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Constancy of c

  1. Mar 7, 2005 #1
    No matter how fast an observer moves, a light ray will always travel at the constant speed of 2.997*10^8 m/s.
    Therefore, for what reason do we say light travels at a 'velocity'? What is the meaning of this word for a ray of light?
    Surely from the perspective of the light, there would be no conception of space and there would be no intrinsic velocity for the light ray.

    Could not the 'velocity' of light be a byword to a form of action, which an inertial observer interprets as a velocity? After all, we cannot know a velocity exists without refering to a background frame of reference (i.e the rest of the mass in the universe).

    However, in the Lorentz transformations, the mass of a body increases as the 'velocity aproaches c'. If c is unity, then '0.98 of c' would indicate a large increase of mass. But how can this be, if a light ray always recedes away from the observer at the same speed?

    Perhaps someone could clarify this issue for me. On the one hand, light seems to have no 'velocity' - only a form of action, expressed as a speed for inertial observers. But on the other (Lorentz) it has an intrinsic velocity which an inertial observer can approach, but never reach exactly.
  2. jcsd
  3. Mar 7, 2005 #2


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    We say light travels at a velocity for a fairly simple reason: we can measure the time (t) it takes for a pulse of light to travel a certain distance (d). Then v=d/t.
    The 'perspective of light' is very ill defined, as has been mentioned around here before: all space is contracted to one point, and there is no proper time. It is safe to say there is no physical frame of reference for light, which is further enforced by the L.T. which prohibits massive particles to travel at the speed of light.
    The observer never feels himself getting heavier. It is more correct to say that if one defines an inertial frame to make measurements and an object is moving wrt that frame, the measured kinetic energy of the object approaches infinity as the measured speed approaches the speed of light. That object never experiences anything strange happening to itself.
  4. Mar 8, 2005 #3
    This is an accepted fact that a ray of light or accelerated radiation will travel at the 'constant speed' c. But what I am saying is this should be reitterated as what an inertial observer sees rather than being intrinsic to the wave of radiation.

    But why should we elevate the motion of an inertial observer over what the world appears to be from the perspective of a light ray? If all reference frames are to be treated on the same basis, surely the universe 'from a photon' should be regarded just as correct as the universe from a mass?
    It would solve many problems if we abandon the notions of absolute distance, space and velocity and instead regard the universe as a 'point'. The reason why distances, space and inertia arise is because of relative relationships with other things in the universe. We cannot know something is moving unless it interacts and without a reference frame of other bodies, everything is at rest. Absolute rest implies no extent for space and hence the universe becomes a 'point'.

    Therefore, with the abandoning of the notions of absolute 'distance' and 'velocity', we can arrive at a more fundamental decription that conforms to the principles of relativity.
  5. Mar 8, 2005 #4


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    There is no such confusion - Special Relativity is quite specific in what it is claiming. Speed is measured by an observer who considers himself stationary:

    In fact, since speed must always be measured against an arbitrarily defined stationary reference frame, this isn't unique to Einstein's Relativity. By the very definition of "speed," the speed of light (and the speed of your car relative to a cop) is measured relative to an arbtrary stationary reference frame.
    The first is one of practicality: we're not in light's reference frame, so its impossible to directly measure what is going on in it. But more than that...
    ...as A/C's post indicated, light does not have a valid inertial reference frame. So no, light's reference frame cannot be "treated on the same basis" as other frames.
    Fine with me - next time I get a speeding ticket, I'll tell the cop that the universe is a point, so speed does not exist, so I couldn't be speeding..... er, uh.... hmmmmm.....
    I think you may misunderstand what SR says: absolute distance, velocity, and time, are abandoned for everything except light (light has absolute speed). That's not "staying true to" Relativity, that is Relativity.
  6. Mar 8, 2005 #5


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    No, it shouldn't. This is because unlike other inertial frames, physics doesn't tell you how to TRANSFORM to the frame of the photon. We do NOT know what is going on in the photon's frame, because we do not have the physics to be able to do that.

    ALL we know is that we can measure the velocity of light IN OUR FRAME, and that this value is identical in ALL inertial frames that we can boost ourselves to. And if we look at this ability to do the transformation, you will notice that it does NOT allow the boosting of ourselves into a frame moving at c. Thus, anything you wish to say about what you can or cannot observe in the reference frame of light is merely speculation/guess work, because there are no theories of any kind to justify such things.

  7. Mar 8, 2005 #6


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    Your argument might make sense if light had a "frame". But it doesn't, as various peole have explained.

    Intrestingly enough, the speed of light is locally equal to 'c' even in an accelerated frame. If the light travels a long distance, errors start to creep in, because all clocks in an accelerated frame do not tick at the same rate. Right at the origin of the frame, though, the errors are small if the distance is small, and the speed of light is equal to 'c'.
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