Can a Human See a Single Photon? (1996)
math.ucr.edu
math.ucr.edu
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.
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?
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.
+---------------------+
| Heralding detector |
| |
+---------------------+
/
/
+----------+ / Photon 1
| | /
Laser beam ------> | Crystal | ------------> Outgoing laser beam
| | \
+----------+ \ Photon 2
\
\
To human eyeLeonard 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.
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.
Would humans sense an interference pattern? 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.
Does that process also count as an "interaction", or is that different to what happens when the photon hits the retina?
Don't forget that the interference pattern is a statistical one, you need to average over a number of photons to "see" it emerge.
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.
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.
https://physics.stackexchange.com/questions/34993/reversing-...
[1] https://physics.stackexchange.com/questions/147826/how-much-...
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.
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.
You can check my math here if you want to convince yourself: https://www.spectralcalc.com/blackbody_calculator/blackbody....
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.
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.
...in a vacuum, of course!
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
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.
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?
Obviously everything is a blackbody, but those photons surely can't be 'single' visible ones.
Luminous beings are we, not this crude matter.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.)
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.
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.)
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.
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. :)
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.
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
In post-modern (quantum) physics and the discovery/conception of photons by Einstein, they have been brought together as the same concept (and more generally for non-light by de Broglie as quantons).
A wavepacket is probably a good way to think of single quantons : https://upload.wikimedia.org/wikipedia/commons/9/92/Wavelet....
N.B.: In sufficiently weird and/or high energy situations, photons can also interact with one another ! :
https://www.quora.com/Do-photons-interact-with-other-photons...
(via a quick google: https://www.newscientist.com/article/2276384-your-finger-can...)
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
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).
We need 5-9 photons to merit perception. For us to see one atom, we would need 5+ photons to bounce off of that atom directly to our eyeballs in 100ms. Since both these things are so tiny, most photons miss the atom altogether. It's only when you have large numbers of atoms, densely arranged, that conditions create visibility.
I 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
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...
There are even visible bacteria! https://www.science.org/content/article/largest-bacterium-ev...
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?
> 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.
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.
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
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.
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.
also, I don't think it's fair to say "the upside doesn't seem that big" when the vast majority of users report life-alteringly positive experiences, including Steve Jobs and many many others:
https://www.google.com/search?q=Steve+Jobs+lsd
https://www.google.com/search?q=famous+people+who+have+tried...
hope this helps.
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.
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.
Like an activation function in artificial neural networks
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.
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.
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.
pretty amazing
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.
Fun video by Steve Mould on the subject: https://www.youtube.com/watch?v=FSYE1T5d9jc
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
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.