# What is Space?

1. Mar 25, 2013

### tolove

edit: Messed up the title! Although, I suppose it's related.
edit: What is space?

Context for this question: I am starting my junior year in physics, so not all that bright on the subject.

In short, what is space? Maybe if someone tries to answer this question I'll get a bit of understanding:

Imagine a universe where there exists nothing but two large masses. Each mass is given a massive velocity 180 degrees apart, far exceeding any gravitational pull back toward each other.

As time approaches infinity, what exists at the initial point where the two masses were located?

2. Mar 25, 2013

### Staff: Mentor

Vacuum.
A framework in our description of the universe.

3. Mar 25, 2013

### BruceW

In general relativity, space (or, really space-time) is not just the backdrop which stuff happens in. Space-time can have curvature. Mass and energy are a source for this curvature, and Newtonian gravity is the limiting case, when there is only a very slight curvature (i.e. Newtonian gravity is only correct when the gravity is weak enough, so it doesn't work for black holes, e.t.c.)

Also, space-time can have non-trivial curvature even when there is no matter around. So space-time is not just 'a place for matter to exist in'. I keep saying space-time, because in general relativity, time is put on equal footing as space, so there are simply 4 'coordinates'.

What I find pretty hilarious is that we don't even know if the universe is closed, or open or flat. A closed universe corresponds to the 'surface of a balloon analogy', where the entire universe has a finite size, but no boundary. The flat universe corresponds to an infinite universe which has no overall curvature, but has curvature 'fluctuations' (for example, a galaxy causes a fluctuation in curvature). Lastly, the open universe is one where there is negative curvature (as opposed to positive curvature in the closed universe), and the geometry is hyperbolic.

I'm not totally sure what your question means. Is your question "do the masses come back to the same place?"

4. Mar 25, 2013

### HomogenousCow

Gravity is the aspect of nature which breaks certain symmetries, for example we don't just fall in any direction but rather towards the center of the earth. Our model for it will change over time as we refine our understanding of nature, so whether it is a force or space time "curvature" just depends on which model you are using right now.

5. Mar 25, 2013

### tolove

And to BruceW,

Does space exists without matter? If matter and space are attached to each other somehow (like everything else seems to be), then what happens when two masses are shot apart? Is there ever a section of 'no space' between them?

The whole concept confuses me!

6. Mar 25, 2013

### Staff: Mentor

Not in current physics.
Matter bends space(time) - without matter, you just have unbent space(time).

7. Mar 25, 2013

### tolove

Is this what is meant by an open model of the universe? Space and time exist in all directions toward infinity? Your use of the word 'framework' makes sense in this context.

If so, then under a closed model, would space be different at that initial point?

What's the terminology for these subjects? I don't want to bother you with too many questions when I'm sure there a wiki page I could read through.

8. Mar 25, 2013

### soothsayer

We know it's pretty darn flat.

We can't know if it is exactly flat or just absurdly close to it one way or another, even with our excellent measurements up to this point, But our "local" universe: everything we'll ever be able to see, is definitely flat for all intents and purposes.

9. Mar 25, 2013

### soothsayer

An open universe means that space and time exist in all directions infinitely and are expanding ever faster forever.

A closed universe is not finite in a four-dimensional sense: You can start at one point, and never reach the "edge" of the universe, but you will loop back around the same point over and over if you travel far enough; imagine traveling on the surface of the Earth...This universe will eventually stop expanding and collapse back on itself, without dark energy, that is. Nothing would be different at the "initial point", since the degree to which a universe is "open" or "closed" is a global quality, and not a local property--nothing distinguishable can really be said about any one localized point in space.

You can also have a flat universe: one that is perfectly balanced between open and closed, so that it expands forever, but at an ever slower rate, such that at infinity, the universe will be in a completely steady state, although it would never actually reach it. Interestingly enough, our own universe would appear to be completely flat, but there is the presence of dark matter, which acts as an anti gravity force on long, long distance scales, which pulls the universe apart. There are differences between an open universe and a flat universe with dark matter that I could explain, if you were interested.

