Is the kilogram no longer defined by a metal object?

For more than a century, the international kilogram was tied to a particular platinum-iridium cylinder kept near Paris. Copies around the world were compared back through a chain of physical standards.

NIST explains that the revised SI instead defines the kilogram in terms of the Planck constant, a fundamental constant used in quantum physics. Realizing that definition in a laboratory requires extremely precise experiments such as a Kibble balance or measurements involving silicon spheres.

The object did not become the wrong mass overnight. It stopped being the thing that ultimately defined the unit.

Why replace the artifact?

A definition tied to one manufactured object inherits that object’s limitations. It can be contaminated, cleaned, damaged, or drift relative to its official copies. A fixed constant of nature is not stored in one vault and does not need to be transported for comparison.

What did not disappear

Physical kilogram masses remain essential for ordinary calibration. NIST explicitly says they are still the practical way to disseminate mass measurements in government, commerce, and laboratories. The change occurs at the top of the traceability chain: those masses are now ultimately connected to a definition based on fundamental constants rather than to one privileged artifact.

The careful wording

It is fair to say that the kilogram is defined using the Planck constant. It is misleading to say that shops or laboratories now “weigh things with quantum mechanics” directly. Everyday measurements still use instruments and physical standards whose calibrations trace back through the new system.

Next card: when a metric reverses the result →