Alternate View Column AV-16
Keywords: quantum, paradoxes,transactional, Copenhagen, interpretation
Published in the November-1986 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 4/4/86 and is copyrighted © 1986, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.
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Quantum mechanics is weird. It has led respectable physicists to spin theories about cats that are half alive and half dead, about worlds which split into alternate universes with each quantum event, about a reality altered because an intelligent observer watches it, about mathematical equations describing "knowledge" rather than physical reality. This month's AV is about my own work, a new interpretation of quantum mechanics which seeks to dispell this weirdness by depicting each quantum event as a "transaction", a sort of handshake across space-time. A long description of this "Transactional Interpretation" has just been published in the July Reviews of Modern Physics (available at most university and major public libraries). It challenges the standard Copenhagen Interpretation of Bohr and Heisenberg which has maintained a shaky dominance as the orthodox interpretation of quantum mechanics for over fifty years.
Quantum mechanics (QM) was invented in the late 1920's when an embarrassing body of new experimental facts from the microscopic world couldn't be explained by the accepted physics of the period. Heisenberg, Schroedinger, Dirac, and others used a remarkable combination of intuition and brilliance to devise clever ways of "getting the right answer" from a set of arcane mathematical procedures. They somehow accomplished this without understanding in any basic way what their mathematics really meant. The mathematical formalism of quantum mechanics is now trusted by all physicists, its use clear and unambiguous. But even now, five decades later, its meaning remains controversial. One hears the platitude that "mathematics is the language of science". Quantum mechanics reminds us that this "language" may lack a proper translation, that formulating a theory is not the same as understanding its meaning.
For orientation, let's start our discussion with some fairly simple questions and answers:
Q: What is quantum mechanics?
A: It's the theory which deals with the smallest scale of physical objects in the universe, objects (atoms, nuclei, photons, quarks) so small that the lumpiness or quantization of physical variables becomes important.
Q: What is quantization?
A: Its the idea that there are minimum size chunks for certain quantities like energy and angular momentum. The minimum energy chunk for light of frequency f is E=hf where h is Planck's constant. We call the particle of light carrying this minimum-size energy chunk hf a photon.
Q: What is meant by "the formalism of quantum mechanics"?
A: Basically, the formalism is mathematics consisting of (1) a differential equation like Schroedinger's wave equation which relates mass, energy, and momentum; (2) the mathematical solutions of that wave equation, called wave functions, which contain information about location, energy, momentum, etc. of some system; and (3) procedures for using wave functions to make predictions about physical measurements on the system.
Q: What's a "system"?
A: It is any collection of physical objects which is to be described by quantum mechanics. It could be a single electron, a group of quarks, an atom, a cat in a box, or the whole universe and all its contents.
Q: Why all the recent fuss about quantum mechanics?
A: Albert Einstein distrusted quantum mechanics because he perceived embedded in its formalism what he called "spooky actions at a distance". The characteristic which worried Einstein is called "nonlocality". The term locality means that separated system parts which are out of speed-of-light contact can only retain some definite relationship through memory of previous contact. Nonlocality means that some relationship is being enforced faster-than-light across space and time. The recent fuss has arisen because the nonlocality of quantum mechanics has been spotlighted by the EPR (Einstein-Podolsky-Rosen) experiments performed in the last decade. These measurements of the correlated optical polarizations for oppositely directed photons show that something very like faster-than-light hand-shaking must be going on within the formalism of quantum mechanics and in nature itself.
Q: Finally, just what is the Copenhagen interpretation?
A: The Copenhagen interpretation of quantum mechanics is a set of ideas and principles devised by Bohr, Heisenberg, and Born in the 1930's to give meaning to the formalism of quantum mechanics and to avoid certain "paradoxes" which seemed implicit in the formalism.
My RMP article lists five independent interpretational ideas which comprise the Copenhagen interpretation:
(1) Heisenberg's Uncertainty Principle, the idea that pairs of "conjugate" variables (like position and momentum or energy and time) cannot simultaneously be measured to "perfect" accuracy, nor can they have well-defined values at the same time;
(2) Born's Probability Law, the rule that the absolute square of the wave function gives the probability (P=|psi|2=psi×psi*) of finding the system in the state described by the wave function;
(3) Bohr's Complementarity Principle, the idea that the uncertainty principle is an intrinsic property of nature (not a just a measurement problem) and that the observer, his measuring apparatus, and the measured system form a "whole" which cannot be divided;
(4) Heisenberg's Knowledge Interpretation, the notion that the wave function is neither a physical wave travelling through space nor a direct description of a physical system, but rather is a mathematically encoded description of the knowledge of an observer who is making a measurement on the system; and
(5) Heisenberg's Positivism, the principle that it isn't proper to discuss any aspect of the reality which lies behind the formalism unless the quantities or entities discussed can be measured experimentally.
The first three elements of the Copenhagen interpretation are needed to connect the formalism with the results of physical measurements. The last two were devised by Heisenberg to deal with Einstein's "spooky actions at a distance" criticism and similar problems which lie in the general area of nonlocality. Let's consider an example of how the knowledge interpretation handles nonlocality.
