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Interaction Between Light and Black Holes

  1. Aug 15, 2015 #1
    Since light is entirely incapable of slowing down, what happens when light approaches a black hole such that it's trajectory passes through the exact center of the black hole? It seems, based on what I currently know, this would mean that the gravitational force pulling on the light would be in exactly the opposite direction that the light is moving, which would not allow for the light to change direction and be kept in that general vicinity.
    I appreciate any help you may offer.
  2. jcsd
  3. Aug 15, 2015 #2
    Nobody knows what happens at the very center of a black hole.
    All we do know is that the math of our best theories starts to give crazy results such as infinite density and curvature.
    This predicament is sometimes called 'a singularity', but what the math describes is very unlikely to be a physical reality.
    The singularity is not 'a thing', what it is, is a sign that out theories are incomplete and cannot predict what happens at such extreme density and temperature.

    In all known circumstances, light cannot exceed the speed of light ever, it's a universal constant according to relativity, and it has been verified experimentally many many times.
    The fact that we have this 'singularity' in a black hole doesn't mean Einstein is wrong, the most well accepted explanation, but still completely speculative at present, is that quantum effects come in to play which banish the singularity.
    Last edited: Aug 15, 2015
  4. Aug 15, 2015 #3
    Sure ? I think that a black hole may change all that stuff, we have tested with normal conditions, not on the real conditions of a black hole, and what if in the inside the rules just change?
  5. Aug 15, 2015 #4
    I was adding to my previous post while you replied ...
    Yes the rules may very well change if quantum effects start to become significant, but we don't know how exactly.
    There is no reason to assume the speed of light changes though, there are many other possibilities.
  6. Aug 15, 2015 #5


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    The first thing to understand is that a black hole cannot be described in terms of classical gravity, where the effect of gravity is modeled as a force. Instead, you need to use General Relativity. In GR, gravity is the result of curved spacetime. Light (and all objects) follows what is known as a geodesic through spacetime. A geodesic is the shortest path between two points. In flat space a geodesic is simply a straight line. However, in curved space a geodesic is no longer a straight line but a curved line. A good analogy is to look at the surface of the Earth. Because the Earth's surface is a sphere, the shortest path between two points on the surface is a curved line known as a great circle.

    The problem with black holes is that these geodesics literally disappear at the center of the black hole, at the singularity. This presents a problem because in order for GR to make any sense the path of geodesics must be smooth and continuous. An object cannot jump around in an abrupt, discontinuous way. A singularity breaks this smooth continuity, so we don't really know what lurks at the center of the black hole. Many believe that the singularity is an artifact of our incomplete knowledge of physics.
  7. Aug 15, 2015 #6


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    Llight maintains a constant speed in vacuum, gravity like motion, witll shift it frequency without affecting velocity. Your hypothetical photons streaming toward a black hole will be blue shifted by as much as a factor of 2 as it approaches. It is, however, doomed upon crossing the event horizon. It will never exit that region of space..
  8. Aug 16, 2015 #7
    that sounds like the light, will stop completely (at least with reference to the black hole) which I thought was impossible. How does that work?
  9. Aug 16, 2015 #8


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    Yes, the light would be absorbed by the black hole just like matter gets absorbed, or eaten, if you prefer.
  10. Aug 16, 2015 #9
    Exactly! - it shouldn't be possible.
    This is the singularity which the math leads us to - something which is very unlikely to be physically real.
    *Something* must be happening to prevent this situation, but we really don't have much clue what it could be.
    'Quantum effects' are the primary suspect, but the truth is that we just don't know (due to lack of any evidence)
  11. Aug 23, 2015 #10
    If you were to take this back to the 1940s, you will see that even Albert Einstein did not approve of such confusion. He stated that nature should not allow this to happen. Theorized mathematical equations suggest that the event horizon is so strong that it would take matter to the center faster than the speed of light. Black holes warp spacetime so much that time by itself is slower than the area around it by much greater factors. Speed can affected how we move through time very much. For example, comparing someone sitting to someone walking 5 mph is a time dilation of 3 to the negative 15 power.
  12. Aug 23, 2015 #11

    Vanadium 50

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    Reference please. I find this claim surprising since the determination that you get a black hole - a region where matter and energy can enter but never exit - was made by Finkelstein in 1958. Einstein died in 1955.
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