Light is a form of energy. It has infinitesimal volume and mass. As a massive object approaches the speed of light, the energy needed to accelerate increases. If you plot the speed of the object and the energy needed to accelerate the object on a graph, you will find that if you connet the points they form a parabolic curve. A parabola never curves back on itself, it continues to expand outward at an ever decreasing rate.The speed of light can only be reached if an infinite amount of energy is used. Light on theother hand is basically massless, so the same principle does not apply. Pure energy can only travel at the speed of light. He was interested in it because, even though it is basically massless, it is affected by gravity. Gravity is the warping of space by energy. Matter is energy in a different form, so it also warps space. Even photons warp space somewhat, so a sufficiently massive body can noticibly interfere with light. The sun, planets, and moon bend the light from the stars. Photons have no measurable mass, so when they are affected by gravity, they don't slow down, yet they are affected by gravity because they are energy. supposedly, there are particles that travel faster than light, but they need energy to slow down, the less massive they are, the faster they would go, but if they were photons, they would travel at the speed of light, because they are pure energy.