> To get to Mars faster than light, you have to be able to travel backwards in time (you could watch yourself start the trip).
I think the issue is that seeing something from the past is not the same as time travel. If I yell, then travel faster than sound, then stop, I'll hear my previous yell, but so what? It's not like I can run back faster than sound and stop myself from yelling.
Yeah, if you use a false analogy it doesn't make any sense. Traveling faster than light isn't the same as (or rather, wouldn't be similar to) traveling faster than sound.
Yes, and that is, specifically, the part that is not apparent. What makes it not the same other than one being light/faster and the other sound/slower?
Everything you say makes sense if you already presume that FTL implies breaking causality, but I'm missing the explanation for why FTL implies breaking causality in the first place.
I mean, it's completely logical that if you get to Mars faster then light, you can watch yourself start the trip, but why does this also mean that you are actually going back in time?
Intuitively it would seem that even if you teleport to Mars with no travel time, observe yourself on Earth, then teleport back to Earth, you should arrive the next instant after you left, not before. So time travel does not follow immediately from the fact that you were able to observe yourself in the past.
Causality propagates at the speed of light. “Speed of light” is just a more intuitive way to say the speed of causality. FTL travel is the same as faster than causality travel. So going faster than light exceeds the speed of causality. To me, that means the order of events is no longer predictable so everything about cause and effect breaks down hence breaking causality.
>If I arrive back on Earth just after I went poof, then I'm pretty sure I went backwards in time a couple of minutes.
How?
Teleport in this context is an instant transportation process. but the image of you going poof is not ists a speed of light.
Like If I throw a ball at a target first THEN Fire a gun at the same target, the bullet will get there first but it didn't time travel to do that.
Lets assume that the teleport process is a wormhole or a folding of space process that reduces the distance between the two locations rather than making you go faster.
You could step through onto mars and back again and not see yourself when you arrive back but then go back to mars again and see yourself disappear and re-appear twice. still no time travel. You're just watching a delayed image.
If you wait 3 minutes on Mars, why would an instant teleport back to Earth result in you arriving just after you went poof rather than 3 minutes after you went poof?
I mean, it's completely logical that if you get to Mars faster then light, you can watch yourself start the trip, but why does this also mean that you are actually going back in time?
I'm the one that introduced the 'start the trip' condition, and they used it in their comment.
I didn't say that traveling faster than light was the same as traveling backwards in time and then, for some reason, that that includes being able to kill a younger version of yourself.
The speed of light is a fundamental aspect of space-time. Time isn't separate from space-time, it is bound up with space into space-time. And then if you goof the rules, goofy things happen.
If you look down in the cousin comments to yours, I had already posted a comment making it clear that I don't think light propagates instantly (3 minutes to Mars and all that).
It's certainly possible that someone will come up with a theory that allows us to set aside the idea of space-time, but it's not worth worrying about until it happens.
Think of it this way: photons don't have to exist, they might just be a model for the propagation of information. In the reference frame of a photon, propagation is instantaneous.
To be clear, you are saying if someone can make it to mars in under three minutes, they can kill their younger self because of the laws of physics that say "if you goof the rules, goofy things happen" ?
To get to Mars in less than 3 minutes you have to either convert mass into energy and back or invent entirely new physics. It'd be pretty cool if either of those things happen.
Specifically, in response to a question about why FTL breaks causality, I said "To get to Mars faster than light, you have to be able to travel backwards in time."
The implication there is actually that you can't go faster than light.
If you went to Mars in 1 minutes instead of 3 you would be going faster than light but not going backwards in time. I'm not sure where you are getting that from.
Like if Mars is reachable by travel though space in one minute, that reachable Mars is inextricably in the past of the Earth that you start from.
If you don't travel through space, then you literally have the ability to jump between points in space-time and are sort of obviously traveling through time.
Like if Mars is reachable by travel though space in one minute, that reachable Mars is inextricably in the past of the Earth that you start from.
Nope, you are mixing up philosophy with science and math.
If you don't travel through space, then you literally have the ability to jump between points in space-time and are sort of obviously traveling through time.
If you can travel backwards in time, you can kill a younger instance of yourself.