Is gravitation faster than light?

  1. As far as I read, gravitation or better: gravitational waves expand with c.

    However, particles like photons which are moving along with c are subject to the curvature of space. Regarding a black hole, light cannot escape with c but gravitation can.

    Is gravitation thus faster than c?

    Carsten
     
    Last edited: Jan 6, 2005
  2. jcsd
  3. dextercioby

    dextercioby 12,314
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    The short answer is:NO.What do you mean,"gravitation can (escape a blackhole)"...???Gravitation IS a black hole (too)...A singularity in the gravitational field,that is...If u want to,a particular solution to the Einstein equations...

    Daniel.
     
  4. Hurkyl

    Hurkyl 16,090
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    The gravitational field outside the black hole is already there; it doesn't need to "escape".

    Of course, anything that happens inside the black hole cannot propagate to the outside... so any gravitational waves that are inside the hole never make it out.

    The same, incidentally, is true of the electromagnetic field of the hole.
     
  5. Hasn't science, so far, proven it be impossible to move faster than c?

    ------ Life is a Problem... SOLVE IT!!!!
     
  6. Hi Problem+Solve,

    Not quite. Theoretical physics do not rule out speeds faster than c (see also: Dirac theory). Mass cannot move as fast or faster than c.

    Particles faster than c are called tachyons. However, no one has found tachyons so far.

    Hm, Hurkyl, is it really true that gravitational waves cannot leave a black hole?

    Changes in a gravitational field are supposed to be "communicated" by gravitational waves. If gravitational waves (which have not been detected so far) were not able to leave a black hole, nothing outside a black hole would notice changes in the gravitation of the black hole.

    Since black holes of different masses are observable, are we not able to conduct that they can send out gravitational waves with this information?

    Otherwise, black holes should all show the same mass (= gravitation): the mass which is necessary to initially form a black hole. Afterwards, information on the increase of its mass would not be able to be "communicated" outside the black hole...

    Moreover, would there be only a 0-or-1 approach to the escape of gravitational waves from a black hole? A gradual influence on the gravitational waves seems more likely, since gravitational fields and the curvature of space also gradually influence c.

    So far I read that the existing theories of gravitation (including SRT, GRT) and EM show explanatory gaps in situations of singularity (big bang, black holes). Is this one of the gaps or which are they?

    Carsten
     
    Last edited: Jan 6, 2005
  7. Alright, thank you.... Do you know a good referece page on the Dirac Theory? I understand the basics of it, but want to know more. Again, thankyou...

    ---- Life is a Problem.... SOLVE IT!!!!!
     
  8. Well, I do not know the profound reference pages but here are some links:

    A simple introduction:
    http://encyclopedia.thefreedictionary.com/Tachyon

    A simple introduction into the equation:
    http://www.iscmns.org/iccf11/ppt/FilippovDtachyon.pdf

    Not bad at all:
    http://www.cbloom.com/physics/2d_dirac.html

    More detailed, with coupling of branes:
    http://www.iop.org/EJ/abstract/1126-6708/2001/02/002
    http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:hep-th/0003122

    Have fun reading!

    Carsten
     
  9. Hurkyl

    Hurkyl 16,090
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    In GR, I'm quite sure -- otherwise we'd be able to get information from inside the black hole.

    The resolution is this: the increase of mass doesn't occur inside the hole -- it occurs on its boundary as massive objects fall from the outside to the inside.
     
  10. That is a good point!

    The boundary must be the Schwarzschild radius, right?
     
  11. Hurkyl

    Hurkyl 16,090
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    The boundary is the event horizon. It doesn't have to be spherical... (but maybe it quickly changes into a spherical shape? I don't recall for sure)
     
  12. As far as I know the Schwarzschild radius is the event horizon since the escape speed at this point becomes c.
     
  13. Something I realized a day or two back is that in the sum-over-histories version of quantum theory, if a particle is near a black hole then some of the paths being summed up must cross over the event horizon and come back out again! :biggrin:

    These are obviously faster-than-light histories which form part of the sum-over-histories that result in the light-speed-or-less particles that make up the universe.

    I knew there were faster-than-light histories but this crossing over the event horizon was just an amusing thing I realized. Thought I'd mention it. :smile:
     
  14. Thanks, Caribou, that is another interesting point. Maybe the particle will not be able to escape the black hole once it has passed the Schwarzschild radius - even if its probability inside the radius was just a fraction above zero. If it interacts with the black hole inside the radius its probability becoms 100% (like interaction with a measuring device).


