In the post on “Fuel Costs of Point to Point Starship Travel”, our host expressed some frustration with Imperial units:
“… they are in the screwball units of US$ per million British Thermal Units (MBTU), where the British Thermal Unit is a quaint unit of energy defined as the heat input required to raise one pound of water by one degree Fahrenheit, which works out to somewhere between 1054 and 1060 joules in civilised units …”
Please allow me to put that in context. Let’s start with what is a civilized Joule? One Joule (named after an Englishman) is a measure of the work done by a force of one Newton acting through a distance of one Meter. Clear now? If one is interested in how much heat can be produced from burning a given amount of hydrocarbons, obviously there is a need for a conversion factor from work to heat. Complications!
One of the interesting features of the Imperial system of units was that it was designed by humans for humans, with most units intended to allow people to have an intuitive feel for their size. For example, a yard (technically the distance from the king’s nose to his thumb) was approximately the length of a human stride. In contrast, the pointy-headed French intellectuals who defined the Meter originally intended a universality – it was to be one ten-millionth of the distance from the Earth’s equator to the pole along a meridian, a line of longitude. Why that rather strange one ten-millionth? Because that made their new Meter approximately equal to the old yard.
However, those French intellectuals made two mistakes, as described in Ken Alder’s “The Measure of All Things”, ISBN 0-7432-1675-X (2002). First, they erroneously assumed the world was round, and therfore all meridians would be the same length. In reality, the planet is an oblate spheroid, and thus their choice of measuring the meridian through Paris undermined their goal of universality. And then they made a simple arithmetic mistake. (Oops!). Thus, their new Meter was as arbitrary as the old Imperial yard.
An interesting point Alder makes in his book is that before the centralizers imposed the Meter, different units were used in different parts of France – mainly because the amount of land needed for a farm varied across the country due to differing soils, rainfall, & topography. A uniform Meter was convenient for the bureaucrats in distant Paris, but put a burden on the people who actually worked the land and paid the taxes.
Moving along, what is a Newton, named after the English scientist? One Newton is the force required to accelerate a mass of one Gram by one Meter per second per second. Oops! No, the mass to be accelerated is not a logical one Gram but rather one thousand Grams. In this supposedly-rational system of units, why is the base unit of mass one thousand Grams rather than one Gram? Oh look! A cute squirrel over there …
As I was saying, the Newton involves accelerating a mass of one thousand Grams at the rate of one Meter per second per second. If one holds a mass of one thousand Grams (the intellectuals called this a kiloGram) in one’s hand, one experiences a force from gravity of a rather non-intuitive 9.80665 Newtons. Contrast that with the intuitive Imperial units – the force of gravity on a mass of one Pound is … One Pound. The wise souls who devised the Imperial system included some rather subtle conversion factors in the background to enable that intuitive human result.
Another example of the Imperial system’s preference for intuitively-meaningful units is the Horsepower, originally defined by Scottish engineer James Watt for his steam engines, such that a one Horsepower engine delivered slightly more power than one good horse. (Excellent marketing technique!). In French “Systeme International” (SI) units, the comparable unit is named the Watt after him, defined as a power output of one Joule per second. It takes 746 Watts to equal 1 Horsepower – in intuitive terms, we might think of 1 Watt as being the not very useful 1 Hamsterpower.
This tendency of the rather awkward French-derived system to produce units which are neither intuitively meaningful nor usefully sized also appears in the unit for pressure – the Pascal, named after a Frenchman. Atmospheric pressure is 101,325 Pascals. Your tire pressure of a convenient 32 psi in the Imperial system becomes something in excess of 220,000 Pascals. This unit is so inconvenient that even the French turned their back on the SI system and instead adopted for most practical purposes the “Bar” – an unofficial unit equal to 100,000 Pascal. (Why 100,000? Do you really have to ask?) Of course, that makes your tire pressure an inconvenient 2.18 “Bar” – good luck getting those tire pressures balanced!
Strangely, those intellectuals never got round to defining an SI unit of volume. Instead of the physically obvious 1 Barrel of oil, they demand the use of 0.15899 cubic Meters. Exasperated French people tried to plug this hole by inventing another unofficial unit, the “Litre”, equal to one cubic deciMeter. At least it is easier for the European motorist to buy a “Litre” of fuel for his automobile rather than 0.001 cubic Meters.
Even the proponents of French units recognize their limits. When dealing with the very small, it is more convenient to think about the size of a methane molecule as 3.8 (non-SI) “Angstroms” than as 0.38 nanoMeters. When dealing with the cosmologically large, 1 (non-SI) “Light-Year” is intuitively more understandable than 9.46 petaMeters. The creators of a set of units have to choose between developing a system which has convenient understandable human scales or developing an internally-consistent rational system. The pointy-headed French intellectuals turned their backs on the human scale and aimed for rationality – but missed.
Any system of units is really a language. It facilitates communication among those who speak that language while being confusing to those who don’t. Some have argued that English-speakers should instead learn & use the more precise Russian language because English is so riddled with ambiguities. However, those of us who speak English manage to get by, and would be reluctant to give up Shakespeare for a reduction in occasional ambiguities.
The adoption of SI units was driven by the same idealistic multi-nationalism that gave us the League of Nations, the United Nations, and the European Union. In principle, such centralized systems should be better than what went before – but reality has shown us something different. This reliance on changeable bureaucratic consensus points to the Achilles’ Heel of the Systeme International in today’s world – the undeniable fact that most units are named after straight white male Europeans, with not a transgendered single mother of color in sight. Fortunately, the Usual Suspects are mostly technically-illiterate and so far have paid little attention to the SI’s blatant demonstration of sexism, racism, & homophobia. However, it is inevitable that, at some point in the future, the users of the SI units will find themselves compelled to talk about Kardashians instead of Joules and Malcolm Xs instead of Watts. You know it is coming!