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[Innominate]

General point: Space is the volume the Universe exists in. Space is the nothiness, finite or infinite. The Universe is everything else in Space - again, finite or infinite. This is my PoV.

I you draw parallels with the balloon, imagine a reverse balloon. Or a balloon in space, if you want. Space is the balloon, the air in it is the universe. It's not so much the air pushing against the balloon, it's the balloon allowing air to be as large as it can stretch.

What does it mean to be "outside the universe"?

[DreamThorn]

Well, using the balloon example, it actually goes like this:

The balloon is the universe.  The space inside the balloon does not exist.  All space and time and everything in it is on the surface of the balloon.  The balloon is expanding, but, for all points on the balloon, everything else is moving away, therefore, all points on the balloon are the center of the expansion.

This works equally well for infinite planes, hyperboloids and the actual 'shape' of the universe.

So, all points in the universe/space-time are the center of the universe and none of them are special.

[Neruz]

Precisely.


The reason we use the Balloon example is to try and show the difficulty in dealing with more than 3 dimensions. In the Balloon example we pretend a 2 dimensional universe exists on the surface of the balloon, which is expanding in three dimensions. But to a 2 dimensional being in that balloon universe, he would be completely unaware of this expansion and merely see all the other objects in his universe moving away from him.

The interesting bit is that this is exactly what we see in our Universe, except it's in three dimensions, not two. (Possibly more, i havn't dived into the physics of time enough to know if things are expanding on a temporal axis as well as a spatial one. Considering time is a part of the Universe as much as space is, i would assume this is the case.)

[Sean Mirrsen]

That would be true for an infinite universe in a finite looped space, but not for a finite universe in an infinite space.

[Il Palazzo]

Somewhat off the current topic of conversation:

I used to concern myself with a similar problem Sean described, namely finding the one set of frames of reference, in which there would be no inertial forces(e.g.Coriolis' force). Or, in other words, when can I say that I'm not rotating? It's pointless to refer oneself to other bodies in space, as they could be rotating with me. Perhaps applying a super(ideal)-accurate measurement tool to find out that there are no inertial foces at work? But then, doing so would single out some frames of reference as special, no? This wouldn't be so bad, I guess, after all, SR concerns itself only with inertial frames of reference... but what does the general relativity say? Is it meaningfull or meaningless to look for the non-rotating frame of reference?
I hope I made some sense.

(to clarify, for those who are not in the know, inertial forces are forces which, somewhat confusingly, appear in the non-inertial reference frames, like the force pushing you onto the side of a car on a road bend, when you pick the car as your frame of reference)

[Starver]

And finally, why does everyone use the words Universe and Space like they're interchangeable? What, the concept that the Universe exists within the Space is that impossible? It would even explain acceleration of the universe's expansion.

In my concept of the universe (may or may not chime with more learned exponents of cosmological theory) you can envision the unviverse as the surface of a balloon, as can you the 'space' as we know it.  The three space dimensions and one time of our common experience (and maybe more) being analogised into the two dimensions of the balloon surface.  And while the balloon surface curves in a third dimension, this is not relevent to the perceivable universe and what we call 'space', except perhaps as something the universe 'curves' into.  'Spacetime' in the 'Universe' [ii]is[/i] the surface of the balloon.  Both of them the same (and maybe more of the dimensionalities).

Maybe gravitational bending of space time is depressions (or bumps, there's virtually no difference in topology at the small scale at least[1]) upon the surface of the Universal Balloon Surface, or maybe that's some other quality to the surface (much as electricla potential, etc, might be, although ultimately it all probably unifies) and that surface topology means something else (maybe the texture dictates the 'unificication' quality), especially as frames of reference carry their own interpretations of various different potentials.

