**Quantum Theory**

In quantum mechanics, energy of a quantum system is described by a self-adjoint (Hermite) operator called Hamiltonian, which acts on the Hilbert space (or a space of wave functions ) of the system. If the Hamiltonian is a time independent operator, emergence probability of the measurement result does not change in time over the evolution of the system. Thus the expectation value of energy is also time independent. The local energy conservation in quantum field theory is ensured by the quantum Noether's theorem for energy-momentum tensor operator. Note that due to the lack of the (universal) time operator in quantum theory, the uncertainty relations for time and energy are not fundamental in contrast to the position momentum uncertainty principle, and merely holds in specific cases (See Uncertainty principle). Energy at each fixed time can be precisely measured in principle without any problem caused by the time energy uncertainty relations. Thus the conservation of energy in time is a well defined concept even in quantum mechanics.

Read more about this topic: Conservation Of Energy, Mechanics

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**Quantum Theory**

**Quantum theory** may mean:

In science:

- Quantum mechanics: a subset of physics explaining the physical behaviours at atomic and sub-atomic levels
- Old quantum theory under the Bohr model
- Quantum field theory, a generic type of relativistic quantum theory, which includes:
- Quantum electrodynamics
- Quantum chromodynamics
- Electroweak interaction

- Quantum gravity, a general term for theories intended to quantize general relativity
- Quantum optics

In popular culture:

- "Quantum Theory", the final song from
*Jarvis (album)*, by Jarvis Cocker - Quantum Theory (video game) is also the name of a console video game for PS3 and Xbox 360, released in 2010

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### Famous quotes related to *quantum theory*:

“But how is one to make a scientist understand that there is something unalterably deranged about differential calculus, *quantum theory*, or the obscene and so inanely liturgical ordeals of the precession of the equinoxes.”

—Antonin Artaud (1896–1948)