4 ms·

It’s not a matter of perception or “wrong” order: events that are close enough in time and distant enough in space that it is impossible to be present at both,

by dghf 5y ago

It’s not a matter of perception or “wrong” order: events that are close enough in time and distant enough in space that it is impossible to be present at both, even travelling at the speed of light — so-called “spacelike separated events” — do not have a single correct order. Imagine you’re riding on top of a moving train carriage, standing exactly in the middle of its roof. Suddenly you see lightning strike the front end of the carriage, and simultaneously you see it strike the rear end. You know the strikes must actually have been simultaneous, because you’re standing exactly halfway between them: if the flashes arrived at the same time, they must have left at the same time. However, from the perspective of someone standing at the trackside, you are moving towards the source of the front flash and away from the source of the rear flash. So the rear flash has further to go to reach you than the front flash. So the flashes can only reach you simultaneously if the rear strike happens first. Who’s right? Do the strikes happen simultaneously or not? Answer: both observers are right. Simultaneity, and the order of spacelike separated events, are relative, not absolute.

knome 5y ago
This doesn't at all explain why short wormholes can't exist. The reasoning seems to be heavily about how an observer would see events transpire, but the observer is pretty inconsequential. The events happen. If they seem to come from two directions, or if we see them out of order, who cares? It's not like we can't see a nova from three different points with the light coming after three different delays due to gravitational lensing. If we had three black holes in a circle, with some light coming through the middle and the rest lensing around, the middle would seem like a shortcut compared to the lensed light that went around the three, but I fail to see how that, or the wormhole instance, matters at all other than to confuse the observer if they are unaware of the foreshortened distance.
- dendriti 5y ago
  You're assuming an objective reality. Events may not truly occur without an observer.
- dghf 5y ago
  Everything’s probably fine if there’s only one FTL wormhole, and no other method of FTL travel or communication: in that situation, the weird event order is a mere curiosity. “Look at that! In our reference frame, that spaceship left the wormhole before it entered it! Cool!” But if a hostile observer sees the spaceship emerge from the wormhole before it enters, and they have access to FTL weaponry (or another FTL wormhole to fire missiles through), they could conceivably destroy the ship before it enters the wormhole, which would be a paradox. (I think, anyway: this is my layman’s understanding.)
  - Andrew_nenakhov 5y ago
    But when the hostile observer will arrive to another end of a wormhole, the spaceship that had entered on that end has already exited from the other end. What I don't understand is this time travel thing. Two events can happen simultaneously here and on Proxima Centaur, and both observers here and there will see that event in another system with 4 year delay. So what? Just like a speed of sound lags behind the speed of light, speed of light will just lag behind the actual events that take place. How is this time travel?
    - dghf 5y ago
      > But when the hostile observer will arrive to another end of a wormhole, the spaceship that had entered on that end has already exited from the other end. Not necessarily. If the wormhole allows FTL travel, then the spaceship's entry into and exit from the wormhole are by definition spacelike-separated events. Such events have no absolute, universally applicable order. Therefore there is at least one reference frame in which the ship leaves the wormhole before it enters it. Note: it does not just appear to have done so; in the chronology of that reference frame, the ship leaves the wormhole, then sometime later it enters it. See https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
      - knome 5y ago
        They're seeing old light. Even if things are moving at FTL, they still have rational cause-and-effect. Your weapon might reach the location where the ship had entered the wormhole faster than any telescope there could see light emitted from your star, but the ship will still have moved into the wormhole prior to exiting. Assuming that to be able to send or receive FTL data implies breakage in causality itself seems pretty preposterous. Seeing billions of years into the past when looking at distance galaxies doesn't imply that if we had FTL we would be able to affect them billions of years in the past. They have continued to move and emit light in all the billions of years since. It just hasn't reached us yet.

by dghf 5y ago

knome 5y ago

This doesn't at all explain why short wormholes can't exist. The reasoning seems to be heavily about how an observer would see events transpire, but the observer is pretty inconsequential. The events happen. If they seem to come from two directions, or if we see them out of order, who cares? It's not like we can't see a nova from three different points with the light coming after three different delays due to gravitational lensing. If we had three black holes in a circle, with some light coming through the middle and the rest lensing around, the middle would seem like a shortcut compared to the lensed light that went around the three, but I fail to see how that, or the wormhole instance, matters at all other than to confuse the observer if they are unaware of the foreshortened distance.

dendriti 5y ago
You're assuming an objective reality. Events may not truly occur without an observer.
dghf 5y ago
Everything’s probably fine if there’s only one FTL wormhole, and no other method of FTL travel or communication: in that situation, the weird event order is a mere curiosity. “Look at that! In our reference frame, that spaceship left the wormhole before it entered it! Cool!” But if a hostile observer sees the spaceship emerge from the wormhole before it enters, and they have access to FTL weaponry (or another FTL wormhole to fire missiles through), they could conceivably destroy the ship before it enters the wormhole, which would be a paradox. (I think, anyway: this is my layman’s understanding.)
- Andrew_nenakhov 5y ago
  But when the hostile observer will arrive to another end of a wormhole, the spaceship that had entered on that end has already exited from the other end. What I don't understand is this time travel thing. Two events can happen simultaneously here and on Proxima Centaur, and both observers here and there will see that event in another system with 4 year delay. So what? Just like a speed of sound lags behind the speed of light, speed of light will just lag behind the actual events that take place. How is this time travel?
  - dghf 5y ago
    > But when the hostile observer will arrive to another end of a wormhole, the spaceship that had entered on that end has already exited from the other end. Not necessarily. If the wormhole allows FTL travel, then the spaceship's entry into and exit from the wormhole are by definition spacelike-separated events. Such events have no absolute, universally applicable order. Therefore there is at least one reference frame in which the ship leaves the wormhole before it enters it. Note: it does not just appear to have done so; in the chronology of that reference frame, the ship leaves the wormhole, then sometime later it enters it. See https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
    - knome 5y ago
      They're seeing old light. Even if things are moving at FTL, they still have rational cause-and-effect. Your weapon might reach the location where the ship had entered the wormhole faster than any telescope there could see light emitted from your star, but the ship will still have moved into the wormhole prior to exiting. Assuming that to be able to send or receive FTL data implies breakage in causality itself seems pretty preposterous. Seeing billions of years into the past when looking at distance galaxies doesn't imply that if we had FTL we would be able to affect them billions of years in the past. They have continued to move and emit light in all the billions of years since. It just hasn't reached us yet.