5 ms·

ETA: my comment was more than a little confused. Original stands below. What I should have said is that different observers can already, in the absence of FTL,

by dghf 5y ago

ETA: my comment was more than a little confused. Original stands below. What I should have said is that different observers can already, in the absence of FTL, disagree about the order of events with space-like separation, but it doesn't matter because by definition anything that happens at one such event can't affect anything happening at another. FTL removes that restriction -- if something can travel faster than light, it can be present at two space-like separated events -- and that's what threatens causality, assuming something like the Novikov consistency conjecture doesn't hold. ======== Original comment: > Only in the sense that different observers could not agree on the order of events. But that's already true of events with space-like separation anyway. You don't need FTL travel for that. With FTL, you run the risk of observers disagreeing about the order of events with time-like separation, so you are getting into grandfather-paradox territory.

Andrew_nenakhov 5y ago
I still don't see why this would be a problem. Substitute light with sound and sight with hearing. We can exceed the speed of sound and produce noises that some observer would perceive in the incorrect order (in fact, happens all the time with supersonic jets). And the world doesn't seem to break because of that, no?
- foerbert 5y ago
  It's not really about light. It's not so much the case that the speed of light is itself special, so much as it is that light (in a vacuum) travels at a special speed. That speed is effectively that of causality, in the 'speed of the ability to cause' sense. Trying to make conclusions about that by using simple analogies to light isn't likely to get you anywhere. You're not dealing with the actual thing you're interested in in the first place.
  - dghf 5y ago
    I try to think about the speed of light as a geometric property of the universe, similar to how you can’t draw a straight line between two points on a circle that’s longer than the circle’s diameter. I don’t know if that’s correct, but the analogy helps me (until I think about it too hard, and then my head hurts).
- 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.)

by dghf 5y ago

Andrew_nenakhov 5y ago

I still don't see why this would be a problem. Substitute light with sound and sight with hearing. We can exceed the speed of sound and produce noises that some observer would perceive in the incorrect order (in fact, happens all the time with supersonic jets). And the world doesn't seem to break because of that, no?

foerbert 5y ago
It's not really about light. It's not so much the case that the speed of light is itself special, so much as it is that light (in a vacuum) travels at a special speed. That speed is effectively that of causality, in the 'speed of the ability to cause' sense. Trying to make conclusions about that by using simple analogies to light isn't likely to get you anywhere. You're not dealing with the actual thing you're interested in in the first place.
- dghf 5y ago
  I try to think about the speed of light as a geometric property of the universe, similar to how you can’t draw a straight line between two points on a circle that’s longer than the circle’s diameter. I don’t know if that’s correct, but the analogy helps me (until I think about it too hard, and then my head hurts).
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.)