Frogs See Photons

Arrgh, I have really wanted to hop back on the blogging bandwagon, but this travel/work reality has made it tough. Next week, though, I plan to be blogging like a banshee. If banshees could blog. And if, when they did blog, they did so frequently and with enthusiasm.

Just got back from the North Carolina Science Festival in Charlotte, where I talked about the Higgs boson. You can find some live-tweeting of the event by searching the hashtag #HiggsTalk. Among all the deep and inspirational points I tried to make, one seemed to create the biggest impression: frogs can see individual photons.

This is an example I got from David Deutsch’s book The Fabric of Reality. It’s an attempt to connect our underlying fundamental description of the world, which is in terms of fields, to what we see when we make a quantum observation, which is in terms of particles — at least if we look closely enough. Deutsch’s point is that human vision is a bit too crude to detect just one photon at a time, but frogs (and presumably other animals) are sensitive enough to see single photons.

Such a fun and quirky fact naturally raised the skeptical instincts of some folks in the room, and to the internet they all went. Is it actually true?

Turns out, to the extent that a few minutes of googling around can reveal, it’s not an easy question to answer. It’s certainly true that the photoreceptors in a frog’s eye are sensitive enough to trigger on individual photons — indeed, researchers are using frog’s eyes to help fashion hybrid light-detector technology. But on the other hand, human photoreceptors are also sensitive enough to trigger on individual photons — and yet, we don’t as a matter of fact actually see photons one at a time. The presumption is that we would be seeing too much noise if our brains actually responded to such low levels of light; in practice, it seems to take several dozen photons before a human will say they see something.

So maybe the same is true for frogs? I wasn’t able to find a definitive-sounding word on the subject, but there is good reason to believe that frogs are at least much more sensitive than we are. The point is that noise we filter out is roughly proportional to body temperature. In a warm-blooded creature, simple thermal motions are constantly jostling the rhodopsin molecules in the eye, which could mimic the act of seeing something. A cold-blooded frog isn’t as susceptible to this problem, so its vision can be usefully much more efficient at low light levels.

Of course none of this matters to the actual point being made in my lecture, which is that light is really a vibration in the electromagnetic field, but careful observations (be they by frogs or artificial photodetectors) reveal individual energy packets call photons. It’s not that the field is “made of” photons, it’s that what we see when we perform measurements in a world governed by quantum mechanics is different from what the world is “actually made of,” to the extent that it’s okay to think about such a concept. Which, with all due respect to my croaky amphibious friends, is more amazing to me than all the eyeballs in the world.

  1. Shouldn’t it be possible to test this? I thought we could generate single photon sources already.

    Lock someone in a dark room with the source. Ask them to say when they think they see something. See if it correlates with the emission of photons. It’s just like ESP tests. Only – yanno – real.

  2. Two additional details that might be of interest to those who want to think deeply about the problems discussed in this post:

    1. At the human threshold of visual perception, the “several dozen photons” (relevant literature suggests the number is closer to one dozen) can be spread out over hundreds or thousands of rod photoreceptors in a brief flash, so that no rod is likely to capture more than one photon at a time. Thus, the perceptual signal is composed of rod single photon responses (lasting about 200 ms each), which converge in downstream retinal neurons (Hecht et al., 1942).
    2. At mammalian body temperature, the rate of shot noise (thermal activation of rhodopsin) is remarkably low given the ~100,000,000 rhodopsin molecules per rod — about one thermal activation per 100 seconds per rod (Baylor, Nunn, and Scnhapf, 1987). Downstream convergence of rod signals is rather severe, however, so that noise will be integrated across thousands of rods.

  3. In optics, you learn that the luminance of an image coming out of a telescope is less than or equal to, but never greater than, the luminance of the object itself. Hence you should never be able to see an extended deep sky object in a telescope that you can’t see directly with the eye. But clearly this is false. Anyone who’s looked through a telescope will tell you that they can see hundreds of galaxies, clusters, and nebulae that they can’t see with the unaided eye. It all comes down to the brain and picking the signal from the noise. A small, dim object covers just a few rods on the retina with weak photons -noise to the brain. But when you magnify the image, many more rods report the same weak signal. Hence the brain decides it IS a signal vice noise and produces an image. Gotta remember that the eye AND the brain are part of the optical system.

