Scientists from 57 nations will gather next week at the General Conference on Weights and Measures in Versailles, France, to redefine the International System of Units (SI) in one of the most fundamental updates of the metric system since it was proposed in 1790.
The 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 process began when 17 nations signed the Treaty of the Meter in 1875. The current SI was established in 1960. The new definitions will be based on the values of physical constants that have been identified and measured with increasing precision since then.
Some changes already have been made. The basic unit of time, the second, now is based on the frequency of radiation of a cesium atom and the meter is based on the speed of light. It is now time for a more general update, said Peter J. Mohr, physicist at the National Institute of Standards and Technology’s Physical Measurement Laboratory, one of the major contributors to the conference. “It’s all based on practical things,” Mohr said.
The standard units Their defining constants
• Second Frequency of a cesium atom
• Meter Speed of light
• Kilogram Planck constant
• Ampere Elementary electric charge
• Kelvin Boltzmann constant
• Mole Avogadro constant
• Candela Luminosity of a particular color and power of light
Many of the defining constants are interconnected, Mohr said. For example, the meter is defined as the distance traveled by light in a vacuum (299,792,458 meters per second) in a given period of time, so the definition of the meter also involves the definition of a second. The definition of the kilogram will involve the Planck constant, the meter, and time
If adopted as expected on Nov. 16, the changes will go into effect in 2019 on May 20 – World Metrology Day, the 144th anniversary of the signing of the Treaty of the Meter.
You’re using it now
The SI is a direct descendant of the metric system and is the standard used by nearly every developed country in the world, including the United States, whether we know it or not. We use inches and pounds and miles and gallons every day, but standards for these 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 that provides more of the details and the math of the proposed changes for educators.
A meter is still a meter; the fundamental things apply
The 2018 SI won’t make a big impact on everyday life. The definitions for the meter, the second, and the candela already are tied to physical constants and will not change. Changes in the ampere will be too small to impact our electric bills, but will change the way the industrial electrical industry measures activity in their distribution systems.
The most significant impact will be that everyone can use these standard units and be on the same—more accurate—page. The meter, for instance, originally was one ten-millionth of the distance from a pole to the equator, but because of the difficulty of surveying this accurately it was actually 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 for the meter—the official bar and a more precise de facto standard based on wavelengths.
Other standards had similar shortcomings. By tying them all to values in nature that can be measured with increasing accuracy and that remain constant everywhere and at all times, the definitions should be unchanging. Scientists and others requiring precise measurements will not have to depend on one-of-a-kind artifacts for their standards. Any lab that can measure the constants can access the standards.
Will adopting the new SI mean that our quest for standard units of measure 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.”