(1) In his introduction to Dyson's summary (mailed on 14 Feb) Paul Davies writes:

"The delayed choice experiment... demonstrates how the actions of an observer now can help determine the nature of reality that was - in the past.... Wheeler emphasized the fact that quantum observations made today can have a hand in determining the nature of reality that was - billions of years ago. Such ideas led to his famous notion of the "participatory universe" in which observers - minds, if you like - are inextricably tied to the concretization of the physical universe emerging from quantum fuzziness."

People hesitate to accept the possibility of influencing/determining the past ("retrocausation") because they tend to confuse it with the possibility of *changing* the past. The latter is clearly impossible: Change is a transition from an earlier state of affairs to a different later state of affairs. A state of affairs that obtains at a given time t cannot be replaced by a different state of affairs obtaining at the same time t. QM tells us that no state of affairs obtains unless it is an indicated state of affairs, as stressed in various terms by Niels Bohr and John Wheeler. Hence there are two times to be considered, the time at which the indicating event occurs and the time at which the indicated state of affairs obtains. More often than not the latter precedes the former. If in addition the indicating event is (partly) determined by the choice of an experimenter then so is the earlier indicated state of affairs.

The relation between indicated properties or values and property- or value- indicating facts has no counterpart in classical physics, where all properties and values are *intrinsic*. This means that, at any time, out of any complete set of mutually contradictory properties or values, every system or observable possesses exactly one (Kant's "principle of complete determination"). The possessed properties can change, or be changed, but otherwise the system possesses them *by itself* - no "measurement" is needed. The kinematical properties of quantum systems and the values of quantum-mechanical observables, on the contrary, are *extrinsic* - not possessed unless their possession is indicated by an actual event or state of affairs.

The Bohr-Wheeler mantra that "no phenomenon is a phenomenon unless..." characterizes a relation between indicating facts and indicated properties/values. It has nothing to do with observers. The challenge is to understand the coexistence of extrinsic properties with the indicating properties of property-indicators - for indicating properties must be intrinsic if an infinite regress is to be avoided - and to understand it without characterizing the intrinsic properties as perceived or known (that is, without invoking consciousness, knowledge, or information).

Wheeler asked (with Misner and Thorne), "may the universe [including the participators] in some strange sense be 'brought into being' by the participation of those who participate?" The extrinsic nature of the properties of the constituents of property-indicators, which have intrinsic properties, suggests a loop just as perplexing. The challenge is to understand the mutual dependence of intrinsic and extrinsic properties without invoking some mysterious matrix from which actuality "emerges." For a possible way of meeting these challenges see [1-4].

(2) Subsequently Paul Davies mentions "the famous 'quantum eraser' experiments in which the choice is not only delayed, but the observer can change his or her mind afterwards!"

If this means that a measurement once made can be undone and another measurement (incompatible with the former) can be made, it is incorrect. Take the experiment of Englert, Scully, and Walther [5,6]. Either which-way information can be obtained (by ascertaining the cavity, left or right, containing the photon left behind by an atom) or phase-relation information can be obtained, but once either information is obtained, it cannot be "erased," and the other information can no longer be obtained. What can be undone (by opening the shutters separating the cavities) is merely the *possibility* of obtaining which-way information [7].

(3) Freeman Dyson asserts that QM is not a complete description of nature, while Bill Unruh (both 14 Feb) asserts that QM describes the past just as well as the future, and (later on) that it describes probabilities. While I largely agree with Unruh's comments, I feel that much confusion is created by the doubly ambiguous phrase "QM describes..." Does "QM" stand for (i) the wave function, (ii) a tool for assigning probabilities to possible events on the basis of actual events and on condition that one of a complete set of mutually exclusive events happens, or (iii) a theory that includes the ontological implications of the formalism of QM? And in what sense does QM describe? Ordinarily one means by a description the representation of an actual state of affairs. One doesn't "describe probabilities," one *assigns* them. Such inappropriate phraseology (like the infamous "destruction of past knowledge" through the acquisition of fresh knowledge or the "preparation" of a probability algorithm) reveal inconsistent ways of thinking. Probabilities are assigned to possible events or states of affairs. Neither probabilities nor possibilities constitute a describable second kind of reality from which the genuine article "emerges" - "a quantum potentiality [which] becomes transformed into physical actuality," as Paul Davies writes.

Freeman Dyson points out that we cannot describe by means of a wave function the statement of a fact. How could we? One doesn't use a probability algorithm to describe a fact. "QM" in sense (i) doesn't "describe" anything. "QM" in sense (iii), on the other hand, can be said to describe nature, and to describe it completely. What is incomplete is not the description but nature herself - but only in relation to an inadequate conceptual framework, one that is "overcomplete". We believe in the absolute reality of our conceptual distinctions, but they only have a contingent reality. Particles are not distinct unless the possession of distinguishing properties is indicated. Regions of space are not distinct unless they are realized (made real) by position indicators ("detectors"), and the distinctions between regions are not real for a particle unless the question "Which region contains the particle?" has an answer (that is, unless an answer is indicated).

We want to construct reality from the bottom up, out of intrinsically distinct individuals, be they particles or infinitesimal spacetime regions ("points"). QM is trying to tell us that this doesn't work. Reality is constructed from the top down, by a differentiation that does not start from but arrives at multiplicity - a limited and contingent multiplicity [1- 4].

