Thed , my friend, you are a self-important windbag, and even after all of your excrescent verbiage I am still at a loss as to what your point is exactly. Going back to what you said above, "E=pc=hf doesn't mean that photons have mass", I would remind you that "the 'm' in E = mc^2 is the dreaded 'relativistic mass'. When dealing with massless particles, it's best to avoid such notions" and also "E = hf = mc^2 is essentially meaningless." That you fail to grasp the difference between this and the statement "The source of the gravitational field in GR is not 'E' alone but (loosely speaking) 'E^2 - p^2'" only further demonstrates your ignorance in matters of relativity. You've a lot more reading to do... I suggest you get to it. Surely reading a textbook puts your time to much better use than posting uninformed blather.

If we now turn to the question 'Does light have mass?' this can be taken to mean different things if the light is moving freely or trapped in a container. The definition of the invariant mass of an object is m = sqrt{E2/c4 - p2/c2}. By this definition a beam of light, is massless like the photons it is composed of. However, if light is trapped in a box with perfect mirrors so the photons are continually reflected back and forth in the box, then the total momentum is zero in the boxes frame of reference but the energy is not. Therefore the light adds a small contribution to the mass of the box. This could be measured - in principle at least - either by an increase in inertia when the box is slowly accelerated or by an increase in its gravitational pull."

Also per your website, which is full of errors, let me point out one thing. Isotropy does not mean constant pressure for God's Sake.

• Re: Thed per your statements below - Human I. Scientist - December 18, 1998 - 00:34 UTC
• Re: Thed per your statements below - Thed - December 18, 1998 - 04:02 UTC