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Some questions about light

  1. Nov 23, 2014 #1
    As a writer, light is as crucial as it is to physicists. But I'm struggling with some conceptual understanding surrounding light. If you could supply any answers to this science layman I would be very grateful.

    1) Without light there is no way of observing and measuring. So light is both itself data and the medium for data to be conveyed at the same time?
    2) The speed of light is the speed limit of the universe. But light can't escape a black hole, because the speed of tumbling matter once past the event horizon is greater? Or is it that light can't overcome the huge gravity in a black hole, in which case how does this work? Past the inner horizon and the tidal sweep through the black hole's rotation heats up the interior higher than even the core fusion energy produced in stars - is this the energy that counters that of light preventing its escape?
    3) Doesn't light itself behave relativistically? That is, in Einstein's gedankenexperiment a man travelling astride a beam of light does not experience light as moving because he is moving at the same speed. Light is effectively stilled, he will always remain astride the same 'block' of light, or is this not a legitimate way to conceive of light? Is there any usefulness in conceiving of blocks or bits or segments of light - we know that it travels at 186,000 miles per second, and we have a concept of only receiving now the light transmitted from stars x light years away, but light is light unless we can somehow differentiate it or have I grossly misunderstood this? Again there seems to me the dual nature of light as data itself and the medium by which other data is transferred to us.

    Thanks for your help
  2. jcsd
  3. Nov 23, 2014 #2


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    absolutely not, even if you use the term "light" to include the entire electromagnetic spectrum (infrared, etc) not just visible light. It certainly is by far the major method but observing/measuring things like neutrinos does not depend on light.

    Technically, it's the other way around. There is a universal speed limit and light obeys it. That is, there is a universal speed limit that is not dependant on light. If we found out that photons had mass (it would have to be staggeringly tiny or we would have found it already) that would not change the universal speed limit, it would just mean that light doesn't go that fast.

    the "escape velocity" at the Event Horizon of a BH is c, so nothing can escape.

    the escape velocity is determined by the space-time curvature caused by the mass inside the EH.

    This is not a legitimate way to think of light because it implies that light has a reference frame but a reference frame is by definition a frame in which an object is at rest and ligth is never at rest.

    I'm not clear on what you are asking here. Light is DETECTED as the particle nature of the quantum object which is light, so yeah, it's useful to think of "bits" of light (specifically the particle characteristic of photons, which are neither particle nor wave but quantum objects that exhibit some characteristics of both without being either).
  4. Nov 23, 2014 #3
    many thanks for your responses.

    Just one follow up, if c is the escape velocity of the event horizon, doesn't light travel at c, so couldn't light in the black hole escape? I mean I kn ow it can't, but why not?
  5. Nov 23, 2014 #4


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    You have to go FASTER than the escape velocity to escape. A photon emitted exactly at the EH would just sit there, moving at c locally but globally constrained by the space-time curvature of its particular position. A photon emitted just outside the EH would EVENTUALLY move away from the BH, it would just take a long time to do so because, again, although it's moving at c locally, it is constrained globally but the space-time curvature (but less and less constrained as it gets away from the gravity well of the BH).

    By the way, "escape velocity" is a bit of a tricky concept. It is a BALLISTIC concept. That is, the escape velocity at the surface of the Earth is the velocity at which you would have to shoot a bullet in order for the bullet to escape to infinity. Rockets that go to the moon never go anywhere near that fast. What they do is use a lot of force to move away from the Earth and as they do so, the escape velocity from where they are gets to be less and less.
  6. Nov 23, 2014 #5
    ah that makes perfect sense.

    Many thanks once again
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