Many worlds on trial: one champion and two skeptics
Sean Carroll defends the many-worlds interpretation as simply taking the Schrödinger equation seriously, while Sara Walker says she is not a many-worlds person and Adam Frank argues it invents infinite unobservable universes, preferring QBism. The disagreement shows that the meaning of quantum mechanics is still wide open among working scientists.
What is the measurement problem in quantum mechanics? Show answer
The theory says a wave function evolves smoothly, yet measurement seems to follow different rules, without a clear account of when or why.
How does Sean Carroll describe the many-worlds interpretation? Show answer
As taking the Schrödinger equation seriously for the whole universe, observer included, without adding a collapse.
What happens when you measure an electron's spin, on Carroll's account? Show answer
You become entangled with it. There is one branch where you saw each outcome, and the branches no longer interact.
Where does Carroll say the other worlds are? Show answer
They do not exist in space. Space exists within each world, and we cannot interact with the others.
Why does Carroll call many-worlds the austere option? Show answer
It uses just one equation and adds no extra ingredients. The space of quantum states was always large enough to contain the branches.
What challenge does Carroll put to skeptics? Show answer
If you do not trust the Schrödinger equation everywhere, name the equation you believe in and show that it fits the data.
What does Carroll concede is the best reason to doubt many-worlds? Show answer
It demands new philosophy about identity, probability and prediction, a large extrapolation from what we know.
How does Adam Frank read the measurement rule? Show answer
As the theory telling us from the beginning that measurement matters, not as a defect to explain away.
What is Frank's main objection to many-worlds? Show answer
It invents an infinite number of unobservable parallel universes to avoid the lesson that measurement matters.
What is QBism, as described here? Show answer
An interpretation, developed by Chris Fuchs and colleagues, that puts the measurement rule, the agent and information at the center.
What is Sara Walker's position? Show answer
She is not a many-worlds person. She sees the branches as bundled into the deterministic macroscopic object that is you, within a random quantum background.
What do Carroll and Frank each mean by simplicity? Show answer
Carroll means the fewest equations. Frank means not positing entities that can never be observed.