4 ms·

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 m

by 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.
    - dghf 5y ago
      It’s nothing to do with old light. It’s to do with the relativity of simultaneity. Have a look at the Wikipedia article I linked to. Or read these posts on Ask a Mathematician / Ask a Physicist: - this one explains the basis of the phenomenon: https://www.askamathematician.com/2010/12/q-according-to-relativity-two-moving-observers-always-see-the-other-moving-through-time-slower-isnt-that-a-contradiction-doesnt-one-have-to-be-faster/ https://www.askamathematician.com/2010/12/q-according-to-rel... - this one explains the implications for FTL travel (and why it can result in time-travel): https://www.askamathematician.com/2011/09/q-hyperspace-warp-drives-and-faster-than-light-travel-why-not/ https://www.askamathematician.com/2011/09/q-hyperspace-warp-...
  - Andrew_nenakhov 5y ago
    Even if it simultaneity is relative, I still don't quite understand why it is a problem. Every local causality is observed. Take this [1] laborous attempt to explain this issue. They explain that 'you can see the message before it was sent'. NO! You can see the message before light from a distant star system delivers you info about it being sent! Like, a bullet arrives to target's head before he has a chance to hear the gunshot. Never a problem for snipers, shouldn't be a problem for FTL messaging either. [1]: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
    - dghf 5y ago
      OK, try this one: Lt Cdr Worf is testing the USS Defiant's impulse engines after a refit. He is currently coasting along in a straight line at a constant 0.6c, and passes Deep Space Nine. We'll call time on board the defiant t and time on board DS9 t', and say that the Defiant flies by DS9 at t = t' = 0. At t = 10 minutes, Worf decides to reward his efforts with a cup of bahgol, only to discover the replicator is on the blink. After about another 10 minutes of pressing various buttons, he gives up and logs a ticket with the DS9 help desk over the subspace network. Because the Defiant is travelling in a straight line at a constant speed of reference, it is in an inertial frame reference, and it is just as valid to see DS9 as moving away from the Defiant at 0.6c as the other way round, and this in fact is what we must do to work out what time on DS9 corresponds to "now" on the Defiant. At 0.6c, the Lorentz factor is 1.25: so for every 1.25 minutes that passes on the Defiant, a single minute passes on DS9 -- or put another way, for every minute that passes on the Defiant, only 0.8 minutes pass on DS9. So when Worf submits his ticket at t = 20 minutes, on DS9 it's t' = 16 minutes. Subspace comms are instantaneous, or so close to it as makes no difference, so O'Brien picks up the ticket very soon after t' = 16 minutes. He replies straight away -- "Have you tried turning it off and on again?" -- and sends the response back over the subspace network. This is where relativity of simultaneity comes in. DS9 is also in an inertial frame of reference, and in that frame it is the Defiant that is moving away at 0.6c, that has a Lorentz factor of 1.25, and where only 0.8 minutes pass for every minute on DS9. So for O'Brien, t' = 16 minutes is simultaneous with t = 12.8 minutes (not t = 20 minutes): allowing for time for O'Brien to read the ticket and write his response, Worf will receive it somewhere around t = 15 minutes -- while he's still tinkering with the replicator. Somewhat surprised, he tries the solution, it works, and he goes off to enjoy his cuppa, never sending the ticket he's just received a response to. This is why FTL risks messing with causality.

by dghf 5y ago

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.
  - dghf 5y ago
    It’s nothing to do with old light. It’s to do with the relativity of simultaneity. Have a look at the Wikipedia article I linked to. Or read these posts on Ask a Mathematician / Ask a Physicist: - this one explains the basis of the phenomenon: https://www.askamathematician.com/2010/12/q-according-to-relativity-two-moving-observers-always-see-the-other-moving-through-time-slower-isnt-that-a-contradiction-doesnt-one-have-to-be-faster/ https://www.askamathematician.com/2010/12/q-according-to-rel... - this one explains the implications for FTL travel (and why it can result in time-travel): https://www.askamathematician.com/2011/09/q-hyperspace-warp-drives-and-faster-than-light-travel-why-not/ https://www.askamathematician.com/2011/09/q-hyperspace-warp-...
- Andrew_nenakhov 5y ago
  Even if it simultaneity is relative, I still don't quite understand why it is a problem. Every local causality is observed. Take this [1] laborous attempt to explain this issue. They explain that 'you can see the message before it was sent'. NO! You can see the message before light from a distant star system delivers you info about it being sent! Like, a bullet arrives to target's head before he has a chance to hear the gunshot. Never a problem for snipers, shouldn't be a problem for FTL messaging either. [1]: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
  - dghf 5y ago
    OK, try this one: Lt Cdr Worf is testing the USS Defiant's impulse engines after a refit. He is currently coasting along in a straight line at a constant 0.6c, and passes Deep Space Nine. We'll call time on board the defiant t and time on board DS9 t', and say that the Defiant flies by DS9 at t = t' = 0. At t = 10 minutes, Worf decides to reward his efforts with a cup of bahgol, only to discover the replicator is on the blink. After about another 10 minutes of pressing various buttons, he gives up and logs a ticket with the DS9 help desk over the subspace network. Because the Defiant is travelling in a straight line at a constant speed of reference, it is in an inertial frame reference, and it is just as valid to see DS9 as moving away from the Defiant at 0.6c as the other way round, and this in fact is what we must do to work out what time on DS9 corresponds to "now" on the Defiant. At 0.6c, the Lorentz factor is 1.25: so for every 1.25 minutes that passes on the Defiant, a single minute passes on DS9 -- or put another way, for every minute that passes on the Defiant, only 0.8 minutes pass on DS9. So when Worf submits his ticket at t = 20 minutes, on DS9 it's t' = 16 minutes. Subspace comms are instantaneous, or so close to it as makes no difference, so O'Brien picks up the ticket very soon after t' = 16 minutes. He replies straight away -- "Have you tried turning it off and on again?" -- and sends the response back over the subspace network. This is where relativity of simultaneity comes in. DS9 is also in an inertial frame of reference, and in that frame it is the Defiant that is moving away at 0.6c, that has a Lorentz factor of 1.25, and where only 0.8 minutes pass for every minute on DS9. So for O'Brien, t' = 16 minutes is simultaneous with t = 12.8 minutes (not t = 20 minutes): allowing for time for O'Brien to read the ticket and write his response, Worf will receive it somewhere around t = 15 minutes -- while he's still tinkering with the replicator. Somewhat surprised, he tries the solution, it works, and he goes off to enjoy his cuppa, never sending the ticket he's just received a response to. This is why FTL risks messing with causality.