# Space-time (in laymans terms)

1. Jan 1, 2016

### themagician

Can anyone explain space-time to me, and how it disproves Newtons theory of gravity please? I've heard of the analogy that mass bends space-time like a bowling ball on a rubber sheet, but I don't understand how this happens, nor how this contradicts Newtons theory of gravity.

2. Jan 1, 2016

### Orodruin

Staff Emeritus
It does not contradict Newton's theory. It has Newton's theory as a limiting case.

You will not be able to fully understand the theory other than in popularised versions without learning graduate level physics and mathematics.

3. Jan 1, 2016

### themagician

Thanks for the response.

You are right in that I probably won't be able to fully understand it, which is why I asked in laymans terms, which might be what you mean by the popularised versions (if they are accurate?). Can you elaborate on these?

Thank you for relabeling the thread.

4. Jan 1, 2016

### PeroK

Here's something that might be useful:

5. Jan 1, 2016

### Fredrik

Staff Emeritus
General relativity is too complicated a subject to be explained in a forum post. I would suggest that you pick up a popular science book about it. The only one I know is "Black holes and time warps: Einstein's outrageous legacy" by Kip Thorne. But it would be even better if you make an effort to understand special relativity first. I suggest "Spacetime physics" by Taylor and Wheeler.

GR doesn't "disprove" Newton's theory of gravity. It just makes more accurate predictions about results of experiments. One example is that Newton's theory says that the orbit of a planet is an ellipse, and GR says that it's not quite an ellipse. Observations have confirmed that GR's prediction is better.

6. Jan 1, 2016

### Staff: Mentor

The rubber sheet analogy is terribly misleading. If you search posts in this forum you'll soon find a video made by our own A.T. that is a much better analogy for a layman.

General Relativity predicts slightly different results than Newton's theory of gravity and measurements (precession of Mercury's orbit, deflection of light passing near a massive object, small corrections required in the GPS system) match the predictions of GR.

7. Jan 1, 2016

### Ibix

Newton's theory of gravity has no propagation term - it's effect is simultaneous with it's cause. However, Einstein had made "simultaneous" a relative concept. Like "here" means a different thing to you and to me, "now" in relativity depends on who is talking. So whose "now" does Newtonian gravity use?

Nobody managed to come up with an answer to this question and, in the end, Einstein developed General Relativity. This looks like Newtonian gravity when you can ignore relativistic effects, but works properly when you can't. It treats gravity as a change in the geometry of spacetime (casually, it curves spacetime) rather than a force.

As others have said, this is very much a sketch that lacks several hundred pages of explanation. If you want to understand it properly you will need maths. Lots of maths.

8. Jan 1, 2016

### FactChecker

This video is based on Epstein's book "Relativity Visualized". The book is full of good illustrations like this. I recommend it.

9. Jan 4, 2016

### alw34

I don't think anybody 'understands it'. Other than via Einstein's mathematics; that's it. Einstein came up with some principles, developed mathematical models and made predictions which have been verified many times over. GR works. There is no 'one liner' I've ever seen that results in an "aha moment".

It was the genius of Einstein to figure out space and time were not immutable and fixed as everyone else thought in the early 1900's. And it took him about ten years after special relativity to arrive at GR. Maxwell noted that his electromagnetic equation phenomena did not fit the scheme of Newtonian mechanics....but like others, could not unlock the exact nature of the disparity.

As already noted, GR is a refinement of Newton's gravity.

[Mentor's note: part of this post raised different issues, so has been moved to a new thread: https://www.physicsforums.com/threads/are-sr-effects-caused-by-spacetime-distortion.850863/] [Broken]

Last edited by a moderator: May 7, 2017