Rest mass of photon is exactly zero.
Theory implies that all particles which
move with speed of light have zero rest mass.
Since photons always move with speed of light,
the rest mass of the photon is a theoretical construct.
Scientists neverthless made measurements to obtain direct evidence.
To see such reasoning and empirical evidence, you may read
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html
Here is evidence based on astronomical observations from 1996
that concludes that upper limit is less then E-47 kg
that is 10 to the power -47 kg
which can be also written as 0.00 ...1 with 47 zeros after the decimal
point
"..If we assume that photons have mass, then we can use the equations
of relativity to construct an equation that relates the mass of a
photon to its speed and its wavelength. This equation predicts that
the speed of a photon in empty space is related to its wavelength,
unless the photon has zero mass. Basically, if the photon has mass,
then short wavelength photons should travel at much lower speeds than
long wavelength photons..."
http://www.madsci.org/posts/archives/dec96/844824361.As.r.html
Here, recent articles are putting that upper limit
to E-54 and later to E-57 kg
http://arxiv.org/abs/physics/0305090
http://www.aip.org/enews/physnews/2003/split/625-2.html
For comparison, rest mass of the proton,
which within 1% is also mass of a hydrogen atoms is 1.6 E-27 kg
http://www.tcaep.co.uk/science/constant/mtom.htm
There is more activity in somewhat related issue of
the neutrino mass, long considered to be zero, and recently
probed by big scale experiments in connection with search
for 'dark mass' in the universe.
http://www.inr.troitsk.ru/~trdat/
But that is a different question.
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CODATA,
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