Quantum Mechanics: A Transactional Interpretation

 

Every once in a while, as regular readers know, I write about quantum mechanics. I don't write as an authority -- I was a solid "C" student in an adult-education course in calculus about 20 years ago and will never make a pretense to understand particle physics. The sphere diagrammed in this image has something to do with quantum theory, and something to do with calculus. And that is all I can tell you about it!  

But quantum mechanics is relevant to many of the philosophical questions in which I have a vested interest in my capacity as a thinking human. So I do need to keep struggling with it. 

I recently came across an interpretation I had not heard before: the transactional interpretation. 

By way of preface: The mainline interpretation is known simply as "Copenhagen" after a period in the mid 1920s when Heisenberg worked as assistant to Bohr at the University of Copenhagen. The Copenhagen view accepts the complementarity of wave and particle readings, accepts indeterminacy at the quantum level, and maintains that everything washes out to coincide with classical physics as the scales involved get larger and pass a threshold. Beyond that, Copenhagen doesn't believe in interpretive theorizing. A phrase often used in this connected is "shut up and calculate."

Another important interpretation, and one that has made its way into pop culture, is the many-worlds view. Some people find the indeterminism difficult to accept. Some of THEM adopt the view that a seemingly indeterminate result, which turned out to be A but could have been B, is actually a fully determined result in which the universe forked -- A on one timeline, B on the other. The only indeterminism here is one over which half of that fork carries the observer. Do I see the dead cat or the living one? Both cats are real, and the fact that there will be such a fork was fully determined.  

Yet another interpretation, and one I have written about here, is Quantum Bayesianism, or QBism, sometimes pronounced "cubism" as in art history classes. This builds from Bayesian probability theory, and accepts a necessarily subjective component to knowledge. 

But let's get back to where we started. There is a "transactional interpretation" of quantum mechanics.  It amounts to taking it as a given that, on a quantum scale, causes can come later in time than their effects. The causal arrow points both ways. 

One point with which all interpretations must deal, after all, is this: a wave-function representing the probable position of a photon spreads out in all directions. But the wave instantly collapses once an observation is made, (or, we might also say, in the presence of a detector). It collapses everywhere, so that the consequences of this collapse can be observed/detected at points far distant from one another.  Or that is how the Copenhagen school would explain what happens.

The TI explains it differently. The source of a photon sends out an "offer wave" moving both forward and backward in time. Consider the wave moving forward in time. This arrives at detectors. Perhaps the "first" time this happens (first is in quotation marks there for a reason) it affects the different detectors differently. Each of the detectors sends back a "confirmation wave." This is going backward in both space AND time to the source. If there is no agreement among the detectors then there is a back and forth echoing in a sort of meta-time, as if the source and the detectors are bargaining. Hence the term "transactional interpretation." 

Eventually the back-and-forth reaches a point at which all the detectors agree. The probability for one detection event is 1, the probability for the rest is 0. Since we don't live in the meta-time in which this bargaining-like echoing takes place, we perceive only one event. The source has sent out a photon and the wave has collapsed in a determinate result, and the various detectors agree on that event.  

So, that is the transactional interpretation. As I understand it. If you don't think I DO understand it, you may be right. Here is more.  

I will close with a quotation from one of my favorite episodes of The Big Bang Theory. 

Howard, who is an engineer and is the only member of the group of four central men who does NOT have a Ph.D., "only" an MS from MIT, is considering going for the doctorate. He talks to Sheldon about it. Sheldon will be teaching a course Howard will have to take. 

Sheldon asks, "What is the correct interpretation of quantum mechanics."

Howard, "As an engineer, I observe that their predictions of experimental results coincide and I need not choose between them. But I know that you believe in the many-worlds view, so on a test I would give that answer." I once joked to one of my sisters about this line. (Yes, we're a pretty nerdy family.) I said, "two pragmatisms expressed in a single sentence!"