Why must the speed light be constant?

Discussion in 'Physics & Math' started by Saint, May 30, 2021.

  1. phyti Registered Senior Member

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    forum;

    There are actually two questions.

    1. Why is the propagation speed of light constant?

    I am not aware of any unanimous answer.
    If Maxwell used permittivity and permeability, em properties of space/vacuum to calculate the propagation speed, it would seem reasonable that space has a structure, since a void/nothing having properties is nonsensical. That structure could determine the propagation speed, just as a medium does for other forms of energy.

    2. Why is the measured speed of light constant?

    Special Relativity predicts motion induced phenomena of time dilation and length contraction, which alters perception and measurement. The two complementary effects result in a constant measure of light speed relative to the observer. Since measurement is the verification tool of science, it is the experimental basis for a constant independent light speed.

    I would question whether photons have energy vs photons are energy.
     
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  3. Ssssssss Registered Senior Member

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  5. Pintsize Registered Member

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    Ssssssss, is it Einstein?
     
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  7. Ssssssss Registered Senior Member

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    Is what Einstein? The book? Yes it's made of 100% naturally sourced free range Einstein.
     
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  8. Write4U Valued Senior Member

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    Planet Einstein.....

    Please Register or Log in to view the hidden image!

     
  9. arfa brane call me arf Valued Senior Member

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    Actually, since photons are quantum particles, and since quantum particles can tunnel through a barrier, it's possible for light to exceed c in a medium (but not in a vacuum; we can classify a vacuum as a kind of limit).

    There is also a well-documented effect, "photon bunching", which does occur in a vacuum. This effect implies that pairs of photons can 'get closer' over long distances, so one of them must exceed c.

    The first tunneling effect applies to single photons, not pairs. As if that's some sort of clue . . .
     
    Last edited: Jun 4, 2021
  10. Q-reeus Banned Valued Senior Member

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    Photon bunching/anti-bunching has nothing to do with any two parallel propagating photons 'catching up to each other' or 'moving further apart' in vacuo, but merely to the statistical distributions produced ab initio at the source(s). See e.g.
    https://www.becker-hickl.com/applications/antibunching-experiments/
    Heroic attempts at producing nonlinear photon-photon scattering requires humongous beam intensities and only is possible with intersection of counterpropagating beams - never in a purely unidirectional one. See e.g.:
    https://indico.cern.ch/event/129952...0189/129085/RAL_Scattering_Meeting_110305.pdf
     
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  11. exchemist Valued Senior Member

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    Both assertions sound quite wrong to me.

    Quantum bunching in no way implies one photon catches another one up. It is to do with them being emitted in bunches.

    The speed of light is not a QM potential barrier. Even if it were, tunnelling probabilities are related to the width and height of the barrier. Since the barrier would effectively have infinite height, and the width would - presumably - be determined by the distance travelled superluminally, the tunnelling probability would be zero.

    Photons do not exceed c in a medium. It is possible for the phase velocity to exceed c, under certain circumstances, but information transfer is no faster than c.
     
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  12. exchemist Valued Senior Member

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    Your last sentence appear to have no connection to the rest of your post. Can you explain its relevance?
     
    Last edited: Jun 4, 2021
  13. Pintsize Registered Member

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    Ssssssss, is Einstein the book?
     
  14. Ssssssss Registered Senior Member

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    No. Das buch is the book. Einstein is one stone.
     
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  15. phyti Registered Senior Member

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    On page 1 the issue of mass, energy, appears (as usual). It also relates to Einstein's definition of gravitational and em fields as independent entities playing the role of an ether.
     
  16. Pintsize Registered Member

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    Phyti, is gravity caused by mass?
     
  17. exchemist Valued Senior Member

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    Eh? Surely GR does away with the concept of a gravitational field entirely, doesn't it?

    And I still don't see why you raise the Star Trekkie notion of photons "being"energy. We've been all round the houses on that one some time back and I don't specially want to revisit it.
     
  18. Q-reeus Banned Valued Senior Member

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    GR is still a field theory but unlike a traditional one based on overlaying a field e.g. classical EM field on a flat spacetime background, it's 'dynamical' in nature. The background spacetime IS in a sense the field also. There is more than one way of formulating it. Noticed you have expressed thoughts on this before:
    http://www.sciforums.com/threads/is-spacetime-a-field.161644/#post-3567538
    See also quotes given in post #17 there.
     
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  19. Write4U Valued Senior Member

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    Can we say that GR is an abstract axiom, not so much a field as a relational condition between all things?
     
  20. Q-reeus Banned Valued Senior Member

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    No. Try typing e.g. 'gravitation explained' into YouTube search bar. There are many vids on offing. One that looks interesting at a glance because of it's graded levels of difficulty has title: "Astrophysicist Explains Gravity in 5 Levels of Difficulty | WIRED" but I won't vouch for its usefulness to you.
     
  21. Write4U Valued Senior Member

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    That was not the question, but your interpretation of the question. The question was about relativity, not gravity.
    Where does this specifically address gravity? I was merely interpreting your post from a more general perspective, based on your use of the term "dynamical", which does not have anything to do with gravity per se.
     
    Last edited: Jun 5, 2021
  22. Q-reeus Banned Valued Senior Member

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    I cannot understand your thought processes. If #55 and link(s) to other posts therein, and suggested course of action in #57 are of no help, sorry I give up.
     
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  23. exchemist Valued Senior Member

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    Ah yes, thanks for the reminder. GR effectively implies a tensor field, in place of Newton's vector gravitational field.
     
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