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Seriously... what's the difference between 'universe expansion' and 'travel'? A particle can't travel faster than c but it can effectively if the universe expa

Seriously... what's the difference between 'universe expansion' and 'travel'? A particle can't travel faster than c but it can effectively if the universe expands in addition to its travel speed? Are there any established sci-fi concepts that replace the concepts of wormholes with 'universe compression'?

namirez 7y ago
Yes, it's called Alcubierre Drive, where spacetime is compressed in front and expanded behind a ship to drive it through the spacetime like a gravitational wave. https://en.wikipedia.org/wiki/Alcubierre_drive https://en.wikipedia.org/wiki/Alcubierre_drive
- Filligree 7y ago
  Or the Krasnikov tube, where time is tilted such that travel through it (in one direction) takes you back in time (from a stationary perspective). That's very similar to a wormhole, but without the topological flaw.
haecceity 7y ago
The first one means the space time metric changes
- semiotagonal 7y ago
  Is it possible that the universe stays the same size and everything within it shrinks?
  - BurningFrog 7y ago
    I think those are just two equivalent ways of thinking of the same thing. The standard way is probably much easier to reason about.
  - tux1968 7y ago
    Don't think there is a way to tell the difference without stepping outside the universe to look back on the system relative to another frame. But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space. Not that i'm truly able to wrap my head around it, but at the moment of the big bang, time and space themselves exploded into existence.
    - Filligree 7y ago
      > But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space. Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. (But even if that's the case, that still has nothing to do with the expansion we've been talking about.)
      - wlesieutre 7y ago
        > Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. I don't think that's right. The fact that space gets bigger doesn't mean that space is inside something, it just means space gets bigger. Disclaimer: not an astrophysicist
        Filligree 7y ago
        Space does get bigger, but that's separate. What eternal inflation is claiming is that some fourteen billion years ago there was a false vacuum collapse, and we're living inside the bubble that resulted, but said bubble is itself still expanding. It won't ever get anywhere, since space outside the bubble is inflating much, much faster than inside. It means there's a border to our universe, which is expending at lightspeed, but it's far outside the Hubble horizon and thus entirely unreachable.
    - edf13 7y ago
      The “nothing” is the one thing I find fascinating ~ maybe it’s our human brains that can’t get it but it just doesn’t work for me! There has to be something doesn’t there?
ebg13 7y ago
> what's the difference between 'universe expansion' and 'travel'? Travel happens inside the universe and expansion doesn't? If a rock is sliding along a stretchy sheet, stretching the sheet will make other things on the sheet move farther away from the rock, potentially faster than the fastest that the rock can slide, even though the rock doesn't move any faster.
samplatt 7y ago
Just because the universe is expanding at c doesn't mean everything inside it is getting bigger at c. Due to the way mass curves spacetime, mass stays more or less the same size (in comparison to the rest of the empty universe, anyway) - it is mostly only the EMPTY parts of space that get stretched out at c. This means that distances between things are getting bigger all the time... BUT things are still able to travel through space, so they get further away even faster. Imagine two people (which are galaxies) walking in the same direction on a travelator (which is spacetime). Ordinarily one person would reach the end, and then in a predictable amount of time so would the other. But if the travelator is expanding evenly in both directions, then the distance between the two people will increase, even though they're walking at the same speed.
sytelus 7y ago
One of the interesting things is that if everyone of us did manage to travel almost at the speed of light then time stops and you can be in universe wherever you want in matter of your seconds. In other words, whole universe just becomes giant wormhole and you teleport from wherever to wherever in matter of seconds at whim. If you reach to the edge of time dimension, the space dimension basically seize the exist.
- easymodex 7y ago
  I'm not sure that's right since things are light-years away it means you also need to travel just as long (or is this just as far as outside obeservers are concerned?). But even so, the universe around you would still get older while you travel like that so in a few trips you will reach the heat death of the universe.
  - mensetmanusman 7y ago
    That is just for outside observers. With time and space dilation, things do become instantaneous at light speed. It is actually kind of fun to think about that in the frame of reference of a photon which ‘is’ constantly coming in and out of existence as it interacts with charged mass.

