Light With an Expiration Date

You may remember from school that light is made up of what we call photons. Photons sometimes behave like waves and sometimes like particles, in a phenomenon called "complementarity." It means if your experiment is set up to see if photons act like waves, they will. But if it's set up to see if they act like particles, they will do that. Think of an annoying sibling who repeats everything you say -- that's kind of like how photons act.

For much of the time since their discovery in the 1920s, photons were thought to have no mass (If you want to ask how something that acts like a particle can have no mass, well, you will have to live in a universe that's not as weird as this one). But some physicists have begun speculating that photons do have mass, because they act like they do when they are at rest.

If they have mass, that means they will eventually decay into smaller particles -- either one of the kinds of neutrinos zipping around the universe or something that hasn't even been discovered yet. And if they decay into other particles, how long will it be before that happens?

German physicist Julian Heeck wondered about that and is using data from several new experiments to find out. Within its own frame of reference, a photon lives about three years, he calculates. He's not yet sure what it decays into; the data don't show that yet.

Now, that three years is from the photon's point of view (if photons have a point of view). When Albert Einstein started noodling around in physics and developing his theories of relativity, one of the ideas that developed from that was that the closer something approaches the speed of light, the more time slows down. Time moves slower for me when I am walking than when I am sitting down, although the difference is so tiny it makes no difference in my perceptions. Add up all the slowdown that motion has produced for me in my life and I may have lengthened it by something less than a second. But that second is only from my point of view. From the perspective of those who aren't me, my life is exactly as long as the calendars and clocks say it is. As for the perspective of those who listen to me preach, you'll have to ask them how long 20 minutes seems like.

A photon moves much faster than we do, so this time dilation effect is a lot larger. So how long do we see a photon living? According to Heeck's figuring, about a billion billion years. In numerals, this is 1,000,000,000,000,000,000, and is also called a "quintillion." Since the best guesses for the age of the universe these days are in the 14 billion to 17 billion year range, it seems clear we are in little danger of running out of photons.

But I'm sure Al Gore will scold us about wasting them anyway.