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The Reacceleration of Light

  1. Sep 28, 2008 #1
    Slowing light


    This is my first post here, but it seems like there are a lot of very nice people here, so I have a question for you (forgive my ignorance, I was a failure in high school math and never took physics, but as with anything, with age you start to appreciate things more.)

    From what I've been reading of Einsteins work, time is relative. The only constant thing in the universe is the speed of light.

    Einstein stated that when light passed an object of great mass, the objects gravity effects the speed and direction of light. It bends it.

    If this is true, then as light is slowed down, does it regain its speed, or does it remain forever changed?

    My question might be confusing for no other reason than I'm having a hard time explaining my question.

    I hope someone can help!
  2. jcsd
  3. Sep 28, 2008 #2
    Re: Slowing light

    HI, and welcome to PF (i'm new too)
    Since light is always constant, it will stay constant, meaning that it will always travel at 3x10^8. Some may say that it isnt true, if you look at shining light through glass, it will slow down. In reality, shining light through a material or medium with a index of refraction greater than 1 is like adding to its distance the light has to travel (imagine a cube with a long maze, it takes longer to get to the other side than when there are no obstacles, amiright?)
    Likewise, the gravitational pull is nothing more than bending and increasing the length that light might have to travel
  4. Sep 28, 2008 #3
    Re: Slowing light

    Thanks for your reply. I was reading about how a telescope is actually slowing down light and moving you further along the dimension of time, which would make sense.

    But like all things, they react when acted on by an outside force, and if light can be bent, it can slow down. If this is true, then it would have to remain at the reduced speed, OR it would have to reaccelerate, but what outside force is acting on the light to return it to its velocity?
  5. Sep 28, 2008 #4
    Re: Slowing light

    That is partially true, when light leaves the material when it travels through (n>1), then until it changes material, it will stay at that speed, so in glass, after light passes through, light will retain its speed at 3x10^8
  6. Oct 1, 2008 #5
    Re: Slowing light

    Try posting this Q in “Special & General Relativity”. You might get better answers.

    I’m an amateur too. So welcome. Since gravity’s involved, you’re asking a General Relativity question. The speed of light can be constant only because time and space are not. But how do you know if time and space change for you too? It all depends on your point of view. In relativity, they call that the reference frame. You’re in a lower gravity reference frame and the light is in a higher gravity reference frame. More gravity makes time look like it’s running slower. But if you were right down there with the light, you wouldn’t notice. You could measure the speed of that light and it would be full speed. But from way up hear, it looks a little slower.

    (Does it regain its speed?) Only if it gets back to less gravity. It might not if it’s near a black hole.

    I love how ChrisPeace put a QM view on your question by using glass as your object of great mass. It’s not great but any mass has a gravitational effect. Do you know the inverse square law? (Look it up.) Even a small mass can have a strong gravitational effect if you get really close. In QM, it’s no good to treat particles as points. But let’s do that just for fun. In this way, even an electron can bend light if the light particle (photon) gets really close. So let’s have some fun with that idea. If a photon goes through glass, it must pass really close to a lot of electrons. They must slow it down. And what if a photon hits an electron? Since there’s the conservation of energy, the energy of the electron increases. (I have no idea if that’s true.)
  7. Oct 1, 2008 #6
    Hello everyone,

    This is kind of a repost because I think I posted my question in the wrong forum...or at least someone was kind enough to point out my mistake.

    First off, I apologize if my question isn't clear. I'm not a physics person (yet), but I've been doing some reading.

    In Relativity Einstein says that when a beam of light passes an object of great mass that it not only bends, but it actually slows down.

    My question is this. If light does indeed slow down, does it reaccelerate? If so, HOW if there is no force acting on it to cause it to do so?


  8. Oct 1, 2008 #7
    Re: Slowing light

    Light can NOT bend. It travels in a perfectly straight line. However, light travels through space-time continuum and massive objects can bend space-time itself. So in essence light doesnt bend but space-time does. These are in perfect contradiction with what we see in the universe today because we see light bending behind massive planets and stars every now and then, so whats happening??

    The answer is simple... light has to "bend" whenever and wherever space-time bends to still be in the continuum. Try imagining a situation where light continues to travel in a straight line even after space-time had been distorted by some degree in the other direction. It doesn't make sense because light has to travel through space... in essence, light has to be in agreement with the continuum for it to be able to go from point A to B. If light refuses to bend even when space time does, then it will never ever be able to travel to anywhere and mathematically speaking: its just inconclussive and meaningless. Light has to bend wherever and whenever spacetime bends... :)
  9. Oct 1, 2008 #8


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    As tct college said, in the sense of general relativity, light does not "bend", it always follows a geodesic path- a "straight line" in the local geometry.