namirez 7y ago

Yes, it's called Alcubierre Drive, where spacetime is compressed in front and expanded behind a ship to drive it through the spacetime like a gravitational wave. https://en.wikipedia.org/wiki/Alcubierre_drive https://en.wikipedia.org/wiki/Alcubierre_drive

Filligree 7y ago
Or the Krasnikov tube, where time is tilted such that travel through it (in one direction) takes you back in time (from a stationary perspective). That's very similar to a wormhole, but without the topological flaw.

haecceity 7y ago

The first one means the space time metric changes

semiotagonal 7y ago
Is it possible that the universe stays the same size and everything within it shrinks?
- BurningFrog 7y ago
  I think those are just two equivalent ways of thinking of the same thing. The standard way is probably much easier to reason about.
- tux1968 7y ago
  Don't think there is a way to tell the difference without stepping outside the universe to look back on the system relative to another frame. But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space. Not that i'm truly able to wrap my head around it, but at the moment of the big bang, time and space themselves exploded into existence.
  - Filligree 7y ago
    > But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space. Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. (But even if that's the case, that still has nothing to do with the expansion we've been talking about.)
    - wlesieutre 7y ago
      > Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. I don't think that's right. The fact that space gets bigger doesn't mean that space is inside something, it just means space gets bigger. Disclaimer: not an astrophysicist
      - Filligree 7y ago
        Space does get bigger, but that's separate. What eternal inflation is claiming is that some fourteen billion years ago there was a false vacuum collapse, and we're living inside the bubble that resulted, but said bubble is itself still expanding. It won't ever get anywhere, since space outside the bubble is inflating much, much faster than inside. It means there's a border to our universe, which is expending at lightspeed, but it's far outside the Hubble horizon and thus entirely unreachable.
  - edf13 7y ago
    The “nothing” is the one thing I find fascinating ~ maybe it’s our human brains that can’t get it but it just doesn’t work for me! There has to be something doesn’t there?

ebg13 7y ago

> what's the difference between 'universe expansion' and 'travel'? Travel happens inside the universe and expansion doesn't? If a rock is sliding along a stretchy sheet, stretching the sheet will make other things on the sheet move farther away from the rock, potentially faster than the fastest that the rock can slide, even though the rock doesn't move any faster.

samplatt 7y ago

Just because the universe is expanding at c doesn't mean everything inside it is getting bigger at c. Due to the way mass curves spacetime, mass stays more or less the same size (in comparison to the rest of the empty universe, anyway) - it is mostly only the EMPTY parts of space that get stretched out at c. This means that distances between things are getting bigger all the time... BUT things are still able to travel through space, so they get further away even faster. Imagine two people (which are galaxies) walking in the same direction on a travelator (which is spacetime). Ordinarily one person would reach the end, and then in a predictable amount of time so would the other. But if the travelator is expanding evenly in both directions, then the distance between the two people will increase, even though they're walking at the same speed.

sytelus 7y ago

One of the interesting things is that if everyone of us did manage to travel almost at the speed of light then time stops and you can be in universe wherever you want in matter of your seconds. In other words, whole universe just becomes giant wormhole and you teleport from wherever to wherever in matter of seconds at whim. If you reach to the edge of time dimension, the space dimension basically seize the exist.

easymodex 7y ago
I'm not sure that's right since things are light-years away it means you also need to travel just as long (or is this just as far as outside obeservers are concerned?). But even so, the universe around you would still get older while you travel like that so in a few trips you will reach the heat death of the universe.
- mensetmanusman 7y ago
  That is just for outside observers. With time and space dilation, things do become instantaneous at light speed. It is actually kind of fun to think about that in the frame of reference of a photon which ‘is’ constantly coming in and out of existence as it interacts with charged mass.