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

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

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 :-/

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! 🎉

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...

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.

"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.

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.

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. 🤯

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

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!

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.

As an eager physics undergraduate 50 years ago I struggled more than I ever expected with the explanations of relativity theory I was given, and this was the final straw that convinced me I did not want to ever be a physicist after all. I kind of understand it now, but I still can't quite get my head around the idea that the earth is making me accelerate upwards even though I'm motionless and the earth is not visibly expanding. The explanation, so I understand, is that the earth, due to its internal pressure, is expanding into space at the same rate at which space-time is collapsing inwards due to gravity, but why are these two things happening at the same rate? Why should there be an equivalence between the earth's internal pressure and the curvature of space-time?

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."

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.

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.

wow! your explanation is the clearest I heard so far and I checked a lot of videos on you tube. thank you!

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.

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.

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

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!

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

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...

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?

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.

I concentrated very hard and still have so many questions. It seems more like a semantic trap than an actual Physics problem.

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.

Love the lesson with the quizwithit! added level of fun~

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.

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 :)

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

It seems like a semantics argument. As a human observer, we usually experience a "force" transmitted through the electromagnetic force, which keeps up from passing through other objects, as you point out. Hopefully physics will come up with a better description of gravity, but it appears to me that the "force" of gravity is caused by entities with mass tend to want to go towards each other such that "time" (another nebulous concept not fully explained by physics) passes at a slower rate compared to space further away from other entities with mass. The ultimate goal of any mass is to move towards the event horizon of a black hole, where time appears to stop in this universe. The Cavendish experiment does measure something that appears to be a force.

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)?

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

If you are perpetually “free-falling” into the black hole and there is no force acting… what causes the probable spaghettification? Falling towards earth only hurts/is fatal once you collide with the accelerating earth. So is it actually possible to survive falling into a black hole? This is a legitimate, searching question. Thank you Sabine!

This was so much clearer to me than Veritasium's attempt on the same subject which left me confused and with a reduced will to live.

Merry Christmas Sabine! 🎄🎅🌌🌟

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. :)

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.

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?

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.

Nice video Sabine! It's so interesting that completely different views can describe facts from different perspectives. I like the beauty of the underlying mathematics and its symmetries. A paradox glimpse what space and time really are. Some facts always connecting and some doesn't fit together. So sad we will never completely understand a fractal universe.

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.

Before high school physics I thought Albert E adapted his math because spent a lot of time and energy walking around hilly terrain. He just naturally thought in terms of curved landscape. Then since inertia and gravity seemed so similar I thought everything must be expanding. Now this!

@sabine if an object is in free fall and if we track its motion at 1sec, 2sec etc. then how is the position of the object explained without acceleration?

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

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.

What I think most people have trouble with, and me too, is the idea that earth is accelerating upwards towards you, it's a strange concept because obviously the earth isn't expanding yet you feel the earth pushing against you and you can indeed measure the acceleration upwards. Very hard to wrap my head around that part. Maybe it could be explained by the spacetime that earth is in is making it seem like it expands outwards, while it's actually the spacetime that bends inwards (gets "smaller"/"thinner" making the earth for all intents and purposes expand outwards, at least as defined by the spacetime it is in).

This gives me a headache because if a rocket is trying to accelerate but is anchored in place you're not gonna be, like, pinned against it, so the idea that the earth is accelerating in every direction yet motionless is weird. If we were standing on the surface of an infinitely expanding bubble, THAT applying a force makes sense, but if it's being held back then it's not applying a force.........

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

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!

We are nowhere near as intelligent as we think we are.

Oh I love knowing new things I listen to you a lot and your smarts makes me wish that when I was younger I could have learned from you. Thank You fo promoting understanding in science!

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.

So the mystery of gravity as a force between masses is replaced with curved space-time between masses. Ah, so clear now.

My daughter and I like to share interesting facts with each other every day. I will send her the link to this video because I never knew that I was accelerating upward. 🤓 Thanks, Sabine! New subscriber.

Whether something is a force depends on your reference frame... If your reference frame is something relative to which you are accelerating, then you are experiencing a force. If your reference frame is such that you are *not* accelerating, then it is not a force within that frame.

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 KZfaq videos. There have been counter videos on this as well, which is why I think you weighing in would be a great arbiter. Thanks!

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 KZfaq!! ❤

Fall off the building and the fall won't kill you, rather, it's the sudden stop at the end of the fall that will kill you. That said, you probably wouldn't give a fig about gravity. Well done Sabine, keep em coming.

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

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

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

I once understood gravity from a unique perspective. That understanding is long gone.

