Physics Today Observant Readers Take the Measure of Novel Approaches to Quantum Theory; Some Get Bohmed Murray Gell‐Mann, James Hahtle, Robert B. Griffiths, Anton Zeilinger, Robert T. Nachtrieb, James L. Anderson, Allen C. Dotson, William G. Hoover, Henry M. Bradford, and Sheldon Goldstein Citation: Physics Today 52(2), 11 (1999); doi: 10.1063/1.882512 View online: http://dx.doi.org/10.1063/1.882512 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/52/2?ver=pdfcov Published by the AIP Publishing This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.215.225.131 On: Mon, 24 Aug 2015 23:19:45 LETTERS Observant Readers Take the Measure of Novel Approaches to Quantum Theory; Some Get Bohmed n "Quantum Theory without Ob- beit discrete, intervals of time. How- DH, if two such quantities at the I servers—Part One" (PHYSICS TODAY, ever, he seems to think that we start same time do not commute, measure- March 1998, page 42), Sheldon Gold- with the union of many different fam- ments of them have to take place in stein discusses our work on the deco- ilies (with the possibility of inconsis- different alternative histories of the herent histories (DH) approach to tencies in statements connecting the universe.2 Our work is not com- quantum mechanics and the related probabilities of occurrence of various pletely finished, but the research work of Robert Griffiths and Roland histories) and are trying to find con- is not plagued by inconsistencies. Omnes. He describes correctly many ditions that will shrink this set to a aspects of the research and makes a single realm and its associated fam- References number of favorable remarks, such as ily thus eliminating inconsistencies. 1. R. Griffiths, Phys. Rev. A 54, 2759 "it seems likely that the program of That is not the case. Rather, we are (1996). R. Omnes, The Interpretation DH can be brought successfully to comparing the properties of different of Quantum Mechanics, Princeton completion." However, he seems to realms or families, while restricting U. P., Princeton, N. J. (1994). R. Grif- have misunderstood one important our statements in each case to a sin- fiths, J. B. Hartle, Phys. Rev. Lett. 81, gle family, thus encountering no in- 1981(1998). point, and as a result he mistakenly 2. M. Gell-Mann, The Quark and the Jag- attributes certain "inconsistencies" to consistencies along the way. uar , W. H. Freeman, New York (1994), the program at its present stage. It is worth mentioning that the fig- p. 172. We always consider a "realm"—a ure caption on the last page of the ar- MURRAY GELL-MANN set of mutually exclusive decoherent ticle is misleading. The photograph ([email protected]) histories with probabilities adding to shows Richard Feynman and one of Santa Fe Institute 1—and we typically impose some fur- us (Gell-Mann), and the caption de- Sante Fe, New Mexico ther conditions on a given realm. (A scribes Gell-Mann as "one of the most JAMES HARTLE "family," as discussed by Goldstein, sensible critics of orthodox quantum ([email protected]) consists of a realm and all its coarse theory" and Feynman as "one of its University of California, Santa Barbara grainings.) It is essential to restrict most sensible defenders." In fact, statements relating the probabilities both physicists held very similar heldon Goldstein's two-part article of occurrence of histories to a given views of quantum mechanics. Some S contains much valuable material. family containing them. 'Here, we months before Feynman's death in Unfortunately, his discussion of consis- have in mind statements such as the 1988, Gell-Mann described to a class tent histories is, in certain respects, following: If B happens at time U at Caltech the status of our work on misleading; at the very least, it is out and C at time t then A must have decoherent histories at that time. of date. (Goldstein, following Murray 3i Feynman was in attendance, and at happened at time tv) The restriction Gell-Mann and James Hartle, uses is necessary despite the fact that the the end of the class, he stood up, and the term "decoherent histories" for numerical probability of a given his- some of the students expected an ex- what Roland Omnes and I call "con- tory belonging to more than one fam- citing argument. But his comment sistent histories.") ily is independent of the family. This was, "I agree with everything you The logical structure of the consis- point has been stressed very strongly said." tent histories approach has been by Griffiths and Omnes.1 Inconsisten- There is no question that the "or- worked out in considerable detail by thodox" Copenhagen interpretation Omnes, and paying serious attention cies can arise if statements relating l the probabilities of