What is a Barn-megaparsec?

In physics, a derived unit that is a product of two other units is often denoted by simply attaching the names together, sometimes in hyphenated form, as in "newton-meter" (unit abbreviation Nm). A "barn-megaparsec" (unit abbreviation bMpc) is the product of a barn and a megaparsec.

A barn is a tiny unit of area measurement exactly equal to 1x10^-28 m^2. This is about the area of a uranium nucleus. In the 1940s, during the Manhattan Project, researchers needed new units, and they called this area a "barn" both to obscure its true meaning, and because quite literally this area is "big as a barn" on the subatomic scale. The barn is a non-SI unit that still retains familiarity to those in particle physics.

A Megaparsec is equal to one million parsecs. A parsec is a distance corresponding to a parallax of one arcsecond. To put another way, it is the distance between the Sun and a distant star if, when viewing the distant star from Earth relative to other distant stars, there is detected one arcsecond of apparent parallax between the star and other distant stars as Earth rotates around the Sun. A parsec is on the scale of trillions of miles.

In the real world, if you were to ever see a volume of any material with these dimensions, it would not be visible. In principle, if you could see it, it would be a great approximation of a straight line- extending almost infinitely in one dimension, with almost no width to the line itself.

The humor lies in the ridiculous juxtaposition of two totally unamangeable quantities, one far too large and one far too small. However, the product of these quanities cancels out the opposing extremeness of each, and the result is an amazingly human-scaled volume of approximately 2/3 teaspoon.