Wheeler's Delayed Choice Paradox
The physicist John Wheeler thought about the double slit experiment and asked himself: what would happen if we did not decide whether to check which way the photons went until after they had passed the slit, would they still respond so as to show only their particle or wave identity but never both at once? In a lab light moves so quickly it is hard to delay the decision on how we are to measure it, but supposing that we used a light source that was a distant quasar, billions of light years away. The light could have been travelling towards us for longer than the World exists and yet apparently it still does not decide whether to behave like a wave or a particle until it encounters some measuring apparatus. This experiment, although it is probably impossible to carry out does not sound to physicists as impossible as you might imagine. Some objects in the universe bend light, indeed any large mass will. There are quasars known that lie directly behind other galaxies so that their image is distorted and magnified by passing a nearer galaxy on the way to our telescopes. We would have to find a quasar where there were just two paths around the intervening lensing object. The observer can adjust the relative path length to get the light in phase by delaying the light arriving along the shorter path by passing it through a suitable fibre optic. In practice this might be very difficult since the two paths might differ in length by light years but no matter. This is a thought experiment. It is hard to perform but perfectly logical.
Wheeler's delayed choice paradox has been with us for since 1978. Th first partial confirmation was carried out by Carroll Alley in 1986. Others have carried out increasingly sophisticated tests since. The most recent I have found carried out by Scully and others in 2001 again confirms that the light really does not 'decide' how it is going to travel until it gets there. As before this was a laboratory setup. It did not involve starlight and the time period is only nanoseconds for the light to pass through the apparatus. A short time but significant because the effect precedes the cause.
Wheeler's paradox is the first clear sign that retrocausal connections may exist. The experiments to test the delayed choice paradox are similar to Dopfer's experiment, which is another experiment in the same class. One might wonder why Wheeler missed this step and did not predict retro causal communication. He believed in it what he called the participatory universe, a universe that only comes into existence as we observe it. This idea arises from his interpretation of quantum mechanics and gave him an alternative understanding of his paradox.