Absolute Zero
Absolute zero is the lowest possible temperature, 0 K or -273.15 degrees Celsius, where particles have the minimum possible energy.
Definition
Absolute zero is the coldest temperature that could ever exist, the point at which the particles in a substance would have the minimum possible energy of motion left. It is defined as $0\ \text{K}$, equal to $-273.15^\circ\text{C}$ or $-459.67^\circ\text{F}$, and it marks the starting point of the Kelvin scale; there is a coldest possible temperature but no hottest one, because particles can only slow down so much. In the language of quantum mechanics, absolute zero is the limit $T \to 0$ at which a system reaches its quantum ground state, though particles retain zero-point energy required by the uncertainty principle, so motion never fully ceases. The third law of thermodynamics states that the entropy of a perfect crystal approaches zero as $T \to 0$ and that no finite sequence of processes can reach $0\ \text{K}$ exactly, and near this limit matter shows quantum behavior such as superconductivity, superfluidity, and Bose-Einstein condensation.
Example
Absolute zero is $-273^\circ\text{C}$, far colder than Antarctica ($-89^\circ\text{C}$ at its coldest) or the surface of Pluto (about $-230^\circ\text{C}$); liquid nitrogen boils at $77\ \text{K}$ ($-196^\circ\text{C}$), liquid helium at $4.2\ \text{K}$, and deep space, warmed only by leftover radiation from the early universe, sits at $2.7\ \text{K}$. Absolute zero itself was first predicted from a graph, not a thermometer: cooling a gas shrinks its volume in a straight line, and a sample with volume $1.00\ \text{L}$ at $0^\circ\text{C}$ and $1.37\ \text{L}$ at $100^\circ\text{C}$ changes by $0.0037\ \text{L}$ per degree, so extrapolating to $V = 0$ gives $T = -1.00/0.0037 \approx -270^\circ\text{C}$, close to the accepted $-273.15^\circ\text{C}$.
Key Insight
Scientists have cooled atoms to within billionths of a kelvin of absolute zero using laser cooling and evaporative cooling, reaching below $100\ \text{pK}$, colder than anything found in nature; at those temperatures thousands of atoms can share one quantum state and behave as a single super-atom, a Bose-Einstein condensate.