I was perusing a back issue of Physics Today, reading an article about events at the Large Hadron Collider, when I noticed the word, “femtobarn”. It was defined as 10^-39 sq.cm. or a decimal followed by 38 zeros then a one. This is pretty small.

As a midwesterner, I thought I knew the size of a barn. So I multiplied out the femto (you know, milli-, micro-, nano-, pico-, femto-) to get the quadrillion times larger 10^-24sq.cm. This is a barn? I had to Google this, and found a clear article at Stanford defining it for lay-folk.

In brief, physicists in the 1940’s were often discussing the cross-sectional area of the Uranium Nucleus. They thought of calling it the Oppenheimer (too many syllables) or the Bethe (too likely to be heard as Beta). Well, most of the research was being done in the midwest, and slamming protons into this target made them think of tossing tomatoes at a barn. The name “barn” was used in a reviewed article, accepted, and it stuck. Now, why femtobarns as the standard unit? It’s just practical for their purposes, like kilometers or megabytes.

By now, if you’ve read this far, you are probably wondering, “why should we care?” Sure, it’s fun to say “femtobarn”, but what use is it in everyday life?