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Lorenz transform equations (LTEs) are invoked to preserve the principle of relativity by resolving paradoxes arising therefrom due to the invariance of light-speed in all frames of reference. Said resolution of those paradoxes manifests itself as the framework of Special Relativity (length contraction, time dilation and relativity of simultaniety). Accordingly, LTEs fail to explain the reason for the invariance of light-speed or the reason the set speed is a certain value.

Mathematics is a necessary tool to prove the accuracy of hypotheses. But we must always return to the original source of any given hypothesis, once proven, in order to appreciate, intuitively, what we have learned. That source is through the lens of everyday human experiences, experiences stored in the brain's memory bank (particularly visual) where our imagination allows us to manipulate them every which way to hopefully attain fuller understandings of how our newfound hypothesis relates to them. It is by such means where we access meaning and value to things. So, mathematics is simply a tool to get us where we want to be. Some who study physics are satisfied in resorting to a compendium of mathematical equations to answer questions without fully understanding the subject matter in an intuitive way. I am not satisfied with mathematical explanations alone; I need to understand the bases for them. So, please explain your answer in the same verbal (non mathematical) context which I am about to present the problem (below).

At the outset, my question does not concern a relationship between a given "stationary" frame and the corresponding "moving" frame. Instead, it concerns only a "stationary" frame. Now, to begin, here is my preliminary question. What precise factors come into play to make the "stationary" observer record the same speed of light whether the light is approaching them or receding from them?

If the recorded speed (distance per unit time) is invariant for this "stationary" observer, then as the light approaches, the space between it and the observer would have to be stretching by just enough to offset the decreasing distance in order to maintain the recorded light-speed ("c"). And, as the light is receding, the space between it and the observer would have to be shrinking by just enough to offset the increasing distance in order to, once again, maintain the recorded light-speed ("c").

But said stretching and shrinking of space would not apply for the "stationary" frame because observed effects of length contraction of space in SR would only apply to the "moving" frame's perspective (which in this case would be the light itself as opposed to a "moving" mass). Time dilation and relativity of simultaniety also wouldn't apply here because, again, we are concerned with only the elapsed time recorded on the "stationary" observer's clock.

The following is, I think, a plausible explanation for this seeming conundrum. Unlike a moving mass which has already gained its momentum before being subjected to a force which accelerated it to its subsequent greater speed (the latter speed which, by analogy, we would be measuring), light is not a mass subject to Newtonian mechanics but is, instead, a form of energy created at the very moment it is emitted from the moving mass. The light could have no history of motion before it was created. So, its speed is independent of the speed of the moving mass.

So, we are always measuring the speed of light, not a "moving" mass from which the light was emitted. All that said, my argument might explain why a "stationary" observer will record the same speed of light whether it is approaching or receding from them, but now my ultimate question which is why is light-speed invariant in the first place and why is this invariant speed 299,792,458 meters per second (as opposed to any other speed)?

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    $\begingroup$ "whether the light is approaching them or receding from them". You can't see light that's receding from you. Do you mean to ask about light sources that are approaching or receding from the stationary observer? $\endgroup$
    – PM 2Ring
    Commented Feb 8, 2023 at 7:10
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    $\begingroup$ In a sense, 299,792,458 metres has the same magnitude as 1 second. Here's one of my answers on this topic: physics.stackexchange.com/a/291346/123208 Also see physics.stackexchange.com/q/3644/123208 & links therein. $\endgroup$
    – PM 2Ring
    Commented Feb 8, 2023 at 7:20
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    $\begingroup$ @PM 2Ring: Yes, the light sources. $\endgroup$
    – user150908
    Commented Feb 8, 2023 at 7:20
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    $\begingroup$ Does this answer your question? Special Relativity Second Postulate $\endgroup$ Commented Feb 8, 2023 at 8:00
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    $\begingroup$ "That source is through the lens of everyday human experiences" No, that only gets us so far. That's why scientists had to start doing experiments. $\endgroup$
    – J.G.
    Commented Feb 8, 2023 at 14:30

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The invariance of light speed and its particular value are both observed facts, which cannot be derived from any current physical theory. Any theory we create is built up around these facts. That is possibly a simpler and a less fulfilling answer than you were looking for. It is possible that a future theory will show how one or both of these facts arise from some deeper physical principle.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Physics Meta, or in Physics Chat. Comments continuing discussion may be removed. $\endgroup$
    – Buzz
    Commented Feb 13, 2023 at 21:24
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By making electrodynamics Lorenz-invariant, we enforce causality. In a universe where the velocity of light is added to that of its source, there would then exist frames of reference in which we would experience effects before their causes had happened.

