Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help a rookie understand lightspeed

  1. Feb 1, 2012 #1
    Although I do have a degree in chemical engineering, I am studying relativity/cosmology on a beginning level just as a hobby. I'm currently attempting to wade through Einstein's relativity equations. However I remain troubled by the underpinning of the whole thing - the constancy of the speed of light. It is easy to accept that light always propagates from its source with a constant velocity similarly to how sound propagates from its source with the same velocity given the same atmospheric conditions. My question is this:
    Is light emitted by a source in motion relative to an observer moving at the same velocity relative to the observer as light emitted by a source that is stationary relative to the observer?
    For the moment, pretend that Einstein was never born and his theory of relativity does not exist. How do we know that light received from celestial objects is moving at a constant velocity relative to us on earth? How do we know that the observed redshift and blueshift of light received from distant galaxies is not due to varying speeds of light received from galaxies in in rapid motion toward or away from us? What is the experimental evidence? I'm sure it's a dumb question but maybe some patient person would be willing to contribute to my enlightenment.
  2. jcsd
  3. Feb 1, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

    Sound propagates through its medium, air, at a constant speed and does not depend on the relative velocities of the source or the receiver. In fact we can determine the "rest" state of this medium by doing experiments involving different motions of the source and receiver which will create the equivalent of "red" and "blue" shifts in the sounds received. The equation for this normal kind of Doppler shift takes into account the velocities of the source and the receiver with respect to the medium.
    The Doppler shifts that we get with light only depend on the relative velocity between the source and the receiver and don't have a factor regarding any "rest" state of a medium. This is called Relativistic Doppler. So no matter how much the source and receiver have accelerated, as long as they are traveling in the same direction at the same speed (relative to their starting condition), there will never be any Doppler shifts or other effects due to their speed. It will be the same as if they were at rest and had never accelerated. (This is ignoring any effects due to gravity.)
    There will be Doppler shifts causing redshift and blueshift as a result of relative velocities but this is different from the speed at which the light propagates.

    Experiments have been done on binary stars where one is blue shifted and one is red shifted to see if both beams of light travel at the same speed and they do. Although we can measure that those two speeds are the same, we cannot measure what that speed is. That is an example of the one-way speed of light which is what Einstein specified in his second postulate to be equal to the measured two-way speed of light.

    Look up "One-way speed of light" in wikipedia for more information.
    Last edited: Feb 1, 2012
  4. Feb 2, 2012 #3
    George -
    Thank you for the information. The "One-way speed of light" led tp some interesting reading. I note that most experiments have only verified the "two way" or roiund trip velocity of light which is really just its velocity relative to its source. The success of the relativity equations notwithstanding, there does not seem to be an abundance of evidence for such an anti-intuitive concept as the constancy of the speed of light between sources and observers in motion relative to each other.
  5. Feb 2, 2012 #4
    Then you will now understand that the (one-way) "speed of light between sources and observers" is largely by definition; and a definition has little to do with intuition or evidence. For example, if you define yourself in motion with velocity v towards the light source, then automatically the speed of the light relative to you is (c-v) as determined with a standard "rest" system. In modern jargon that is called "closing speed".

    Note also that the speed of the source only plays a role in the direction of the light rays; their speed is independent of the motion of the source.

  6. Feb 2, 2012 #5
    You seem to refer to particle models of light which were effectively disproved by Fresnel and Fizeau: the speed of light is unaffected by the medium through which it has passed, and even while it passes through a moving medium such as fast running water, its speed is only a little affected by that motion - its speed is not constant relative to the medium.

    Wave theory as developed by Maxwell-Lorentz perfectly explained those phenomena, and that success was the basis for special relativity.
  7. Feb 2, 2012 #6


    User Avatar
    Science Advisor
    Gold Member

    You said in your first post that "It is easy to accept that light always propagates from its source with a constant velocity similarly to how sound propagates from its source with the same velocity". I thought you meant that once the sound is emitted by the source, its speed is independent of the speed of the source, correct? And, in that regard, didn't you mean that it is easy to accept that light is the same way? In other words, once it is emitted by the source, its speed is independent of the speed of the source.

    The article on the one-way speed of light points out that there have been experiments to prove that this is the case. This is not just a postulate of Special Relativity, it is also a postulate of Lorentz Ether Theory and an experimentally verifiable fact.
  8. Feb 4, 2012 #7
    Though I don't quite understand the "by definition" part, what I was referring to is what Harald mentioned above,
    except that it seems that if I were moving toward the source, the closing speed of the light would be (c+v), and if I were moving away from the source, the closing speed relative to me would be (c-v), no? This agrees with intuition or common experience, though it seems to conflict with what George is saying and with my understanding of relativity theory.
    My intuition has no problem accepting that there is no ether or medium - light could be similar to water squirting out of a hose, which doesn't require any medium to travel through - in fact water squirting from a hose could probably be made to have waye-like properties. There is a matter that seems to me to require a universal frame of reference, but I will leave that for another post. Regarding Doppler, red shift, blue shift. would not a wave traveking faster relative to an observer appear to have a shorter wavelength, and one traveling slower appear to have a longer wavelength in a manner indistinguishable from actually having a shorter or longer wavelength if differences in the velocty of the wave are unknown?
  9. Feb 4, 2012 #8
    Actually what George says
    is in total agreement with intuition, once sound leaves the speaker, or water leaves the hose, the motion of the speaker or hose no longer has any effect on it, but this does not necessarily put any constraint on its speed relative to an observer. If the hose is mounted on a fire truck approaching me at 60 miles/hr then the velocity of the water approaching me would seem to be velocity of the water coming out of the hose plus the velocity of the truck.
  10. Feb 4, 2012 #9
    I believe I understand what you all are trying to impart, which is that sound travels at a certain velocity through its medium regardless of the motion of the source or observer, and light works the same way without the medium. I still wonder though, other than the De Sitter double star measurements, what evidence is there that light doesn't act instead like water streaming from a hose which velocity does depend on the motion of the source and observer.
  11. Feb 4, 2012 #10
    Evidence for this dates back to 1725 in the astronomical phenomenon of stellar aberration, in which stars viewed along directions parallel to the earths axis appear to shift position in an annual cycle (see http://en.wikipedia.org/wiki/Aberration_of_light and http://www-spof.gsfc.nasa.gov/stargaze/Saberr.htm)

    James Bradly attributed it to the finite speed of light and gave an early estimate for that speed. If the speed of light varied with the speed of the light source, stars viewed along a particular direction would not shift, as they do, by equal amounts.
  12. Feb 4, 2012 #11


    User Avatar

    Staff: Mentor

    It's not a dumb question, because... We don't KNOW this.

    We believe it to be true based on:
    1) A huge amount of experimental evidence about the behavior of light at smaller scales.
    2) No worrisome experimental evidence suggesting that light behaves differently at cosmological scales and at local scales.
    3) There is a fairly convincing logical argument that the speed of light should be constant for all observers, so neither #1 nor#2 should be surprising.

    My favorite way of expressing the argument in #3 is (pasted from another thread) is:
    Einstein's big insight was that if we assume that the speed of light in vacuum is constant for all observers, then we can construct an internally consistent theory that matches the available experimental evidence and reconciles the principle of relativity with the laws of classical electricity and magnetism.
    Last edited: Feb 4, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook