the physics without an explanation, again. . First she says: Somehow the surface of Earth causes a force. Then: "if you took away the surface of Earth, you'd fall." With no explanation of why. Contradiction. . This is a self contradictory story, not an explanation. It's just a different model that is shoe-horned in to sound like it is an explanation. . . This totally ignores an important concept that what Newton's First law was supposed to teach you. WHY does the spring "measure" acceleration"? Why would Earth "collapse" with out a force? What causes Earth's "internal" pressure"? . Velocity is not meaningless. It has meaning relative to other things - terrible justification. . Yes, this thing we call gravity is indistinguishable from acceleration, so is acceleration real, or is it all gravity. . Then she says you need a force to a accelerate something; so she does go back to Newton's First law. . Also the spring would change going into the Black Hole, depending on the gradient of the space-time curvature being different on the two ends. . Falling from the building you ARE accelerating in the absolute sense. yet she says you're not - DIRECT contradiction. It doesn't appear she understands the words she speaks, but isd merely parroting a script. . . Sorry. Doesn't work. Too much contradiction. I ask why it can't it be modeled as an attractive phenomenon and call it an force between all atoms.

[@maddi62]

Why does the chunk of earth have a different acceleration?

[@TheOneMaddin]

@@maddi62 Roughly, because the acceleration depends on the gravitational attraction which depends on the distance from the center of earth and the amount of earth above and below you.

[@piyushpanchal7573]

Could you also elaborate the differences in the two situations where one is standing on a planet that is not rotating vs one that is rotating? I am wondering how the acceleration of a rotating frame factors in

[@TueSorensen]

It's not much different. Gravity is about the same.

[@chriskennedy2846]

@@TueSorensen I weigh less on the equator than I do on the North Pole.

[@TueSorensen]

@@chriskennedy2846 Meh. Not by much.

[@marco1anziano84]

So you re telling that g is not a force because it generates acceleration(or relative movement) of an object by deforming space-time. But if it possible to compensate its effect on acceleration/movement using a proper force (forces and g clearly interact in the sense that they can produce partially compatible effects), what is the link between the two entities? What do they have in common that makes them able to interact? Also,1) what would an accelerometer read in deep space (no g acting)? 2) would then a spring whith very low mass extend if placed vertically on the surface of a planet with no atmosphere?

[@shaunswett6684]

Great explanation. I noticed you said a "small box". How small? If you were in a box on a planet, wouldn't the acceleration be slightly lower near the top because that is further from the center of the planet? Whereas in a rocket, wouldn't the acceleration be the same, no matter where it is measured? The box would need to be just a single point!

[@el-vado]

bingo! there's no true equivalence between the inertial and gravity forces . it's a myth. gravity field vanishes infinitely far from a body, while inertial fields don't.

[@woobilicious.]

a point like spherical cow-box in a vacuum obviously :P . pragmatically speaking being in the ISS would only be 90% difference. You can get away with a fairly large box depending on the accuracy of your tools.

[@inciaradible7144]

Very informative video; clears up a lot for me. A question: Are there parallels to be drawn to forces, which are oftentimes referred to as fictitious forces or psuedo-forces such as the centrifugal force or, in the case of Earth, the coriolis force?

[@user-zz3sn8ky7z]

Yes, there are, Einstein even said that it can be viewed that way in certain contexts. The thing that distinguishes the two is that you can't get rid of this "gravitational force" by changing a reference frame - because all reference frames, as far as we understand, are inherently under the influence of gravity

[@eeronat]

I knew it as the Coriolis Effect. First time I've seen it referred to as Coriolis force. Apparently it is indeed called that. Not a force, of course.

[@inciaradible7144]

@@eeronat Yeah, I've heard both effect and force, the latter which I found very confusing after knowing it is, indeed, not a force.

[@hamzahbakouni6208]

Wonderful analogy and presentation. As a fan of physics I may please ask whether the illustrated example of falling into a blackhole without noticing anything, may apply specifically to smaller objects and maybe in context of bigger blackholes in order to limit the tidal effects, as spacetime curvature may vary between adjacent points. Thanks. 🙏

[@landonian1223]

this is my favorite thing to teach about relativity because you can get people to really think about what gravity feels like, which is nothing. i always start with the question, "can you actually FEEL gravity?" basically same as sabine's accelerometer example

[@rosewood1]

That's a false analysis. No you cannot feel gravity. But that doesn't mean it's not there. You don't feel air around you when it's still. If your submerged in the sea you don't feel the sea around you. But in fact as you decend to deeper depths your body is compressed. Equally on the moon where gravity is much this has an effect.

[@PedroPedruzzi]

Thanks, Sabine! You know, I've seen a bunch of intro GR videos and I got everything that's explained. However I still miss a graphical exemple showing how the curved spacetime causes mass to fall. I know non accelerated matter follows geodesic paths but how geodesics can be free-falling ones?

[@Gunni1972]

The "Gravity is not a force" theorem(eme). Acelleration is only absolute to it's starting point. NOT TO CURVED SPACETIME EITHER.

[@scitor]

Great video as always, thank you for that! One thing though, despite being in the title I feel you could have mentioned a little more often that _gravity is not a force_ 🤣 ❤

[@laestrella9727]

😂😂😂😂😂😂😂

[@garymarkowitz5059]

That car didn't just run over you. You accelerated up into it!

[@greatPretender79]

Thank you for this particularly engaging video! But is gravity a force?

[@lordkancer6962]

I absolutely love your videos. I am interacting in the comments section specifically so that it gets pushed out to more people.