Quantum Foundations and Quantum Gravity: From Double Slits to Cosmic Cycles

Overview

In this New Scientist conversation, physicist Sean Carroll guides a tour of quantum mechanics and its most stubborn questions, from the double slit experiment to the nature of reality, and from interpretations like Copenhagen and Many-Worlds to the role of gravity and cosmology in the quantum world.

Key insights

• Quantum probabilities differ from classical ones and raise foundational questions about measurement and reality.
• Interpretations such as Copenhagen, Many-Worlds, and pilot-wave offer different answers to what the wave function represents and what happens during measurement.
• The intersection of quantum mechanics with gravity and time leads to deep puzzles like the problem of time and how space-time might emerge from quantum theory.
• A novel cyclic cosmology is explored, where quantum mechanics could underlie a bounded, repeating universe with a bounce rather than a singular Big Bang.

Quantum foundations and the double slit

The discussion opens with the double slit experiment as a touchstone for understanding wave rather than particle behavior in quantum systems. Through repeated single-particle trials, interference patterns emerge only when no which-slit information is obtained. Once which-slit information is measured, the interference disappears and the particle behaves classically. This leads to the standard Copenhagen view: the wave function describes the system until a measurement collapses it to a definite outcome. Yet this raises the measurement problem and the reality problem: what exactly constitutes a measurement, and what is real in the quantum world when no one is looking?

Probability, interpretation, and philosophy

The program then shifts to probability in quantum mechanics, contrasting frequentist and Bayesian viewpoints and noting that some interpretations treat quantum probabilities as reflecting beliefs rather than intrinsic features of reality. These questions feed into broader debates about what experiments can and cannot settle in science, and how philosophy informs scientific inquiry.

The measurement problem, the role of the observer, and Wigner's friend

The conversation delves into the role of the observer and boundary between quantum and classical descriptions. The Copenhagen cut posits a division between quantum systems and classical observers, but many-worlds challenges this by treating observers as quantum systems that become entangled with what they measure. The Wigner's friend thought experiment surfaces the tension: different observers may assign different quantum states to the same situation, depending on whether systems are treated as collapsed or still evolving under quantum laws.

Alternate interpretations and the physics landscape

Carroll surveys a spectrum of interpretations including hidden-variable or pilot-wave theories, objective collapse models, and the many-worlds perspective. He emphasizes that there is no universal consensus; the decisions often depend on how one weighs mathematical elegance, ontological commitments, and experimental consequences.

Quantum gravity, time, and the Wheeler–DeWitt problem

The conversation then connects quantum foundations to gravity. Gravity resists straightforward quantization in the same way as other forces and small-scale quantum field theory must be reconciled with dynamic spacetime. The Wheeler–DeWitt equation, describing the “wave function of the universe,” famously lacks explicit time evolution, prompting the problem of time and debates about whether time is emergent or fundamental.

Time's arrow, entropy, and initial conditions

Turning to cosmology, the discussion explains how entropy provides an arrow of time and why the early universe began in a highly ordered (low entropy) state. Boltzmann’s ideas on entropy depend on microscopic configurations consistent with macroscopic observations, leading to questions about why the Big Bang started with low entropy and how this may relate to quantum gravitational dynamics.

A cyclic, quantum-informed cosmology

Finally, Carroll presents a novel approach in which quantum mechanics is employed from the outset to model a cyclic universe. Two possibilities emerge: either the universe is infinite in time and restless, or its evolution is constrained to a finite, deterministic quantum space that allows periodic cycles. In the latter case, a bounce replaces a singularity, and entropy behaves in a way that permits a repeating history with a bounded set of states. The arrow of time is handled locally in each cycle, and observers in different cycles will experience time in distinct but coherent ways depending on their position in the entropy evolution.

What lies ahead

The episode closes by reflecting on the role of experiments in advancing foundations in physics. While concrete experiments are challenging for resolving fundamental questions, exploring these ideas leads to testable predictions and informs the pursuit of a consistent theory of quantum gravity. The conversation invites listeners to consider the deep connection between quantum mechanics, time, and the cosmic scale.