Absolute zero is the theoretical point where all molecular motion ceases and they are at complete rest (except for quantum mechanical motion). At absolute zero, matter contains no heat energy. By international agreement, absolute zero is a temperature of precisely -273.15 °C.
According to kinetic theory, there should be no movement of individual molecules at absolute zero, so any material at this temperature would be solid. In a monatomic gas, most of the energy is in the form of translational motion, and the temperature can be measured in terms of the distribution of this motion, with slower speeds corresponding to lower temperatures, perhaps even down to absolute zero. But this is contrary to experimental evidence, as helium will never solidify at normal pressures, regardless of temperature.
Because of quantum-mechanical effects, the speed at absolute zero is larger than zero and depends, along with the energy, on the volume within which a particle is confined. At absolute zero, the molecules and atoms in a system are all in their ground state, the state of lowest possible energy, and a system has the least amount of kinetic energy allowed by the laws of physics. The lowest possible zero-point energy for a confined particle in a box, however, is not zero. Rather than being fixed and non-moving, the equation for the energy levels shows that no matter how low the temperature gets, even when the quantum number takes its minimum value of one, a particle still has some translational kinetic energy and motion. This is a reflection of Heisenberg's uncertainty principle, which states that one cannot measure values (with arbitrary precision) of certain conjugate quantities (e.g., the position and momentum), which are pairs of observables of a single elementary particle.
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