I Constancy of Speed of Light: Postulate or Assumption?

[Peter Strohmayer]

The axiom (postulate, assumption etc.) of the constancy of the "speed of light" is misleading in my opinion, as far as it is based on the Newtonian concept of "velocity" (resp. his concept of space and time).

If one derives new concepts of space and time (and thus also of velocity) from the mentioned axiom of the constancy of the „speed of light", which was actually the case so far, one changes the most important premise of his former axiom. You are sawing off the branch you are sitting on.

If, on the other hand, one bases the "postulate" on a "relativistic" concept of velocity, then the "postulate" becomes a matter of course.

Not the given (Newtonian) terms of space, time and velocity describe the propagation of the light, but the propagation of the light describes the terms of space, time and velocity. Not rigid scales and ticking clocks are the basis for the concepts of time, space and velocity, but the length of the propagation of a light pulse from the point of view of the observer who has emitted this light pulse (in other words: the light clock). Time is what passes when a light pulse propagates from event E1 of its emission to event E2 of its arrival. Space is what is bridged when a light pulse propagates from event E1 of its emission to event E2 of its arrival.

From the point of view of the particular observer, the length of propagation of a light pulse from its start from a light source at rest with him (event E1) to its arrival at a target (event E2) is both the length of time and the space that lies between these two events from his point of view (events E1 and E2 have a "light-like distance" from each other). The speed of the light pulse as a ratio of the space covered to the time required for it must by definition always be "1". This is not a postulate, but follows from the concepts of space and time.

 

[phinds]

Maybe the short answer regarding the reason for the constancy of the speed of light is that the instruments used to measure it are made of atoms/molecules, held together by electromagnetic forces, and that the EM force carrier is also travelling with the speed of light?

Is that intended to be a joke? I hope so.

 

[DanMP]

Is that intended to be a joke? I hope so.

No, it is an honest question. See here this:

A photon (from Ancient Greek φῶς, φωτός (phôs, phōtós) 'light') is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless,[a] so they always move at the speed of light in vacuum, 299792458 m/s

 

[strangerep]

If one derives new concepts of space and time (and thus also of velocity) from the mentioned axiom of the constancy of the „speed of light", which was actually the case so far, one changes the most important premise of his former axiom.

Blind people can set up coordinate systems in space and time, at least in their local vicinity. They don't need light propagation to accomplish that. Similarly, they can sense whether they're accelerating, regardless of whether they can sense their surroundings.

My point is that physical equivalence of inertial frames is a more fundamental principle than the tool of light propagation. [One doesn't have to be "sighted" to be an "observer". ]

 

[robphy]

In modern formulations of relativity, the emphasis is on causality and causal structures based on a finite upper limit of signal speeds, which was historically motivated by “light” and “electromagnetic phenomena”.

If the photon were found to have a nonzero invariant mass, then relativity would survive… with many references to light being replaced by maximum-speed signals.

 

[Sagittarius A-Star]

Not rigid scales and ticking clocks are the basis for the concepts of time, space and velocity, but the length of the propagation of a light pulse from the point of view of the observer who has emitted this light pulse (in other words: the light clock).

But the light clock contains a ruler (=rigid scale).

 

[Peter Strohmayer]

"But the light clock contains a ruler, what, I think, you mean with ‚rigid scale‘.“

The unit with which to measure is determined by a material basis in the form of a period of time found in nature that can be reproduced as accurately as possible (e.g. the decay of a caesium atom, the rotation of a pulsar, etc.).

From this arbitrarily determined unit of the time "1", during which a light pulse spreads out temporally between two events (start and arrival), follows as spatial length of this spreading out between the two events necessarily the unit of the space "1".

This could be used to construct a light clock with the length "1". But this "rigid scale" is not the actual material basis of the system of units.

 

[PeterDonis]

The unit with which to measure is determined by a material basis in the form of a period of time found in nature that can be reproduced as accurately as possible (e.g. the decay of a caesium atom, the rotation of a pulsar, etc.).

No, that's not what a light clock is. A cesium clock is not a light clock.

A light clock is a clock whose "ticks" are the bouncing of a light pulse between two mirrors that are held rigidly a fixed distance apart. In the context of SR, this works fine because spacetime is flat and the mirrors can simply be placed in free fall at rest relative to each other, and they will then stay at rest relative to each other, the same distance apart, forever.

However, as soon as you either allow spacetime to be curved (GR), or accelerate the clock, it is no longer a simple matter to keep the mirrors the same distance apart. But that is what is required for the light clock to work properly.

This could be used to construct a light clock with the length "1". But this "rigid scale" is not the actual material basis of the system of units.

Yes, it is. See above.

 

[Peter Strohmayer]

Two mirrors in free fall are a nice and useful idea, but not a "rigid scale" in the conventional sense ("primal meter") in which I used it above.

 

[PeterDonis]

Two mirrors in free fall are a nice and useful idea

Yes, one which goes by the name "light clock".

but not a "rigid scale" in the conventional sense ("primal meter") in which I used it above.

Then you're going to need to explain what, exactly, you think "rigid scale" means. Note that you weren't the first to use that term in this thread, as far as I can tell: @Sagittarius A-Star was. He is welcome to correct me if I'm wrong, but I took him to mean by "rigid scale" the fact that, as I described, the two mirrors of a light lock must remain the same distance apart at all times for the light clock to work properly.

 

[Peter Strohmayer]

In #51 I replaced the "rigid scale" (conventionally of solid matter) with a light clock. If you also call the spatial distance in an ideal light clock with two free-falling mirrors a "rigid scale", then we agree not in the terms used, but in the matter.

The underlying question is still the same: What could be the material basis of the unit system found in nature: a spatial distance between two simultaneous events (= the existence of two resting points of matter) or a temporal distance between two events at the same place?

