How 'bathed in cosmic rays' are we, really?

I was lying in bed with my eyes closed, trying to sleep and looking at the back of my eyelids, when a single pixel flashed. This happens now and then and I jokingly assume this is caused by a neuron being struck by a cosmic ray of some sort that snuck past all the defenses the Earth provides, but I still couldn’t sleep so I’m asking the title question.

It’s highly variable.

Wikipedia also has an article on the cosmic ray flux.

For the extreme example: I saw a calculation once that, given the mass of all of the human eyeballs on the planet, there were probably a few people that saw a neutrino from Supernova 1987a.

I did work on parts for a neutrino detector once. I’ve had a varied career. But when a neutrino interacts with common matter, like in an eye, it would show up as a single pixel flashing, right?

There are several ways to detect neutrinos. One method, the first ever used, involves scintillators to detect the photons produced. So that’s a single photon and would be a tiny little flash.

But there also can be Cherenkov radiation which is a bit more spread out. Also, some detectors detect isotopes produced, e.g., argon from chlorine. So no glow at all.

The scintillator-detected photons require fairly high energy neutrinos and a rare type of interaction. Not sure how often those occur naturally at the Earth’s surface.

Astronauts have reported flashes in their eyes due to cosmic rays. Most cosmic rays don’t make it through our atmosphere, but some do manage to reach the ground, or sometimes particles caused by their collision with the atmosphere reach the ground. So it is possible that at least some of these flashes are due to cosmic rays that managed to make it to your eyes.

However, it is much more likely that the single “pixel” flashes in your eye are caused by your eye itself than something external like a neutrino. The most likely explanation is that the slight change in pressure throughout your eye due to the change in orientation from standing or sitting to lying in bed is causing a tiny bit of mechanical stress on your retina, which is triggering a small number of retinal neurons. Retinal neurons will also occasionally just fire off spontaneously for no real reason. It’s also possible that a few neurons are firing in response to one spontaneous trigger, since you probably wouldn’t see a noticeable flash if it were just a single neuron firing.

By “saw,” do you mean that people literally saw something in their field of vision, or just that a particle hit their retina?

Or that the neutrino interacted with the matter in the fluid in the eyeball, producing enough visible EM radiation to cause an optic nerve to fire ?

Neat video about making your own particle detector with isopropyl alcohol and dry ice.


The local science museum has (or more probably used to have) a working cloud chamber. It was the most unsexy thing, just sitting there and you’d walk right past it 9 times of 10. But it was sitting there, registering cosmic several times a minute. I suppose if you waited long enough you could catch a particle decaying, but I never could.

Missed edit window. OTOH, occasional flashes of light were the first symptom I had of my developing diabetes. You might mention that to your doctor the next check-up.

By “saw”, I meant a detectable flash. Which the person who saw it would probably write off as nothing, since there are a great many other explanations for such a flash, which are both more mundane and far more common. But they still would have seen it.

The Museum of the Rockies used to have a spark-chamber detector on display (a stack of alternating parallel plates, filling perhaps a cubic meter, charged up just short of the breakdown voltage of the air gaps). Every few seconds, a cosmic ray would pass through and draw a series of sparks on a line through the plates. But it was a very fiddly bit of equipment to keep operating properly, and was down for maintenance more often than it was up. Last I saw it, it was collecting dust in the back of one of the university’s labs.

If we were really bathed in them wouldn’t we either be able to stretch, flame on, turn invisible, or be covered with orange rocks?