Photons and Relativity
Photons are elementary particles that carry the electromagnetic force, including light, and they’re much cooler than many give them credit for. It’s common knowledge that photons travel at the speed of light—the universe’s maximum speed—so the light we see from distant stars and galaxies is ancient, sometimes taking billions of years to reach us. But amazingly, photons don’t actually experience this time. Relativity tells us that as an object approaches the speed of light, it experiences two things: increased mass, and time dilation (i.e., time slows down). Because photons are massless, they are the only things able to reach the speed of light, and at that speed, time dilation is infinite, which means that time effectively stops for them. From their point of view, photons are emitted and reabsorbed instantaneously. This holds true for a photon emitted in the Sun that is reabsorbed a fraction of a millisecond later, and is equally true for a photon emitted in a distant star that travels for 13 billion years before it’s reabsorbed by your eye. Either way, photons think they arrive instantaneously. Futhermore, time and space are just two aspects of the same thing—spacetime—and so photons don’t experience distance either. From their perspective, the time and location of their emission are exactly the same as the time and location of their absorption—which is mind-bending, but incredibly awesome.