Where Does the Quantum World End and Ours Begin?

Quantum mechanics is a scientific framework that describes how particles and energy behave at very small scales, such as atoms and subatomic particles. Unlike the predictable behavior of everyday objects, quantum systems exhibit unusual properties like wave-particle duality, where particles can act as both particles and waves. For example, in the double-slit experiment, a single electron or photon can create an interference pattern as if it passed through both slits simultaneously, a phenomenon impossible for classical objects. This was demonstrated even when only one particle was present in the experiment at a time, showing that quantum behavior is not dependent on large numbers of particles. Quantum mechanics is the most precisely tested theory in science, with experiments confirming its predictions to an accuracy of one part in 10^12. Despite its precision, there is no universally accepted interpretation of what quantum mechanics tells us about reality, making it one of the most debated topics in physics.

Quantum cosmology applies quantum mechanics to the universe as a whole, particularly to the conditions at the beginning of the universe, such as the Big Bang. Traditional models of the Big Bang suggest a singularity, a point of infinite density and curvature, where classical physics breaks down. Quantum cosmology attempts to describe this early universe using quantum principles, including the gravitational field. One influential idea is the no boundary proposal, developed by Stephen Hawking and James Hartle, which suggests the universe began in a quantum state without a sharp singularity. Instead of a cone-like shape, the geometry resembles a smooth hemisphere, often called a shuttlecock geometry. This approach avoids the need for an external observer, as the universe itself is the quantum system. Jonathan Halliwell, a professor at Imperial College London, began his career in quantum cosmology before focusing on the foundations of quantum mechanics.

Decoherence is a process in quantum mechanics where the fragile quantum behavior of a system is disrupted by interactions with its surrounding environment. This interaction causes the wave-like properties of quantum systems to dissipate, leading to the emergence of classical behavior, where objects appear solid and predictable. For example, a quantum particle in a superposition of two states (like being in two places at once) will lose this property when it interacts with air molecules, light, or other environmental factors. Decoherence explains why we don’t observe quantum effects in everyday objects like the moon or humans. It resolves the paradox of how the quantum world transitions to the classical world without requiring a conscious observer, as the environment itself acts as the 'observer.' Halliwell emphasizes that decoherence is key to understanding the boundary between quantum and classical behavior.

The histories approach to quantum mechanics, developed by James Hartle, Murray Gell-Mann, and others, provides a way to understand quantum systems without relying on external observers or the collapse of the wave function. Instead, it focuses on records or measurements made in the present and looks for correlations between them to infer what happened in the past. This approach avoids the need to assume that the past 'actually happened' in a definite way, which is problematic in quantum cosmology where there is no external observer. For example, in quantum cosmology, we cannot observe the early universe directly, but we can use present-day records, like the microwave background radiation, to infer its quantum state. The histories approach is part of the broader decoherent histories framework, which aims to provide a consistent description of quantum systems without paradoxes.

Schrödinger’s cat is a thought experiment designed to illustrate the strange consequences of quantum superposition. In the experiment, a cat is placed in a sealed box with a device that has a 50% chance of releasing poison, triggered by the decay of a radioactive atom. According to quantum mechanics, the atom is in a superposition of decayed and undecayed states until observed. This implies the cat is simultaneously alive and dead until someone opens the box and observes it. The paradox highlights the tension between quantum superposition and our classical intuition about reality. Halliwell notes that while the thought experiment is useful for discussion, the use of a cat is outdated and ethically problematic by modern standards. A simpler example, like the double-slit experiment, can convey the same idea without involving animals.

The transition from quantum behavior to classical behavior is a major unsolved question in physics. Quantum systems, such as electrons or atoms, exhibit wave-like properties and can exist in superpositions of multiple states. However, macroscopic objects like humans, cats, or the moon do not show these properties. Decoherence explains part of this transition by showing how interactions with the environment destroy quantum superpositions. For example, a macroscopic object like a human is constantly interacting with air molecules, photons, and other particles, which rapidly decohere any quantum superposition. This process is so effective that quantum effects are nearly impossible to observe in large objects. Halliwell emphasizes that this transition does not require a conscious observer, as the environment itself acts as the 'observer' that collapses quantum states into classical ones.

Albert Einstein famously questioned whether the moon is 'really there' when no one is looking, highlighting the tension between quantum mechanics and classical intuition. In quantum mechanics, particles exist in superpositions until measured, which suggests that reality may not be definite until observed. Einstein, who preferred a deterministic view of the universe, found this idea troubling. The histories approach to quantum mechanics provides a way to address this question without invoking observers. Instead of relying on measurement, it focuses on the correlations between records in the present to infer what happened in the past. For example, we can infer the position of the moon based on the gravitational effects it has on Earth, even if no one is looking at it. This approach avoids the need for a conscious observer and aligns with the idea that reality is defined by consistent records rather than observations.