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my question does not concern a relationship between a given "stationary" frame and the corresponding "moving" frame. Instead, it concerns only a "stationary" frame ...

What precise factors come into play to make the "stationary" observer record the same speed of light whether the light is approaching them or receding from them?

Since we are dealing only with a single frame, your "stationary" frame, we merely need to look at the laws of physics in that frame.

Specifically, Maxwell's equations govern the behavior of electromagnetism. Maxwell's equations, in turn, admit a set of solutions called "vacuum solutions" which are the kinds of electromagnetic fields that you can have without any sources (currents or charges) nearby. These solutions are electromagnetic waves that propagate at the definite velocity $c=1/\sqrt{\mu_0 \epsilon_0}$ where $\epsilon_0$ describes how much electric field a charge density produces in vacuum and $\mu_0$ describes how much magnetic field a current density produces in vacuum.

With Maxwell's equations in vacuum, that is the only speed that electromagnetic waves can travel. The speed of the thing emitting the waves does not change the speed of EM waves, just like the speed of an object emitting sound or the speed of a boat producing a water wave does not change the speed of sound waves or water waves. In all cases the waves propagate isotropically in the "stationary" frame regardless of the emitter characteristics.

If the recorded speed (distance per unit time) is invariant for this "stationary" observer, then as the light approaches, the space between it and the observer would have to be stretching by just enough to offset the decreasing distance in order to maintain the recorded light-speed ("c"). And, as the light is receding, the space between it and the observer would have to be shrinking by just enough to offset the increasing distance in order to, once again, maintain the recorded light-speed ("c").

But said stretching and shrinking of space would not apply for the "stationary" frame because observed effects of length contraction of space in SR would only apply to the "moving" frame's perspective (which in this case would be the light itself as opposed to a "moving" mass). Time dilation and relativity of simultaniety also wouldn't apply here because, again, we are concerned with only the elapsed time recorded on the "stationary" observer's clock.

None of this is relevant in a single frame.

my ultimate question which is why is light-speed invariant in the first place and why is this invariant speed 299,792,458 meters per second (as opposed to any other speed)?

That is two additional questions which are different from the above primary question. The question about why $c$ is invariant is answered here:

Why does speed of light have to be constant?

Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

The question about why it is 299792458 m/s is answered here:

Why is the speed of light defined as 299792458 m/s?

Standard Definition of speed of light and metre

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  • $\begingroup$ How are electric and magnetic field charge densities produced in a vacuum determined mathematically? Specifically, do mathematical operations determining their values require the speed of light ("c") which, perhaps, is obscured by more fundamental (derivative) subsets of equations? Also, the second part of my question was not why the invariant value of "c" is defined by the metric system but, instead, why the value is what it is using any unit of measure. This might be irrelevant depending on how you answer my first question. $\endgroup$
    – user150908
    Commented Feb 8, 2023 at 17:38
  • $\begingroup$ @Rob yes, the mathematical operations (Maxwell’s equations) describing the electric and magnetic fields automatically lead to electromagnetic waves that propagate at c. I would normally have posted it, but I avoided the math per your specific instructions in the question. Regarding the second part, you cannot talk about the value of c without talking about units. Without talking about units all that you can say is that it is invariant and finite. $\endgroup$
    – Dale
    Commented Feb 8, 2023 at 18:58
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The special thing about the "speed of light" is the speed, not the light. That is, the effects of relativity come about if there is any invariant speed that is measured to be the same by all observers. There is such a speed in our universe, and it happens that light, gravity, and the strong nuclear force all travel at that speed.

