# Gravity question: Why do things fall?

• staballoy
In summary, the bowling ball/rubber sheet analogy does not explain what causes an object to accelerate toward another mass. Einstein's theory of relativity explains how mass curves space, and how this causes objects to follow geodesics in curved spacetime. This is why objects near a massive object seem to accelerate towards it from the perspective of an observer on the surface.

#### staballoy

I understand that mass warps spacetime, and am well familiar with the bowling ball/rubber sheet analogy, but that to me is a depiction without explanation of the process. It doesn't explain for me what actually causes an object, Newton's apple for example, to accelerate toward another mass, in this case the Earth. I recall seeing an explanation on a PBS show years ago called Einstein's Universe, that involved gravitational time dilation causing acceleration, IIRC, but can't remember the details. I am an interested non-mathematical layperson, so any explanation is appreciated.

staballoy said:
It doesn't explain for me what actually causes an object, Newton's apple for example, to accelerate toward another mass, in this case the Earth.
Basically we don't know.
Newton's laws say objects attract each other because of their mass, Einstein's theory says mass curves space - but these are just ways of saying 'because it does'.

staballoy said:
I understand that mass warps spacetime, and am well familiar with the bowling ball/rubber sheet analogy, but that to me is a depiction without explanation of the process. It doesn't explain for me what actually causes an object, Newton's apple for example, to accelerate toward another mass, in this case the Earth.
The ball/rubber sheet analogy doesn't explain Newtons gravity. This is explained in this post (check also the links there):
https://www.physicsforums.com/showpost.php?p=2046692&postcount=4
staballoy said:
I recall seeing an explanation on a PBS show years ago called Einstein's Universe, that involved gravitational time dilation causing acceleration, IIRC, but can't remember the details. I am an interested non-mathematical layperson, so any explanation is appreciated.
Gravitational time dilation and mass attraction are both effects of a distorted time dimension:
http://www.physics.ucla.edu/demoweb...alence_and_general_relativity/curved_time.gif

Are you asking why mass curves spacetime in the first place, or are you asking why, given that mass curves spacetime in this way and that objects follow geodesics in curved spacetime, it so happens that geodesic paths near a massive object will have the property that the smaller objects seem to accelerate towards the larger one from the perspective of an observer on the surface?

JesseM said:
Are you asking why mass curves spacetime in the first place, or are you asking why, given that mass curves spacetime in this way and that objects follow geodesics in curved spacetime, it so happens that geodesic paths near a massive object will have the property that the smaller objects seem to accelerate towards the larger one from the perspective of an observer on the surface?

I'll take answers to both. The diagram from the link offered above (http://www.physics.ucla.edu/demoweb...alence_and_general_relativity/curved_time.gif) helps a bit. If I understand correctly, the passage through time slows the closer an object is to a another mass, but passage through space accelerates, like approaching the speed of light, where the faster an object moves through space, the slower it moves through time.

staballoy said:
I'll take answers to both. The diagram from the link offered above (http://www.physics.ucla.edu/demoweb...alence_and_general_relativity/curved_time.gif) helps a bit. If I understand correctly, the passage through time slows the closer an object is to a another mass, but passage through space accelerates, like approaching the speed of light, where the faster an object moves through space, the slower it moves through time.
Well, there isn't really an answer to why mass curves spacetime, every theory of physics just has some basic ground assumptions which aren't derived from anything else, and which are justified solely in terms of the accuracy of the resulting theory. I think you can show that curved spacetime is a necessity if you start out with some other basic assumptions like the equivalence principle, though, but those other assumptions have to be taken as given.

As for the second question, if you found the diagram A.T. posted useful, I remember A.T. posted a bunch more diagrams based on the "curved time" visualization in post #4 of this thread, so you may want to take a look at those links.

staballoy said:
The diagram from the link offered above (http://www.physics.ucla.edu/demoweb...alence_and_general_relativity/curved_time.gif) helps a bit. If I understand correctly, the passage through time slows the closer an object is to a another mass, but passage through space accelerates, like approaching the speed of light, where the faster an object moves through space, the slower it moves through time.
Yes but this are two different time dilation effects: 1) advancing more along the space dimensions and less along time dimension. 2) having longer distances along the time dimension to go. You can examine both in closeup here: