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#### Quantum Theory

Posted by Maddalena Frau on October 14, 2013 at 9:45 AM |

Quantum Theory (or Quantum Mechanics or Quantum Physics - the terms are interchangeable) is an extension of physics in order to cover the behaviour of microscopic objects. Physics as it was before Quantum Theory is called Classical Physics.

On some versions Quantum Theory includes Classical Physics as a special case. From the start the theory was subject to controversy and developed into a wealth of different forms, mostly agreeing at the level of practical calculation but disagreeing wildly as to the interpretation.

The question "what is quantum theory" is therefore a difficult one.

Both Classical and Quantum Physics describe how the observable ** properties ** of a ** system **
change with time.

The "system" (which here means "thing") can be anything from an atom to the universe; its properties are quantities like position, momentum, energy, the internal arrangements of its parts and so on.

In Classical Physics there is a set of properties for any given system (namely the positions and velocities of all its parts) which completely determines its time-development and the properties at any later time.

**In Quantum Physics there is no such complete set of
properties. Instead**

1. At any given time there are many different possible sets of properties, any one of which sets can be observed; but it is not possible to observe all the properties simultaneously. For instance, position and velocity cannot be observed simultaneously; the first gives a particle-picture the second a wave-picture.

2. Any properties at a later time cannot (except in special circumstances) be determined by observing properties at an earlier time. Only their probabilities are fixed by the earlier observation.

The term ** observed ** means different things in
different versions: e.g. "manifested," "recorded by a macroscopic
instrument," "brought to (human?) consciousness" and so on. The last
possibility links quantum theory with theories of mind. At any given
time there is a well defined specification of the probability of
observing any given property. This collection of probabilities is fixed
by (or in some versions is identical with) the ** quantum state, **
but this state is not itself observable. Interpretations differ as to
whether the state is real or a mathematical abstraction, with profound
consequences for the whole notion of reality in physics.

The earliest interpretations, dating from workers in Copenhagen, used a two-tier world: a small system obeying non-Classical Physics and an observing laboratory obeying Classical Physics. The many pre-1965 theories tend to call themselves "The Copenhagen Interpretation." Later interpretations tried to achieve a more unified view. This history introduced a succession of alternative structures: the collapse of the state, many worlds, environmental diffusion and so on. These have almost all been superseded.

Systems with infinitely many degrees of freedom (in particular, fields such as the electromagnetic field) are described by ** quantum field theory ** whose states can all be constructed out of a special state of the field in question called the ** vacuum ** for that field. The vacuum has zero energy (except in Dirac's theory which enjoyed brief popularity).

*Prof. Chris Clarke is visiting Professor in the Faculty of
Mathematical Studies, University of Southampton. He is the author of
'Reality Through the Looking Glass' (1995).*

Categories: Quantum Physic, Research, news

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