Now, consider that either there is a maximum speed attainable by anything, or else there isn't. Ultimately that's a question for experiment, but either way there are some strange effects:

(1) If there is no maximum speed then an object can move arbitrarily quickly between two points, and effectively occupy the entire space between those points. Also, an event anywhere in the universe could potentially affect us here on Earth immediately. A universe like that would be pretty odd.

(2) If, on the other hand, there is a maximum speed then that speed would have to be the same for all observers, regardless of their relative motion. From the existence of an invariant speed you can derive the Lorentz transforms and all the results of the special theory of relativity, which are also pretty odd.

So either way we'll get some counter-intuitive effects. As it happens experiment shows that there is a maximum speed, usually denoted by $c$. Light happens to travel at that speed (the reason for this is that photons are massless).

As for why $c$ has the value it does, that's really just a choice of units. If there is an invariant speed (the same for all observers) that establishes a fixed relation between space and time, and one may as well use that relation to define $c$ to be 1. That's the physicists preferred choice for $c$, but in SI units it has a different value that's really basically arbitrary.

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  • $\begingroup$ I have a problem understanding your last paragraph. Since there IS a fixed relationship between space and time, the reason that we arrive at a precise value in units, regardless of whatever unit system we use, is still unknown. So, yes; the choice of units we use is arbitrary. But, the precise value obtained using the system of our choice is what I'm curious about. $\endgroup$
    – user150908
    Commented Feb 10, 2023 at 1:08
  • $\begingroup$ The "natural" value of c is 1. Systems of units in which c has a different value just indicate the degree to which that particular system of units has a mismatch between units of time and units of space. So if you measure space in meters and time in seconds, your time units are skewed by 299,792,458 times relative to the space units. That's purely an artifact of the choice of units; in a sense it's just a reflection of how "unnatural" the system of units is. $\endgroup$
    – Eric Smith
    Commented Feb 10, 2023 at 2:33
  • $\begingroup$ Ok. Both the left and right sides of Maxwell's equation are measuring light ("c" and EM are the same thing). Mathematical operations performed on EM (right side) don't change the speed of "c" (left side) whatever that speed may be since, based on said mathematical operations, it turns out that the speed of "c" = 1 times speed of EM, or simply 1 - which proves invariance of light-speed even though the equation doesn't establish a value for light-speed itself. $\endgroup$
    – user150908
    Commented Feb 10, 2023 at 23:19
  • $\begingroup$ The invariance of the speed of light doesn't come from Maxwell's equations. It comes from requiring that the laws of physics (including those for electromagnetism) be the same for observers moving at constant speed relative to one another. $\endgroup$
    – Eric Smith
    Commented Feb 11, 2023 at 1:31
  • $\begingroup$ Here's another way of thinking of it: the speed of light has the particular value it does in (e.g.) SI units because SI units are chosen based on everyday human experience, and our experience stretches far more in time than in space. So essentially asking why the speed of light has the value it does is equivalent to asking why humans are so small. $\endgroup$
    – Eric Smith
    Commented Feb 11, 2023 at 1:35
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The constancy of the speed of light is a necessary consequence of the universe being four dimensional and defined by the Minkowski Metric (which is just a 4 dimensional version of Pythagoras' Theorem).

There is an excellent intermediate level explanation at https://en.wikibooks.org/wiki/Special_Relativity/Spacetime#The_modern_approach_to_special_relativity

"A space-time interval of zero only occurs when the velocity is c (if x>0). All observers observe the same space-time interval so when observers observe something with a space-time interval of zero, they all observe it to have a velocity of c, no matter how fast they are moving themselves."

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The speed of light OBVIOUSLY varies with the speed of the observer:

https://youtube.com/watch?v=bg7O4rtlwEE

The speed of the light pulses relative to the stationary observer is

c = df

where d is the distance between subsequent pulses and f is the frequency at the stationary observer. The speed of the pulses relative to the moving observer is

c'= df' > c

where f' > f is the frequency at the moving observer.

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