Great video! I have one question. In the example of the person jumping from the top of a building, around 12:00, why does the person suffer an impact, a larger acceleration or something, if it wasnt accelerating in the first place. In general, how can free falls cause collisions with consequences if the bodies did not gain velocity or energy (if energy or velocity even matter in this context) ?

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.

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

great talk! One of the pitfalls is to think all the time relative to our earth. Just imagine being "in space", far from a reference object like earth. It would be hard to measure your velocity. Velocity relative to what? Earth? But earth itself is running fast around the sun. And our solar system is in orbit through the Milky Way, etc. This way I am starting to get the picture of relativity

Hello, greetings from a layman here who admires you and your videos, thank you for your work!

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!!!

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.

Thanks Sabine. I have a number of your books. I'm a physics graduate so maybe I can understand most of your concepts.

The notion of force itself is an abstraction that is arbitrarily created to measure changes in states of motion. What happened with Einstein is just that he saw behind this construct in relation to gravity. I wouldn't be surprised if the same twist would turn out with the other three fundamental interactions, meaning that there are no forces at all.

This is the most useful 15 minutes I have spent on KZfaq. Thanks, Sabine.

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

Fyi: Every phone nowadays has an accelerometer on it. It's how it determines orientation

Hi Sabine Could you tell me where I have gone wrong….Let’s say you take the case of the train and flashing lights and put the flashing lights one up high and one on the floor in a rocket ship with Alice. Have the rocket ship accelerate upwards constantly the light basically blue shifts from bottom to top which is opposite of what the gravitational potential does. Now as the rocket ship comes into a landing back on earth it now shifts as the gravitational potential does. Would this not indicate a difference to be noticed between acceleration and gravity thus a flaw in the equivalence principle in this scenario? Try drawing the different scenarios out as I did one just considering acceleration up and down and another scenario of just using gravitational potential (which would not matter if going up or down as it is always red shifted upwards) curious where I have gone wrong. Love your videos

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.

The equivalence principle is a simplification. You can distinguish between forces due to gravity and a constant acceleration because gravity is inhomogeneous. If you are a 1D point equivalence holds, but a 3D object experiences tidal forces.

Doesn’t the box and spring stretch when they enter the black hole as space is getting bigger one end in comparison to the other ? Or does this depend on the frame of reference you observe it from ?

In using words such as "Force" which Gravity is not, it would be good to define what a "Force" is. It is also good to know that we won't accelerate towards a black hole, when that time comes (or the space-time exists)

"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.

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 ...

Such a nice explanation of a complicated (well not anymore) concept. I remember seeing Veritasium's video on this topic but I remember I understood much less.

I figured, an accellerometer registers change in motion. You're moving at 1000mph at the equator, along the surface and your motion changes direction downwards, 15 degrees per hour. I was thinking, this is why an accellerometer registers an accelleration upwards.

Finally, I was always wondering the acceleration i felt on this earth, without things moving. Thanks.

Sabine…one of a kind. Another great tutorial.

Thank you Sabine for bringing science into youtube so we can try to become smarter.

So I must've missed it, as I always seem to, every time someone explains how gravity is a geodesic distortion of space, not a force... but by what physical [phenomenon; effect] does aggregate matter/mass accrue the *shear influence* to cause space to distort as described? Thanks in advance.

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. 🙏

Thank you so much for finally making this clear to me! I've spent years trying to understand why the upward acceleration of the Earth did not allow me to spring into the air and fly!

it is so hard to explain this to people, i've been trying for years.. everytime im trying people think im either wrong or dont understand physics

Thanks for teaching us the Spring Theory! 🙂

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.

Sabine forced me to have an interaction with something or another relative to something else. The gravity of her excellent discourse about the myth of the force of gravity has left me wanting a half gallon of chocolate ice cream and another look at the video! And she’s one of the few reasons the internet and KZfaq are worthwhile. ❤

"If you're in the enclosed box, you can't tell if you are accelerated or not." Are you sure? Because in an elevator, I sure can tell when the lift starts accelerating upwards, and when it slows down (As a little child, I used to to jump up when the elevator was about to reach the destination floor; and I feel that could jump higher when the lift was decelerating.

This is a great video as you see gravitational force everywhere in science media all the time. I think people just have a hard time wrapping there head around the fact that normal space is not curved but space near earth is and how that would actually look like. The marble on rubber is a good visual aid for 2d space but then trying to translate that into 3d space can give people trouble. Another interesting thing is when you think about when spacecraft use a planet's gravity well for slingshot maneuvers. Where does the energy come from if gravity is not a force that can give the spacecraft more energy then when it started with? I know the answer to that one but curious if others here do :)

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?

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

My comprehensive college physics book put it this way: "There is mass and spacetime fabric, mass tells spacetime how to curve, spacetime tells mass how to move under its curvature". A subjective interpretation of the cosmos in modern physics.