Question

Gravity is basically curvature of space-time fabric. Really?

I don't understand the naive explanation of how you imagine a massive object creates a depression in a stretched bed-sheet, and things just roll into it.

That makes no sense. Things roll into it is not due the depression of the bed sheet, but it is because WE ARE IN A GRAVITATIONAL field.

So if we were in the outer space, and did the same experiment, then even though bed sheet may get deformed (some how) but that doesn't mean objects will roll in. They would just stay there.

Answer 1

So even though, masses do bend space time, which are proved by many gravitation ally lensed images, I still don't understand why objects have to follow that curvature

Why can't objects just stay still.?
I understand that if the object tries to move, then he will have to follow that curvature, but objects should have a choice of staying still. So that doesn't explain why i cannot walk against gravity? cause i ll be just walking against that curvature (note: you can't say walking against curvature requires energy, cause there is no OTHER gravity, the theory ITSELF says curvature is gravity, so I must be able to just walk the other way)

Answer 2

lol I have no idea what ur talking about xD

Answer 3

who asked you to comment then? :P

Answer 4

:3

Answer 5

@Abhisar

Answer 6

@Kainui

Answer 7

interesting post :)

Answer 8

i think the rubber sheet bowling ball example conveys one aspect of the full relativity model, which in fact is a lot more complex .

Answer 9

there is an interesting physics debate going on (well not really debate)

Answer 10

Is gravity the exchange of gravitons, or the curvature of space-time? (or both)

Answer 11

I hope we can draw more experts in this field to openstudy.

Answer 12

also i think we should distinguish between asking what causes gravity to occur, and how we can adequately describe the effects of gravity

Answer 13

Forget about gravitons, :P

that is some weird quantum mechanical approach to unify the forces,

But General relativity has been proved, atleast to some extent. And we do know that space time bends. But it doesn't make sense, how that curvature is what we call as gravity :P
Cause Einstien said gravity ISN'T A FORCE.

Answer 14

yea we sure do need more experts .. who can give explanations without equations :D

@Vincent-Lyon.Fr

Answer 15

oh he is going to say something

Answer 16

\(\color{blue}{\text{Originally Posted by}}\) @Mashy
Why can't objects just stay still.?
I understand that if the object tries to move, then he will have to follow that curvature, but objects should have a choice of staying still.
\(\color{blue}{\text{End of Quote}}\)

Objects are never "still" they can only ever appear to be from your point of view. Aren't we all moving quickly around the sun right now, despite being still?

Galileo realized that an object in motion stays in motion unless acted upon. So being "still" is the same as having a constant velocity of 0. So consider things in terms of constant velocity.

So when it comes to gravitational fields, the more gravity there is, the slower time goes. Perhaps you might have heard that the speed of light is a constant for all reference frames? This is the same concept, except applied to all objects, really everything has a 4 vector that points in the x, y, z, and time direction and is conserved.

Since it is conserved, if you move into a gravitational field, the component in the time direction gets smaller causes there to be an increase in the vector in a spatial direction, which is acceleration!

Why does the change of velocity point towards increasing gravity? Gotta study tensor calculus. ;P

Answer 17

@Kainui
That sorta, kindaish, making some sortof ish, sense to me.. hmmm :D

Answer 18

I'm trying to get myself up to speed to learning the relativistic Schrodinger equation, but this is sort of part of the basics of relativity you might be able to find some good videos on this on youtube. Actually, wio gave me a link to some, but I lost it. @wio help!

Answer 19

btw, when I said still, i mean't being still with respect to a planet (whose gravitational field is what we are interested in)
So imagine a universe, with jsut earth, and me about 5000km from the surface of the earth. Its possible that i can be still with respect to earth right? :P

Then my velocity AS SEEN by the earth is zero.

Answer 20

So in that scenario, how do i get an acceleration?

Answer 21

@Kainui Most videos I saw, basically start from that stretched elastic sheet kind of example. and take off from there

Answer 22

Here's a cute video I saw a while back. https://www.youtube.com/watch?v=DdC0QN6f3G4

Answer 23

What do you mean, how do you get acceleration?

if you are 5000km from the surface of the earth, how did you get there?

Either something is applying a force holding you there, or you are in free fall.

Just "floating" there "still" is not real scenario.

Answer 24

Oh my god

OMG.. that video is mind boggling.. !! I m just .. m just fascinated by it.. and yet I just can't seem to understand it

I mean i get it.. but its taking time to digest!!

Answer 25

@Kainui
Forget about how i get there

why can't we start with a problem in which at time t=0 I am 5000km from the surface of earth and my speed with respect to earth is zero? :O

Answer 26

classically a little time later i get an acceleration due to the gravitational force

but using GR, there is no force.

Answer 27

Now I am just trying to picture using the content of that video!!

Answer 28

Oh ok I see what you're trying to say.

But it doesn't matter that you have 0 velocity relative to the earth. You are still travelling through spacetime with your constant 4-vector. The gravity of earth causes time to slow down and the gradient of your spatial 4-vector points towards the center of the earth and causes it to increase.

Your question is like asking this:

This is what you're saying. You're at the top of a parabola in regular looking newtonian motion where the velocity=0. "starting" to accelerate towards to earth is just the same as "continuing" to accelerate after you've started falling just like when something is thrown up and "slows" down.

The point is, gravitational fields slow down time. All objects have conserved 4-vectors. To make up for it, you stop going through time and start moving through space towards increasing gravity causing gravity to increase more, time to slow down more, and your spatial velocity to increase even more, giving you nice looking acceleration.