Editing Transactional interpretation (section)

{{Short description|Interpretation of quantum mechanics}}
{{Quantum mechanics|cTopic=[[Interpretation of quantum mechanics|Interpretations]]}}
The '''transactional interpretation of quantum mechanics''' ('''TIQM''') takes the [[wave function]] of the standard [[Mathematical formulation of quantum mechanics|quantum formalism]], and its complex conjugate, to be retarded (forward in time) and advanced (backward in time) waves that form a quantum interaction as a [[Wheeler–Feynman absorber theory|Wheeler–Feynman handshake]] or transaction. 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 [[Physical paradox|paradoxes]].<ref>
{{cite book
 |last1=Cramer
 |first1=John
 |author1-link= John G. Cramer
 |title=Compendium of Quantum Physics
 |chapter=Transactional Interpretation of Quantum Mechanics
 |date=July 2009
 |editor1= Daniel Greenberger
 |editor1-link= Daniel Greenberger
 |editor2= Klaus Hentschel
 |editor2-link= Klaus Hentschel
 |editor3= Friedel Weinert
 |publisher= Springer
 |pages=795–798
 |doi=10.1007/978-3-540-70626-7_223
 |chapter-url=https://www.researchgate.net/publication/226312851
 |isbn=978-3-540-70622-9
}} {{open access}}</ref><ref name="Cramer 1986">
{{cite journal
 |last1=Cramer |first1=John G.
 |date=July 1986
 |title=The Transactional Interpretation of Quantum Mechanics
 |journal=Reviews of Modern Physics
 |volume=58
 |issue=3
 |pages=647–688
 |doi=10.1103/RevModPhys.58.647
 |url=https://www.researchgate.net/publication/280926546
 |bibcode=1986RvMP...58..647C
}} {{open access}}</ref><ref name="Cramer 1988">
{{cite journal
 |last1=Cramer |first1=John G.
 |date=February 1988
 |title=An Overview of the Transactional Interpretation
 |journal=International Journal of Theoretical Physics
 |volume=27
 |issue=2
 |pages=227–236
 |url=http://ws5.com/spacetime/Cramer%20-%20Transactional%20Interpretation.pdf
 |doi=10.1007/BF00670751
 |bibcode=1988IJTP...27..227C
 |s2cid=18588747
}}</ref> TIQM formed a minor plot point in his science fiction novel [[Einstein's Bridge (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.<ref name="Cramer Analog 2005">
{{cite journal
 |last1=Cramer |first1=John G.
 |date=December 2005
 |title=A Farewell to Copenhagen?
 |journal=Analog
 |series=The Alternate View
 |publisher=Dell Magazines
 |url=https://www.npl.washington.edu/av/altvw125.html
}}</ref>

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 [[Klein–Gordon equation|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. A quantum event occurs when a "handshake" exchange of advanced and retarded waves triggers the formation of a transaction in which energy, momentum, angular momentum, etc. are transferred.  The quantum mechanism behind transaction formation has been demonstrated explicitly for the case of a photon transfer between atoms in Sect. 5.4 of [[Carver Mead]]'s book ''Collective Electrodynamics''. 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]].{{citation needed|date=November 2017|reason=Others disagree about 'physically' real}} Philosopher and writer [[Ruth Kastner]] argues that the waves exist as possibilities outside of physical spacetime and that therefore it is necessary to accept such possibilities as part of reality.<ref>George Musser and Ruth Kastner; [https://blogs.scientificamerican.com/critical-opalescence/can-we-resolve-quantum-paradoxes-by-stepping-out-of-space-and-time-guest-post/ "Can We Resolve Quantum Paradoxes by Stepping Out of Space and Time?"], ''Scientific American'' blog, June 21, 2013.</ref>

Cramer has used TIQM in teaching quantum mechanics at the [[University of Washington]] in [[Seattle, Washington|Seattle]].