As Einstein said, Matter tells spacetime how to curve, and spacetime tells matter how to move... Light is a quantum particle which is said to have a massless gauge symmetry,Abelian U(1). But Phenomena like curvature of light, radiation pressure and Gravitational redshift requires light to be massive(in my theory(look below for it!!!)). As I thought a while about these phenomena, another question arised.... I found that light is emitted by matter(electrons are the main particles involving in these emissions... I am not sure that protons will emit...).So, if matter are made up of particles, light is surely made up of particles, moreover if light is an electromagnetic wave, light can't travel because electric and magnetic fields are produced only in the vicinity of an electron and also they have limits i.e. these fields can't exist to infinity.
To understand the second thing, look carefully at Einstein's words(at the beginning of this article). It tells us that massless objects will not curve i.e. massless objects will only propagate rectilinearly. Again, experimental observations show that light curves around gravitational bodies. This again shows that light is massive.
The tertiary thing is gravitational redshift. Many of them are misunderstanding as a consequence of spacetime dilation but what is the reason behind the 'gravitational blueshift?' Again, light have to be massive to fulfill this explanation. Here is where my massive photon theory plays. My explanation is like this... suppose a photon leaves a massive star. Because of the gravitational pull of the star, the photon have to lose its speed. Light can travel only at c(\(299792458\) m/s) so, to maintain that speed, light loses some of its energy to maintain that speed and \(E=hv\) where \(h\)=planck constant and \(v\)=frequency. As there is a decrease in energy, there is a corresponding decrease in its wavelength. This is how redshift occurs. Also gravitational blueshift occurs when a photon is travelling towards a planet. The explanation is the same... due to the gravitational pull of the planet, its speed tend to increase. so, to maintain the speed if light, that energy is given to its wave part i.e. absorb energy. since \(E=hv\), frequency will increase.
Radiation pressure is another one which requires light to have mass. To learn about radiation pressure,[click here] (http://en.wikipedia.com/wiki/Radiation_Pressure) Another thing is that if light goes near a black hole, it loses all its energy to escape, and finally becomes another particle which doesn't have charge, and also can't interact electromagnetically, because all its energy is lost but its mass will always be there. so, it will interact gravitationally. this may be the theory to dark matter.
Thank you for reading this article