12 ms·

Can a Human See a Single Photon? (1996)

167 points by omarfarooq 4y ago

ljlolel 4y ago
Yes
formerly_proven 4y ago
Yes but
sbaiddn 4y ago
Question in the title -> No. TL/DR: No, we cant. The retina itself can respond to a single photon, but we are not retinas. Instead, the eye filters out this low light sensitivity lest we go crazy with the resulting visual noise at night.
- high_byte 4y ago
  Not 1 but 5-9 photons, still less than billions :) pretty amazing
  - bitexploder 4y ago
    It’s cool our brains have that as a heuristic for whatever reason. Wonder if you could train yourself to see 1-3?
    - jvanderbot 4y ago
      If you tune up your sensitivity to 1-3 photons, you'll probably find a maddening TV white noise overlaid on your vision.
      - rini17 4y ago
        I do see exactly such a noise in dark rooms, am not unnerved by it at all.
        jvanderbot 4y ago
        Well that's expected when SNR is low. But in a dark room it's harder to see because SNR is low. I was just saying that I wouldn't want to artificially lower SNR in all environments, but maybe it wouldn't and maybe others do.
        rini17 4y ago
        I think the noise is suppressed not merely because it's harder to see, but mainly because neural networks tend to hallucinate given such an input. In other words, there's fine tuning between losing weak signals and hallucinations.
      - kentiko 4y ago
        Is there a link with Visual snow syndrome?
- ilyt 4y ago
  So that would be yes but our noise filter in brain doesn't let us see it
  - sbaiddn 4y ago
    You are not a retina. You cant see photons.
- Asraelite 4y ago
  Is this also acknowledging the more recent study linked in one of the top comments?
superposeur 4y ago
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.)
- pavlov 4y ago
  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.
  - jonplackett 4y ago
    Weird that humans can do this when our eyes aren’t even that great at night vision compared to a lot of other animals.
    - mlindner 4y ago
      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.
    - kayodelycaon 4y ago
Eddy_Viscosity2 4y ago
You can't see a photon, but you can taste a proton.
- metacritic12 4y ago
  Is this true or just a pun? Like your acid taste buds are literally sensitive enough to register a single H+ binding?
  - Eddy_Viscosity2 4y ago
    It is true you can taste protons. Whether you can taste a single one, I'm not sure, nor even how to test it. Fun video by Steve Mould on the subject: https://www.youtube.com/watch?v=FSYE1T5d9jc https://www.youtube.com/watch?v=FSYE1T5d9jc
    - tmearnest 4y ago
      I’m convinced you cannot taste single protons. Water self ionizes, so there will always be acidic species (H+, H3O+, …) way way above the concentration of single molecules. https://en.wikipedia.org/wiki/Self-ionization_of_water https://en.wikipedia.org/wiki/Self-ionization_of_water
      - amelius 4y ago
        Maybe the OH- parts neutralize the H+ sensation?
  - elil17 4y ago
    Our detection threshold for citric acid is like 0.04 mM (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629876/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629876/), or almost 2.3*10^17 protons per 10 mL sample used in the study I linked to. Obviously you're not going to be able to taste a single proton in your mouth. But maybe only a very small fraction of those protons are actually interacting with the receptors. Figuring out the threshold on the cellular level would be complex because it would depend on the rate of diffusion of H+ through the relevant channels and the number of relevant channels.
lebubule 4y ago
Recent experiments have put the claim to test Overview https://www.nature.com/articles/nature.2016.20282 https://www.nature.com/articles/nature.2016.20282 The actual work https://www.nature.com/articles/ncomms12172 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.
- mensetmanusman 4y ago
  Would be fun to see if it’s possible to set up the experiment as a double slit. Would humans sense an interference pattern? Is the back of an eye a sufficient observer? :)
  - d_tr 4y ago
    > Is the back of an eye a sufficient observer? :) It definitely is. The retina "measures" photon positions, which is why you see images. Observation is just interaction, no need to consider whether A can observe B. If they interact, some kind of observation takes place.
    - HPsquared 4y ago
      When the photon passes through the lens and the watery liquid part, its path is bent by refraction. Does that process also count as an "interaction", or is that different to what happens when the photon hits the retina?
      - d_tr 4y ago
        It does count as interaction. The systems are different.
  - phailhaus 4y ago
    Yep! Any interaction counts. If something interacts with a photon, and that interaction can only happen if it goes through one of the two slits (i.e., it tells you "which way" the photon went), then the interference pattern will disappear.
weinzierl 4y ago
What is the comparison to modern camera sensors nowadays? Can they detect single photons?
- callesgg 4y ago
  There are Photon-counting Cameras but they can not be used when there are allot of photons. As an example; the max nr of photons per second and pixel from one of the commercial ones is 10^7 photons/s/pixel.
