9 Influencing the past?

Logical flow chart for the ESW experiment

Figure 1.10.1 A log­ical flow chart for the ESW experiment

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Before rejecting the assump­tions that lead to this con­clu­sion, we need to be clear about what they amount to. Saying that a “green” atom went through L cannot mean that this very atom would also have gone through L if the cavity con­taining the photon emitted by it had not been ascer­tained. For if this cavity had not been ascer­tained, the exper­i­menters could have checked how the atom went through both slits (in phase or out of phase), and regard­less of the out­come they would have found that it went through both slits.

Rather, a “green” atom went through L only because this is what was indi­cated by a mea­sure­ment. An atom goes through a par­tic­ular slit only if the appro­priate mea­sure­ment is made, and it goes through L only if this is the out­come. It goes through both slits with a par­tic­ular phase rela­tion only if a dif­ferent mea­sure­ment is made, and it goes through both slits in phase only if that is the out­come. If nei­ther mea­sure­ment is made, it doesn’t go through the left slit and it doesn’t go through the right slit and it doesn’t go through both slits in phase and it doesn’t go through both slits out of phase. To para­phrase a well-​​known dictum by John Wheeler[1] — a tow­ering figure of 20th Cen­tury physics — no prop­erty (or behavior) is a pos­sessed (occur­rent) prop­erty (behavior) unless it is a mea­sured (or indi­cated) prop­erty (behavior). Mea­sure­ments do not merely reveal pre-​​existent prop­er­ties or behav­iors that occurred inde­pen­dently of mea­sure­ments; they create their outcomes.

Seen in this light, the pos­si­bility of par­tially deter­mining an atom’s past behavior no longer seems pre­pos­terous. If (as sug­gested by some of the exper­i­ments pre­vi­ously dis­cussed) the prop­er­ties of the quantum world exist only if, and only to the extent that, they are mea­sured, then the behavior of a quantum system at an ear­lier time can depend on a mea­sure­ment per­formed at a later time, and thus also on a deci­sion taken at a later time. Nothing like the backward-​​in-​​time cau­sa­tion of some fan­tasy novels, which changes the past, is sug­gested. By choosing to per­form one of the two exper­i­ments, the exper­i­menters cause no change in the atom’s past behavior. Rather, they con­tribute to create or deter­mine its past behavior.

The world has exactly one his­tory. Its state at any given time is what it is; it cannot be changed. But it can be what it is because of a mea­sure­ment per­formed at a later time. (In fact, events indi­cating the pos­ses­sion of a prop­erty occur quite gen­er­ally after the time of pos­ses­sion of the indi­cated prop­erty, albeit usu­ally a fairly short time after.)

But still, if no behavior is an occur­rent behavior unless it is a mea­sured or indi­cated behavior, we have rea­sons to doubt that the “green” atoms actu­ally went through L. For, strictly speaking, the con­clu­sion that is war­ranted by the detec­tion of a photon in the left cavity is only this coun­ter­fac­tual (contrary-​​to-​​fact) state­ment: if a direct mea­sure­ment of the slit taken by the cor­re­sponding atom had been made, then this would have been found taking the left slit. But here we need to take into account that no mea­sure­ment is direct. Mea­sure­ment out­comes are inferred from pointer posi­tions, dig­ital dis­plays, detector clicks, com­puter print­outs, and what not. If we had no right to infer the posi­tion of a par­ticle from such an indi­cator — if we could only con­clude ad infinitum that a par­ticle would be found if the appro­priate mea­sure­ment were made — then it would be impos­sible to mea­sure the posi­tion of a par­ticle, or any­thing else for that matter. But if we do have the right to infer the posi­tion of a par­ticle from the click of a detector, then it appears that we also have the right to infer the slit taken by an atom from the click of a pho­to­sensor (if one is placed in each cavity).

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1. [↑] “No ele­men­tary phe­nom­enon is a phe­nom­enon until it is a reg­is­tered (observed) phe­nom­enon.” — Wheeler, J.A. (1983). Law without law, in Wheeler and Zurek (eds.), Quantum Theory and Mea­sure­ment, Princeton Uni­ver­sity Press, 182–213.