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The transactional interpretation of quantum mechanics (TIQM) describes quantum interactions in terms of a standing wave formed by both retarded ("forward-in-time") waves, in addition to advanced ("backward-in-time") waves. It was first proposed in 1986 by John G. Cramer, who argues that it helps in developing intuition for quantum processes. He also suggests that it avoids the philosophical problems with the Copenhagen interpretation and the role of the observer, and also resolves various quantum paradoxes.^{[1]}^{[2]} TIQM formed a minor plot point in his science fiction novel Einstein's Bridge.
More recently, he has also argued TIQM to be consistent with the Afshar experiment, while claiming that the Copenhagen interpretation and the many-worlds interpretation are not.^{[3]} The existence of both advanced and retarded waves as admissible solutions to Maxwell's equations was explored in the Wheeler–Feynman absorber theory. Cramer revived their idea of two waves for his transactional interpretation of quantum theory. While the ordinary Schrödinger equation does not admit advanced solutions, its relativistic version does, and these advanced solutions are the ones used by TIQM.
In TIQM, the source emits a usual (retarded) wave forward in time, but it also emits an advanced wave backward in time; furthermore, the receiver, who is later in time, also emits an advanced wave backward in time and a retarded wave forward in time. The phases of these waves are such that the retarded wave emitted by the receiver cancels the retarded wave emitted by the sender, with the result that there is no net wave after the absorption time. The advanced wave emitted by the receiver also cancels the advanced wave emitted by the sender, so that there is no net wave before the emitting time either. In this interpretation, the collapse of the wavefunction does not happen at any specific point in time, but is "atemporal" and occurs along the whole transaction, and the emission/absorption process is time-symmetric. The waves are seen as physically real, rather than a mere mathematical device to record the observer's knowledge as in some other interpretations of quantum mechanics.
Cramer uses TIQM in teaching quantum mechanics at the University of Washington in Seattle.
TIQM is explicitly non-local and, as a consequence, logically consistent with counterfactual definiteness (CFD), the minimum realist assumption.^{[1]} As such it incorporates the non-locality demonstrated by the Bell test experiments and eliminates the observer dependent reality that plagues the Copenhagen Interpretation. Greenberger–Horne–Zeilinger state the key advance over Everett's Relative State Interpretation^{[4]} is to regard the conjugate state vector of the Dirac formalism as ontologically real, incorporating a part of the formalism that, prior to TIQM, had been interpretationally neglected. Having interpreted the conjugate state vector as an advanced wave, it is claimed that the origins of the Born rule follow naturally from the description of a transaction.^{[1]}
The transactional interpretation has similarities with the two-state vector formalism (TSVF)^{[5]} which has its origin in work by Yakir Aharonov, Peter Bergmann and Joel Lebowitz of 1964.^{[6]}^{[7]}
Being "atemporal", TIQM assigns ontological priority to events in pseudo-time. This appears to have acted as the foremost inhibition to mainstream acceptance of the interpretation and underpins Maudlin's (1996, 2002) objection.^{[8]} Kastner (2010) has determined that inclusion of pseudotime is not a requirement of the transactional mechanism.^{[9]}
TIQM faces a number of common criticisms. The following is partial list and some replies:
1. “TI does not generate new predictions / is not testable / has not been tested.”
TI is an exact interpretation of QM and so its predictions must be the same as QM. Like the many-worlds interpretation (MWI), TI is a ‘pure’ interpretation in that it does not add anything ad hoc but provides a physical referent for a part of the formalism that has lacked one (the advanced states implicitly appearing in the Born rule). Thus the demand often placed on TI for new predictions or testability is a mistaken one that misconstrues the project of interpretation as one of theory modification.
2. “It is not made clear where in spacetime a transaction occurs.”
One clear account is given in Cramer (1986), which pictures a transaction as a four-vector standing wave whose endpoints are the emission and absorption events. Other possible accounts are being explored in which the formation of a transaction is an aspatiotemporal process, or one taking place on a level of possibility rather than actuality.
3. “Maudlin (1996, 2002) has demonstrated that TI is inconsistent.”
Maudlin (see below) raised an interesting challenge for TI which has been addressed by (at least) four different authors, all of which have presented ways for TI to remain viable in the face of this challenge:
4. It is not clear how the transactional interpretation handles the quantum mechanics of more than one particle.
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