| A harvest of questions |
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| Discussion, first conclusions, interpretational strategy | |
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Page 3 of 3 Questions arising from the experiment of Englert, Scully, and Walther: We asked: are we entitled to believe that an atom that was found going through L would also have gone through L if we had not ascertained the slit through which it went? The answer was negative. If we had not checked through which slit the atom went, we could have checked how (and made sure that) it went through both slits.
Interpenetrating Surfaces (detail) by Eric J. Heller. In other words, the experimenters have a choice. They may leave the shutters closed and determine the cavity containing the photon, thereby not only ascertaining the slit taken by the atom but also making sure that the atom went through a single slit. Or else they may open the shutters and not only learn from the behavior of the sensor how the atom went through the slits — in phase or out of phase — but also make sure that it went through both slits. It follows that an atom goes through a single slit only if the appropriate measurement is made, and it goes through both slits only if a different measurement is made. If neither measurement is made, it doesn't go through the left slit and it doesn't go through the right and it doesn't go through both slits in phase and it doesn't go through both slits out of phase. A measurement does not simply reveal something that is the case — something that would have been the case even if no measurement had been made. The properties of the quantum world exist only if, only when, and only to the extent that they are measured. Measurements create their outcomes. In the quantum world, to be is to be measured.
As said, by choosing either of two measurements, the experimenters choose between two possibilities: the atom went through a single slit or the atom went through both slits. Since this choice can be made long after the atom has made its mark on the backdrop, it contributes to determine the atom's past. It is therefore possible to influence the past! This does not mean that the past can be changed. The world has exactly one history. Its state at any given time is what it is, was what it was, or will be what it will be — period. Stories that permit making changes in the past inevitably run into irresolvable paradoxes, inasmuch as such changes have consequences in the present that are inconsistent with the actual present. (If you were to cause your grandfather to die in his cradle, you wouldn't exist. Changing the past is a logical impossibility.) The experimenters' influence on the atom's past, however, does not have consequences that are inconsistent with the actual present. Nor do they change the past; they contribute to create it. This possibility exists for two reasons:
(As a matter of fact, a property-indicating event occurs quite generally after the time of possession of the indicated property.) |
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