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Photon questions

  1. Oct 15, 2007 #1
    1. What and why changes the shape of the the object as it comes closer to the speed of light.

    2. Why does the time slow down when something travels closer to the speed of light?

    3. Why is light absolute from all pts. of references?

    4. What is the rest mass photon?(since photons can be slowed down if they travel underwater, etc, Is it possible...and what would happen if we can...slow down the photon to 0 velocity from our reference?

    I do not want mathematical proof...but conceptual answer such as: the reason an object can not travel to the speed of light is b/c it will take infinite energy for it to speed up to there.(why btw?)
  2. jcsd
  3. Oct 15, 2007 #2


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    OK, I'm going to try to explain it a bit; with a disclaimer for errors and inaccuracies in my own text or the linked pages (we all know that Wikipedia is not always trustworthy, right? :smile:)

    1) Because all distances in one direction (along the direction of movement) are contracted. You should probably study (special) relativity to understand it.

    2) Again, you should study special relativity. The effect of time dilation makes clocks in relatively moving frames run slower. You can intuitively understand it though, by considering a clock made of two mirrors and a light beam reflecting between them, as in this applet.

    3) I assume you mean the speed of light is absolute from all reference frames. This is an axiom of special relativity based off physical observations (like this one).

    4) A photon is a massless particle, it's rest energy is given by [itex]\hbar \nu[/itex] with [itex]\nu[/itex] the frequency. You can convert this to kilograms if you like (m = E / c^2, though I doubt that makes much sense). Indeed light can be slowed down, and has been.

    For the reason an object can not travel to the speed of light, be aware that energy and mass are actually the same thing in SR. Moreover, to accelerate a massive object, energy is needed (a force should act which means work is done - even in SR :smile:). For some reason, when the speed approaches c, any extra energy you put in is no longer used for increasing the velocity, but is being stuck in the rest mass (rest energy) of the object.

    The problem with SR is basically that it is often counter-intuitive. It is based on some axioms which seem reasonable (the speed of light is constant for all observers, for any equivalent observation frame the same physical laws hold, etc) but many odd consequences follow from it. I think the best way would be to buy a good book, there are probably tons of "pop-sci" books on relativity; though I would recommend taking the math approach as well.
    Last edited: Oct 15, 2007
  4. Oct 15, 2007 #3


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    Conceptually, what stays the same when an object moves is the space-time interval. (Clink on the link for a wikipedia defintion).

    So when you ask why the shape doesn't mean the same on a moving body. I assume you mean why the distance between a pair of points doesn't remain the same. The answer is because it is the above space-time interval, also known as the "Lorentz interval" which remains the same (i.e. is invariant). Time intervals also don't remain the same (are not invariant) - this shows up as time dilation, which you also asked about. It is only the Lorentz interval which remains the same, it is only the Lorentz interval which remains invariant. Philosophically it is convenient to regard the Lorentz interval as being "more fundamental" than distances and times, just because of this invariance, because everyone in any inertial frame agrees on its value regardless of their velocity.

    Photons may have an average propagation speed slower than 'c' in a medium, but this has more to do with the physics of media than it has to do with relativity. The photons inter-convert back to lattice vibrations (phonons) and are otherwise absorbed and re-emitted, changing their average propagation speed. PF has a FAQ entry on this written by ZapperZ, which I will hunt up if there is a need (you can probably find it yourself if you look hard enough).
  5. Oct 16, 2007 #4
    pervert, you are just stating facts, not telling me why. For instance, why do thing's happen the way they do in space-time interval?
  6. Oct 16, 2007 #5


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    Every chain of "why" questions ultimately terminates in an appeal to some fundamental principle which does not have a further justification of "why" beyond it. It just is.

    In relativity, this chain of "why" terminates (IMO, anyway) with the invariance of the Lorentz interval. (It might be possible to use some sort of argument based on the anthropic principle to go further, but it's not clear if this is possible, and if it turns out to be possible it's not clear that it is a good idea.)

    It's certainly possible to imagine a universe that works in the Newtonian manner, with distances being invariant and the Lorentz interval not. It just so happens that the universe we live in appears to be the sort that makes the Lorentz interval invariant rather than distances.
  7. Oct 16, 2007 #6
    That chain theoretically ends with the posultates of the theory. In practice, perhaps not.
    The invariance of the spacetime interval can be derived from the Lorentx transformation and the Lorentz transformaton can be derived from time dilation and length contraction, both of which can be derived from the basic posultates of special relativity.

  8. Oct 16, 2007 #7
    space-time invariance

    Hi Pete
    I consider that the invariance of the space-time is used in the derivation of the LT, LT can be derived from length contraction only without using time dilation, length contraction and time dilation are complemenatry effects , the addition law of speeds can be derived without using the LT...
    All that facts are the starting points for an increasing number o papers with guessed titles x derived from y, x derived without using y.... That is one of the nice faces of SRT.
  9. Oct 16, 2007 #8


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    Ideally, such papers should be titled "x derived from y [using w]" and "x derived without using y [but using z]" where w and z are the [usually implicit] assumptions used. Admittedly, sometimes it's not easy to identify w and z... but for a true derivation they must be identified.
  10. Oct 16, 2007 #9


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    In fact, the M M experiment provides a good basis for the fact that in inertial frames the speed of light is constant.It's not clear ithat Einstein was fully aware of MM, but he definitely glommed, sorry 'bout that technical term, onto the invariance of c. This pushed him to examine Maxwell's EQ, space and time and the rest is history.

    There's no better reference than Einstein's book, Relativity -- very clear, very readable. Also, many freshman text covered SR, as do more advanced books on mechanics and E*M, and, of course QM and QFT. It's hard stuff, and takes time to absorb. In 8 years of undergraduate and graduate physics, I probably had 8 or 9 courses that covered SR, and then I taught SR numerous times. Frankly, I'm still not sure that I really understand the subject.

    Study, study, study.
    Reilly Atkinson

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