How many electrons does it take for a human to feel a static electricity discharge?
Can a Human See a Single Photon? (1996)
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Re: Can a Human See a Single Photon? (1996)
#122Recent 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.
> 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 av…
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.
Re: Can a Human See a Single Photon? (1996)
#123This 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 dar…
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.
Re: Can a Human See a Single Photon? (1996)
#124Earlier quoted context omitted.
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.
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…
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.
Re: Can a Human See a Single Photon? (1996)
#125Earlier quoted context omitted.
...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.
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 se…
Yeah, if that's your mindset, stay away from hallucinogens!
Re: Can a Human See a Single Photon? (1996)
#126Earlier quoted context omitted.
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 h…
What if the room was made out of perfect mirrors?
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
Re: Can a Human See a Single Photon? (1996)
#127If 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.
Re: Can a Human See a Single Photon? (1996)
#128Earlier quoted context omitted.
> 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 av…
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…
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Re: Can a Human See a Single Photon? (1996)
#129Earlier quoted context omitted.
> 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 av…
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…
[deleted]
Re: Can a Human See a Single Photon? (1996)
#130Earlier quoted context omitted.
> 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 av…
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…
[deleted]