Chemiotics: The unbearable weirdness of quantum mechanics (with apologies to Kundera)

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Much of the training of budding neurologists in the 60s was concerned with how to perform a good neurological exam and interpret the results. Various constellations of abnormalities pointed to different regions of the nervous system and the history often told us what sort of trouble was present there. Essentially we were inferring abnormalities of structure from abnormalities of function.

Why not just look? We had only two ways to do so back then (1) sticking a needle in an artery and injecting a dye which X-rays couldn’t pass through (radio-opaque dyes – if you don’t already know what they are, think of what you’d want to synthesize) – this had a 1–5 % stroke rate at the time (2) injecting air via a spinal tap and taking X-rays subsequently (I’m not kidding).

The advent of computerized axial tomography (CAT scans) and MRIs (magnetic resonance imaging) changed all that. We were able to directly look at structure without a decent exam. Not only that – problems could be picked up before they produced changes in function (e.g., earlier).

Naturally, neurologists were panicked, thinking that we would soon become the buggy whip manufacturers of medicine. Somehow, telling my colleagues that MRIs showed the essential correctness of quantum mechanics didn’t help, producing only blank stares and decreased referrals.

Telling the man in the street that spectroscopy alone shows the correctness of quantum mechanics (sharp absorption and emission lines show that only certain energies of molecules and transitions between them are permissible) just doesn’t cut it. But everybody knows what an MRI is.

Forget the wave nature of light (for today). Think of photons as baseballs travelling at various speeds (I know, light has but one speed and its frequency determines its energy just as the speed of a baseball determines its kinetic energy). Throw the baseball at a window. If you throw it fast enough (high kinetic energy) it goes through, if you throw it slowly it doesn’t. Everybody knows that.

Not so with the light used for MRI. They are radiowaves and contain around one millionth of the energy of visible light, yet they go right through our skull and brain rather than bouncing back. Why? The only way we can get them absorbed by our brains is to place ourselves in a strong magnetic field in the scanner. The magnetic field essentially creates two new energy levels so close together in energy that the tiny energy difference between them matches the energy of the radiowave permitting it to be absorbed. Without absorption, no pictures. Certainly counterintuitive, but used every day all over the world. Quantum mechanics rules (but weirdly).


Stu Cantrill

Chief Editor, Nature Chemistry, Springer Nature