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Why are black holes black ?

  1. Apr 23, 2013 #1
    Why are black holes black even though light(photons) has no mass? Gravity shouldn't affect it.
  2. jcsd
  3. Apr 23, 2013 #2
    Gravity does affect light. I think even Isaac Newton suspected that. Albert Einstein calculated the effect of gravity on light, and he was right. As far as we can tell, black holes behave quite as he predicted, even though he neither knew nor believed they existed and didn't like the idea.

    The modern view is that mass bends space. Everything is effected by that, mass or no mass. In fact, all masses move through space the same way. It doesn't matter how heavy or dense they are, what shape, etc. Space bends the same for everything, light included.

    It takes a little getting used to, but it makes sense.
  4. Apr 23, 2013 #3
    Thinking about a typical representation of gravity in the classroom, just picture a trampoline with a bowling ball on it. The mass warps the fabric of space time. As stated above by ImaLooser, Einstein calculated the effect of gravity on light. Since light is affected by gravity, it is then easy to see how black holes are black. Once you can understand that light, although massless, is affected by gravity, the rest of your question falls into place.
  5. Apr 23, 2013 #4


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    Welcome to PF!

    Hi Mabcs! Welcome to PF! :smile:
    energy is mass, mass is energy

    light has energy, so light has mass

    (it doesn't have rest-mass :wink:)
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