  4. Sean,
    As someone who has always had a great interest in the entirety of the electromagnetic spectrum, I still have no idea, by however many magnitudes, of the approximate number of photons we may see (or may pass through the pupil) in even the tiniest detectable flash of light. Or to another extent, what might be the total number of photons emitted in, say, a camera flash?

    I have never had any concept of quantity in this regard.

  5. Adding to the two previous comments, I remember being told (during an undergraduate course in nuclear physics — sorry, no references) that a human eye can indeed detect a single photon. This was known since the early days of nuclear physics — when people were being used as parts of scintilator detectors for low activity nuclear radiation.

    Namely, after spending 3-4 hours in a completely dark room, the experimenter would uncover the scintilating crystal detector mounted on one of the walls of the room, activate the stopwatch and start counting scintilations. The scintilators would typically emit one photon per particle detection, and the experimenter would count individual photons, a dozen per minute or so.

    Of course, with the advent of electronics, photomultipliers and computers to do the counting, this practice is now long gone. :-)

    But as a curious side-effect of some history of experimental nuclear physics, it is well known in nuclear-physics circles that humans can indeed detect and count individual photons, given proper preparation, environment and single-photon sources.

    Best, :-)

  6. I plan to be blogging like a banshee. If banshees could blog.

    So… lots of wailing, and a blog post means someone nearby is doomed to die in the near future? Yikes.

  7. For those of you who’d like to learn more about the detection limit of vision, I strongly recommend Chapter 2 (“Photon Counting in Vision”) in Bill Bialek’s fantastic book “Biophysics: Searching for Principles”.

  8. “…light is really a vibration in the electromagnetic field, but careful observations (be they by frogs or artificial photodetectors) reveal individual energy packets called photons. It’s not that the field is “made of” photons, it’s that what we see when we perform measurements in a world governed by quantum mechanics is different from what the world is “actually made of,” to the extent that it’s okay to think about such a concept.”

    It sounds from this as if photon detection and photons themselves are artifacts of the observational set-up. That is, the field-like character of the world as it is “in itself” (per QM), merely *appears* particle-like under certain conditions. But presumably the generation of such appearances is itself accounted for by physical theory, such that they are also what the world is “really like” in some respect.

  9. I found your point interesting about our filtering above the level at which rhodopsin would be triggered. This reminds me that our ears are almost sensitive enough to hear the impact of air molecules from Brownian motion at room temperature. I wonder in what ways our body self-limits the sensitivity of our other senses.

  10. 20 deg C (the upper end of the temperatures used for frogs in that 1988 article) is 3.5% higher than 10 deg C (the lower end of the temperatures). I’ve not read the original article, but was the work careful enough to be able to claim that frogs are 3.5% less sensitive to photons at the higher temperatures? To capture enough samples to really claim that would seem to take _years_.

    Humans are only about 8% warmer than these particular frogs. If sensitivity to individual photons is roughly inversely proportional to temperature, then… what? The discrete nature of the events have to be deconvolved from the statistics, but shouldn’t humans trigger on 11 photons when frogs trigger on 10? Something seems VERRRRY suspicious about claiming “noise we filter out is roughly proportional to body temperature”.

  11. Regarding human detection of individual photons, here at the University of Illinois, Professor Paul Kwiat is in fact working on this very problem:

    From the little I’ve heard of the project, my understanding is that they think the limits of human perception are not really that rigorously mapped yet, and they hope with some trickery to stop our brains from filtering out their single-photon signals.

  12. The comment I posted earlier was not asking about what any minimum detectable quantity of photons may be. It was more of a naively general question as to how many photons might be emitted by something familiar such as a 100w light bulb, or maybe even a candle flame, in a time frame such as one second. I have no idea as to what magnitude of photons this might represent. Even a ballpark figure would be nice as I have no clue whatsoever. I assume the number to be massive. Would anyone know?

  13. The reason a person generally can’t see a single photon is both from what is described above and the fact that you damage your eyes throughout your life. Like your ears; there is a decibel limit that you can’t go past without permanently damaging your ears, generally somewhere between 125 – 160 dB. When you attend some sort of concert or experience something extremely loud, you notice that your ears ring for a while after the event because they’ve been damaged and your brain is raising the noise floor while trying fix what it can. The problem being that once you damage your senses to a certain degree, you simply do not have the ability to completely repair them. If you take a second to listen in a quite room, I’ll bet you can hear a very high pitched background noise, and that’s the permanent damage you’ve already done.

