[HN Gopher] Can a Human See a Single Photon? (1996)
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Can a Human See a Single Photon? (1996)
Author : omarfarooq
Score : 146 points
Date : 2023-01-20 10:58 UTC (12 hours ago)
(HTM) web link (math.ucr.edu)
(TXT) w3m dump (math.ucr.edu)
| Eddy_Viscosity2 wrote:
| You can't see a photon, but you can taste a proton.
| metacritic12 wrote:
| Is this true or just a pun? Like your acid taste buds are
| literally sensitive enough to register a single H+ binding?
| elil17 wrote:
| Our detection threshold for citric acid is like 0.04 mM
| (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.
| Eddy_Viscosity2 wrote:
| 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
| tmearnest wrote:
| 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
| amelius wrote:
| Maybe the OH- parts neutralize the H+ sensation?
| [deleted]
| dekhn wrote:
| This page is old- it's been established people can see single
| photons. I know a scientist at Berkeley who did this exact
| experiment.
| amelius wrote:
| How many electrons does it take for a human to feel a static
| electricity discharge?
| beefield wrote:
| 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 wrote:
| 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.
| tlhunter wrote:
| 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 wrote:
| I thought that's astigmatism, which distorts different
| wavelengths differently. Some LEDs have very few, widely
| separated wavelengths, and your astigmatism separates them.
| weinzierl wrote:
| What is the comparison to modern camera sensors nowadays? Can
| they detect single photons?
| formerly_proven wrote:
| 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 wrote:
| 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.
| callesgg wrote:
| 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.
| cozzyd wrote:
| I'm sure you could "see" a TeV photon showering in your eye?
| mk_stjames wrote:
| 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
| stainablesteel wrote:
| 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
| fallingfrog wrote:
| 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.
| vjulian wrote:
| 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?
| wyager wrote:
| No, eliminating the photon count filter would just make your
| vision look noisy/staticky in dark conditions.
|
| Most of the effects of LSD (patterns etc) are actually pretty
| well understood. It's probably caused by a stable geometric
| pattern generated by a reaction-diffusion process in your
| visual cortex. https://plus.maths.org/content/uncoiling-spiral-
| maths-and-ha...
|
| Related to your question (although minus drugs) - I very
| recently figured out how to "turn off" shot noise suppression
| in my visual processing. I woke up a few months ago and was
| struggling to understand something I was looking at in the room
| (it was a low-contrast scene with confusing shadows) and
| something clicked, and all of a sudden I could see the shot
| noise from my eyes. Very bizarre! I can now detect it just by
| paying close attention in moderately dim conditions.
| mrguyorama wrote:
| In the dark I perceive an extremely "noisy" visual field,
| with lots of different color dots like on a shitty camera
| sensor at low light. Is that not normal?
| LastTrain wrote:
| It is too bad the risks (criminal) are so high, LSD is an
| interesting enough experience that is worth trying at least
| once.
| mlindner wrote:
| 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.
| [deleted]
| asah wrote:
| ...or the opposite.
|
| the VAST majority of people who have taken LSD under
| reasonable circumstances have an incredible, life changing
| experience.
|
| I personally know 100s who have, not a single one of them
| regrets it.
| mrguyorama wrote:
| The big problem for me is that everyone I know who has
| done psychedelics and enjoyed it says "yeah it was fun,
| not a big deal" while the people who have bad trips have
| _REALLY FUCKING BAD TRIPS_. The upside doesn 't seem that
| big, and the downside is possibly going insane for a few
| years like someone I know of. It's an extremely small
| chance and you basically have to be predisposed and
| making poor choices about your setting, but it's still
| there.
| klyrs wrote:
| Yeah, if that's your mindset, stay away from
| hallucinogens!
| moloch-hai wrote:
| Any profound experience (and many a mundane one) causes
| permanent change. We call that "memory".
|
| But people who are borderline schizophrenic should never do
| LSD. They may tip into full-on psychosis. They _should_ ,
| however, take MDMA, and experience _quiet_ , maybe for the
| first time.
