Why must the speed light be constant?

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

  1. irk Registered Member

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    9
    Take e.g. Multi World interpretation of QM, it claims there is no collapse of of wave function but it is branching into different worlds, our perception is just limited to a single world. Multi World thus results in gargantuan Universe. It is not excluded that at some point somebody proves the Multi World results in logical contradiction of some kind or that such Universe is impossible on physical grounds, the same could be with other interpretations. This preprint provides condensed overview of different interpretations and some state-of-the-art issues as of June 9th 2021.

    Returning to the original question: Constancy of the speed of light in vacuum is a postulate ('axiom') of the relativity theory, so don't ask why. Speed of light is also simply related to the vacuum magnetic permeability and electrostatic permittivity in the classical theory of electromagnetism and thus constant, so rather ask why vacuum has those properties with those specific values and no classical theory can give any hint on this. Now comes the quantum theory in its heavyweight edition: Quantum Field Theory in which vacuum is an exquisitely special quantum object with shockingly rich structure. Knowing its properties one can attempt to calculate vacuum permeability and permittivity from the picture and tools provided by the theory. The 2013 paper The quantum vacuum as the origin of the speed of light provides such calculations in apparently good agreement with established values. But it also predicts tiny, tiny fluctuations in the speed of light due to how photon is propagating through quantum vacuum. However, Quantum Field Theory is so intricate that one should consult opinions of a bunch of superexperts what they think about this paper before earmarking the original question answered, or maybe better to say clarified since then immediately questions like why the quantum vacuum is such as it is pop out.
     
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  3. Beaconator Valued Senior Member

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    It travels at c ipso facto.

    it has always been measured at this constant. That is why it is at this constant. Measuring the speed of light through different elements and we come up with c and the diffuse element. All times we have tried to measure it even on moving parts measures the speed of light at its constant.

    through the heaviest and lightest “atmosphere “ it has always been measured at this constant.

    that is why the light train dynamic has always been so confusing.

    how does something moving at c repeat the same velocity? Well we don’t know. But it does and we can prove it

    now how did Einstein know the speed of light was constant before he could measure it through any medium is beyond anyone except him and his belief in god. Just like mine in waking someone up from a coma by saying, “ think of the best time in your life, think of the worst time, and think of something simple”
     
    Last edited: Jun 12, 2021
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  5. Q-reeus Valued Senior Member

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    The authors of 2nd linked-to article in #81 have imo a very questionable basis for their physical model 'explaining' light speed in terms of interactions with 'vacuum virtual Fermion pairs'.
    Below eqn (27) is this choice passage:
    "Also the interaction of a real photon with a pair must not exchange energy or momentum with the vacuum (for instance, Compton scattering is not possible)."
    Suppression, by fiat, of normally expected randomness of photon absorption-then-re-emission (destruction-then-creation in QFT parlance)? That's handy! Especially given the supposed virtual Fermion pair will have an arbitrary translational direction and speed relative to the given photon in some specified frame. Not to mention a randomly oriented angular momentum. Seems artificial to me. There is afaik still zero experimental support for the author's theoretical model. Not a surprise. Anything otherwise would be a huge surprise and require careful scrutiny.

    I have numerous times now posted the following piece strongly challenging the notion of a vacuum seething with 'virtual particle pairs':
    https://arxiv.org/abs/hep-th/0503158v1
    Given he establishes that the known and experimentally confirmed Van der Waals/London dispersion interactions account for 100% of the Casimir forces, what's left for 'vacuum fluctuations'?
    Deferring to an expert in QM/QFT who reinforce that viewpoint:
    https://www.physicsforums.com/insights/misconceptions-virtual-particles/
     
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  7. irk Registered Member

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    This is why I mention one has to consult superexperts what they think

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    . That speed of light is tied to vacuum permeability and permittivity is already in classical theory. The vacuum begs then for explanation on the grounds of quantum theory, no?

    In recent times indeed opinion seems to be shifting on the Casimir effect as not having relation to quantum vacuum. But quantum vacuum belongs to the canon of QFT with vacuum polarisation and its role in charge screening and Lamb shift explanation. Though these are effects in strong external fields, with no such fields quantum vacuum is more nebulous, it seems it is invoked as necessity due to the Heisenberg uncertainty principle. "Physicists have speculated about the properties of the quantum vacuum for at least8 5 years; however, only recently have they understood the quantum vacuum sufficiently well to begin making testable predictions" to cite the preprint from February 2021: Properties of the Quantum Vacuum calculated from its structure
     
  8. Q-reeus Valued Senior Member

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    4,618
    I realize there are two basic opposed schools of thought on this basic issue. Read the entire articles by Jaffe and Neumaier. Lamb shift and similar all have an explanation that doesn't invoke vacuum fluctuations/virtual particle pairs. A bonus is the notorious 120 ('only' ~ 68 by some optimistic accounts) order of magnitude vacuum catastrophe problem is nonexistent.
     
