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‘Wheeler's hunch is that the universe is built like an enormous feedback loop, a loop in which we contribute to the ongoing creation of not just the present and the future but the past as well. To illustrate his idea, he devised what he calls his "delayed-choice experiment," which adds a startling, cosmic variation to a cornerstone of quantum physics: the classic two-slit experiment....
By the time the astronomers decide which measurement to make— whether to pin down the photon to one definite route or to have it follow both paths simultaneously— the photon could have already journeyed for billions of years, long before life appeared on Earth. The measurements made now, says Wheeler, determine the photon's past. In one case the astronomers create a past in which a photon took both possible routes from the quasar to Earth. Alternatively, they retroactively force the photon onto one straight trail toward their detector, even though the photon began its jaunt long before any detectors existed.
It would be tempting to dismiss Wheeler's thought experiment as a curious idea, except for one thing: It has been demonstrated in a laboratory. In 1984 physicists at the University of Maryland set up a tabletop version of the delayed-choice scenario. Using a light source and an arrangement of mirrors to provide a number of possible photon routes, the physicists were able to show that the paths the photons took were not fixed until the physicists made their measurements, even though those measurements were made after the photons had already left the light source and begun their circuit through the course of mirrors.
Wheeler conjectures we are part of a universe that is a work in progress; we are tiny patches of the universe looking at itself— and building itself. It's not only the future that is still undetermined but the past as well. And by peering back into time, even all the way back to the Big Bang, our present observations select one out of many possible quantum histories for the universe.’
Discover Magazine - Does the Universe Exist if We're Not Looking?