1986: Transactional interpretation
The Transactional Interpretation (TI) was developed by John Cramer and originates from about 1986. This interpretation is technically quite complex and therefore this discussion will only provide the most basic of introduction of its underlying concepts. The interpretation is based on a more objective approach that employs an explicitly non-local `transaction' which describes any quantum event as a `handshake` executed through an exchange of `advanced` and `retarded` waves, which are respectively associated with `forward-in-time` and `backward-in-time` processes. An advanced wave is a valid solution to a wave equation, but the wave is required to a have negative energy and propagate backwards in time. As this behaviour is not observed in nature, these solutions are not normally used, but are retained in the transaction interpretation.
As understood, an advanced wave at , as shown in the diagram above, is offered by the emitter and can be considered to be moving backwards in time. The absorber then responds via a retarded wave as a form of confirmation. This advanced-retarded handshake is the basis of the transactional interpretation of quantum mechanics. When the retarded wave is received at , i.e. at some time in the future, the process effectively erases all traces of the advanced wave, thereby preserving causality. The very nature of the quantum world means that this mechanism cannot be observed at the level of the classical world.
- The double-slit experiment:
Again, only a basic interpretation is attempted. The advanced waves passes through both slits, but are moving backwards in time. The retarded waves are subject to interference in the sense that the confirmation is either minimal or maximal in-line with interference eventually produced.
- The EPR paradox:
The correlation of spin within the EPR experiment is guaranteed by the advanced wave moving backwards in time, which is then confirmed by a retarded wave moving forward in time. This transaction process then accounts for the apparent non-locality of what appears to be an instantaneous synchronisation of the emergent particle properties.
However, this interpretation would seem to raise some fairly important issues concerning causality, even if we assume the effects cannot be directly observed. There are two basic principles of causality, which are described in terms of the strong and the weak principle. Basically, the strong principle states that a cause should always precede its effect in all realities, while the weak principle is confined to the classical world. Given that observation of the quantum world is restricted, it would appear that this interpretation is not subject to even conceptual verification, which is consistent with many of the more esoteric interpretations.
Note: See Cosmic Interpretation in right inset for a specific problems of this interpretation in respect to an expanding universe.