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Using Black Holes for FTL travel

  1. Apr 12, 2010 #1
    So... my first post, and bear in mind, all I know of Black Holes is what I've read of Black Hole and Penrose Effect articles on Wikipedia. (don't hate me)

    Would it be possible to use Black holes' Ergosphere for FTL travel?
    And this is all on the "if" that we, or some other race, achieved decend space travel technology, and materials and construction technology that would withstand the gravitational waves of the Black Hole.

    If a ship entered the Ergosphere, moving to the direction it spun, then lowering itself to the edge of the Event Horizon - Ergosphere border, went around once, then using the Penrose Effect, left the Ergosphere and towards whatever it's destination was.

    Also, I would like to apologise if I posted this in the wrong area, since I have basically no idea of anything physics related (other than c = Light speed and Black Holes haven't been confirmed totally)
  2. jcsd
  3. Apr 12, 2010 #2
    No. You can't travel as fast as light or faster.
  4. Apr 12, 2010 #3
    I was expecting a more detailed answer :|
  5. Apr 12, 2010 #4
    According to special relativity you need an infinite amount of energy to accelerate to c. A black hole does not have infinite energy. The Penrose Process would only take a small fraction of the energy of the black hole so you could not use it to travel at c .
  6. Apr 12, 2010 #5
    The previous answers are wrong. A large enough rotating black hole can be used as you say to travel backwards in time. Then you go forward in time and distance, and reach your destination before the light from your entering the ergosphere arrives at your destination.

    There are only a couple of caveats, and one is a real biggie. You can not go back in time before the black hole existed. (Technically before it started rotating, but that will usually be the same point in time.) You, and your space ship would be subject to extreme gravitational gradients, measure in g's per meter. And the big one, that close to any black hole we know about that can be used this way, the radiation is also extreme. You are going to need protection from X-rays, gamma rays, and relativistic neutral particles, among other things. The accretion disk provides a lot of this radiation, but not all of it. Radio waves falling into the black hole will be blue shifted into the X-ray spectrum and beyond, and the light from any star even a few parsecs away can spoil your trip.

    So while physicists may run this sort of experiment in the future, if it is going to do you any good, the round trip distance travelled will be in the hundreds of light years, even if you return home the day after you left.

    Doesn't this violate causality? The joke among physicists a few dozen years ago was, "General Relativity, Quantum Mechanics, and causality. Pick any two." By now though physicists are not willing to abandon GR or QM without a lot of currently non-exsisting proof.

    So is causality dead? Stephen Hawking recently published a paper about closed time-like loops (CTLLs). Crossing a CTLL may be required for causality violations, and crossing a CTLL puts you in a strange land with respect to Heisenberg Uncertainty. Since time does not pass for photons or particles traversing the loop, the amount of energy in the loop can be infinite--assuming gravity propagates at light speed. So anything can happen when crossing a CTLL, and causality is not "really" violated. (Your space ship emerges from the CTLL in the past, but that is all right. At any point in the past, any object can appear out of the CTLL, including a random spaceship with an arbitrary crew.)
  7. Apr 12, 2010 #6
    Well, if black wholes are just stretching space and time then you wouldn't really be traveling faster than the speed of light considering c is constant. However, that would all depend on if you were assuming space time to be a function of the density of the free energy field, because if you weren't dependent on that to define the space you were traveling through then you would be traveling faster than c used to be when the free energy field was at it's normal state.
  8. Apr 12, 2010 #7
    Well, theoretically you could travel FTL if you fell into the black hole, though practically your spaceship would be torn apart.
  9. Apr 12, 2010 #8


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    Ok cool, but doesn't this imply that random particles entering the black hole would be send back in time? If there are any CTLL's possible for something as large as a spaceship, then certainly a photon or other particle could traverse the same path and wind up in the past.

    Certainly that would lead to a loss in material that could probably be measured externally, or the appearance that the black hole is radiating particles. Perhaps experimental confirmation?
  10. Apr 12, 2010 #9
    Yes, you can travel faster than light. You just can't travel faster than light "locally". But, global FLT is possible. You can "jump" from one spacetime point to another spacetime point across the universe, but you take your local environment with you, so you still travel slower than the flashlight beam in your hands.
  11. Apr 12, 2010 #10
    The appearance that a black hole emits particles is called Hawking Radiation for small or non-rotating black holes. If you have a (mathematical, well worked out) theory of where the polar jets from stellar mass and supermasive black holes is generated, you too can be a guest at a Nobel Prize ceremony. ;-)

    My opinion is that the jets start out as particles which approach the black hole directly instead of getting swept up into the accretion disk. These particles will be subject to very intense radiation pressure from the accretion disk as the particle gets close to the black hole. The combination of gravitation acceleration and radiation pressure should put the particles, if they don't pass within the event horizon, in a hyperbolic orbit. In essence the particle will do a ninety degree turn at the black hole.

    What have I left out? Radiation pressure can accelerate a particle out of the event horizon even if it is already inside. (Lots of photons being absorbed and re-radiated by a particle can accelerate it away from the black hole.) Does it happen? It probably requires geometric arguments and renormalization of some sort. Photons falling into the black hole will be blue shifted, inside the event horizon, and there are infinities in most frames of reference. Also the ergosphere can touch the inner edge of the accretion disk, depending on the size of the black hole, and its spin. Frame dragging will produce very strange effects near a rotating black hole and there are reasons to suspect that all large black holes are spinning as fast as physics allows.

    Will there be a separate signature for time shifting in this very intense environment? My feeling (not even strong enough to be called a theory) is that symmetry arguments will make such radiation undetectable near most black holes. The radiation--and that term includes everything from photons to planets here--from random time travel into the past will be balanced by the (loss of) radiation from time travel into the future. Now if you can watch two black holes merge the effect should be noticeable. I've often thought that quasars may be the signature of two black holes about to collide. (Timelike orbits disrupted by the encounter would result in extra radiation near the black hole merger time. The effect could last for thousands to millions of years if one of the black holes is large enough.)
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