I feel comfortable imagining atoms, the quantal units of matter. But I’m not sure how to think about photons, the quantal units of light. Things get stranger when quanta of light interact with quanta of matter. As Feynman says (in his famous QED): “Reflection and transmission are really the result of an electron picking up a photon, ‘scratching its head’ so to speak, and emitting a photon.”
This quote opens Chapter 6 of Sonke Johnsen’s Optics of Life. The opening paragraph is precious! “Light does not bend in a lens, it doesn’t bounce of the surface of glass, and it doesn’t spread out after passing through a small hole. It doesn’t even travel in a straight line. The happiest day of my scientific life came when I read Feynman’s QED and learned that refraction, reflection, and diffraction – things I had known since fifth grade – were all lies. More accurately, they were illusions. It appears that light bends, bounces, and spreads out. The illusion is so good that you can base solid mathematical predictions on them, but carefully thought and further experiments show that more is going on… They come from the fact that photons interact with one another in an unusual way. The interaction is often referred to as wave interference because it can be described mathematically much like the interference of water waves, but the reality of it is considered stranger.”
Johnsen will circle back to this strangeness in the final chapter of his book; but meanwhile he carefully goes through why we seem to observe light traveling in straight lines, and then bending, bouncing, or spreading out, under the appropriate conditions. I’ll say this. I wish I learned a bit more optics in high school physics (having never taken a physics college class). But it’s unlikely that learning it in school would have come with the interesting biological examples that Johnsen provides. So, if you ever wanted to learn some of these things, Johnsen’s book might at least be motivating. He tries to scale down the math to give you an intuitive feel for what’s going on. But some math is unavoidable, and for good reason: It’s dastardly useful.
In my P-Chem quantum chemistry course (and briefly without equations in G-Chem), I discuss the strangeness of the experiment where electrons are fired at a two-slit grating creating an interference pattern. We think of electrons as tiny particles (but not as waves) and therefore imagine we’re shooting tiny bullets through our electron-gun (or cathode ray tube, as found in old computer monitors and TVs). Maybe we’re firing them too fast and they’re bouncing off each other and thus “interfering”. But if we slow down the rate of firing electrons to essentially one at a time, we still get the build-up of the interference pattern (that you’d see if using light waves). This is very odd. Does a single electron go through both slits at once and interfere with itself somehow? Now, if we try to figure out which slit the electron is “passing through” by using a detector, the interference pattern disappears and the electrons “behave” like little bullets being shot out of a gun. Take away the detector and the interference pattern re-establishes. It’s all very mysterious! Students get a kick out of hearing this story – with no clear resolution other than making the point that the act of observation can screw up the experiment.
Light, on the other hand, is essentially treated as a wave, except when it interacts with matter. This is exemplified by the photoelectric effect, which I go through in great detail in G-Chem – although I’m convinced most of the students still don’t “get” it. Maybe it’s not their fault. This idea of light acting as quantal packets or photons is rather mysterious. And those equations in optics where light is treated as a wave are so much easier to work with, and the equations have awesome predictive power. The math always works out when you consider light to be a wave: it bends, bounces, and spreads out exactly the way we observe in nature. Johnsen considers the possibility that perhaps “photons are not a useful or accurate way to describe light” and he used the wave-related equations throughout the book. They work, and they work well! Using photons is “messier and far less intuitive”.
But since the interaction of light with matter is what chemists (and biologists) care about, where we observed quantal changes, we have to think about photons. Johnsen provides a memorable framework to think about this: “A sensitive light meter will record individual events, leading us to think of photons as insects flying into a bug zapper. However, just because something interacts with matter in a quantal fashion doesn’t mean that the thing itself is packaged in discrete units. Suppose you shake a crib with a sleeping baby. If you shake it hard, the baby always wakes up. However, if you shake it gently, the baby might wake up. The waking up itself is a quantal event – the baby is either awake or asleep – but the probability of this happening depends on how hard you rock. As a child, I remember a similar process working at the dinner table with my little brother. Kick him lightly under the table long enough and he would eventually blow his stack. You never knew quite when, though.”
Another reason why I’ve been thinking about the quantal nature of light is when I consider energy transduction and the second law of thermodynamics in the context of the biosphere. Ignoring the small amount of geothermal energy as a source for the moment, the vast majority of life on Earth is directly or indirectly supported by energy influx from the sun. I tell my students that these photons are high-quality energy, especially if they’re higher frequency and shorter wavelength. Ultra-violet and blue light are certainly high energy photons compared to red and infra-red. We typically treat infra-red and “heat” synonymously. I tell my students that “heat” is crappy low-quality energy. Technically, I will be more careful and say “thermal energy” rather than heat. Thermal energy is the kinetic energy of particles due to random motion, and as such, isn’t useful unless it can be directed in some way (which in itself will require some other energy source).
What does this have to do with the biosphere? Essentially, life absorbs a smaller number of high-energy photons (“light”), and then releases a larger number of low-energy photons (“heat”). Thus, entropy, if you were counting photons, increases in line with the second law of thermodynamics. (The Earth-Sun dyad can mostly be treated as almost a closed thermodynamic system.) Molecules and their electrons interact with photons. The photons are picked up, something is done, and photons are released. Sometimes the photons released seem to have the same energy as the original impinging ones. Sometimes they don’t. Usually the photons emitted are lower energy and larger in number, and they eventually dissipate into the coldness of space along a temperature gradient. Thus, the quantal nature of light, in obedience to the second law of thermodynamics, allows beautiful myriad structures to form, at least on Goldilocks-planet Earth.
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