Feynman suggested that the way to interpret quantum phenomena such as the double-slit experiment was to assume that when a particle travels from point A to point B, it doesn’t simply take one path - it takes every possible path simultaneously; the photon travels through both slits at the same time and interferes with itself, for example…
In this scheme, when a photon travels from a lamp to your eye it moves in a straight line, but it also dances about in twists and swirls, travels to Jupiter and back, and ricochets off the Great Wall of China. The obvious question, then, is why do we see only ever see one path, straight and simple? Feynman’s answer was, because all the other paths cancel each other out…
In the sum-over-histories interpretation, each path can be mapped out as a wave. Each wave has a different phase (effectively a starting time), and all the waves added together create an “interference pattern”, building upon one another where their phases align and cancelling each other out where their phases are mismatched. The sum of all the waves is one single wave, which describes the path we observe…
Applied to the universe, this idea has an obvious implication. Just as a particle travelling from point A to point B takes every possible path in between, so too must the history of the universe. In one history, the Earth never formed. In another, Al Gore is president. And in yet another, Elvis is still - well, you get the idea. “The universe doesn’t have a single history, but every possible history, each with its own probability,” Hertog says…
But there is a twist: the history that we see depends on the experimental setup. In the double-slit experiment, it has been shown time and again that if we use a photon detector to find which of the two slits the photon went through, it no longer creates an interference pattern, just a single spot on the film. In other words, the way you look at the photon changes the nature of its journey.
The same thing happens in Hawking and Hertog’s universe: our observations of the cosmos today are determining the outcome - in this case, the entire history of the universe. A measurement made in the present is deciding what happened 13.7 billion years ago;
by looking out at the universe, we assign ourselves a particular, concrete history.