History of Science

# Using Particle Physics for Measurement Shows Just How Far Measurement Has Come

Measurement is literally the most basic tool science has, and yet it's fraught with more drama than daytime television. You've got most of the world using the metric system while the U.S. clings to imperial measurements, weight standards that are losing weight, and calendars that require leap years to make the dates line up. But in 2018, the voice of reason will step in: the General Conference on Weights and Measures is set to adopt new definitions for the seven base measurements, most likely using particle physics to do it.

## Must Be 3 Cubits Tall To Ride

In the beginning, when people needed to measure things, they used what they had: their bodies. One of the oldest units of measurement was the cubit, which was defined by the length of the arm from the tip of the middle finger to the elbow. That was divided further into smaller units: six palms, 24 digits.

Things got a little more advanced in the 1790s with the introduction of the meter, which was originally defined as one ten-millionth of the distance between the Equator and the North Pole. That's quite a tricky measurement, so the International Bureau of Weights and Measures (BIPM) in Paris kept an official representation of its length in the form of an iron bar — an iron bar that happens to be roommates with the platinum cylinder that represents the weight of a kilogram.

But as technology progressed, measurements got more precise. Take the measurement of time, for instance: seconds were defined as a certain fraction of a day until astronomers realized that the length of a day changes throughout the year. Around the turn of the 20th century, they compromised by defining the day as the mean (average) length of the day on January 1st, 1900. In 1967, scientists used their knowledge of the atom to give the second more precision: one second is 9,192,631,770 periods of the radiation for a Cesium-133 atom. Sounds complicated, but scientists can make that measurement anywhere, any time, regardless of the amount of daylight this time of year. That's key. For ideal precision, a scientist has to be able to check her measurements in the lab, not against some abstract concept.

## Seven SIs for Seven Brothers

Today, there are seven base units of measurement in the metric system, a.k.a. The International System of Units (SI). In 2018, the 26th General Conference on Weights and Measures (CGPM) will convene, and experts predict that they'll create new definitions for most of these units. Here's how they're likely to change, according to Symmetry Magazine:

• Distance: In 1983, the CGPM redefined the meter from the length of an iron bar in Paris to the distance light travels in a vacuum in 1/299,792,458 of a second. That's not likely to change.
• Mass: The lowly kilogram did not get the modern facelift of the meter. We're still using that platinum-iridium cylinder kept under three bell jars that's slowly losing weight over time. Scientists hope that this time around, the CGPM will redefine the kilogram in terms of Planck's constant, which is the smallest amount of quantized energy possible and widely used in particle physics.

• Time: What, making a second 9,192,631,770 periods of the radiation for a Cesium-133 atom isn't good enough for you? Atomic time is where it's at, and that's probably how it will stay.
• Temperature: Kelvin, every scientist's favorite unit of temperature, is currently based on water's triple point, when it's simultaneously a liquid, solid, and gas. But impurities in water can throw off its triple point, so scientists would rather not bother. Instead, they'd like to use Boltzmann's constant, a number that links the movement of gas molecules to the temperature of that gas. It's already used heavily in physics, so why not?

• Electric current: the basic unit of electric current is known as the ampere, and it currently has this fantastical definition: imagine you have two infinitely long and thin conductors placed parallel to each other, one meter apart, in a vacuum. One ampere is is the amount of current it takes to create a force of 0.2 micronewtons between them. Because you can't have an infinitely long conductor, this obviously can't be measured in the lab, so scientists want to make the ampere based on the fundamental charge of an electron. Much cleaner.

• Luminosity: the candela has been defined and redefined over the years: first it was the intensity of an actual candle, then of a light bulb filament. For a long time it was based on the intensity of what's known as blackbody radiation. The current definition requires four different units of measurement to explain, but suffice it to say, it's probably not going to change. (One candela is still roughly the brightness of a candle.)

• Quantity: 18th-century scientist Amedeo Avogadro was the first to notice that mass was related to number of atoms — something he didn't yet have a name for, which is why Avogadro's constant didn't get its name until after his death. More generally known as the mole, this heavily used number is defined as the quantity of atoms in 12 grams of carbon-12. Ah, but that relies on grams, which adds an extra step to the measurement. Scientists hope to redefine the mole as a single quantity: 6.022140857×1023, to be exact.

Shoe sizes, however, will remain completely unreliable. Some things never change.

#### The SI Base Units

##### SI Base Units
– National Physical Laboratory
Ashley Hamer