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Is the speed of light truly constant?

  1. Nov 19, 2014 #1
    The question that ponders upon the thought if the speed of light really is constant is a fairly common question. However, I was wondering about something and MAYBE this can provide some backup to the theory that the speed of light isn't constant, please point out any mistakes I make in stating theories wrongly, using the wrong theories in the wrong context of use, etc as I am just learning about physics. If Einstein's General Relativity states (just as an example that uses General Relativity) that light bends when it gets close to the Sun because of the Sun's huge gravitational pull, then wouldn't the speed of light (when close to the Sun and is bent) increase because the gravity is pulling it down faster as it gets closer? Is it the path of the light or is it light itself that is "bending"? Is light simply following the contour of space-time which doesn't change it's speed? It's like when you put the bowling ball on a trampoline, then you put a billiards ball on there, then the billiards ball falls towards the bowling ball cause it has more mass and so if we let the trampoline symbolize the fabric of space-time, the bowling ball as perhaps the Sun or any star for that matter, and the billiards ball as light (photons), the billiards ball increases momentum and speed as it reaches the bowling ball, same thing with light, it increases momentum and speed as it reaches the Sun (any star), and for it to have momentum, it must have mass, if it has mass, then it's affected by gravity, therefore, gravity affects light changing its speed at certain times, am I correct?
  2. jcsd
  3. Nov 19, 2014 #2


    Staff: Mentor

    Locally the speed of light is constant. However, when you are in a curved spacetime the global notions of speed and distance can vary due to the curvature.
  4. Nov 19, 2014 #3
    What makes special relativity work is that the speed of light in a vacuum is invariant. An object can curve and keep a constant speed, look at spacecraft in orbit. I think you are confusing velocity and speed.
  5. Nov 19, 2014 #4
    I never really thought of that spacecraft example, it's really good. I have another question though, if photons are just tiny packets of energy, in order for them to have speed, shouldn't they have some kind of mass? Also, what gives light its speed?
  6. Nov 19, 2014 #5
  7. Nov 19, 2014 #6


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    Staff: Mentor

    Photons do indeed have mass (in the sense that a box containing a switched-on light source will have slightly more mass than the same box with no light inside it), and if you know the energy of a photon you can calculate its mass from the famous ##E=mc^2##.

    However while you're studying relativity,just about the best thing you can do is to try to forget that you ever heard the word "photon" :) Save that concept for quantum mechanics, and think in terms of flashes of light for your relativity studies.

    As for what gives light its speed: Light is an electromagnetic wave, and the speed at which it propagates in a vacuum can be calculated from Maxwell's laws of electricity and magnetism (google for "Maxwell's equations"), discovered in 1861.
  8. Nov 20, 2014 #7

    Wes Tausend

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    Gold Member


    Yes, the speed of light at least always measures constant so far.

    If you are, "just learning about physics", I found Einstein's description on gravity the easiest to follow. Earlier today, another forum member asked about gravity and how it affects everyday life, so I included a link to Einstein and also gave him my version of Einstein's "thought experiment", about how Einstein started assembling his General Relativity (GR) gravity theory. The bending of light is certainly part of the same thought experiment. But read the first part linked below first.

    The first part of how Einstein more-or-less said plain old gravity acts is HERE.

    Below I will describe in my own words how Einstein realized light should bend in gravity.

    In brief, the above link refers to gravity being equal to an elevator being drawn up in otherwise empty space. Two scientists do simple experiments to see if they are in a gravitational "field". They've already dropped an object to see if it falls (in the other, above referred post link). Actually the floor rises to meet the floating item, but it appears exactly as though it has fallen in a firmly founded room resting on earth.

    Anyway, now the pair of scientists are ready to do a gravity vs light test. Science should be fun, by the way.

    In their remarkably well equipped elevator, the science team has a lithium powered drill. So their fondness for wrecking things makes them decide to drill a hole in the wall (all real scientists have this trait, atom smashers, etc). They choose an expensive diamond tipped titanium 1/8 inch bit and it immediately breaks. They blame the fake gravity. Science takes persistence and total lack of remorse, so they grab a more robust cheap 1/4 inch bit. After nearly melting this bit, they are finally successful in perforating a nasty hole in the formerly perfectly good wall of the costly elevator.

    It just so happens that there is a light beam hitting the outside of the damaged elevator at exactly the right spot. And at a handy 90° angle! (Science requires a lot of luck too.)

    Doing good science begs that scientists also be opportunists, so they decide to see what properties the light has, because they are curious, and in case there is lecture circuit money in it. Decades of previous data indicate light travels in straight lines, so they are pretty sure light travels in straight lines. At first there seems nothing remarkable about the light beam streaking through the room and hitting the opposing wall. Fortunately for them, the air in the elevator is full of floating sheet rock dust from wrecking the wall. Suddenly one of them notices that the light beam traces a curve in the dusty room atmosphere, and the beam hits the wall a bit lower than it should on the opposing wall. They argue who saw it first. Career politics in the middle of nowhere, very typical.

    Then they suspect that the curve results from the acceleration of the elevator, but this time neither wants sole credit in case this ghastly crooked light theory is incorrect. They finally reason since the light takes a period of time to travel across the width of the room, the accelerating elevator steadily moves up a bit during the delay interval and this causes the light to keep drooping lower on it's path along the dusty way to the good wall. They reason that if the elevator had been moving at a steady speed instead of accelerating, the dusty beam path would have been straight. They conclude only acceleration "bends" light. They argue who concluded first.

    If the scientists had a ladder, they could drill another hole nearer the ceiling and that light beam would not bend quite as much. Because they are accelerating, it is as though artificial gravity is weaker further from the floor, just like real gravity weakens high above earth. With an incredibly accurate bathroom scale, the scientist on top of the ladder would also weigh less at the top of the ladder (than on the floor) also because of the weaker upper pseudo-gravity. I know I have that simple logical explanation why this is so around here somewhere on another post, if you need it.

    Back on earth the two scientists eagerly prepare to tell everyone who will listen, that gravity bends light. But, since he sent them there, Einstein hands them a broom while he takes all the credit. And besides, he knew that would happen. He always said, "I knew that"... and his predictions were always right, at least so far.

    In all seriousness, this is a very simplified rendition of how gravity and light interact. Einstein was able to use the very same principle to predict light would bend in a gravitational field. Naturally, he convinced other people go to the middle of nowhere to observe, and prove, the phenomenon during an eclipse. Of course.... the master was truly ingenious.

  9. Nov 20, 2014 #8
    In fact that was not a good example, as Nugatory already pointed out: light is modeled as a wave in GR, just as in SR. The elevator thought experiment nicely illustrates that, based on the equivalence principle, light should bend in a gravitational field - but it does not tell us why.

    Einstein's physical inference was the following: as measured from Earth, the speed of light is reduced in the Sun's gravitational field (which is rather the opposite of your speculation). His calculations of how much light bends (first in 1911, when he ignored a term, and next in 1916*) were fully based on that interpretation. As a matter of fact, if he had used the assumption of SR according to which the speed of light is a universal constant, he would have predicted zero bending! Not surprisingly, the effect is also known as "gravitational lensing".

    * you can find the calculation here: (oops link malfunction! - scroll down to the end, p.821)
    Last edited: Nov 20, 2014
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