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Thread: If the universe is spatially finite and flat, could you fly 'out of space'?

  1. #1 If the universe is spatially finite and flat, could you fly 'out of space'? 
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    Ever since reading Lawrence Krauss' book 'A Universe From Nothing', I've been very curious about something. We have apparently determined that the universe is flat, meaning that if a light beam (or presumably any other object moving in a straight line) will never return to where it began. And we also know that space is expanding. To properly convey what I want to know, I'll use a thought experiment.



    Let's say you blasted off from earth at trillions of times the speed of light, such that you were moving through space faster than the space in front of you was expanding. And let's for the sake of it discount any time dilation effects. My question is, what would happen eventually? If the universe is finite and flat, would you eventually run out of space and literally fly into nothingness? Or am I thinking about it wrong altogether? I suspect I probably am, but can someone clarify?


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  3. #2  
    mvb
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    Nobody knows for sure. After all, we cannot even in principle see that far. If there is an edge, it is well beyond the distance that light can have reached us even if it left at the time of the Big Bang. However, we know that an infinite universe is consistent with the physical laws we know, and no one has had an idea of how to handle, much less produce, any "edge to the universe." Therefore, the easiest guess is that there is no edge.

    Please note that your "thought experiment" is impossible as far as anyone knows. [I'm sure you knew that, but just in case anyone else doesn't or prefers to ignore that liklihood.]


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    Yes, I know. However, there is seemingly no reason why the expansion rate of the universe (the value of dark energy) has to be what it is, and in principle there could be universes out there in which the value of dark energy is almost zero, in principle eliminating the need to faster than light travel in my thought experiment, as such a universe would by definition be expanding very slowly and be nowhere near as big as our universe is. We simply don't know, as you said.
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    Just to add my two cents, I expect that our experience in respect to the universe is similar to that of a being within a black hole. There simply is no direction that leads out. "You can't get there from here."
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    We have apparently determined that the universe is flat
    That isn't really correct. We determined only that the region of the universe which we can observe is very nearly flat ( within the observational error margin ). The thing is that the universe may be much much larger than the region we can observe; it may appear flat to us simply due to its size, much like the earth appears flat to us since its radius of curvature is very much larger than most objects around us.
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    As I understand it, it is possible, for the universe to be flat but still curved back on itself. I think the surface of a torus, for example, is flat in the Euclidean sense (e.g. the angles of a triangle add up to 180, the fifth postulate is true) but it is of finite area and unbounded and if you head off in some (all? maybe not) directions you will end up back where you started.
    ei incumbit probatio qui dicit, non qui negat
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    KJW
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    Quote Originally Posted by Strange View Post
    I think the surface of a torus, for example, is flat in the Euclidean sense (e.g. the angles of a triangle add up to 180, the fifth postulate is true)
    The integral of the curvature over the entire surface (a topological invariant) is zero, but the local curvature of the surface of a doughnut isn't everywhere zero (the Gaussian curvature is negative in the vicinity of the hole and positive around the outer rim). However, I suspect that the zero integral of the curvature over the entire surface is what allows a flat surface to have the topology of a torus.
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    To make the thought experiment possible in theory, you would have to suggest your space craft was travelling a little slower than light speed. If that was the case, then the expansion of the universe is fast enough to ensure that, even if an 'edge' existed, the spacecraft would never reach it. This hypothetical 'edge' would recede at a faster rate than the space craft could travel.

