I think I will drop into this group a brief passage fom Heisenberg's
"Quantum Theory" translated into English in 1930.
"Dirac has set up a wave equation which is valid for one electron and is
invarient under the Lorentian transformation. It fulfills all
requirements of the quantum theory, and is able to give a good account
of the phenomena of the "spinning" electron, which could previously only
be treated by, ad hoc assumptions. The essential difficulty which arises
with all relativistic quantum theories is not eliminated however. This
arises from the relation
1/c^2 = u^2C^2 + p^2 in x + p^2 in y + p^2 in z
between the energy and the momentum of a free electron. According to
this equation there are two values of E which differ in sign associated
with each set of P in z, P in y and P in Z. The classical theory could
eliminate this by arbitrarily excluding the one sign, but this is not
possible according to the principles og quantum theory. Here
spontaneuos transitions may occure to the states of negative energy; as
these have never been observed, the theory is certainly wrong. Under
these conditions it is very remarkable that the positive energy-levels
(at least in the case of one electron) coincide with those actually
observed."
So what was wrong? What occured that proved both theories were correct?