Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Theory of FTL

  1. Oct 6, 2003 #1

    drag

    User Avatar
    Science Advisor

    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.
     
    Last edited: Oct 6, 2003
  2. jcsd
  3. Oct 6, 2003 #2
    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.
     
  4. Oct 6, 2003 #3

    drag

    User Avatar
    Science Advisor

    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.
     
    Last edited: Oct 6, 2003
  5. Oct 6, 2003 #4

    russ_watters

    User Avatar

    Staff: 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.
     
    Last edited: Oct 6, 2003
  6. Oct 7, 2003 #5

    drag

    User Avatar
    Science Advisor

    Greetings !
    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.
     
  7. Oct 7, 2003 #6

    selfAdjoint

    User Avatar
    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.
     
  8. Oct 8, 2003 #7

    russ_watters

    User Avatar

    Staff: Mentor

    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.
     
  9. Oct 8, 2003 #8
    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?
     
  10. Oct 12, 2003 #9
    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.
     
  11. Oct 20, 2003 #10

    drag

    User Avatar
    Science Advisor

    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.
     
  12. Oct 20, 2003 #11

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Well here's a discussion of how some people reduced the negative energy requirements . 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.
     
  13. Oct 20, 2003 #12

    drag

    User Avatar
    Science Advisor

    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.
     
  14. Nov 6, 2003 #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?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Theory of FTL
  1. FTL Possible? (Replies: 7)

  2. QFT and FTL (Replies: 4)

  3. Is FTL possible? (Replies: 27)

  4. FTL and causality (Replies: 10)

Loading...