This was originally posted on a horrible site called Myspace. When Myspace underwent a redesign in Fall 2010, hundreds of insightful reader comments that had been left over the years were lost. I have since deleted my account there.
When it comes to the light from distant objects, most people think of the Universe as a giant cosmic shooting gallery: There’s a star over here, a star over there, maybe a galaxy that way, all spitting out tiny packets of light called “photons.” These photons careen through space at an amazing speed, we are told, and a few of them reach Earth after their lengthy travels, allowing us to see the stars and galaxies they came from. Even prominent science writers sometimes describe a photon “racing along” for billions of years, only to go splat against someone’s retina or a photographic plate on a telescope. This isn’t too surprising; things in our everyday experience go fast and go splat, so depicting bits of light like this makes intuitive sense.
But it’s just wrong. The “common sense” view of the Universe as a shooting gallery of light — while somewhat easy to grasp — has been out of date for over 100 years. It’s as incorrect as saying that life on Earth appeared fully formed from the Creator’s hand within the span of a week. Yes, creation is easier to grasp than evolution by natural selection. But just because something is easy to comprehend doesn’t mean it resembles the truth.
Photons cannot be said to “race,” “speed,” or “careen” through space like bullets, in any manner at all. They may seem to race, and we may get “splatted” by them — but they do not, themselves, race. In 1905, Albert Einstein showed that as an object’s speed through its environment goes up, its relationship with that environment changes: distances become shorter (a phenomenon known as Lorentz contraction), and durations of time also become shorter (known as time dilation). Special relativity turns the speed of light into a kind of “cosmic speed limit”: Nothing can go faster than that, because for anything that travels at that speed, the distances traveled contract to exactly zero, and the duration of travel similarly contracts to exactly zero. No time or distance is “experienced” by a photon, ever. It is therefore wrong to say that a photon races anywhere (“races” being an intransitive verb describing what it, itself, does), or that it “spent five billion years traveling” through space. Such anthropomorphisms are our own invention; they don’t reflect the photon’s reality, as defined by special relativity.
If a photon isn’t a little particle flying at a terrific speed, what is it, then? Here’s where it gets odd: A photon’s path is a line that connects all points in space and time that are equivalent as far as relativity is concerned. Starting from any point on the line, 186,000 miles away from that point, the cosmic clock is one second earlier. So, the line could connect the following equivalent points in space and time: Your eyeball right now, the Moon 1.2 seconds ago, the Sun 8 minutes ago, the star Sirius 8.6 years ago, and the Andromeda Galaxy 2.5 million years ago. A photon “traveling through space” could cross all five of these points. Except that it isn’t traveling. It’s a dimensionless “thing” which an observer at one of the similarly correlated points in familiar space and time will encounter. Because in the bigger picture of relativistic spacetime, there’s no difference between these points at all.
Photons are objects which all observers seem to witness moving — they are seen at different places in space at different times — but which don’t, themselves, travel anywhere or experience any time. This has profound implications for the way we view the Universe as a whole, but that’s a topic for another time. Suffice it to say, we should abandon the archaic idea that particles of light race ontologically through space for billions of years. They don’t.