Why helium won't freeze, even at absolute zero

Helium is the only element that cannot be frozen at normal atmospheric pressure, however cold it gets: cooled toward absolute zero (0 K, or −273.15 °C) it stays a liquid. Los Alamos National Laboratory notes that it also has the lowest boiling point of any element, about 4.2 K.

The reason is zero-point energy. Quantum mechanics says atoms can never be perfectly still, even at the lowest temperature physics allows. Helium atoms are very light and attract one another only weakly, so that leftover jiggling is enough to stop them settling into a crystal. Heavier, stickier atoms lock into place long before.

The only way to make solid helium is to squeeze it: about 25 atmospheres, more than 20 times the air pressure at sea level, applied within about a degree of absolute zero.

That is why search results for helium’s “freezing point” can mislead. Figures such as −458 °F describe helium held under roughly 25 atmospheres; at ordinary pressure there is no freezing point at all, and Los Alamos’s periodic table leaves helium’s melting point blank. Even “liquid at absolute zero” is an extrapolation: as Oxford physicists point out, absolute zero itself can be approached but never actually reached.

How it was finally frozen

• 1908: Heike Kamerlingh Onnes liquefies helium in Leiden and measures it at about −268.8 °C, earning his lab the title “the coldest place on Earth”.
• 1910s–1920s: he pushes his apparatus to roughly one degree above absolute zero, yet solid helium stays out of reach.
• 1926: Willem Hendrik Keesom, his successor as director of the Leiden laboratory, finally produces solid helium, by adding pressure rather than more cold.

Helium remains the textbook case of quantum mechanics deciding the behaviour of an everyday substance, not just of atoms in isolation.