I have a question not about artificial photosynthesis, but about "ordinary" natural photosynthesis. I was reminded that we have a thread on photosynthesis when I saw this in another thread:
Thermodynamics is like that. We lose energy at every step ...
It is my understanding that photosynthesis and some of the other metabolic chains in living cells are NOT like that: the losses in each step are often quite small. (Do these processes avoid losses by operating molecule-by-molecule instead of macroscopically?)
Here is my understanding, almost certainly confused, of the first step in photosynthesis. Please help correct it rather than just laughing at me.
A photon is absorbed by a Mg (magnesium) atom in a chlorophyll molecule. This produces an "exciton" (electron and electron-hole pair), the two pieces of which start bouncing hither and yon. Or rather, it produces a
quantum superposition of such excitons which, like the qubit data in a quantum computer, can explore various paths. The excitons bounce (like a ping-pong ball) among the nearby paddle-shaped chlorophyll molecules. In classical physics, the electron and electron-hole would usually find and cancel one another before long, producing some waste heat. But "if lucky" the electron of an exciton might eventually reach the Mn (manganese) atom in a reaction center and trigger the first key step in the photosynthesis sequence.
The electron reaches ("tunnels to") and interacts with the Mn atom MUCH more often than would be expected if it were following a "drunkard's random walk." Like the qubits following Grover's Algorithm in a quantum computer, the exciton created when a quantum of sunlight hits a living green leaf seems to be attracted toward a useful goal!
There is a famous story about this comparison between a quantum computer and a blade of green grass. I found part of the book excerpt without paywalls or Google-walls elsewhere:
“Molecular biology and quantum mechanics developed in parallel, rather than cooperatively. Biologists hardly attended physics lectures and physicists paid little attention to biology. But in April 2007, a group of MIT-based physicists and mathematicians who worked in a rather esoteric area called quantum information theory were enjoying one of their regular journal clubs (with each member taking a turn at presenting a new paper they had found in the scientific literature) when one of the group arrived with a copy of the New York Times carrying an article which suggested plants were quantum computers (more on these remarkable machines in chapter 8). The group exploded into laughter. One of the team, Seth Lloyd, recalled first hearing about this “quantum hanky-panky.” “We thought that was really hysterical . . . It’s like, ‘Oh my God, that’s the most crackpot thing I’ve heard in my life!’” The cause of their incredulity was the fact that many of the brightest and best-funded research groups in the world had spent decades trying to figure out how to build a quantum computer, a machine that could carry out certain calculations much faster and far more efficiently than the most powerful computers available in the world today. It relies on digital bits of information that are normally either 0 or 1, to be both 0 and 1 simultaneously and therefore able to pursue all possible calculations at once—the ultimate in parallel processing. The New York Times article was claiming a humble blade of grass was able to perform the kind of quantum trickery that lay at the heart of quantum computing. No wonder these MIT researchers were incredulous. They might not be able to build a working quantum computer but, if the article was right, they could eat one in their lunchtime salad!”
My question is:
What the F***?!!! Can anyone explain the "goal-oriented" behavior of photosynthesis (or Grover's Algorithm for that matter) in an
intuitive way? (Don't just summarize quantum mechanics. I've already read seven versions of
Quantum Physics for the Intelligent Layman and am starting in on
Quantum Physics for the Utter Imbecile.)
