# Using the Entire Universe as a Quantum Laboratory: An Interview with Daniel Sudarsky [Video]

### To test the interpretation of quantum mechanics, look to cosmology.

Quantum physicists do love their beer. By day they build instruments, measure numbers, solve equations. By night they retire to the bar or pub and muse about the philosophical puzzles of quantum theory. By “philosophical” they mean “fun but impractical." Maybe quantum theory betrays the existence of parallel universes, maybe it exposes causal influences coming from the future, but the matter is surely unresolvable. Drink up.

Daniel Sudarsky has nothing against beer, but disputes the common assumption that the interpretation of quantum mechanics is of no practical import. He thinks it has measurable consequences for cosmology, including the seeds of galaxies, black holes, and gravitation in general. Sudarsky and his colleague Elias Okon, both at the National Autonomous University of Mexico, spelled out their argument in an earlier guest post on this blog. There they focused on a specific interpretation in which the collapse of the wavefunction is a physical process as opposed to a mathematical contrivance, a concept I elaborated on in a Nautilus magazine article last year.

I caught up with Sudarsky in June at a conference on the philosophy of quantum gravity, organized by Christian Wüthrich at the University of Geneva and Nick Huggett at the University of Illinois in Chicago.

His talk at that conference, also available on YouTube, delved into the related question of how our present theories dovetail with a putative quantum theory of gravity. He suggested that quantum gravity should be thought of not as a single theory, but as a ladder of progressively more elaborate descriptions, descending rung by rung from the observed world to a deep level where spacetime itself dissolves:

classical continuum (e.g. fluids, general theory of relativity)

classical particles (e.g. Newton’s law of universal gravitation)

quantum field theory in curved spacetime (e.g. cosmological inflation)

semiclassical theory (e.g. Hawking’s analysis of black holes)

Planck scale (e.g. string theory, loop quantum gravity)

Although theorists naturally gravitate to the puzzles of the deepest level, the higher levels are no breeze, either. We observe the world to consist of objects following definite paths, but this commonsensical view must give way, and Sudarsky analyzed how that might happen.