In a recent contribution to this journal [Am. J. Phys. 64, 1468–1475 (1996)] I wrongly asserted that retrocausation in the Englert, Scully, and Walther (ESW) experiment (a double-slit interference experiment with atoms) can occur only until the atom arrives at the screen. In their response, Englert, Scully, and Walther [preceding paper] point out my fallacy but give an incomplete analysis of its origin. In this paper I trace this fallacy to a deep-seated preconception about time and reality. I show that among the two possible realistic interpretations of standard quantum mechanics, the reality-of-states view and the reality-of-phenomena view, only the latter is viable. It follows that retrocausation is a necessary feature of any realistic account of the ESW experiment based on standard quantum mechanics. Finally I eludicate the sense in which the spatial properties of quantum systems are objective, and show that they are extrinsic rather than intrinsic.
REFERENCES
1.
Ulrich
Mohrhoff
, “Restoration of interference and the fallacy of delayed choice: Concerning an experiment proposed by Englert, Scully and Walther
,” Am. J. Phys.
64
, 1468
–1475
(1996
).2.
Berthold-Georg
Englert
, Marlan O.
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Walther
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Marlan O.
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, Berthold-Georg
Englert
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Walther
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,” Nature (London)
351
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(1991
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Berthold-Georg Englert, Marlan O. Scully, and Herbert Walther, “Quantum erasure in double-slit interferometers with which-way detectors,” Am. J. Phys. 67 (1999).
5.
quoted in Abraham Pais, Niels Bohr’s Times, In Physics, Philosophy, and Polity (Clarendon, Oxford, 1991), p. 433.
6.
Michael Redhead, Incompleteness, Nonlocality and Realism (Clarendon, Oxford, 1987), Sec. 2.3.
7.
Yakir
Aharonov
, Peter G.
Bergmann
, and Joel L.
Lebowitz
, “Time symmetry in the quantum process of measurement
,” Phys. Rev. B
134
, 1410
–1416
(1964
);reprinted in John Archibald Wheeler and Wojciech Hubert Zurek, Quantum Theory and Measurement (Princeton U.P., Princeton, NJ, 1983), pp. 680–686.
8.
Niels Bohr, Essays 1958–62 on Atomic Physics and Human Knowledge (Wiley, New York, 1963), p. 3.
9.
Bernard d’Espagnat, Conceptual Foundations of Quantum Mechanics (Benjamin, Reading, MA, 1976), 2nd ed., p. 251.
10.
“I must discard here a disastrous, but very frequent, misunderstanding concerning advanced causality. Many people think that advanced causality would imply the possibility of reshaping the past—for example, killing one’s grandfather in his cradle. This is sheer nonsense. The world history is one, and cannot be rewritten. Advanced causality does not mean reshaping, but it does mean shaping the past from the future.”—Olivier Costa de Beauregard, “Space–time and probability: Classical and quantal,” in The World View of Contemporary Physics, edited by Richard F. Kitchener (SUNY Press, New York, 1988), pp. 104–124 (original emphasis). For further publications on retrocausation prior to 1988 see the references in this article.
11.
ESW’s “quantum erasure” terminology is potentially misleading. The photon does not carry information about the atom but only enables us to obtain such information. In this respect it is like a telescope which does not by itself carry information about the moon but only enables us to obtain such information. To obtain information about the moon, we must point it at the moon, and to obtain information about the atom, we must subject the photon to a measurement. What is “erased,” accordingly, is not existing information but merely the possibility of obtaining (a particular type of) information. The creation of a matter of fact cannot be undone.
12.
Hilary Putnam, Representation and Reality (MIT, Cambridge, MA, 1988), p. 113.
13.
Bernard d’Espagnat, Veiled Reality (Addison–Wesley, Reading, MA, 1995).
14.
N. David Mermin, “What’s wrong with this sustaining myth?” Phys. Today 11–13 (March 1996).
15.
Albert Einstein, quoted by Wolfram Schommers, “Space-time and quantum phenomena,” in Quantum Theory and Pictures of Reality, edited by Wolfram Schommers (Springer, Berlin and Heidelberg, 1989), p. 230.
16.
Hermann Weyl, Philosophy of Mathematics and Natural Science (Princeton U.P., Princeton, NJ, 1949), p. 116.
17.
David Bohm, Quantum Theory (Prentice-Hall, Englewood Cliffs, NJ, 1951).
18.
Albert
Einstein
, Boris
Podolsky
, and Nathan
Rosen
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” Phys. Rev.
47
, 777
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(1935
);reprinted in John Archibald Wheeler and Wojciech Hubert Zurek, Quantum Theory and Measurement (Princeton U.P., Princeton, NJ, 1983), pp. 138–141.
19.
Particle 1 thus provides an example of an ensemble that is both pre- and postselected: postselected (determined backward in time) by being eventually found in the state and preselected (determined forward in time) by being initially prepared in the state —a “preparation” that capitalizes on the entanglement of the two particles and is effected by the postselection of particle 2 in the state
Yakir
Aharonov
and Lev
Vaidman
[“Complete description of a quantum system at a given time
,” J. Phys. A
24
, 2315
–2328
(1991
)] proposed that pre- and postselected ensembles be represented by a two-state vector.The two-state vector is to time-symmetrized quantum theory (TSQT) what the state vector is to standard quantum theory [Lev Vaidman, “Time-symmetrized quantum theory,” invited lecture, Fundamental Problems in Quantum Theory workshop, University of Maryland Baltimore County, 4–7 August 1997 (unpublished);
R. E. Kastner, “Time-symmetrized quantum theory, counterfactuals, and ‘advanced action,’ ” forthcoming in Stud. Hist. Phil. Mod. Phys.). TSQT is based on the seminal paper by Y. Aharonov, P. G. Bergmann, and J. L. Lebowitz (Ref. 7). It takes due account of the fact that the maximally specified state of a system contains information based not only on initial but also on final measurements.
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