Photon is the sub-atomic particle that light is made of. The properties of Photon: Mass = 0, Charge = 0, Spin = 1. According to Einstein's relativity, light travels fastest at 300,000 km / sec. When any other object travels at the speed of light, the universe shrinks to zero distance. But, we know that it takes 8.3 minutes for light from the sun to reach earth. So, even for sun light, the universe does not shrink to zero. I find it hard to believe that photon has no mass. The energy of a photon is h x λ (Planks constant multiplied by Frequency) is roughly 10-20 Joules, small but finite. According to Einstein, E = mc2. In a nuclear reaction, the mass lost is converted to energy which is carried by photon. Consider a hypothetical experiment, where photon is converted to a truly mass-less energy "particle". In the experiment, the mass of a photon is converted to pure energy. In short, the mass of the photon is lost. The lost mass is equivalent to the energy calculated by Planks formula (proportional to its frequency). So, the mass of a photon is 10-36 (small but finite). If there was a mass-less particle, it would travel at infinite speed and can move from one end of the universe to the other end of the universe in zero time. For a truly mass-less particle, the universe would truly shrink to zero.