Hi

All electromagnetic waves travel with same speed, where speed is, v=fλ. For instance, radio waves have longer wavelength and low frequency but gamma waves have shorter wavelength and high frequency, and therefore both waves have same speed. Light is just one type of EM waves. Then, why is special emphasis laid on 'light' while talking of the equation E=mc^2 and Einstein's theories of relativity? In other words, would it be wrong to say that in the equation, E=mc^2, "c" is speed of a EM wave, or saying that speed of a EM wave is a universal constant? I hope you get my question. Thank you.

Regards
PG

 

The speed in a vacuum is different than the speed in materials like copper, water, glass, or plastic. Using light, in common parlance, sounds less pretentious than "Electromagnetic Wave". We all know what the phrase "speed of light" means so I'm not really sure if there is any real value in the alternate choice of words.

 

It wouldn't be any less correct, but it arguably wouldn't be any more correct, either. It's somewhat like saying that 2+2 is 4 and then having someone assert that it could be 3+1, as well, and therefore we should include that, too. That there are other correct ways to express something doesn't mean that any way that fails to capture all of them is wrong. Now, if the claim where that c must be only the speed of "light", then the beef is valid.

In either case, it should be stipulated that the speed in question is the speed in a vacuum.

 

My understanding of c is that it's the universal speed limit and that massless particles (photons) always move at that speed even in matter.
http://profmattstrassler.com/articl...r-particles-with-math/7-particles-are-quanta/

A single photon in matter is either passed or not but the phase velocity of an EM wave is shifted by photon atomic scatters slowing the EM waves energy (information) movement.