# What Shall We Say, Shall We Call It By A Name?

February 2, 2014

An aptronym is a name that serendipitously reflects the character of its referent. Familiar examples include Chris Moneymaker, Anthony Weiner, Marilyn vos Savant, Price Club, Usain Bolt, and almost everybody who worked for Car Talk.

There are a few quantities in physics which have aptronyms.

The most familiar, since it is typically taught in introductory electromagnetism, is the Poynting vector, which poynts in the direction that energy flows in an electromagnetic field (eg. in the direction light is shining).

However, there are other good, less-familiar aptronyms:

1. The Schwarzschild radius is the size of the event horizon of a black hole. “Schwarzschild” translated literally means “black shield”, which is apt, as the outside universe is… shielded from anyone who has had the misfortune to fall pass the horizon.
2. The Heaviside function $\theta(x)$, or the unit step function, is 0 if $% $ and 1 if $% $ (the definition at $x=0$ is sometimes taken to be $\frac{1}{2}$, 1, and sometimes 0). The name can be considered apt for two reasons: some say it looks like an imbalanced two pan balance (in which case $% $ is the “heavy part”), while others say that the mass of the function is where $% $ in the sense that
$$\\ \int_{-\infty}^x \theta(x)\ dx = x \theta(x).$$
3. The Unruh effect is the fact that an accelerating observer will measure that the vacuum of space has a non-zero finite temperature, whereas an observer at rest (or, by relativity, an observer coasting at a constant velocity) will measure the vacuum to be at absolute zero. “Unruhe” translates to “unrest” from German, but was actually named after W.G. Unruh. This was pointed out to me by my friend Will Donnelly.

February 2, 2014

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