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Black Holes

  1. May 10, 2007 #1
    So black holes are incredibly dense points of matter that affect everything in their area with very powerful gravity field, including light. \

    So, asuming that gravity affects only matter, that by default says that light is a physical particle.

    So why don't the regular laws apply to it? When light moves through the air in the atmosphere, why doesn't it slow due to air resistance like all other objects with mass?
  2. jcsd
  3. May 10, 2007 #2


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    When light travels through the atmosphere its speed is slightly lower than in vacuum.
  4. May 10, 2007 #3
    You assumtion is wrong. Gravitation affects mass and energy. Under gravitation mass and energy is equivalent.
  5. May 10, 2007 #4
    As has been stated, your assumption is incorrect. If you look at general relativity, gravity is a manifestation of the curvature of spacetime. Light, or photons, travel along null geodesics, which are essentially a generalization of straight lines in curved space. So if a photon is traveling along spacetime it is going to follow that generalized straight line path, which in curved spacetime means it will appear to be bent (giving rise to gravitational lensing).

    And at any rate, light does behave as it should. In mediums denser than vacuum, the speed of light is less than it is in the vacuum (this gives rise to index of refraction for example).
  6. May 11, 2007 #5
    thanks for the help. I have two more question:

    Does light continue slowing down as it moves through more of an atmosphere?

    If light is in the electromagnetic spectrum, then all other wavelengths of the spectrum travele at the same rate. Correct?
  7. May 11, 2007 #6
    - No, the velocity only changes at the interface.
    - In vacuum, yes. In media, no, it depends. Otherwise there would be no rainbows..
  8. May 11, 2007 #7
    If light is bent around a black hole, does it bend around all points of gravity, however small?

    And even if it doesn't, do the other waves of the spectrum bend around black holes?
  9. May 11, 2007 #8
    yes! light bend around all points of gravity!

    yes! the other waves of the spectrum also bend around black holes!
  10. May 13, 2007 #9
    As the medium becomes denser then in general yes, the effective speed of light decreases. For example light travels fastest in a vacuum, slower in air and slower still in water.

    And correct, light is just an electromagnetic wave. What makes it special to us is we can see a sliver of the electromagnetic spectrum and have called it light. Light is the same thing as x-rays or radio waves. All that differs is the frequency (or wavelength).

    Yes, as stated it will move around all sources of gravity. But the effects will be very very very small for small sources of gravity.
  11. May 22, 2007 #10
    you have to remeber that black holes have one of the strongest gravitational pulls. they pull in a lot of thing, including light.
  12. May 22, 2007 #11
    The bending of light around the periphery of Sun was one of the experimental confirmations of General Relativity.
  13. May 22, 2007 #12
    physically speaking light really slows down a bit
  14. May 22, 2007 #13
    So melodramatic.. won't you enlighten us all then to the error of our ways?
    Last edited: May 22, 2007
  15. May 22, 2007 #14
    In the presence of a medium of refractive index n, the speed of light is reduced to c/n. Since the atmosphere has an refractive index of around 1.000295 at sea level for visible light, it is not slowed down much.

    You may interested to know that light beams have been slowed down to a walking pace and even stopped completely (http://www.afrlhorizons.com/Briefs/Jun02/OSR0201.html [Broken] just from google).
    Last edited by a moderator: May 2, 2017
  16. May 22, 2007 #15


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    Wont it just be easier to tell us? I'm eager to know why what mathman said is wrong.
  17. May 22, 2007 #16

    Chris Hillman

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    Oh, it wasn't so bad

    Somewhat to my surprise, people care. Hmm... it wasn't nearly as bad as I said... sorry!.

    Oops, nothing. I was "kicking" about a bunch of posts which came later on. Here are my pedantic corrections/comments:

    I think madphysics is trying to ask whether light bends around any compact object, or indeed any concentration of mass-energy in some region, or just around black holes. The answer is of course "yes".

    Light bending does not depend upon the frequency of the light--- this is very important, if it weren't true, we'd see colored fringes in those Hubble images of lensed galaxies. But in any case I suspect madphysics meant to ask if "massless" radiation which is not electromagnetic radiation--- in particular, gravitational radiation, also experiences bending. The answer is "yes", because wave packets of such radiation have (roughly speaking) world lines which are null geodesics (in vacuum).

    Well, from what I just said you can see that I wouldn't put it this way.

    Sorry, Brad, that looks OK too.

    All objects with mass M have pretty much the same "gravitational pull". The interesting thing about neutron stars and black holes is that they are so much more compact than ordinary objects, which means that you can get a lot closer without striking "the surface" (or encountering the horizon). Since the tidal forces scale like M/r^3, smaller r for given M means much larger tidal forces.

    sanjeeb is thinking of the "Shapiro light delay effect", but this name is potentially misleading since, at the level of tangent spaces, light always travels at c (in vacuum). This effect is really a global effect; in a sense "the effective speed of light" over a large course can be different from c. One way to understand that this is no contradiction is to realize that even in flat spacetime there are multiple operationally significant notions of distance in the large for accelerating observers (which at very small scales all reduce to the notion given by the metric tensor). In particular, radar distance, the notion relevant to the Shapiro effect, can exhibit odd behavior when a light path travels near a massive object such as the Sun.

    These corrections are probably incomplete since at least two of the other posters are in my ignore list, in part because I tired of correcting their frequent misstatements concerning gtr.

    OK, shame on me for not reading more than the last half of the thread. I'll try not to make such sweeping statements in future :blushing:
    Last edited: May 23, 2007
  18. May 23, 2007 #17
    I think people were a little offended by your comments Chris because it just came across as very arrogant that you accused *everyone* of being wrong and you wouldn't even justify your reasoning, instead we would have to ask you for the privlidge of divulging your greater widsom upon us.

    In scientific circles, these kind of sweeping statements are highly offensive. But your apology is welcome and I won't hold it against you in future!
    Last edited: May 23, 2007
  19. May 23, 2007 #18

    Chris Hillman

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    Thanks, natski, I appreciate that!

    OK, we now return you to the original thread
  20. May 29, 2007 #19
    Sorry i hope you dont mind me posting this here rather than starting a new topic

    I believe i'm right in saying that light cant escape a black hole due to it following a pathway through bent space which starts and ends in the blackhole.

    so if somehow we could survive the trip through the EV of a blackhole and then tryed to look out through the EV back into normal space what would we view the EV from inside as being ,bright due to all light trying to escape following pathways back in or black.
    Last edited: May 29, 2007
  21. May 30, 2007 #20

    Chris Hillman

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    If you are picturing null geodesics inside the event horizon, any outgoing ones must fall back without ever getting outside the horizon.

    See Andrew Hamilton's website (see my sig) for some pointers, then ask again.
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