If the universe was round?

[tpeezy]

Once again here I am with another moronic question so please bear with me.....Oh by the way this is totally hypothetical, so please dont say this is impossible blah blah blah......ok here it goes. If somehow the entire universe was round like the earth, and somehow I know this is impossible, but somehow if we had a telescope to see all of the way around the universe and look at ourselves from behind, would we be able to see earth in the past, or back in time like the big bang or whatever because that would be as far as you could look without coming full circle. Please just bear with me on this question I know its dumb but I was just curious.

 

[ramparts]

Well, "round" is an extremely vague term, but the thing I think you're thinking about - where the Universe is curved in three dimensions the way the Earth's surface is curved in two - isn't impossible at all. Well, it's almost certainly ruled out by observations as of only about 10 years ago, but it's theoretically possible, an important model of the universe called a closed universe.

If the Universe had this shape, you would in fact be able to see back in time (or see objects in space at multiple times) if the radius of curvature were small enough that light would have had time to make a complete circuit (it isn't; we done a search for just those kinds of repeated images). Wiki has a good article on the shape of the Universe, and this particular section is especially relevant:

http://en.wikipedia.org/wiki/Closed_uni ... l_universe

 

[tpeezy]

its not the shape that really concerns me in the question, lets assume it was round just for the sake of the question. If u were able to look around it until you end up seeing the back of your head, that is if u were there. would you be able to see earth back in time, or where the earth is now but hadnt yet been created? Like lets just say again for the sake of the question this is all theoretical now, lets say that the round universe was 10 billion light years long, I know its not but lets jusay it is for this question, and you looked all the way to the end and back around so that you could see the back of your head. I know this too is probably impossible, but if it was would you be seeing earth back in time, or would it just be the back of your head that you would indeed see?

 

[BlackHoleAndromeda]

Hmm... So if the universe is 3-Dimensional, which there is no doubt in my mind that it might be, then perhaps it rotates as well. But even in the awesome realm of space, even if you could see the Earth from the Earth :shock: :? , I would argue that the only history you would see is where this galaxy was about 1.2 billion years ago.

 

[tpeezy]

why 1.2 billion years?

 

[BlackHoleAndromeda]

why 1.2 billion years?

Well don't quote me on that, that's just an imaginary estimate. I would imagint that even that would be a significant venture into the past.

 

[ramparts]

its not the shape that really concerns me in the question, lets assume it was round just for the sake of the question. If u were able to look around it until you end up seeing the back of your head, that is if u were there. would you be able to see earth back in time, or where the earth is now but hadnt yet been created? Like lets just say again for the sake of the question this is all theoretical now, lets say that the round universe was 10 billion light years long, I know its not but lets jusay it is for this question, and you looked all the way to the end and back around so that you could see the back of your head. I know this too is probably impossible, but if it was would you be seeing earth back in time, or would it just be the back of your head that you would indeed see?

In theory you could see the Earth back in time, if the curvature of the Universe were strongly curved enough. You wouldn't be able to see the back of your head unless either the Universe were very very small (about the size of the Earth) or you stood still for a very long time! Remember that light travels at a finite speed, so if you were to see the Earth you wouldn't see it in its present state since the light would have to travel all the way around the Universe before getting back to you. The problem is that if the Universe is curved in this way, it's so big (that is, the curvature is so small) that the light from any one place would take longer than the age of the Universe to return to where it was emitted.

 

[emperor_of_localgroup]

Once again here I am with another moronic question so please bear with me.....Oh by the way this is totally hypothetical, so please dont say this is impossible blah blah blah......ok here it goes. If somehow the entire universe was round like the earth, and somehow I know this is impossible, but somehow if we had a telescope to see all of the way around the universe and look at ourselves from behind, would we be able to see earth in the past, or back in time like the big bang or whatever because that would be as far as you could look without coming full circle. Please just bear with me on this question I know its dumb but I was just curious.

I always see this type of questions about 'seeing our past' by telescopes or by other means.
Telescopes use incoming light to observe objects, how can we observe receding light (our past) with telescopes? Am I missing something here? To observe our past, we must have something that travels 'faster than light'.
If you are saying, if the universe is round past light from earth will be reflected back into the universe for us to observe? Then the obvious question will be asked "do we know the edge of the universe is made of reflecting materials?"

I agree with 'BlackholeAndromeda', the universe is probably round and it's spinning. But the size of the universe is 'unthinkable' but 'not infinite'.

 

[ramparts]

Well, emperor, imagine that light were constrained to the Earth's surface. Then by looking far enough, we'd see "around" the world and back in time....erm, 2.5 seconds or so, I believe.

Anyway, that obviously doesn't work because light isn't constrained to the Earth's surface. But if the universe were "round" and were sufficiently small, kind of like a 3D analogue of the Earth's surface (which is 2D) then just this thing would happen, because light is constrained to move along least-time paths in spacetime. The fact that we don't see repeated images of anything (or ourselves in the past!) means that either the universe is flat, or it's curved but has such a huge radius of curvature that light hasn't had time to make it around since the Big Bang.

Make sense?

 

[emperor_of_localgroup]

Well, emperor, imagine that light were constrained to the Earth's surface. Then by looking far enough, we'd see "around" the world and back in time....erm, 2.5 seconds or so, I believe.

Anyway, that obviously doesn't work because light isn't constrained to the Earth's surface. But if the universe were "round" and were sufficiently small, kind of like a 3D analogue of the Earth's surface (which is 2D) then just this thing would happen, because light is constrained to move along least-time paths in spacetime. The fact that we don't see repeated images of anything (or ourselves in the past!) means that either the universe is flat, or it's curved but has such a huge radius of curvature that light hasn't had time to make it around since the Big Bang.