    But still my initial question remains. Maybe I should state it differently:

    In the theory of gravitational waves: Are these waves influenced by relativity and the curvature of space (as photons and other particles are) or not?

    If yes, I would consider it paradox because gravitation would influence itself.

    If no, I would consider time dilation, lenght contraction and the like to behave differently with gravitational waves as they behave with other particles like photons. (But this would also imply that c is not the proper speed for gravitational waves.)

    Does anybody know the answer?

    Carsten
     
  15. I just looked up Jim Hartle's book on gravity and noticed something about a definition.

    Gravitational waves are "ripples in spacetime curvature" caused by "any mass in nonspherical, nonlinear motion". But a mass in spherical and/or linear motion must still affect other masses.

    So I assume this means gravitational waves and gravitons are different things, with the former emitted in many situations and being made up of the latter.

    I think gravitational waves from a black hole are like ripples when a stone is moved back and forth in water. The stone doesn't really change in any significant way.

    Gravitons would be smeared out quantum particles with wave properties that can make up these ripples when they occur. So a graviton is a gravitational wave in a different sense.

    My guess is that with gravitons we'd be talking about them in smeared out orbits outside and inside the black hole's horizon with them being in increasing and increasing numbers closer to the center. And this is where the gravity comes from, with these orbiting gravitons affecting the paths of other gravitons and anything else.

    A black hole is a big cloud of gravitions going out from the singularity to the event horizon and beyond? And something which disturbs a black hole causes a whole lot of gravitons to be knocked out of orbit and form a gravitational wave?

    I'm speculating well beyond my own level of knowledge now. :bugeye: :biggrin:
     
    Last edited: Jan 7, 2005
  16. selfAdjoint

    selfAdjoint 8,147
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    If you accept a graviton theory of gravity, a gravity wave would be a coherent stream of gravitons, i.e. their wave functions would be in macroscopic synch.
     
  17. gravity is curvature

    According to the GRT, curvature is gravity and vice versa.
    Gravitational waves should not be imagined like electro magnetic waves, but rather as curvature ripples which move across the 4 dimensional space.

    As such, gravity does not escape from a black hole. It is created by the black hole, in this sense that the black hole curves the space around it.

    Greetz,
    Leo
     
  18. Thank you caribou, selfAdjoint and Leo!

    But I believe I still do not have an answer to my question. I am sorry for bugging you, but maybe I have missed the clue.

    Let me put the question differently:

    If we think of gravitons instead of gravitational waves, do they move with the speed of c?

    And are they influenced by relativity as well as the curvature of space?

    Example: Are gravitons from a mass m deflected if they pass nearby the space curvature of another mass M?

    Carsten
     
  19. As far as I remember, fields are considered to be established when you do a problem. They travel at the speed of light, so if you don't have mass or charge in the infinite, then the fields in the infinite are zero always.
     
  20. Leo said it well,
    Light cannot escape a black hole because the force of gravity i.e gravitons, are forcing it them back. Gravity is the black hole (though the singularity is mass).

    When we say gravity waves travel at c, a good example of this is such: If the sun were to disapear right now, it would take ~8mins for earth's orbit to be affected (aswell for us to see it). So picture a wave through 4 dimensional spacetime traveling at c. So if a black hole was to disapear, let's say a black hole powering a galaxie, the gravitons would travel at c, so each star would lose its orbit at different moments.
     
  21. Hi DB,

    that is correct. By theory, the observer on such a star (or an observer orbiting it) should notice ("watch") the black hole vanish at the same time his star loses the orbit around it.

    Also correct. But are gravitons or gravitational waves the same as curvature of space? The curvature itself is static in the sense of resting in space. The wave (or gravitons) propagate with a speed (c?). So it must be two different effects even if they have the same origin.

    But this would imply, that

    a) gravitons are either able to propagate at a higher escape speed from the black hole than photons. (However it would mean our observer´s star would lose orbit before the observer notices the black hole vanish.)

    or

    b) gravitons propagate at c but we do not know how they escape a black hole. (And does the relativity of c apply to gravitons as well: speed, time, length contraction?)


    I do not want to be impolite. My insisting on this point might seem like it but it is truly not and I sure do not want it to look like it. I am just a little curious.


    Hurkyl, is this an accepted theory or rather a personal assumption?

    If gravitons or gravitational waves occur from the boundary of a black hole (Schwarzschild radius), why wouldn´t photons do the same? But wouldn´t we see a white hole instead of a black one?


    Your curious

    Carsten
     
    Last edited: Jan 8, 2005
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