  9. irk Registered Member

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    9
    Look at this paper Properties of the Quantum Vacuum calculated from its structure, it goes good distance to clarify confusions around this topic. One has to differentiate between 'real' and 'virtual' (internal Feynmann diagrams) vacuum fIuctuations. Neumaier also explain this. Treatment with real fluctuations, the case how photon interacts with them (Neumaier does not mention this), allows for the calculation of the speed of light and fine structure constant with high precision. They also point why Casimir problem was approached incorrectly applying quantum vacuum and also deal with vacuum catastrophe and old cosmological problem. Regarding Lamb shift the mainstream is with fluctuations. Do you consider vacuum polarisation also as unreal?
     
    Last edited: Jun 13, 2021
  10. Q-reeus Valued Senior Member

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    4,618
    I'm not at all up to understanding the intricacies of QFT, but the theorized real interactions between a real photon and off-shell virtual Fermion pairs comes across as rather contrived.
    Lorentz invariance of the vacuum requires the same enormous spread of not only vp pair energies but momenta in any chosen inertial frame. Given the Fermion vp pairs acquire fleeting dipole moments, the claimed zero angular momenta implies that entirely unlike for a diatomic gas, the vp pairs are completely transparent to each other. Each pair only capable of interacting with real particles/photons.

    Given that, there is still the question of how a say exactly z-direction propagating photon remains exactly z-direction propagating after absorption/emission by a vp pair having arbitrary transverse to z-axis relative momentum? Evidently by mathematically imposing the requirement that vp momentum before must equal vp momentum after. Except that when the energy the vp pair has momentarily 'borrowed' from the vacuum is 'payed back' - that potentially enormous momentum must disappear also. Just where this mysterious reservoir of vacuum energy-momentum that vp pairs can borrow from and pay back to is far from clear to me. I'll leave this topic for others to ponder further!
     
  11. irk Registered Member

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    I do not see problem with angular momentum. Photon energy is in it (zero mass) and so it must be preserved. Energy borrowed and pay back is allowed due to the uncertainty principle between energy and time. Why this is as it is is of course mysterious but this is also touching cosmology and existence of the Universe in the form we are trying to understand. One issue which is unclear to me is that photon is treated as particle, where it is as a wave, in particular how to treat interference in quantum vacuum?

    Going back to the constancy of the speed of light. I think the best answer we have at present that it is due to the properties of vacuum elaborated in the Quantum Field Theory. The last point is illuminated in this bunch of papers Usual disclaimer is that Quantum Field Theory is an enormous edifice and one should seek opinion of superexperts. Superexperts council could be composed of Nobelists in this area plus guys who produced volumes >1000 pages in this topic

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    . Note also that by invoking quantum vacuum we also defer the answer to the next level: so why quantum vacuum have such properties?

    Another variation on the constancy of the speed of light is how it would be if the speed of light is constant but different, e.g. half or twice of its present value? I think that on the basis of classical theory including relativity theory, all kind of parameters would rescale, time would be flowing faster/slower, lengths would be longer/shorter, masses bigger/smaller and when living inside such world one would not notice anything different. But on the basis of quantum theory rescaling could have serious consequences and likely the world would not be stable or could not exist at all which implies that our speed of light is a result of fine tuning of parameters providing extreme stability. Usual disclaimer, I am not even an expert...
     
    Last edited: Jun 14, 2021
  12. Q-reeus Valued Senior Member

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    4,618
    Assuming reality of vp picture, Lorentz invariance of vacuum means vp pairs will blink in and out of existence with not just ephemeral energy but random linear momenta drawn from a huge and continuous spectrum of possible values. Angular momentum must otoh be quantized and that is a basic distinction. True also of a diatomic gas where nonetheless angular KE contributes typically ~ 2/5 of net molecular KE. Evidently no counterpart in vp pairs case. Very nonintuitive.
     
    Last edited: Jun 14, 2021
  13. irk Registered Member

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    9
    Not only this but the pairs can be any kind of particles with mass and even virtually anything else, some speculations are talking virtual mini black holes.
     
  14. TonyYuan Gravitational Fields and Gravitational Waves Registered Senior Member

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    In the traditional method of measuring the speed of light on the earth, the measured speed of light will never change. It's like measuring the speed of sound in an airplane. The speed of sound is constant in any direction. The reason is that the sound is dragged by the air in the plane.

    The light is dragged by the gravitational field. Assuming that the speed of light (in the case of earth rotation velocity is 0) is x, then the speed of light measured in any direction on the earth is x, for the same reason as sound.
     