    Note that there is no theoretical barrier to two objects moving apart at greater than light speed, as long as there can be no energy, matter, or information travelling between them at more than light speed. It has been realised for a long time that some parts of our universe, at vast distances apart, have a relative velocity (relative to each other) greater than light.
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  10. #9  
    KJW
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    Quote Originally Posted by skeptic View Post
    Note that there is no theoretical barrier to two objects moving apart at greater than light speed, as long as there can be no energy, matter, or information travelling between them at more than light speed. It has been realised for a long time that some parts of our universe, at vast distances apart, have a relative velocity (relative to each other) greater than light.
    It is actually the local speed of light that is the relativistic speed limit. It is possible for the light from a distant object never to reach us, that is for our future world-line to lie entirely outside the future light-cone of the distant object (or alternatively, that the object lies entirely outside our past light-cone). In this case, the object lies on the other side of a horizon relative to us. This is the more correct way of saying that a distant object is travelling faster than the speed of light relative to us, assuming that the object is not travelling faster than its local speed of light.
    Last edited by KJW; July 30th, 2013 at 04:47 AM.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by skeptic View Post
    To make the thought experiment possible in theory, you would have to suggest your space craft was travelling a little slower than light speed. If that was the case, then the expansion of the universe is fast enough to ensure that, even if an 'edge' existed, the spacecraft would never reach it. This hypothetical 'edge' would recede at a faster rate than the space craft could travel.Note that there is no theoretical barrier to two objects moving apart at greater than light speed, as long as there can be no energy, matter, or information travelling between them at more than light speed. It has been realised for a long time that some parts of our universe, at vast distances apart, have a relative velocity (relative to each other) greater than light.
    That completely changes what I'm trying to figure out. Fine, let's say that we're in a hypothetical universe that is much smaller than ours and which is expanding at around 50% the speed of light. Then what? Could you in principle 'catch up' with the universe? What would that even mean?
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    Quote Originally Posted by Fanghur View Post
    That completely changes what I'm trying to figure out. Fine, let's say that we're in a hypothetical universe that is much smaller than ours and which is expanding at around 50% the speed of light. Then what? Could you in principle 'catch up' with the universe? What would that even mean?
    It would have to be a very tiny universe.
    We can see objects from earth using telescopes that are receding at many times the speed of light.

    "expanding at around 50% the speed of light" I asume you mean the edge is moving away at 50%
    Objects nearer you will be moving more slowly.

    If space ends at the edge of the universe, then there is no room/space for your spaceship outside the universe.

    If space continued and not stars, galaxies or matter, then the edge conditions would be interesting.
    The gravity from the entire mass of the univere pulling you back
    I believe in nothing, but trust gravity to hold me down and the electromagnetic force to stop me falling through
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  13. #12  
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    To Fanghur

    I know this is not the answer you want, but realistically, it is all you are going to get.

    Quite simple, a thought experiment that requires we break the fundamental laws of physics becomes meaningless.

    You might as well say "if I had a magic wand, then......."

    If your thought experiment requires travel at FTL, or changes the basic nature of the universe, then there is no realistic result possible.
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    Zogg From Betelgeuse Explains Why the Universe Has No Edge

    i don't know whether this series of videos will help.
    Sometimes it is better not knowing than having an answer that may be wrong.
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    I am happy to participate in the journey with you all. I am, also, thankful to share. I have not read Mr. Krause's book. Based on context, he offers that the universe is "flat." I believe that this measurement of restrictive dimension defies the concept of expansion in a strong degree. This is not to suggest that expansion cannot occur within the restriction of such a plane. However, the inquiry is distinctly relative to the circumstances participating within the measurement, and equally pertinent, the nature of each colliding objects.
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    Pardon the bump admin
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  17. #16  
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    If it were possible to fly off the edge of the universe, then almost all of the Big Bang Theory about the early universe would fail. Instead of the constricted space producing giant amounts of pressure, so much that matter as we know it couldn't even form, a lot of that pressure would have been relieved by stuff falling over the edge.


    [wild conjecture]
    . .... either that or maybe stuff falling off the edge provides an alternative to the inflation hypothesis? Instead of the universe needing to expand rapidly, it was expanding slowly and stuff was falling off the edge?
    [/wild conjecture]



    Quote Originally Posted by PetTastic View Post
    Quote Originally Posted by Fanghur View Post
    That completely changes what I'm trying to figure out. Fine, let's say that we're in a hypothetical universe that is much smaller than ours and which is expanding at around 50% the speed of light. Then what? Could you in principle 'catch up' with the universe? What would that even mean?
    It would have to be a very tiny universe.
    We can see objects from earth using telescopes that are receding at many times the speed of light.
    I was under the impression that the "Hubble Sphere" was supposed to be when the rate of something receding exceeds the speed of light and so the light can never arrive.
    Some clocks are only right twice a day, but they are still right when they are right.
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