Skip to Content
Uncategorized

The kilogram is being redefined as a fundamental constant, not just a chunk of metal

November 19, 2018

Scientists from more than 60 countries agreed to change the definition of the kilogram during a conference on Friday in Versailles, France.

The history: The kilogram was originally defined as being equivalent to the mass of a liter of water. Since 1889 scientists have based its definition on a physical object: a platinum alloy cylinder known informally as “Le Grand K,” which was kept in a safe in Paris. An obvious drawback was that if the cylinder changed, so did the entire global system of measurement. When it was measured in 1980s, it weighed several micrograms less than it was supposed to, forcing manufacturers who made products based on the standards to reissue their weights.

What happens now? A kilogram will now be measured in terms of the amount of electricity needed to counteract the weight. The quantity that relates weight to electrical current is named Planck’s constant, after the German physicist Max Planck, and denoted by the symbol h. A super-accurate set of scales, called a Kibble balance, can measure mass to an accuracy of about one part per billion by using a fixed value of h, which scientists have already measured extremely precisely.

The new system’s advantages: Rather than measuring all kilograms against one single object—Le Grand K—the new definition means anyone with a Kibble balance can check their weights. And this means that rather than being based on an arbitrary, physical object that can change over time, definitions are based on the fundamental constants of nature.

Correction: An earlier version of this post incorrectly described the use of the Kibble balance.

Deep Dive

Uncategorized

Embracing CX in the metaverse

More than just meeting customers where they are, the metaverse offers opportunities to transform customer experience.

Identity protection is key to metaverse innovation

As immersive experiences in the metaverse become more sophisticated, so does the threat landscape.

The modern enterprise imaging and data value chain

For both patients and providers, intelligent, interoperable, and open workflow solutions will make all the difference.

Scientists have created synthetic mouse embryos with developed brains

The stem-cell-derived embryos could shed new light on the earliest stages of human pregnancy.

Stay connected

Illustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at customer-service@technologyreview.com with a list of newsletters you’d like to receive.