The National Institute of Standards and Technology recently published its US version (PDF) of the 8th edition of the International System of Units (abbreviated SI from the French Le Système International d'Unités): now the metric system of choice for the US. Rush out and pick up your "Americanized" booklet to get the scoop on which units made the cut, which ones didn't, plus how to spell "meter" and "liter" the American way.
The new brochure is out just shy of 20 years after the Omnibus Trade and Competitiveness Act of 1988 made the metric system "the preferred system of weights and measures for United States trade and commerce," and just more than a year after the 2007 America Competes Act (PDF) cleared things up and legally updated the definition of the metric system to the International System of Units (SI). Prior to 2007 certain US metric values were still on the centimeter-gram-second system, which was mostly abandoned by the rest of the world by the 1970s when the first SI Brochure was published. So while metric is still not the official system of units used in the United States, the government seems to at least know that it's an option.
America seems fairly content to be one of only three countries in the world (not even enough to constitute the metaphor of a handful) whose official measurement system is still English units, rather than metric. Of all the races America has been in to be first at something, we seem to be pretty content hanging back on this one, along with our English Unit compatriots Liberia and Myanmar.
While a legal switch to SI might mean big changes for importers and exporters, and certainly physics-based industry, it would probably mean very little change for your average Joe. Even if America switches to SI, it doesn't mean local usage will immediately change as well. In European countries where SI is the official system, people get their five-day forecast in Celsius rather than Kelvin, and their speed limits in kilometres per hour, not per second. This isn't due to stubbornness, but local practicality. The most practical system of units for a given place and situation is not only determined by things like which system has a base unit of 10, but by factors such as the scale of what is being measured, the way in which it must be measured, or simply by how well everyone else understands it. Practicality can changed based on geography: sailors still use the non-SI units knots, fathoms, and particularly nautical miles.
Sometimes the only thing that matters is a common understanding. Whether your grocer prices potatoes by the kilogram or by the pound or by their equivalent weight in AA batteries, what is important is that the buyers understand whichever one he uses (by the way, to see such a silly equivalent check out http://www.sensibleunits.com/). MIT grads or Cambridge locals may know of the Smoot: a unit of distance invented in 1958 when a group of MIT students measured the length of Harvard Bridge using a kid named Oliver R. Smoot ('62). The bridge is 364.4 smoots long "plus an ear." Rumor has it the unit is actually used by locals to roughly measure distances.
Even physics, the science that practically wrote the book on SI, has its own colloquial units that are used commonly because they are more practical. The ångström, the bar, and the barn are not SI units but can be found in published papers and reports and are used with varying frequency among different sections of physics. The decibel, which is technically unit-less but still used in order to understand a quantity, isn't officially SI either. The term "fermi" was used in physics to describe 10-15 meters before SI caught up with the advancing science and created the new prefix femto.
Don Groom of the Particle Data Group at Lawrence Berkeley National Laboratory adds, "In general, physicists remember and use numbers of order one. The binding energy of a hydrogen atom is 13 eV, the 'radius' of an electron is about 2.8 fm. Cross sections (areas) are measured in barns, where 1 barn = 10-24 cm2. If you ask somebody at SLAC what the integrated beam luminosity is, you will be told 1.2 inverse picobarns or something. It is simply convenient--and also customary."
Physicists use enough units to make your head spin, but sometimes those strange units have an important benefit: they mean scientists can talk about size and scale in a convenient way. The astronomical unit (AU), which is not officially SI, is used for these reasons; while you may not know and could never remember how far 299.19 million kilometers from the Sun is, you can grasp and visualize twice the distance from the Earth to the Sun (2 AU). As Groom says: "Why make it hard?"
In high-energy physics, the most prominent of the non-SI units is the electronvolt (eV). An eV is defined as the kinetic energy acquired by an electron passing through one volt of energy in a vacuum. The eV is used far more often in particle physics than the SI unit of energy the joule, and adopts many of the SI prefixes. It's argued by some that using joules would be simpler, and Richard Feynman even apologized in one of his recorded lectures to young scientists for the creation of this confusing unit. But when you are constantly talking about the energy of small particles such as the electron, it makes sense to use a unit that you can then express in a simple form.
The SI Brochure states that the eV is not accepted officially by SI because it has to be obtained experimentally and can only be expressed approximately in terms of SI units. In actuality, all units must somehow be obtained experimentally. Measuring a standard mass to define the kilogram is still technically an experimental result. Ideally, those experiments are based on natural constants, such as the speed of light which is used to calibrate length. Physicists will admit that experimental values are always improving and always changing on some small scale. Just take the radiocarbon year. In the early days of radiocarbon dating it was thought that the ratio of carbon-14 and carbon-12 was constant in the atmosphere, which it isn't, and the value of carbon-14's half life has been measured more precisely since the late 1940s. Rather than changing the entire algorithm, there are simply charts for converting radiocarbon years into calendar years (right now 1200 radiocarbon years is approximately equal to 1400 calendar years).
Colloquial units can serve a purpose better than the officially accepted ones because of their local practicality, but the United States' "local" environment has already grown to include the rest of the world, so why are we acting like Libya and Myanmar are the only ones in our local circle? Why don't we act like we're a part of the global community that we've played such a large part in creating?
Beyond practicality and commonality, the United States should also be aware of common politeness. As we participate in global activities involving a system of units, it is completely inconsiderate to bring with us a system of units that is neither more practical nor more popular than that which everyone else is using. On that note, NASA recently announced it would make all of its missions on the lunar surface operate in metric. From NASA's Web site:
Jeff Volosin, strategy development lead for NASA's Exploration Systems Mission Directorate said, "I think NASA has been seen as maybe a bit stubborn by other space agencies in the past, so this was important as a gesture of our willingness to be cooperative when it comes to the Moon."
Maybe NASA is getting the right attitude, but this decision to stop being a bit stubborn is still a bit late considering it comes eight years after a units miscommunication caused a navigation error with the Mars Climate Observer, sending a few million taxpayer dollars into the Martian atmosphere in flames. Putting that aside, the change will not only be a polite gesture to other space agencies, but significantly advantageous for NASA as well. Now no one has to do conversions every time they need to borrow a socket wrench from someone else's tool box. However, NASA continues to use both SI and English units on the international space station. If anyone can shed some light on that executive decision, I'd love to hear it.
So until the United States nudges itself off the cliff and into SI oblivion, for the sake of culture and community I'll keep measuring distances in Smoots and my beer in pints; but for the sake of international collaboration, I'll write a letter to my congressman about pricing astronaut ice cream by the kilogram.
