What is absolute zero, and can anything reach it?

Absolute zero is 0 kelvin, equal to minus 273.15 Celsius or minus 459.67 Fahrenheit, and it represents the point at which particles hold the minimum possible energy. It is not a state of literal zero motion, because quantum mechanics requires residual zero-point energy that cannot be removed. It is unreachable in principle, a consequence of the third law of thermodynamics: each step of cooling removes a fraction of the remaining energy, so reaching zero would require infinitely many steps. Laboratories have come extraordinarily close, cooling atomic gases to within billionths of a kelvin using laser cooling and evaporative techniques.

What happens near that limit is why physicists chase it. Matter enters states with no everyday analog: superfluid helium flows without viscosity and creeps up the walls of its container, superconductors carry current with zero resistance, and Bose-Einstein condensates form when a cloud of atoms collapses into a single shared quantum state, behaving as one entity, a phenomenon predicted in the 1920s and finally produced in 1995. These are not curiosities, since superconductivity underlies MRI machines, particle accelerator magnets, and quantum computing hardware. The coldest known naturally occurring place is the Boomerang Nebula at about 1 kelvin, colder than the cosmic microwave background, cooled by its own rapid gas expansion, which means the coldest spots in the universe are in laboratories on Earth.