  15. Q-reeus Valued Senior Member

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    4,618
    A more consistent 'aether drag' analogy to your sound measured inside an aeroplane would be one measured by an open cockpit flyer. If aether entrainment were real all of astronomy and cosmology and established relativistic physics would be thrown into turmoil. Spinning galaxies and spinning stars within them should all show e.g. unsymmetric aberration of light patterns but don't.
     
  16. TonyYuan Gravitational Fields and Gravitational Waves Registered Senior Member

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    Gravitational field is not ether.
     
  17. Q-reeus Valued Senior Member

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    4,618
    Combining the second last and last sentences in #91 seems clear enough to me. Gravity is posited as the means to entrain the aether, and this entrainment is in relation to both translational and rotational motion of a gravitating body e.g. the Earth. It implies a viscosity to space resulting in presumably eddies and drag force that would surely have long ago materially affected e.g. planetary orbits. Apart from before mentioned aberration patterns. Of light received from distant sources passing close to or through nearer rotating ones.
     
  18. TonyYuan Gravitational Fields and Gravitational Waves Registered Senior Member

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    There's no ether, just gravitational fields and gravitational waves. Gravitational wave has Doppler effect, it will affect the gravity around the planet. The light will be affected in the gravitational field, so it will be dragged by the gravitational field.

    https://photos.app.goo.gl/RAfMhZhvNPqFVNXo8
     
  19. Q-reeus Valued Senior Member

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    4,618
    Uh huh. On p2 #36, #38, here:
    http://www.sciforums.com/threads/gr-is-a-static-gravity-model.164397/
    results flatly in contradiction to claimed perihelion precession for Venus were pointed to. Digging in at all costs isn't always a good strategy.
     
  20. TonyYuan Gravitational Fields and Gravitational Waves Registered Senior Member

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    Do you think something is wrong? Please point out, thank you.
    This post is about the constant speed of light. We can focus on this topic. I don't want to lead the topic to my theory. If you are interested, you can post your opinion on my post.
     
  21. TonyYuan Gravitational Fields and Gravitational Waves Registered Senior Member

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    635
    Forward:
    Einstein's attempt at a general theory is not valid, because he approached the project believing that general relativity would just be a simple generalisation of special relativity's physics.

    And it's not. A general theory is based on fundamentally different assumptions to the special theory.

    • Under a general theory, all observer-masses must have associated curvature, and any relative motion of observers must then physically alter the shape of spacetime (=shape of the light-metric), due to gravitomagnetic effects.
    • Under special relativity, we assume perfectly empty space, and we assume that the introduction of observers does not affect the status of that space as perfectly empty. All observers can move how they like without affecting the shape of spacetime.
    So SR observers have zero curvature and zero effect on the metric, while GR observers must have curvature and their motion must affect the metric, otherwise the general principle doesn't work.

    "SR" and "GR" observers are mutually incompatible, and GR observers cannot obey the physics derived for SR observers. GR observer-masses cannot obey SR equations of motion. These are different geometries.

    Consequently, Einstein's general theory is riddled with logical faultlines and inconsistencies, that we are only able to deal with by avoiding exact solutions (which would expose the mismatches) and embracing approximations, or by arguing that certain irresolvable problems will have to wait until a theory of quantum gravity comes along.

    Currently, Einstein;s general theory doesn't even work properly for the simplest possible idealised mass (a spherically-symmetrical mass whose specific details are hidden behind a curvature horizon), moving inertially, in a straight line at constant speed, against an arbitrarily-flat background, arbitrarily-distant from any other disturbing matter.
    Preprint Gravitomagnetic horizons and the comprehensive failure of Ei...

    If a gravity-well has any motion at all with respect to a distant observer then ... pooft ... Einstein's general theory self-destructs.
     
  22. Sherlock Holmes Registered Member

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    50
    The speed if light is constant because it is a law or arises from laws, this isn't being dismissive either.

    The speed of light can be calculated from what are called Maxwell's equations of electromagnetism, these equations describe relationships between electrical and magnetics fields.

    These equations are regarded as laws, "laws of physics" or "laws of nature" and are considered to be the same for all observers, whoever you are, wherever you are, how ever your moving, these laws are always true.

    So, because the laws - the equations - are always true for all observers then the speed of light is also always the same for all observers.
     
  23. TonyYuan Gravitational Fields and Gravitational Waves Registered Senior Member

    Messages:
    635
    Light travels through a long glass. (take the ground as reference)
    If the speed of the glass is 0, the speed of light is v0;
    If the speed of the glass is 100m/s, the speed of light is v1.
    What is the relationship between v0 and v1? Will it be equal or not equal?

    @Sherlock Holmes, can you give the answer?
     

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