The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. 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.
If more than one ground state exists, they are said to be degenerate. Many systems have degenerate ground states. It turns out that degeneracy occurs whenever a nontrivial unitary operator commutes with the Hamiltonian of the system.
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. It is also possible for the highest excited state to have absolute zero temperature for systems that exhibit negative temperature.
Read more about Ground State: Examples
Famous quotes containing the words ground and/or state:
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—Gertrude Stein (1874–1946)
“That popular fable of the sot who was picked up dead-drunk in the street, carried to the duke’s house, washed and dressed and laid in the duke’s bed, and, on his waking, treated with all obsequious ceremony like the duke, and assured that he had been insane, owes its popularity to the fact that it symbolizes so well the state of man, who is in the world a sort of sot, but now and then wakes up, exercises his reason and finds himself a true prince.”
—Ralph Waldo Emerson (1803–1882)