| culi wrote:
| 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
| LastTrain wrote:
| Who said it altered brain chemistry or that it would change
| you forever? I called it an interesting experience, that is
| it. Maybe for some people it is life changing, but then
| again some people say seeing the northern lights was life
| changing.
| lukan wrote:
| "A chemical that causes irreversible random brain chemistry
| changes from a single use sounds like damage has happened"
|
| From what I understand, LSD does not change your brain
| chemistry directly, but you make deep experiences, and
| those might change your deep thinking (and therefore the
| brain chemistry). Opening up your perception to other
| information channels. But sure, that doesn't mean it must
| be beneficial. And whether it is a damage I would say
| depends on the outcome.
|
| I know people where LSD was beneficial (solving cluster
| headache or overcoming addiction) and people where it
| clearly wasn't (became spaced out weirdos). But everything
| you do or don't do, comes with a risk. If LSD does not
| appeal to you, then just don't do it.
| gameshot911 wrote:
| 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
| moistly wrote:
| There are single-cell organisms up to an inch or so in
| diameter. https://en.m.wikipedia.org/wiki/Valonia_ventricosa
|
| There are even visible bacteria!
| https://www.science.org/content/article/largest-bacterium-ev...
| maxbond wrote:
| 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-spir...
|
| [2] https://images.squarespace-
| cdn.com/content/51511761e4b0323b0...
| zerotheliger wrote:
| have you seen them? well now you have :3
| sbaiddn wrote:
| 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.
| Asraelite wrote:
| Is this also acknowledging the more recent study linked in one
| of the top comments?
| high_byte wrote:
| Not 1 but 5-9 photons, still less than billions :)
|
| pretty amazing
| bitexploder wrote:
| It's cool our brains have that as a heuristic for whatever
| reason. Wonder if you could train yourself to see 1-3?
| jvanderbot wrote:
| If you tune up your sensitivity to 1-3 photons, you'll
| probably find a maddening TV white noise overlaid on your
| vision.
| kentiko wrote:
| Is there a link with Visual snow syndrome?
| rini17 wrote:
| I do see exactly such a noise in dark rooms, am not
| unnerved by it at all.
| jvanderbot wrote:
| 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 wrote:
| 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.
| ilyt wrote:
| So that would be yes but our noise filter in brain doesn't let
| us see it
| sbaiddn wrote:
| You are not a retina. You cant see photons.
| jll29 wrote:
| 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.
| notfish wrote:
| 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....
| [deleted]
| reef84_ wrote:
| Why "nonzero"? That's already explicit.
| Sharlin wrote:
| 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 wrote:
| 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 wrote:
| 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.
| de6u99er wrote:
| What if the room was made out of perfect mirrors?
| qwezxcrty wrote:
| 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
| ASalazarMX wrote:
| 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.
| dylan604 wrote:
| >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.
| devindotcom wrote:
| >Assuming a spherical isotropically radiating human
|
| ...in a vacuum, of course!
| Sharlin wrote:
| Of course.
| ASalazarMX wrote:
| In a perfectly dark room, isolated from cosmic rays, at
| zero Kelvin.
| bigbluedots wrote:
| Comments like yours are one of the reasons I love HN. Thank
| you!
| ask_b123 wrote:
| Do we have to account for blink rate?
| notfish wrote:
| 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.
| Someone wrote:
| I don't understand the physics, but clicking "Calculate"
| produces a graph with x axis between 8 and 12 mm.
|
| 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?
| eternauta3k wrote:
| Does an infrared photon have enough energy to stimulate a rod?
| thesimonlee wrote:
| Definitely. 20 years ago when homes were not littered with
| LEDs all over, I would wake in the night to total blackness.
| I could still find my way around the house just able to see
| based on what is I suppose a noise floor of ambient
| temperature. Go into the kitchen & behold the stove top
| emitting 'bright' white light, from I suppose a temperature
| of around 30degC /90F.
|
| Obviously everything is a blackbody, but those photons surely
| can't be 'single' visible ones.
