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http://www.physlink.com/Education/askexperts/ae180.cfm
My science data book says that the mass of a photon is zero. But how
can photon exert radiation pressure, as it occurs in the sun?
Asked by: Suraj
Answer
Actually what your science book says or is trying to say anyway,is
that the photon's rest mass is zero. The concept of the rest mass
derives from special relativity. The rest mass is the mass of a
particle (in our case the photon) as measured by an observer who sees
the particle still and with zero speed. In other words, the particle
is at rest as far as this observer is concerned. Thus comes the term
REST mass. But according to special relativity, light ALWAYS travels
with the light speed c, and is NEVER at rest. And so it has zero REST
mass. (...) ...though photons don't have rest mass, they do have
energy and thus they have mass. The photons are wave particles. This
means that they act as waves and as particles as well. This is the
duality of the nature of light (and of every particles). And so as
particles they have mass, and as waves they have frequency. The
pressure they exert is due to the particle nature of light. It's easy
now to understand the mechanism that causes this pressure.
Answered by: George Moustris, M.S., Electrical and Computer Engineer,
AUTH, Greece

http://www.usatoday.com/weather/resources/basics/wonderquest/photonmass.htm
Question: Do photons have mass? If not, why does the gravitational
field of a star bend passing light?
Answer: "A particle like a photon is never at rest and always moves at
the speed of light; thus it is massless," says Dr. Michael S. Turner,
chair of the Department of Astrophysics at the University of Chicago.
(...) You ask how a star's gravitational field can bend the path of a
massless photon. This takes us into the realm of Einstein's general
theory of relativity. The mass of the photon isn't attracted to the
star's mass under Einstein's theory. Rather, the star's mass distorts
space and the photon's path changes because the space is curved, says
Paul Hewitt in his book Conceptual Physics.

When the variability/constancy of the speed of light is concerned,
"mass of photons" is a red herring and the above twists and turns show
that Einsteinians do know that. What is relevant and what Einsteinians
are trying to camouflage can be presented as follows:

A light source on top of a tower of height h emits light with
frequency f, speed c (relative to the source) and wavelength L. A
receiver on the ground receives light with frequency f', speed
c' (relative to the receiver) and wavelength L'. Einstein's 1911
solution (which is in accordance with Newton's emission theory of
light) gives:

f'=f(1+gh/c^2); c'=c(1+gh/c^2); L'=L

A rocket of length h accelerates with acceleration g. A light source
at the front end emits light with frequency f, speed c (relative to
the source) and wavelength L. A receiver at the back end receives
light with frequency f', speed c' (relative to the receiver) and
wavelength L'. At the moment of reception, the receiver has speed v
relative to the light source at the moment of emission. Einstein's
equivalence principle gives:

f'=f(1+v/c); c'=c+v; L'=L

All those results given by Newton's emission theory of light were
experimentally confirmed in 1960 when Pound and Rebka measured the
gravitational redshift factor to be 1+gh/c^2. Nobody noticed this
important confirmation of the emission theory (rather, Einsteinians
announced a glorious confirmation of Divine Albert's Divine Theory)
because Einstein, after 1911, had managed to confuse everything:

http://www.speed-light.info/speed_of_light_variable.htm
"Einstein wrote this paper in 1911 in German (download from:
http://www.physik.uni-augsburg.de/annalen/history/einstein-papers/1911_35_898-908.pdf
). It predated the full formal development of general relativity by
about four years. You can find an English translation of this paper in
the Dover book 'The Principle of Relativity' beginning on page 99; you
will find in section 3 of that paper Einstein's derivation of the
variable speed of light in a gravitational potential, eqn (3). The
result is: c'=c0(1+phi/c^2) where phi is the gravitational potential
relative to the point where the speed of light co is measured......You
can find a more sophisticated derivation later by Einstein (1955) from
the full theory of general relativity in the weak field
approximation....For the 1955 results but not in coordinates see page
93, eqn (6.28): c(r)=[1+2phi(r)/c^2]c. Namely the 1955 approximation
shows a variation in km/sec twice as much as first predicted in 1911."

Pentcho Valev
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