A excited atom gives up energy by spitting out a photon. The QM formalism represents this event as a wave function which spreads out from the atom in an ever-widening spherical wave front resembling the ring of ripples from a stone thrown into a pond. The absolute square of this spreading wave function at a particular point in space-time gives the probability of finding the photon there. Finally the photon hits a silver atom in a photographic plate, giving up its energy and leaving a black spot on the plate. Instantaneously the photon's wave function undergoes a process called "collapse" which resembles the pricking of a soap bubble. The wave function completely disappears from all of space except in the immediate vicinity of the struck atom. The photon has now delivered its energy to the silver atom and has no probability of existing elsewhere. The wave function which had just been expanding through time and space has abruptly vanished.
This vanishment is part of Einstein's "spookiness" criticism. In 1929 at a physics conference he questioned how the remote parts of the wave function could possibly know that it was time to vanish when the photon was detected. Heisenberg's explanation was that the spreading wave function was not a real wave moving through space at the speed of light but rather a representation of the knowledge of an observer. When the observer had not yet detected the photon, it has an equal probability of being anywhere on the spreading spherical wave front. But as soon as the photon is detected it is know to have travelled to the silver atom and its probability of being elsewhere must become zero.
The problem with the knowledge interpretation comes when we try to stretch it to the EPR experiments, a system of two polarization-correlated photons travelling in opposite directions. Now there are two observers making measurements and gaining information about two photons which are out of speed-of-light contact, and yet the two measurements remain correlated in a "spooky" way. The nonlocality which enforces this correlation cannot be dismissed by attributing it to changes in knowledge. Something else must be goin on, and the Copenhageners can only retreat behind the shield of Heisenberg's positivism in dealing with the problem.
The transactional interpretation meets the nonlocality problem head on, using a "transaction" model for quantum events which is itself nonlocal because it uses advanced waves which have negative energy and travel backwards in time. Advanced waves were the subject of a previous AV column ["Light in Reverse Gear II", August-1985 Analog]. This transaction model is based on the "absorber theory" originated by Richard Feynman and John Wheeler.
In the absorber theory description any emission process makes advanced waves on an equal basis with ordinary "retarded" waves. But when the retarded wave is absorbed (sometime in the future) a cancellation process takes place which erases all traces of advanced waves and their "advanced" effects. The absorber manages to absorb the retarded wave by making a second retarded wave identical to but exactly out of phase with the retarded wave from the emitter. Thus the two cancel and we say that the retarded wave from the emitter is absorbed. However, the absorber also must make an advanced wave. This advanced wave backtracks the retarded wave, travelling backwards in time along the path taken by the retarded wave and reaching the emitter at the instant of emission. It continues backward in time, but now it is accompanied by the advanced wave from the emitter. The two waves are exactly out of phase, so they also cancel, removing all "advanced" effects in the process.
An observer not privy to these inner mechanisms of nature would perceive only that a retarded wave had gone from the emitter to the absorber. The absorber theory description, unconventional though it is, leads to exactly the same observations as the conventional one. But it differs in that there has been a two-way exchange, a "handshake" across space-time which led to the transfer of energy from emitter to absorber.
This advanced-retarded handshake is the basis for the transactional interpretation of quantum mechanics. It is a two-way contract between the future and the past for the purpose of transferring energy, momentum, etc. It is nonlocal because the future is, in a limited way, affecting the past on the same basis that the past affects the future. When you stand in the dark and look at a star a hundred light years away, not only have the retarded light waves from the star been travelling for a hundred years toward your eyes, but also advanced waves from your eyes have reached a hundred years into the past to encourage the star to shine in your direction. In my RMP paper this model is used to explain the accumulation of curiosities and paradoxes (the EPR paradox, Schroedinger's cat, Wigner's friend, Wheeler's delayed choice, etc.) which have lain in the quantum mechanics Museum of Mysteries for decades. The need for half-and-half cats, schizophrenic universes, observer-dependent reality, or "knowledge" waves has been eliminated.
In this column we usually spotlight recent physics developments and then consider their science fiction implications. The transactional interpretation unfortunately pulls the rug from under a number of excellent SF works based on the weirder aspects of quantum mechanics. Examples are Pohl's "The Coming of the Quantum Cats" and Hogan's The Proteus Operation, both of which use the many-worlds or Everett-Wheeler interpretation of quantum mechanics [See "The Alternate View: Other Universes II", November-1984 Analog]. The transactional interpretation addressed the same problems which prompted development the many-worlds interpretation and solves them in a more satisfactory way.
There are SF possibilities in the transactional interpretation. Advanced waves could perhaps, under the right circumstances, lead to "ansible-type" FTL communication favored by LeGuin and Card and to backwards in time signalling of the sort used in Benford's Timescape and Hogan's Thrice in Time. There is also the implication implicit in the transactional interpretation that Possibility does not become Reality along that sharp knife-edge that we call "the present". Rather, Reality crystallizes along a much fuzzier boundary which stitches into both future and past, advancing somehow in a way which defies sharp temporal definition. There must be a story in that.
John G. Cramer, Reviews of Modern Physics 58, #3 (1986).
John G. Cramer, International Journal of Theoretical Physics 27, 227 (1988).
S. J. Freedman and J. F. Clauser, Phys. Rev. Letters 28, 938 (1972);
A. Aspect, J. Dalibard, and G. Roger, Physical Review Letters 49, 91 (1982);
A. Aspect, J. Dalibard, and G. Roger, Physical Review Letters 49, 1804 (1982).
J. A. Wheeler and R. P. Feynman, Reviews of Modern Physics 17, 157 (1945);
J. A. Wheeler and R. P. Feynman, Reviews of Modern Physics 21, 425 (1949).
This page was created by John G. Cramer on 7/12/96.