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Comment by superposeur | Hacker News Reader

I’ve been aware of this fact as Feynman points it out in the course of the lectures, but find it totally nuts — there is some sense of scale I’m missing.

Matter consists of discrete chunks, atoms, but these chunks are so infinitesimal and numerous that there is no question of seeing them and any effect involving a handful of them is far, far below the human scale.

Light also comes in chunks, and the number of these chunks should be comparable to number of atoms since, e.g every single atomic transition generates a photon. Actually they should be far more numerous as they are massless and easily created, destroyed.

Yet, somehow, they are not far below the human scale of detection — as few as 5 (!) results in a perceivable flash.

What am I missing here? (Obviously this boils down to a numeric estimation and the numbers are what they are — my question is why do the numbers wind up even remotely in the human ballpark.)

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Another piece of this puzzle is that in a lot of cases, chemical reactions have an exponential sensitivity to energy levels.

The visible light spectrum corresponds to photons with energies of roughly 1.6 to 3.4 electron volts. That's a non-trivial amount of energy, enough to break the weaker bonds in many kinds of molecules.

In comparison, our environment is awash in huge numbers of thermal infrared photons, with energies on the order of 0.025 eV. But each of these photons has much less than 1/100th the effectiveness when it comes to interacting with atomic bonds.

As for the photopigments themselves, you can think of them kind of like atomic-scale mousetraps. Once a photosensitive molecule has been put into a highly energetic state, it only takes a tiny stimulus to make it release that energy, eventually leading to a much larger nerve impulse. Biological photosensors have been optimized by evolution to take advantage of this effect.

The visible photons have enough energy above the room temperature noise, that each one can be detected and amplified. Even when that energy is minuscule, it is sufficient to affect a molecule of pigment enough to cause cascade of amplifying reactions leading to detection.

I think we almost agree, but I want to clarify that visible photons and room temperature photons have a very different energy and frequency, and the molecules that detect them in the eye are very sensitive to frequency. So you see the "visible" photons that have a frequency in the correct range, but don't see the other photons.

I guess you can see a few visible photons even if the background has a lot of ultraviolet photons that have more energy and frequency. (It may hurt your eyes. Don't try it at home.)

Maybe seeing in the dark is simply so valuable that evolution has created extremely light-sensitive eyes for many species?

If there was a molecule whose detection in tiny amounts would give a species a competitive edge, we might have noses capable of detecting 5-molecule amounts of this substance.

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

Unless there's something else involved, most other animals that have good night vision is simply because they have bigger eyes that collect more photons. A single photon test would be the same with those animals eyes versus our own. I think the bigger determining factor is visual acuity as that corresponds to how easy it is to activate a photo receptor area which is easier to do the smaller those areas are.

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

photography perspective: Lets say hunting (any species) is most effective when the hunter has a wide field of view from a small pupil. So any hunting carnivore animal, will hunt better under non-ideal light if their pupils are smaller, which requires more sensitive eyes.

Higher sensitivity should lead to wider range of sharp focus, that should lead to more hunting success, more dinners, then more descendants.

I suppose on the prey side being able to see the wolf later during sunset should result in becoming the wolf's dinner less often leading to generally more descendants.

I am not a physicist but fun fact, In transit there is no such thing as a photon(electro magnetic waves can transit at any energy level) however light can only interact with matter at discrete energy levels, so the photon(a discrete energy level of light) only exists there, at the interaction with matter.

For some very good practical optics you can do a lot worse than the Huygens Optics youtube channel. https://www.youtube.com/watch?v=SDtAh9IwG-I

Particle and waves were just two seemingly incompatible models the modern natural philosophers used.

In post-modern (quantum) physics and the discovery/conception of photons by Einstein, they have been brought together as the same concept (and more generally for non-light by de Broglie as quantons).

A wavepacket is probably a good way to think of single quantons : https://upload.wikimedia.org/wikipedia/commons/9/92/Wavelet....

N.B.: In sufficiently weird and/or high energy situations, photons can also interact with one another ! :

https://www.quora.com/Do-photons-interact-with-other-photons...

https://en.wikipedia.org/wiki/Two-photon_physics

Its my understanding that the human finger, run across an extremely flat surface, can detect a < few atoms.

(via a quick google: https://www.newscientist.com/article/2276384-your-finger-can...)

It looks like that has to do with surface texture. I can reliably feel a step of 0.005" (0.13mm). I doubt if steps under 0.001" (0.03mm) can be felt. Nowhere near atomic scale.

Not true.

It's one atom per molecule, changing the friction.

> those who touched the surfaces could differentiate them based on chemical differences, including the substitution of one atom within each silane molecule for another, because of subtle changes in friction

You're not detecting the overwhelming majority of photons - only those few with an energy high enough to cause chemical reactions.

If you were constantly bombarded with atoms at the same energy level (1.5-3.5eV), you would notice them as well. This just occurs less frequently (unless you are on fire).

Really interesting. I'm not an expert, but I'll armchair speculate here.

We need 5-9 photons to merit perception. For us to see one atom, we would need 5+ photons to bounce off of that atom directly to our eyeballs in 100ms. Since both these things are so tiny, most photons miss the atom altogether. It's only when you have large numbers of atoms, densely arranged, that conditions create visibility.

I'd need to verify my sources, but I recall the way photon detection works is by cascade; one photon comes in, and causes a couple of more "things" (I can't remember what - electrons, probably) to be dislodged, each of which causes a couple more... and boooom whooooosh very quickly you have something which can be easily noticed by a sensor.

Welcome to the beauty of logarithmic scales.

Similarly surpising, large cities can be seen from orbit if you know where to look.

Seeing a photon is more like seeing a city on Pluto from Earth.

biological op-amps

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