Do Molecules have Shadows?

Do molecules have shadows?

I don’t know. It’s a question I wrote on the white board in my office two weeks ago. Several students have since asked me about this, leading to some interesting discussions! The impetus for the question comes from reading Carving Nature at Its Joints. Chapter 6, written by Roy Sorensen, is titled “Para-Natural Kinds”.

What are para-natural kinds?

Here is the author’s definition. “A para-natural kind is an absence defined by a natural kind. For example, cold is defined as the absence of heat and shadow as an absence of light.” This constrains the definitions to connect with physical measurements involving heat and light, rather than more abstract concepts – evil is the absence of good, for example.

Sorensen argues that “absences are not substances and so are not natural kinds”. One neat visual example he provides is from The Tomb and Shade of Washington by James Merritt Ives. Shade takes two meanings in reference to the tomb and the trees in the landscape. There are the shades or shadows ‘cast’ by these structures, but between the two trees is an outline of Washington, a spirit shade perhaps. Very clever!

Let’s tackle the issue of heat. Technically, as a physical chemist, I would define heat as the transfer of thermal energy. Thermal energy arises from the motion of atoms and molecules. The amount of thermal energy is measured by temperature – itself a tricky concept with extensive treatment by the philosopher of science Hasok Chang. I don’t use the word ‘cold’ when I’m teaching chemistry; we do talk about higher and lower temperatures as a measure of thermal energy. I haven’t done so consciously because of the philosophical implications; I’ve simply imbibed the culture and language of my discipline – we use the words heat, temperature and thermal energy often, but rarely if ever use the word cold. I’ve never brought up this issue with my students, simply because I’ve never thought about it philosophically, but I might do so next semester since I’m teaching thermodynamics in both my G-Chem 2 and P-Chem 2 courses.

The issue of light is stranger. When light falls on an object, depending on the source direction of light and the shape of the object, it casts a well-defined shadow. The shadow moves, grows, shrinks (as the object or the light source is moved) in ways that can be calculated and predicted exactly. Things are more complicated however. If an object reflects some light, then it also casts a ‘para-reflection’. Sorensen provides visual illustrations to distinguish these two cases. He also provides a practical example of why one might care, comparing a white beach ball and a black beach ball sitting on the surface of a pond. “A hot duck that wants to cool off will paddle to the ball’s shadow, not to its para-reflection.”

Sorensen makes a further distinction between para-natural kinds and artifacts when he discusses shadows. I can’t summarize his text more clearly than what he has already written so here are snapshots of the relevant text.

If astronomers care about shadows, should chemists? After all, we’re interested in the interaction of light and matter. (And if magic is tied to the electromagnetic spectrum, a spell-caster should care greatly.) This is what led me to the original question: Do molecules have shadows?

In a sense, the simple answer is yes. Molecules have mass and shape. Some molecules are particularly ‘good’ at absorbing light of particular wavelengths. The entire dye and color industry is predicated on such properties. So, perhaps if you shine a swath of light at a molecule, then it should ‘cast’ a shadow. Do molecules reflect light? A macromolecular solid certainly would. A liquid-ish colloid would provide interesting dynamic patterns. But our ‘real world’ experience is built on the fact that the size of photons (if they have a ‘physical’ measurable size) is much smaller than the size of everyday objects. Analog-ously (pun intended) one might say that the wavelength of visible light is much smaller than the lengthscale of everyday objects.

What happens when a single photon interacts with a single molecule? There is a quantum mechanical interaction, and it can be calculated. (I won’t go into the details here.) But I don’t know if this would lead to a shadow in any meaningful sense that we can conceive. So I don’t know the answer to the original question.

What is a hole? Or a gap? If it is the absence of matter (be it atoms, electrons or photons), is a hole a para-natural kind?

The physical chemist Peter Atkins has suggested that all of chemistry can be boiled down to just four processes: proton transfer (Bronsted acid-base), electron transfer (redox), sharing unpaired electrons (free-radical), sharing electron pairs (Lewis acid-base). You could boil down chemistry essentially into one reaction type: moving electrons into ‘holes’. I think I know what an electron is, but what’s an electron-hole? The absence of an electron? The shade of an electron? I like this latter definition – it sounds ghostly to me!