# Theory of FTL

## Main Question or Discussion Point

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 proccess. Seems like SR has no effect here,
so we should go FTL ?

Live long and prosper.

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MathematicalPhysicist
Gold Member
when you mean FTL do you mean faster then light if so i dont 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 dont 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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russ_watters
Mentor
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.
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.

Staff Emeritus
Gold Member
Dearly Missed
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.

russ_watters
Mentor
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?

neutroncount
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.

Greetings !

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

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.

idea you mentioned (I've heard of it, in general)
requires some kind of negative energy ?

Thanks people !

Live long and prosper.

Staff Emeritus
Gold Member
Dearly Missed
Well here's a discussion of how some people http://members.tripod.com/da_theoretical1/lwh/ESAAwarp.html [Broken] . 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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Thanks !
But, for the less educated of us (for now) :
Would it be correct of me to say that basicly
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.

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?