Immortality???

 

By firing a pulse of light at a semi-transparent/semi-reflective thin medium, researchers can measure the time it must take for these photons to tunnel through the barrier to the other side. Although the step of tunneling itself may be instantaneous, the traveling particles are still limited by the speed of light, and although photons can be absorbed and re-emitted, it's not so easy to destroy photons in any way at all. (Credit: J. Liang, L. Zhu & L.V. Wang, 2018, Light: Science & Applications)

To live forever, a false hope for all constituents of existence save for perhaps one, the photon, the carrier of the electromagnetic force, a massless particle forever traveling at the speed of light in a vacuum. is most likely immortal but then again, is it really?

In the expanding Universe, for billion 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.

The Red Shift beckons ...

The farther a galaxy is, the faster it expands away from us and the more its light appears redshifted. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years (multiplied by the speed of light) that it took the light emitted from it to reach us. But we can only understand redshifts and blueshifts if we attribute them to a combination of motion (special relativistic) and the expanding fabric of space (general relativistic) contributions both. If the light simply got “tired” instead there would be a differing series of observable consequences. Credit: Larry McNish/RASC Calgary)

Although no light can escape from inside a black hole’s event horizon, the curved space outside of it results in a difference between the vacuum state at different points near the event horizon, leading to the emission of radiation via quantum processes. This is where Hawking radiation comes from, and for the tiniest-mass black holes, Hawking radiation will lead to their complete decay in under a fraction-of-a-second. For even the largest mass black holes, survival beyond 10^103 years or so is impossible due to this exact process. Credit: The EU’s Communicate Science

Hence ... photons live on.

End result?

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,
• and even if you wait for all of those photons to reach arbitrarily low energies due to the Universe’s expansion, the Universe still won’t be devoid of photons.

Why’s that?

Because the Universe still has dark energy in it. Just as an object with an event horizon, like a black hole, will continuously emit photons due to the difference in acceleration close to versus far away from the event horizon, so too will an object with a cosmological (or, more technically, a Rindler) horizon. Einstein’s equivalence principle tells us that observers cannot tell the difference between gravitational acceleration or acceleration due to any other cause, and any two unbound locations will appear to accelerate relative to one another owing to the presence of dark energy. The physics that results is identical: a continuous amount of thermal radiation gets emitted. Based on the value of the cosmological constant we infer today, that means a blackbody spectrum of radiation with a temperature of ~10–30 K will always permeate all of space, no matter how far into the future we go.

Immortality it is ... for now. :)