WHAT IS QUANTUM THEORY ABOUT?

Indeterminacy

• Thinking in terms of particles, it appears that the passage of the photon or electron through the slit screen involves some kind of NONLOCAL interaction, since its behaviour depends on the state of the distant slit through which it does not pass.
• If we invent any way of watching the particles to identify which slit each passed through, then we reduce the uncertainty in position of the particle as it goes through the slit screen, and the direction of its momentum becomes so uncertain that the pattern is wiped out. If the interference pattern is seen, then the path of each particle must be sufficiently uncertain that we can't tell which slit it passed through.
Many "non-demolition" experiments have been devised, in which the paths of the particles are identified indirectly, so that there is no direct physical mechanism to affect the path. But the interference pattern still vanishes unless the path is sufficiently uncertain.

The uncertainty in the path of a particle contributing to an interference pattern is not simply an uncertainty in our knowledge of its path. It is more fundamental than that. The particle cannot have a precise path, any more than a wave can be associated with a unique linear track. If interference is observed, the path of the particle is INDETERMINATE - it is not physically well-defined rather than just unknown.

Radioactive decay

Indeterminacy is a general feature of the behaviour of quantum particles. Radioactive atoms decay with a characteristic half-life T. This means that in a time T half the atoms in any sample will decay. If we watch any particular atom, then there is a 50:50 chance that it will decay in time T. The time when a specific atom decays is random, and the process can only be described statistically. During any time interval the state of the atom - decayed or undecayed - is indeterminate. It has a certain probability of being decayed and another of being undecayed. These two aspects of its state must be combined in the same way as the amplitudes of waves. The probabilities for the two components of the state are found from the wave intensity. This quantum wave has an amplitude represented by the Greek letter psi, written .

It is only when we detect the decay products that we know the atom has decayed, and its state turns into the decayed state. The wave (or wave function) collapses from being a sum of two waves, one representing the undecayed state and the other the decayed state, and becomes that representing the decayed state.