Quantum Mechanics

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Quantum mechanics is a theory that describes the interactions of all particles and systems. It underlies all physical phenomena, including scattering.


Wavefunction

A quantum system is completely specified by its Wave Function:

Integral Notation Dirac Notation
     

The wavefunction is typically normalized:

Integral Notation Dirac Notation
     

The distribution of the particle described by is given by:

Integral Notation Dirac Notation
     

In the Copenhagen Interpretation, is the probability of finding the particle at location . In Universal Wave Function interpretations (e.g. MWI), can be thought of as the spatial distribution of the particle. The wavefunction contains all the information one can know about a system. It can thus be thought of as 'being' the particle/system in question. However, the wavefunction can be described in an infinite number of different ways. That is, there is not a unique basis for describing the wavefunction. So, for instance, one can describe the wavefunction using position-space or momentum-space:

These representations can be inter-related (c.f. Fourier transform):

State

Note that the wavefunction describes the state of the system; there are various choices of basis one can use as an expansion.

This can also be viewed as a vector in the Hilbert space. The Dirac notation (bra-ket notation) is useful in this regard. A particular state is a (column) vector:

Which is a 'ket'. We define a 'bra' (the 'final state') as a (row) vector:

And note that the 'bra' is the conjuagte transpose of the 'ket':

Wave packet

TBD

Heisenberg Indeterminacy Relations

(Also known as Heisenberg Uncertainty Principle.)

Superposition

If and are both allowed states for a given system, then the following state is also allowed:

This leads to a notable consequence:

Notice that the final terms represent 'interference' between the two constituent states. This interference has no classical analogue; it is a quantum effect. Thus a superposition is not merely a 'joining' of the two states (e.g. "the particle can be in state 1 or state 2"), but a truly coherent interference between the two states. The superposition may be more generally written as:

Integral Notation Dirac Notation
     

The distribution of the particle described by is given by:

Integral Notation Dirac Notation
     

Measurement

The 'probability' for the wavefunction to collapse into state is:

Integral Notation Dirac Notation
     

See Also