Why must the speed light be constant?

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

  1. exchemist Valued Senior Member

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    Er, yes, quite. I think the appropriate expression is, "Do not adjust your set."

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  3. Write4U Valued Senior Member

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    Who is talking about gravity? You are not reading what I write which was based on this post by you;
    Where does it mention gravity?

    But thanks for that link.
     
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  5. Write4U Valued Senior Member

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    So no gravity field ? More like a tensor field, a relational condition of everything?

    Tensor field
    https://en.wikipedia.org/wiki/Tensor_field#
     
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  7. arfa brane call me arf Valued Senior Member

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    Research since the 80s implies that when any quantum particle (photons included) tunnels through a barrier, it does so at superluminal speed. The particles appear to spend most of their time at the entrance and exit of a barrier, and hardly any in between.

    Why this is is still an open question, but as you also say it does not imply that information can be transmitted superluminally. A signal, to contain information, needs a certain structure. What that is exactly is still under discussion, it seems, because obviously if "tunneling time" is real, we don't fully understand it.
     
  8. exchemist Valued Senior Member

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    Citation required.

    I am sceptical it makes any sense to speak of a "speed" of tunnelling. What you have is a wave function that is not damped to zero by the potential barrier, so the entity spends part of its time on the far side of it, i.e there is non-zero probability of detecting it there.

    Trying to determine its "speed" seems to me like trying to determine the "speed" of an electron in an atomic orbital. It has no real meaning. The most you can do is associate a "speed" with the expectation value of the momentum, in a semi-classical way.

    There is a description of the issue here:

    https://www.quora.com/What-is-the-speed-of-quantum-tunneling?share=1
     
    Last edited: Jun 5, 2021
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  9. Q-reeus Valued Senior Member

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    The whole Gunter Nimtz thing: https://en.wikipedia.org/wiki/Günter_Nimtz
    As you say there is no 'advanced superluminal tech' to be had, although the last section in that article gives Nimtz just a smidgen of wiggle room so to speak.
     
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  10. exchemist Valued Senior Member

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    Thanks for this - something I didn't know. It seems Nimtz's interpretation as involving superluminal "speed" is strongly contested, as I would have expected.

    The fact that it involves microwaves already makes me suspicious, as that is an area where the crossover to classical EM physics is fairly common to see.

    It seems to me to be likely to be analogous to the so-called instantaneous (="superluminal") "communication" between QM objects in entanglement, i.e. not in fact what it seems to be. Since, in tunnelling, the wave function is not damped to zero by the potential barrier, it means the object does not actively "travel" through the barrier from one to the other, it is simply present on both sides with a non-zero probability. So it's hardly surprising that it seems to "go through" faster than light, as it is on the far side already!
     
  11. Q-reeus Valued Senior Member

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    Yes. There is a folklore proposing 'faster than light communication' via quantum nonlocality, but none have afaik ever mounted a credible case for other than standard position of tiny random noise as best possible scenario. I suppose we should humbly acknowledge 'at least so far'. Hedging one's bets is after all arguably (I don't want to appear reckless and irresponsibly on this) the safest play here.

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  12. Q-reeus Valued Senior Member

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    Dupe error
     
  13. exchemist Valued Senior Member

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    There's a good explanation of entanglement in Carlo Rovelli's "Helgoland", which gets rid of the notion of any "communication". But that's another story............
     
  14. Q-reeus Valued Senior Member

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    Thanks for lead there. I was unaware of the human level struggle in the early days leading to new QM till reading Rovelli's synopses at:
    https://www.penguinrandomhouse.com/...-translated-by-erica-segre-and-simon-carnell/
    Thanks. Lead to a fascinating review I cannot recover the URL to, summarizing the struggle of pioneers of QM - Bohr, (especially) Heisenberg, Born, Dirac et. al. to formulate QM as a consistent theory. Personal intellectual struggles with no smooth easy path to an eventual and still evolving 'enlightenment'.
     
