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

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

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 :-/

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

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...

"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.

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.

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.

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!

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.

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?

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."

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

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.

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

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.

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!

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

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?

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.

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.

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?

This is insane! I'm definitely keeping the 'force of gravity' in my toolbox. Each year when the clocks change, old people fall off ladders trying to change them. At the hospital we don't tell them "the earth accelerated towards you and broke your hip". Relatively speaking - the earth didn't move.

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

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 :)

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

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) ?

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.

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...

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.

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

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

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.

Are a free fall and having every particle in a system (for example, a spring) accelerated the same amount (and direction) distinguishable? I assume the spring expands because only one end of it is accelerated, and the rest of the spring is pulled by the internal interactions between its particles.

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.

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

@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?

Hi Sabine, great video as always. I have some questions that are bugging me. 1) you say Einstein said an elevator in a box with you and a spring on Earth is the same as in a rocket and indistinguishable. But how can this be true? If we actually do this on Earth you will feel nothing and the spring will not stretch. However in an accelerating rocket you will feel being pushed down and you will see the spring stretch. So this analogy make no sense to me ? 2) Not sure if Einstein explained how the curvature of space-time works. For example is it the case that somehow there are 'magically' lots of sheets or rail tracks holding up massed objects ? This is what the analogy of rubber sheet suggests, but how? Whete does this cone from? Eg is it a field of some sort (like magnetic fields create a force, is this series of 'rubber sheets' the gravitational field or something? It just seems to me that we went from unexplained action at a distance from Newton to more unexplained actions with Einstein. Is this being researched currently? 3) If an apple is falling off a tree I have seen some explanations say it is not the apple necessarily falling but the Earth accelerating upwards to meet the apple. I dont understand this. For example this explanation may work for a single apple but whete we have multiple same experiments all over the world at same time then which way is Earth accelerating now to meet every apple? Seems like we are causing the Earth to stretch in all directions? 4) if gravity truly is not a force then why does Newtons gravitational equation even work at all ? If Newton was so completely and utterly wrong then his equation has no right to work at all. It should be completely wrong. However it does work and extremely well, exceot in some cases. His equation concept is based on a force and on interacting masses. ? Are Einsteins equations very different eg do not involve mass and distance or fairly similar to Newton? 5) how can acceleration be absolute ? Acceleration is defined as the rate of change of velocity. Velocity is relative, time is relative, but somehow acceleration which is the derivative of velocity is absolute, just dont see why mathematically? Or conceptually ? For example we talk abot inertial frames in Special Relatively, and say velocity is relative. However if you accelerate within an accelerating frames at a different rate to the frame itself, why is it conceptually any different to the relative concept in an inertial frame ? Ie why wouldnt acceleration also be relative?

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)?

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.

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!

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"?

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 RU-vid!! ❤

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.

Very interresting, could u explain about the recherch on the graviton?

Great indeed. Now the famous "Black square" by K. Malevich could be surely considered as illustration of the scientific equivalence principle in Fine Arts. The part about pressure from inside the Earth is my favorite.😂.

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.

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

@4:09, what exactly means "would freely fall in the curvature that it itself creates". So does it fall or not? I'd love to hear a more detailed explanation of that part of the video.

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.

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. 🙏

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

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 :)

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 ?

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

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!

Merry Christmas Sabine! 🎄🎅🌌🌟

Thanks for teaching us the Spring Theory! 🙂

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.

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

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

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.

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

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.

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!

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!

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!

I think when we imagine (or even physically hold) a spring or slinky it becomes easy to see. We just have to change our perspective of what is acting our applying force. When the slinky sits in a pile on my hand with no gaps between the coils, because my hand is applying acceleration pushing it up. If I hold it from the top coil and it opens up into loose coils, it's because of my hand applying acceleration holding it up. If I drop it (and we imagine wind resistance and air turbulence having no effect) it will go to a position where it's neither fully compressed, nor under tension being stretched out. Additionally, it would be straight, the coils would all be in line, whereas if I went back to holding it, with one end in each hand, the middle coils would sag, because of the acceleration applied by my hands. If I apply more acceleration by lifting it above my head, the sag would be greater while I moved it. And if I suddenly pushed it towards the ground, the bend would be upwards rather than downwards. Simply put, the sag/bend in the coils is opposite to the direction of acceleration.

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

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

Question, in order for there to be Mass to bend space the atoms would’ve needed to be attracted to each other to group sufficiently, since most of the suns and planets in the universe were created in the first few billion years appears to me that there would be insufficient time to create the universe if gravity only relied on bent space????

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

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!!!

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.

Sorry if this type of question has been asked already. I imagine it's difficult to describe a phenomenon that involves 4-dimensional spacetime since our brains are evolved to perceive reality in 3 dimensions. I'm interested in the concepts of "falling" and "internal pressure." But mostly falling, since that's what we perceive to be the effect of gravity from our naturaliistic point of view. If a mass of atoms in any configuration is not near any other such mass (near being an admittedly vague word), is the mass "falling?" Is it falling toward the nearest other mass which deforms spacetime the most with respect to the location of the mass? If the mass is between two other large masses, in such a position that each large mass distorts spacetime to an equivalent degree with respect to the small mass, what happens to the mass? Does it stop "falling?" Does it undergo some type of acceleration?

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! 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?

Thanks you so much for this Sabine. One of your earlier videos about this topic, which had a simple "gravity is not a force" and the raw explanation has been used by flat earthers as evidence of their position. This fully qualifies what you meant, and removes one more "justification" they can pull out of the bag

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

Very informational, I'll need to watch this one a couple of times for it to sink in and I've had undergraduate level Physics, although I was not that good at it.

Sabine…one of a kind. Another great tutorial.

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

brilliant. thank yo Sabine. They say that satellites are always "falling toward the earth" but are "moving too fast to hit the earth". I'm gonna rewatch this and try to map out a more correct and precise explanation.

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'?

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!

Great video as always. I thought the definition of force was anything that can cause acceleration (which is ultimately also acceleration but atomic scale). By this definition anything that causes you to accelerate is a force and so is gravity. I am now wondering so how force is actually defined.

As you fall into a black hole even in a box, the space time is not equal inside the box, there is always a gradient relative to the masses around you. The gravitational gradient increases exponentially as you get closer and closer, eventually the force between your head and feet will rip you apart. The gradient will continue to get greater up to a point where even your molecules will be shredded.

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!

This is the most useful 15 minutes I have spent on RU-vid. Thanks, Sabine.

I feel this video needs a follow up about what's really going on with this internal pressure. This means from the perspective of spacetime you are continuously moving upwards in space time, but the curvature of spacetime precisely counteracts that movement so that you remain "still" relative the the surface of the planet which is also moving upwards due to the internal pressure of earth? Is this a correct interpretation?

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

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

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

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.

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.........

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 ...

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.

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