If you couldn't get
your hands on a timepiece with a second hand, the utility
of **seconds** would seem limited to the (non-coincidental)
fact that they are about the same as a resting heartbeat period.
**Years** and **days** might seem less arbitrary to us,
but we would have trouble convincing our friends on
Tau Ceti IV.^{3.3}
Remember, our perspective in Physics is universal,
and in that perspective all units are arbitrary.

We choose all our measurement conventions for convenience, often with monumental short-sightedness. The decimal number system is a typical example. At least when we realize this we can feel more forgiving of the clumsiness of many established systems of measurement. After all, a totally arbitrary decision is always wrong. (Or always right.)

Physicists are fond of devising "natural units" of
measurement; but as always, what is considered "natural"
depends upon what is being measured.
Atomic physicists are understandably fond of
the **Angstrom** (Å), which equals 10^{-10} m,
which "just happens" to be
roughly the diameter of a hydrogen atom.
Astronomers measure distances in **light years**,
the distance light travels in a year
(
m),
**astronomical units** (a.u.), which I think have
something to do with the Earth's orbit about the sun,
or **parsecs**, which I seem to recall
are related to seconds of arc at some distance.
[I am not biased or anything....]

Astrophysicists and particle physicists tend to use units
in which the velocity of light (a fundamental constant)
is dimensionless and has magnitude 1; then times and
lengths are both measured in the same units.
People who live near New York City have the same habit, oddly enough:
if you ask them how far it is from Hartford to Boston,
they will usually say, "Oh, about three hours."
This is perfectly sensible insofar as the velocity of
turnpike travel in New England is nearly a fundamental constant.
In my own work at TRIUMF, I habitually measure distances
in **nanoseconds** (billionths of seconds: 1 ns = 10^{-9} s),
referring to the distance (29.9 cm)
covered in that time by a particle
moving at essentially the velocity of
light.^{3.4}

In general, physicists like to make *all*
fundamental constants dimensionless; this is indeed
economical, as it reduces the number of units one must
use, but it results in some oddities from the practical
point of view. A nuclear physicist is content to measure
distances in *inverse pion masses*, but this is not apt
to make a tailor very happy.

Jess H. Brewer - Last modified: Wed Dec 16 11:30:10 PST 2015