# Constancy of Speed of Light: Postulate or Assumption?

• I
• Aromalsp
In summary, the theory of relativity is based on two key principles: the principle of relativity and the constancy of the speed of light. The constancy of the speed of light is one of the fundamental postulates of the theory of relativity.
@DanMP you should take a step back and think about this. If you have found something that you think overturns what has been found and confirmed by many tens of thousands of professional physicists over many decades, do you REALLY want to decide that you are right and they are all wrong?

You would be better served by approaching the issue from the point of view that you know you are wrong but you are trying to understand WHY you are wrong.

russ_watters, berkeman and vanhees71
PeterDonis said:
The relevance of the fine structure constant to measuring the speed of light, taken literally--i.e., we are actually measuring the speed of light beams, not trying to theoretically understand why there is a particular speed that is invariant based on the geometry of spacetime--is that light is electromagnetic radiation and the fine structure constant is the relevant physical constant for electromagnetic behavior. But there could be some other field that is also massless (for example neutrinos were believed to be massless until a few decades ago), and its speed would also be the invariant speed even though it would not be electromagnetic. Understanding why such a field would have the same invariant speed as light would require understanding the geometry of spacetime.
The finestructure constant is the coupling constant of electromagnetic fields with matter. As you say yourself correctly, the speed of light is a fundamental constant related to the relativistic spacetime description and has thus conceptually nothing to do with the finestructure constant. Empirically the electromagnetic field is massless and thus its phase velocity is the speed of light. Indeed that holds for any massless field if there were another one giving rise to asymptotic free states in the vacuum, which to the best of our knowledge seems not to exist though: Indeed, at least two of the three (known) neutrino flavors are not massless, and the only other massless field in the Standard Model, the gluon field, cannot be observed as a asymptotic free state due to color confinement.

phinds said:
It's true but it doesn't tell you anything meaningful or useful.
You don't know that. Maybe it does, in conjunction with other information.

phinds said:
(1) We measure distance with a ruler --- are there any distances that are forced to be constant by virtue of the fact that we measure them with rulers?
How exactly is a ruler affected by a distance?

phinds said:
(2) You are not required to use a light clock to measure the speed of light, you could use a mechanical clock.
In a mechanical clock there are force carrier photons, so the clock is, in a way, similar with a light clock.

PeterDonis said:
We can imagine measuring devices made out of something else, whose behavior would not depend on the fine structure constant, but such devices would still have to measure the same invariant speed.
Please name/describe such a clock and its (performed?) measurements.

PeterDonis said:
But there could be some other field that is also massless (for example neutrinos were believed to be massless until a few decades ago), and its speed would also be the invariant speed even though it would not be electromagnetic.
Neutrinos are not massless. Please name another "field that is also massless".

phinds said:
@DanMP you should take a step back and think about this. If you have found something that you think overturns what has been found and confirmed by many tens of thousands of professional physicists over many decades, do you REALLY want to decide that you are right and they are all wrong?
Regarding this matter, the constancy of the speed of light in vacuum, the "tens of thousands of professional physicists" were happy to just accept it as a postulate. I, on the other hand, want to understand the reasons, so I asked some questions. I never said that they are wrong.

russ_watters
DanMP said:
How exactly is a ruler affected by a distance?
They teach you how to use a ruler in elementary school.

Distance affects a ruler by making it either too long or too short to reach exactly between two stationary points.

Possibly you should be asking a better question. Like "how does a rigid object such as a length of steel maintain its rigidity".

bhobba and russ_watters
DanMP said:
In a mechanical clock there are force carrier photons, so the clock is, in a way, similar with a light clock.
Not in the way you seem to think.

Your whole approach to the issue in this thread is becoming ridiculous.

bhobba and vanhees71
DanMP said:
You don't know that. Maybe it does, in conjunction with other information.
No, we do know. You are just waving your hands.

DanMP said:
How exactly is a ruler affected by a distance?
Um, it measures it?

DanMP said:
In a mechanical clock there are force carrier photons
No, there aren't. At best you could say there are virtual photons, but that concept has many limitations and is not at all useful in this context.

DanMP said:
so the clock is, in a way, similar with a light clock.
No, it isn't. See above.

DanMP said:
Please name/describe such a clock and its (performed?) measurements.
Oh, you want an example? Ok, here's one: a pendulum clock depends on gravity to work, not electromagnetism.

DanMP said:
Neutrinos are not massless. Please name another "field that is also massless".
Gluons, as has already been pointed out.

phinds

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