Parallel Worlds Probably Exist. Here's Why
The Mysteries of Quantum Mechanics
Classical mechanics is a straightforward field - if you know the state of a system, such as the position and velocity of a particle, you can use an equation like Newton's second law to calculate what that particle will do in the future. In quantum mechanics, however, the situation is more complex. If you know the quantum state of a particle, represented by its wave function, you can use the Schrödinger equation to evolve that state over time. But the wave function often spreads out, rather than behaving like a discrete particle.
This poses a challenge: how do we reconcile the spread-out wave function with the point-like particle detections we observe in experiments? The founders of quantum theory grappled with this issue, ultimately proposing the idea of "wave function collapse" - the notion that the wave function suddenly and irreversibly changes when a measurement is made.
Schrödinger's Cat and the Many Worlds Interpretation
Schrödinger himself was uncomfortable with this formulation, which is why he invented the famous Schrödinger's cat thought experiment. In this scenario, a cat is placed in a box with a radioactive atom, a detector, and a mechanism that will release poison if the atom decays. According to quantum mechanics, the atom exists in a superposition of decayed and not-decayed states, which becomes entangled with the detector and the cat. This means the cat is also in a superposition of alive and dead states.
Schrödinger's point was to show that the idea of wave function collapse is problematic - the cat cannot be truly both alive and dead at the same time. But there is a better way to think about this, proposed by physicist Hugh Everett in the 1950s: the Many Worlds interpretation of quantum mechanics.
The Many Worlds Interpretation
The key insight of the Many Worlds interpretation is that measurement is not a special process that requires a separate set of rules. Instead, measurement is simply the result of a quantum system (like the detector) becoming entangled with another quantum system (like the cat). There is no need for wave function collapse - the entire system, including the observer, evolves smoothly according to the Schrödinger equation.
The apparent paradox of Schrödinger's cat arises because we think of the observer (the person opening the box) as a single entity. But in reality, the observer also becomes entangled with the state of the cat. When the box is opened, the observer's wave function splits into two branches - one where the cat is alive, and one where the cat is dead. Both of these outcomes occur, but the observer only experiences one of them.
Branching Universes and Environmental Decoherence
This branching of the observer's wave function is a key feature of the Many Worlds interpretation. It suggests that the universe is constantly splitting into parallel versions, or "worlds," as quantum systems become entangled with their environments. This process of "environmental decoherence" is what causes the wave function to branch, leading to the proliferation of parallel worlds.
The rate at which this branching occurs is not precisely known, but it is likely happening very frequently - perhaps even infinitely often. This may sound implausible, but it is simply a consequence of taking the mathematics of quantum mechanics seriously. Rejecting the existence of these parallel worlds requires additional assumptions, like the collapse of the wave function, which introduce their own problems.
Implications and Objections
The Many Worlds interpretation has several important implications. First, it restores determinism to quantum mechanics - every possible outcome happens, it's just that we only experience one branch of the wave function. Second, it eliminates the need for the mysterious "measurement problem" and the associated issues with wave function collapse.
However, the idea of parallel worlds existing in this way does raise some objections. For example, how is energy conserved across all these worlds? And how many worlds are there, exactly? While we don't have definitive answers to these questions, the Many Worlds interpretation provides a more consistent and elegant framework for understanding quantum mechanics than the traditional Copenhagen interpretation.
Ultimately, the Many Worlds interpretation may seem counterintuitive, but it is a natural consequence of taking the mathematics of quantum mechanics seriously. As physicist Sean Carroll notes, "the universe branches whenever a quantum system in superposition becomes entangled with its environment." This branching is happening all the time, leading to the proliferation of parallel worlds that we can never directly observe, but which are nonetheless a fundamental part of reality.