- formerly_proven 4y ago
  Yes, modern CMOS image sensors have quantum efficiencies of more than 80 %, and modern CCDs are well above 90 %. This depends on wave length, particularly in the NIR spectrum. This means that 80 or 90 % of the incident photons are converted into electrons. This number does not include photons being reflected from the surface of the sensor or the filter stack on top. There's also SPADs "single photon avalanche diodes". These are sort of like a geiger counter for light, i.e. a single interacting photon triggers an impulse that can be counted or timed. These are very commonly used for time-of-flight sensors.
  - wyager 4y ago
    NB that CMOS and CCD sensors might have a QE approaching 1, but they cannot (yet) be used to detect a single photon due to internal noise (unless you get a really expensive sensor and make it super cold). For detecting single photons, you need really high in-sensor amplification. Options include SPADs or photomultipliers if you don't need an image, or image intensifier tubes (used in night vision) if you do need an image. Semiconductor cameras that use IIT-like operational principles are in development now, undergoing testing for next-gen (digital) night vision.
cozzyd 4y ago
I'm sure you could "see" a TeV photon showering in your eye?
- mk_stjames 4y ago
  Somewhat related- high energy cosmic rays (not photons, but ultra high energy protons, or helium nuclei) are theorized to cause flashes of light and other visual phenomenon in astronauts. https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena
zerotheliger 4y ago
have you seen them? well now you have :3
jll29 4y ago
It's amazing what happens in an eye: retinal (C₂₀H₂₈O), an aldehyde of Vitamin A, exists in two forms: all-trans-retinal gets hit by photos, flips and becomes 13-cis retinal (simplifying slightly). If enough photons are involved, the info gets transmitted to the train. If the photons were in a particular constellation, we see a "rectangle" or "circle"; quite a miracle if you think about it.
danbruc 4y ago
On a related note, you can hear sounds that displace your eardrums on the order of an atomic diameter. At least that is something I heard years ago, a quick search turned up this StackExchange question [1] so there may be some truth to this, I did however not read the answers too carefully. They actually seems to suggest that the threshold is even quite a bit smaller which makes me somewhat cautious of the claim, but then again you are essentially integrating over really many atoms in your eardrum getting displaced together. [1] https://physics.stackexchange.com/questions/147826/how-much-air-needs-to-be-displaced-to-generate-an-audible-sound https://physics.stackexchange.com/questions/147826/how-much-...
- jiveturkey 4y ago
  > something I heard
  - danbruc 4y ago
    Pun not intended but noticed while typing.
- sandworm101 4y ago
  >> sounds that displace your eardrums on the order of an atomic diameter. That isn't all that surprising really. This isn't one atom moving a tiny bit. It is billions and billions of atoms all shifting back and forth together.
  - jiveturkey 4y ago
    AIUI, we "hear" that motion. It's not that it takes billions+ of atoms in order to effect a motion at all, it's that the amount of motion here, _of the sensor_, is just an atomic diameter. If it takes billions of atoms to move the sensor, the sensor itself is comprised of trillions+ (I'd guess, decillions+) of atoms. That we can "hear" the movement of just a single atomic diameter in distance of such a relatively massive structure doesn't seem amazing? I'm even more amazed that with such sensitivity, we aren't inundated with the noise from that, and can still hear normal volume sounds, whose movement must be absolutely massive compared to a single atomic diameter. I guess that effect is similar to the eye's. We cannot see a single photon in the presence of daylight. That only works in a darkened (pitch black in this case) room.
vjulian 4y ago
Genuine question: I’ve never taken LSD, but could some of its purported visual effects be explained as seeing photons via a depressed neurological filter?
- LastTrain 4y ago
  It is too bad the risks (criminal) are so high, LSD is an interesting enough experience that is worth trying at least once.
  - mlindner 4y ago
    As someone who's never taken LSD and have talked to people who have, the number of times people express irreversible changes in how they think about things just from trying things once makes me think that it is not a very good idea to even try once. A chemical that causes irreversible random brain chemistry changes from a single use sounds like damage has happened and the user simply interprets that damage as an improvement of some sort. In the same way we used to cut out pieces of people's brains in the early 1900s and they would report more comfortable lives.
    - culi 4y ago
