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If You Traveled Far Enough Through Space, Would You Return to Your Start Point?

51 points by rising-sky 8y ago

IIAOPSW 8y ago
No. If you could there would be a few galaxies which would be about equidistant from us along both paths. Therefore we would notice a few double copies of the same galaxies. It would be similar to standing in the middle of two mirrors. IIRC scientists have already ruled out a universe with a "radius" less than some ridiculous number (like billions of light years). There. Just saved you a click. This is old and well know. Why is Forbes writing about it now?
- craftyguy 8y ago
  >Why is Forbes writing about it now? The general public doesn't know this, it's a very slow news week, or both.
  - stcredzero 8y ago
    In the case of things like this, it's as much because people have erroneous ideas left over from outdated science journalism, as it is from people just not having any information or inclination to care. (Sherlock Holmes's reaction to the information that the Earth goes around the sun.)
  - gyre007 8y ago
    > Why is Forbes writing about it now? My totally random guess is the reason why theyre writing about it now is that it might be part of PR for the First Man movie about Neil Armstrong being the first man on the moon.
- heavenlyblue 8y ago
  Nope. Our visible universe is bounded by the expansion of space that happens at the speed of light. That means we can only see a subset of the universe at any time.
  - gpderetta 8y ago
    Not only that, due to expansion, the subset is getting smaller by the second!
  - ikeboy 8y ago
    But if you travel at less than the speed of light you'd never pass that anyway
    - isaiahg 8y ago
      Because of expansion we'll likely never be able to travel to any of the galaxies in the universe except our local neighborhood. But it's still worth studying those galaxies from a distance to learn more about our universe.
    - PopeDotNinja 8y ago
      If you traveled at the speed of light, you'd still never make it to the edge of the universe. The edge of the observable universe is moving away from us faster than the speed of light. [1] [1] https://en.wikipedia.org/wiki/Observable_universe https://en.wikipedia.org/wiki/Observable_universe
  - charlesism 8y ago
    So a beam of light with a head start, half-way across the universe from you, could "wrap around" and eventually reach you. But the beam of light couldn't "wrap around" and reach itself, because the universe constantly moves the goal post before light beam gets the chance?
    - heavenlyblue 8y ago
      Yes. The fact is that does it not prove or disprove the fact that our universe is wrapped around - it may be either way. The parent's point is invalid because we wouldn't be able to see the wrapped around light even if it existed.
  - karmakaze 8y ago
    Expansion is not limited to the speed of light. Two points in space may be 'moving apart' faster than the speed of light because they're not moving, space is being created in-between. The more space there is in between, the faster the rate of expansion between the points.
- empath75 8y ago
  Believe it or not, everyone doesn’t know everything that everyone else knows.
  - deleted 8y ago
    [deleted]
- bmurphy1976 8y ago
  > Why is Forbes writing about it now? Why not? Look who the author is: Ethan Siegel. He has a blog (https://medium.com/starts-with-a-bang https://medium.com/starts-with-a-bang) and a podcast (https://soundcloud.com/ethan-siegel-172073460 https://soundcloud.com/ethan-siegel-172073460). He basically does a Dear Abby type column based on astronomy and physics topics. He takes difficult concepts and tries to break them down into something the lay person can understand. Quite frankly, I think he's pretty damn good at it and we should be happy Forbes is publishing his stuff for a wider audience.
