How Fast Does a Photon Move?

A photon is one of a plethora of elementary particles, but the photon is particularly noteworthy because it is the particle that carries light--or, in effect, it is light itself. The photon is a key concept in modern physics with some very peculiar characteristics, such as having no mass. Though this may sound counterintuitive, it is precisely this feature that is responsible for the photon's speed.

Features

Photons carry electromagnetic radiation of all types, from radio waves to gamma rays, but most people associate the photon with light. Unlike other particles such as electrons, photons have no mass while in a vacuum and therefore travel at the speed of light, which is an astounding 67,100,000,000 mph (186,000 miles per second). This speed is a physical constant, meaning that it holds true anywhere in the universe, and is designated in physics by a lower-case "c."

History

The early Greeks were the first to propose that energy is composed of particles. However, later experiments suggested that energy behaved more like waves than particles. In the 1860s, the physicist James Clerk Maxwell discovered that light was a form of electromagnetic waves. In 1900, Max Plank, a German physicist, suggested that light was made up of particles. Aided by famous physicists such as Albert Einstein, it was finally decided that light could be both a wave and a particle: It is a particle when scientists use instruments to detect photons, and also a particle when it interacts with other particles, but acts as a wave while traveling through a medium such as a vacuum.

Benefits

Since distances in outer space are so enormous, the speed of light serves as a measuring unit to help astronomers and other scientists--not to mention science fiction writers--gauge how far something is in space by using the amount of time it takes for light (photons) to travel to a certain point. For instance, a photon from the surface of the sun will take 8.3 minutes to reach Earth. Greater distances, such as those between stars or even galaxies, are measured in terms of light-years, which is the distance covered by photons traveling in a vacuum for one year--roughly 10 trillion km.

Identification

Scientists use the equation E=hv to calculate the energy content of a photon, where the energy (E) is equal to an unchanging number that was determined by Max Plank (called "Plank's constant") times the frequency, or how fast the photon vibrates (v). This equations helps determine how much energy any particular photon carries; X-rays, for example, have more energy than radio waves, even though both are "carried" by photons.

Theories/Speculation

Einstein famously stated that nothing can travel faster than light, since according to his theory of relativity, faster-than-light travel would literally mean going back in time, and whatever attempts this type of travel will actually have negative mass. However, the speed of photons has been an endlessly fascinating subject for scientists and science fiction writers, and some speculate that particles that are faster than photons, called tachyons, can theoretically exist (particles that travel slower than light are called tardyons).