      LSD as well as psilocybin and a number of other natural psychedelics exhibit what's called a "classical psychedelic effect". I've tried a couple of them and consider LSD to be essentially the same It's worth noting that psychedelic fungi and plants are absolutely GLOBAL in distribution. Psilocybe can be found basically everywhere and there are a few related genera of fungi that also contain psilocybe. South America has a ton of psychedelic plants, but even Native America has some psychedelic grasses used to make "prairie ayahuasca". There's a ton of psychedelic cacti species including a very famous cactus (which I shall not name) that very few people know is psychoactive because the Native American group that uses it wishes to protect it from being overharvested. Some very poisonous plants like nightshade are also psychoactive. The fact that indigenous people throughout the Americas have managed to develop techniques to safely utilize these plants suggests close study and attention. There's a famous "Jesus was a mushroom" take by an archeologist that the early cults from which Christianity emerged dreamed up Jesus through their use of psychedelic mushrooms. The take doesn't hold much water but what is agreed upon is that even those groups regularly consumed psychedelics. Christmas also comes from cultural usage of psychedelics. Amanita muscaria, perhaps the most famous mushroom, is that red mushroom with the white spots (kinda looks like Father Christmas, don't it). It's poisonous and very psychedelic. To consume the psychoactive, indigenous European groups would drink reindeer piss. The piss neutralizes the poison but leaves the psilocybin. And so these reindeer allow Amanita muscaria to fly around dropping gifts for us. I could go on about sweatlodges, the tens of thousands of different psychedelic plant species known, and other ways psychedelic experiences shaped cultures around the world. My point is is that psychedelics are likely a universal cultural norm. In fact I'd argue we're living in a very strange culture where their use is not very widespread. I personally didn't try a psychedelic until after highschool. My first experience cured my debilitating social anxiety and I went from my heart racing any time anyone talked to me to organizing community events within the span of a year. I've had similar effects with my depression and other mental health struggles. I'm not trying to suggest they're for everyone, but I'd say the fact that there's "irreversible changes in how [you] think about things" is exactly what makes them such a useful medicine
qikInNdOutReply 4y ago
So, can a autist, with a deficient noise filter see it?
- michaelcampbell 4y ago
  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?
  - qikInNdOutReply 4y ago
    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.
dekhn 4y ago
This page is old- it's been established people can see single photons. I know a scientist at Berkeley who did this exact experiment.
notfish 4y ago
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/blackbody_calculator/blackbody.php https://www.spectralcalc.com/blackbody_calculator/blackbody....
- deleted 4y ago
  [deleted]
- Sharlin 4y ago
  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.
  - gnulinux 4y ago
    What if you're in a pitch-black dark room of mirrors? Would it make it more likely notice the other person through photons?
    - Sharlin 4y ago
      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.
  - notfish 4y ago
yu3zhou4 4y ago
> sensors in the retina can respond to a single photon. But neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms Like an activation function in artificial neural networks
Sprakle 4y ago
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/ 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.
gameshot911 4y ago
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 https://www.wikiwand.com/en/Blue_field_entoptic_phenomenon
- moistly 4y ago
  There are single-cell organisms up to an inch or so in diameter. https://en.m.wikipedia.org/wiki/Valonia_ventricosa https://en.m.wikipedia.org/wiki/Valonia_ventricosa There are even visible bacteria! https://www.science.org/content/article/largest-bacterium-ever-discovered-has-unexpectedly-complex-cells https://www.science.org/content/article/largest-bacterium-ev...
- maxbond 4y ago
  One time a fellow intern and I were isolating single colonies of spirulina (many bright green algae* cells in a spiral [1], which is roughly 50 micron wide and maybe 15 micron tall, if memory serves), and I got one colony on it's own in a drop of water under a microscope (using a special pipette & the wick effect for suction). I took my eyes off of the eyepiece, and looked at the drop of water. I could see a bright green dot in the middle, right where I'd left my spirulina. I asked the other intern if they saw it too; they did. It was a magical moment. I had a similar experience when I was doing something with H. pluvalis (an algae that turns bright red and forms a film under stress [2], you've likely seen it in a birdbath or shallow puddle) and I put this film under the microscope, looked at it, and saw it was 1 cell thick. I'm confident if I had gotten a single one of those cells on it's own, I could've seen it. Can't confidently remember how big they were, but large on the scale of cells you find in a drop of water (maybe 25 micron in diameter?), but smaller than most animal cells. * Technically a cyanobacteria and not an algae, but if a lay person looked at a pond of it they'd say to themselves, "that's a bunch of algae" [1] https://images.fineartamerica.com/images-medium-large/1-spirulina-cyanobacteria-light-micrograph-sinclair-stammers.jpg https://images.fineartamerica.com/images-medium-large/1-spir... [2] https://images.squarespace-cdn.com/content/51511761e4b0323b0cd00500/1497555135471-V5Q9LG80KNZGOHWTBCN4/DSC02300.JPG?format=1000w&content-type=image%2Fjpeg https://images.squarespace-cdn.com/content/51511761e4b0323b0...
