- Einstein's theory of relativity says that we age more slowly as we approach the speed of light.
- As it turns out, photons themselves have an expiration date—about one billion billion years.
Everything dies. To many, this seems to be the one absolute truth in the universe. Plants and animals rot and decay, stars explode and grow dark, planets crumble or are burned, and even black holes may radiate away. Indeed, our very atoms, which are the same atoms that make up everything else in the universe, decay into lighter elements as time marches on.
But what about light? Can it die, or will it exist into infinity?
In order to understand the answer to this question, we first need to understand how atoms work. You can get the basics in our previous article, but in short, anytime that you have a heavy atom, there is some risk that it will spontaneously start to break down into smaller particles. This is known as “radioactive decay,” and it is the process that ultimately leads to the demise of atoms.
But does a similar process exist in light?
Ultimately, this question hinges on whether or not photons (the carriers of the electromagnetic force, i.e. light) can have mass.
Typically, photons are said to have zero mass. This is the standard solution that is accepted by a majority of scientists (in other words, it is the scientific consensus). However, as always seems to be the case in science, things get tricky when we start adding other variables and thinking about the “what ifs.”
So, let’s see, what if photons did have mass?
If a photon did have a non-zero rest mass, that means that it can decay into lighter elements, so the photon would break down into either some known elementary particles that are lighter, like neutrinos and antineutrinos, or an as-yet-undiscovered-particle.
The problem with this idea is that, according to current understanding, photons cannot be brought to rest. As a result, the idea of rest mass does not really apply to them. But for the sake of physics, let’s continue to assume that photons do have non-zero rest mass.
Thanks to previous experiments, we know what the upper limit of this mass is, so taking this into consideration, how long can photons live?
Julian Heeck of the Max Planck Institute for Nuclear Physics set out the tackle this issue, and in research published in Physics Review Letters B, Heeck calculated the minimum amount of time photons could live.
Since photons are moving at such excessive speeds, time dilation comes into play and must be accounted for. Once this is taken into consideration, according to the photons’ frame of reference, Heeck found that its lifetime would be a rather short three years (however, from our frame of reference, light would live about one billion billion (1018) years).
Those years look a little something like this: 10,000,000,000,000,000,000. For comparison, the universe is only 13,800,000,000 years old. That puts the lifetime of a photon at nearly 700 million times the age of the universe. This excessive gap means that, for all intents and purposes, the photon lives forever.
In order to come up with the figure, as previously mentioned, Heeck needed to know what the upper limit rest mass is for photons. Thanks to previous analyses, Heeck had a figure that constrained this to 10–18 eV (10–54 kg). Then, Heeck used the spectrum for the Cosmic Microwave Background Radiation (CMB), the most precisely measured black-body spectrum in nature, to complete his research. If the photon does have mass, and it can decay into lighter particles, the number density of photons in the CMB should decrease as the photons travel.
By utilizing a combination of the known constraints for the aforementioned mass and the CMB, Heeck’s analysis showed that a visible wavelength photon is stable for, as you already saw, at least 1018 years.