I was thinking about the many interpretations of quantum physics, and one thing that never made sense to me was the many world's interpretation. Basically at any given moment for which something exists in a superposition, isn't this interpretation basically saying that there would have to exist a new universe for every possible discrete outcome that could result once the measurement is made?
So let's say an electron's location is a superposition of two distinct locations. Doesn't that mean there would at that moment have to be two distinct universes for each distinct location that is superposed? If so, then clearly the amount of universes being spawned every moment must be astronomical.
There could be trillions upon trillions of electrons(for instance) having superposed states, and then every moment in time (I suppose every planck time?), it seems there would have to be yet another set of universes spawning for every superposition.
How could this be? Surely there must be some kind of thermodynamic cost to replicate an entire universe. What would fuel or supply the additional mass and energy for every universe at every moment for every superposed state?
To my lay person's mind, this seems to be a very big weakness of this interpretation, but I needed to ask people with the physics background required to answer this question. Could there be some kind of loophole that could allow alternate universes to spawn so rapidly? Otherwise, doesn't this present a bit of a problem for taking this interpretation seriously?
Answer
The many worlds interpretation has a lot of problems, but this isn't one of them. You're imagining "creating alternate universes" as some energetic event, like a mini Big Bang, but what really happens is a smooth splitting of the wavefunction.
For example, suppose we have a spin in a superposition of up and down states, $$\frac{1}{\sqrt{2}} (| \uparrow \rangle + | \downarrow \rangle).$$ Now, suppose we measure the spin with a macroscopic detector that displays $1$ if the spin is up and $0$ if the spin is down. Then under the Copenhagen interpretation, the final state is $$|1, \uparrow \rangle \text{ or } |0, \downarrow \rangle$$ each with 50% probability. Under the many worlds interpretation, the final state is $$\frac{1}{\sqrt{2}} (|1, \uparrow \rangle + |0, \downarrow \rangle)$$ and these two terms are the two "worlds". Energy is still conserved. Though there are now two branches of the wavefunction, each has $1/\sqrt{2}$ times the amplitude, so half the energy.
All standard interpretations of QM agree this is the right final state for two microscopic objects interacting; many worlds just extends it a macroscopic object. The difference is that interaction with a macroscopic system is irreversible by the Second Law, so you'll never be able to unentangle the detector and get back to your original state.
As a result, we call the two branches of the wave function different "worlds", since they can never interfere with each other. More conservative interpretations, like Copenhagen, just say that the other world doesn't exist at all. It's a matter of taste, since the difference is unobservable.
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