4 ms·

If you’re saying traveling faster than the speed of light is possible, you’re also saying traveling backwards in time is possible. And yet people tend to be ve

If you’re saying traveling faster than the speed of light is possible, you’re also saying traveling backwards in time is possible. And yet people tend to be very skeptical of backwards time travel claims.

eutropia 6y ago
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.
spazrunaway 6y ago
I've never seen this explained in a way that made any sense to me, ELI5?
- 8fGTBjZxBcHq 6y ago
  If you reframe "speed of light" as "speed of the propagation of information" the traveling faster than light means information about an event can be received before the event happens from the perspective of the observer. Remember we know it to be true that there is no universal authoritative "now" all observers will see things happening at different times and none of them are wrong or seeing an illusion in that. Why this is effectively time travel is harder to explain without getting into relativity and I'm not going to try right now. The main thing to get though is that you just can't observe events before they happen. It violates causality and if you allow for that (like you totally can with math) then nothing makes any sense and you can't use that math to describe how we know the world to work in any useful way.
  - spazrunaway 6y ago
    > "the traveling faster than light means information about an event can be received before the event happens from the perspective of the observer" Why is that a problem? So let's say I'm on the moon and you're on Earth, and you're watching me through a telescope. I use an FTL cell phone to text you "Hi". Your phone buzzes, you read the message, then visually observe me through the telescope typing the message before you've received it. I guess I just don't understand why that really matters or violates causality. You're just seeing outdated information. There's no way you can interact with my "past" self that you can see through the telescope, it's just a visual artifact.
    - ghodith 6y ago
      That’s true until you factor in relativity, which the person above you was trying to avoid. Basically, if someone were traveling near the speed of light (and in the right direction) they would see the person get the text on the moon before the person on the earth sent the message. And it’s not just a matter of perspective either, if they had an FTL phone they could see the message received on the moon and then text the person on earth before he sent the message, preventing him from sending it in the first place. The explanation for this is complicated, however, so I would suggest you look at the links provided earlier in these comments for a more comprehensive explanation.

eutropia 6y ago

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.

spazrunaway 6y ago

I've never seen this explained in a way that made any sense to me, ELI5?

8fGTBjZxBcHq 6y ago
If you reframe "speed of light" as "speed of the propagation of information" the traveling faster than light means information about an event can be received before the event happens from the perspective of the observer. Remember we know it to be true that there is no universal authoritative "now" all observers will see things happening at different times and none of them are wrong or seeing an illusion in that. Why this is effectively time travel is harder to explain without getting into relativity and I'm not going to try right now. The main thing to get though is that you just can't observe events before they happen. It violates causality and if you allow for that (like you totally can with math) then nothing makes any sense and you can't use that math to describe how we know the world to work in any useful way.
- spazrunaway 6y ago
  > "the traveling faster than light means information about an event can be received before the event happens from the perspective of the observer" Why is that a problem? So let's say I'm on the moon and you're on Earth, and you're watching me through a telescope. I use an FTL cell phone to text you "Hi". Your phone buzzes, you read the message, then visually observe me through the telescope typing the message before you've received it. I guess I just don't understand why that really matters or violates causality. You're just seeing outdated information. There's no way you can interact with my "past" self that you can see through the telescope, it's just a visual artifact.
  - ghodith 6y ago
    That’s true until you factor in relativity, which the person above you was trying to avoid. Basically, if someone were traveling near the speed of light (and in the right direction) they would see the person get the text on the moon before the person on the earth sent the message. And it’s not just a matter of perspective either, if they had an FTL phone they could see the message received on the moon and then text the person on earth before he sent the message, preventing him from sending it in the first place. The explanation for this is complicated, however, so I would suggest you look at the links provided earlier in these comments for a more comprehensive explanation.