Another reason why QM might be considered incomplete is that it does not allow us to predict that, or when, a measurement will take place. The probability that a variable Q has the value v at the time t is the product of two probabilities - the probability that any one of the possible values of Q is indicated for t, and the probability that the indicated value is v given that a value is indicated for t. QM is exclusively concerned with probabilities of the latter type. It does not assign a probability to the occurrence of a value-indicating event, nor does it specify sufficient conditions for such an event. If QM is a fundamental and universal theor, this means that the value-indicating events presupposed by QM are uncaused, as stressed by Ulfbeck and A. Bohr in a recent article [11]. (For my response to this article see [4].)

(4) Freeman Dyson writes: "the role of the observer in quantum mechanics is solely to make the distinction between past and future.... the quantum- mechanical description of an event ceases to be meaningful as the observer changes the point of reference from before the event to after it.... All we need is a point of reference, to separate past from future, to separate what has happened from what may happen, to separate facts from probabilities."

If we want to arrive at the conception of a free-standing reality - and I believe that this is what physics is about - the distinction between past and future is not available. The temporal modes past, present, and future can be characterized only by how they relate to us as conscious subjects: through memory, through the present-tense immediacy of qualia, or through anticipation. In the world of physics we may qualify events or states of affairs as past, present, or future *relative to* other events or states of affairs, but we cannot speak of *the* past, *the* present, or *the* future. The proper view of physical reality therefore is not only what Thomas Nagel [8] has called "the view from nowhere" (the physical world does not contain a preferred position corresponding to the spatial location whence I survey it); it is also what Huw Price has called "the view from nowhen'' [9]: The physical world does not contain a preferred time corresponding to the particular moment (the present) at which I experience it.

The formalism of QM is a probability algorithm. (If the ontological implications of the occurrence of probabilities in a fundamental theory are regarded as part of QM then QM itself is of course more than a probability algorithm.) This algorithm assigns probabilities to possible measurement outcomes on the basis of any other relevant measurement outcome or set of outcomes, via the Born rule if probabilities are assigned on the basis of past *or* future outcomes, via the ABL rule if probabilities are assigned on the basis of past *and* future outcomes [10]. The idea that QM only assigns probabilities to future outcomes on the basis of past outcomes is a prejudice, based on an illegitimate importation of our temporal outlook into the physical world. QM treats (or at least allows us to treat) all events past, present, and future on an equal footing, and it allows us assign probabilities to them on the basis of *any* set of relevant events -
not just the special set of past events.

The following notions militate against this point of view: (i) There is something like an evolving state of affairs; (ii) the existence of a property-indicating fact involves a transition from an earlier (potential) state of affairs to a later (actual) state of affairs. The probability that something happens at a given time is not something that exists at that time, any more then the probability of detecting a particle in a given region is something that exists inside that region. The time dependence of probabilities (and therefore of state vectors as well) is not the time dependence of an evolving state of affairs but a dependence on the time of an actually or counterfactually performed measurement. A "prepared" state vector is not something that exists before a measurement (performed at the time t) and then "collapses," but an algorithm for assigning prior probabilities to the possible results of (i) measurements counterfactually performed at times earlier than t and (ii) the actual measurement at t. This has nothing to do with a transition from potential to actual.

(5) Dieter Zeh wrote (on 15 Feb): "I do not understand what postselection... means if there is no ensemble to select from." To the comments by Jonathan Oppenheim (16 Feb) (and Zeh's reply of 19 Feb) I would like to add the following: While probabilities can be measured only as relative frequencies, quantum-mechanical probabilities are more basic than relative frequencies. There is absolutely no reason why we cannot assign probabilities on the basis of future (or past and future) events. We need an ensemble only for measuring probabilities, and the right ensembles for measuring probabilities assigned on the basis of future events is a postselected one.

[1] U. Mohrhoff, What quantum mechanics is trying to tell us, Am. J. Phys. 68, 728 (2000); quant-ph/9903051.

[2] U. Mohrhoff, The world according to quantum mechanics (Or the 18 errors of Henry P. Stapp), Secs. 7-9, to appear in Found. Phys.; quant-ph/0105097

[3] U. Mohrhoff, Against "knowledge," quant-ph/0009150.

[4] U. Mohrhoff, Making sense of a world of clicks, quant-ph/0202148.

[5] B.G. Englert, M.O. Scully, and H. Walther, The duality in matter and light, Scientific American, December 1994, 56.

[6] M.O. Scully, B.G. Englert and H. Walther, Quantum optical tests of complementarity, Nature 351, 111 (1991).

[7] U. Mohrhoff, Objectivity, retrocausation, and the experiment of Englert, Scully and Walther, Am. J. Phys. 67, 330 (1999).

[8] T. Nagel, The View from Nowhere (Oxford UP, New York, NY, 1986).

[9] H. Price, Time's Arrow & Archimedes' Point (Oxford UP, New York, NY, 1996).

[10] U. Mohrhoff, Objective probabilities, quantum counterfactuals, and the ABL rule - A response to R.E. Kastner, Am. J. Phys. 69, 864 (2001); quant- ph/0006116.

[11] O. Ulfbeck and A. Bohr, Genuine Fortuitousness. Where did that click come from?, Found. Phys. 31, 757 (2001). _______________________________________________

Ulrich Mohrhoff Sri Aurobindo International Centre of Education Pondicherry 605002 Indiaujm@satyam.net.inhttp://members.tripod.com/ujmvjm/ujm.htm _______________________________________________

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Published   2002.02.28