How Everything Works
How Everything Works How Everything Works

Question 524

Does light have mass? If so, then how can it travel at the speed of light? Doesn't the mass of an object (particle) approach infinity as its velocity approaches the speed of light?
Light has precisely zero mass and that makes all the difference. You're right that taking a massive particle up to the speed of light is impossible because doing so would, in a certain sense, give the particle an infinite mass. But the more important issue here is that doing so would require an infinite amount of energy and momentum.

Most physicists use the word mass to mean a particle's mass at rest—its rest mass—and as you bring the particle to higher and higher speeds, its rest mass doesn't change. However, the relationship between the particle's energy and its momentum does change with speed and the particle's momentum begins to increase more rapidly than it should according to the older, pre-relativistic mechanical theories. In an effort to explain this anomalous increase in momentum while retaining the old Newtonian laws of motion, people sometimes assign a fictitious "mass" to the particle; one that equals the rest mass when the particle is stationary but that increases as the particle's speed increases. As a particle approaches the speed of light, its momentum increases without limit and so does its "mass." Not surprisingly, the limitless rises in energy, momentum, and "mass" prevent the massive particle from ever reaching the speed of light.

As for light, it really does have zero mass and therefore can't be described by the Newtonian laws of motion. All light has is its momentum and its energy. In fact, light can't travel slower than the speed of light because that would require it to have a mass! So the world of particles is divided into two groups: massless particles that must travel at the speed of light and massive particles that can never travel at the speed of light.


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