fractallyte 4y ago
This is a snippet from Scientific American, October 1993, 50 and 100 years ago: October 1893: "It now does not seem improbable that, when by the power of thought an image is evoked, a distant reflex action, no matter how weak, is exerted upon certain ends of the visual nerves, and, therefore, upon the retina. Helmholtz has shown that the fundi of the eyes are themselves luminous, and he was able to see, in total darkness, the movement of his arm by the light of his own eyes. This is one of the most remarkable experiments recorded in the history of science, and probably only a few men could satisfactorily repeat it, for it is very likely that the luminosity of the eyes is associated with uncommon activity of the brain and great imaginative power. It is fluorescence of brain action, as it were." --Nikola Tesla, in a paper read before the Franklin Institute Can anyone - uh - shed some light on it?
- duskwuff 4y ago
  For context, it's important to remember that Tesla was an accomplished experimental scientist, but he was prone to wild claims and flights of fancy which weren't based in scientific reasoning, even by the standards of the day. Not everything he said or wrote was literally true. > Helmholtz has shown that the fundi of the eyes are themselves luminous... This is erroneous -- no part of the eye emits light. Some animals, like cats and dogs, have retroreflective surfaces within the eye, and I wouldn't be surprised if that's what got Helmholtz (or possibly Tesla in quoting him?) tripped up. > ... and he was able to see, in total darkness, the movement of his arm by the light of his own eyes ... If this experiment occurred, it's far more likely that there were low levels of light present in the room, and/or that the experimenter was imagining the position of his arm as sensed through proprioception. > ... it is very likely that the luminosity of the eyes is associated with uncommon activity of the brain and great imaginative power And this is one of the flights of fancy I was talking about. :) There's no basis for this claim.
tlhunter 4y ago
This reminds me; if I look at a red LED in the dark, my eyes seem to add a purple shape around it, kinda like an infinity symbol. Does this happen to anyone else?
- Derbasti 4y ago
  I thought that's astigmatism, which distorts different wavelengths differently. Some LEDs have very few, widely separated wavelengths, and your astigmatism separates them.
stainablesteel 4y ago
its an interesting space to try and inspire engineering from biology, a quick search says eagles have the best eyesight i just saw a video about a dog's nose being used for particle detection in crime scenes, i'm wondering if there's already been, or when there will be work on recreating the basis of an eye as a detector or if maybe this has already been subverted by lens manufacturing
beefield 4y ago
As a layman in modern physics, I struggle to understand the shape and size of a photon as a wave. As far as I understand, e.g double slit experiment would implicate that the size of a single photon wave would be clearly macroscopic in nature. Is there any text available that would discuss this without a need of Ph.D in physics?
- brummm 4y ago
  So I think one conceptual issue that makes this so hard to understand is that we tend to imagine a physical reality where light is a wave or a stream of particles. However, that is absolutely wrong. Both the wave and the particle concept are mental models we have constructed to explain light as a phenomenon and to calculate things. And in their respective realms, they work exceptionally well. However, where things start to get complicated is when the models give different results. However, this doesn't mean something is wrong, it just means the mental model we use to make sense of nature is stretched beyond where it's applicable. So to preface, light is neither a stream of particles or some sort of wave like the ones on the surface of water. Light is the excitation of the electromagnetic field and is described by a quantum field theory called the standard model. So. photons are 0-dimensional (so no size) excitations of this field and they interact with matter like our eyes, surfaces, sensors in an experiment and produce physical effects.
amelius 4y ago
How many electrons does it take for a human to feel a static electricity discharge?
fallingfrog 4y ago
If you look carefully, you can see a little bit of noise in very low lighting, probably because of the small number of photons. However, the amount of noise you see with your eyes is orders of magnitude less than the camera in your phone picks up. Camera technology is still well behind biological cameras. To get similar performance you’d probably have to cool the ccd with liquid nitrogen.
hulitu 4y ago
I was expecting sonething like "if it has enough energy ... " , but the article is great in explaining vision.

