Are photons really eternal?

In the entire Universe, only a few particles are eternally stable. A photon, a quantum of light, has an infinite lifetime. Or not?

One of the most persistent ideas in the entire universe is that everything that exists now will someday cease to exist. The stars, galaxies and even black holes that occupy the space of our Universe will someday burn out, dim and disintegrate, going into a state that we call “thermal death”: when it will be impossible to extract energy in any way from a uniform, equilibrium state with maximum entropy. But perhaps there are exceptions to this general rule, and some things really will last forever.

One such candidate for a truly stable entity is the photon, a quantum of light. All electromagnetic radiation that exists in the Universe consists of photons, and photons, as far as we can tell, have an infinite lifetime. Does this mean that the light will truly live forever? The answer to this question is not so simple. We can imagine circumstances in which they will indeed live forever, but we can also imagine cases in which they decay, change into other particles, or even change into something new or unexpected. This is a big and interesting question that challenges us to the limits of everything we know about the universe. This is the best answer that science has to date.

As Vesto Slifer first noted in the 1910s, some objects we observe have spectral signatures of absorption or emission from certain atoms, ions, or molecules, but with a systematic shift toward the red or blue portion of the light spectrum. Combined with measurements of the distance to these objects, this data led to the original idea of an expanding universe: the further away a galaxy is, the more red its light will appear to our eyes and instruments.

For the first time, the question of whether a photon has a finite lifetime has arisen for a very good reason: we have just discovered key evidence for an expanding universe. Spiral and elliptical nebulae in the sky turned out to be galaxies, or, as they were then called, “island universes,” far beyond the Milky Way. These clusters of millions, billions, or even trillions of stars were located at a distance of at least millions of light years, which made it possible to place them far beyond the Milky Way. Moreover, it quickly became clear that these distant objects were not only far away, but appeared to be moving away from us, since the further they moved away, the more their light was systematically shifted towards redder and redder wavelengths.

Of course, by the time this data became widely available in the 1920s and 1930s, we had already learned about the quantum nature of light, which showed us that the wavelength of light determines its energy. We were also well aware of the special and general theories of relativity, from which it followed that as soon as light leaves its source, it can only change its frequency:

due to interaction with any form of matter and/or energy,
due to the movement of the observer either towards him or away from him,
due to changes in the curvature properties of space itself, for example, as a result of gravitational red/blue shift or expansion/contraction of the Universe.

The first potential explanation in particular has led to the formulation of a fascinating alternative cosmology: tired light cosmology.

In a long enough time, the light emitted by a distant object will reach our eyes even in the expanding Universe. However, if a distant galaxy's recession speed reaches and remains above the speed of light, we will never be able to reach it, even if we can catch light from its distant past. — In a long enough time, the light emitted by a distant object will reach our eyes even in the expanding Universe. However, if the recession speed of a distant galaxy reaches and remains above the speed of light, we will never be able to reach it, even if we can catch light from its distant past.

First formulated in 1929 by Fritz Zwicky—yes, the same Fritz Zwicky who coined the term “supernova,” first formulated the dark matter hypothesis, and once tried to “calm” turbulent atmospheric air by shooting a rifle down a telescope tube—the tired light hypothesis put forward the idea that propagating light loses energy as a result of collisions with other particles present in the space between galaxies. The larger the space through which light travels, the more energy is lost through these interactions, and it is this, rather than special speeds or cosmic expansion, that explains why light appears more red to more distant objects.

However, for this scenario to hold, two predictions must be true.

When light passes through a medium, even a rarefied one, it slows down from the speed of light in a vacuum to the speed of light in that medium. This slowdown affects light of different frequencies to different degrees. Just as light passing through a prism is split into different colors, light passing through an intergalactic medium that interacts with it should slow down light of different wavelengths by different amounts. When this light enters a real vacuum again, it will again travel at the speed of light in a vacuum.

In the vacuum of space, all light, regardless of wavelength or energy, travels at the same speed: the speed of light in a vacuum. When we observe light from a distant star, we are observing light that has already traveled from the source to the observer.

And yet, by observing light coming from sources at different distances, we did not find a dependence of wavelength on the amount of redshift that the light exhibits. On the contrary, at all distances, all wavelengths of emitted light are shifted by exactly the same factor as all the others; There is no wavelength dependence for redshift. Because of this null observation, the first prediction of tired light cosmology is refuted.

But there is a second prediction that also needs to be taken into account.

If more distant light loses more energy by traveling a greater distance through the “lossy medium” than less distant light, then more distant objects should appear blurred by a larger and greater amount than less distant ones. Once again, when we test this prediction, we find that it is completely unsupported by observation. More distant galaxies, when observed next to less distant galaxies, appear as clear and high resolution as less distant galaxies. This is true, for example, for all five galaxies in the Stefan Quintet, as well as for the background galaxies visible behind all five members of the quintet. This prediction also turned out to be refuted.

