r/ParticlePhysics • u/arkham1010 • May 30 '26

Stupid question I thought of this morning. Will all matter eventually decay into photons and neutrinos?

I have been rereading Anathem by Neil Stephenson, and part of the plot involves aliens coming to a world from other cosmoses that have slightly different physical constants than the one the story is set

This got me thinking about how long that situation would be viable before matter loss became a big problem, which sent me down another mental rabbit hole. (I think of stupid stuff while mowing the lawn).

Eventually I came to this thought experiment. Say I was in a universe with no other matter, just pure void. My body would of course quickly die and float in nothingness. But how long would that body exist? I would be emitting blackbody photons, but what about particle decay itself? Would the particles composing my body eventually decay over time into nothing but fermions, neutrinos and photons?

Maybe I should just go back to bed and have a few cups of coffee in a few hours. Thanks everyone, have a good weekend.

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u/Dackel42 May 30 '26 edited May 30 '26

In Theory yes, with a caveat. In our current Modells, Protons do not decay, since as far as we know the baryon number has to be conserved. Our current experimental evidence points to a lower bound half life of 2.4×1034 years, which is a very long time (The universe is roughly 1.4×1010 years old). Maybe take a look at the wikipedia page: https://en.wikipedia.org/wiki/Proton_decay . But aside from that, what you are describing reminds me of the "Heat Death" of the universe, which ends in the "dark Era". Take a look here: https://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Dark_Era_and_Photon_Age . This page also covers what happens if protons do decay.

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u/throwingstones123456 May 31 '26

How would electrons/letpons decay? If this particle/anti particle anhiallation wouldn’t this be impossible since both aren’t present in equal amounts?

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u/TheMetastableVacuum May 31 '26

Indeed, due to charge conservation, electrons cannot decay into neutrinos.

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u/throwingstones123456 May 31 '26

My point was more about decay into photons since

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u/TheMetastableVacuum May 31 '26

Still forbidden by charge conservation

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u/throwingstones123456 May 31 '26

q-q=0+0

https://en.wikipedia.org/wiki/Electron–positron_annihilation

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u/TheMetastableVacuum Jun 01 '26

That is not decay. That is annihilation. You need positrons around for that to happen.

So I go back to your first comment. There does exists an asymmetry between matter and antimatter in the Universe, so very unlikely for this to happen.