While traditionally the surface-of-the-balloon example is used to show expansion of the universe (by pumping up the balloon) pushing galaxies apart without them 'travelling' over the surface, an interesting by-product of the analogy is that you can instead imagine it as a 'static' history-encompassing model of the universe where 'latitude' on the balloon's surface as represents time, and the longitude (a single dimension in our "balloon", but representing all other dimensions in the 'metaverse' version.

In this version you can see 'slices' of the universe for any given 'universal' time.  (Noting that you can also cut your time slices diagonally across the surface, or perhaps represent them as an intersection of a conic or double-conic shape, its origin possibly at the North Pole but angled accordingly, which would give numerous 'slices' across the universe that may be indistinguishable to slice inhabitants.

The slice intersecting the North Pole of the 'balloon' shows a singularity that is 'the start of time', as does the 'South Pole'[2] which might be your 'end of the universe', complete with Restaurant if you so wish.  Everywhere inbetween is a ring of universe, without border but finite, causality (trivially) feeding southward, though at every point other than at either pole causality can 'head' directions generally southward but not directly south.

(The analogy might break down when you consider the geometry of non-longitudinal Great Circle journeys, which at their northern and southern extremes find themselves angling past the 'causality limit' angle, and eventually back northward, but imagine a 'drip down' force that probably explains 'expansion' and any inexorably leading into the end-of-time contraction.)

Additionally, considering time in such a way (as lines of latitude, or similar) and considering the Universal Balloon as an entity sat within the (out of our experience[3]) meta-universal dimensions, it means that the the question "What Is North Of THe North Pole?" is very much an apt response to the age-old "So What [Caused|Happened Before] The Big Bang?" question that Big Bang sceptics tend to roll out.


But maybe hyperdimensional systems aren't your bag, or you can't envision them.  If so, sorry.  And I hope you haven't read all the above for nothing.



[1] Though if the above is a good analogy, a sufficiently large scale (say across 1/4 the size of the universe, in all dimensions including time) might reveal whether any distortions such as this bulge 'in' or 'out' of the nominal 'meta-space' balloon shape.

[2] Although if you're not too keen on the "Big Crunch" ending of the universe and think it expands forever, the balloon could be considered an infinite 'bell' shape, with no lower extreme.

[3] The whole M-Dimensions theory where we are 20 dimensions in 21 (or however many it is, these days) would fit that.  A 20-dimensional 'balloon surface' in a 21-dimensional meta-universe.  And with who knows how many other universal balloons are eternally sat there, differing sizes and compositions in each bubble, with wildly different bubble surfaces giving universes variously viable for intelligences like ourselves or totally unlike ourselves or totally devoid of intelligence or even totally devoid of 'features'.  Now, that's something worth speculating about.  In my opinion, of course.

[Starver]

I you draw parallels with the balloon, imagine a reverse balloon. Or a balloon in space, if you want. Space is the balloon, the air in it is the universe. It's not so much the air pushing against the balloon, it's the balloon allowing air to be as large as it can stretch.

Ah, now, first of all you wrote this before I wrote my own 'balloon' analogy, so I hope there's no confusion there.

Secondly, the usual analogy is that the universe is the balloon (surface) and that 'meta-'space is the air within and without the balloon itself.  But space==balloon.  Which indicates that you've not seen too many Royal Institution Christmas Lectures or locally equivalent educational broadcasts on the subject.

[Starver]

That would be true for an infinite universe in a finite looped space, but not for a finite universe in an infinite space.

That probably depends on your definition of 'space', and I don't know which one you're using (your original or the conventional, or possibly the 'meta-space' one).

[Starver]

...frames of reference, in which there would be no inertial forces(e.g.Coriolis' force)...would single out some frames of reference as special, no?

Briefly (there's a lot that can be said) my opinion is that there are an unlimited number of frames of reference not locally perceived as accelerating or rotating which are 'special' in this way.  i.e. they really aren't that special.  Unless you want to argue that there is a greater (unlimited times the original unlimited number, I suppose) amount of total frames which are 'non-special' in this manner.  But that's Aleph-Nul and Aleph-Prime territory.  You still can't call the number 2 any more special than the number 5, 101 or (2^43,112,609)-1 if you consider prime numbers to be special.  (There may be other reasons why 2 is more special, like being the only even prime, or the smallest[1], or the 'first', but that's outside the scope of this analogy.





