"Many people, when they encounter the words "quantum mechanics," go on the alert for esoteric paradoxes. And there are certainly plenty of those on offer. But sometimes, as my brilliant friend the physicist Sidney Coleman put it in a famous lecture at Harvard, quantum physics is "in your face."
Vision, especially our perception of color, is an outstanding example of that. Its most basic features reflect quantum principles and would be incomprehensible with them. This emerges clearly if we compare vision with hearing, where quantum effects aren't in play.
To hear, we sense pressure waves, commonly called sound waves, which impinge on our eardrums. Channeled through some impressive natural mechanical engineering, sound waves set off vibrations on the membranes of our inner ears. Those membranes work like the keyboards of a pair of inverse pianos: The sounds play the keys! Neurons fire in response to the keys' motion, generating the signals that our brains interpret as music, speech or whatever.
Two things are noteworthy in this process. First, we naturally deconstruct the incoming wave pattern into its component of pure tones. Mathematicians learned how to use equations to perform that feat in the 19th century and they call it Fourier analysis. It is similar to what spectrometers, ranging from Isaac Newton's prisms to sophisticated modern instruments -- but not our eyes -- do to separate light into its component frequencies.
Second, the response is graded: The louder a tone, the more forceful the motion of the corresponding key. This is like a proper piano, where the pressure on a key determines whether it gives a louder or softer response, as opposed to a harpsichord, whose strings can only be plucked at a constant volume.
Vision differs radically from hearing in both ways. Light vibrates faster than mechanical engineering can handle, but our visual apparatus can exploit the fact that it comes in packets of energy -- photons -- which can trigger changes in the shapes of molecules [1]. Now we're talking quantum theory.
For most people, color vision involves three kinds of receptor proteins in the cone cells of the retina. Photons either induce shape changes or don't; the effect is all-or-none, not graded. And, typically for quantum mechanics, they are chancy: We can't predict exactly whether a given photon will trigger a given receptor, but only supply odds. Those odds depend on the photon's wavelength -- that is, the color tone it represents -- and which type of receptor protein is involved.
What visual neurons get to "see," compared with the robust dynamics of the inverse piano of hearing, is more like the keyboard of a poorly tuned harpsichord with only three keys.
Since many different combinations of photons can produce the same pattern of probabilities, many physically distinct patterns of illumination produce the same color perception. In this way, we are all profoundly colorblind.
In dim light, we run into another limit of our vision, stemming from the unpredictable behavior of photons. When there are only a few photons to work with, the cone cells become unreliable, and we switch over to night vision based on different cells, the rods. The nocturnal harpsichord has only one key, so we perceive only shades of gray, lighter or darker, according to how frequently that key triggers.
Fundamental limitations of vision follow from its reliance on quantum processes. Yet such is the gush of information from the external world that even an attenuated stream supplies enough material for our brains to manufacture a splendid motion picture. Far from being remote and esoteric, quantum mechanics is very much "in your face" -- in your retina, to be precise." [1]
How do retinal isomers look is shown in this picture: https://commons.wikimedia.org/wiki/File:1415_Retinal_Isomers.jpg#/media/File:1415_Retinal_Isomers.jpg
2. REVIEW --- Wilczek's Universe: We Need Quantum Physics to See. Wilczek, Frank.
Wall Street Journal, Eastern edition; New York, N.Y. [New York, N.Y]. 20 May 2023: C.4.
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