# I Does gravity change density of space?

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1. Nov 9, 2017

### grahas

I was watching a movie called Everything and Nothing. It got me thinking. For the sake of argument pretend that you could make the sun pop in and out of existence. If you were to measure the distance between two points on the other side of the solar system with no sun, then if the sun were to just appear would the distance between the points shrink?

2. Nov 9, 2017

### Orodruin

Staff Emeritus
The question makes no sense. You cannot ask "what does physics say about a situation where I explicitly assume that physics does not hold?" and expect meaningful answers.

3. Nov 9, 2017

### MarekKuzmicki

Well, answer is yes.
You are measuring space density by using light. Mass objects are creating gravity which affects light.
But there is a 'but'. Sun is quite massive. Mass creates gravity. But what in essence is mass and how it is connected to space in the first place no one knows. So maybe sun is dense space and you are putting it in less dense space. Maybe not. How it works... well, we know that without gravity space is 'flat'.

4. Nov 9, 2017

### Ibix

Einstein's field equations have the concept of local conservation of energy built in to them. It therefore is not possible to describe the effect of the Sun popping in and out of existence (as @Orodruin said) in terms of general relativity since that would be a massive violation of energy conservation, so is inconsistent with general relativity.
This is not correct. The answer is not defined because the scenario is internally inconsistent.
Do you have a reference to support either of those positions?

5. Nov 9, 2017

### MarekKuzmicki

https://en.wikipedia.org/wiki/General_relativity#Definition_and_basic_properties
Definition and basic properties
General relativity is a metric theory of gravitation. At its core are Einstein's equations, which describe the relation between the geometry of a four-dimensional, pseudo-Riemannian manifold representing spacetime, and the energyâ€“momentum contained in that spacetime.[36] Phenomena that in classical mechanics are ascribed to the action of the force of gravity (such as free-fall, orbital motion, and spacecraft trajectories), correspond to inertial motion within a curved geometry of spacetime in general relativity; there is no gravitational force deflecting objects from their natural, straight paths. Instead, gravity corresponds to changes in the properties of space and time, which in turn changes the straightest-possible paths that objects will naturally follow.[37] The curvature is, in turn, caused by the energyâ€“momentum of matter. Paraphrasing the relativist John Archibald Wheeler, spacetime tells matter how to move; matter tells spacetime how to curve.[38]
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Following.
Mass is curving spaces. Curved space is more dense (than flat one).

Surely in GR there is no poping on and out of existence.

:)

6. Nov 9, 2017

### Staff: Mentor

No, it isn't. The concept of "density of space" makes no sense. There is no way to compare a curved space with a flat one in the way you are thinking.

Wikipedia is not a good primary source. You need to look at a textbook. Sean Carroll's online GR lecture notes are a good start:

https://arxiv.org/abs/gr-qc/9712019

7. Nov 9, 2017

### Staff: Mentor

The OP question is based on misconceptions, which have been corrected. Thread closed.