Why the speed of light is constant?

[Sturk200]

Sorry, I was not very pleased with my previous post and tried to delete it to make my objection to your reasoning clear, but you were too fast and had already responded

The pratfalls of instantaneous communication!

The objection is that you are applying a double-standard by NOT asking for the mechanism behind Euclidean geometry. Do you not see the clear inconsistency in your position? In your own words you accept Euclidean geometry with no mechanistic explanation simply "because I already believe", but you require such an explanation of Lorentzian geometry.

Let me see if I can try to clear this up. I do not ask for a mechanism for any geometrical procedure -- geometry is geometry and does not require forces. What I "already believe" is not that Euclidean geometry requires no mechanism, but rather that objects in the real world are constituted by their lengths and durations, both of which features happen to be preserved by a Euclidean rotation, neither of which are preserved by a Minkowski rotation.

My gripe, then, is not mathematical or geometrical. Rather, it is ontological. The battle between Euclid and Minkowski is really over the question of what constitutes an object -- is it length and duration or is it the space-time interval? By explaining, for instance, length contraction using a Minkowski rotation, and insisting that this geometrical explanation is the full physical explanation, we are awarding ontological priority to the space-time interval. This, again, is because the rotation does not invoke a mechanism (therefore does not change the underlying object), and yet alters the length and duration of an event.

What grounds have we for awarding ontological priority to the space-time interval? Simply that the speed of light is constant (on which the entire theory is based). Might we find a non-geometrical explanation of length contraction by further inquiring as to why the speed of light is constant? My contention, considering how little was known about the composition of matter in Einstein's day as compared with our own, is that we might. And apparently I am not alone in this hope (http://www.euregiogymnasium.ch/alumni/images/pdf/aeneas_wiener-lorentz_contraction.pdf). The idea is that the physical reason for the constancy of the speed of light may also explain length contraction without invoking space-time geometry.

I believe that there is a mechanism (per your definition) which explains the geometry of spacetime in relativity.

Is this not going to get you in trouble when I ask you, in the interest of consistency, to provide a mechanism for Euclidean geometry?

Flat spacetime is the solution of the EFE in the special case where there is no significant mass present.

My understanding is that special relativity was "written in" to the EFE, but I probably don't know enough about GR yet to hold my own.

 

[Dale]

My gripe, then, is not mathematical or geometrical. Rather, it is ontological.

Then there is nothing to discuss here. This forum is for science, not philosophy. You are welcome to any ontology that you like, provided that it is consistent with experiment.

Thread closed.

 

[PeterDonis]

Does the principle of relativity require that the speed of light be measured to be the same for all inertial observers, or does it require that the speed actually be the same in all inertial frames.

The principle of relativity does not draw the implicit distinction you are making. The measured speed is the "actual" speed.

My understanding is that special relativity was "written in" to the EFE

I don't understand what you mean by this. The statement that flat Minkowski spacetime is a solution of the Einstein Field Equation for a zero stress-energy tensor is easily proved; textbooks on GR often assign this as a homework problem. There's nothing "written in"; it's a simple consequence of the equation.

clearly no moving frame will carry free space along with it.

This amounts to assuming what you are trying to prove, that Maxwell's Equations don't require $c$ to be the same in all inertial frames. You can't just help yourself to this assumption; you have to justify it. Can you? To me it's obviously false, since "free space" does not have a state of motion at all, so it doesn't have to be "carried along".

Now note that a frame traveling in the same direction in which light is propagated will augment the necessary distance traveled by the light beam between coordinates of that frame, as compared to a frame traveling in a direction opposite to the propagation (i.e. into the beam). Now if the actual speed must be the same, then the measured speeds are different.

I don't understand this reasoning. How are you defining the "actual" speed?

Also, are you taking into account length contraction? Remember that the Michelson-Morley experiment was done in the 1880's, well before Einstein published his SR papers, and Einstein was not the first one to propose length contraction as an explanation of that result; Lorentz and Fitzgerald both did so in, IIRC, the early 1890's. Also, Mach made a similar claim to the one I made above: that there is no such thing as the "actual length" of an object apart from its measured length.