And so Wheeler's thought experiment and the predictions of quantum mechanics were brought to the laboratory for testing. [2] This is what happens.

"If what you say is true," he said (in effect), "then I may choose to know a property after the event should already have taken place." [1] Wheeler realized that in such a situation, the observer's choice would determine the outcome of the experiment â€“ regardless of whether the outcome should logically have been determined long ago.

J ohn Archibald Wheeler is one of those thinkers who takes the ideas of quantum mechanics seriously. After studying the Copenhagen explanation of the double slit experiment â€“ with its emphasis on what the observer knows and when it is known â€“ Wheeler realized that the observer's choice might control those variables in a test.

5. If the screen is removed, we reveal two telescopes. The telescopes are tightly focused on, watching, observing, just the narrow space of one slit only. The left telescope watches the left slit; the right telescope watches the right slit. (The mechanism/metaphor of the telescope assures that if you are looking through the telescope, you will see a flash of light if the photon went either wholly or in part through the slit on which it is focused; otherwise not. Therefore, you will obtain "which-path" information about the photon hit.)

Or the screen can be left in place. This is the experimenter's choice, which is delayed until after the photon has passed the slits (2) in whatever manner it happens to do so.

The screen can be removed, as indicated by the dotted line. It can be removed quickly, very quickly, after the photon has passed the double slits but before the photon reaches the plane of the screen. That is, the screen can be removed while the photon is in transit in region 3.

4. First, we have a screen (or other detection system that can measure the horizontal placement of a photon hit, but is not able to distinguish where the photon came from).

2. Photon passes through double slit unobserved, logically either through one, through the other, or through both. To obtain an interference pattern, we surmise that something must pass through both slits; to obtain a particle distribution, we surmise that the photon must pass through one or the other. Whatever the photon does, it presumably does it now when it passes through the slits.