Interview, Fireside Chat
Sean Carroll: Many-Worlds Interpretation of Quantum Mechanics
- The speaker rejects the textbook quantum mechanics premise that measurement or observers play a fundamental role in physical laws, citing a lack of empirical evidence.
- The speaker notes that while it would be "infinitely cool" if mental cogitation affected reality, current evidence points toward observation being non-fundamental.
- The Many Worlds Interpretation (MWI) is identified as the speaker's preferred alternative, originally proposed by Hugh Everett III in the 1950s.
- MWI posits that observers are quantum systems obeying the Schrödinger equation like all other matter, possessing a wave function rather than holding a special status.
- Measurement is redefined as a process of entanglement where the observer's wave function becomes entangled with the system's wave function.
- This results in distinct branches where the observer perceives a specific outcome (e.g., electron located here vs. elsewhere) without the branches interacting or influencing one another.
- The interpretation asserts that "worlds" are created whenever a superpositioned system becomes entangled with its environment, regardless of an observer's presence.
- The speaker argues that MWI is the most mathematically simple interpretation, as it requires no additional rules beyond the Schrödinger equation.
- The complexity of MWI lies in mapping the formalism to observed reality, specifically in "carving up" the universal wave function into non-interacting branches.
- Critics often find MWI counter-intuitive because the branching process is difficult to reconcile with everyday experience, though the speaker views this as a mapping issue rather than a theoretical flaw.
- The speaker calculates the finite dimensionality of the Hilbert space within our observable universe horizon.
- This calculation assumes the universe's accelerating expansion (discovered in 1998) creates a cosmic horizon that limits the number of possible states.
- The estimated dimensionality of this Hilbert space is $10^{10^{122}}$.
- For context, this number vastly exceeds the age of the universe ($10^{17}$ to $10^{18}$ seconds) and the total number of particles ($10^{88}$).
- A finite Hilbert space implies a limit to wave function branching, corresponding to the universe's eventual cooling and expansion into an empty state.
- The speaker clarifies that "splitting" in MWI should not be confused with "copying" to avoid misconceptions about energy conservation.
- In the splitting model, the pre-existing universe divides into thinner branches, conserving the total "vector" of the wave function.
- The speaker definitively states that energy is conserved in MWI, as the total wave function evolves unitarily without the creation of new energy.
- Three primary alternatives to MWI are outlined as the current "frontrunners" in quantum mechanical interpretation debates.
- Hidden Variable Theories: Propose that the wave function is real but incomplete, with additional variables (typically particles) governing dynamics.
- These theories struggle to extend beyond non-relativistic systems to quantum field theory or quantum gravity.
- Spontaneous Collapse Theories: Suggest that wave functions collapse randomly at a low probability for single particles but rapidly for macroscopic objects due to entanglement.
- Variants include induced collapse theories (e.g., Roger Penrose's model where gravity triggers collapse).
- Epistemic Interpretations: View the wave function strictly as a predictive tool for experimental outcomes rather than a physical reality.
- These interpretations struggle to define what physical entity exists prior to or independent of the prediction.
- Hidden Variable Theories: Propose that the wave function is real but incomplete, with additional variables (typically particles) governing dynamics.
- The speaker favors MWI for its applicability to advanced physics theories like quantum gravity and holography.
- Other interpretations carry "classical baggage" that privileges specific concepts like spatial location, requiring re-engineering for new theories.
- MWI is described as "plug and play," allowing immediate application to any underlying theory, making it the most robust foundation for understanding the emergence of space-time.