Category: Science

“Consider a bar of soap. Has it ever struck you that soap lather is always white no matter what color the soap is? That isn’t because the soap somehow changes color when it is moistened and rubbed. Molecularly, it’s exactly as it was before. It’s just that the foam reflects light in a different way. You get the same effect with crashing waves on a beach—greeny-blue water, white foam—and lots of other phenomena. That is because color isn’t a fixed reality but a perception.” (Bill Bryson, The Body)

I have been thinking about soap. Not the smell of it, not the brand, not the dish it sits in by the sink, but its color, and what becomes of that color the moment the soap is wet and worked between the hands. However dark the bar the foam that comes off it is white. It is always white.

The soap has not changed. This is the part that unsettles me a little, still, though I understand the physics of it well enough to explain it to a child. Nothing has been dissolved out of the soap, no pigment washed down the drain with the first rinse. The bar, examined afterward, is exactly what it was — same weight, same tint, same faint milled scent. What has changed is only the shape of the surface presented to the light. A solid surface reflects color; a foam of small bubbles scatters light in every direction at once, and the scattering reads, to the eye, as white. The water does this too, at the shoreline, where the sea that looked green or the color of a bruise all the way to the horizon turns to white foam the instant it breaks against the sand. Nothing in the ocean has been added or subtracted. Only its surface has been shattered into a million small mirrors, and a million small mirrors, taken together, show us nothing but the light itself.

There is a machine built on the same principle, aimed at the opposite end. A stealth aircraft is not invisible in the way a child means invisible — it has not been made transparent, or absent, or small. It has been shaped. Every facet of the old F-117, every curved plate of the B-2, is angled to take an incoming radar signal and send it away, off at some oblique angle, into empty sky, anywhere but back to the dish that sent it out looking. Where the geometry alone cannot manage it, a coating finishes the job, converting the beam into a small, undetectable warmth rather than an echo. The aircraft has not changed what it is. It has changed only where its reflection goes. The operator at the radar screen sees nothing, and concludes, wrongly, that there is nothing there.

The foam and the aircraft are the same trick, pointed in opposite directions. Both break a surface’s geometry until the return signal — light, in one case, radar, in the other — stops describing the object honestly. The foam scatters light everywhere at once and reads, to the eye, as brilliant and total. The aircraft scatters radar everywhere except back to the observer and reads, to the screen, as nothing at all. One hides by flooding the return. The other hides by starving it. Neither the soap nor the aircraft has changed what it is. What has changed is the transaction between the object and whatever signal goes out to find it — and it is that transaction, not the object, that a color or a presence turns out to describe.

I do not think this makes color, or presence, less real. I think it makes real a smaller and stranger category than we had assumed. We want our perceptions to be reports from the world, filed accurately, awaiting only our attention. It is more honest, and harder to sit with, to admit that a great many of our perceptions are not reports at all but events — things that happen at the meeting point of a self and a surface, and that would not happen in quite the same way to anyone standing an inch to the left.

I rinse the soap and set it back in the dish. In a moment the foam is gone, down through the drain, and the bar is only itself again, the color it always was, waiting for the next hand to come along and change it into something briefly, purely white.