In quantum mechanics and quantum field theory, the <answer>vacuum</answer> is defined as the state (that is, the solution to the equations of the theory) with the lowest possible energy (the ground state of the Hilbert space). In quantum electrodynamics this <answer>vacuum</answer> is referred to as 'QED <answer>vacuum</answer>' to distinguish it from the <answer>vacuum</answer> of quantum chromodynamics, denoted as QCD <answer>vacuum</answer>. QED <answer>vacuum</answer> is a state with no matter particles (hence the name), and also no photons. As described above, this state is impossible to achieve experimentally. (Even if every matter particle could somehow be removed from a volume, it would be impossible to eliminate all the blackbody photons.) Nonetheless, it provides a good model for realizable <answer>vacuum</answer>, and agrees with a number of experimental observations as described next.
The state with the lowest possible energy in quantum mechanics defines what ?
vacuum