This is all really under the subject of Cosmology. You can wikipedia that and find a wealth of related information.

Have fun!

Soothsayer

10. Mar 26, 2013

### BruceW

I think this is a misconception that is propagated by proponents of the inflationary theories. For inflationary theories to give a good description of our universe, they require that the universe is pretty darn flat. The current 'standard cosmological model' is that the universe is pretty darn flat, and inflation happened. I guess that's because this is judged to be the simplest working model. (But there are other models).

But this doesn't mean that the universe is pretty darn flat. In fact, according to Jerzy Plebanski and Andrzej Krasinski, in their book "An introduction to general relativity and cosmology", they say "Current real knowledge gives no grounds to assume that $\rho_0$ is greater than $0.3 \rho_{crit}$" (where rho_0 is observed density and rho_crit is the density at which the universe is flat).

But then the supernovae observations imply that dark energy can account for over 1/2 of the total density of the universe (which will bring up the total density to something greater than $0.3 \rho_{crit}$). Of course, this doesn't mean that the observed density is even close to $\rho_{crit}$. As far as I know, a flat universe has not been ruled out. But also, as far as I know, they have not measured the total density of the universe to be even close to the critical density.

Maybe they have measured the density of the universe with greater accuracy than I thought. I have not read any recent journals. Only textbooks which are half a decade old. Do you know much about this? I might do a google search...

11. Mar 26, 2013

### BruceW

I thought that in a universe without matter, there is still 'non-trivial' solutions. And that the maximally symmetric solution is a 'de Sitter universe'

12. Mar 26, 2013

### Staff: Mentor

There are more solutions, but I don't see any reasons why this space should have any significant deviation from the trivial solution.

13. Mar 26, 2013

### WannabeNewton

I think there's a wording misunderstanding here. Bruce you are correct that a region of space time can have curvature even if there is no local mass energy distribution (vacuum) given there is mass energy somewhere else. This curvature is codified by the Weyl curvature tensor. The most familiar example is of course the Schwarzschild solution. I think mfb was talking about no mass energy anywhere in space time.

14. Mar 26, 2013

### soothsayer

Really? I've had several Cosmologists tell me that verified observational evidence has put Ω at 1 to within a surprisingly small experimental error.

15. Mar 26, 2013

### BruceW

I think that is only true if we assume inflation. I might be wrong.

16. Mar 26, 2013

### BruceW

Yeah, I mean the same as mfb. That there is no matter anywhere. So then there is only dark energy. And the maximally symmetric solution (of a universe with no matter anywhere) is a 'de Sitter universe'. Is this right? I don't know much about GR and cosmology. Always learning though :)

17. Mar 26, 2013

### WannabeNewton

yeah, see paragraph 2: http://en.wikipedia.org/wiki/De_Sitter_space

Or see problem 3 of chapter 5 in Wald (in particular part b), if you have access. It would be more exciting than reading a wiki article haha.

18. Mar 26, 2013

### petm1

What is space, like the question what is time, can no longer be answered one without the other. Myself I have been wondering what part of time is space, other than the present we all share.

19. Mar 27, 2013

### soothsayer

I'm pretty sure you're wrong about that. I don't see anything having to do with Inflationary assumptions factoring in to the observational methods that astronomers are using to determine Ω. WMAP uses CMB anisotropies to determine the geometry of the universe based on the FLRW model alone, and the results are within 0.4% of flat.

http://arxiv.org/pdf/astroph/0302209.pdf

Last edited: Mar 27, 2013
20. Mar 27, 2013

### soothsayer

Also, Bruce, remember that the main reason Inflation was theorized in the first place was to explain why it was that the universe seemed to be so flat. It wasn't a "I believe Inflation is correct, so the universe MUST be flat" sort of thing, it was a "The universe is so flat that we need some sort of theory that can explain why", and that's how Inflation was born.

http://en.wikipedia.org/wiki/Flatness_problem