In the beginning God
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14-03-2014, 06:04 PM
RE: In the beginning God
(14-03-2014 05:46 PM)itsnotmeitsyou Wrote:  I was thinking more about advanced life that exists now and in our galaxy. There's enough time and space that there could have been countless interstellar empires across the universe that never got farther than their arm of their galaxy. Hell, there could be an interstellar war going on somewhere right now that we'll never know about. We've only just directly photographed an exoplanet, and those are relatively close.

Yes, but relatively close takes on a whole new meaning with faster-than-light travel...

(14-03-2014 05:46 PM)itsnotmeitsyou Wrote:  Think about how a species that has FTL travel would spread. They'd colonize the closest habitable planets first. Kepler has found 69 potentially habitable planets in the small bit it has searched. If that's pretty standard, there could easily be an advanced race somewhere in our arm of the galaxy that has no idea we exist because they haven't searched this far yet.

Which is indeed fair enough - if, one is assuming intelligent life to be so rare. If an assumed capability would allow distinguishable contact within a certain distance, then either there is nothing within that distance or the assumption of capability is invalid. Well, but that's just the simplest two answers.

(14-03-2014 05:46 PM)itsnotmeitsyou Wrote:  "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space."
Douglas Adams, The Hitchhiker's Guide to the Galaxy

In short, who knows what's out there and if we'll ever know about it?

"It is known that there are an infinite number of worlds, simply because there is an infinite amount of space for them to be in. However, not every one of them is inhabited. Therefore, there must be a finite number of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is also zero, and that any people you may meet from time to time are merely the products of a deranged imagination."

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14-03-2014, 06:17 PM
RE: In the beginning God
(14-03-2014 01:39 PM)itsnotmeitsyou Wrote:  
Quote:Do we know where the universe ends?

This requires a bit of clarification. When most people talk about "the universe", they're actually talking about the "observable universe". This is the furthest distance from earth that we can see (approx 13.8 billion light years). We can only see this far because light from further out hasn't had time to reach us.

The entire universe (furthest reaches of expansion) is much larger. I can't remember exactly, but I want to say something like 70 billion light years. across.

I have a question:
I've heard this explanation before and often wondered about the "end" of the universe. Though, before we [humans] knew earth was a sphere, we often talked about the "ends of the earth". Couldn't it be possible that the universe is just a giant sphere, and what we are seeing now is just our perception of the curve?
I is not good with universal-physics-mumbo-jumbo-stuff and this is a legit question; don't hate.

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14-03-2014, 07:17 PM
RE: In the beginning God
(14-03-2014 06:17 PM)Im_Ryan Wrote:  I have a question:
I've heard this explanation before and often wondered about the "end" of the universe. Though, before we [humans] knew earth was a sphere, we often talked about the "ends of the earth". Couldn't it be possible that the universe is just a giant sphere, and what we are seeing now is just our perception of the curve?

Well, what they say is true, from a certain point of view.

Whether or not the universe is infinite (though it probably is) is a separate question to whether or not it is bounded. That is a matter of curvature. Closed curvature produces a sphere analogue. Flat curvature produces a plane analogue. Open curvature produces a hyperbolic saddle analogue.

Suffice to say this is quite complicated and very much an open question.

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14-03-2014, 07:38 PM
RE: In the beginning God
(14-03-2014 07:17 PM)cjlr Wrote:  
(14-03-2014 06:17 PM)Im_Ryan Wrote:  I have a question:
I've heard this explanation before and often wondered about the "end" of the universe. Though, before we [humans] knew earth was a sphere, we often talked about the "ends of the earth". Couldn't it be possible that the universe is just a giant sphere, and what we are seeing now is just our perception of the curve?

Well, what they say is true, from a certain point of view.

Whether or not the universe is infinite (though it probably is) is a separate question to whether or not it is bounded. That is a matter of curvature. Closed curvature produces a sphere analogue. Flat curvature produces a plane analogue. Open curvature produces a hyperbolic saddle analogue.

Suffice to say this is quite complicated and very much an open question.

Could you elaborate on the difference between 'infinite' and 'bounded'?
I mean, say the universe is a sphere. Wouldn't it seem infinite to us?

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14-03-2014, 07:43 PM
RE: In the beginning God
(14-03-2014 07:38 PM)Im_Ryan Wrote:  Could you elaborate on the difference between 'infinite' and 'bounded'?
I mean, say the universe is a sphere. Wouldn't it seem infinite to us?

They're two distinct properties. A closed topology is bounded; it may or may not be infinite (consider: on a finite closed surface one might end up where one started). An open topology, if finite, possesses edges.

The universe seems very likely to be infinite, so that's that part settled. Its shape, not so much.

There is then the matter of traversability - which is to say, our (or some other beings') capacity to travel. S'not like we'd be able to go sightseeing to the 'edge' of the universe even if there were one.

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14-03-2014, 07:48 PM
RE: In the beginning God
(14-03-2014 07:43 PM)cjlr Wrote:  
(14-03-2014 07:38 PM)Im_Ryan Wrote:  Could you elaborate on the difference between 'infinite' and 'bounded'?
I mean, say the universe is a sphere. Wouldn't it seem infinite to us?