    From a Euclidean point of view, light DOES "bend" but Einstein never said the the speed was changed! The speed of light is always "c".

    When light passes through a medium, like glass, in which the observed speed of light is less than c, strictly speaking what happens is that light passes between atoms at c, is "captured" by an electron, the retransmitted. It is the time lag between capture and retransmission that causes the apparent reduction in the speed of light: in fact, light is not slowed nor stopped- it simply does not exist between capture and retransmission.
  10. Oct 1, 2008 #9
    I don't think the intrinsic speed of light is ever altered however the distance it has to travel is alterable. Distance is NOT fixed. I will explain below with 2 scenarios.

    Assume there exists points A and B in space such that it takes light exactly 1 minute to cover the distance.

    Now assume that a super massive blackhole forms between points A and B such that the straight line distance between A and B is at least tangential to the circumfrence of the blackhole but not in the event horizon so that light can successfully travel the distance.

    Since blackholes have the effect of taking discreet points in space and making them a singularity, i.e they shorten distances, it will take light less than 1 minute to travel from point A to point B simply because space time has been altered (compressed) but the speed of light itself always remained constant.

    Remember that space-time is like an elastic fabric. It can be stretched and warped. The time it takes light to travel strictly depends on the distance it has to travel because its speed is always constant. In essence, if point A and B are at a distance d apart; and since the time it takes light to travel a given distance is t = d/c; distance, d, can always be altered by sufficiently massive objects. For example, if d lies at least tangential to the circumference of a very very massive planet, then from an outsider's perspective d increses and as such the time it takes light to travel also increases.
  11. Oct 2, 2008 #10
    Re: Slowing light

    Light does not "slow down" picture this.

    You walk to school everyday, it usually takes you 15 minutes, but because there is construction going on it takes you 20 minutes now. In reality, you still walk the same speed, but because there is construction going on now, it takes you longer, picture gravity as the construction going on, and light as the traveler. Get it?

    People seem to be confused about light, just a bit though. Let us all remember light has its given velocity because of the way it's constructed in this universe, not because it's some god like being. In order to "slow" down light, you'd have to increase its mass, that at the moment is still impossible.
  12. Oct 2, 2008 #11
    If I may just expand a little bit on the idea of space being warped and stretched...I just want to see if I'm comprehending a concept.

    So what you're basically telling me is that if I take a sheet of stretchy rubber, and I get a ruler and a pen, and draw a straight line on it, no matter if I stretch the rubber in any direction, or make it rippled or whatever the case may be...the line still remains straight.

    To me that still seems like an adaptation of Euclidean Geometry without disturbing its principles.
  13. Oct 2, 2008 #12
    What kind of description are you using to say all this? I'm asking you because it doesn't match with what is written in the Physics Forums FAQ here:
    "Do Photons Move Slower in a Solid Medium?"
  14. Oct 2, 2008 #13
    Yes. Thats exactly it. If you draw a straight line on a stretchy rubber and stretch, warp or ripple it, the line you drew would seem to twist or stretch relative to an external observer. However, relative to the rubber itself, the line is still very very straight.
  15. Oct 2, 2008 #14


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    It might be easier to see/comprehend this if you first imprint the rubber sheet with an xy grid.

    Then draw your line perfectly diagonally across the grid and the line crosses every xy vertex on its diagonal path.

    Now you can plainly see that, no matter how you twist and distort the rubber sheet, the line still crosses the xy vertices.

    And those vertices double nicely as units of measurement too. Everywhere you measure, from the flattest plain to the most tortured loop-de-loop, the line will go up one unit and across one unit. i.e. perfectly straight.
  16. Jan 1, 2009 #15
    This may be necroposting and I don't know if this is against the terms of the forum, but I'll give it a go.

    I understand the idea that time doesn't bend, space does, but I know I've been getting a lot of people saying that light doesn't slow down.

    In Relativity Einstein says "a curvature of rays of light can only take place when the VELOCITY of the propagation of light VARIES with position".

    Anyone care to translate? To me it says that when curved lights velocity can vary.
  17. Jan 2, 2009 #16


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    In physics, velocity is a vector quantity, that is, it has both magnitude (which we call "speed") and direction. An object's velocity can change by changing either the speed alone, or the direction alone, or both speed and direction simultaneously.

    The speed of light is constant, but its velocity can change (by changing direction).
  18. Jan 2, 2009 #17


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    Time does bend too. Here is my favorite visualization of curved time vs. curved space:
    I depends where the clock is, that measures the velocity. The local stationary clock at the lights position measures always c. A stationary clock far away of the gravitation source runs faster and measures less than c.
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