Can Faster-Than-Light Travel Break Causality in General Relativity?

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Discussion Overview

The discussion revolves around the theoretical implications of faster-than-light (FTL) travel in the context of general relativity (GR) and its potential to break causality. Participants explore various concepts related to FTL, including the Alcubierre "space warp," the role of negative energy, and the relationship between light, matter, and relativistic constraints.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that FTL could be achieved by converting light into matter and back, suggesting that this process might bypass relativistic constraints.
  • Others argue that conservation of momentum would not hold in such a scenario, raising questions about the feasibility of this approach.
  • A participant mentions the Alcubierre "space warp," which theoretically allows for FTL travel by manipulating spacetime, but notes that it requires unknown technology involving negative energy.
  • There is a discussion about whether the Alcubierre solution allows for instantaneous travel after building a field, with some suggesting that synchronization issues would arise if two fields were created simultaneously.
  • One participant introduces the idea of an antigravity field potentially allowing FTL travel by altering the perception of time relative to the rest of the universe.
  • Another participant counters that the concept of antigravity is fundamentally different from the Alcubierre warp bubble, which does not involve linear movement through space.
  • Concerns are raised about the implications of FTL travel on causality, with a participant questioning whether such travel would break causality in GR as it does in special relativity.

Areas of Agreement / Disagreement

Participants express a variety of views on the feasibility of FTL travel and its implications for causality. There is no consensus on the validity of the proposed methods or their consequences, indicating ongoing debate and uncertainty.

Contextual Notes

Limitations include unresolved questions about the nature of negative energy, the specifics of momentum conservation in proposed FTL scenarios, and the technological feasibility of the concepts discussed.

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Greetings !

Let me see if I got the theoretical idea of FTL right:
We take some huge amount of light in front of the spaceship
turn it into matter and then back into light and so on -
we repeat the process. Seems like SR has no effect here,
so we should go FTL ?

Live long and prosper.
 
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when you mean FTL do you mean faster then light if so i don't see the connection between the repetition of light into matter and vice versa and faster then light travel.
 
Originally posted by loop quantum gravity
when you mean FTL do you mean faster then light if so i don't see the connection between the repetition of light into matter and vice versa and faster then light travel.
Conservation of momentum would not apply here.
Matter would attract the spaceship, then turn back
into light without conserving momentum. Since
it'll be space-time curvature itself that'll accelerate
the spaceship - there, theoreticly, shouldn't be a limmit
to its velocity according to GR, or should there be one ?

Live long and prosper.
 
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Matter accelerating due to gravity still gains relativistic mass and accelerates slower the faster it gets. Holding a carrot on a stick in front of a horse in this case doesn't help you any.
 
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Greetings !
Originally posted by russ_watters
Matter accelerating due to gravity still gains relativistic mass and accelerates slower the faster it gets. Holding a carrot on a stick in front of a horse in this case doesn't help you any.
Holding a carrot in front of a horse won't work at all. :wink:
But, that's not the type of effect I'm talking about.
I don't know, maybe you're right, but as far as relativistic
mass goes - we can't see a spaceship emmit enough mass
to ever reach c, but here we're not emmiting anything
just periodicly warping space-time itself. (Fundumentally
we know that places in the Universe are moving away at faster
than c - because space-time itself expands.) In short, maybe
it won't work, but I wan'na here a better excuse.

Live long and prosper.
 
Are you thinking of the Alcubierre "Space warp"?

That exploits the fact that in GR spacetime can expand faster than light and this doesn't violate the c limit because it is not a speed and enters the math in a completely different way. So Alcubierre found a solution of Einstein's field equations where the space is contracted ahead of the ship and expanded behind it, making a kind of down hill all the way. The ship never itself travels faster than light but this sort of spacetime soliton carries it along at arbitrarily high speeds.


EMPHASIS! This is only the solution of an equation and it looks like there is unknown (impossible?) technology involved to realize it, namely harnessing negative energy.
 
Originally posted by selfAdjoint
Are you thinking of the Alcubierre "Space warp"?

That exploits the fact that in GR spacetime can expand faster than light and this doesn't violate the c limit because it is not a speed and enters the math in a completely different way.
This is essentially the way it works in Star Trek, but you still run into the speed limit because whatever energy/negative energy you use to cause the space warp can only propagate at the speed of light. To get around taht, you could wait while you build the field along the entire path, then make the trip virtually instantaneously.

To use a destination 5 ly away as an example, you would spend 5 years building the field (basically a wormhole) before sending the spacecraft through instantaneously.

Maybe you could have two fields - one from the destination and one from your origin building simultaneously and cut your time in half. Of course, syncronizing the two fields would require communication - at the speed of light.
 
Hmm... If we could get an antigravity field somehow we could achieve FTL, because while in its field our time would speed up relative to the rest of the universe's time... Right?
 
No, as I stated in your antigravity thread, it's just an inverse. Instead of an "attracting" effect, there would be a "repulsive" effect. The whole point of Alcubierre's warp bubble is to curb the effects of relativity since the ship doesn't actually move linerally through space.
 
  • #10
Greetings !

(Sorry, I took a little trip and couldn't be online
for the past couple of weeks. I'm back now ! :smile:)

Let's, for the moment, just talk about sub-light
travel. So, this concept should work as I see it.
(Right ?)(Not that we have the technology to
implement it with any acceptable level of efficiency,
but at least we know it works.)

Now, first of all no negative energy or other bogous
stuff required here. Second, why wouldn't this allow
us FTL travel ? I mean, a normal space vehicle has
exhaust mass - propellant, and of course relativistic
constraints apply here - like Xeno's paradox. However,
I can't quite see how or why that should apply in this
case.

selfAdjoint, could you please expand on why the
idea you mentioned (I've heard of it, in general)
requires some kind of negative energy ?

Thanks people ! :smile:

Live long and prosper.
 
  • #11
Well here's a discussion of how some people http://members.tripod.com/da_theoretical1/lwh/ESAAwarp.html . Also google on David Waite. He has a site that goes through a lot of GR math, and there's a subsection on FTL schemes.
 
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  • #12
Thanks !
But, for the less educated of us (for now) :
Would it be correct of me to say that basically
the idea is to add to the idea in this thread also
somekind of (unknown for now) "negative" space-time
curvature ? And if that is mostly the case - why ?
I mean, take some light, turn it into matter, then
turn it back and back again. Momentum shouldn't be
conserved here, I think, so you get exhaustless
propulsion. And... of which I'm not at all certain, but
I'm trying to find out here - movement which
is not subject to the laws of SR and can violate
it - FTL travel. Where does negative energy come
in here, or did I mix-up these different ideas ?

Peace and long life.
 
  • #13
Hope that someone can clarify this:
I know that in special relativity, faster than light travel breaks causality, for example, imagine that there's an archer shooting an arrow to an apple. If the arrow is going to travel faster than light, then there's some observer that will see the arrow hit the apple before it was launched
Does it also happens in GR, that is, does faster than light travel breaks causality in GR?
 

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