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Comment by Sharlin | Hacker News Reader

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.

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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.

>Assuming a spherical isotropically radiating human

...in a vacuum, of course!

In a perfectly dark room, isolated from cosmic rays, at zero Kelvin.

my home office setup

Of course.

What if the room was made out of perfect mirrors?

It's still more likely to be reabsorbed by other part of the body than the other's retinas. Wonder if anyone will dare to do the math.

>Wonder if anyone will dare to do the math.

Warning! Madness this way lies.

I'm thinking back to the movie Real Genius during one of the montage scenes where they're cramming for finals in a common area when one student just stands up from his material and starts screaming in madness before fleeing. Everyone else, just behaves as if this is normal as someone takes the now vacant place at the table.

That guy screaming in madness? That's me after just thinking of doing the math without even grabbing a pencil.

No, this will not help. Thermodynamic law forbids this.

Otherwise one can create a machine concentrating all the thermal radiation of one black object at temperature T1, onto another perfectly white object also at T1 with a tiny black hole, heat up the second and then violate the second law of thermodynamics.

Keyword for further Googling: etendue

Comments like yours are one of the reasons I love HN. Thank you!

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.

Do we have to account for blink rate?

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