167 points by omarfarooq 4y ago

ljlolel 4y ago

Yes

formerly_proven 4y ago

Yes but

sbaiddn 4y ago

Question in the title -> No. TL/DR: No, we cant. The retina itself can respond to a single photon, but we are not retinas. Instead, the eye filters out this low light sensitivity lest we go crazy with the resulting visual noise at night.

high_byte 4y ago
Not 1 but 5-9 photons, still less than billions :) pretty amazing
- bitexploder 4y ago
  It’s cool our brains have that as a heuristic for whatever reason. Wonder if you could train yourself to see 1-3?
  - jvanderbot 4y ago
    If you tune up your sensitivity to 1-3 photons, you'll probably find a maddening TV white noise overlaid on your vision.
    - rini17 4y ago
      I do see exactly such a noise in dark rooms, am not unnerved by it at all.
      - jvanderbot 4y ago
        Well that's expected when SNR is low. But in a dark room it's harder to see because SNR is low. I was just saying that I wouldn't want to artificially lower SNR in all environments, but maybe it wouldn't and maybe others do.
        rini17 4y ago
        I think the noise is suppressed not merely because it's harder to see, but mainly because neural networks tend to hallucinate given such an input. In other words, there's fine tuning between losing weak signals and hallucinations.
    - kentiko 4y ago
      Is there a link with Visual snow syndrome?
ilyt 4y ago
So that would be yes but our noise filter in brain doesn't let us see it
- sbaiddn 4y ago
  You are not a retina. You cant see photons.
Asraelite 4y ago
Is this also acknowledging the more recent study linked in one of the top comments?

superposeur 4y ago

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.)

pavlov 4y ago
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.
- jonplackett 4y ago
  Weird that humans can do this when our eyes aren’t even that great at night vision compared to a lot of other animals.
  - mlindner 4y ago
    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.
  - kayodelycaon 4y ago

Eddy_Viscosity2 4y ago

You can't see a photon, but you can taste a proton.

metacritic12 4y ago
Is this true or just a pun? Like your acid taste buds are literally sensitive enough to register a single H+ binding?
- Eddy_Viscosity2 4y ago
  It is true you can taste protons. Whether you can taste a single one, I'm not sure, nor even how to test it. Fun video by Steve Mould on the subject: https://www.youtube.com/watch?v=FSYE1T5d9jc https://www.youtube.com/watch?v=FSYE1T5d9jc
  - tmearnest 4y ago
    I’m convinced you cannot taste single protons. Water self ionizes, so there will always be acidic species (H+, H3O+, …) way way above the concentration of single molecules. https://en.wikipedia.org/wiki/Self-ionization_of_water https://en.wikipedia.org/wiki/Self-ionization_of_water
    - amelius 4y ago
      Maybe the OH- parts neutralize the H+ sensation?
- elil17 4y ago
  Our detection threshold for citric acid is like 0.04 mM (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629876/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629876/), or almost 2.3*10^17 protons per 10 mL sample used in the study I linked to. Obviously you're not going to be able to taste a single proton in your mouth. But maybe only a very small fraction of those protons are actually interacting with the receptors. Figuring out the threshold on the cellular level would be complex because it would depend on the rate of diffusion of H+ through the relevant channels and the number of relevant channels.

lebubule 4y ago

Recent experiments have put the claim to test Overview https://www.nature.com/articles/nature.2016.20282 https://www.nature.com/articles/nature.2016.20282 The actual work https://www.nature.com/articles/ncomms12172 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.

mensetmanusman 4y ago
Would be fun to see if it’s possible to set up the experiment as a double slit. Would humans sense an interference pattern? Is the back of an eye a sufficient observer? :)
- d_tr 4y ago
  > Is the back of an eye a sufficient observer? :) It definitely is. The retina "measures" photon positions, which is why you see images. Observation is just interaction, no need to consider whether A can observe B. If they interact, some kind of observation takes place.
  - HPsquared 4y ago
    When the photon passes through the lens and the watery liquid part, its path is bent by refraction. Does that process also count as an "interaction", or is that different to what happens when the photon hits the retina?
    - d_tr 4y ago
      It does count as interaction. The systems are different.
- phailhaus 4y ago
  Yep! Any interaction counts. If something interacts with a photon, and that interaction can only happen if it goes through one of the two slits (i.e., it tells you "which way" the photon went), then the interference pattern will disappear.

