SI base units physical constants artifact definitions

The kilogram is no longer defined by a metal cylinder

For more than a century, the kilogram was defined by a particular metal cylinder kept in France. Since 2019, the SI system has instead defined it through a fixed value of the Planck constant. Other base units are also linked to fundamental constants or physical phenomena. Measurement standards no longer depend on preserving a single object that might change over time.

The asteroid that killed the dinosaurs created a hot underground habitat

The Chicxulub asteroid impact shattered rock and left enormous amounts of heat beneath the crater. Water circulated through the fractured ground, forming a hydrothermal system that may have lasted for a very long time. Researchers think such environments could have supported microorganisms even after the surface suffered catastrophic destruction.

The immune system has to learn not to attack your own body

Young immune cells that strongly attack the body's own molecules are normally destroyed or held back. Autoimmune disease can develop when this protection fails. Not attacking your own body is therefore something the immune system must actively maintain.

Like this? There’s a whole feed of it. Replace empty scrolling with things worth knowing.

Download Hence for free

SI base units physical constants artifact definitions

The kilogram is no longer defined by a metal cylinder

Keep exploring