The recent experiments with direct detection of gravitational waves showed that they arrived 1.7 second before the gamma-ray burst from the neutron stars merger [1]. While this is tiny deviation it posts a question about the speed of light - is it really not c (Lorentz speed from special relativity) but a tiny bit below it? And how absolute is principle of relativity?
While there were many attempts to overcome the principle of relativity (and all failed) they all came from the perspective of theoretical re-interpretation of Michelson-Morley experiment and futile attempt to save the theory of classical ether. Here is the attempt to evaluate it from the point of view of experimentalist not theoretician.
What is any physical law from the point of view of modern theoretician? This is most frequently the mathematical law beyond it which is from the philosophical point of view holds supreme, that is absolute from the point of view of mathematics. Indeed, the energy conservation law means no energy may be created or lost, absolute mathematical zero. Principle of relativity means that speed of light in vacuum is exactly Lorentz speed with absolute mathematical accuracy. In General Theory of Relativity the constant G is absolute and may not deviate nowhere, just absolutely fixed constant. Any such idea creates a possibility to express such law in quite simple mathematical form. The mathematical equations are written and new phenomena are predicted.
But from the point of view of experimentalist any physical law is just a fit to the existing experimental data and nothing more. It is obligatory approximate and by no means absolute - one day it will be rejected and become the simplified approach of more general law. And physics seems to confirm this point of view: Newtons law were considered as absolute, but finally became just a simple limit version of both quantum mechanics or theory of relativity depending upon the scale of research. The same is true with any physical law: it is just fit to the existing experimental data and nothing philosophically absolute is beyond it. They all temporary and just waiting to be revoked and replaced by some more general laws, which are in there's turn are to be replaced by even more general laws etc. However, some of such fits are enormously good! Energy conservation law is one example. Another example is principle of relativity and postulate that speed of light in vacuum is exactly Lorentz speed. That particular law is so accurate fit to the existing data that by many is considered as absolute. But that may be changed soon. Of course, even if it is not absolutely correct it is very close to absolute, and the deviation is only possible to be very (enormously) small and revealed on very large (enormously large) distances, like in space.
The idea of the non-absoluteness of the speed of light in vacuum originates mainly from the quantum mechanics and especially from the idea of quantum vacuum. For example, due to the presence of virtual particles in quantum vacuum it is responding to the external field and thus the Kerr effect in the vacuum is possible (quite serious publication in the very serious journal is here [2]). The author emphasizes the fact that the principle of relativity is respected in the sense that the received velocity of light is obligatory below Lorentz speed but from philosophical point of view the existence of such effect means that the principle of relativity is not an absolute any more, but rather an extremely good fit to the existing experimental data. The calculated deviation of the speed of light from Lorentz speed is enormously small (and well below measurability level) but it means that small deviations of speed of light from Lorentz speed are not a big deal anymore - the relativity principle is not absolute, rather a fit and may be replaced by more general law.
From the point of view of experimentalist it opens the new venues of search for the experimental data, which would contradict to the existing paradigms of science and would create a "new physics". One of the ideas is that the red shift is not due to the Big Bang, but rather due to the inherent but not discovered properties of light which allow it to loose the energy while propagating the enormous distances of billions of light years which the observer at the Earth see like a red shift (of course it is red, means that the energy is lost, not obtained, which would contradict to another general law of energy conservation which is from experimentalist point of view not absolute too, but for right now and in this phenomenon is valid - say that law is better overall fit compare to principle of relativity).
From my point of view any matter including light is both particle and wave and thus the light obligatory has non-zero rest mass (despite of course enormously small) [3]. In this approach every particle including photon is considered as having rest mass and photon is ultra-relativistic particle for which the simplified energy relation E=p*c is valid (pulse times speed of light), as for any other ultra-relativistic particle like electron for example. In this approach of course that is approximate relation even for photon E~p*c, because in reality speed of light is a little bit below Lorentz speed c, which was possibly revealed by the experiment with gravitational waves (see the beginning of the post). The speed of gravitational waves is not Lorentz speed too, of course, from the same consideration as before, but the gravitational wave propagating with Lorentz speed is better fit compare to speed of light (and the Lorentz speed itself is not absolute, philosophically absolute value too, but that would be for future generations to discuss).
But would be the origin of this mass for photon? Is it really the particle with some finite but very small mass (in this case the registration of non-relativistic photon in vacuum would be very difficult since it would have the De-Broglie wavelength of enormous value - around 100 thousands meters for the photon moving with velocity of 1 m/s, from the evaluations done in [3]). The detector to register this particle as a particle should have a similar dimensions of 100 km and not feasible even in the far future.
In this case the red shift may be explained very similar to the energy loss of an ultra-relativistic particle moving in the field along the curved trajectory - the electron in cyclotron is circling around and as an accelerating particle shedding away photons (synchrotron radiation) thus loosing energy. The photons are subject to the gravitational fields (light is bent by masses) and since the mass is not absolutely zero, they are shedding away gravitons and are losing energy too. In this case since they are still ultra-relativistic this is not revealed as the change in speed (it is extremely small and unmeasurable) but as red-shift. Being propagated in the non-uniform gravitational fields for billions of years they are obtaining finally red shift observable on Earth.
