文中鏈接的我的英文文章：

Constant Light Speed in Vacuum was not a Mere Postulation

Rongqing Dai

As a theoretical premise of the theories of relativity of Einstein, the constant vacuum light speed to all observers has been presented to the public as a postulation in all text books. However, if we carefully review the relevant theoretical preparations before the advent of the special relativity, we might see a clear logical thread leading to the conclusion of the constant vacuum light speed to all observers, and thus it was more of a logical conclusion than a mere postulation.

First, in 1865 Maxwell published[1] his deduction that the propagation speed of electromagnetic waves is

v =1/                                 (1)

where ε is permittivity and μ is permeability, and, which are denoted as ε_o and μ_o in vacuum (empty space).

When Maxwell calculated speed of light using (1) with the values of ε_o and μ_o known to him, he found it matched the recent measured speed of light at that time very well and thus believed that light be electromagnetic waves and equation (1) be the formula for the speed of light c. Obviously, Maxwell's formula for the speed of light does not contain any information about the frame of reference.

Second, the independence of the speed of light from the selection of frame of reference according to (1) was further supported by the principle of relativity that the laws of physics have the same form in all admissible frames of reference. This was a simple generalization of the Galileo's principle of equivalence of mechanical motion in inertial frame (which was first generalized to inertial system for all physics laws and then extended to all admissible frames of reference). With this principle, the Maxwell formula together with the values of ε_o and μ_o would not change by selecting different frames of references that are moving relative to each other.

However, the above two conditions are still not enough to make a type of speed constant to all observers. For example, the formula for the speed of shallow water waves is, and it does not involve frame of reference and it conforms to the principle of relativity, but the speed of water waves are not independent of the selected frame of reference. This is because water is a movable medium, and thus the speed of water waves might not only be superposed by the speed of the water flow but also be altered by the observer’s speed relative to the medium. At the time of Maxwell, the medium for light to propagate was believed to be the luminiferous ether. Therefore, we need one more reason to complete the needed conditions for the conclusion of constant speed of light.

Third, beginning with the report of Michelson-Morley’s famous failed experiment in 1887[2], a series of experiments showed that the supposed luminiferous medium ether did not exist, which seemed to leave the supposed empty space a complete empty vacuum, and thus provided the important third condition for the constancy of speed of light to all observers. This is because now people could not find any reason for the speed of light to take a value in a frame T of reference different from its value in a frame S of reference since a) it is given by equation (1) in both frames of reference and b) there is no medium movement in any sense to account for any possible difference based on any knowledge known to any human at that time.

One might ask why the speed of an asteroid moving in the empty space is not constant to all observers while it does not need any medium either. The simplest answer would be that there is no formula like the Maxwell’s formula for speed of light that would govern the speed of all the asteroids without involving the selection of frame of reference.

Therefore, scientifically speaking, the constancy of the speed of light to all observers is not a mere postulation but rather a logical conclusion as long as one admits that a) light is electromagnetic waves and its speed can be given by formula (1), and b) physics laws should be the same in all admissible frames of reference and c) vacuum is empty space without any medium for light to propagate.

However, when it comes to this 21^st century, some new discovery might make the historically logical conclusion of constant speed of light to all observers truly become a postulation.

For the past decades, scientists have found that vacuum is not empty but instead full of energy[3]. What makes matters further complicated is that the energy in vacuum does not seem to be light-phobic but instead could produce photons, which would make it philosophically unsound to completely exclude the possibility for light to interact with the energy when it travel is in the vacuum unless we can have a clear proof to support that, which does not seem to be available yet. If light does interact with the energy in vacuum in some manner, then we might ask what if a confined vacuum itself moves in a broader open space as mentioned in [3]? Will the speed of light still be constant in that situation?

Of course, we can also look at this issue from another angle: we don’t have any reason to tell us that light will interact with the energy in vacuum yet and thus we do not need to worry about it until we have some reason for it. In this way, the constant speed of light would truly become a postulation instead of logical conclusion, philosophically and scientifically.

Final Remarks

The discussion in this short essay shows that whether the constancy of the speed of light to all observers would be merely a postulation or a logically sound conclusion is indeed more of a philosophical and psychological than a scientific issue. Scientifically the logical thread leading to the conclusion of the constancy of light speed was historically clear and sound; but psychologically and thus philosophically the issue is subtle and intricate. Historically, when the constancy was proposed by Einstein, it took great courage to stand out against the common perception of hundreds of millions of people because deviant new theories in sciences have always been at the mercy of public opinions like it is today in 21st century. On the other hand, a concealed unknown could often offer more sense of solidity than something with clear thread of logical root, and thus a so-called empirically proved postulation would stand more firmly than a logical conclusion with clear historical background. This is good for helping the public to receive the benefit of the postulated claim, but in case new challenges appear, it could also function to hinder the reexamination of the possible subtlety in the postulation under the new condition. With new evidences showing that the vacuum might not be a complete empty space without anything, it might raise philosophical concerns about the influence of the vacuum energy upon the speed of light in vacuum; hence reviewing the logical thread leading to the claim of the constancy of the speed of light might help reexamine the claim with some new possible conditions. Although the output of the reexamination could most probably very well be a reassurance of the validity of the claim, philosophically and scientifically it is still a right attitude to hold when facing new challenges.

 

 

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[1] e.g. Wikipedia (2022) Electromagnetic wave equation. https://en.wikipedia.org/wiki/Electromagnetic_wave_equation

[2] e.g. Wikipedia (2022) Michelson–Morley experiment. https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

[3] Choi, Charles Q. (2013) Something from Nothing? A Vacuum Can Yield Flashes of Light. https://www.scientificamerican.com/article/something-from-nothing-vacuum-can-yield-flashes-of-light/#:~:text=For%20instance%2C%20photons%E2%80%94packets%20of,that%20pushes%20the%20mirrors%20together.