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

Just exactly what does it mean that gravity is not a force? In this video I will revisit the question and explain why you are currently accelerating upwards, and how Einstein's equivalence principle works.

00:00 Intro
00:42 Acceleration is absolute
02:17 How gravity works in general relativity
04:21 Einstein's Equivalence principle
11:39 From Einstein back to Newton
13:48 Learn Science with Brilliant

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#science #physics

[dbracer]

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

[binqnjq]

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 :-/

[waynesaban]

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

[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! 🎉

[richtheobald]

"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.

[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!

[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.

[billmcdonald]

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"?

[jeremypearson]

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.
🤯

[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

[dougjamesberwick]

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

[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.

[thehadster]

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!

[__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.

[Mobri]

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

[LuvHrtZ]

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

[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)?

[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.

[markoszouganelis]

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

[jonathandavid]

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.