[1] If you don't accept '1' as a prime, from either the POV that it is not a 'constructive' prime (multiplication by 1 does not yield a new non-prime) or as not having exactly two distinct integer divisors (1 and... nothing else in the case of '1').

[dreiche2]

@Ampersand

What "there is no universal frame of reference" means, is that there is no such thing as standing still, i.e. there is no such as thing as "moving only in time".

What is time in one frame of reference would be a combination of space and time in another frame of reference, unless the two frames of reference are stationary relative to each other.

Maybe you should read up on Lorentz transformations for more clarity on what I am saying above.

There is no one special space-time.

Yeah, moving along the time axis just means being stationary with regards to the frame of reference the time axis refers to. It's relative.

I used to concern myself with a similar problem Sean described, namely finding the one set of frames of reference, in which there would be no inertial forces(e.g.Coriolis' force). Or, in other words, when can I say that I'm not rotating? It's pointless to refer oneself to other bodies in space, as they could be rotating with me. Perhaps applying a super(ideal)-accurate measurement tool to find out that there are no inertial foces at work? But then, doing so would single out some frames of reference as special, no? This wouldn't be so bad, I guess, after all, SR concerns itself only with inertial frames of reference... but what does the general relativity say? Is it meaningfull or meaningless to look for the non-rotating frame of reference?
I hope I made some sense.

But acceleration (which causes these inertial forces) is not relative, and this includes rotation. You can measure it from inside your frame of reference without referring to anything external at all exactly because of these forces.

The basic relativity principle is really nothing about Einstein, just Galileo. If you don't look outside, you cannot measure your own velocity. You can, however, measure your acceleration.

Like you said, SR does not involve non-inertial frames.

Is it meaningfull or meaningless to look for the non-rotating frame of reference?
I hope I made some sense.

But the non-rotating, non-accelerating frames are exactly the inertial frames, by definition, so of course it makes sense to talk about them (and of course, there are infinitely many of them all moving relative to each other).

[Il Palazzo]

Oh well. I had a feeling it was a silly question.

[Starver]

The basic relativity principle is really nothing about Einstein, just Galileo. If you don't look outside, you cannot measure your own velocity. You can, however, measure your acceleration.

Which may be due to rate of change of motion or a gravitional field (or, of course, both together).  The equivalence of which  is another part of relativity.  (c.f a perfect freefall in a plummeting lift and the same setup drifting in space without any other masses [anywhere!] acting on you).

(Of course, you could always measure the tidal difference between furthest and nearest point to any assumed mass, or a 'parallax' angle of pull across the perpendicular, with sensitive enough equipment, I would say.)

But I feel that this is an order of pedantry beyond what's necessary, and yet still short of the Ultimate Truth.

[Virex]

To toss a completely different but also related ball into the ballpark:

Newton's laws acceleration and gravity appear when thermodynamics, holographic theory and information theory are combined:
http://motls.blogspot.com/2010/01/erik-verlinde-comments-about-entropic.html

This means that gravity could very wel be an entropic force that only exists on the macroscopic scale, and has at least a very different meaning on the quantum scale.

[Neruz]

Well we already know that Gravity is the odd one out of the forces.

[eerr]

To toss a completely different but also related ball into the ballpark:

Newton's laws acceleration and gravity appear when thermodynamics, holographic theory and information theory are combined:
http://motls.blogspot.com/2010/01/erik-verlinde-comments-about-entropic.html

This means that gravity could very wel be an entropic force that only exists on the macroscopic scale, and has at least a very different meaning on the quantum scale.

Don't you mean entropic on the quantum scale, but averaging as gravity on the full-scale?