- EGreg 8y ago
  Can you explain how one implies the other?
- jobigoud 8y ago
  > scientists have already ruled out a universe with a "radius" less than some ridiculous number (like billions of light years) How is billions of light years a ridiculous number? Even the observable universe is bigger than that, so the full universe could be many many times bigger. Since you can't see past the observable universe you wouldn't be able to see the double copies.
  - stcredzero 8y ago
    We know from analysis of the cosmic background radiation, that the universe is at least 150 times the size of the observable universe, which puts the minimum diameter of actual universe in the range of 10 to 15 trillion light years. It could be much larger than that, however.
williesleg 8y ago
Sure. The main thing is that space-time is not linear or even understandable. So infinitely traveling through an infinite number of directions and dimensions could intersect with your start point.
scottmsul 8y ago
In cosmology, there's a parameter called "curvature". If the curvature were slightly less than 1, the topology would be "closed", meaning it would be finite, and traveling in a straight line ends back where you started. This is like the surface of the Earth, but in 3 dimensions instead of 2. Triangle angles add up to greater than 180 degrees. If the curvature were equal to 1, the universe would be "flat", meaning it would be infinite and triangles would add up to 180. If the curvature were greater than 1, it would be "hyperbolic", meaning it would be infinite and triangles would add up to less than 180. If I recall correctly, currently the best measurement of the curvature is 1 +/- .02, so it's probably flat. What's interesting is if the shape was flat and the universe were truly infinite, there would be an infinite number of perfect copies of Earth, the closest of which would be within 10^10^23 meters, as noted by Max Tegmark (source: http://space.mit.edu/home/tegmark/crazy.html http://space.mit.edu/home/tegmark/crazy.html)
- jessriedel 8y ago
  > If the curvature were greater than 1, it would be "hyperbolic", meaning it would be infinite There are periodic (finite-volume, homogeneous) hyperbolic spaces. Also, I think you can have infinite-volume spaces with positive curvature if they aren't homogeneous.
- emilfihlman 8y ago
  >Level I: A generic prediction of cosmological inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions - including an identical copy of you about 10^10^29 meters away.
- jacobush 8y ago
  Why copies? By sheer probability or some other mechanism. It indeed is crazy. Is it possible that is the nature of the universe?
- colanderman 8y ago
  Why would an infinite universe necessarily contain infinite matter?
- 8y ago
deleted 8y ago
[deleted]
joh6nn 8y ago
Per Brock et al, 2000, yes: the universe is shaped exactly like the Earth. If you go straight long enough, you'll end up where you were.
- 317070 8y ago
  Do you have a link? I cannot find your reference.
  - libertymcateer 8y ago
    https://www.youtube.com/watch?v=yGxSExvJ2L0 https://www.youtube.com/watch?v=yGxSExvJ2L0 "The Universe is shaped exactly like the Earth, if you go straight long enough, you'll end up where you were." Isaac Brock is the lead singer of Modest Mouse.
tibyat 8y ago
modest mouse said it in 2000. “the universe is shaped exactly like the earth, if you go straight long enough you end up where you were.” physicists are a bit slower to pick up on it, but they’ll get there.

