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B Blackhole breaking the speed limit of light?

  1. Jan 15, 2017 #1
    So, a black hole has infinite gravity that even light cant escape from it,
    my question is,
    the gravitational field of a blackhole can even pull light into it,
    then it means it is even faster than light, if not, light can escape from it.

    Does this arguement make any sense, please tell me!! thanks
  2. jcsd
  3. Jan 15, 2017 #2


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    No, it doesn't!
    Nothing can locally go faster than light and this means the trajectory of any particle(massive or not, so light or anything else) can only be of two special kinds(null for light and time-like for other particles). The reason nothing can get out of a black hole, is that inside a black hole particles can have no trajectory of those kinds that leads to outside the black hole.
  4. Jan 15, 2017 #3
    Sorry for my stupidity, but can you explain in easier words,
    so, light is unable to escape from a blackhole because of the blackhole puts light on a trajectory that doesn't allow light to escape?
    From my understanding the blackhole has a infinite strong gravitational force that pulls light to it once it enters the event horizon. So this proves the gravitational force of the blackhole is indeed breaking the speed limit of light?

    I understand that speed should not exceed the speed limit but i was trying to think of situations,
    Last edited by a moderator: Jan 15, 2017
  5. Jan 15, 2017 #4


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    Its ok, this is pretty advanced stuff and you don't have the background. Its not called stupidity.

    To put it in a simpler way, gravity is the same as deformations of spacetime from being flat. This means that at every point of space and at any instant of time, its gravity that determines what it means to go forward in time. And actually sometimes there isn't even a unique time, but that doesn't concern us here.

    A black hole changes the structure of spacetime in a way that inside its horizon, going forward in time means getting closer to the singularity. So inside the horizon of a black hole, its the definition of time to get stuck in there and get closer to the singularity.
  6. Jan 15, 2017 #5


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    Your notion that a gravitational field has a speed isn't correct. The gravitational field of a black hole is static. What makes it a black hole is that its gravitational pull is so strong that even light can't escape. At its surface called event horizon light emitted there moves locally with c. There it just isn't pulled towards the singularity and just doesn't escape either.
  7. Jan 15, 2017 #6


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    Thread moved to the relativity forum as it is more appropriate there.
  8. Jan 15, 2017 #7


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    Light moves faster than anything with a rest mass, in the sense that if you have a fair race between light and whatever else (i.e. something with rest mass) that you're racing against, the light will finish the race first. This does not mean that there aren't some race courses that even light can't finish, specifically light can not always "catch up" to something if that "something" has a head start.

    So while there isn't anything that's "faster" than the light, this statement in no way implies that light can escape a black hole. It should be understood that when we say the speed of light is the limiting speed, we simply mean that nothing can beat light in a fair race. It doesn't say anything about "unfair" races.

    It's a bit mathematical, but one can gain a lot of insight by studying the "Rindler Horizon" that occurs in accelerated space-ships. Using the principle of equivalence, one can make useful analogies between the behavior of light in an accelerating spaceship and the behavior of light in a gravitational field. It's helpful to read up on the "principle of equivalence" and "Einstein's elevator" to understand how an accelerating elevator can create something that looks like "gravity". I don't have any great beginner level references on the Principle of equivalence handy, and it's a bit of a digression, so I won't be more specific other than to mention it as something that could be useful to learn more about.

    For an intermediate level treatment (with calculus) of the Rinlder horizon,you might try Greg Egan's treatment at http://www.gregegan.net/SCIENCE/Rindler/RindlerHorizon.html. I'm not aware of any good non-calculus beginner level treatments of the Rindler horizon, unfortunately. Wiki's treatment of the topic, for instance, seems to be at least at the same level as Egan's, and to my mind much more terse and harder to follow.

    To summarize: light is the fastest thing around, but under circumstances such as the black hole event horizon, or the Rindler horizon on an accelerating space-ship, it's simply not "fast enough". Lght is the fastest thing that exists according to the laws of physics as we currently understand them, so what this means is that if light can't escape a black hole, nothing can.
  9. Jan 15, 2017 #8


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    There is no "gravity leaving the black hole". The apparent gravitational attration is there because spacetime is deformed. In the easiest case, the deformation is static - nothing changes, nothing moves. Nothing breaks the speed of light.

    Many features of black holes can be understood graphically in Kruskal–Szekeres coordinates. Image from "Dr Greg" there:


    Region I is the world outside the black hole, region II is inside the black hole. Region III and IV don't matter here. The upper hyperbola labeled "r=0" is the center of the black hole. The future is upwards, space is left/right.
    Light always travels upwards parallel to one of the two orthogonal lines. It can go towards the upper left, going from region I to region II: it can enter the black hole. Light starting in region II cannot leave it and will hit the center.

    Particles with mass have to travel "between" the two light directions: they have to move upwards, and sidewards slower than light. They can also enter the black hole, but cannot leave it.
  10. Jan 15, 2017 #9


    Staff: Mentor

    The logic is flawed. Light is also trapped by a black body or a wall or a mirror, without any of those things going faster than light. They were already present when the light ran into them. Similarly the static gravitational field was already there when the light entered it. It didn't have to go faster than light, the light came to it.
  11. Jan 16, 2017 #10
    thankyou for helping me out,
    so the
  12. Jan 16, 2017 #11
    Thankyou everyone for such kind and complete answers, appreciate it !
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