Commentary on

"The Observer in the Quantum Experiment"
Bruce Rosenblum and Fred Kuttner

Department of Physics, University of California at Santa Cruz

e-print at http://www.arxiv.org/pdf/quant-ph/0011086

By Ross Rhodes

[This essay assumes familiarity with the discussion of the double slit experiment in The Reality Program, Chapter 2.]

Rosenblum and Kuttner present the philosophical problem of measurement in quantum mechanics by analyzing the relationship between the experimenter’s choice that leads to a particular result; and the inferences that may be drawn from that result, concerning external (to the experimenter) physical developments that would have led to such a result. The paper attempts to be "theory neutral," by which the authors mean that their focus is on the experiments and the results in the first instance. A "theory" of how these results come about is less important to their discussion than a basic understanding of the conflict represented by the results themselves. First we must understand the conflict; then we may theorize how to resolve the conflict.

On the model of the double slit experiment, the experimenter’s choice is whether to obtain which-path information.

• If the experimenter chooses to obtain which-path information, the "clumping" result will lead to the inference that a particle took a definite path through only one of the two slits, and that nothing went through the other slit.
• On the other hand, if the experimenter chooses to forego which-path information, the "interference" result will lead to the inference that something about the particle went through both slits.

Note that we need not await a build-up of the pattern itself to be confident that the pattern will develop according to the availability of which-path information or not. This is because this phenomenon – clumps for which-path information, interference pattern for no-which-path information – is well established after many years of consistent experimental results. Therefore, although the detection of a single quantum unit does not, by itself, constitute a "pattern," nevertheless in the context of a double slit experiment we know that the single detection is part of a larger pattern that will develop as surely as the sun will rise tomorrow (and perhaps more so). Accordingly, with each single result from the passage of a single quantum unit through the double slit experiment, we can say that the result is part of a particular pattern – "clumping" or "interference," depending on whether the experimenter chooses to obtain which-path information.

The experimenter knows that with each choice made, the result will say something about the logically inferable prior state of affairs as the quantum unit traveled through the region of the double slit. (Rosenblum and Kuttner point out that any attempt to infer a prior physical state from a quantum mechanical measurement is contrary to the orthodox Copenhagen interpretation. Even so, the inferences are there for the naïve realist.)

• The "clumping" pattern is explainable by simple geometry – essentially a straight line running from the source, through one slit or the other, and terminating at the detection device. The inference from a clumping result (even a single detection event) is that the quantum unit traveled in a more-or-less straight line from the source, through one slit, to the detection device. A clumping detection does not betray any influence whatsoever of the slit not traveled. For an individual clumping detection, we may as well have a single slit experiment – the other slit might as well be closed off or just not there.

• The "interference" pattern is explainable by wave mechanics, and in a particular way. The pattern that eventually develops will be directly (mathematically) related to the wavelength of the quantum unit and the separation of the slits. That is, if you change the distance between the slits, the pattern changes. Therefore, the placement of a single detection event betrays the distance between the slits. And if the detection event is determined by the distance between the slits, then we can say that the quantum unit, at the time of detection, is betraying the influence of both slits. The inference, working backwards, is that the quantum unit went through both slits, and that it did so in a wave-like fashion. At the very least, we can say that its placement is a function of the physical presence of two slits.

If the experimenter chooses to know which path the quantum unit took through the slits, then the result (a detection consistent with an eventual clumping pattern) will imply that the quantum unit took one and only one path through one and only one slit. On the other hand, if the experimenter chooses not to know which path the quantum unit took through the slits, then the result (a detection consistent with an eventual interference pattern) will imply that the quantum unit took two paths, i.e., that some part of the quantum unit went through each slit.

Accordingly, there would appear to be a relationship between the experimenter’s choice and the manner in which the quantum unit physically traveled through the region of the double slit. What is more, the two possible methods of travel appear to be physically inconsistent – either through one slit by the entirety, or through both slits by some kind of divisibility. This physical inconsistency itself leads to something very like the Copenhagen interpretation, as Rosenblum and Kuttner later point out.

It is the relationship between the experimenter’s choice, on the one hand, and the inferred physical state of affairs at the slits, as implied by the result, on the other, that Rosenblum and Kuttner discuss in this paper.

To illustrate the relationship between choice and result (actually, between choice and the entire course of the experiment), Rosenblum and Kuttner imagine what would happen in the classical world if similar events took place. We have two characters – the Demonstrator and the Experimenter.

The Demonstrator produces a large number of boxes, arranged in pairs. The Experimenter is told that each pair of boxes holds one marble, and that the Experimenter is to find out which box contains the marble by opening first one box, then the other.

The Experimenter does so. For each pair of boxes, it turns out that there is a marble in one box, and no marble in the other box. About half the time, the marble will be in the first box, in which case there is no marble in the second box. Similarly, if there is no marble in the first box, there will always be a marble in the second box. The Experimenter concludes that each pair of boxes has been prepared ahead of time by the Demonstrator, who has placed a marble randomly in one box or the other of the pair.

The Demonstrator next produces more boxes, again arranged in pairs, and shows the Experimenter a new kind of marble that can come apart into two halves – a white half and a black half. For each pair of boxes, the Experimenter is to find which box contains the black half, and which box contains the white half, and this must be done by opening both boxes at the same time.

The Experimenter does so. For each pair of boxes opened simultaneously, there is a white half in one box, and a black half in the other box. The Experimenter concludes that each pair of boxes has been prepared ahead of time by the Demonstrator, who has randomly placed the white half of the marble in one box, and the black half in the other box.

Now, the Demonstrator produces more boxes, again arranged in pairs. The Experimenter is told to open each pair of boxes however he likes – either one at a time, or both at the same time.

