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.