Irreconcilable differences

The white whale of theoretical physics is the unified theory of everything, a theory that would unite the two pillars of modern physics, Einstein's theory of general relativity and quantum mechanics. While both explain a lot about our universe, they are also incompatible with one another. Now, James Quach at the Barcelona Institute of Science and Technology in Spain has proposed a way to potentially reconcile the two, and his idea begins with an iconic experiment.

Thomas Young's 1801 double slit experiment involves sending particles through two slits and examining the pattern they make when they hit a screen behind the slits. Classical views on particles dictate that they should form two parallel bars on the screen, and most do, until we get to quantum particles — when we send electrons through the slits, they behave like waves, producing alternating bands of light and dark.

While we don't know why quantum particles create the patterns they do in the double slit experiment, we do know how to predict where they will hit the screen thanks to the Born rule, which thus far works in all its real tests. But the thing is, physics doesn't know why it works — it just does.

Quach's study suggests looking at assumptions from the double slit experiment to locate current gaps in our knowledge of quantum mechanics, which would bring us one step closer to reconciling it with the theory of general relativity. He suggests we engineer an experiment in which the Born rule doesn't work, as this will illuminate any weak points in our understanding of quantum mechanics and, hopefully, help us reconcile it with general relativity.

He proposes taking theoretical physicist Richard Feynman's assertion that one has to consider all paths, even the non-sensical ones, in accounting for the probable paths of a particle in the double slit experiment. Instead of just considering two potential paths for the quantum particle (through slit A or through slit B), physicists should consider a third path: through slit A, then to slit B, and finally to the screen.

Quach says this could be tested if two detectors are placed after the double-slit board. One of the detectors would indicate whether the quantum particle has traveled through slit A or B, while the other would detect whether the particle went through just one or both slits without knowing which one.

A Middle Ground

The probabilities you get when you account for interference between Quach's three paths is different from the probabilities predicted by the Born rule, which only accounts for two paths. This crack in the Born rule could indicate which assumptions within Einstein's theory of general relativity and quantum mechanics must give way, leading us down the path toward the coveted theory of everything.

While Quach's proposal hasn't been formally peer-reviewed or tested in real-world experiments yet, it could be the first step toward solving a problem that has plague the scientific community for half a century and lead to a more complete understanding of the universe in which we live.

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