Ground state

In physics, the ground state of a quantum mechanical system is its lowest-energy state. An excited state is any state with energy greater than the ground state. The ground state of a quantum field theory is usually called the vacuum state or the vacuum.

Related Topics:
Physics - Quantum mechanical - Energy - State - Excited state - Quantum field theory - Vacuum state - Vacuum

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If more than one ground state exists, they are said to be degenerate. Many systems have degenerate ground states, for example, the hydrogen atom. It turns out that degeneracy occurs whenever a nontrivial unitary operator commutes with the Hamiltonian of the system.

Related Topics:
Hydrogen atom - Unitary operator - Commutes - Hamiltonian

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According to the third law of thermodynamics, a system at absolute zero temperature exists in its ground state; thus, its entropy is determined by the degeneracy of the ground state. Many systems, such as a perfect crystal lattice, have a unique ground state and therefore have zero entropy at absolute zero (because ln(1) = 0).

Related Topics:
Third law of thermodynamics - Absolute zero - Temperature - Entropy - Crystal

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