| Luc wrote:
| Did you consider you were seeing (very low amounts of)
| visible light? Temperature had nothing to do with it.
| [deleted]
| yetanotherloser wrote:
| Gosh. I used to have pretty good night vision and recognise
| what you mean about "see based on the noise floor" - but I
| never had an experience like seeing the stovetop heat.
| Remarkable. I wonder how much variation there is between
| people? Unfortunately mine's faded with age so I don't
| think it's worth my trying to see if I can see that myself
| now...
| fractallyte wrote:
| I added a relevant comment elsewhere on this page:
| https://news.ycombinator.com/item?id=34457309
| yu3zhou4 wrote:
| > 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
| ljlolel wrote:
| Yes
| superposeur wrote:
| 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.)
| somat wrote:
| 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
| philipswood wrote:
| Welcome to the beauty of logarithmic scales.
| rini17 wrote:
| 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.
| gus_massa wrote:
| 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.)
| [deleted]
| teraflop wrote:
| 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.
| [deleted]
| pavlov wrote:
| 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.
| VLM wrote:
| 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.
| jonplackett wrote:
| Weird that humans can do this when our eyes aren't even that
| great at night vision compared to a lot of other animals.
| kayodelycaon wrote:
| 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. :)
| not2b wrote:
| Humans can echolocate:
| https://en.wikipedia.org/wiki/Human_echolocation
| mlindner wrote:
| 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.
| wyager wrote:
| 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).
| casenmgreen wrote:
| 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.
| Kinrany wrote:
| Similarly surpising, large cities can be seen from orbit if you
| know where to look.
| MacsHeadroom wrote:
| Seeing a photon is more like seeing a city on Pluto from
| Earth.
| darkerside wrote:
| 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.
| ranie93 wrote:
| biological op-amps
| mjburgess wrote:
| 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...)
| mhb wrote:
| 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.
| hgsgm wrote:
| 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
| formerly_proven wrote:
| Yes but
| qikInNdOutReply wrote:
| So, can a autist, with a deficient noise filter see it?
| michaelcampbell wrote:
| 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 wrote:
| 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.
| Sprakle wrote:
| 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.
| lebubule wrote:
| Recent experiments have put the claim to test
|
| Overview https://www.nature.com/articles/nature.2016.20282
|
| The actual work 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.
| xhrpost wrote:
| > To optimize this ratio in practice, we used the multi-pixel
| sensor of an electron multiplying charge-coupled device (EMCCD)
| camera as our idler detector (Fig. 1a). As the EMCCD can detect
| multiple photons simultaneously, it allowed us to identify and
| reject, that is, post-select, all events other than those where
| a single-photon pair was generated with a higher efficiency
| than with more traditional single-photon avalanche diodes
| (SPAD)
|
| How is it possible to both detect a photon and then allow it to
| travel to the human eye? Wouldn't detection require absorption
| of the photon?
| abdullahkhalids wrote:
| There is a process called spontaneous parametric
| downconversion (SPDC), where if you shine laser of frequency
| f at a crystal, it will with small probability p emit two
| photons of frequency f/2 (energy conservation in play here).
|
| If done correctly, the outgoing laser light and the two
| photons all travel in different directions and so can be
| separated and further directed using mirrors or optical fiber
| cables.
|
| Because the process is non-deterministic, what we usually do
| is direct one of the photon beams towards a "heralding" [1]
| detector, while the other is directed towards the optical
| setup where we need a single photon [2]. If at a given moment
| a photon pair is produced, then the heralding detector will
| click; which tells us that is also a photon currently in our
| optical setup.
|
| Finally, there is a ~p^2 probability that two photon-pairs
| will be produced at the same time by this process (and p^3
| etc). To eliminate this possibility, in this experiment their
| heralding detector can detect how many photons landed on it
| any given moment. So if they see 2 or more photons in their
| heralding detector, then they discard this run, because now
| there are multiple photons heading towards the human eye.
|
| [1] Herald as in the guy who announced that the King was
| approaching.