When the Experimenter does this, he finds that whenever he opens the boxes one at a time, there is always a single marble in one box or the other – just like in the first demonstration. On the other hand, whenever the Experimenter opens both boxes at the same time, there is always half a marble in each box of the pair – just like in the second demonstration.

The Experimenter must now puzzle out how this thing works.

• Did the Demonstrator know in advance how the Experimenter would choose to open the boxes? It doesn't seem likely, since the Experimenter is doing his best to choose his method randomly.
• Is the Demonstrator using mental telepathy to control the Experimenter's choice of method? The Experimenter isn't aware of it, and tries to avoid it – say, by thinking of his childhood pet's name and addressing each pair of boxes in order, opening the boxes one at a time or simultaneously depending on whether the next letter of the pet's name is a vowel or a consonant. This should avoid even reverse telepathy, since the Demonstrator cannot substitute boxes based on reading the Experimenter's mind after the box pairs have all been laid out and the session has begun.
• Is the marble (or two half-marbles) being put in the box(es) after the Experimenter begins to open the box(es), and before the Experimenter can see inside? If so, it is not obvious how this is done, and the Experimenter finds he can pick up the box pair and hold them in his hand before he decides how to open them, and the results are the same.
• Perhaps each pair of boxes is prepared for either experiment – with a marble in one box and nothing in the other when the boxes are opened one at a time; and a trap door springing to reveal half a marble in each box when they are opened simultaneously. Close inspection reveals no such trap door, nor any connection between the two boxes of any pair.
• Perhaps it is magic – really and truly magic. But the Experimenter doesn't believe in real and true magic.
• Perhaps there is an invisible field that is completely undetectable, which is disturbed in different ways upon opening one or both boxes, and this field arranges the marbles in a way that is completely undetectable while it is doing the arranging, but which results in one marble when the boxes are opened one at a time, and two half-marbles when opened simultaneously. But, come to think of it, this hypothesis is basically another way to say real and true magic.

The Experimenter's puzzlement – and the physicist's puzzlement – arises from the fact that the result appears to be so strongly correlated to his free-will choice. And yet, it seems that the result had to have been prepared in advance, before he had made his choice, because the prior conditions for the two different results conflict – they cannot both have been prepared ahead of time except in the trap-door scenario, which has been ruled out.

This is exactly the situation confronting the quantum experimentalist in the double slit experiment. If the Experimenter chooses to gain which-path information, then the result is clumps, indicating that the quantum unit at one time was passing through one and only one slit, just like the marble appearing in one and only one box. If, on the other hand, the Experimenter chooses to forego which-path information, then the result is interference, unmistakably indicating that the quantum unit has managed to acquire attributes related to both slits, just like the two half-marbles appearing in both boxes. Either way, the historical path of the quantum unit appears to be determined by the Experimenter's choice. Yet, as we know, the Experimenter may make his choice after the quantum unit has passed the region of the slits, in the same way that the Experimenter chooses after the boxes have been presented to him by the Demonstrator.

It appears that the Experimenter has alternative conclusions that would fit the results. First, the Experimenter may conclude that his choice somehow caused the boxes to exist in a certain state, i.e., that the boxes were not prepared in advance with one result or the other, but everything including history was established depending on his choice. Either the boxes were prepared in advance with both results, and one of these results magically disappeared when he made his choice; or the boxes had nothing in them until he made his choice, at which point the marble (or half-marbles) magically appeared in the box(es).

The logical extension of this possibility leads to the further conclusion that the nature of existence at the time of the Experimenter's choice is not dictated by anything in existence prior to that choice. Existence in all of its glory can be explained only as the result of many choices, brought about by an unknown mechanism connected in some manner to the choice itself. This "unknown mechanism" is the magic that makes the marbles appear when the boxes are opened – Abracadabra, presto!

Second, the Experimenter may conclude that the boxes were prepared with foreknowledge of how he would choose; or that the prior preparation of the boxes somehow forced his choice. Either way, his "choice" was an illusion – determined by something that was knowable in advance by the Demonstrator, or forced as a direct result of the advance preparation of the boxes. He may have thought that he was thinking independently – and making his choice based on some private rule unknown by any other, or even out of sheer cussedness – but in fact he was not "thinking" at all in the usual sense. In fact, he was just playing out some pre-determined rule related to how the boxes were prepared. And if this were the case, then there would be no reason to suppose that the Demonstrator, in choosing the preparation, was acting any differently or with any greater degree of free-will choice.

The logical extension of this possibility leads to the further conclusion that all things, including the illusion of choice, are completely deterministic. "Thinking" and all of the experiential aspects of the human condition can be explained only as artificial intelligence that gives rise to awareness by an unknown mechanism that is related in some manner to complexity. That is, it must be possible to sculpt a sand figure in such a way that the arrangement of the grains of sand themselves give rise to self-awareness and the illusion of choice on the part of the sculpture. This process, too, must be counted as magic on the order of Pinnochio's Fairy Godmother.

There is a basic conflict between these two common postulates: (1) that the Experimenter makes free will choices; and (2) that the Experimenter "discovers" a state of affairs that pre-existed the measurement, which is to say that reality exists independent of the Experimenter's choice. It seems that free choice is incompatible with existence in the absence of choice, because either existence determines the "choice"; or the choice creates an existence which did not pre-exist. To put it another way, if there is existence in the absence of choice, then that existence must dictate the specifics of every choice, and so no choice is free; if there is free choice, then that choice must dictate the nature of existence. There is a "conflict between the belief in free choice and the belief in a physical reality prior to that choice." [at p. 15.]

The correlation between choice and prior existence seems inescapable, as is the correlation between choice and present existence. Either the prior existence determines the choice; or the choice determines the present existence, supplying the inference of a definite and distinguishable prior existence.

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