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  15. phyti Registered Senior Member

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    Pintsize;

    exchemist;
    [quotes from A. Einstein, lecture at U. of Leiden, 1920.
    Comparing a coordinate system K at rest in the Lorentzian ether, with a coordinate system K' moving in the ether,]

    "Why must I in the theory distinguish the K system above all K' systems, which are physically equivalent to it in all respects, by assuming that the ether is at rest relatively to the K system? For the theoretician such an asymmetry in the theoretical structure, with no corresponding asymmetry in the system of experience, is intolerable."

    [After removing the mechanical properties of the ether, and building on the work of Lorentz in separating em processes from processes of matter,]

    "The ether does not exist at all. The electromagnetic fields are not states of a medium, and are not bound down to any bearer, but they are independent realities which are not reducible to anything else, exactly like the atoms of ponderable matter."

    [Moving on to GR]

    "But inertial resistance opposed to relative acceleration of distant masses presupposes action at a distance; and as the modern physicist does not believe that he may accept this action at a distance, he comes back once more, if he follows Mach, to the ether, which has to serve as medium for the effects of inertia. But this conception of the ether to which we are led by Mach's way of thinking differs essentially from the ether as conceived by Newton, by Fresnel, and by Lorentz. Mach's ether not only conditions the behaviour of inert masses, but is also conditioned in its state by them."

    "The ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities, but helps to determine mechanical (and electromagnetic) events"

    [The gravitational field (determined by the distribution of matter), conditions the spatial environment.]
     
  16. exchemist Valued Senior Member

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    Helgoland is a model of clarity of thought. What I had not realised is that Heisenberg made a conscious effort to copy Einstein's approach to relativity, by ruthlessly adhering to what the observations tell us, and them alone, without any prior or additional assumptions, and then seeing where that might lead.

    Rovelli is an advocate of the relational interpretation of QM. After reading his book, I must say I am fairly well persuaded it makes the most sense of any of the alternatives.

    P.S. I ordered a copy from Waterstones and got a signed copy! I was well chuffed.

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  17. exchemist Valued Senior Member

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    Except that none of the passages quoted mention the idea of a field at all. "Field" is a word inserted by you in square brackets at the end.

    However, since I wrote the post to which you are responding, the conversation has moved on and I have been reminded, by Q-reeus, that GR replaces Newton's vector field with what is effectively a tensor field. So I accept that GR can be seen as a form of field theory.
     
  18. phyti Registered Senior Member

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    I only posted some of the relevant statements. You have to read the whole paper for an understanding of its contents.
    Here he does use 'field'.
    "As to the part which the new ether is to play in the physics of the future we are not yet clear. We know that it determines the metrical relations in the space-time continuum, e.g. the configurative possibilities of solid bodies as well as the gravitational fields; but we do not know whether it has an essential share in the structure of the electrical elementary particles constituting matter."
     
  19. exchemist Valued Senior Member

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    OK, perhaps you should have quoted that, then. But anyway the issue is resolved.
     
  20. irk Registered Member

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    To be up-to-date you may find useful the following discussion. A serious attempt in Assessing Relational Quantum Mechanics: We find that it fails to address the conceptual problems of standard quantum mechanics, and that it leads to serious conceptual problems of its own. In A response... by Rovelli: ...presupposes assumptions that are precisely those questioned in the Relational Interpretation... turns out to be an exercise in misunderstanding. My take from this is that, like anything in physics, an interpretation of QM can not be persuasive until other options are shown having logical errors or/and are excluded by experiment.
     
  21. exchemist Valued Senior Member

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    Oh I don't know, it depends how easily persuaded one is, I think.

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    In this case experiment does not help, as it is just the interpretation of the theory, rather than the theory itself, that is in question. Similarly, there are no logical errors in this or the other interpretations, or none that seem to be generally agreed upon as such. Perhaps it is matter of philosophical or aesthetic preference, as much as anything else.

    But many thanks for the references. It will take me some time to read them.
     
  22. sculptor Valued Senior Member

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    Do neutrinos travel faster than the speed of light?
     
  23. Janus58 Valued Senior Member

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    No. It was original assumed that neutrinos had a 0 rest mass and thus, like light, traveled at c. Later it was determined that they they have a very small, but non-zero, rest mass and thus traveled at less than c.
     

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