The theorem

(88) |

(89) |

For example, consider non-interacting spinless particles
of mass
confined in a cubic box of dimension . According to standard
wave-mechanics, the
energy levels of the th particle are given by

(90) |

(91) |

Consider, now, a statistical ensemble of systems made up
of weakly interacting particles. Suppose that this ensemble
is initially very far from equilibrium. For instance,
the systems in the ensemble might only be distributed
over a very small subset of their accessible states.
If each system starts off in a
particular stationary state (*i.e.*, with a particular set of quantum
numbers) then, in the absence of particle interactions, it will remain
in that state for ever. Hence, the
ensemble will always stay far from equilibrium, and the principle of
equal *a priori* probabilities will never be applicable. In reality,
particle interactions cause each system in the ensemble to make
transitions between its accessible ``stationary'' states. This allows the
overall state of the ensemble to change in time.

Let us label the accessible states of our system by the index . We can
ascribe a time dependent probability of finding the system
in a particular approximate stationary
state at time . Of course, is proportional
to the number of systems in the ensemble in state at time . In general,
is time dependent because the ensemble is evolving towards an
equilibrium state. We assume that the probabilities are properly
normalized, so that the sum over all accessible states always yields

(92) |

Small interactions between particles cause transitions between the approximate
stationary states of the system. There then exists some transition probability
per unit time that a system originally in state ends up in state
as a result of these interactions. Likewise, there exists a probability
per unit time that a system in state makes a transition to
state . These transition probabilities are meaningful in quantum mechanics
provided that the particle interaction strength is sufficiently small, there is
a nearly continuous distribution of accessible energy levels, and we consider time
intervals which are not too small. These conditions are easily satisfied for
the types of systems usually analyzed via statistical mechanics (*e.g.*,
nearly ideal gases). One important conclusion of quantum
mechanics is that the forward and inverse transition probabilities between two
states are the same, so that

Suppose that we were to ``film'' a microscopic process, such as two classical particles approaching one another, colliding, and moving apart. We could then gather an audience together and show them the film. To make things slightly more interesting we could play it either forwards or backwards. Because of the time reversal symmetry of classical mechanics, the audience would not be able to tell which way the film was running (unless we told them!). In both cases, the film would show completely plausible physical events.

We can play the same game for a quantum process.
For instance, we could ``film'' a group of photons
impinging on some atoms. Occasionally, one of the atoms will *absorb* a
photon and make a transition to an ``excited'' state (*i.e.*,
a state with higher than
normal energy). We could easily estimate the rate constant for this process
by watching the film carefully. If we play the film backwards then it will appear to
show
excited atoms occasionally *emitting* a photon and
decaying back to their unexcited state. If quantum mechanics possesses
time reversal symmetry (which it certainly does!) then both films should appear
equally plausible. This means that the rate constant for the absorption of
a photon to produce an excited state must be the same as the rate constant for
the excited state to decay by the emission of a photon. Otherwise, in the backwards
film the excited atoms would appear to emit photons at the wrong rate, and we could
then tell that the film was being played backwards. It follows, therefore, that
as a consequence of time reversal symmetry, the rate constant for any process in
quantum mechanics must equal the rate constant for the inverse process.

The probability of finding the systems in the ensemble
in a particular state changes
with time for two reasons. Firstly, systems in another state can make transitions
to the state . The rate at which this occurs is , the probability
that the systems are in the state to begin with, times the rate constant
of the transition . Secondly, systems in the state can make
transitions to other states such as . The rate at which this occurs
is clearly times . We can write a simple differential equation
for the time evolution of :

(94) |

where use has been made of the symmetry condition (93). The summation is over all accessible states.

Consider now the quantity (from which the theorem derives its name),
which is the mean value of over all accessible states:

(96) |

(97) |

(98) |

(99) |

Note, however, that is a monotonically increasing function of . It follows that whenever , and

(101) |

The theorem tells us that if an isolated system is initially not in
equilibrium then it will evolve under the influence of particle interactions in
such a manner that the quantity always *decreases*. This process will
continue until reaches its minimum possible value, at which point
, and there is no further evolution of the
system. According to Eq. (100), in this final equilibrium state
the system is equally likely to be found in any one of its accessible
states. This is, of course, the situation predicted by the principle
of equal *a priori* probabilities.

You may be wondering why the above argument does not constitute a mathematically
rigorous proof that the principle of equal *a priori* probabilities
applies to many particle systems. The answer is that we
tacitly made an unwarranted
assumption: *i.e.*, we assumed
that the probability of the system making a transition
from some state to another state is *independent* of the past history
of the system. In general, this is not the case in physical
systems, although there are many
situations in which it is a pretty good approximation. Thus, the
epistemological status of the principle of equal *a priori* probabilities is that it
is *plausible*, but remains *unproven*. As we have already mentioned,
the ultimate justification for this principle is *empirical*: *i.e.*,
it leads to theoretical predictions which are in accordance with
experimental observations.