weinzierl 4y ago

What is the comparison to modern camera sensors nowadays? Can they detect single photons?

callesgg 4y ago
There are Photon-counting Cameras but they can not be used when there are allot of photons. As an example; the max nr of photons per second and pixel from one of the commercial ones is 10^7 photons/s/pixel.
formerly_proven 4y ago
Yes, modern CMOS image sensors have quantum efficiencies of more than 80 %, and modern CCDs are well above 90 %. This depends on wave length, particularly in the NIR spectrum. This means that 80 or 90 % of the incident photons are converted into electrons. This number does not include photons being reflected from the surface of the sensor or the filter stack on top. There's also SPADs "single photon avalanche diodes". These are sort of like a geiger counter for light, i.e. a single interacting photon triggers an impulse that can be counted or timed. These are very commonly used for time-of-flight sensors.
- wyager 4y ago
  NB that CMOS and CCD sensors might have a QE approaching 1, but they cannot (yet) be used to detect a single photon due to internal noise (unless you get a really expensive sensor and make it super cold). For detecting single photons, you need really high in-sensor amplification. Options include SPADs or photomultipliers if you don't need an image, or image intensifier tubes (used in night vision) if you do need an image. Semiconductor cameras that use IIT-like operational principles are in development now, undergoing testing for next-gen (digital) night vision.

cozzyd 4y ago

I'm sure you could "see" a TeV photon showering in your eye?

mk_stjames 4y ago
Somewhat related- high energy cosmic rays (not photons, but ultra high energy protons, or helium nuclei) are theorized to cause flashes of light and other visual phenomenon in astronauts. https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena

zerotheliger 4y ago

have you seen them? well now you have :3

jll29 4y ago

It's amazing what happens in an eye: retinal (C₂₀H₂₈O), an aldehyde of Vitamin A, exists in two forms: all-trans-retinal gets hit by photos, flips and becomes 13-cis retinal (simplifying slightly). If enough photons are involved, the info gets transmitted to the train. If the photons were in a particular constellation, we see a "rectangle" or "circle"; quite a miracle if you think about it.

danbruc 4y ago

On a related note, you can hear sounds that displace your eardrums on the order of an atomic diameter. At least that is something I heard years ago, a quick search turned up this StackExchange question [1] so there may be some truth to this, I did however not read the answers too carefully. They actually seems to suggest that the threshold is even quite a bit smaller which makes me somewhat cautious of the claim, but then again you are essentially integrating over really many atoms in your eardrum getting displaced together. [1] https://physics.stackexchange.com/questions/147826/how-much-air-needs-to-be-displaced-to-generate-an-audible-sound https://physics.stackexchange.com/questions/147826/how-much-...

jiveturkey 4y ago
> something I heard
- danbruc 4y ago
  Pun not intended but noticed while typing.
sandworm101 4y ago
>> sounds that displace your eardrums on the order of an atomic diameter. That isn't all that surprising really. This isn't one atom moving a tiny bit. It is billions and billions of atoms all shifting back and forth together.
- jiveturkey 4y ago
  AIUI, we "hear" that motion. It's not that it takes billions+ of atoms in order to effect a motion at all, it's that the amount of motion here, _of the sensor_, is just an atomic diameter. If it takes billions of atoms to move the sensor, the sensor itself is comprised of trillions+ (I'd guess, decillions+) of atoms. That we can "hear" the movement of just a single atomic diameter in distance of such a relatively massive structure doesn't seem amazing? I'm even more amazed that with such sensitivity, we aren't inundated with the noise from that, and can still hear normal volume sounds, whose movement must be absolutely massive compared to a single atomic diameter. I guess that effect is similar to the eye's. We cannot see a single photon in the presence of daylight. That only works in a darkened (pitch black in this case) room.

vjulian 4y ago

Genuine question: I’ve never taken LSD, but could some of its purported visual effects be explained as seeing photons via a depressed neurological filter?

