Apr 17, 2024 05:54 PM
https://bigthink.com/starts-with-a-bang/...-lifetime/
KEY POINTS: In the expanding Universe, for billions upon billions of years, the photon seems to be one of the very few particles that has an apparently infinite lifetime. Photons are the quanta that compose light, and in the absence of any other interactions that force them to change their properties, are eternally stable, with no hint that they would transmute into any other particle. But how well do we know this to be true, and what evidence can we point to in order to determine their stability? It's a fascinating question that pushes us right to the limits of what we can scientifically observe and measure.
EXCERPTS: . . . 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.
The first arises simply from the fact that we have a cosmological redshift. Each and every photon that’s produced, irrespective of how it’s produced, whether thermally or from a quantum transition or from any other interaction, will stream through the Universe until it collides and interacts with another quantum of energy. But if you were a photon emitted from a quantum transition, unless you can engage in the inverse quantum reaction in rather rapid fashion, you’re going to begin traveling through intergalactic space, with your wavelength stretching due to the Universe’s expansion as you do. If you’re not lucky enough to be absorbed by a quantum bound state with the right allowable transition frequency, you’ll simply redshift and redshift until you’re below the longest possible wavelength that will ever allow you to be absorbed by such a transition ever again.
However, there’s a second set of possibilities that exists for all photons: they can interact with an otherwise free quantum particle, producing one of any number of effects. [...] In other words, even very low-energy photons can be converted into other particles — non-photons — by colliding with another high-enough-energy particle.
There’s yet a third way to alter a photon beyond cosmic expansion or through converting into particles with a non-zero rest mass: by scattering off of a particle that results in the production of still additional photons...
[...] The fourth-and-final factor of two comes from the cosmological expansion, which stretches the wavelength to double its original wavelength, thereby halving the energy-per-photon. On long enough timescales, this will cause the energy density of the Universe in the form of photons to asymptotically drop toward zero, but it will never quite reach it...
[...] So will they ever die out? Not according to the currently understood laws of physics. In fact, the situation is even more dire than you probably realize. You can think of every photon that was or will be:
Why’s that? Because the Universe still has dark energy in it... (MORE - missing details)
KEY POINTS: In the expanding Universe, for billions upon billions of years, the photon seems to be one of the very few particles that has an apparently infinite lifetime. Photons are the quanta that compose light, and in the absence of any other interactions that force them to change their properties, are eternally stable, with no hint that they would transmute into any other particle. But how well do we know this to be true, and what evidence can we point to in order to determine their stability? It's a fascinating question that pushes us right to the limits of what we can scientifically observe and measure.
EXCERPTS: . . . 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.
The first arises simply from the fact that we have a cosmological redshift. Each and every photon that’s produced, irrespective of how it’s produced, whether thermally or from a quantum transition or from any other interaction, will stream through the Universe until it collides and interacts with another quantum of energy. But if you were a photon emitted from a quantum transition, unless you can engage in the inverse quantum reaction in rather rapid fashion, you’re going to begin traveling through intergalactic space, with your wavelength stretching due to the Universe’s expansion as you do. If you’re not lucky enough to be absorbed by a quantum bound state with the right allowable transition frequency, you’ll simply redshift and redshift until you’re below the longest possible wavelength that will ever allow you to be absorbed by such a transition ever again.
However, there’s a second set of possibilities that exists for all photons: they can interact with an otherwise free quantum particle, producing one of any number of effects. [...] In other words, even very low-energy photons can be converted into other particles — non-photons — by colliding with another high-enough-energy particle.
There’s yet a third way to alter a photon beyond cosmic expansion or through converting into particles with a non-zero rest mass: by scattering off of a particle that results in the production of still additional photons...
[...] The fourth-and-final factor of two comes from the cosmological expansion, which stretches the wavelength to double its original wavelength, thereby halving the energy-per-photon. On long enough timescales, this will cause the energy density of the Universe in the form of photons to asymptotically drop toward zero, but it will never quite reach it...
[...] So will they ever die out? Not according to the currently understood laws of physics. In fact, the situation is even more dire than you probably realize. You can think of every photon that was or will be:
- created in the Big Bang,
- created from quantum transitions,
- created from radiative corrections,
- created through the emission of energy,
- or created via black hole decay,
Why’s that? Because the Universe still has dark energy in it... (MORE - missing details)