    Or you just live a naturally noisy life and the noise floor is always high as a result. An example with light is when you stare at a light bulb or the reflection off of a car window, or the sun of course, then the image is literally burned into your retina for a few seconds / minutes. You know what happens when you tell kids they shouldn’t stair at the sun? They stare at the sun to see what happens. The sun is friggin’ amazing; every kid stares at it at least once, so we’ve all got some permanent damage to some extent.

    But like you were saying, if we were able to see a single photon and not have the ability to turn it off, then we would basically loose our sense of sight or suffer in some other way because there would be too much noise. Luckily, our brain takes care of the automation of our senses and turns that crap down when we’re in traffic or when our spouses ask us to do something that they could easily do but just don’t feel like doing; WHAT, so I’m just your slave now? It’s sexist to assume that a woman can do the same jobs that men do yet when it comes time to take the trash to the street every week I have to do it…

  14. Ron– It’s easiest to think in terms of energy. The energy of a single photon is E=hf, where h is Planck’s constant and f is the frequency. If you have a light bulb emitting a characteristic frequency with a certain amount of energy per time, it’s straightforward to figure out the number of photons. A 100W bulb (which converts only about 2% of its energy into photons, the rest going into heat) emits approximately 1018 photons per second.

  15. What’s your take on the baryon asymmetry problem? Do you have any articles written on this?

  16. I have read that some human eyes may have detected neutrinos, as a blue flash, from the 1987 supernova. How would that be possible?

  17. I don’t understand how this post doesn’t contradict what Sean said here. (Note: I’m probably one of the less physics-educated people commenting here…)

    If infrared photons have the wrong amount of energy to activate rhodopsin, then how can this be true: “In a warm-blooded creature, simple thermal motions are constantly jostling the rhodopsin molecules in the eye, which could mimic the act of seeing something.”?

    I understand (I think) that infrared radiation is different from molecules simply bumping into each other, but wouldn’t those “bumps” still have to have just the right amount of energy to activate rhodopsin?

  18. “A 100W bulb (which converts only about 2% of its energy into photons, the rest going into heat) emits approximately 10^18 photons per second.”

    Don’t you mean 2% of the energy into visible light photons, the rest being emitted as infrared photons?

  19. At Neil.
    I was intrigued by this question so I did a rough back of the envelope calculation. In 1987 there were 5 billion people on earth. The average human eye has 6 cm^3, so the total volume of eyeball fluid (assuming 2 eyes per person) is 60,000 m^3 or 60,000 metric tons of water. (That is going to give me nightmares tonight) Kamiokande-II has a total mass of water of 3000 tons and detected 11 anti-neutrinos that day. So theoretically 220 events might have been seen that day. Of course it would be much weaker than any light you would be seeing, and background radioactivity would overwhelm any neutrino signal, so the odds of anyone actually noticing a blue flash and have it be a real neutrino interaction would be nearly impossible.

  20. Thanks, Chris. Great explanation! It is similar to the fact that the static on an untuned tv screen is in some small part caused by the CMB, but you can’t tell the signal from the noise.

  21. Chris– Yes, that would have been a more accurate way to say it.

    Tim– I’m not an expert, but I presume the point is that thermal motions have a spectrum of energies, and some fraction of them would have to activate the rhodopsin.

  22. Reading made me think specifically how an animals frontal lobes first, specifically people with serious mental disorders. In case studies, reports of seeing subatomic particles, mostly report the obvious visual disturbances (Walls melting, objects ‘breathing’, black and white dots.) Indeed alot of this has to do with a decrease or increase of serotonin and other chemicals in the brain, but the corralation of frogs seeing photons and a schizophrenic seeing atom-like things or things reported that require some warped interrpetation of science, I personally think theres a strong connection due to light dissemination in both situation, say the pupuls dialated in a mental ill patient and the frogs ablity to see photons with no chance of having a mental illness.
    I probably sound insane. I dont think paranoid schizoprenia effects frogs. It would be cool to find out, for sure.

  23. *reading this made made me think of how an animals frontal lobes work…

    What small keys on my Android! With no edit option! Die Samsung!

  24. Regarding the thermal activation of vision — a number of things other than visible light photons can make you “see” things. Surely you have rubbed your knuckles in your eyes and seen irregular shapes and flashes of “light”.