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fun fact about the human visual system: the minimum number of photons that need to enter the eye to be perceived is 1
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(but you gotta sit in a completely dark box for 40 minutes and have a precisely calibrated laser shine into your eye & then you're able to guess whether the laser went off at a rate a little better than chance. but, y'know, one photon, what do you expect)
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“The most amazing thing is that it’s not like seeing light. It’s almost a feeling, at the threshold of imagination,” says Alipasha Vaziri, a physicist at the Rockefeller University in New York City, who led the work and tried out the experience himself. nature.com/news/people-can-sense-single-photons-1.20282