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How do we know the speed of light is constant?

  1. Jun 14, 2004 #1
    How do we know the speed of light is constant?

    If the behavior of light can be described by particle theory, and if the percieved speed of a non-light particle is affected by the relative motion of the observer, then it doesn't make sense to me how light is always observed to have the same speed, reguardless of the speed of the emitter or the speed of the observer.

    What experiments have been conducted in the past to show the speed of light to be an absolute constant, reguardless to relative motion?
  2. jcsd
  3. Jun 14, 2004 #2
    The Michelson-Morley Experiment is a historical example. The constancy of the speed of light is also a prediction of Maxwell's Equations.
  4. Jun 14, 2004 #3
    And the value of this speed is used in modern technologies such as the global positioning system.
  5. Jun 14, 2004 #4


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    It was predicted (derived from Maxwell's equations) and, more importantly, is measured to be constant.
    The behavior of light can be described by particle theory only in certain cases.
    The first to directly address the question of relative motion was the Michelson/Morley experiment in 1890-something. Since then, there have been hundreds of others. Today, not a whole lot of experimentation is being done to confirm its constancy, but there are a number of practical devices (GPS) that wouldn't work if it wasn't constant.
  6. Jun 14, 2004 #5
    russ watters said, "It [constant light speed] was predicted (derived from Maxwell's equations)...."

    TALewis said, "The constancy of the speed of light is also a prediction of Maxwell's Equations."

    Maxwell's equations are consistent with an ether?

    Constant light speed is inconsistent with an ether.

    So how can Maxwell's equations predict constant light speed?
    Last edited: Jun 14, 2004
  7. Jun 14, 2004 #6
    In order for the technology of laser to work, the speed of light must be a constant.

    In order for the principle of reflection and refraction to work, the speed of light must be a constant in the medium.

    In order for the principle of spectroscopy to work, the speed of light must be a constant.
    Last edited: Jun 14, 2004
  8. Jun 14, 2004 #7
  9. Jun 15, 2004 #8


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    This I'm not sure about. I *think* its because Maxwell's equations predicted a constant speed through the ether and SR just discards the "through the ether" part.

    There are, of course, some observations that ether theories can explain identically to SR (mathematically).
  10. Jun 15, 2004 #9
    thanks for all the replies, fellas. But tell me something, has GPS or laser technology ever been tested at relativistic speeds?
  11. Jun 15, 2004 #10

    matt grime

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    What makes you think that the speed of the appartus involved (wrt to which inertial frame?) affects the speed of light?
  12. Jun 15, 2004 #11

    Tom Mattson

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    On Maxwell's equations...

    Maxwell's equations do not predict that the speed of light is the same in every frame of reference.

    Before the turn of the 20th century, theoretical physics was in the following state.

    1. The laws of mechanics were well-verified.
    2. The laws of EM were well-verified.
    3. The laws of mechanics were invariant under Galilean relativity.
    4. The laws of EM were not invariant under Galilean relativity.

    Here's the problem: According to the best theory of the day, absolute motion could not be detected by performing mechanical experiments in an inertial frame, but it could (in principle) be detected by performing electrodynamic experiments in an inertial frame. But this was not corroborated experimentally.

    One of the following had to be true:

    1. EM theory was wrong.
    2. Mechanics was wrong.
    3. There are two correct relativitity theories, one that leaves mechanics invariant and one that leaves EM invariant.
    4. There is one relativity theory, and Galileo's ain't it.

    Since the first two were so well agreed with by experiment that physicists were loathe to consider them (note: I'm not addressing the necessity of quantum theory here). The third one just seemed absurd. Why should one spatiotemporal transformation hold while doing mechanical experiments, and another while doing electrodynamic experiments? How does nature "know" which type of experiment you are doing? And what transformation holds when you do an electro-mechanical experiment (as all EM experiments really are, anyway!)?

    #4 is the only choice, and it leads to SR. With a rejection of Galilean relativity, we obtain slight corrections to mechanics (which have been confirmed experimentally), and no change in EM theory, and balance is restored to the universe.
  13. Jun 15, 2004 #12


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    Are there any experiments that verify the constancy of c. If c were to change by a micron/second over a century, could we detect it?

    What would such an experiment entail? Most experiments to find small changes involve the null concept, but I can't think of a way to do that in this situation.

  14. Jun 15, 2004 #13
    Njori asked, "Are there any experiments that verify the constancy of c. If c were to change by a micron/second over a century, could we detect it?"

    Absolutely not. It could be drifting around a LOT more than that, and we couldn't tell.

