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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
Our eyes perceive objects by having light bounce off that object and into our eyes. Studies seem to show that human eyes can detect as few as ~10 photons. I doubt you would call that "seeing" your hand though, just being more right than not that it was there is you guessed. And those photons need to be in the visible spectrum for our eyes to detect them though. In earth orbit there is clearly plenty of light for this to work since we can see on earth. Even much farther out in the solar system there are plenty of visible spectrum photons for us to see our hands in front of us.
So, deep space. The average photon density of the universe (which corresponds to deep space) is reasonably high at ~few hundred per cm^3, but the problem is most of those photons are not detectable by the human eye since they are not in the visible spectrum. I'm having trouble finding a good data source to get a better answer, but there's so little stuff in general in the universe that its likely close enough to pitch black in the darkest places in the universe (far from any galaxy) that it would be similar to being in a cave with little spots of light being distant galaxies. Your hand would be visible by blocking those pin-prinks of light
Comment by [deleted] at 27/04/2023 at 04:59 UTC
2 upvotes, 0 direct replies
Studies seem to show that human eyes can detect as few as ~10 photons.
Source
The average photon density of the universe (which corresponds to deep space) is reasonably high at ~few hundred per cm3,
Source.