How many photons are there?
Refinement: in the "potentially" observable universe? [There are some that have perhaps never have interacted... even since the Bing Bang]
Related: How many have there ever been? How to even think about this?
There are estimates for the number of protons at around 1E80. https://en.wikipedia.org/wiki/Eddington_number
How about all the mass particles? Using other modern methods. And simplistically multiplying by three to take account of neutrons and a few other exotics... 3E80. https://www.popularmechanics.com/space/a27259/how-many-particles-are-in-the-entire-universe/
This is the observable universe "now". One can argue that there are more "now" but largely unobservable... in that they went outside of our spacetime "hyper cone".... and have been doing so for all of the history of the universe. This means the hyper spacetime volume is roughly eight times bigger (would be sixteen if time had +-) and the ones observable plus the ones that have left would be 2E81
Photons are a different beast. Involved not only in the static existence of matter but in every interaction. AND some are not observable even within our spacetime hyper cone. That is to say they have not interacted, or many not ever interact. Imagine a neutrino but more ephemeral. And interacting purely because of their sheer weight numbers. https://en.wikipedia.org/wiki/Neutrino
BTW the number of neutrinos are estimated to be on the order of 1E80 (or more) as well. And the thinking around this is also helpful https://socratic.org/questions/how-many-neutrinos-are-in-the-universe
https://en.wikipedia.org/wiki/Massless_particle
Well here is a number for the photons produced by stars during the universes lifetime..
https://www.sciencealert.com/scientists-have-determined-number-of-photons-produced-by-stars-in-the-entire-universe
Star made photons 4E84
BUT it is about interacting photons. And again in the spacetime cone. Let us again multiply by eight for the ones that "went the other way"... and set aside "primordial" or "native" photons that have never interacted.... even though that could be quite a sizeable constituency. Interacting 3E85.
https://en.wikipedia.org/wiki/Elementary_particle
https://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg
Now let us over reach and make a wild guess that native photons are actually over eight but under ten more orders of magnitude. E95 for round numbers. Interestingly this is about one to ten moles of photons for every other single particle.
New question: What mass could a photon have? Be completely undetectable by current methods? And yet make up about eight times more mass energy than all the others put together? That is fairly simple from the above. The lightest particle is the electron neutrino at around .1 eV. Call a photon 23 orders of magnitude less than that or about 1E-25 eV
Does a photon mass of 1E-25 eV fit?
