# Can there be an area with no gravity?

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1. Dec 1, 2014

### Pjpic

Can there be an area without gravity? Are branes the only places with gravity?

2. Dec 1, 2014

### Staff: Mentor

Thereis nowhere in the universe where there is no gravity. There many places where the gravitational effects are so weak that we can ignore them, and when we're in a hurry we often say "no gravity" instead of "gravity so weak that we can ignore it".

I'm not sure what branes have to do with anything here.

3. Dec 1, 2014

### pervect

Staff Emeritus
It depends on what you mean by "gravity". People usually think of "gravity" in GR in terms of the Riemann tensor, the connection coefficients, or the metric. (Maybe more, that is what first comes to mind). You could have a region where the Riemann tensor was zero or the connection components $\Gamma^{ij}_k$ were all zero. There isn't any way I can think of to have no metric, or a zero metric, though you could have a flat metric. Test particles in any such region would move in straight lines. Sorry to be so technical, but I am not sure I understand the point of the question. Branes are not part of GR, I am not sure of how to relate them to your question.

4. Dec 1, 2014

### Pjpic

Does that mean if space is not curved there is no gravity?

5. Dec 1, 2014

### Staff: Mentor

Not "space" but "spacetime", but with that qualification, yes.

6. Dec 1, 2014

### pervect

Staff Emeritus
That should be "space-time" as Nugatory points out, not space. That said, the answer again depends on what you mean by "gravity". I assume you've heard of Einstein's elevator thought experiment, where you have someone standing on the floor of an elevator that's accelerating upwards in otherwise empty space. If you call what the elevator passenger experiences "gravity" then there is "gravity" but no space-time curvature in this case. Mathematically, we would say the Christoffel symbols are non-zero. Thus if the passenger was standing on a scale, the scale would have a nonzero reading which one could call the passenger's weight. However in this case the Riemann curvature tensor, which describes the curvature of space-time, is zero.

7. Dec 3, 2014

### William Donald

If there was only one star in the whole universe, and you were placed billions of light years away from it; there would still be gravity. It's defiantly going to be smaller than a micro amount; however, it would still pull no matter where you were in the universe. It would most likely would take billions, maybe even trillions of years to fall into the star from that distance. But with space-time expansion and such; you might not ever even fall into the star. Who knows!

8. Dec 5, 2014

### Staff: Mentor

If there were only one star in the universe, it would not be expanding; more precisely, the concept of "expansion of the universe" would not be well-defined, since it requires multiple massive objects that can move away from each other. (In the standard model used in cosmology, the matter in the universe is idealized as a continuous fluid, with the individual galaxies in the universe being like the individual "particles" of the fluid. But this model won't work either with just a single star.)