How Long is a Day? How Heavy is a Kilogram? - In Praise of Metrology

Every once in a while I notice a story about metrology, or the science of measurement, that reminds me how many of our concepts about the world are carefully constructed and contingent.

A few months ago I asked just how long ago the civil war was after hearing a story about a book stolen during the civil war and just recently returned. On the scale of a human life or generation the civil war is very recent history and yet America seems to have completely buried it in the past. Americans aren’t very good at remembering the past so the personal distance to the civil war is not really surprising.

Today I read a story about the kilogram at Chad Orzel’s blog. It seems that defining the kilogram in terms of a fundamental physical constant has proven to be much more difficult than for other constants like the length of the meter or the time of a second.

Attempts to redefine the kilogram have yet to yield anything, though. The problem, as always, is the gravity is so damnably weak.

Gravity may not seem like a weak force, but it is. The simplest illustration of gravity’s weakness is the old “rub-a-balloon-on-your-hair-and-stick-it-to-the-ceiling” trick. When you do that, the attractive force of maybe ten billion extra electrons on the balloon is enough to hold it up against the gravitational pull of the entire Earth pulling on a billion trillion atoms in the balloon. Gravity is preposterously weak compared to the electromagnetic force.

At one of the philosophy meetups I remember Harland arguing that a tree ring entails the age of a tree. But it’s not at all clear that our time words are ever that clear. Even a simple question like “How long is a day?” is really nested inside of an incredible network of assumptions and beliefs. Are we talking about mean solar day? Is a day from dawn to dusk? If I say that an activity took me a day to finish do we mean 24 hours, 8 hours, some other length of time.

With meaning there is a struggle/tension between precision (this is what scientists need in order to do their calculations or run the GPS system), perception (the fact that most people if asked to sit still for a minute would probably guess wrong at the length of time), and usability the exchange of information and coordination that takes place between people and individuals.

So I say long life and success to the metrologists who are constantly trying to improve our measurements.

Todd Suomela
Associate Director for Digital Pedagogy & Scholarship Department

My interests include digital scholarship, citizen science, leadership, and communications.