Relativity and FTL information: Is there really a conflict ?

[Nerdfish]

It's pretty easy to explain why FTL information travels to the past. According to this article anyway:
http://www.theculture.org/rich/sharp...es/000089.html

Some people say that FTL is impossible because it is time travel, which violate causality.
But is causality necessarily true ?

There is a interpretation of quantum mechanics under which time travel does not lead to paradoxes: The many world interpretation.
http://en.wikipedia.org/wiki/Many_worlds_interpretation

Time travel, in this case, merely cause the timeline to split into two: in one the FTL information is present, and the other it's not.

Therefore a FTL signal containing message "don't send this message" would be sent as it was never received. It would arrive in the past of a timeline where it was never sent, and resulting in it not being sent out from that timeline. As a result there is no paradox.

Is the conflict between FTL information and relativity, or FTL information and causality ?

Perhaps this will clear up as we gain more understanding of the structure of time. Since I am not an expert on this matter, I'd like to hear what others think of it.

[SquidDNA]

That's a creative interpretation of the many worlds interpretation.

[Langy]

The conflict is basically that FTL travel implies time travel; it isn't that this is necessarily 'bad', just very annoying. There are also a number of ideas about why time travel won't exist - paradoxes are just one of them, another being something about quantum self-correction or something that I don't really understand, etc.

[Black]

The many worlds interpretation is scientifically equivalent to any other valid interpretation of quantum mechanics, particularly the Copenhagen interpretation. It is all philosophical.

[Derivative]

This might shed some light on the whole causality thing.

Though I always viewed the whole time travel thing being a broken record. Go back in time and make a change (I.E. remove yourself from history), time will continue on a path not containing you, or a path that wouldn't lead to the creation of the reason for making the change. Once time gets to the point where you had gone to the past, time skips and lands back where you landed in time. But without you there making a change, time reverts to the path that contained you. Thus time is self-correcting!
There's probably some holes in that thought however.

[Paladin]

My personal and completely unsupported belief is that time isn't linear in the first place. It's a malleable dimension, and we just only perceive one slice of it at a time. And if time isn't linear, then it eliminates the problem that paradoxes pose.

[Black]

My personal and completely unsupported belief is that time isn't linear in the first place. It's a malleable dimension, and we just only perceive one slice of it at a time. And if time isn't linear, then it eliminates the problem that paradoxes pose.

Do these sentences mean anything?

[Paladin]

Yes, actually.

[Derivative]

So time is what you make of it?

[TDATL]

If time must follow linear causality (the cause of pretty much every paradox) then it is impossible to travel or send information back in the first place. The information/object/individual sent back must break linear causality to travel through time in the first place.

example. I leave from the year 2012 and travel back to 1912. In 1912 "I" only existed as scattered atoms spread across the planet. For me to appear there, either through rearranging "my" 1912 atoms to fit into my 2012 configuration or from bringing "my" 2012 atoms with me, energy has to be exerted for which there is no casual link. The effect has to be placed before the cause.

If I can break causality once then I don't need to worry about maintaining a non-existent casual link to the point I left from. By traveling back in time I have already broken it in regards to me. I don't need to worry about a grandfather paradox because as far as 1912 is concerned I popped out of nowhere. My existence at a point in that point in time is a casual paradox. There was no cause to the effect of me appearing as far as linear causality is concerned. I don't have to maintain a cause for my life because I have preempted the need for it through whatever device allowed me to travel through time in the first place. It is a required aspect of traveling back in time. If I can't break causality I can't do it in the first place and thus there is no worry about me needing to maintain causality either way.

All of that is ignoring another big issue that lots of time travel theory's gloss over (because it is very inconvenient.) The Earth is moving. It is moving very fast and spinning. On top of that the galaxy itself is moving and there is no real way of measuring exactly where The Earth was at any given point in time and space. If you are off by even the tiniest degree you could end up stuck inside of something or, more likely, floating in space. So before you can safety move back in time you could very well need a space vessel that can cross interstellar (perhaps intergalactic) distances.

Edit: There is mainly mass guessing but there is another potential complication. Who is to say that there is anything in the past to go back to? If time is simply a dimension like depth, length, or width then who is to say that matter moving through it doesn't only exist in a certain "space" in time.

You could might could travel back in time but there wouldn't be anything there to see. Existence as you know it may only exist in the present. It hasn't reached the future yet and it has already left the past.

Edit2: I should mention that the last bit is more or less the plot of The Langoliers minus the things eating the past.

[Stripe7]

There is also the slight issue of it taking all the energy of the universe to achieve FTL. As you are approach light speed, your mass increases and it takes ever more energy to accelerate. So as far as we know it is quite impossible not just from the causality issue. If there are FTL spaceships out there we should be able to see them from Earth since as they decelerate there should be a burst of Cherenkov radiation.

[Whiskey144]

I feel the need to point out that FTL doesn't necessarily take more energy than is required in the universe. Realspace FTL, wherein you accelerate past c and into a superluminal region, is impossible as considered by Einstein's relativity. Cheating by using wormholes or similar "cut the distance out" methods runs full-tilt into causality issues.

Cheating by using a preferred reference frame that magically applies to only FTL travel and/or communication actually gets around all of the usual beefs taken with FTL. Star Wars hyperspace is, surprisingly enough, an example: hyperspace is its own topology, so presumably isn't bound by quite the same physical constraints as 4D space. Thus, it can have a preferred reference frame [in this case, the entirety of 'hyperspace'], without breaking relativity.

It's worth noting that preferred reference frames makes FTL work anyway you look at it, it's just that most of those ways also break relativity [but not causality]. As the saying goes: Relativity, Causality, FTL- pick any two.

An aside: there would be no Cherenkov burst on deceleration. Cherenkov radiation occurs when an object goes faster than the local velocity of light. So the Cherenkov burst would actually be seen upon a ship's acceleration past c. Further, there's no guarantee it would be visible from the Earth. You can pick out lots of stuff from Earth, and there are some theoretical spaceship propulsion systems which would be effectively visible from lightyears away *cough* Valkyrie antimatter rocket *cough*, but even a magic fusion torch would be visibility-limited to the Solar system.

[Mantaray]

actually, even as you decelerate you are going to emit Chekhov radiation up to the point at which you cease to move faster then light. but the concentration of it will increase as you cease to move so fast through it.

whereas at the point of acceleration past FTL you would emit an amount of radiation proportional to the increase of speed past FTL, but you'd move away from it in such a way that it'd hardly be considered a burst
at least that's my understanding anyway.

[Nerdfish]

WE know that FTL cannot be achieved by accelerating. The question is whether FTL without accelerating past C is possible. I agree with Whiskey on "Relativity, Causality, FTL- pick any two".
This has interesting ramifications. Does it mean if we detect photons passing through wormholes, we disprove causality ?
Edit: Previously though wormholes were observed, but they are just a theoretical construct according to wiki.