    EDIT: On further thought (suggested by Tom Mattson in post #14) the following statement is obviously wrong.

    In fact if the theory of realivity is correct, I'm pretty sure the speed of light could double tomorrow and we'd never know. I'll have to think about that a little, but I'm pretty sure it's right
    Last edited: Jun 15, 2004
  15. Jun 15, 2004 #14

    Tom Mattson

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    Sure you could tell: by measuring it.

    Remember that the weird effects of SR only become apparent under Lorentz boosts. That means that, if all our measuring apparatus is in the same inertial frame, the doubling of the speed of light isn't going to be hidden by length contraction or time dilation. If you have a light source that is situated 3m from a photodetector and you measure the time between emission and absorption to be 5 nanoseconds, you will calculate a speed of 6E8 m/s, or 2c.
    Last edited: Jun 15, 2004
  16. Jun 15, 2004 #15
    Tom Mattson,

    Oops, you're right! :redface:

    In fact if it slowed down enough, relative simultaneity would be part of our intuititve experience, and all those dizzy posts claiming it can't be right wouldn't exist. The universe would be very different!
  17. Jun 15, 2004 #16


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    "Relativistic speed" is whatever speed relativistic effects become noticeable for your device. GPS satellites operate at relativistic speed (and gravity effects), meaning if the engineers who designed/built the system hadn't calibrated the clocks in the satellites according to Relativity, the system would lose accuracy by about 2km per day.

    The Relativity is so important to the GPS system and the GPS system is so good at measuring its effects, it has actually been used for scientific testing of Relativity.
    Due to astronomical observations, there are limits on how much/fast it could be changing as the universe ages. The fact that we see stars/galaxies in space that look similar to our own is evidence that the laws of the universe remain constant with changing time and space.
  18. Jun 16, 2004 #17
    All the SOL experiments have ignored some crucial physics. If your eye is looking at an oncoming wave stream and you move into the stream the frequency of the light you see will increase. This is the doppler shift. But, ask yourself, "does your eye shorten the wave length of the light you are moving against? Are you applying a pressure, a force on the wave that contracts the wave length?"

    There is a more reasoned and newly *discovered dynamic. As you move against the light your eye moves faster over the wave lengths you perceive. If you are moving against a moving train of boxcars that you count as you pass them, do the boxcars get shorter?

    Or does your data show only that the frequency of the measured boxcars increases?

    *see Grounded's Post# 1 in the Measurement of the SOL Thread. in this forum.
    One does not need wave length shortening to explain anything.

    All the light measurement answers that are thrown at questions like yours haven't considered the unterpertubed characteistic of wave length subject to moving observers. In fact the squished wave length doctrine is doctrinally implicit and, you get a constant speed of light, no matter what and, SR remains the Wicked ***** of Science, saved!
  19. Jun 16, 2004 #18
    Sorry to interrupt, but you don't measure that train's speed constant either. Isn't light a special case? Speed=frequency x wavelength, so if speed is constant and frequency increases, wavelength must decrease, right?
  20. Jun 16, 2004 #19
    yes you do measure the train speed constant, how could you not? You are measuring the frequency opf box cars passing your eyes as you count. You do see, don't you, that the number of box cars counted passing by you is different depending on whether you are moving with or away from the moving box cars, don't you?

    Why do I anticipate that your answer will be steeped in SR dogma, mantras, like your knee jerk interruption in your post? Why do I know that to be the case?

    Try to see what the question that is presented to you, not just the question you waqnt to answer. You mesure the number of boxcars per second entering your eye, just like you measure the nomber of wavelengths passing through your eye in a second. The assumption of the SR theorists, at al, is that when you move against the stream of photons that the observer's motion compresses the wavelength, applies a force, a pressure, when all that is occuring is the increase of the number of full wave lengths passing through your eye. The increase in frequency, or decrease, is the observers velocity wrt the photons' wave lengths which may be considered invariant.

    The speed relative to the source is fxlambda - C, whee f is the mesured frequency.

    Wespe, don't believe or disbelieve this trash, but before you do anything try to understand it.

    Hey Wespe it is something new. Do they have somethings new where you come from?
  21. Jun 16, 2004 #20
    So you say no matter how fast you are moving against a train, you measure its speed constant? Surely there must be a misunderstanding.

    For trains, the frequency increases, the wavelength (length of a car) remains same, so the measured speed increases. (considering train speed << c)

    For light, the frequency increases, the wavelength decreases, so the measured speed remains constant.

    Are you ignorant of relativistic doppler formula supported by experiments?
    Last edited: Jun 16, 2004
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