Science & Technology

Can You See a Single Photon With Your Naked Eye?

It isn't always easy to wrap your mind around how light works. It's a particle, but it's also a wave, and that wave can be powerful. Photons generally travel en masse — we're talking densities in the range of quadrillions per square inch if they're coming from the sun. But what if instead of the whole wave crashing over you, you were only hit by a single drop of light — would you know? In other words, is it possible for the human eye to see a single photon?

Got a Light?

The answer, in a word, is "yes." Unfortunately, the more accurate answer, in two words, might be "yes, kinda." In a 2016 study published in Nature Communications, subjects who were asked to identify the flash of a single photon did so correctly at a rate higher than chance. That means that even if it's difficult, the brain does seem capable of processing that tiny piece of data. And really, "processing" is probably a better word for it than "seeing." As physicist Alipashi Vaziri put it, "The most amazing thing is that it's not like seeing light. It's almost a feeling, at the threshold of imagination."

In the experiment, three male participants went through a total of 30,767 trials. The trials went like this. The man would be given 40 minutes to let his eyes adjust to the darkness. Then, he'd be hooked into the optical system, which centered on a photon gun that created two light particles at a time — one would go into the eyeball, the other to a machine sensor. When he'd press a button, he would hear two sounds separated by one second. One of those sounds might also be accompanied by a single photon. He'd have to answer whether he thought the photon fired, and on which sound. Then, he'd find out if he was correct. Those answers, it turned out, were better than chance: The participants guessed right 51.6 percent of the time.

Okay, so that might not sound super impressive, but there were enough trials to reach the level of statistical significance — that is, the math shows that it likely wasn't random chance. One unexpected result of the test might be even more interesting, though: The experiment showed that some degree of "priming" might make it easier for the brain to catch the photons. Whenever the men correctly identified a photon, they were more likely to correctly identify the next photon they saw within 10 seconds — their success rate increased to 56 percent for that particular subset.

Casting Shadows

Now, don't start working on your single-photon flashlight quite yet. Some researchers have suggested that the 2016 experiment wasn't nearly large enough to draw any universal conclusions. After all, they might have just happened to select the three guys who are really good (or really bad!) at spotting tiny lights. Still, we've known since 1942 that the eyes' rod cells only need a single photon to make them react, so the question isn't so much about the sensitivity of the eye as the brain's tendency to tune out what it identifies as background noise. The actual ability to detect a single photon isn't likely to come in handy anytime soon, especially since the particles aren't often found on their own, and can't tell you much by themselves anyway. It's already been shown, however, that human brains are perfectly capable of detecting three photons at a time, and those two more photons of visual acuity aren't likely to make a huge difference. Still, the fact that that this degree of sensitivity is possible in biological systems bodes well for the capabilities of future technology.

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A little light goes a long way. All the way back to the beginning of history, in fact, and further still. In "Light: A Radiant History from Creation to the Quantum Age" (free with your trial membership to Audible), Bruce Watson explores how this fundamental power in the universe has interacted with humanity. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Written by Reuben Westmaas May 25, 2018

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