A problem with how curved spacetime explains gravity

Question

I've read quite a few online explanations of how curved spacetime is the reason objects are drawn to each other, and that gravity is an illusion. Most of them follow the same path:

explain what geodesic is, and how it can actually be a straight line.
explain how matter warps spacetime.
make a conclusion that an object is not pulled by gravity, but instead it tries to follow the straight line which is actually a geodesic.

What i don't understand is the following theoretical situation. Lets suppose we have Earth that doesn't spin, doesn't move around the Sun, or in the Galaxy, or Universe, Earth is absolutely stationary. And there is a person on its surface holding a stone. The stone is then released. If there are no forces that are attached to it, why would it start moving along any geodesic at all? At this point it doesn't even matter how warped the geodesic is if there is nothing to push it along the way in the first place.

In your example, the earth is also following a geodesic. Why ask about the stone but not about the earth? — WillO, Jan 24 '18 at 17:10
The stone is moving because it's moving in time. Remember that a geodesic is a curve in spacetime not just a curve in space. — John Rennie, Jan 24 '18 at 17:15
Possible duplicate of How does “curved space” explain gravitational attraction? — John Rennie, Jan 24 '18 at 17:17
Possible duplicate of How does "curved space" explain gravitational attraction? — Gonenc, Jan 24 '18 at 17:32

score 0 · Answer 1 · answered Jan 24 '18 at 18:50

0

The stone doesn't accelerate. The observer standing on Earth is accelerated upward by the electrostatic binding and repelling forces of his matter and the matter of Earth.

See this demonstration:

https://www.youtube.com/watch?v=sbSxxsb30_E

answered Jan 24 '18 at 18:50

Steven Thomas Hatton

1,965

score 0 · Answer 2 · answered Jan 24 '18 at 21:19

At first I would like to say that the explanation you will read now is only for undersanding the "essence" of the idea of spacetime-curvature, it won't be accurate mathematically VERY NOT. This question was a problem for me too when I first read about general relativity. I like to imagine that space is literally falling towards massive objects, and when you don't move in either direction you are falling together with the space around you. So when you are falling towards the Earth you are not moving reletively to the space around you, but from another reference frame it seems that you are falling. Let's suppose that we are not speaking of Earth. Imagine a spere made of dust with constant density and negligible drag force when moving in it. If you are falling towards it you will accelerate until you reach it's edge and after it you will decelerate until you reach it's center. If you graph your position $x$ towards time $t$ and mirror it you will get a line with a bump on it. This bump represents the curvature of the spacetime. I hope I helped you to get an idea of what is going on in these kind of fenomena.

A problem with how curved spacetime explains gravity

2 Answers2