Helium Is the Only Element That Cannot Be Solidified by Pressure Alone

While squeezing most substances is enough to force their atoms into an orderly, solid state, helium steadfastly refuses to play by these rules. This unique resistance stems from a quantum mechanical principle known as zero-point energy. Because helium atoms are so incredibly lightweight and the attractive forces between them are exceptionally weak, they are in a constant state of motion. Even at the theoretical coldest temperature of absolute zero, where all classical atomic motion should cease, helium atoms still vibrate with enough energy to prevent them from locking into the rigid crystal lattice of a solid. This perpetual quantum jiggle is simply too powerful for pressure alone to overcome.

To finally coax helium into its solid form, a discovery made by Willem Keesom in 1926, scientists must use a two-pronged attack. They must apply immense pressure, over 25 times that of Earth's atmosphere at sea level, while simultaneously chilling the element to temperatures below 1 Kelvin (-458°F). Even then, the resulting solid is anything but ordinary. It can exist as a "supersolid," a bizarre state of matter that is both rigidly ordered like a solid and can flow without any friction like a superfluid. This strange behavior makes solid helium a unique natural laboratory for studying the counterintuitive laws that govern the quantum world.