Abstract Despite investigations for over 70 years, the absolute limits of human vision have remained unclear. Rod cells respond to individual photons, yet whether a single-photon incident on the eye can be perceived by a human subject has remained a fundamental open question. Here we report that humans can detect a single-photon incident on the cornea with a probability significantly above chance. This was achieved by implementing a combination of a psychophysics procedure with a quantum light source that can generate single-photon states of light. We further discover that the probability of reporting a single photon is modulated by the presence of an earlier photon, suggesting a priming process that temporarily enhances the effective gain of the visual system on the timescale of seconds.
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> Sirius isn't barely visible at the detection threshold, it's the brightest star in our sky.
And it's entirely washed out during the day. The full Moon is very bright, but it's still 400,000 times dimmer than the Sun when seen from Earth, and that's only ten different. The brightest star in our sky is simply not very bright; our eyes are just pretty awesome.
That star you are seeing is 25 orders of magnitude dimmer.
https://astro.wku.edu/labs/m100/mags.html
"While you may perceive one star to be only a few times brighter than another, the intensity of the two stars may differ by orders of magnitude. (Light intensity is defined as the amount of light energy striking each…
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square cm of surface per second.) The eye is a logarithmic detector. While the eye is perceiving linear steps in brightness, the light intensity is changing by multiplicative factors. This is fortunate; if the eye responded linearly instead of logarithmically to light intensity, you would be able to distinguish objects in bright sunlight, but would be nearly blind in the shade! If logarithms are a faint memory, you should peruse a refresher on logs and logarithmic scales before continuing."
https://physics.stackexchange.com/questions/329971/how-many-... says looking up at a sunny sky lets you take in 3×10^14 photons per second per eye. Yet you can see a single photon! https://www.nature.com/articles/ncomms12172
Or, we can conclude the entire field in dozens of countries simply can't do math. Your choice.
The instantaneous "HDR" capability of biological eyes is really quite amazing. About 5 orders of magnitude for human eyes, about 2-3 for most cameras.
By the way, there's a really cool in its simplicity medium term adaptation mechanism in eyes as well, they measure light intensity by photo-decay of a chemical substance that is produced slowly. If there is much light, the substance decays a short time after production. If there is little light, it accumulates for about half a minute, massively increasing sensitivity. The quantum efficiency (the inverse of how many photons it takes to produce a signal) of a dark-adapted eye is about 0.3: https://www.nature.com/articles/ncomms12172#MOESM482
The title made me think of the minimum number of photons detectable by human vision. Apparently, we can detect single photons:
https://www.nature.com/articles/ncomms12172
Recent experiments have put the claim to test
Overview
https://www.nature.com/articles/nature.2016.20282
The actual work
https://www.nature.com/articles/ncomms12172
> Here we report that humans can detect a single-photon incident on the cornea with a probability significantly above chance.
> In any case, our human eyes are actually really, really good.
A dark-adapted human eye may be able to detect light levels as low as a single photon:
https://www.nature.com/articles/ncomms12172
(Not reliably, mind you. But the fact that this works at all is incredible.)
Keep in mind the human eye is very good at detecting low level lighting, better than a camera. Some studies say it can even detect a single photon. It was likely easier to see in person.
[1] https://www.nature.com/articles/ncomms12172
> our eyes aren't very sensitive instruments
I take umbrage with that statement! Our eyes are exquisitely sensitive, and most importantly, have staggering dynamic range.
Our eyes are capable of perceiving a single photon [1], albeit noisily (I've been lucky enough to have performed this experiment myself!).
But the greatest thing about our eyes is the dynamic range: the difference in brightness between a moonless, starry night (which we are perfectly capable of navigating by eyesight) and a bright sunny day is nine orders of magnitude. A bright day is a billion times brighter!
Show me an RF receiver or light camera with that dynamic range!
The one place our eyes are limited is in frequency range.
[1] https://www.nature.com/articles/ncomms12172
> "until the subjects guesses were no better than random"
Crucially, no: They analyzed performance at 60% and made their argument based on the photons delivered at that light intensity. If your objection is about that statistical argument (" ... the photons would have been spread over about 350 rods"), such an indirect method was necessary because the experimenters didn't have an apparatus that could emit single photons in 1942. The 2016 article mentioned upthread [1] improves on this by providing the simple and direct experiment: emit single photons and check if the subjects can beat a coin-toss over large numbers of trials.
1. https://www.nature.com/articles/ncomms12172
Yeah, people get really messed up by just how good our eyes are. (For a close-to-home example, people think indoor plants get a lot closer to sunlight-level amounts than they really do.)
We can spot a single photon in the right conditions. https://www.nature.com/articles/ncomms12172
The Fermi paradox is the conflict between an expectation of a high {\em ex ante} probability of intelligent life elsewhere in the universe and the apparently lifeless universe we in fact observe.
Despite their massive size, successful deep artificial neural networks can exhibit a remarkably small difference between training and test performance.
In honor of Alan Turing's hundredth birthday, I unwisely set out some thoughts about one of Turing's obsessions throughout his life, the question of physics and free will.