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 interference, so the photon had to pass though both slits, but if the telescopes are used, only one fires, so the photon just took one path. The inevitable conclusion is that a detector turned on after the photon passes 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 the physical path a photon takes can be altered after it sets off, then its future can affect its past! The distances involved are irrelevant, so a photon could travel from a distant star for a billion years, then it hits a telescope on earth, decide if it physically came 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.
This is a big problem, as that time can flow backwards puts all physics in doubt, but now consider the alternative, that the photon takes every path and only picks one when it arrives. Computing calls this tactic, of leaving choices until the last possible moment, just-in-time management. For example, it lets supermarkets restock based on point-of-sale data rather than historical estimates.
In Young’s experiment, just-in-time management makes the photon immune to delayed events. It goes through both slits, as usual, and if a screen is there, gives interference, but if not, it just carries on until it hits a telescope, which restarts it with a path that went through one slit. If the screen is there, we conclude the photon went through both slits, but if the telescopes are there, we conclude it went through one slit. Yet swapping the screen in and out after the photon goes through the slits doesn’t matter at all because the physical event that defines the path occurs on arrival.
If light is made of particles, the delayed choice two slit experiment implies backwards causality, but if it is a processing wave, time doesn’t reverse and causality remains intact.