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Can a Human See a Single Photon? (1996)

math.ucr.edu

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Re: Can a Human See a Single Photon? (1996)

#71

Post by Sharlin »

notfish wrote: post #65

This fun fact gets even more fun when you consider that a human radiates a nonzero amount of visible light via blackbody radiation - on the order of a photon/minute - which means that in theory, with some luck, in an otherwise cold and completely dark room, two humans might be able to notice each other through vision alone. You can check my math here if you want to convince yourself: https://www.spectralcalc.com/blac…

A bit more than just “some” luck needed for one of those photons to be emitted straight into the other person’s pupil!

The angular diameter of a human pupil (~5 mm) at a distance of ~2 m is about 0.2 degrees, taking the small angle approximation. So the area of the pupil is maybe 0.04 square degrees, or around one millionth of a full solid angle (~41,000 square degrees).

Assuming a spherical isotropically radiating human, the probability of hitting a pupil from any distance is thus pretty unlikely.

Re: Can a Human See a Single Photon? (1996)

#72

Post by qikInNdOutReply »

michaelcampbell wrote: post #60
qikInNdOutReply wrote: post #58

So, can a autist, with a deficient noise filter see it?

I'm not sure if your reply is meant to be snarky or not, because I don't understand it. If I've missed a pun it wouldn't be the first time, but ELI5?

Autism is described to not affected as in missing "filters", the inability to process faces for example or to shut out noise. If the noise level is higher for sound, why should the noise levle for visuals not be higher and thus percieved lightsources as small as a photon be visible.

Re: Can a Human See a Single Photon? (1996)

#73

Post by gnulinux »

Sharlin wrote: post #71
notfish wrote: post #65

This fun fact gets even more fun when you consider that a human radiates a nonzero amount of visible light via blackbody radiation - on the order of a photon/minute - which means that in theory, with some luck, in an otherwise cold and completely dark room, two humans might be able to notice each other through vision alone. You can check my math here if you want to convince yourself: https://www.spectralcalc.com/blac…

A bit more than just “some” luck needed for one of those photons to be emitted straight into the other person’s pupil! The angular diameter of a human pupil (~5 mm) at a distance of ~2 m is about 0.2 degrees, taking the small angle approximation. So the area of the pupil is maybe 0.04 square degrees, or around one millionth of a full solid angle (~41,000 square degrees). Assuming a spherical isotropically radiating h…

What if you're in a pitch-black dark room of mirrors? Would it make it more likely notice the other person through photons?

Re: Can a Human See a Single Photon? (1996)

#74

Post by Sprakle »

There's a fun way you can try seeing individual photons yourself! With a Nuclear Spinthariscope, somewhat popularized in this XKCD comic https://xkcd.com/2568/

You can buy them for about $50 from some science education sites. It's quite a conversation starter, as long as you're ready to sit in the dark for 15 minutes.

Re: Can a Human See a Single Photon? (1996)

#75

Post by notfish »

Sharlin wrote: post #71
notfish wrote: post #65

This fun fact gets even more fun when you consider that a human radiates a nonzero amount of visible light via blackbody radiation - on the order of a photon/minute - which means that in theory, with some luck, in an otherwise cold and completely dark room, two humans might be able to notice each other through vision alone. You can check my math here if you want to convince yourself: https://www.spectralcalc.com/blac…

A bit more than just “some” luck needed for one of those photons to be emitted straight into the other person’s pupil! The angular diameter of a human pupil (~5 mm) at a distance of ~2 m is about 0.2 degrees, taking the small angle approximation. So the area of the pupil is maybe 0.04 square degrees, or around one millionth of a full solid angle (~41,000 square degrees). Assuming a spherical isotropically radiating h…

We probably need to get more precise for a real answer.

The number I’m getting is about 2 photons/minute per steradian; a steradian has solid area 1m^2 at 1m away. With fully dilated pupils (8mmish) you get an area of pi x 2 x r^2 = 50mm^2, which gives us a rate of about .15 photons/day.

So… Maybe. If you’re very lucky. It’d be interesting to do the same math for a room temp object to see if you’re above the noise floor at all or if this is completely impossible, but I should go do my actual job instead of letting myself get nerdsniped by this.

Re: Can a Human See a Single Photon? (1996)

#76

Post by [deleted] »

notfish wrote: post #65

This fun fact gets even more fun when you consider that a human radiates a nonzero amount of visible light via blackbody radiation - on the order of a photon/minute - which means that in theory, with some luck, in an otherwise cold and completely dark room, two humans might be able to notice each other through vision alone. You can check my math here if you want to convince yourself: https://www.spectralcalc.com/blac…

[deleted]

Re: Can a Human See a Single Photon? (1996)

#77

Post by Sharlin »

gnulinux wrote: post #73
Sharlin wrote: post #71

Earlier quoted context omitted.

A bit more than just “some” luck needed for one of those photons to be emitted straight into the other person’s pupil! The angular diameter of a human pupil (~5 mm) at a distance of ~2 m is about 0.2 degrees, taking the small angle approximation. So the area of the pupil is maybe 0.04 square degrees, or around one millionth of a full solid angle (~41,000 square degrees). Assuming a spherical isotropically radiating h…

What if you're in a pitch-black dark room of mirrors? Would it make it more likely notice the other person through photons?

So you can probably fit three to five one-bounce reflections in your field of view. Then there’s the “infinite” number of higher-order reflections, but given that the probability for a photon to be absorbed rather than reflected is maybe 5% every time it hits a mirror, the additional reflections don’t really contribute that much beyond maybe 20 bounces. So I guess the odds could improve to something like 1/10,000 as a really rough guesstimate.

Re: Can a Human See a Single Photon? (1996)

#78

Post by Someone »

notfish wrote: post #65

This fun fact gets even more fun when you consider that a human radiates a nonzero amount of visible light via blackbody radiation - on the order of a photon/minute - which means that in theory, with some luck, in an otherwise cold and completely dark room, two humans might be able to notice each other through vision alone. You can check my math here if you want to convince yourself: https://www.spectralcalc.com/blac…

I don’t understand the physics, but clicking “Calculate” produces a graph with x axis between 8 and 12 μm.

Isn’t that 8,000 to 12,000 nm and thus outside of visible range for humans (400 to 800 nm)?

Decreasing the lower limit to zero shows things drop of rapidly near the visible range for humans.

Or do I have to subscribe and interpret the text file version?

Re: Can a Human See a Single Photon? (1996)

#79

Post by not2b »

kayodelycaon wrote: post #63
jonplackett wrote: post #23

Earlier quoted context omitted.

Weird that humans can do this when our eyes aren’t even that great at night vision compared to a lot of other animals.

Humans can navigate in darkness quite well if trained. For vision, you look for shadows. Shadows are generally things that absorb light and reflect it, like trees. Hearing can also tell you how close things are. In still air, a tree absorbs sound. Moving air flows around objects. Then there is the feel of terrain underfoot. :)

Humans can echolocate: https://en.wikipedia.org/wiki/Human_echolocation

Re: Can a Human See a Single Photon? (1996)

#80

Post by gameshot911 »

Another cool fact is that the human eye can see individual cells! [1] In this example it's a white blood cell, but the eye can also see human ovums as well.

I was convinced this was the case as a kid, but no one ever believed me!

[1] https://www.wikiwand.com/en/Blue_field_entoptic_phenomenon

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