The Nature of Light

I had this epiphany long ago, but it took this line from Surely You’re Joking, Mr. Feynman to remind me of it.

They gave out dark glasses that you can watch [the first atomic bomb test] with. Dark glasses! Twenty miles away, you couldn’t see a damn thing through dark glasses. So I figured the only thing that could really hurt your eyes (bright light can never hurt your eyes) is ultraviolet light. I got behind a truck windshield, because the ultraviolet can’t go through class, so that would be safe, and so I could see the damn thing. (134)

And I was thinking about the bold part, and I came to realize he was right. You see things when light of some wavelength enters your eye and hits a molecule called retinal (the aldehyde form of the most common form of Vitamin A). The 11-cis form of retinal is isomerized to its most stable all-trans form, and the conformational change ultimately leads to the nerve impulse that registers as vision.

The key to realizing what Feynman says is that retinal absorbs at a specific wavelength. If you think about the photoelectric effect, electrons in a metal are “knocked out” by light only of a certain minimum energy or above. Likewise for a certain piece of light to be registered by retinal photoisomerization, it must have a certain amount of minimum required energy. Isomerization has a certain activation barrier.

As it turns out the intensity of light has nothing to do with its energy. From studies done of the photoelectric effect, it was determined that the frequency of light is what determines its energy, and that these come in discrete “lumps” (as Feynman puts it), known as photons. Each photon has a set frequency – a set energy. The intensity, on the other hand, is determined by the probability of finding a photon. A brighter light is simply more photons of the same energy. So it cannot possibly hurt your eye, because you’re still seeing photons of energies that your body already knows how to deal with – just many more than you’re used to.

But that wasn’t my epiphany. Let me explain what exactly is.

In church we are taught to bring light in a world of darkness. And there’s this nice, optimistic belief that while we can light up the darkness, the darkness cannot smother the light. Now this is great and all – but is it true? We investigate with the photon idea.

In [my favorite interpretation of] quantum theory, we see that classical light waves are actually propagating probability distributions of photons. Classical physics teaches us that the intensity of a classical wave is proportional to the square of its amplitude, which itself is proportional to the wave’s energy. If we allow energies to be quantized in discrete particles, and the wave to be propagating in an arbitrary space, then we can see how amplitude becomes related to the number of particles per unit space. We can imagine how this in turn is related to the probability of finding a particle. So with this established…

In darkness, the intensity is zero, so the probability of finding a light particle – a photon – is equivalently zero. From here I deduce that if we add light to darkness, we add photons, increasing that probability to a nonzero number. Yay we can do this! If I want to shine my light onto others, I simply add some of my unique photons. That’s quite nice.

Now let’s look at the reverse operation. Can darkness smother light? Darkness is defined as a lack of photons. If there already are photons, you have to actively remove the photons by hand; they won’t go away by themselves (that’s violating conservation of energy!). “Diffusion” (not sure how you can do that with photons “bound” by a probability wave, but for the sake of argument–) will reduce the probability towards zero, but never equivalently zero. You can smother light, but darkness by itself can never smother out light. On the other hand, light can kill darkness.

Optimistic thinking? Yes. Reality? Yes. Hooray! The upshot is, you see aspects of God all around you.

Even in quantum theory.

(And yes, I know that Einstein said that God does not play dice.)

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