*Next month will be the 100th anniversary of Bohr's model of the atom, one of the foundations of the theory of quantum mechanics. And look where we are now: we still don't know what the darned theory really means. One of the most radical interpretations (which is saying something) has got to be the so-called Transactional Interpretation, whereby particles send a type of signal backward in time. This past fall, University of Maryland philosopher Ruth Kastner published a book that tries to make sense of it. I've invited her to guide us through it.*In the June issue of Scientific American, physicist and writer Hans Christian von Baeyer describes the current state of "deep confusion about the meaning of quantum theory" and discusses one proposal--a denial that the theory describes anything objectively real--for rendering some of the quantum perplexities "less troubling." Von Baeyer also lists several other possible interpretations, but leaves out what I think is the most promising approach. The idea, known as the Transactional Interpretation, was first proposed by University of Washington physicist John Cramer in the 1980s and has its roots in the ideas of renowned physicists John Wheeler and Richard Feynman. This interpretation makes use of a concept known technically as "advanced action," which is characterized not by the usual positive energy but by

*negative*energy. Though it may seem counterintuitive at first, it turns out to provide a natural way to understand certain aspects of the theory that currently seem arbitrary or ad hoc, such as the rule for calculating the probabilities of measurement outcomes. In the transactional picture, the entities described by quantum states, which are characterized by positive energy, are only half the story. The other half of the story is the absorption of those emitted states, which is accompanied by a negative-energy (advanced) response. Cramer himself compared his account to the handshake of a financial transaction: the emitted state is the offer and the response state is the confirmation. Absorption is the key to untying the interpretational Gordian Knot presented by quantum theory, which has given rise to such perplexities as the famous Schr?dinger's Cat thought experiment. It is absorption that collapses the quantum superposition and saves the poor cat from the fate of being both dead and alive at the same time. Cramer's original version of the interpretation, although promising, did not receive widespread acceptance. Physicists and philosophers had trouble making sense of advanced propagation, which is usually considered synonymous with back-in-time propagation and therefore seemed to raise the possibility of causal-loop paradoxes, such as being able to go into the past and kill one's own parents. In addition, some critics felt that the notion of absorber was not well-defined. My research is aimed at resolving these types of challenges and providing a clear account of what constitutes an absorber. By incorporating principles from relativistic quantum theory, which were absent from the original transactional picture, I have been able to obtain a clear criterion for the boundary between the microscopic quantum realm and the macroscopic classical realm, which is the point at which collapse is overwhelmingly likely to occur (although the collapse process is fundamentally indeterministic). My development of the Transactional Interpretation makes use of an important idea of Werner Heisenberg: "Atoms and the elementary particles themselves ... form a world of potentialities or possibilities rather than things of the facts." This world of potentialities is not contained within space and time; it is a higher-dimensional world whose structure is described by the mathematics of quantum theory. The Transactional Interpretation is best understood by considering both the offer and confirmation as Heisenbergian possibilities--that is, they are only potential events. That removes the possibility of causal-loop inconsistencies, since neither the positive-energy offer wave nor the negative-energy confirmation wave carries real energy, and neither is contained in spacetime. It is only in the encounter between the two that real energy may be conveyed within spacetime from an emitter to an absorber--and when this occurs, all the energy is delivered in the normal future direction. The Transactional Interpretation, in this new possibilist version, provides not only a clear physical account of measurement but also a new understanding of quantum reality in which dynamic possibilities give rise to observable physical events through the transactional process. It also renders harmless the "spooky action at a distance" that troubled Einstein. Quantum correlations do not violate the relativistic speed limit because these correlations exist only at the level of possibility. The transactional picture is conceptually challenging because the underlying processes are so different from what we are used to in our classical world of experience, and we must allow for the startling idea that there is more to reality than what can be contained within spacetime. As is evident from von Baeyer's article, quantum theory truly challenges us to think outside the box--and, in this case, I submit that the box is spacetime itself. If this seems farfetched, consider the eloquent point made by physicist and philosopher Ernan McMullin: "Imaginability must not be made the test for ontology. The realist claim is that the scientist is discovering the structures of the world; it is not required in addition that these structures be imaginable in the categories of the macroworld." Only if we face the strange non-classical features of the physical world head-on can we have a physical, non-observer-dependent account of our reality that solves longstanding puzzles such as the problem of Schr?dinger's Cat.

*Images courtesy of Ruth Kastner*