A new way to take the measure of our universe

By: William Jackson
February 1, 2017

William Jackson
William Jackson

Scientists at the National Institute of Standards and Technology are helping to refine the way we measure our universe by redefining our standard units of measure. The international project is the culmination of more than a century of effort to establish a system of measurements that are derived from nature rather than arbitrary values or physical artifacts.

The new International System of Units (SI) will be based on the values of physical constants that have been identified and defined with increasing precision since the Treaty of the Meter was signed in 1875. The current SI was established in 1960 and it is now time for an update, said Peter J. Mohr, physicist at the NIST Physical Measurement Laboratory.

“It’s all based on practical things,” Mohr said. Practical, but not necessarily simple.

The standard units
• Second
• Meter
• Kilogram
• Ampere
• Kelvin
• Mole
• Candela

and their defining constants
• Frequency of a cesium atom
• Speed of light
• Planck constant
• Elementary electric charge
• Boltzmann constant
• Avogadro constant
• Luminosity of a particular color and power of light

The defining constants do not line up one-to-one with the base units of measure, Mohr said. “For example, the speed of light involves the relation between the meter and the second and fixes their ratio. More extreme, the Planck constant has units of kilogram meter squared/per second, involving mass, length and time.”

The changes are expected to be adopted in November 2018 by the General Conference on Weights and Measures and would go into effect in 2019 on May 20 – World Metrology Day, the anniversary of the signing of the Treaty of the Meter.

A meter is still a meter; the fundamental things apply

Don’t expect big changes. “The values are chosen so that the impact will be minimal,” Mohr said. “It will be imperceptible to almost everybody.” The definitions for the meter, the second, and the candela—already tied to physical constants – will not change.

But that does not mean there won’t be some changes. Once the ampere is tied to the elementary electric charge, “your electric bill could change by a power of 10 to the negative seven,” Mohr said, laughing. More significantly, it will change the way the industrial electrical industry measures activity in their distribution systems. “The community knows about this and is preparing for it.”

The most significant impact will be that everyone using these standard units will be on the same page. The meter originally was to be one ten-millionth of the distance from the pole to the equator, but because of the difficulty of surveying this accurately it was defined by a platinum rod. This was replaced by a platinum-iridium bar in 1875. But the ability to measure wavelengths of light outstripped the accuracy of the prototype bar and by 1960 there were two standards—the official bar and a more precise de facto based on wavelengths.

Other standards had similar shortcomings. By tying them all to values in nature that can be measured with increasing accuracy, the definitions will remain the same as units are refined. And scientists will not have to depend on a metal cylinder in Paris for the standard kilogram. Any lab that can measure the constants accurately can access the standards.

You’re using it now

The SI is a descendant of the metric system and is used by nearly every developed country in the world except the United States. But although most people in the U.S. use inches and pounds every day and government regulations for fuel efficiency are measured in miles per gallon, that does not mean we are not using the SI. The standards for U.S. (or English) units of measure are defined by NIST using the SI. The definition for an inch, for example, is 2.54 cm.

Mohr and a couple of colleagues—NIST scientist William D. Phillips and high school physics teacher Sandra Knotts—have written an interesting introduction to the new SI in the journal The Physics Teacher. You can check that out for more of the details—and the math—of the proposed changes.

Will adopting the new SI mean that our quest for standard units of measure derived from nature is finally finished? Maybe; but maybe not.

“You can never say never,” Mohr said. The prototype kilogram cast in 1879 was supposed to be the standard for 10,000 years. It has lasted a little more than a century. “You have to keep an open mind about it.”