I think this question is self-explanatory.

In this sense I agree with Sagittarius A-Star in #56: There is a "rigid scale" in the light clock, but originally only in the form of the time distance between two events in the same place.

The term "rigid scale" for a reproducible constant time interval at the same place does not seem appropriate to me. Perhaps "constant scale" would be better.

 

[Sagittarius A-Star]

In #51 I replaced the "rigid scale" (conventionally of solid matter) with a light clock.

The purpose of a light clock is not to measure a spatial distance, but to measure it's proper time.

 

[Peter Strohmayer]

Since a light clock connects the temporal distance "1" between two light-like distant events and the spatial distance "1" between these events, it can be used not only to measure the proper time, but also to measure a spatial distance between two mass points resting on top of each other (laser measuring device), and also to measure a possible relative speed between the mass points (radar).

 

[Sagittarius A-Star]

Since a light clock connects ...

Usually, time is measured with clock(s) and distance with a ruler.
You can measure distance also with the combination of a clock and light propagation (radar).
You can measure time also with the combination of a ruler and light propagation (light clock).

 

[Peter Strohmayer]

Ok, thank you.

 

[bhobba]

This is an interesting thread.

As a proponent of the so called one postulate version of SR, I must point out for it to work you need to be careful in defining what an inertial frame is. Most just say it's a frame where Newtons first law holds without mentioning its symmetry properties. I dont know why, but Landau is the only author I know that does that - and it is not even in his relativity book, its in his Mechanics textbook.

Thanks
Bill

 

[vanhees71]

Of course, you can't even formulate Newton's Lex I, if you don't have a minimal spacetime model to begin with. The usual one-postulate derivations of the Poincare group assume that there's a class of inertial frames, where Lex I holds ("special principle of relativity"), where there is a time which is assumed to be described by the oriented $\mathbb{R}$ with the usual topology (the Archimedian ordered Cauchy-complete field of real numberes) and that for any inertial observer at any moment of time space is described as a 3D Euclidean affine manifold. Taking then all the symmetries of this "preliminary spacetime model" (i.e., translation invariance of time + ISO(3) of euclidean space) you are lead to either the Galilei group and thus Newtonian spacetime with the notion of an absolute time, the Poincare group and Minkowski spacetime or ISO(4), but the latter possibility is ruled out by the additional assumption of the possibility to establish a "causality structure". See, e.g.,

V. Berzi and V. Gorini, Reciprocity Principle and the Lorentz
Transformations, Jour. Math. Phys. 10, 1518 (1969),
https://doi.org/10.1063/1.1665000

 

[DanMP]

Well, you can "explain" the reason for the constancy of the speed of light by saying that it is a consequence of the fine structure constant.

When you wrote this, did you ponder a moment on why the constancy of the speed of light through vacuum can be explained using the fine structure constant? Do you have any explanation, other than the fact that the instruments we use to measure it (clocks, rulers) are structures made of atoms/molecules, held together by the electromagnetic force, and that the force carrier for the electromagnetic force is a photon, moving with the same speed as the light we measure?

If you measure the speed of light on a mountain top using a clock on sea level, you won't get the same value as you get using a local clock.

Another thing, how this speed of light can be "enforced"? Are the photons carrying speedometers? And/or the photons are acting like that because they were programmed to do so? Don't you think that this constancy of the speed of light is simply a consequence of the fact that we measure it using instruments affected by it?

 

[phinds]

If you measure the speed of light on a mountain top using a clock on sea level, you won't get the same value as you get using a local clock

If you make ANY local measurement regarding time but using a non-local clock, you would likely get the wrong results. This is what is called a "vacuous truth". It's true but it doesn't tell you anything meaningful or useful.

Don't you think that this constancy of the speed of light is simply a consequence of the fact that we measure it using instruments affected by it?

Absolutely not.
(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?
(2) You are not required to use a light clock to measure the speed of light, you could use a mechanical clock.
(3) How would the fact that we use a clock to measure the speed of light explain the fact that if an object is approaching us slowly and shines a light at us, we see the speed of that light as c, and if it is approaching very fast and shines a light at us, we also see the speed of that light as c?

 

[PeterDonis]

how this speed of light can be "enforced"?

By the geometry of spacetime. The geometry of spacetime is such that any massless object will move at what we have been calling "the speed of light". But that term is really a misnomer as it's a property of spacetime geometry and not of light; it should be called "the invariant speed" or something like that.

Another way of looking at it is that what we have been calling "the speed of light" is really just a unit conversion factor: it converts from time units to distance units (or, if you use its reciprocal, it converts from distance units to time units). It's no different from, for example, sailors measuring horizontal distance in nautical miles but depth of water in fathoms, and having to convert between them sometimes. The conversion is just a matter of geometry.

Don't you think that this constancy of the speed of light is simply a consequence of the fact that we measure it using instruments affected by it?

The fact that all of our current instruments are made of atoms whose structure and behavior is primarily determined by the electromagnetic interaction, and hence depends on the fine structure constant, is an artifact of our current technology, not a law of physics. 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.

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.

 

[phinds]

@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.

 

[vanhees71]

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.

 

[DanMP]

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.

(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?

(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.

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.

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".

 

[DanMP]

@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.

 

[jbriggs444]

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".

 

[phinds]

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.

 

[PeterDonis]

You don't know that. Maybe it does, in conjunction with other information.

No, we do know. You are just waving your hands.

How exactly is a ruler affected by a distance?

Um, it measures it?

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.

so the clock is, in a way, similar with a light clock.

No, it isn't. See above.

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.

Neutrinos are not massless. Please name another "field that is also massless".

Gluons, as has already been pointed out.

 

[PeterDonis]

Thread closed for moderation.

 

[berkeman]

Thread will remain closed.