The main galaxies of the Stefan Quintet, discovered by Webb on July 12, 2022. The distance to the galaxy on the left is only ~15% of the distance to the other galaxies, and the background galaxies are many tens of times further away. But the image of them all is equally clear, demonstrating that the Universe is full of stars and galaxies almost everywhere we look.

While these observations are good enough to disprove the tired light hypothesis—and, in fact, they were good enough to disprove it as soon as it was proposed—it is only one possible way that light can be unstable. The light can either fade away or turn into some other particle, and there are some interesting ways to look at these possibilities.

The first follows simply from the fact that we have a cosmological redshift. Each generated photon, regardless of how it was generated – thermally, as a result of a quantum transition or any other interaction – will spread throughout the Universe until it collides and interacts with another energy quantum. But if we are talking about a photon emitted as a result of a quantum transition, then if it cannot quickly enter into a reverse quantum reaction, it will begin to travel through intergalactic space, and its wavelength will be stretched due to the expansion of the Universe as it travels. If he is unlucky and is not absorbed by some atom in a quantum bound state with a suitable permissible transition frequency, he will move further and further into the red part of the spectrum until he crosses the threshold of the maximum wavelength, after which he can no longer be absorbed .

This synthesis of three different sets of spectral lines from a mercury vapor lamp shows the effect a magnetic field can have. In (A) there is no magnetic field. In (B) and (C) there is a magnetic field, but it is oriented differently, which explains the different splitting of the spectral lines. Many atoms exhibit such fine or even hyperfine structure without the application of an external field, and these transitions are very important when it comes to creating functional atomic clocks. Many transitions, such as those shown here, are discrete rather than continuous processes.

However, there is a second set of possibilities that exists for all photons: they can interact with a free quantum particle, causing one of many effects.

This could be, for example, scattering, where a charged particle—usually an electron—absorbs and then emits a photon again. In this case, energy and momentum are exchanged, and either the charged particle or the photon can achieve a higher energy level due to the fact that the other particle has less of this energy.

At high enough energies, a collision of a photon with another particle—even another photon if the energy is high enough—can spontaneously produce a particle-antiparticle pair if there is enough energy for them both to pass through Einstein's E = mc². In fact, the highest-energy cosmic rays can do this even with surprisingly low-energy photons that are part of the cosmic microwave background (CMB), the residual glow from the Big Bang. For cosmic rays with energies above ~10¹⁷ eV, one typical RI photon has a chance to produce electron-positron pairs. At even higher energies, more likely ~10²⁰ eV, the RI photon has a much greater chance of turning into a neutral pion, which quite quickly deprives cosmic rays of energy. This is the main reason why there is a sharp decline in the population of the highest energy cosmic rays: they are above this critical energy threshold.

Energy spectrum of the highest energy cosmic rays, broken down by the collaborations that discovered them. All results are incredibly consistent from experiment to experiment and show a significant drop off at the GZK threshold of ~5 x 10^19 eV. However, many such cosmic rays exceed this energy threshold, indicating a flaw in the most simplistic understanding of these cosmic rays.

In other words, even very low-energy photons can be converted into other particles when colliding with another particle with sufficiently high energy.

There is a third way for a photon to change, besides cosmic expansion or transformation into particles with a non-zero rest mass: scattering from the particle, which results in the formation of additional photons. In virtually every electromagnetic interaction, or interaction between a charged particle and at least one photon, there are so-called “radiative corrections” that arise in quantum field theories. For every standard interaction in which there are the same number of photons at the beginning and at the end, there is just under 1% chance—1/137, to be exact—that you will end up emitting an extra photon beyond the number you started with.

And whenever you have an energetic particle that has a positive rest mass and a positive temperature, those particles will also emit photons, losing energy in the form of photons.

Photons are very, very easy to create, and although they can be absorbed, causing corresponding quantum transitions, most excitations cease to exist after a certain time. As in the old adage, “As goes up, comes down,” quantum systems that are excited to higher energies by absorbing photons eventually also decay, producing at least the same number of photons, usually with the same net energy that was absorbed at the beginning.

When a hydrogen atom is formed, the electron and proton spins are equally likely to be aligned or misaligned. If they are aligned, then no further transitions will occur, and if they are aligned, then they can quantum tunnel to a lower energy state, emitting a photon with a very specific wavelength (21 cm) in a very specific and quite long time interval. This transition has been measured to an accuracy of better than 1 part per trillion and has remained unchanged for the many decades during which it has been known. This is the first light that appeared in the Universe after the formation of neutral atoms and even before the formation of the first stars. It appeared after that: when new stars form, ultraviolet radiation ionizes hydrogen atoms, creating this signature again when these atoms spontaneously reform.

