# Speed of Light is a Property of Massless Particles or Space?

1. May 2, 2015

### Islam Hassan

The speed of light is a parameter that attaches itself to what exactly, an inertial frame of reference or a massless particle moving therein?

IH

2. May 2, 2015

### wabbit

Interesting question. I would say to both. We cannot speak of speed without a reference - at the very least something to which that speed is relative. And of course we can only speak of the speed of something

And the invariance of the speed of light requires of course at least two frames and a particle, since it is the property that any two observers will measure the same speed relative to them for a massless particle.

But this doesn't even really need space, which can be thought as an abstraction of such measurements.

3. May 2, 2015

### Staff: Mentor

It doesn't "attach itself" to anything, except spacetime itself; it's a fundamental property of spacetime.

4. May 3, 2015

### m4r35n357

Very interesting question, I'd like to have a go at answering this. Space-time itself specifies a top speed (possibly infinite), whereas Maxwell's equations tell us what the speed of EM waves is (so this determines the speed of light, c).
It just so happens that experimentally the top speed is the same as the value of c.

5. May 3, 2015

### wabbit

Hmm is that right ? If the photon had mass wouldn't this modify Maxwell's equations ? And if it is massless it must travel at the invariant speed. I may be wrong here but I think Maxwell's equations as such are only compatible with the speed of light being both finite and equal to the invariant speed, no ?

Trying to recall SR derivation here, I think it goes Maxwell -> invariant speed of light -> SR with top speed =speed of light.

Last edited: May 3, 2015
6. May 3, 2015

### bcrowell

Staff Emeritus
The question is much too vague to say anything about.

7. May 3, 2015

### m4r35n357

I'm not sure how to answer a question like "If the photon had mass . . .".
I'm just saying that as I understand things spacetime itself cannot define either the top speed or the value of the speed of light. Whether those two speeds can be identified is subject to experimental observation. Maxwell's equations on the other hand actually define the (invariant) speed of light but say nothing about whether it is the top speed.

8. May 3, 2015

### wabbit

What I am saying is that there can be only one invariant speed and this is equal to the "top speed" since this top speed is invariant - once light speed is known to be invariant (which must be true if Maxwell equations hold), then it is also the top speed.

Now I agree this may be an overstatement: if we found experimentally two invariant speeds, then there would be the need for another non-SR theory of spacetime. I don't know if there are such theories where Maxwell's equations still hold, perhaps it is the case - that experiment would kill SR, but maybe not Maxwell, I don't know.

Last edited: May 3, 2015
9. May 3, 2015

### m4r35n357

I don't think we are disagreeing here. The light postulate says (I think) that the top speed = the invariant speed = the value of the speed of light. Finding two invariant speeds would mean that one of them isn't the top speed, and in any case Minkowski space only caters for one invariant speed, so I agree that would spell trouble for SR! BTW I think I've reached the limits of my understanding in this area, so I'll wait to find out if I've misunderstood something fundamental . . .

10. May 3, 2015

### Staff: Mentor

I think that we need some clarification from the OP before we can continue. @Islam Hassan if you could reach out to me by PM and clarify what you mean by the speed of light "attaching" itself to things, then I can reopen the thread.

11. May 4, 2015

### Islam Hassan

Perhaps a clearer formulation of my question is as follows: is the speed of light a constant ascribed to the i) the movement of a particle proper due to inherent characteristics of this particle or some property of spacetime that puts a limit on the speed of this movement? Is it theoretically possible to have a different "species" of spacetime (alternative universes and the like...) where c has a different value? To my mind, if the answer is yes, then c is dependent on spacetime and is a property of spacetime and not the particle itself.

Did I make myself any clearer?

12. May 4, 2015

### Staff: Mentor

The value of c is entirely determined by your system of units. What matters is whether or not the invariant speed (aka c) is finite. If it is infinite then you get Galilean relativity, and if it is finite then you get Einstein's relativity.

13. May 4, 2015

### Staff: Mentor

As I said in post #3, it's a property of spacetime.

14. May 4, 2015

### nitsuj

Playing devil's advocate, we don't measure spacetime. How is it certain it's a property of spacetime itself.

Also for the OP's question...I'm seeing this as merely two interpretations, and not being of physical significance.

15. May 4, 2015

### Staff: Mentor

We measure the motion of freely falling objects, which marks out the geodesics of spacetime and tells us its geometry. That amounts to measuring spacetime, at least in the required sense for this discussion.

Because all massless particles travel at the same speed. If it were a property of particles, we would expect to see different values of $c$ for different particles and different interactions. But we don't.

16. May 4, 2015

### nitsuj

The choice to use the phenomenon of a geodesic as describing the geometry of spacetime is just that...a choice. It is also that gravity is a field, and fair enough to infer that any changes in measure are due to a "field interaction" of sorts.

Of course I am splitting hairs, but yes my point is that it's (as the OP proposed) a matter of interpretation, and that's it. I think what you described is a field interaction, and yes a "gravity" unique one. (gravitons)

Why is that necessary? To be clear the difference here is "Spacetime limits propagation of fundamental forces." vs "Fundamental forces have a property; propagating at an invariant speed." Why would you not consider the "property" of the "particle" is traveling at an invariant speed? They've tried to directly measure spacetime (in this context), but failed.

This is good fun, I never thought of a speed as being a fundamental property of something but in the case of an invariant speed it is. Also enjoying thinking in the context of causation that Fundamental forces are the only "things" (force carriers) that can travel at an invariant speed. So across distances in spacetime causation isn't merely cause/effect, it's cause - force carrier at invariant speed - effect. Due to property of spacetime or implicit / fundamental property of physics in a continuum?