51 points by rising-sky 8y ago

No. If you could there would be a few galaxies which would be about equidistant from us along both paths. Therefore we would notice a few double copies of the same galaxies. It would be similar to standing in the middle of two mirrors. IIRC scientists have already ruled out a universe with a "radius" less than some ridiculous number (like billions of light years). There. Just saved you a click. This is old and well know. Why is Forbes writing about it now?

craftyguy 8y ago
>Why is Forbes writing about it now? The general public doesn't know this, it's a very slow news week, or both.
- stcredzero 8y ago
  In the case of things like this, it's as much because people have erroneous ideas left over from outdated science journalism, as it is from people just not having any information or inclination to care. (Sherlock Holmes's reaction to the information that the Earth goes around the sun.)
- gyre007 8y ago
  > Why is Forbes writing about it now? My totally random guess is the reason why theyre writing about it now is that it might be part of PR for the First Man movie about Neil Armstrong being the first man on the moon.
heavenlyblue 8y ago
Nope. Our visible universe is bounded by the expansion of space that happens at the speed of light. That means we can only see a subset of the universe at any time.
- gpderetta 8y ago
  Not only that, due to expansion, the subset is getting smaller by the second!
- ikeboy 8y ago
  But if you travel at less than the speed of light you'd never pass that anyway
  - isaiahg 8y ago
    Because of expansion we'll likely never be able to travel to any of the galaxies in the universe except our local neighborhood. But it's still worth studying those galaxies from a distance to learn more about our universe.
  - PopeDotNinja 8y ago
    If you traveled at the speed of light, you'd still never make it to the edge of the universe. The edge of the observable universe is moving away from us faster than the speed of light. [1] [1] https://en.wikipedia.org/wiki/Observable_universe https://en.wikipedia.org/wiki/Observable_universe
- charlesism 8y ago
  So a beam of light with a head start, half-way across the universe from you, could "wrap around" and eventually reach you. But the beam of light couldn't "wrap around" and reach itself, because the universe constantly moves the goal post before light beam gets the chance?
  - heavenlyblue 8y ago
    Yes. The fact is that does it not prove or disprove the fact that our universe is wrapped around - it may be either way. The parent's point is invalid because we wouldn't be able to see the wrapped around light even if it existed.
- karmakaze 8y ago
  Expansion is not limited to the speed of light. Two points in space may be 'moving apart' faster than the speed of light because they're not moving, space is being created in-between. The more space there is in between, the faster the rate of expansion between the points.
empath75 8y ago
Believe it or not, everyone doesn’t know everything that everyone else knows.
- deleted 8y ago
  [deleted]
bmurphy1976 8y ago
> Why is Forbes writing about it now? Why not? Look who the author is: Ethan Siegel. He has a blog (https://medium.com/starts-with-a-bang https://medium.com/starts-with-a-bang) and a podcast (https://soundcloud.com/ethan-siegel-172073460 https://soundcloud.com/ethan-siegel-172073460). He basically does a Dear Abby type column based on astronomy and physics topics. He takes difficult concepts and tries to break them down into something the lay person can understand. Quite frankly, I think he's pretty damn good at it and we should be happy Forbes is publishing his stuff for a wider audience.
EGreg 8y ago
Can you explain how one implies the other?
jobigoud 8y ago
> scientists have already ruled out a universe with a "radius" less than some ridiculous number (like billions of light years) How is billions of light years a ridiculous number? Even the observable universe is bigger than that, so the full universe could be many many times bigger. Since you can't see past the observable universe you wouldn't be able to see the double copies.
- stcredzero 8y ago
  We know from analysis of the cosmic background radiation, that the universe is at least 150 times the size of the observable universe, which puts the minimum diameter of actual universe in the range of 10 to 15 trillion light years. It could be much larger than that, however.

williesleg 8y ago

Sure. The main thing is that space-time is not linear or even understandable. So infinitely traveling through an infinite number of directions and dimensions could intersect with your start point.

scottmsul 8y ago

In cosmology, there's a parameter called "curvature". If the curvature were slightly less than 1, the topology would be "closed", meaning it would be finite, and traveling in a straight line ends back where you started. This is like the surface of the Earth, but in 3 dimensions instead of 2. Triangle angles add up to greater than 180 degrees. If the curvature were equal to 1, the universe would be "flat", meaning it would be infinite and triangles would add up to 180. If the curvature were greater than 1, it would be "hyperbolic", meaning it would be infinite and triangles would add up to less than 180. If I recall correctly, currently the best measurement of the curvature is 1 +/- .02, so it's probably flat. What's interesting is if the shape was flat and the universe were truly infinite, there would be an infinite number of perfect copies of Earth, the closest of which would be within 10^10^23 meters, as noted by Max Tegmark (source: http://space.mit.edu/home/tegmark/crazy.html http://space.mit.edu/home/tegmark/crazy.html)

jessriedel 8y ago
> If the curvature were greater than 1, it would be "hyperbolic", meaning it would be infinite There are periodic (finite-volume, homogeneous) hyperbolic spaces. Also, I think you can have infinite-volume spaces with positive curvature if they aren't homogeneous.
emilfihlman 8y ago
>Level I: A generic prediction of cosmological inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions - including an identical copy of you about 10^10^29 meters away.
jacobush 8y ago
Why copies? By sheer probability or some other mechanism. It indeed is crazy. Is it possible that is the nature of the universe?
colanderman 8y ago
Why would an infinite universe necessarily contain infinite matter?
8y ago

deleted 8y ago

[deleted]

joh6nn 8y ago

Per Brock et al, 2000, yes: the universe is shaped exactly like the Earth. If you go straight long enough, you'll end up where you were.

317070 8y ago
Do you have a link? I cannot find your reference.
- libertymcateer 8y ago
  https://www.youtube.com/watch?v=yGxSExvJ2L0 https://www.youtube.com/watch?v=yGxSExvJ2L0 "The Universe is shaped exactly like the Earth, if you go straight long enough, you'll end up where you were." Isaac Brock is the lead singer of Modest Mouse.

tibyat 8y ago

modest mouse said it in 2000. “the universe is shaped exactly like the earth, if you go straight long enough you end up where you were.” physicists are a bit slower to pick up on it, but they’ll get there.