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Two speed of light experiments at different altitudes

  1. Apr 8, 2006 #1
    Different link: http://www.gutenberg.org/etext/5001

    If you assume a standard clock, say the second on earth at sea level, you will notice a difference between the second of this clock and the second of a clock on top of Mt. Everest - assuming that these devices are identical. An experiment measuring the speed of light on top of Mt. Everest will give the same result as an experiment measuring the speed of light at sea level. But, now it is obvious that the experiments themselves are being executed at different rates. They're different rates because of how initally synchronized clocks will desynchronize after being subjected to a change altitude - Gravitational Time Dilation.
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  2. jcsd
  3. Apr 8, 2006 #2


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    Please don't site someone's website and use it as "evidence". If you wish to challenge established physics, doing it like this will only earn you either a scoff, or dismissal. Find an experimental evidence that has been published in peer-reviewed journals. I can cite several websites that claim to have evidence of ghosts.

    I'm hoping that everyone here would at least have some standard in considering the SOURCE of the information. If one learns nothing else from PF, this would be the most valuable piece of awareness that we can provide.

  4. Apr 8, 2006 #3
    Ok. Maybe using Google wasn't a good idea. But I think this is a legitimate quote from Einstien - I've looked up the quote months ago, and it appears to be authentic - I have the looked at Einstiens paper on GR (but I haven't seen the detailed formulas he wrote). Note that I didn't say Einstein was right. Most scientists agree that Einstein was wrong about black holes and quantum mechanics.

    Here is something more appropriate:

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  5. Apr 8, 2006 #4

    Here is the context of that quote made in this thread (not copyrighted):

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  6. Apr 8, 2006 #5


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    If you expect an answer, ask something.
  7. Apr 8, 2006 #6
    I am curous about this part of the quote:

    "In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlinlited domain of validity ; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light)."

    In particular, is Einstein saying unambiguously that the G field changes the speed of light - or simply its direction (velocity vector) .. do we know whether an individual photon can be deflected w/o altering its speed?
    Last edited: Apr 8, 2006
  8. Apr 8, 2006 #7


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    The short version is that,the term "speed of light" is ambiguous. If you use a single clock and ruler at the origin of a coordinate system, the speed of light, thought of as the rate of change of the distance coordinate with respect to the time coordinate, in general changes.

    But if instead of using one clock and ruler (at the origin of the coordinate system) one uses multiple clocks and rulers, each located where the light actually is, the speed of light is constant.

    Basically one constructs "local inertial frames" around these multiple clocks and rulers, and shows that the speed of light is constant in each of these local frames.

    The modern interpretation is to say that the speed of light is actually constant.

    Earlier interpretations attached more physical significance to the coordinate speed than modern interpretations do. Einstein's quote is an example of one of these earlier interpretations. Some people like to "stop the clock" at Einstein, I'm not one of them, physics has progressed and changed a little since his time. I like to think it's for the better, even if one disagrees anyone who wants to read a modern textbook will be need to be aware of the modern interpretation.

    See for instance


    which discusses this very quote of Einstein's

    Last edited: Apr 8, 2006
  9. Apr 8, 2006 #8


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    There can be no misunderstanding of the quote if you understand that when Einstein says VELOCITY he means velocity. Velocity is a vector quanity so is composed of 2 components, speed (magnitude) and direction. The magnitude of the velocity of light is ALWAYS c. However the direction of the light is changed by massive objects, so the VELOCITY of light is not always constant.
  10. Apr 8, 2006 #9


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    To put it another way, physics is not like literature. When you study Shakespeare's plays, you always study Shakespeare's own words. Anything else just isn't Shakespeare!

    Physical theories are different. Einstein deserves great respect as the primary original creator of relativity theory, but the theory, after its creation, does not depend on Einstein's own words. Physicists have been working with relativity for over a hundred years, over half of them since Einstein died. At this point, Einstein's writings on relativity are mainly of historical interest. This doesn't mean they're worthless, of course, but we have to put them in the context of our current knowledge.
  11. Apr 9, 2006 #10
    pervect - thanks for the tutorial. I see from the article I was not the only one to be puzzled by what Einstein was trying to say, or whether he was even correct in what he was attempting to state.
  12. Apr 9, 2006 #11


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    There is a more fundamental ambiguity issue here, though, which goes beyond what Einstein's intentions were when he wrote this quote. Baez, in his GR tutorial, talks about this issue a little bit:


    This is exactly the heart of the problem with the ambiguity of the velocity of light. There is no problem as long as one is restricted to measuring the velocity of light in a "small" inertial frame. One can safely say that the velocity of light, locally, is always equal to 'c' - because one can compare velocities if they are at the same point. When the frame is small enough, the curvature effects become irrelevant.

    The very concept of comparing a velocity at one point in space-time to the velocity at another distant point is not in general unambiguously defined.

    In practice, people (cosmologists, for instance) often adopt some specific coordinate system, and use the rate of change of a distance coordinate with respect to a time coordinate to find the 'velocity' of a distant object. This procedure can and does result in the 'velocity' of light at a distant location as not being equal to 'c'. The procedure depends entirely on the adoption of some specific coordinate system, so it is not very "physical" as the results are coordinate dependent.

    However, any procedure that locally measures the speed of light (by setting up a small, local coordinate system at the same point in space-time where the light beam itself is located) will always get 'c', without running into this issue.

    The short version is that the speed (coordinate speed) of light is always locally 'c', but not necessarily globally equal to 'c'.

    The modern interpretation is to focus on what is invariant (the local velocity of light as measured by the observer in a small-enough frame with his clocks and rulers), and not worry about what changes (the velocity of light at a distant location in some specific coordinate system).
    Last edited: Apr 9, 2006
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