The Problem with Physical Standards

For most of modern history, the international standard for the meter was a physical object: a bar made of platinum-iridium alloy, kept under carefully controlled conditions at the International Bureau of Weights and Measures (BIPM) near Paris, France. Any nation that needed to calibrate its measurements had to ultimately trace its standards back to this single object.

The problem was obvious: physical objects change. The platinum-iridium bar, despite being one of the most stable materials available, could theoretically be damaged, altered, or simply drift slightly over time. Scientists and metrologists had long sought a better foundation — one based on the unchanging laws of nature rather than a manufactured artifact.

The Spectral Precision of Krypton-86

The solution came from atomic physics. When a gas is excited by an electrical discharge, its electrons jump to higher energy states and then fall back down, emitting photons of very specific wavelengths — a unique "fingerprint" for each element. These spectral lines are extraordinarily reproducible: identical atoms anywhere in the universe emit light at exactly the same wavelengths.

Among all candidate elements, krypton-86 stood out. Its orange-red spectral line — produced by the transition between specific electronic energy levels — was exceptionally sharp, meaning it had a very narrow bandwidth. This made it highly suitable as a precision length standard because a narrow spectral line means wavelength can be measured with greater accuracy.

The 1960 Redefinition

At the 11th General Conference on Weights and Measures (CGPM) held in 1960, member nations adopted a new definition of the meter based entirely on krypton-86:

"The metre is the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p₁₀ and 5d₅ of the krypton-86 atom."

This was a profound shift. The meter was no longer a physical bar — it was now a count of light waves. Any suitably equipped laboratory anywhere on Earth (or anywhere in the universe, in principle) could reproduce the standard independently.

Why the Number 1,650,763.73?

The seemingly arbitrary number was chosen to match the old physical bar as closely as possible. Scientists carefully measured the platinum-iridium meter bar using krypton-86 interferometry and arrived at 1,650,763.73 wavelengths as the best fit. This ensured continuity — the new definition didn't change the length of a meter, it just redefined how it was expressed.

How the Standard Was Implemented

National metrology institutes used Krypton-86 spectral lamps — sealed glass tubes containing isotopically pure Kr-86 at low pressure — to realize the standard. By exciting the gas and using an interferometer (a device that measures distances by counting interference patterns of light waves), labs could achieve remarkable precision in length measurements.

The technology required:

  • Isotopically pure krypton-86 (the other isotopes produce different, interfering spectral lines)
  • Precise temperature control of the lamp (typically operated in a liquid nitrogen bath at 64 K)
  • Laser interferometry to count the wavelengths

The Transition to Speed-of-Light Definition (1983)

The krypton standard served science well for over two decades, but by the early 1980s, laser technology had advanced to the point where the speed of light could be measured with even greater precision than krypton's spectral line could deliver. In 1983, the CGPM adopted a new definition of the meter based on the speed of light:

"The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second."

This definition remains in use today. The speed of light (c = 299,792,458 m/s exactly) is now a fixed constant by definition, and the second (defined via cesium atomic transitions) anchors the whole system.

Krypton's Legacy in Metrology

The krypton-86 era was a pivotal chapter in the history of measurement. It proved that atomic spectral properties — not physical objects — could serve as the ultimate foundation of human measurement systems. This philosophical shift, from artifacts to natural constants, continues to shape how the SI (International System of Units) is defined today.

Krypton's role in defining the meter is a powerful reminder that seemingly obscure scientific properties of "inert" gases can have profound practical consequences — not just in industry, but in the very way humanity organizes and communicates about the physical world.