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There is nothing particularly unique about the transmission and absorption of photons
through a polarizing film. Exactly the same conclusions as those outlined in the previous section
are obtained by studying other simple experiments involving photons or microscopic particles. For instance, the interference of photons [108,32], or the Stern-Gerlach experiment [54,46,95]. An examination of
these experiments leads us to formulate the following fundamental principles of
quantum mechanics:
**Dirac's Razor:**
- Quantum mechanics can only answer questions regarding the
outcome of possible experiments. Any other questions lie beyond the realms of
physics.

**Principle of Superposition of States:**
- Any microscopic system (e.g., an atom, molecule, or particle) in a given state can be regarded
as being partly in each of two or more other states. In other words, any state
can be regarded as a superposition of two or more other states. Such superpositions can be performed in an infinite number
of different ways.

**Principle of Indeterminacy:**
- An observation made on a
microscopic system causes it
to jump into one or more particular states (that are related to the type
of observation). In general, it is impossible to predict into which final
state the system
will jump. However, the probability of a given system jumping into a given final
state can be predicted.

The first of these principles was formulated by quantum physicists (such as Dirac)
in the 1920's
to fend off awkward questions such as
``How can a microscopic system suddenly
jump from one state into another?''
or
``How does a microscopic system decide into which state to
jump?''
As we shall see, the second principle is
the basis for the mathematical formulation of quantum mechanics.
The final principle is still rather vague. We need to extend it
so that we can predict into which possible states a system can jump after
a particular type of observation, as well as the probability of
the system making a particular jump.

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** Up:** Fundamental Concepts
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Richard Fitzpatrick
2016-01-22