Original by Philip Gibbs 1997.

The short answer is "no", but it is a qualified "no" because there are odd ways of interpreting the question which could justify the answer "yes".

Light is composed of photons, so we could ask if the photon has mass. The answer is then definitely "no": the photon is a massless particle. According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons, it was known that light carries momentum and will exert pressure on a surface. This is not evidence that it has mass since momentum can exist without mass. (For details see the Physics FAQ article What is the mass of a photon?).

Sometimes people like to say that the photon does have mass because a photon has energy
*E = hf* where *h* is Planck's constant and *f* is the frequency of
the photon. Energy, they say, is equivalent to mass according to Einstein's famous
formula *E = mc ^{2}*. They also say that a photon has momentum, and
momentum

If we now return 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{E ^{2}/c^{4}
- p^{2}/c^{2}}*. 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 both directions
symmetrically in the box, then the total momentum is zero in the box's 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 the greater force
required to accelerate the box, or by an increase in its gravitational pull. You
might say that the light in the box has mass, but it would be more correct to say that the
light contributes to the total mass of the box of light. You should not use this to
justify the statement that light has mass in general.

Part of this discussion is only concerned with semantics. It might be thought that it would be better to regard the mass of the photons to be their (nonzero) relativistic mass, as opposed to their (zero) invariant mass. We could then consistently talk about the light having mass independently of whether or not it is contained. If relativistic mass is used for all objects, then mass is conserved and the mass of an object is the sum of the masses of its parts. However, modern usage defines mass as the invariant mass of an object mainly because the invariant mass is more useful when doing any kind of calculation. In this case mass is not conserved and the mass of an object is not the sum of the masses of its parts. Thus, the mass of a box of light is more than the mass of the box and the sum of the masses of the photons (the latter being zero). Relativistic mass is equivalent to energy, which is why relativistic mass is not a commonly used term nowadays. In the modern view "mass" is not equivalent to energy; mass is just that part of the energy of a body which is not kinetic energy. Mass is independent of velocity whereas energy is not.

Let's try to phrase this another way. What is the meaning of the equation
*E=mc ^{2}*? You can interpret it to mean that energy is the same
thing as mass except for a conversion factor equal to the square of the speed of
light. Then wherever there is mass there is energy and wherever there is energy
there is mass. In that case photons have mass, but we call it relativistic
mass. Another way to use Einstein's equation would be to keep mass and energy as
separate and use it as an equation which applies when mass is converted to energy or
energy is converted to mass--usually in nuclear reactions. The mass is then
independent of velocity and is closer to the old Newtonian concept. In that case,
only the total of energy and mass would be conserved, but it seems better to try to keep
the conservation of energy. The interpretation most widely used is a compromise in
which mass is invariant and always has energy so that total energy is conserved but
kinetic energy and radiation does not have mass. The distinction is purely a matter
of semantic convention.

Sometimes people ask "If light has no mass how can it be deflected by the gravity of a
star?". One answer is that all particles, including photons, move along geodesics in
general relativity and the path they follow is independent of their mass. The
deflection of starlight by the sun was first measured by Arthur Eddington in 1919.
The result was consistent with the predictions of general relativity and inconsistent with
the newtonian theory. Another answer is that the light has energy and momentum which
couples to gravity. The energy-momentum 4-vector of a particle, rather than its
mass, is the gravitational analogue of electric charge. (The corresponding analogue
of electric current is the energy-momentum stress tensor which appears in the
gravitational field equations of general relativity.) A massless particle can have
energy *E* and momentum *p* because mass is related to these by the
equation * m ^{2} = E^{2}/c^{4} -
p^{2}/c^{2}*, which is zero for a photon because

For an alternative viewpoint of relativistic mass, see the article
by T.R. Sandin in the American Journal of Physics, **59**, 11 (November 1991).