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Quantum Foundations

Quantum mechanics is one of our most successful theory. It underlies all of chemistry, all of condensed matter physics, all of modern technology. And yet its foundations remain deeply puzzling. What does it mean for a physical system to be in a superposition? What happens during a measurement? Does quantum mechanics apply to the universe as a whole? And what happens to space and time themselves at distance and time scales when quantum gravitational effects become important?

Quantum Decoherence

The process by which a quantum system becomes classical is one of the deepest unsolved problems in physics. Quantum decoherence occurs when a quantum system becomes entangled with its environment, causing the coherence between different quantum states to be dispersed into environmental degrees of freedom and making the system appear classical, but its precise formulation to complex systems and to the Universe has remained highly challenging.

The Centre has also led the development of the decoherent histories approach to quantum theory, which provides a rigorous framework for assigning probabilities to entire histories of the universe. This is a crucial element for quantum cosmology, where there is no external observer to perform measurements.

Causal Sets

Some members of the Centre are proponents of the causal set approach to quantum gravity which proposes that spacetime is not a smooth continuum but a network of causally related "atoms" of spacetime. The smooth continuum we experience emerges as an approximation after coarse-graining the underlying discrete structure.

This idea gives a different perspective with potential implications for cosmology, the cosmological constant problem, black hole physics, and the nature of time.

Quantum Entanglement

A further remarkable bridge between quantum gravity and quantum foundations comes from the work of members of the Centre who discovered how entanglement of quantum information is identical to the mathematics describing certain black holes in supergravity. The same algebraic structures appear in both contexts, suggesting a deep and still poorly understood connection between the quantum information content of spacetime and the structure of quantum gravity itself.

Quantum Cosmology

A question critical to the foundations of quantum cosmology is the nature of the No-Boundary Wave Function of the Universe. This is a critical direction, led in particular by Hawking and many of his former students, multiple of whom are now members of the Salam Centre.

Members of the Centre explore the quantum nature of black holes, and investigate the possibility that the laws of physics themselves evolved in the early universe, a cosmogony in which order and matter emerge from chaos through a process of evolution in the fundamental constants and laws of nature. Alternative approaches to the quantum nature of the universe is further being studied using unimodular gravity, providing a framework in which time is fundamentally the canonical dual of the constants of nature.