They're two distinct properties. A closed topology is bounded; it may or may not be infinite (consider: on a finite closed surface one might end up where one started). An open topology, if finite, possesses edges.

The universe seems very likely to be infinite, so that's that part settled. Its shape, not so much.

There is then the matter of traversability - which is to say, our (or some other beings') capacity to travel. S'not like we'd be able to go sightseeing to the 'edge' of the universe even if there were one.

Okay, I think I'm following along...
But, if it was a closed topology, then the 'ends' would have to connect. Right? So wouldn't that make a sphere? Consider
I mean, why would space just 'end'? Does that even make sense? (seriously asking)

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14-03-2014, 07:57 PM
RE: In the beginning God
(14-03-2014 07:48 PM)Im_Ryan Wrote:  Okay, I think I'm following along...
But, if it was a closed topology, then the 'ends' would have to connect. Right? So wouldn't that make a sphere? Consider

Yes.

Only not a sphere 'cause spacetime is a 4 (or N if you like) dimensional manifold twisting onto itself instead of two dimensions folded over within three.

But yeah, that's the analogy.

(14-03-2014 07:48 PM)Im_Ryan Wrote:  I mean, why would space just 'end'? Does that even make sense? (seriously asking)

In the broadest theoretical sense - sure. It could be a finite surface. If it's flat, for instance - a plane is infinite. A disc wouldn't be.

There would then remain to be settled the question of how the boundary would be experienced by whatever happened to be within it. It certainly couldn't be just a big wall; I'd imagine a sort of infinitely curved boundary, if you can conceptualise that. Which is to say, one wouldn't hit a wall so much as only ever asymptotically approach a theoretical limit.

Which would rather be infinite in practice for many purposes. Hmm.

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14-03-2014, 08:21 PM
RE: In the beginning God
(14-03-2014 07:57 PM)cjlr Wrote:  
(14-03-2014 07:48 PM)Im_Ryan Wrote:  Okay, I think I'm following along...
But, if it was a closed topology, then the 'ends' would have to connect. Right? So wouldn't that make a sphere? Consider

Yes.

Only not a sphere 'cause spacetime is a 4 (or N if you like) dimensional manifold twisting onto itself instead of two dimensions folded over within three.

But yeah, that's the analogy.

(14-03-2014 07:48 PM)Im_Ryan Wrote:  I mean, why would space just 'end'? Does that even make sense? (seriously asking)

In the broadest theoretical sense - sure. It could be a finite surface. If it's flat, for instance - a plane is infinite. A disc wouldn't be.

There would then remain to be settled the question of how the boundary would be experienced by whatever happened to be within it. It certainly couldn't be just a big wall; I'd imagine a sort of infinitely curved boundary, if you can conceptualise that. Which is to say, one wouldn't hit a wall so much as only ever asymptotically approach a theoretical limit.

Which would rather be infinite in practice for many purposes. Hmm.

I should've said 'sphere-like', but yeah you get what I'm saying.

This whole 'end' thing is just confusing. Could be like...hmm, I can't think of the word right now, but like in math when you graph a number infinitely divided by two, where there is no 'end' per se, but it just keeps getting smaller?
(sorry for the weird questions, I'm just trying to get a grasp on this)

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14-03-2014, 08:31 PM
RE: In the beginning God
(14-03-2014 08:21 PM)Im_Ryan Wrote:  I should've said 'sphere-like', but yeah you get what I'm saying.

Yep. Spacetime analogies are always two-dimensional. 'Cause that's what we can naively visualise in three dimensions. Balls, balloons, trampolines, planes, donuts, etc.

(14-03-2014 08:21 PM)Im_Ryan Wrote:  This whole 'end' thing is just confusing. Could be like...hmm, I can't think of the word right now, but like in math when you graph a number infinitely divided by two, where there is no 'end' per se, but it just keeps getting smaller?
(sorry for the weird questions, I'm just trying to get a grasp on this)

That'd be an asymptote. Which is, er, what I said.

I don't really know, s'just that that was my first guess. That's a solution set I'm not sure anyone has probed too deeply because it's very unlikely to be applicable to our universe.

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14-03-2014, 08:46 PM
RE: In the beginning God
(14-03-2014 08:31 PM)cjlr Wrote:  
(14-03-2014 08:21 PM)Im_Ryan Wrote:  I should've said 'sphere-like', but yeah you get what I'm saying.

Yep. Spacetime analogies are always two-dimensional. 'Cause that's what we can naively visualise in three dimensions. Balls, balloons, trampolines, planes, donuts, etc.

(14-03-2014 08:21 PM)Im_Ryan Wrote:  This whole 'end' thing is just confusing. Could be like...hmm, I can't think of the word right now, but like in math when you graph a number infinitely divided by two, where there is no 'end' per se, but it just keeps getting smaller?
(sorry for the weird questions, I'm just trying to get a grasp on this)

That'd be an asymptote. Which is, er, what I said.

I don't really know, s'just that that was my first guess. That's a solution set I'm not sure anyone has probed too deeply because it's very unlikely to be applicable to our universe.

Now that I remember the word, yeah you did. My bad Blush
Well at least I feel like I'm grasping the subject...thanks Thumbsup

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