A meter must be represented by a particular physical bar kept in a vault.

In 1889, international metrology gave the meter a physical embodiment: the distance between marks on a carefully manufactured platinum-iridium bar stored near Paris. Compared with regional rods, body-based measures, and inconsistent local standards, this was an enormous improvement.

But any artifact has a weakness as a definition. A bar can be damaged, contaminated, strained, or subtly altered, and copies can drift relative to one another. The ideal standard would be reproducible anywhere without depending on one irreplaceable object.

In 1960, the meter was redefined using a specified wavelength of krypton-86 light. That replaced the metal bar with an optical phenomenon laboratories could reproduce independently.

The next step came in 1983. Because the speed of light in vacuum had been measured with extraordinary precision, the SI fixed its numerical value and defined the meter as the distance light travels in vacuum during a specified fraction of a second.

The prototype bar was not a scientific mistake. It was a highly successful technology that became unnecessary once nature itself supplied a more stable reference.