5 ms·

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 You're just seeing outdated information.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.
  - nostromo 6y ago
    If I got into a FTL spaceship, I could go out in space, turn around, come back, and interact with my previous self. I could even tell my past-self that I didn't find anything of value -- and that there's no reason to go at all. Maybe the original me decides not to go, and now there's two copies of myself on Earth. These are the sort of contradictions that break our understand of causality once you introduce FTL.
  - 8fGTBjZxBcHq 6y ago
    So the main thing and again I'm not really equipped to explain the relativity behind this but I do think I understand it somewhat. The main thing in the situation you're describing is that it's one way. Make it two way, imagine getting a message saying "don't send that message" from someone who has already seen the message you're about to send. You can imagine this and that's part of the problem understanding the issue I think. But if this combination of events was possible very many things that we know to be true about spacetime would have to not be true.
  - sabujp 6y ago
    You got your sentence wrong. You receive the message BEFORE you see the person through the telescope typing the message, i.e. it takes 2.5 seconds for light to reach us from the moon, you would get the message 2.5 seconds before you see the person typing it assuming the FTL messaging system was instantaneous.

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.
- nostromo 6y ago
  If I got into a FTL spaceship, I could go out in space, turn around, come back, and interact with my previous self. I could even tell my past-self that I didn't find anything of value -- and that there's no reason to go at all. Maybe the original me decides not to go, and now there's two copies of myself on Earth. These are the sort of contradictions that break our understand of causality once you introduce FTL.
- 8fGTBjZxBcHq 6y ago
  So the main thing and again I'm not really equipped to explain the relativity behind this but I do think I understand it somewhat. The main thing in the situation you're describing is that it's one way. Make it two way, imagine getting a message saying "don't send that message" from someone who has already seen the message you're about to send. You can imagine this and that's part of the problem understanding the issue I think. But if this combination of events was possible very many things that we know to be true about spacetime would have to not be true.
- sabujp 6y ago
  You got your sentence wrong. You receive the message BEFORE you see the person through the telescope typing the message, i.e. it takes 2.5 seconds for light to reach us from the moon, you would get the message 2.5 seconds before you see the person typing it assuming the FTL messaging system was instantaneous.