This is non trivial issue, which is unfortunately not very well described in good sources, but I will try. So my understanding is that the mainstream idea is that photon is massless, meaning that photon has zero invariant mass (rest mass). This was confirmed by experiments which set the lower possible value of photon at incredible low value. That means there was no experiment which would find any invariant mass of photon. But there is also the energy/momentum of photon, which can be also be called effective mass (relativistic mass). From wikipedia: https://en.wikipedia.org/wiki/Mass–energy_equivalence#Massless_particles Massless particles have zero rest mass. Their relativistic mass is simply their relativistic energy, divided by c2, or mrel = E/c2. The energy for photons is E = hf, where h is Planck's constant and f is the photon frequency. This frequency and thus the relativistic energy are frame-dependent. If an observer runs away from a photon in the direction the photon travels from a source, and it catches up with the observer—when the photon catches up, the observer sees it as having less energy than it had at the source. The faster the observer is traveling with regard to the source when the photon catches up, the less energy the photon has. As an observer approaches the speed of light with regard to the source, the photon looks redder and redder, by relativistic Doppler effect (the Doppler shift is the relativistic formula), and the energy of a very long-wavelength photon approaches zero. This is why a photon is massless—this means that the rest mass of a photon is zero. ..... Relation to gravity In physics, there are two distinct concepts of mass: the gravitational mass and the inertial mass. The gravitational mass is the quantity that determines the strength of the gravitational field generated by an object, as well as the gravitational force acting on the object when it is immersed in a gravitational field produced by other bodies. The inertial mass, on the other hand, quantifies how much an object accelerates if a given force is applied to it. The mass–energy equivalence in special relativity refers to the inertial mass. However, already in the context of Newton gravity, the Weak Equivalence Principle is postulated: the gravitational and the inertial mass of every object are the same. Thus, the mass–energy equivalence, combined with the Weak Equivalence Principle, results in the prediction that all forms of energy contribute to the gravitational field generated by an object. This observation is one of the pillars of the general theory of relativity. The above prediction, that all forms of energy interact gravitationally, has been subject to experimental tests. The first observation testing this prediction was made in 1919. During a solar eclipse, Arthur Eddington observed that the light from stars passing close to the Sun was bent. The effect is due to the gravitational attraction of light by the Sun. The observation confirmed that the energy carried by light indeed is equivalent to a gravitational mass. Another seminal experiment, the Pound–Rebka experiment, was performed in 1960. In this test a beam of light was emitted from the top of a tower and detected at the bottom. The frequency of the light detected was higher than the light emitted. This result confirms that the energy of photons increases when they fall in the gravitational field of the Earth. The energy, and therefore the gravitational mass, of photons is proportional to their frequency as stated by the Planck's relation. Another source: http://physics.stackexchange.com/qu...-have-no-mass-how-can-they-have-momentum?rq=1 There are two important concepts here that explain the influence of gravity on light (photons). The theory of Special Relativity, proved in 1905 (or rather the 2nd paper of that year on the subject) gives an equation for the relativistic energy of a particle; $$E^2 = (m_0 c^2)^2 + p^2 c^2$$ where $m_0$ is the rest mass of the particle (0 in the case of a photon). Hence this reduces to $E = pc$. Einstein also introduced the concept of relativistic mass (and the related mass-energy equivalence) in the same paper; we can then write $$m c^2 = pc$$ where $m$ is the relativistic mass here, hence $$m = p/c$$ In other words, a photon does have relativistic mass proportional to its momentum. De Broglie's relation, an early result of quantum theory (specifically wave-particle duality), states that $$\lambda = h / p$$ where $h$ is simply Planck's constant. This gives $$p = h / \lambda$$ Hence combining the two results, we get $$m = E / c^2 = h / \lambda c$$ again, paying attention to the fact that $m$ is relativistic mass. And here we have it: photons have 'mass' inversely proportional to their wavelength! Then simply by Newton's theory of gravity, they have gravitational influence. (To dispel a potential source of confusion, Einstein specifically proved that relativistic mass is an extension/generalisation of Newtonian mass, so we should conceptually be able to treat the two the same.) My comment and important disclaimer: Effective mass of photon means that you can calculate bending of light based on Newton theory. But this calculation gives only the half value compared to value calculated by GR which is also value confirmed by observations. So Im not trying to imply, that effective mass of photon is the only cause of bending of light. I have tried to discuss the components contributing to this bending in previous thread: http://www.sciforums.com/threads/example-about-bending-of-trajectory-near-to-sun.156631/ But unfortunately it seems that some people dont understand what Im trying to say with this example.