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How does FTL imply time travel?

akvadrako 6y ago
A few explanations I've collected: https://www.askamathematician.com/2012/07/q-how-does-instantaneous-communication-violate-causality/ https://www.askamathematician.com/2012/07/q-how-does-instant... 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... http://www.theculture.org/rich/sharpblue/archives/000089.html http://www.theculture.org/rich/sharpblue/archives/000089.htm... The important bit is that it doesn't matter HOW information is transmitted FTL – it can be via wormholes or magic. For observers in another reference frame it will still result in causality violations.
- nostromo 6y ago
  An easy-to-understand example: If I got in a faster than light spaceship and traveled to Jupiter, and then turned around and came right back, I would arrive before I left and could shake my own hand on the launchpad. If we think humans will soon figure out FTL, it makes one wonder why we've not yet seen any human FTL spaceships from the future.
  - diggernet 6y ago
    That...doesn't make sense. Let's say your spaceship can go to Jupiter in one second (Edit: as measured from Earth), which would require FTL speed. Then you turn around and come back. You've still arrived two seconds after you left. At Jupiter, you could look through a super-telescope and see yourself still on Earth, but you still aren't really seeing yourself, because you are not still on Earth. You are just seeing the photons you passed during the trip, catching up to you.
    - jfengel 6y ago
      Yeah, it doesn't make sense because it's not correct. FTL enables a kind of time travel that isn't going back in time to shake your own hand. It allows two people in two different reference frames to disagree about whether A caused B or B caused A. You don't get to go "back in time" but you can send somebody lottery results before they're drawn (from somebody's perspective).
      - akvadrako 6y ago
        > FTL enables a kind of time travel that isn't going back in time to shake your own hand. That is exactly what it is. You should read the links above to understand what's being claimed.
        diggernet 6y ago
        The links discuss FTL communication implying communicating through time. I haven't taken the time to fully understand the arguments, so for the sake of this discussion I'm willing to simply assume that their conclusions are correct. Now... Please explain to me how communicating through time will allow you to literally, physically, shake your own hand as was claimed.
        akvadrako 6y ago
        It's the same argument, just instead of assuming one can send information FTL, one can send matter. Or you assume a teleporter exists.

A few explanations I've collected: https://www.askamathematician.com/2012/07/q-how-does-instantaneous-communication-violate-causality/ https://www.askamathematician.com/2012/07/q-how-does-instant... 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... http://www.theculture.org/rich/sharpblue/archives/000089.html http://www.theculture.org/rich/sharpblue/archives/000089.htm... The important bit is that it doesn't matter HOW information is transmitted FTL – it can be via wormholes or magic. For observers in another reference frame it will still result in causality violations.

nostromo 6y ago
An easy-to-understand example: If I got in a faster than light spaceship and traveled to Jupiter, and then turned around and came right back, I would arrive before I left and could shake my own hand on the launchpad. If we think humans will soon figure out FTL, it makes one wonder why we've not yet seen any human FTL spaceships from the future.
- diggernet 6y ago
  That...doesn't make sense. Let's say your spaceship can go to Jupiter in one second (Edit: as measured from Earth), which would require FTL speed. Then you turn around and come back. You've still arrived two seconds after you left. At Jupiter, you could look through a super-telescope and see yourself still on Earth, but you still aren't really seeing yourself, because you are not still on Earth. You are just seeing the photons you passed during the trip, catching up to you.
  - jfengel 6y ago
    Yeah, it doesn't make sense because it's not correct. FTL enables a kind of time travel that isn't going back in time to shake your own hand. It allows two people in two different reference frames to disagree about whether A caused B or B caused A. You don't get to go "back in time" but you can send somebody lottery results before they're drawn (from somebody's perspective).
    - akvadrako 6y ago
      > FTL enables a kind of time travel that isn't going back in time to shake your own hand. That is exactly what it is. You should read the links above to understand what's being claimed.
      - diggernet 6y ago
        The links discuss FTL communication implying communicating through time. I haven't taken the time to fully understand the arguments, so for the sake of this discussion I'm willing to simply assume that their conclusions are correct. Now... Please explain to me how communicating through time will allow you to literally, physically, shake your own hand as was claimed.
        akvadrako 6y ago
        It's the same argument, just instead of assuming one can send information FTL, one can send matter. Or you assume a teleporter exists.