what is space?

[sanctus]

I often hear theories that state that space is finished, that there are borders to our universe and therefore to space. Isn't that contradictory, I mean to say something is limited doesn't it imply that there is something further. In other words if we say space ends here at the point A, it implies that there is a point A+e (e as small or big as one wants to) in which there is no space, but the existence of a point A+e implies that there is space there and therefore space is not finished in A.

So what is space and how can it be limited?

[Truth]

Keep it simply.Space is just in how many possible directions you can go or travel.LxWxH

[Freethinker]

I am not sure of what "space is finished" is intended to mean. I don't see where it would fit into any of the generally accepted cosmological theories. Perhaps with the exception of some steady state theory. But I do not know of any steady state theories that have serious acceptance.

This does not mean that there is not a question as to what is outside of the "space" which the universe has expanded to at this point.

But the question would seem to be meaningless to the extent that anything that would be outside of our physical existence would exist in such a state as to not correlate to anything we experience in our physical world. Perhaps the only interaction of concern would be in inflationary multi-universe models where mutually exclusive universes are rushing at each other at the speed of light and upon intersection, one destroys the other.

But the question of "what is on the other side of the boundary of our universe" might not have any valid answer.

[Tormod]

It all depends on the definition of "universe", as well as what the universe looks like.

If we say that the universe is everything, then there is nothing on the outside, since that too would be a part of the universe.

If the universe is part of a larger ensemble of universes (say, one of many), then outside our universe could be either another universe (surrounding ours) or some kind of strange space in which all the universes float. The "multiverse" theory (or rather, theories) is based around the idea that our universe is simply one of many, perhaps infinitely many.

Also, "finished" is not the right term here. I think you meant to write "finite".

Consider this simplification: Imagine that our universe is the surface of a large bubble. Wherever we look, we will see the universe continue in every direction - north, south, east, west (but not up and down). No matter where we move on this bubble the horizon would be equally distant from us, just like a ship in the middle of the ocean. The distance to the horizon is determined by the speed of light and the expansion rate of the universe, which limits what we can see.

The universe would look the same in all directions. You could travel acrosse the surface infinitely, but a globe has a finite surface.

If you expand this idea into the third dimension, you get what we see today: space is the same in every direction we look. This is why the Milky Way appears to be in the middle of the universe.

Since we cannot see beyond the current observable universe, we have no way (as of yet) to determine what is outside of it. Most likely, it is simply more of the same.

Most theories I have read suggest that the universe may be endless yet have a finite size, which sounds like a contradiction but is more like a geomtry puzzle.

As to what "space" is, it is a different question altogether.

Tormod

[Bo]

"What is space-time" is a question i have asked several times now to people much more learned then i am (Brian Greene, Robert Dijkgraaf, Noble prize winner Gerard 't hoofd, and some other less famous guys) And nobody was able to give me a satisfying answer... So i wont try it here

as to what the finite size of the universe means: Space time is infinite. But think of 2 photons travelling with the speed of light in opposite direction. They where created at the big bang an had no interference whatsoever. their distence, taking the expension of space time in account (by inflation etc. See my post in ad hoc inflation topic) is taken as the 'size of the universe' By no way whatsoever you can go beyond that point. And beyond no particles/fields/whatever can exist; because they simple dont have the time to travel to that region since the big bang. -Bo

[TeleMad]

The Universe can be larger than the "photon traveling distance" because of inflation. During inflation, the Universe expanded at speeds greater than c. After that brief burst of exponential expansion, the Universe settled down to "standard" expansion, but it had already exceeded the "photon traveling distance".

[sanctus]

Yes Tormod, I meant space is finite. And my question is exactly what you, Bo and Fretthinker said that there is no valid answer. To put it in tormods words, what is outside the "4dimensional " bubble? What happens if we somehow manage to get out of its surface?

[Tormod]

Originally posted by: sanctus
Yes Tormod, I meant space is finite. And my question is exactly what you, Bo and Fretthinker said that there is no valid answer. To put it in tormods words, what is outside the "4dimensional " bubble? What happens if we somehow manage to get out of its surface?

My point is that the bubble idea is simply a way to imagine the universe. Our universe is NOT the bubble but the surface of it in a lower dimension. There is nothing on the outside ("above" the surface) and there is nothing inside the bubble (the bubble has no inside).

The universe is not expanding into anything - rather, space-time is what constitutes the universe. As the universe expands, so space-time expands.

Tormod

[Bo]

Telemad wrote:

The Universe can be larger than the "photon traveling distance" because of inflation.

as ive said: i meant the distence the photon travelled with inflation taken into account.

Sanctus wrote:

Yes Tormod, I meant space is finite. And my question is exactly what you, Bo and Fretthinker said that there is no valid answer.

Let me be clear here: I (and many people with me) dont know a valid answer. That doesn't mean there is no valid answer.
Bo

[BlameTheEx]

There is one interesting possibility based on the hubble constant. This is the ratio of distance of distant objects to the average velocity they are travelling away from us. To put it another way, how fast the universe is expanding. The exact figure is not know, but here are some estimates:

"The current value of the Hubble constant, denoted, is hotly debated, with two opposing camps generally getting values near the high and low ends of 50 and 100 km s-1/Mpc (where Mpc is a megaparsec, equal to 106 parsecs ). Using infrared observations of Cepheid variables , Madore (1992) obtained 83±13 km s-1/Mpc. Using HST observations of Cepheid variables in the Coma cluster of galaxies, van den Bergh (1995) obtained 81± 8 Km s-1/Mpc. Sandage et al. (1996) hold out for a small value of 57± 4 km s-1/Mpc using supernovae "

But is the universe expanding? Observations of the most distant galaxies have produced troubling results. First the hubble constant is apparently rather more constant than expected. Also these galaxies are rather older than expected.

What if the hubble constant were a property of space, and the expansion was just apparent rather than real? The galxies at the (to us) edge of the universe would the be much the same as local ones.

We now have an interesting theory of space. The universe could be described as a singularity, with each point in it defined by the direction of zero velocity.

Put it another way. Space has 4 dimensions with the 4'th being time. We have always guessed that there is no real difference between them. Time is just the direction we are travelling in. Ok. So every point in space has a specific direction for time which defines it. The further away, the great the difference in direction.

A space rocket launched into deep space, with an initial boost, will not travel for ever (assuming it didn't hit anything), but will come to a halt when it reaches V/H Mpc where V is its initial velocity in kms-1 and H is the hubble constant. A photon will travel rather further (same assumption) but reaches an end at C/H Mpc where C is speed of light in kms-1. At that distance Time will be travelling in the opposite direction, and the red shift will be total. The photon is left with zero energy. This would define the size of our universe from one (arbitrary) end to the other.

There are problems with this idea. If light is red shifted just by travelling from A to B, then where does the energy go? Worse a standing wave should also be red shifted over time. It is travelling at the speed of C, just back and forth over the same spot. Fundamental particles, such as electrons are probably no more than standing waves. Unlike photons they can't loose their inherent energy smoothly.

Still I can't help feel that this idea is worth pursuing. Can anybody think of a way of salvaging it?