CAN LIGHT BE CREATED OR DESTROYED??
Saturday, September 24th, 2022
One of the most enduring ideas in all the Universe is that everything that exists now will someday see its existence come to an end. The stars, galaxies, and even the black holes that occupy the space in our Universe will all some day burn out, fade away, and otherwise decay, leaving what we think of as a “heat death” state: where no more energy can possibly be extracted, in any way, from a uniform, maximum entropy, equilibrium state. But, perhaps, there are exceptions to this general rule, and that some things will truly live on forever.
One such candidate for a truly stable entity is the photon: the quantum of light. All of the electromagnetic radiation that exists in the Universe is made up of photons, and photons, as far as we can tell, have an infinite lifetime. Does that mean that light will truly live forever? That’s what Anna-Maria Galante wants to know, writing in to ask:
“Do photons live forever? Or do they “die,” and convert to some other particle? The light we see erupting from cosmic events over a verrrrry long past … we seem to know where it comes from, but where does it go? What is the life cycle of a photon?”
It’s a big and compelling question, and one that brings us right up to the edge of everything we know about the Universe. Here’s the best answer that science has today.
The first time the question of a photon having a finite lifetime came up, it was for a very good reason: we had just discovered the key evidence for the expanding Universe. The spiral and elliptical nebulae in the sky were shown to be galaxies, or “island Universes” as they were then known, well beyond the scale and scope of the Milky Way. These collections of millions, billions, or even trillions of stars were located at least millions of light-years away, placing them well outside of the Milky Way.
Moreover, it was quickly shown that these distant objects weren’t just far away, but they appeared to be receding from us, as the more distant they were, on average, the greater the light from them turned out to be systematically shifted toward redder and redder wavelengths.
Of course, by the time this data was widely available in the 1920s and 1930s, we had already learned about the quantum nature of light, which taught us that the wavelength of light determined its energy. We also had both special and general relativity well in hand, which taught us that once the light leaves its source, the only way you could change its frequency was to either:
1. have it interact with some form of matter and/or energy,
2. have the observer moving either toward or away from the observer,
3. or to have the curvature properties of space itself change, such as due to a gravitational redshift/blueshift or an expansion/contraction of the Universe.
The first potential explanation, in particular, led to the formulation of a fascinating alternative cosmology: tired light cosmology.
First formulated in 1929 by Fritz Zwicky — yes, the same Fritz Zwicky who coined the term supernova, who first formulated the dark matter hypothesis, and who once tried to “still” the turbulent atmospheric air by firing a rifle through his telescope tube — the tired light hypothesis put forth the notion that propagating light loses energy through collisions with other particles present in the space between galaxies. The more space there was to propagate through, the logic went, the more energy would be lost to these interactions, and that would be the explanation, rather than peculiar velocities or cosmic expansion, for why light appeared to be more severely redshifted for more distant objects.
However, in order for this scenario to be correct, there are two predictions that should be true.
1. ) When light travels through a medium, even a sparse medium, it slows down from the speed of light in vacuum to the speed of light in that medium. The slowdown affects light of different frequencies by different amounts. Just as light passing through a prism splits into different colors, light passing through an intergalactic medium that interacted with it should slow light of different wavelengths down by different amounts. When that light re-enters a true vacuum, it will resume moving at the speed of light in a vacuum.
And yet, when we observed the light coming from sources at different distances, we found no wavelength-dependence to the amount of redshift that light exhibited. Instead, at all distances, all wavelengths of emitted light are observed to redshift by the exact same factor as all others; there is no wavelength-dependence to the redshift. Because of this null observation, the first prediction of tired light cosmology is falsified.
But there’s a second prediction to contend with, as well.
2.) If more distant light loses more energy by passing through a greater length of a “lossy medium” than less distant light, then those more distant objects should appear to be blurred by a progressively greater and greater amount than the less distant ones.
And again, when we go to test this prediction, we find that it isn’t borne out by observations at all. More distant galaxies, when seen alongside less distant galaxies, appear just as sharp and high-resolution as the less distant ones. This is true, for example, for all five of the galaxies in Stephan’s Quintet, as well as for the background galaxies visible behind all five of the quintet’s members. This prediction is falsified as well.
While these observations are good enough to falsify the tired light hypothesis — and, in fact, were good enough to falsify it immediately, as soon as it was proposed — that’s only one possible way that light could be unstable. Light could either die out or convert into some other particle, and there’s a set of interesting ways to think about these possibilities.
Originally published on bigthink.com
(Wednesday, September 28th, 2022)
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