Is the speed of light constant?

Firstly, the speed of light has nothing to do with light itself. It might more accurately be called the “cosmic speed limit” as it represents the fastest speed that anything can move in our universe. The reason light can reach this speed is because it is carried by photons, which have no mass. Objects that have mass, any mass at all, would require an infinite amount of energy to accelerate them to the speed of light (thanks, Einstein!), which is not possible.

Photons not only transmit the visible light spectrum but all electromagnetic energy, including infrared, ultraviolet, radio waves, microwaves, x-rays, gamma rays… all of these travel at the “speed of light.”

In addition, any massless particle can also reach this speed. Gravitons, theoretical carriers of the gravitational force, would have no mass but have yet to be discovered. Last month, a team led by Princeton University researchers proved the existence of the Weyl Fermion, a massless particle first theorized to exist in 1929.

The familiar speed of light, 186,000 miles per second, or 671 million miles per hour, or 299,792,458 meters per second, is actually the speed of light in a vacuum. If light passes through anything other than a total vacuum, its speed will be less. Therefore, the maximum value is a constant but the speed of light itself is not. For example, light moves through glass at 33% less than top speed. This is a very significant effect, but as the light is still traveling over 447 million miles per hour – not noticeable to humans.

Humans have, however, slowed light much more than this. In 1999, Harvard University scientists projected a laser beam through a material that had been cooled to 459 degrees (Fahrenheit) below zero. At this temperature, just above “absolute zero,” all particle motion virtually ceases. The light beam in this experiment zoomed along at a breakneck 38 miles per hour.

Bonus Fact: Proponents of the “Virtual Reality” (VR) theory believe that the entire universe is a computer simulation á la “The Matrix.” They suggest that the speed of light represents the maximum computing speed of the computer that is running the simulation (which could, for all we know, be some alien kid’s science fair project).