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Re: [Phys-l] Absolute four-momentum of massless particles

That's what I've said for years, but am frequently argued with. I suppose one can envision a universe with different laws such that the second postulate would be necessary, but in our universe, if electromagnetic phenomena do obey Maxwell's equations in any observer's frame of reference, then the possibility of the existence of electromagnetic waves is inherent in the equations, and it is guaranteed that all inertial observers would observer such waves as travelling at c = 1/sqrt(epsilon0 mu0).

I have read that some of Einstein's early musings involved the question of what a light wave would look like if one could travel at the speed of light. Like a stationary wave, a wave frozen in position forever? No, he decided, all observers must see the light wave wave -- and travel. There should be no way to distinguish between different reference frames by testing how light acts. The constancy of the speed of light almost immediately follows from this line of reasoning. The only other (already excluded) possibility would be infinite light speed.

Ken Caviness
Southern Adventist University
Collegedale, TN 37315

-----Original Message-----
From: [] On Behalf Of Spagna Jr., George
Sent: Friday, 01 October 2010 10:05 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Absolute four-momentum of massless particles

The second postulate is not needed. The first postulate says that all the "laws of physics" apply in an inertial frame, so that you cannot determine by any experiment. The speed of light (whatever it is!) in vacuo is given by Maxwell's equations, so the constant value of c is a consequence of the first postulate.

 "Who cares about half a second after the big bang;
what about the half a second before?"    
                        - Fay Weldon

Dr. George Spagna
Physics Department
Randolph-Macon College
P.O. Box 5005
Ashland, VA 23005-5505

phone: (804) 752-7344
fax: (804) 752-4724

Forum for Physics Educators