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Light bending?

  1. May 30, 2008 #1

    not too sure if this is the right section but i do remeber reading it in a book about astrophysics and cosmology. The reason im asking is that a can't find where i read it.

    It said something like light bends slightly due to gravity or something like that. There was a man in a space craft and the height from which he shone a torch the light hitting a screen on the other side of the space craft was slightly lower and so the light had bent for some reason.

    If anyone has absolutly any idea of what I'm talking about please could you tell me and explain fairly simply what is going on

    Thanks _Muddy_, sorry for the rather poor and vague explanantion
  2. jcsd
  3. May 30, 2008 #2


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    You seem to be refering to the thought experiment dealing with the path of light in an accelerating spacecraft.

    The gist of it is this: If you have an accelerating spacecraft and you shine a light from one side to the other(perpendicular to the acceleration), the beam will follow a curved path as seen by someone in the space craft, due to the fact that the velocity of the ship changes as the beam crosses the width of the ship. Now, according to General Relativity, there is no difference beween the effects of acceleration and gravity(as far as anyone in the ship knows they could be sitting motionless in a gravity field rather than accelerating.
    Given this, it follows that if accleration causes the beam to curve, so will gravity.
    Astronomically, this has been confirmed by noting how the gravity of galaxies can cause the light from further galaxies along the same line of sight, to curve as it passes the nearer galaxy.
  4. May 30, 2008 #3


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    More specifically known as the Principle of Equivalence.
  5. May 30, 2008 #4
    Yes light bends in high gravity fields, for example beams of light traveling in a line near the sun (rectilinear propagation) will bend towards the sun due to the high gravity. If you type "light bending around sun" or something like that in google images you will get some nice diagrams.
  6. May 30, 2008 #5
    I thought that light consisted of photons and that photons had no mass? I didn't know that gravity would have an effect on something without a mass?
  7. May 30, 2008 #6


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    In general relativity a mass warps spacetime and light follows these curves in space. I'm sure you've seen the rubber sheet analogy for general relativity. Confusion comes about because we are originally taught Newton's view of gravity which involves the concept of forces between two objects with mass so it seems absurd that light can be affected by gravity, but as others have mentioned it has been directly observed.
    Last edited: May 30, 2008
  8. May 30, 2008 #7
    It gets kind of complicated if you havent studied the theory of relatively. A better way to word it would be that the light itself does not bend it follows the curvature of space time. Imagine space as a web of fibres, as it nears a large gravitational field such as the sun or a blackhole it becomes warped. So basically the light just follows the space time curvature. Check it out on google and if you go on youtube you can probably find some animated videos which will explain relativety and space time. Or maybe someone else on PF can explain it better as I myself have limited knowledge on the subject.
  9. May 30, 2008 #8
    there we go Kurdt beat me to the post
  10. May 31, 2008 #9
    thanks everyoen and thanks again kurdt
  11. Jun 20, 2008 #10
    The fact that everything travels in straight lines in inertial reference frames, and that these frames accelerate in a gravitational field, shows that everything has mass.
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