Make sense?

Oh, you are thinking or saying, we are on the surface of a round sphere (universe)? . Not inside the sphere?
Good luck with your expanding balloon. ha ha, I get it now.

 

[yevaud]

Ramparts is precisely correct. This very thing was once theorized on by Kurt Godel, who showed that were the universe in the condition(s) stated, then one could travel in time by circumnavigating the periphery of the universe. All other manners of a-causal events would also be permissible. None have ever been seen. Looks like Godel was right.

As to the "surface of an expanding sphere" analogy, please understand that the universe really isn't a bubble or balloon in three (well, four) dimensions: it's a topologically higher order (higher dimensions) "shape." The "expanding balloon" concept is just a visual aid, but doesn't actually represent reality.

 

[ramparts]

Mmm, this is the trick. With the Earth's surface there is an inside, but with the Universe there doesn't have to be. In fact, mathematically speaking we expect there not to be. A surface can have something called intrinsic curvature, meaning it acts like a curved surface but the curvature is part of the surface, not embedded in some higher dimensional space. That's the main problem with the balloon analogy - if you consider two dimensions of the Universe, it will act kind of like the surface of an expanding balloon, but there's no inside of the balloon, and no air that the balloon is expanding into. The surface of the balloon is all there is.

 

[yevaud]

Exactly so.

I recollect in earlier String theories (Heterotic String Theory), the topology of the universe in 10 dimensions was an orbifold, with four dimensions in full expression, and the other six "compactified." Curiously, the four-dimension visual of this was rather like a conic section.

 

[emperor_of_localgroup]

As to the "surface of an expanding sphere" analogy, please understand that the universe really isn't a bubble or balloon in three (well, four) dimensions: it's a topologically higher order (higher dimensions) "shape."

You presented this statement as if this is the ultimate truth about the universe'

When in reality, no one knows the true nature of the universe, shape, size, whatsoever.

 

[Balthazar579]

Mmm, this is the trick. With the Earth's surface there is an inside, but with the Universe there doesn't have to be. In fact, mathematically speaking we expect there not to be. A surface can have something called intrinsic curvature, meaning it acts like a curved surface but the curvature is part of the surface, not embedded in some higher dimensional space. That's the main problem with the balloon analogy - if you consider two dimensions of the Universe, it will act kind of like the surface of an expanding balloon, but there's no inside of the balloon, and no air that the balloon is expanding into. The surface of the balloon is all there is.

That makes absolutely no sense. But it explains the very same thing I couldn't wrap my head around for the past 6 months or so. I understand it now...well, not really, but it...now I'm confusing myself. eedless to say, I now understand why we'd be on the baloon, instead of in it, and why there is no inside.

But to me it seems to raise questions. For example, if two points are moving apart due to expansion of the curve, would it be possible for these parts to see each other following this curve and, at the same time, ignoring the curve altogether? Like looking through a baloon?
...
This is the entire topic, isn't it?

 

[Balthazar579]

As to the "surface of an expanding sphere" analogy, please understand that the universe really isn't a bubble or balloon in three (well, four) dimensions: it's a topologically higher order (higher dimensions) "shape."

You presented this statement as if this is the ultimate truth about the universe'

When in reality, no one knows the true nature of the universe, shape, size, whatsoever.

Not really, no. But there are some which theoretical mathematics suggests are most likely. I only remmeber that at all because one such shape is a torus. Yes. It is likely(on a relative scale at least) that the universe is a torus. I'm just as lost as you are.

 

[tpeezy]

just wanted to say thank you for all the input, and most of all thank you for not calling me a moron for asking such a silly question. You guys are awesome, and this forum contains a lot of very mature, and smart people.

 

[yevaud]

As to the "surface of an expanding sphere" analogy, please understand that the universe really isn't a bubble or balloon in three (well, four) dimensions: it's a topologically higher order (higher dimensions) "shape."

You presented this statement as if this is the ultimate truth about the universe'

Not in the least. The universe can be considered to have many different "shapes" (in higher dimensions), depending on how you scrutinize it.

Mathematics and Physics may not be the "ultimate truths," but they generally do work.

 

[ZenGalacticore]

Yeah, um, if the Universe really were a 3-D expanding balloon, then it would be expanding into a "pre-existing" three dimensional space. And that is a contradiction of the definition of "universe".

I think the analogy confuses people more than anything. They forget that the "balloon" analogy means that all the space and matter of the universe is the surface of the "balloon", which was mentioned above. We lose a dimension here because we can't visualize a higher 5th (or 10th) spatial dimension.

We're like the flatlanders inferring a third dimension. It will always be a riddle, an anomaly. The Universe is an expanding super-atom of energy converted to matter and vice-versa. Warped unto itself, and will eventually "swallow" itself, leading to some other kind of incarnation of matter and energy and space. An orgasm of God.

 

[ramparts]

That makes absolutely no sense. But it explains the very same thing I couldn't wrap my head around for the past 6 months or so. I understand it now...well, not really, but it...now I'm confusing myself. eedless to say, I now understand why we'd be on the baloon, instead of in it, and why there is no inside.

But to me it seems to raise questions. For example, if two points are moving apart due to expansion of the curve, would it be possible for these parts to see each other following this curve and, at the same time, ignoring the curve altogether? Like looking through a baloon?
...
This is the entire topic, isn't it?

No, of course it makes no sense Physics is really wonderful. The universe is, unfortunately, under no obligation to conform to our everyday experience.

Since there's no inside that'd be impossible. As far as the points on the balloon are concerned, the surface doesn't look curved. Imagine you're a flat person living on the balloon's surface - you have no idea what up or down, outside or inside mean. Your sight only exists across the balloon's surface. Same applies in the 3D analogy where there isn't an inside.