|
| [2] In this case, towards the human eye.
| 323 wrote:
| > How is it possible to both detect a photon and then allow
| it to travel to the human eye?
|
| Leonard Susskind explained it like this in one of his
| lectures (they are on YouTube, he's an excellent explainer):
|
| From the moment the photon is emitted, to the moment it's
| detected, the photon exists in entanglement with all the
| intermediary things it "touched". Only at the final location
| it's "absorbed" (with a probability). At the intermediary
| locations the probability ended on the low side so it passed
| through.
| mecsred wrote:
| "SPDC is a quantum optical technique in which correlated
| pairs of photons (called signal and idler) are produced
| probabilistically from a higher energetic pump photon in a
| non-linear crystal following energy and momentum
| conservation16,17 (Fig. 1a). By detecting one of the photons
| (idler) and sending the other (signal) to the observer's
| eye..." I suppose conservation of momentum allows them to
| guarantee pairs, in which case a photon in one direction
| guarantees a photon in the opposite direction.
| mensetmanusman wrote:
| 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 wrote:
| > 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 wrote:
| 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?
| phailhaus wrote:
| 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.
| yamtaddle wrote:
| What escapes my understanding with the whole "interaction
| is observation" thing is that I don't get how anything
| could ever _not_ be interacting with a whole lot of other
| stuff. Gravity and EM fields are everywhere. Even if
| photons are somehow immune to that (which, they 're not,
| because gravity can re-direct photons) it's my
| understanding that we can see the same interference
| patterns with particle streams of ordinary matter, and I
| can't for the life of me figure out how those could ever
| _not_ be interacting with basically everything remotely
| nearby, including the entire test apparatus.
| monktastic1 wrote:
| The short answer (as I understand it) is that decoherence
| is not binary. The gravitational field may be able to
| partially resolve the position, for example, and so you
| only get slight reduction in interference.
|
| https://physics.stackexchange.com/questions/34993/reversi
| ng-...
| phailhaus wrote:
| > Gravity and EM fields are everywhere.
|
| And now you understand why "quantum gravity" is such a
| big question in physics right now! We don't understand it
| all. I actually don't know anything about how EM fields
| affect superposition, perhaps someone else can chime in.
| kulahan wrote:
| The feeling of asking a question and realizing you've
| reached the edge of human understanding is very fun
| abdullahkhalids wrote:
| Yup, everything interacts with everything close by all
| the time.
|
| The important thing is by how much, and what sort of
| interference patterns can this produce.
|
| As it turns out, interference is quite hard to produce
| randomly because two fields only produce wavering
| patterns when their frequency and other parameters are
| almost equal.
|
| So yes, the ball you just threw to your friend is
| actually spread out over a whole region, that spread is
| about 10^-34 m so it impact is not visible at all.
| 323 wrote:
| When you look at the flickering pattern a laser pointer makes
| on a rough surface, that flickering pattern is similar in
| nature to the double slit interference, and you are seeing it
| with your eyes.
|
| Don't forget that the interference pattern is a statistical
| one, you need to average over a number of photons to "see" it
| emerge.
| photochemsyn wrote:
| Interesting study, though if you look at the results
| 'significantly above chance' looks to be 0.60 +/- .05, which is
| indeed better than 0.50 but not what anyone could call reliable
| detection.
|
| I wonder if trained owls could detect single photons, or if
| their night vision is based on just having much larger lenses
| that collect more light?
|
| It seems that all rods in retinas are activated by single-
| photon-absorption, it's just about how many have to be
| activated to generate a neural signal.
|
| https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4629483/
| kstrauser wrote:
| Owl eyes have really cool adaptations (see
| https://abcbirds.org/blog/owl-eyes/). If we can somewhat
| detect single photons, it seems very likely owls could do it
| much more repeatably.
| fractallyte wrote:
| 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 wrote:
| 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.
| danbruc wrote:
| 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-...
| sandworm101 wrote:
| >> 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 wrote:
| > something I heard
| danbruc wrote:
| Pun not intended but noticed while typing.
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