1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Constancy of light? Why the fuss?

  1. Apr 8, 2010 #1
    Sorry for length.

    On what grounds are physicists standing when they demand a constant velocity of light? Or even, on what grounds are others trying to disprove special relativity? 400 years ago it was universally believed that light traveled instantaneously, with infinite velocity. This infinite velocity was also considered fixed in my opinion, fixed at infinity. Over time this train of thought was broken down by many arguments and good reasoning from some of the brightest minds of the day. Now it is believed that light travels finitely, at tangible speeds, and it is further still believed that light travels at a definite, fixed speed, the constant velocity of light (aprox. 3x10^8 m/s). Until reading and researching on Einstein's special theory of relativity, I had always supposed light, or a photon, could vary in speed, perhaps even as a factor of varied frequency. Even immediately after reading Relativity I held this thought, for in his special theory and the pages proceeding it he seems to define a reality where a variable light speed is indeed the case and quite probable, or at the very least relative. Upon further research on light, the only reasoning behind a constant light speed I could find was its use as a physic’s constant. Furthermore upon research, I found that for some time now, modern physicists have been questioning a constant speed of light and have been gathering data that supports their doubts. So I am asking, why the need for a constancy of light? Is it not possible or even probable that something like frequency could vary the linear velocity of an individual photon? And as far as for use as a physics constant couldn’t use c=the avg. velocity of white light in vacuum, or the velocity of the wave front of white light in vacuum, or something similar? At the same time, it is already well known that light travels at different speeds through different mediums.
    In the situation Einstein sets up in his special theory, you have a man-a on a rigid reference body-a, and a man-b on a reference body-b in motion away from and relative to man-a fixed to reference body-a. Man-a shines a light, projects light rays from a source fixed on body-a. Man-a would measure the velocity of the ray as the constant c. Special relativity states that man-b, on the body in motion, would also measure the velocity of the light ray to be c. This postulation was made after "experimental evidence" from Michelson’s and Morley’s experiments which disproved the presence of a luminiferous aether, and supposedly further confirmed the constancy of light. These experiments were done by shining white light from a single source through a lens which splits it, then after deflecting the two beams off mirrors, the light re-converges through the lens and is read by a detector. The system was erected on a marble slab, afloat in mercury, so that it could be rotated and also so that it would be as level, stable, and isolated as possible. The whole process divided and re-converged light beams with the source and detector’s beams, as well as the split beams, at 90 degrees from each other. The experiment confirmed that the earth is not continually dragging through a light medium, luminiferous aether, as none of the beams changed velocity no matter the angle propagated in, in relation to earth’s orbit. Fine, I’ll believe this is evidence that we are not continually passing through an effervescent, static, light medium, but how does this attest to a fixed velocity for photons? No duh the wave front from the single source travels at the same velocity no matter which way you point the light, especially when propagated and detected on the same reference body. (The full details of the Michelson’s and Morley experiment can be found on various internet sources.)
    Furthermore by Einstein’s approximation a photon emitted from where man-a is standing, propagates from him at velocity c towards man-b on body-b moving away from man-a; if man-a where to measure the velocity of the photon he would get c; as the same photon passes man-b he supposedly would also record the photon’s velocity to be c; so now, if man-a where to measure the velocity of that same photon again he would record the speed of the very same photon to be c plus the velocity of man-b, since for some reason the same photon is moving at velocity c relative to both men. This seems like an argument for the instantaneity of light. I found a similar deduction when researching proofs for special relativity at http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/relativity.html. The value c is also thoroughly ingrained as a constant of many equations, notably Maxwell’s equations describing electromagnetic radiation.
    There has been much to-do recently about the constancy of light and whether or not the speed of light can vary, and how. It seems as though physicists have forgotten or ignored the true definition of the constancy of light; it is the constant speed of light in a vacuum. It is assumed that photons traveling unimpeded in a vacuum would travel at maximum velocity. I am at a point where I don’t even know what the debate is anymore. Light has already been shown to change velocity as it passes through different mediums. So is the doubt on lights velocity in vacuum? Or is it merely on the velocity of light in relative motion? It seems to me that both have already been thoroughly debated, deduced, and defined. I have not found any evidence against light’s maximum velocity in vacuum, or supporting the claim that light travels at velocity c relative to all bodies of reference. As far as I’m concerned, if man-b measures the photon’s velocity to be c minus the velocity of body-b relative to body-a, then further accounts for his velocity relative to the reference body from which the light is propagating, he still would record the velocity of the photon to be c with respects to the source. The photon is still traveling with the same energy and doesn’t have to break any physical laws. If light can travel different speeds through different mediums, then why can’t it also travel at varying velocity relative to inertial reference bodies? Also, does it not seem that vacuum is an ethereal medium by which light travels, only slowed by passing through space containing mass? There is no direct evidence present to-date completely and assuredly supporting that light travels at fixed velocity with respect to all inertial reference bodies regardless of motion of source or detector. There is also no direct evidence, however, that the postulates of special relativity are wrong; they might just need to be applied differently.
    If there are flaws in my research or reasoning then please inform me, because I can’t seem to find what all the fuss is about. I think physicists all around just need to get a more solid definition on light and how it applies to vacuum, occupied space, and more importantly relatively to inertial reference bodies. The bottom line remains that these theories, postulates, relations, and equations have done much for our understanding of the physical world and have made many confirmed predictions, whether wholly justifiably founded or not. Acceptance and improvement of our current wealth of knowledge is the only true way to progress.
  2. jcsd
  3. Apr 8, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Well, there's really not much to debate.
    Answer: The accepted theory is that light always has the same speed when in a vacuum. The grounds upon which this theory stands is that it is consistent with numerous observations and precision measurements. It is not something accepted blindly by physicists, it has been checked and tested and retested extensively. This has gone on more than 100 years -- the testing did not end with the Michelson Morley experiment.
    Last edited: Apr 8, 2010
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook