5 ms·

If the ring rotated, and black hole density decreases with size (https://www.youtube.com/watch?v=71eUes30gwc https://www.youtube.com/watch?v=71eUes30gwc), could

If the ring rotated, and black hole density decreases with size (https://www.youtube.com/watch?v=71eUes30gwc https://www.youtube.com/watch?v=71eUes30gwc), could a rotating Gödel universe exist within our universe? Could a region of space be engineered to allow for a limited form of time travel?

pwatsonwailes 2y ago
No, is the short answer. What you'd need is space-time rotating, not something physical rotating. If you could make the things rotate because space-time was rotating, not because they were, then yes, but there's no mechanism we know of which could do that.
- ganzuul 2y ago
  Are you familiar with the equations? I'm not prepared to simply take your word for it. In short, this seems to say the exact opposite of your claim: https://en.wikipedia.org/wiki/Dust_solution https://en.wikipedia.org/wiki/Dust_solution
  - pwatsonwailes 2y ago
    You're on the wrong thing there. I could be wrong but I think you're outside your field on this one.
    - ganzuul 2y ago
      It says that spacetime exists as an interaction of gravity alone. This implies that there is no other frame of reference in this type of solution to GR. i.e. without mass there is no time in such a universe. Not a new idea. > I could be wrong but I think you're outside your field on this one. And in contrast what would that make of you?? I'm saying that if there in some point in the future (because we can see it now) is sufficient mass density in the region of space of that big ring, and it is rotating, we tick every box we know of to theoretically allow for an eternal circle. "Engineering" it would mean that someone wanted some type of eternal existence, which is the profound idea at play here. Engineering things without the technology to manufacture it happens all the time. Just because we can't imagine how to build it does not mean we can't calculate if it could exist.
  - qsi 2y ago
    Where in the Wikipedia page does it seem say so? I can't find anything relevant but then again I don't understand all of it.
    - ganzuul 2y ago
      > A perfect and pressureless fluid can be interpreted as a model of a configuration of dust particles that locally move in concert and interact with each other only gravitationally, from which the name is derived. That "only" is important but unintuitive. It means space and time can not be separated from mass.
      - nyssos 2y ago
        It absolutely does not. "Interact with each other only gravitationally" has its plain and ordinary meaning: we're ignoring other interactions. No charge, no collisions, no radiation, etc.
      - NemoNobody 2y ago
        Are you trying to say the ring is proof that something assembled the stars as such to engineer an eternal circle, intentionally - the intent being the profound thing? Light travels very fast but space is very big, so the light from stars we see is very old - some of the lights have outlived their stars, so I assume you meant the past but tbh I'm unsure
  - deleted 2y ago
    [deleted]
- tomthe 2y ago
  I agree with the no, but you can make space itself rotate because things in space rotate: https://en.wikipedia.org/wiki/Frame-dragging https://en.wikipedia.org/wiki/Frame-dragging And that in turn would rotate things in space... or not?
  - pwatsonwailes 2y ago
    The Lense-Thirring effect is absolutely a thing, and we have direct evidence for it. To be clearer (I totally wasn't clear enough on this tbf), there's nothing we know of which can do it at the required scale to allow for time travel. What we're talking about here are closed timelike curves. There's models which suggest they could exist inside a singularity, but they're not going to outside without something which seriously breaks other areas of physics (Tipler cylinders etc).
    - ganzuul 2y ago
      > There's models which suggest they could exist inside a singularity, but they're not going to outside without something which seriously breaks other areas of physics (Tipler cylinders etc). A singularity is a dimensionless point. It has no inside. Did you mean a black hole? If so, the Kurtzgesagt cartoon explains this. The second part of you sentence seems to have a broken sentence structure. Can't make sense of it.
      - tempestn 2y ago
        To clarify the rest of the sentence for you: ...but [closed time-like curves are] not going to [exist] outside [of a singularity] without something which seriously breaks other areas of physics (Tipler cylinders etc. [are examples of such theoretical instances which would break other areas of physics]).
- deleted 2y ago
  [deleted]
matja 2y ago
Similar to the plot of https://en.wikipedia.org/wiki/Ring_(Baxter_novel) https://en.wikipedia.org/wiki/Ring_(Baxter_novel)
- ganzuul 2y ago
  Yes, though the Kurtzgesagt video seems to allow for a Kerr metric to be habitable in a very normal sense.

pwatsonwailes 2y ago

No, is the short answer. What you'd need is space-time rotating, not something physical rotating. If you could make the things rotate because space-time was rotating, not because they were, then yes, but there's no mechanism we know of which could do that.

ganzuul 2y ago
Are you familiar with the equations? I'm not prepared to simply take your word for it. In short, this seems to say the exact opposite of your claim: https://en.wikipedia.org/wiki/Dust_solution https://en.wikipedia.org/wiki/Dust_solution
- pwatsonwailes 2y ago
  You're on the wrong thing there. I could be wrong but I think you're outside your field on this one.
  - ganzuul 2y ago
    It says that spacetime exists as an interaction of gravity alone. This implies that there is no other frame of reference in this type of solution to GR. i.e. without mass there is no time in such a universe. Not a new idea. > I could be wrong but I think you're outside your field on this one. And in contrast what would that make of you?? I'm saying that if there in some point in the future (because we can see it now) is sufficient mass density in the region of space of that big ring, and it is rotating, we tick every box we know of to theoretically allow for an eternal circle. "Engineering" it would mean that someone wanted some type of eternal existence, which is the profound idea at play here. Engineering things without the technology to manufacture it happens all the time. Just because we can't imagine how to build it does not mean we can't calculate if it could exist.
- qsi 2y ago
  Where in the Wikipedia page does it seem say so? I can't find anything relevant but then again I don't understand all of it.
  - ganzuul 2y ago
    > A perfect and pressureless fluid can be interpreted as a model of a configuration of dust particles that locally move in concert and interact with each other only gravitationally, from which the name is derived. That "only" is important but unintuitive. It means space and time can not be separated from mass.
    - nyssos 2y ago
      It absolutely does not. "Interact with each other only gravitationally" has its plain and ordinary meaning: we're ignoring other interactions. No charge, no collisions, no radiation, etc.
    - NemoNobody 2y ago
      Are you trying to say the ring is proof that something assembled the stars as such to engineer an eternal circle, intentionally - the intent being the profound thing? Light travels very fast but space is very big, so the light from stars we see is very old - some of the lights have outlived their stars, so I assume you meant the past but tbh I'm unsure
- deleted 2y ago
  [deleted]
tomthe 2y ago
I agree with the no, but you can make space itself rotate because things in space rotate: https://en.wikipedia.org/wiki/Frame-dragging https://en.wikipedia.org/wiki/Frame-dragging And that in turn would rotate things in space... or not?
- pwatsonwailes 2y ago
  The Lense-Thirring effect is absolutely a thing, and we have direct evidence for it. To be clearer (I totally wasn't clear enough on this tbf), there's nothing we know of which can do it at the required scale to allow for time travel. What we're talking about here are closed timelike curves. There's models which suggest they could exist inside a singularity, but they're not going to outside without something which seriously breaks other areas of physics (Tipler cylinders etc).
  - ganzuul 2y ago
    > There's models which suggest they could exist inside a singularity, but they're not going to outside without something which seriously breaks other areas of physics (Tipler cylinders etc). A singularity is a dimensionless point. It has no inside. Did you mean a black hole? If so, the Kurtzgesagt cartoon explains this. The second part of you sentence seems to have a broken sentence structure. Can't make sense of it.
    - tempestn 2y ago
      To clarify the rest of the sentence for you: ...but [closed time-like curves are] not going to [exist] outside [of a singularity] without something which seriously breaks other areas of physics (Tipler cylinders etc. [are examples of such theoretical instances which would break other areas of physics]).
deleted 2y ago
[deleted]

matja 2y ago

Similar to the plot of https://en.wikipedia.org/wiki/Ring_(Baxter_novel) https://en.wikipedia.org/wiki/Ring_(Baxter_novel)

ganzuul 2y ago
Yes, though the Kurtzgesagt video seems to allow for a Kerr metric to be habitable in a very normal sense.