🚀 go-pugleaf

Tom Roberts wrote in sci.physics.relativity:
> develope...@hotmail.com wrote:
> > If there is no such thing as absolute motion, it follow there is no
> > such thing as absolute rest.  How, then, can we speak of an object's
> > rest mass?  Rest wrt what?
>
> The underlying concept is of invariant mass. This is an intrinsic
> property of an object, and in relativity is the closest analog of what
> "mass" meant in Newtonian mechanics.
>
> The phrase "rest mass" hearkens back to the old days in which there was
> confusion between mass (aka invariant mass) and "relativistic mass".
> "Relativistic mass" is not really mass, and is frame dependent so it is
> not intrinsic to an object, either; it is numerically equal to the
> energy of the object, and "energy" is a much better name for this
> quantity than "relativistic mass". The "relativistic mass" of an object
> in its rest frame is equal to its invariant mass, or in the usual
> shortcut way of speaking, its rest mass.
>
> So this is "rest relative to the object itself". In modern physics, the
> first word of "invariant mass", and the first word of "rest mass", are
> redundant, because the concept "mass" includes those aspects as well.
> The first word of "relativistic mass" is an oxymoron.
>
> Tom Roberts

Unfortunately Roberts Roberts the relativistic verbiage contains too
many oxymorons. As far as photons are concerned, the only important
question is:

Does the speed of photons vary with the gravitational potential, as
the speed of other particles does, and, equivalently, does the speed
of photons vary with the relative speed of the light source and the
observer, as the speed of other particles does?

Roberts Roberts this question is directly relevant to the axioms of
relativity and for that reason I called it "the only important
question". The question "Do photons have mass?" is important but not
directly relevant.

Roberts Roberts you have already given half of the answer:

Tom Roberts wrote in sci.physics.relativity:
> Pentcho Valev wrote:
> > CAN THE SPEED OF LIGHT EXCEED 300000 km/s IN A GRAVITATIONAL FIELD?
> Sure, depending on the physical conditions of the measurement. It can
> also be less than "300000 km/s" (by which I assume you really mean the
> standard value for c). And this can happen even for an accelerated
> observer in a region without any significant gravitation (e.g. in
> Minkowski spacetime).
> Tom Roberts tjroberts@lucent.com

Pentcho Valev