Last edited: May 4, 2015
17. May 4, 2015

### Staff: Mentor

Yes, but the field option doesn't really amount to an alternative as far as the topic of discussion here is concerned. See below.

A field on what? On flat spacetime. So you still have spacetime structure underneath, and that spacetime structure still has a single invariant speed built into it.

(Also, even on the "gravity is a field" interpretation, the background flat spacetime on which the field is defined is not observable; only the curved spacetime produced by the field is. So even on the field interpretation, there's no way to separate out "measurements of the field" from "measurements of spacetime, separate from the field".)

But on the second interpretation, there is no reason to expect all fundamental interactions to propagate at the same invariant speed. There could be more than one. Only if the invariant speed is a property of spacetime, not the forces, would you expect it to be the same for all interactions.

(Of course, trying to model different invariant speeds for different interactions would require a very different model from the ones we're used to. But so what? Such a model is still logically possible.)

Can you give a reference? I don't understand what you're referring to here.

18. May 4, 2015

### nitsuj

Fundamental interactions are the "in between" of the changes we are performing physics* on. Force carriers move at an invariant speed, and unless I can formulate a well rounded argument for an invariant speed due to causation it'll have to be postulated that c is invariant...as it is now. c isn't the important part, the invariance is.

For my "daily experience" why would I see a difference if the fundamental forces had different speeds? so long as they were invariant. That said my gut feel is causation falls apart if we could measure different invariant values of things that in effect are the passage of happenings that are being measured and in turn comparing those measurements...according to what coordinates?? Suppose coordinates wouldn't have any comparative significance if motion was involved.

Was referring to theories & experiments regarding the invariance of light, looking for properties specific to spacetime, like aether.

*measurements / observations

Last edited: May 4, 2015
19. May 4, 2015

### Staff: Mentor

You probably wouldn't in your daily experience, but it would be straightforward (though possibly technically quite challenging) to do experiments to show the different speeds if they existed.

These experiments were looking for a preferred frame, which could be interpreted as a "property of spacetime", but is certainly not the only possible one.

20. May 4, 2015

### nitsuj

I'm not saying we couldn't measure different values of whatever invariant speed, fundamental interactions cannot happen more slowly then matter, most matter we perform physics on is held together by these fundamental interactions, what of a "force carrier" that can go faster then another type of "force carrier" in order to accelerate a bit of matter faster than the interaction that caused that effect of more velocity? The four forces must have all the same invariant speed, as we can accelerate matter with the strong force too, I'm thinking this is all a matter of interaction :p

Thought preferred frames where a some while after Maxwell. To get a fine tip on my point.. theories/experiments looking for something between "here and there" interacting with the light traveling from here to there causing the invariance of light.

21. May 4, 2015

### Staff: Mentor

Obviously interactions, and the matter that is interacting, would behave differently in a universe which had different speeds for different interactions. My point is not that we have any evidence for such a thing; of course we don't. The interactions in our actual universe all have the same invariant speed, and behave accordingly, as does the matter that is interacting. I'm only pointing out that different behaviors are logically possible. But we are getting pretty far off topic at this point.

22. May 4, 2015

### nitsuj

We did, I enjoyed it. Fun musing, though I'm going to have a hard time saying "it's the geometry of spacetime". Maybe I'll trade that in for "it's the geometry of "xyz" field interactions. meh, tomayto-tomahto*

*edited ......potato-potato didn't read as well

23. May 5, 2015

### strangerep

I think you might like the (less well-known) derivations of SR that don't assume the light principle, (which I'm guessing you're not already familiar with?). Cf. Rindler's SR textbook (and various others -- I could dig out more specific references if needed).

It turns out that if we ask for the most general group of transformations compatible with the relativity principle (i.e., that all inertial observers perceive the same laws of physics), then a universal constant with dimensions of speed emerges as one possibility (among very few).

The properties of unitary irreducible representations (field representations) of the group then enables an identification between massless spin-1 reps and light. That enables the value of "c" to be identified with light speed empirically.

A more general analysis yields fractional linear transformations instead of the usual linear Lorentz transformations, and a universal length constant suspiciously connected to the cosmological constant also emerges.

So I'd answer the OP's question this way: the principle that all inertial observers perceive the same laws of physics (the relativity principle, "RP") admits a universal speed constant. The homogeneous space for the resulting group (Poincare group) is what we call Minkowski spacetime. IOW, Minkowski spacetime is just a convenient (but artificial) stage for representing the (1-particle) action of the Poincare group. Unitary irreducible representations of the Poincare group include the massless spin-1 case, which is seen to correspond to light when the details are analyzed, thus determing the value of "c" empirically.

I.e., RP -> group of transformations with a "c" constant -> Minkowski space as homogeneous space of the group
-> massive and massless field representations ("elementary particles").

In this sense, the "geometry of spacetime" is subordinate to the RP.

(I've skipped over some details, of course, but could add them if needed.)

Last edited: May 5, 2015
24. May 5, 2015

### wabbit

would you have a reference/link ? I have a vague recollection of seeing that but can't remember where, nor the details.

25. May 5, 2015

### nitsuj

So what makes it "turn out" that given one SR postulate we can deduce a "universal constant with dimensions of speed" and is that the same as saying an invariant speed? It doesn't seem like it, just seems to say there would be a max speed and that's it. My musing of it lead me to think it's implicit, that "force carriers" would move fastest and be an invariant speed, in turn coordinate transforms can be done and observation is continuity in causation.