LastTrain 4y ago
It is too bad the risks (criminal) are so high, LSD is an interesting enough experience that is worth trying at least once.
- mlindner 4y ago
  As someone who's never taken LSD and have talked to people who have, the number of times people express irreversible changes in how they think about things just from trying things once makes me think that it is not a very good idea to even try once. A chemical that causes irreversible random brain chemistry changes from a single use sounds like damage has happened and the user simply interprets that damage as an improvement of some sort. In the same way we used to cut out pieces of people's brains in the early 1900s and they would report more comfortable lives.
  - culi 4y ago
    LSD as well as psilocybin and a number of other natural psychedelics exhibit what's called a "classical psychedelic effect". I've tried a couple of them and consider LSD to be essentially the same It's worth noting that psychedelic fungi and plants are absolutely GLOBAL in distribution. Psilocybe can be found basically everywhere and there are a few related genera of fungi that also contain psilocybe. South America has a ton of psychedelic plants, but even Native America has some psychedelic grasses used to make "prairie ayahuasca". There's a ton of psychedelic cacti species including a very famous cactus (which I shall not name) that very few people know is psychoactive because the Native American group that uses it wishes to protect it from being overharvested. Some very poisonous plants like nightshade are also psychoactive. The fact that indigenous people throughout the Americas have managed to develop techniques to safely utilize these plants suggests close study and attention. There's a famous "Jesus was a mushroom" take by an archeologist that the early cults from which Christianity emerged dreamed up Jesus through their use of psychedelic mushrooms. The take doesn't hold much water but what is agreed upon is that even those groups regularly consumed psychedelics. Christmas also comes from cultural usage of psychedelics. Amanita muscaria, perhaps the most famous mushroom, is that red mushroom with the white spots (kinda looks like Father Christmas, don't it). It's poisonous and very psychedelic. To consume the psychoactive, indigenous European groups would drink reindeer piss. The piss neutralizes the poison but leaves the psilocybin. And so these reindeer allow Amanita muscaria to fly around dropping gifts for us. I could go on about sweatlodges, the tens of thousands of different psychedelic plant species known, and other ways psychedelic experiences shaped cultures around the world. My point is is that psychedelics are likely a universal cultural norm. In fact I'd argue we're living in a very strange culture where their use is not very widespread. I personally didn't try a psychedelic until after highschool. My first experience cured my debilitating social anxiety and I went from my heart racing any time anyone talked to me to organizing community events within the span of a year. I've had similar effects with my depression and other mental health struggles. I'm not trying to suggest they're for everyone, but I'd say the fact that there's "irreversible changes in how [you] think about things" is exactly what makes them such a useful medicine

qikInNdOutReply 4y ago

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

michaelcampbell 4y ago
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?
- qikInNdOutReply 4y ago
  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.

dekhn 4y ago

This page is old- it's been established people can see single photons. I know a scientist at Berkeley who did this exact experiment.

notfish 4y ago

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/blackbody_calculator/blackbody.php https://www.spectralcalc.com/blackbody_calculator/blackbody....

deleted 4y ago
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Sharlin 4y ago
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.
- gnulinux 4y ago
  What if you're in a pitch-black dark room of mirrors? Would it make it more likely notice the other person through photons?
  - Sharlin 4y ago
    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.
- notfish 4y ago

yu3zhou4 4y ago

> sensors in the retina can respond to a single photon. But neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms Like an activation function in artificial neural networks

Sprakle 4y ago

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/ 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.

gameshot911 4y ago

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 https://www.wikiwand.com/en/Blue_field_entoptic_phenomenon

moistly 4y ago
There are single-cell organisms up to an inch or so in diameter. https://en.m.wikipedia.org/wiki/Valonia_ventricosa https://en.m.wikipedia.org/wiki/Valonia_ventricosa There are even visible bacteria! https://www.science.org/content/article/largest-bacterium-ever-discovered-has-unexpectedly-complex-cells https://www.science.org/content/article/largest-bacterium-ev...
maxbond 4y ago
One time a fellow intern and I were isolating single colonies of spirulina (many bright green algae* cells in a spiral [1], which is roughly 50 micron wide and maybe 15 micron tall, if memory serves), and I got one colony on it's own in a drop of water under a microscope (using a special pipette & the wick effect for suction). I took my eyes off of the eyepiece, and looked at the drop of water. I could see a bright green dot in the middle, right where I'd left my spirulina. I asked the other intern if they saw it too; they did. It was a magical moment. I had a similar experience when I was doing something with H. pluvalis (an algae that turns bright red and forms a film under stress [2], you've likely seen it in a birdbath or shallow puddle) and I put this film under the microscope, looked at it, and saw it was 1 cell thick. I'm confident if I had gotten a single one of those cells on it's own, I could've seen it. Can't confidently remember how big they were, but large on the scale of cells you find in a drop of water (maybe 25 micron in diameter?), but smaller than most animal cells. * Technically a cyanobacteria and not an algae, but if a lay person looked at a pond of it they'd say to themselves, "that's a bunch of algae" [1] https://images.fineartamerica.com/images-medium-large/1-spirulina-cyanobacteria-light-micrograph-sinclair-stammers.jpg https://images.fineartamerica.com/images-medium-large/1-spir... [2] https://images.squarespace-cdn.com/content/51511761e4b0323b0cd00500/1497555135471-V5Q9LG80KNZGOHWTBCN4/DSC02300.JPG?format=1000w&content-type=image%2Fjpeg https://images.squarespace-cdn.com/content/51511761e4b0323b0...

