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Why is the Speed of light always constant?

  1. Jun 3, 2012 #1
    Why is it constant regardless of how fast the observer or the source was moving.. can anyone explain it to me or give me a good site that explains it?
    Thanks in advance
  2. jcsd
  3. Jun 3, 2012 #2
    See the second postulate. A postulate (axiom) is a claim that cannot be proven, but is taken as factually true and as a starting point of logical deductions.

    As to the special role given to light, I might comment the following: A more modern approach postulates the existence of a limit speed of propagation of interactions in Nature. In Classical Mechanics, the interaction is described via potential energy that is a function of the instantaneous relative positions of the particles. This is known as "action at a distance", which implies infinite speed of propagation of the interactions.

    The existence of a limit speed makes it a fundamental constant of Nature. According to the first postulate of Relativitiy (so called Principle of Relativity), this constant must have the same meaning and value as measured by all inertial observers.

    If one develops Electrodynamics according to Hamilton's Principle of least action, one finds that electromagnetic fields can propagate through empty space with the same limit speed. According to Maxwell's theory, light is just an electromagnetic wave. Thus, this limit speed coincides with the speed of light.

    But, other fields also propagate with the same speed. For example, according to General Relativity, a disturbance in the metric of flat spacetime also propagates as a gravitational wave with the speed of light.

    It also turns out that massless particles necessarily travel at the same speed.
  4. Jun 3, 2012 #3


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    Does it seem to you that the speed of light should be constant in some sense--just not independently of the speed of the observer or the source? If so, can you describe what does make sense to you so we can take you from where you are to where you need to be?
  5. Jun 3, 2012 #4
    Unfortunately there is really no absolute reason WHY. But if you read below you will find that despite everyday appearances TIME and LENGTH [distance] are NOT 'absolute' in the way you might think. For observers moving relative to each other, time is dilated [slowed] and distance is shortened!! That is really difficult to 'understand'.

    Perhaps a way to begin to understand such stuff is to recall that when two observers are far apart, each appears 'small' to the other...yet you learn that when you get close you both appear 'bigger'. That is a 'real' phenomena and so is time dilation and length contraction.

    Here is a very brief and decent place to start:


    For more:

    Last edited: Jun 3, 2012
  6. Jun 3, 2012 #5


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    Thanks for the good explanation Dickfore; the OP and I are lucky to have you on the forums.
  7. Jun 4, 2012 #6
    If the speed of light is constent, then how would it explane what happens when light falls into the event horizen of a black hole wheir you would think the ∞ gravity would speed it up to ∞ speeds?
  8. Jun 4, 2012 #7
    In a gravitational field, the direction of light is changed, not its speed. So, in a black hole, rays of light travel in circles around the inside of the black hole.
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