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Does light have an infinite lifetime?
In all the Universe, only a few particles are eternally stable. The photon, the quantum of light, has an infinite lifetime. Or does it?
As was first noted by Vesto Slipher back in the 1910s, some of the objects we observe show the spectral signatures of absorption or emission of particular atoms, ions, or molecules, but with a systematic shift toward either the red or blue end of the light spectrum. When combined with distance measurements for those objects, this data gave rise to the initial idea of the expanding Universe: the farther away a galaxy is, the greater its light will appear redshifted to our eyes and instruments. We can only be certain of the rules to the extent that we’ve measured them, but unless there’s a big piece of the puzzle missing that we simply haven’t uncovered yet, we can count on the fact that photons might fade away, but they’ll never truly die.
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