Because an individual radioactive nucleus will eventually decay at a specific time, Einstein and others believed that a complete physical theory should allow this time to be predicted exactly, rather than just statistically. Bohm became a leading proponent of the 'hidden variable' theory, which restores determinancy at the quantum level. Whereas in Bohr's interpretation quantum events are predicted probabilistically, in hidden variable theories there exist 'hidden' properties, that is properties which we cannot measure, which determine events about quantum physics.
In Bohm's interpretation, the Schrödinger wave function is treated almost like a classical force field, such as a magnetic force field or gravitation. A term called 'quantum potential' is also introduced, just as we know about electric and magnetic potential. The wave function guides the particles along their paths, just as a magnetic field causes magnetic substances to move in a particular fashion.
The mathematician John von Neumann proved long ago that no hidden variable theory can agree exactly with the predictions of quantum physics. Not all the predictions have been examined, however, so there exists some possibility a hidden variable theory could be formulated that agrees with all observations that have been made. None has yet been produced that physicists find satisfactory.
Based on a work of H. Everett in 1957, J. A. Wheeler and D. DeWitt suggested that all available choices are indeed realised at measurement -- the universe splits into copies of equally real universes, each containing one of the possible choices! Additionally, these almost identical parallel universes coexisting in time and space cannot communicate with each other. This proliferation of copies of the entire universe is repeated each time a measurement is made!
At its face value this incredible suggestion is untestable.
The point of all this is to avoid the 'reduction' or 'collapse'
hypothesis of the standard interpretation of quantum physics.
Another possible virtue is to address the 'fine-tuning problem'
which tries to understand why many of the parameters in our
universe are adjusted in their values to generate the observed
properties of matter. A many-worlds interpretation hypothesis
would be that all possible values of these paramters actually occur
in parallel universes; we happen to reside on the one with
finely-tuned parameters. But the other universes with
different parameters will have very different properties and
evolution processes and cannot be the almost identical copies
needed to avoid the reduction hypothesis!
Quantum physics is now over 70 years old and has been very successful in providing explanations for physical phenomena, with the prevailing Copenhagen interpretation. Much of the criticism it has faced derives from the radical change from earlier theories that quantum physics represents. Some of it involves problems that arise within quantum physics itself. The question of the correct interpretation of the mathematical formalism has remained something of a problem up to the present. For now, we shall leave the matter there, and take up the current status of this subject in the next chapter.