occurrence of histo- works in measurement situations and to his "Rule 4" would have pre- ries are made while referring to differ- accurately predicts the outcomes of vented Goldstein from making the ent families in the course of a given laboratory experiments. It is not erroneous assertion that the consis- argument. That is true even if the wrong. Rather, it is a special case tent histories formalism is rendered histories involve only a single time. of the more general interpretation inconsistent by the results of An- Goldstein mentions our efforts to in terms of decoherent histories of drew Gleason; Simon Kochen and the universe. The Copenhagen pic- understand what is so special about Ernst Specker; John Bell; and Lucien ture is too special to be fundamental, 2 the "usual" realm defined by hydro- Hardy. My own recent work has and it is clearly inadequate for quan- led to a quite systematic treatment of dynamic variables averaged over small tum cosmology. the whole subject, in which consistent volumes and evaluated at short, al- As Goldsteins title suggests, DH is history "beables" (the physical refer- a formulation of quantum mechanics ents of the mathematical terms) are Letters submitted for publication should be in which observers do not play a fun- spelled out in considerable detail, and addressed to Letters, PHYSICS TODAY, damental role. We are working to the formalism is shown to be com- American Center for Physics, One Physics perfect that formulation. However, plete as a fundamental theory, with- Ellipse, College Park, MD 20740-3843 or we are not seeking, nor do we have, a out need of the additional principles to [email protected] (using your sur- formulation that implements Albert that Goldstein seems to think are nec- name as "Subject"). Please include your Einstein's idea of attributing "physical essary. Although the "primitive ontol- affiliation, mailing address and daytime reality" to all quantities for which ogy" (to use Goldstein's term) of con- phone number. We reserve the right to edit there are situations in which they sistent histories was not presented in letters. can be measured with certainty. In the earliest papers in as clear a form This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to © 1999999 AJHAndiCV€ ! Imiiiuu? ui Pli) ^-220-6IP: 131.215.225.131 On: Mon, 24 Aug 2015 23:19:45FEBRUARY 1999 PHYSICS TODAY 11 as is now possible—a quite common to illustrate the inconsistency. that "inconsistencies can arise ... is occurrence when important new ideas For a certain quantum state ?//, say precisely the point of the example I are introduced into physics—the fun- at time t - 0, for a pair of spin-V2 par- used in the article and am using here. damental ideas have not changed, ticles, there are spin components A, Robert Griffiths is more explicit and more recent work has confirmed £, C and D (also at t = 0) for which about the cause of my having made the soundness of the basic strategy the DH approach yields the following "the erroneous assertion that the con- adopted by Gell-Mann and Hartle, four statements concerning joint prob- sistent histories formalism is ... in- Omnes and myself. (Readers inter- abilities P: consistent"—namely, my not "paying ested in pursuing the subject further 1. P(A = 1, B= l) = 0.09. serious attention to [Omnes's] Rule 4." may wish to consult reference 3, 2. P(A = 1, C# l) = 0. Here is the rule, as given on page which contains a response to various 3. P(B = 1, D^ l) = 0. 163 of the reference Griffiths men- criticisms and misunderstandings of 4. P(C = 1, D= l) = 0. tions: "Any description of the proper- consistent histories, as well as simple Corresponding to these four state- ties of an isolated physical system examples that may make some of the ments are four pairs of commuting must consist of propositions belonging ideas easier to follow.) observables and four decoherent together to a common consistent logic. There is one aspect of consistent families (the sort of families to which Any reasoning to be drawn from the histories that was perfectly clear in DH assigns probabilities): the AB fam- consideration of these properties the very first paper on the topic and ily, the AC family, the BD family and should be the result of a valid implica- in all our subsequent work: A quan- the CD family However, these fami- tion or of a chain of implications in tum history consists of a sequence of lies cannot be combined into, say, an this common logic.'1 What Omnes events at successive times, and these ABCD family, and thus DH does not calls a "consistent logic" amounts events correspond to subspaces of the supply us with probabilities for simul- more or less to a (decoherent) family quantum Hilbert space.