Given that there are so many ways to create photons, you're probably salivating at the desire to find a way to destroy them. After all, simply waiting for cosmic redshift effects to drive them to asymptotically low energy and density would take an arbitrarily long time. Every time the Universe doubles in size, the total energy density in the form of photons drops by a factor of 16: 2⁴ times. The factor of 8 appears because the number of photons – despite all the ways they are created – remains relatively fixed, and doubling the distance between objects increases the volume of the observable Universe by 8 times: twice the length, twice the width and twice the depth .

The fourth and final factor of two results from cosmological expansion, which stretches the wavelength to twice its original wavelength, thereby halving the energy per photon. On sufficiently large time scales, this will lead to the fact that the energy density of the Universe in the form of photons will asymptotically tend to zero, but will never reach it.

While matter and radiation become less dense as the universe expands due to its increasing volume, dark energy is a form of energy inherent to space itself. As new space forms in the expanding Universe, the density of dark energy remains constant.

You can try to get smart and imagine some exotic particle of ultra-low mass that combines with photons and into which the photon can turn under appropriate conditions. Some boson or pseudoscalar particle – such as an axion or axino, a neutrino condensate, or some exotic Cooper pair – could lead to just such phenomena, but, again, this only works if the photon has a high enough energy , to turn into a particle with non-zero rest mass through E = mc². Once the photon energy moves below a critical threshold, this no longer works.

In a similar way, one can imagine the ultimate way of absorbing photons: colliding them with a black hole. Once something moves from outside the event horizon inward, not only will it never be able to escape, but it will always increase the rest mass energy of the black hole itself. Yes, over time there will be many black holes in the Universe, and they will increase in mass and size over time.

But even this will only happen up to a certain point. Once the density of the Universe falls below a certain threshold, black holes will begin to decay under the influence of Hawking radiation faster than they grow, which means producing even more photons than originally entered the black hole. Over the next ~10¹⁰⁰ years or so, all black holes in the Universe will eventually decay completely, with the vast majority of the decay products being photons.

The simulated black hole decay results not only in the emission of radiation, but also in the decay of the central orbital mass that maintains the stability of most objects. Black holes are not static objects, but rather change over time. However, black holes formed from different materials should have different information encoded in their event horizons, and it is unclear whether this information is encoded in the outgoing Hawking radiation.

So will they ever die? According to modern laws of physics, no. In fact, the situation is even more dire than you probably think. You can remember every photon that has been or will be created:

created during the Big Bang
created as a result of quantum transitions,
created as a result of radiation corrections,
created as a result of energy radiation,
or created as a result of the collapse of a black hole,

and even if you wait until all those photons reach arbitrarily low energies due to the expansion of the Universe, the Universe will still not be devoid of photons.

Why?

Because there is still dark energy in the Universe. Just as an object with an event horizon, such as a black hole, will continuously emit photons due to the difference in acceleration near and far from the event horizon, so too will an object with a cosmological (or more precisely Rindlerian) horizon. Einstein's principle of equivalence tells us that observers cannot distinguish gravitational acceleration from acceleration due to any other cause, and any two unrelated places will appear to accelerate relative to each other due to the presence of dark energy. The physics involved is identical: a continuous amount of thermal radiation is emitted. Based on the value of the cosmological constant that we assume today, this means that the emission spectrum of a black body with a temperature of ~10^-30 K will always permeate all space, no matter how far into the future we go.

All matter falling from outside into a black hole emits light and is always visible, while due to the event horizon nothing can escape. But if you yourself fell into a black hole, your energy could, in theory, burst out again as part of a hot Big Bang in the newborn Universe.

Even at the very end of its existence, no matter how far into the future we go, the Universe will always continue to produce radiation, ensuring that it will never reach absolute zero, that it will always contain photons, and that even at the lowest energies it ever reaches, the photon will no longer have anything to decay or transform into. Although the energy density of the Universe will continue to fall as it expands, and the energy inherent in each individual photon will continue to fall as time rushes further and further into the future, there will never be anything “more fundamental” that they would move on.

Of course, there are exotic scenarios we can come up with to change history. Perhaps photons actually have non-zero rest mass, causing them to slow down to less than the speed of light when enough time has passed. Perhaps photons are truly unstable in nature, and there is something else, truly massless, such as a combination of gravitons, into which they can decay. And perhaps there is some kind of phase transition that will occur far in the future, where the photon will reveal its true instability and decay into an as yet unknown quantum state.

But if all we have is a photon, as we understand it in the Standard Model, then the photon is truly stable. A universe filled with dark energy ensures that even if existing photons are pushed to arbitrarily low energies, new ones will always be created, causing the universe to have a finite and positive number of photons and photon energy density. We can only be sure of the rules to the extent that we have measured them, but unless there is some big piece of the puzzle that we simply haven't uncovered yet, we can count on the fact that photons may decay, but never will die for real.