Another interesting way for photon to have the finite mass is to have the virtual finite mass. In this case the quantum vacuum may offer such possibility. Quantum vacuum is considered as a special media obeying principle of relativity for the uniform motion and for the motion with constant acceleration [4]. Here is how it is stated in [4]:
"We may emphasize that the motional force does not raise any problem to the principle of special relativity. As a matter of fact, the reaction of vacuum (*) vanishes in the particular case of uniform velocity. The quantum formalism gives an interesting interpretation of this property : vacuum fluctuations appear exactly the same to an inertial observer and to an observer at rest. Hence the invariance of vacuum under Lorentz transformations is an essential condition for the principle of relativity of motion to be valid and it establishes a precise relation between this principle and the symmetries of vacuum. More generally, vacuum does not oppose to uniformly accelerated motions and this property corresponds to conformal symmetry of quantum vacuum [*]. In this sense, vacuum fluctuations set a class of privileged reference frames for the definition of mechanical motions"
Later in the same publication [4] the authors actually predict the existence of subtle effects never observed, connected with Kazimir forces, which creates the problems for dissipative processes in quantum vacuum being considered from point of view of principle of relativity. Essentially they came to the conclusion that if the process is dissipative and valid in quantum vacuum it may a little contradict to the philosophical absolute principle of relativity, like the publication [2]. This is actually expected and well in line with present blog: the principle of relativity is not philosophical absolute, rather a very good fit for the existing data and small (but actually extremely small and not yet possible to observe) deviations are not only possible but rather inevitable.
In the sense of the quantum vacuum the light is actually supported by the popping in and out of the existence virtual particles pretty much as in a condensed matter with huge refraction index (say 10 in ultra-dense plasma) the light is almost completely supported by the real, not virtual, charges and dipoles (indeed, when the index of refraction is larger than 1 part of the energy of the electric and magnetic fields of light is actually in the induced polarization of the matter, not in the initial vacuum hold electric and magnetic fields, that is why the speed of light is smaller and the values of the field are different). If the index of refraction is 100 almost nothing left of the quantum vacuum here, and the propagating light is almost entirely depends upon the properties of the real particles and real charge distributions. In essence the light in this situation is more like the total assembly of orientations and motions of the real particles. Thus the famous experiment of Fresnel dragging may be interpreted as follows.
Einstein's formula u=c/n + v(1-1/[n*n]) or u=c/n - v(1-1/[n*n]) (see [5]), where u is the measured speed of light propagating in the media with refraction index n moving with velocity n in the direction of the light or against it
is because light is partially supported by media. If n=1 (vacuum, no support) the velocity is obviously c (principle of relativity). If the value of n is huge (say 100) the velocity of media is almost completely added or subtracted (in this case the aether theory is valid because the light is essentially nothing more now but some distortion of the moving media and almost nothing left from the vacuum part).
If the media with huge refraction index n is subject to tidal force of gravity near the gravitating body (say moving directly toward the center of the gravitating body) any distortion of the media is subject to such tidal forces too. Since the front part of the oscillation of the light is attracted more strongly toward the body than the other side ( in both cases, whether it is moving toward it or away from it) the light obviously is spread more together with the media (the wavelength is becoming larger). Thus if such media with light passes near the gravitationally attracting body (say the star being ruptured by tidal force passing near the black hole) the red shift must be obviously present and explained by the action of gravity on the media, not on light itself.
The quantum vacuum being almost completely OK with respect to principle of relativity, however, may explain the tiny red shift in the same way: the effective n for quantum vacuum is not exactly 1, but a little deviating from it (n>1, of course). This is exactly the conclusion of [2]. Then when the photon is passing the ominous spaces in the Universe traveling for billions of years this media (quantum vacuum) creates kind of non-zero virtual mass of the photon (despite indeed really very small). During the passage of the light near the gravitational body the light is supported by those virtual particles which are in turn are subject to gravitational field. From quantum mechanics it means that the non-zero mass is accelerating a little and thus obligatory shedding some gravitons and loosing energy. From classical electrodynamics point of view the virtual media of quantum vacuum is stretched by the tidal force, thus elongating a little bit (very small effect of course) the photon, leading to the increase of the wavelength and to the red shift. Once the gravitational body is passed the influence of it on virtual refraction index is over (n=1 with much larger accuracy) so the velocity of photon is restored too, but the stretch is not (because for both directions toward the body and away from the body the gradient is working in the same direction). If it would be real media it may contract back through the electromagnetic forces, but the quantum vacuum is different - it is not passing information from point to point. Light is being now supported by the quantum particles which popped out into the very short existence and have no knowledge about the virtual particles which supported light before the gravitating body.
The effect is enormously small but the photons is to travel billions of years, too. So this creates the slow change of the energy of photon through not change of velocity but rather through the elongation of wavelength - exactly red shift.
References.
1.Ask Ethan: Why Did Light Arrive 1.7 Seconds After Gravitational Waves In The Neutron Star Merger? (forbes.com)
2.Scott Robertson "Optical Kerr effect in vacuum"// Phys Rev. A, 100, 063831
Phys. Rev. A 100, 063831 (2019) - Optical Kerr effect in vacuum (aps.org)
3.Dmitriy S. Tipikin "The quest for new physics. An experimentalist approach"
Published in MoreBooks in 2021:
https://www.lap-publishing.com/catalog/details//store/gb/book/978-620-4-73173-5/the-quest-for-new-physics-an-experimentalist-approach
THE QUEST FOR NEW PHYSICS An experimentalist approach / 978-620-4-73173-5 / 9786204731735 / 6204731734 (lap-publishing.com)
Free version on Vixra.org:
https://vixra.org/abs/2011.0172
The Quest for New Physics: An Experimentalist Approach, viXra.org e-Print archive, viXra:2011.0172
4.“Quantum vacuum
fluctuations” by Serge Reynaud, Astrid Lambrecht,
Cyriaque Genet, Marc-Thierry Jaekel
CERN Research, 10.1016/S1296-2147(01)01270-7
https://core.ac.uk/download/pdf/25312347.pdf
/home/www/ftp/data/quant-ph/dir_0105053/0105053.dvi (core.ac.uk)
5.https://en.wikipedia.org/wiki/Special_relativity#Dragging_effects
https://en.wikipedia.org/wiki/Special_relativity#Dragging_effects