QR3.8.3 Delayed Choice Experiment

Figure 3.21. Delayed choice experiment

That photons travel about a foot per nanosecond allows a delayed choice two-slit experiment. Two detection options are used, either the usual screen or two telescopes that focus on one slit or the other (Figure 3.21). The trick is that the choice of which to use is made after the photon passes the slits, when the screen is either quickly removed or not. If the screen is used, the result is the usual interference so the photon passed though both slits but if the telescopes are used only one fires, so the photon took one path or the other. It follows that a detector turned on after the photon passed the slits decides the path it took before that:

It’s as if a consistent and definite history becomes manifest only after the future to which it leads has been settled.” (Greene, 2004) p189

If an observation made after a photon travels a path decides the path it took before that, physical realism must conclude that the future can affect the past! The distances involved are irrelevant, so a photon could travel from a star for a billion years then decide when it arrives at earth if it physically traveled via galaxy A or B. As Wheeler says:

To the extent that it {a photon} forms part of what we call reality… we have to say that we ourselves have an undeniable part in shaping what we have always called the past.(Davies & Brown, 1999) p67

That time flows backwards puts all physics in doubt but quantum realism concludes that the photon takes every path and chooses a physical event when it arrives. Computing calls leaving all processing choices until the last possible moment just-in-time processing and it lets point-of-sale systems base supply orders on what customers actually buy rather than historical estimates.

Applying this method to Young’s experiment, photon instances go through both slits and if a screen is there they give interference but if it isn’t there, they just carry on spreading until an instance registers a telescope to restart there. Since that instance went through one slit, we call that the photon’s “path”. If on the other hand the screen is left there, we conclude that the photon went through both slits. On the quantum level, swapping the screen in and out after the light goes through the slits doesn’t matter at all because the physical event that defines the path occurs on arrival.

Every photon observation involves a specific instance whose path history becomes that of “the photon”. Photon instances take every path until an observation restarts the photon, making its path the photon’s physical path. According to physical realism, the delayed choice two slit experiment implies backwards causality but in quantum realism there is no time reversal and causality remains intact.