Sean Carroll on the Many-Worlds Interpretation, Quantum Measurement, and the Impossibility of Time Travel

The discussion centers on the Many-Worlds Interpretation (MWI) of quantum mechanics, presented as a proposed solution to the "measurement problem." Sean Carroll argues that MWI explains observed phenomena, such as apparent wavefunction collapse, without invoking special processes. Instead, it posits that the universe continuously branches into multiple realities, with each branch representing a different outcome of a quantum event. The core idea is that the Schrödinger equation governs everything, and what we perceive as collapse is merely our experience of being on a specific branch. A crucial distinction is made between MWI and other interpretations like hidden variables or dynamical collapse theories, emphasizing that MWI doesn't introduce new physics but rather reinterprets what's already there. The independence of these branches is highlighted, making "hopping" between them physically impossible. The concept of determinism within a branch is discussed, contrasting the theoretical possibility of rewinding the universe with the practical impossibility due to the immense information required, akin to trying to un-spill a glass of wine. While not offering direct practical recommendations for daily life, the discussion provides a profound insight into the nature of reality as understood by MWI. It addresses the "fear of missing out" (FOMO) in the context of infinite branching, suggesting an acceptance that one is always missing out on countless other realities. The analogy of the spilled wine glass vividly illustrates the practical irreversibility of macroscopic events, even if theoretically reversible at a fundamental quantum level with perfect knowledge. The conversation delves into the broader implications for concepts like time travel and the universe's memory. It concludes that quantum mechanics, even with MWI, offers no help for time travel. The principle of information conservation is central, implying that if one possessed "Laplace's demon-like" knowledge of the universe's quantum state, one could theoretically rewind it. However, this remains a theoretical construct, practically impossible for large systems, underscoring the limits of our knowledge and computational capacity in understanding and manipulating reality.

we didn't invent many worlds cuz you thought it was cool to have a whole bunch of worlds right we amended it because we were trying to account for what we observe here in our world and what we observe here in our world are wavefunctions collapsing

many worlds is just a proposed solution to that problem and the answer is nothing special is happening it's still just the Schrodinger equation

the entire point of many worlds is to explain what we observe but it tries to explain what we already have observed right

the branches are independent yes is it possible to hop from one to the other no it's so physical limit there's a the theory says it's impossible

the downside of many worlds is that you're missing out on an enormous and that's always what it's gonna be like

it is possible in principle to rewind everything if you start from perfect knowledge of the entire wave function of the universe in practice that's never gonna happen

time travel not possible nope at least quantum mechanics has no help

the fundamental laws of physics in either many worlds quantum mechanics or Newtonian physics conserve information

if you have all the information about the quantum state of the world right now your Laplace's demon-like and your knowledge and calculational capacity you can wind the clock backward but none of us is right

once things get to be large Avogadro's number of particles right bigger than a cell no chance you