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Comment by holmium | Hacker News Reader

But, it's the DM that would provide the heat/glow for the star:

> If the DM particles are their own antiparticles, then their annihilation provides a heat source that stops the collapse of the clouds and in fact produces a different type of star, a Dark Star, in thermal and hydrostatic equilibrium.

> Three key ingredients are required for the formation of DSs:

> 1) sufficient DM density

> 2) DM annihilation products become trapped inside the star

> 3) the DM heating rate beats the cooling rate of the collapsing cloud.

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> If the DM particles are their own antiparticles, then their annihilation provides a heat source

How much if this is speculation? Also, do other particles behave like this?

I didn't realise particles could be their own antiparticles, but it transpires that e.g. photons are, because all photons are neutral, not charged somehow.

However, even though a proton is its own antiparticles, two photons do not annihilate, right?

> Also, do other particles behave like this?

Yes, as you note, photons are their own antiparticles.

The maths doesn't have a preferred time direction, so two photons can annihilate into an electron-positron pair.

I'm not sure if this has actually been observed given how hard it is. That said, my favourite type of supernova is caused by pair creation, though I don't know the proportion of that which comes from 2-photon interactions: https://en.wikipedia.org/wiki/Pair-instability_supernova

There's also Majorna particles, but as I understand it the only known particles that are definitely Majornas are also quasiparticles:

https://en.wikipedia.org/wiki/Majorana_fermion

Notably it does bound the energy of gamma rays over long distances (as the higher the energy the more likely it will annihilate with other photons along the way.)

The wikipedia article on the Breit-Wheeler process has some history of the work on experimental observations, although I don't know how accurate or up to date it is https://en.wikipedia.org/wiki/Breit–Wheeler_process

> The maths doesn't have a preferred time direction, so two photons can annihilate into an electron-positron pair.

But only if their energy is high enough. So by that reckoning, photons below 511 keV don't have antiparticles, and those above it do. That's pretty weird. So maybe it's better to say that photons aren't really their own antiparticle, but they might theoretically destroy each other in some rare circumstances.

> Also, do other particles behave like this?

Nobody is sure, but some people think that neutrinos are they own antiparticle https://en.wikipedia.org/wiki/Neutrino#Majorana_mass I never liked that theory, but some people that know more than me about particle physics liked it.

There were some experiment using atoms that decay ejecting two neutrinos, and hopping that in some case the two neutrinos will annihilate each other. https://en.wikipedia.org/wiki/Neutrinoless_double_beta_decay . IIRC, none of the experiments found the strange annihilation, so perhaps neutrinos are not their own antiparticle :) .

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