fractallyte 4y ago

This is a snippet from Scientific American, October 1993, 50 and 100 years ago: October 1893: "It now does not seem improbable that, when by the power of thought an image is evoked, a distant reflex action, no matter how weak, is exerted upon certain ends of the visual nerves, and, therefore, upon the retina. Helmholtz has shown that the fundi of the eyes are themselves luminous, and he was able to see, in total darkness, the movement of his arm by the light of his own eyes. This is one of the most remarkable experiments recorded in the history of science, and probably only a few men could satisfactorily repeat it, for it is very likely that the luminosity of the eyes is associated with uncommon activity of the brain and great imaginative power. It is fluorescence of brain action, as it were." --Nikola Tesla, in a paper read before the Franklin Institute Can anyone - uh - shed some light on it?

duskwuff 4y ago
For context, it's important to remember that Tesla was an accomplished experimental scientist, but he was prone to wild claims and flights of fancy which weren't based in scientific reasoning, even by the standards of the day. Not everything he said or wrote was literally true. > Helmholtz has shown that the fundi of the eyes are themselves luminous... This is erroneous -- no part of the eye emits light. Some animals, like cats and dogs, have retroreflective surfaces within the eye, and I wouldn't be surprised if that's what got Helmholtz (or possibly Tesla in quoting him?) tripped up. > ... and he was able to see, in total darkness, the movement of his arm by the light of his own eyes ... If this experiment occurred, it's far more likely that there were low levels of light present in the room, and/or that the experimenter was imagining the position of his arm as sensed through proprioception. > ... it is very likely that the luminosity of the eyes is associated with uncommon activity of the brain and great imaginative power And this is one of the flights of fancy I was talking about. :) There's no basis for this claim.

tlhunter 4y ago

This reminds me; if I look at a red LED in the dark, my eyes seem to add a purple shape around it, kinda like an infinity symbol. Does this happen to anyone else?

Derbasti 4y ago
I thought that's astigmatism, which distorts different wavelengths differently. Some LEDs have very few, widely separated wavelengths, and your astigmatism separates them.

stainablesteel 4y ago

its an interesting space to try and inspire engineering from biology, a quick search says eagles have the best eyesight i just saw a video about a dog's nose being used for particle detection in crime scenes, i'm wondering if there's already been, or when there will be work on recreating the basis of an eye as a detector or if maybe this has already been subverted by lens manufacturing

beefield 4y ago

As a layman in modern physics, I struggle to understand the shape and size of a photon as a wave. As far as I understand, e.g double slit experiment would implicate that the size of a single photon wave would be clearly macroscopic in nature. Is there any text available that would discuss this without a need of Ph.D in physics?

brummm 4y ago
So I think one conceptual issue that makes this so hard to understand is that we tend to imagine a physical reality where light is a wave or a stream of particles. However, that is absolutely wrong. Both the wave and the particle concept are mental models we have constructed to explain light as a phenomenon and to calculate things. And in their respective realms, they work exceptionally well. However, where things start to get complicated is when the models give different results. However, this doesn't mean something is wrong, it just means the mental model we use to make sense of nature is stretched beyond where it's applicable. So to preface, light is neither a stream of particles or some sort of wave like the ones on the surface of water. Light is the excitation of the electromagnetic field and is described by a quantum field theory called the standard model. So. photons are 0-dimensional (so no size) excitations of this field and they interact with matter like our eyes, surfaces, sensors in an experiment and produce physical effects.

amelius 4y ago

How many electrons does it take for a human to feel a static electricity discharge?

fallingfrog 4y ago

If you look carefully, you can see a little bit of noise in very low lighting, probably because of the small number of photons. However, the amount of noise you see with your eyes is orders of magnitude less than the camera in your phone picks up. Camera technology is still well behind biological cameras. To get similar performance you’d probably have to cool the ccd with liquid nitrogen.

hulitu 4y ago

I was expecting sonething like "if it has enough energy ... " , but the article is great in explaining vision.