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A comment on Scientists announce a physical warp drive is possible.

Re:Scientists announce a physical warp drive is possible by nostromo on Friday March 05, 2021 @12:12PM (#26359205)
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.
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Re:Scientists announce a physical warp drive is possible by eutropia on Friday March 05, 2021 @12:15PM (#26359239)
How does FTL imply time travel?
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Re:Scientists announce a physical warp drive is possible by akvadrako on Friday March 05, 2021 @01:21PM (#26360042)
A few explanations I've collected:

https://www.askamathematician.com/2012/07/q-how-does-instant...

http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...

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.

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Re:Scientists announce a physical warp drive is possible by nostromo on Friday March 05, 2021 @02:06PM (#26360697)
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.

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Re:Scientists announce a physical warp drive is possible by diggernet on Friday March 05, 2021 @03:46PM (#26361858)
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.

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Re:Scientists announce a physical warp drive is possible by jfengel on Friday March 05, 2021 @04:19PM (#26362258)
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).

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Re:Scientists announce a physical warp drive is possible by akvadrako on Friday March 05, 2021 @04:34PM (#26362416)
> 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.

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Re:Scientists announce a physical warp drive is possible by diggernet on Friday March 05, 2021 @05:32PM (#26363080)
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.

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Re:Scientists announce a physical warp drive is possible by akvadrako on Saturday March 06, 2021 @03:32AM (#26366038)
It's the same argument, just instead of assuming one can send information FTL, one can send matter. Or you assume a teleporter exists.
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Re:Scientists announce a physical warp drive is possible by spazrunaway on Friday March 05, 2021 @12:17PM (#26359264)
I've never seen this explained in a way that made any sense to me, ELI5?
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Re:Scientists announce a physical warp drive is possible by 8fGTBjZxBcHq on Friday March 05, 2021 @01:38PM (#26360242)
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.

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Re:Scientists announce a physical warp drive is possible by spazrunaway on Friday March 05, 2021 @01:55PM (#26360501)
> "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.

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Re:Scientists announce a physical warp drive is possible by ghodith on Friday March 05, 2021 @02:07PM (#26360731)
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.

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Re:Scientists announce a physical warp drive is possible by 8fGTBjZxBcHq on Friday March 05, 2021 @05:31PM (#26363076)
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.

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Re:Scientists announce a physical warp drive is possible by nostromo on Friday March 05, 2021 @02:39PM (#26361116)
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.

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Re:Scientists announce a physical warp drive is possible by sabujp on Saturday March 06, 2021 @11:51AM (#26368639)
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.
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