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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?

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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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    $\begingroup$ Another important remark is that the "number" of worlds is not a definite thing. There's always one wavefunction. You can split it into some number of branches, depending on exactly how 'irreversible' you want the splitting to be. A sane timescale for splitting is on the order of the decoherence time of the system, not the Planck time. $\endgroup$ – knzhou Jul 18 '16 at 17:13
  • $\begingroup$ So the world we do observe, in the multiverse interpretation, has half the energy it had before decoherence, for that part of the universe's energy in that originally superimposed state? If so, I can detect the other world because I lost half my energy (for that state). And I loose that every time a decoherence happens somewhere? Can you explain? $\endgroup$ – Bob Bee Jul 19 '16 at 4:27
  • $\begingroup$ @BobBee Quantum mechanics is linear, so there is no observable difference if your 'world' is scaled down by $1/\sqrt{2}$. One way of imagining this is that everything, including your measuring apparatuses, has"shrunk" equally. (I don't actually advocate that explanation, which can be extremely misleading, but it's a nice soundbite.) $\endgroup$ – knzhou Jul 19 '16 at 4:42
  • $\begingroup$ Makes no sense. Maybe QM is linear and doesn't care about scale but cosmology and gravitation does (and in case you would say well maybe not quantum gravity, the cosmological solution and data we have does not care about quantum gravity). It does not compute. Never thought much of the multiverse anyway, but am interested if there is any math explanations that make any sense. I doubt it. I wish the answers to these questions were more correct, by using the known understanding of decoherence, and not propagate the multiverse unlikely theories. $\endgroup$ – Bob Bee Jul 19 '16 at 5:49
  • $\begingroup$ If we look at the collection of universes as a multiverse, and every time we "split" a universe, the number of universes increases. Does this mean that the mass/energy of the multiverse is growing boundlessly? Is this allowable because every split universe has half the energy, as you described in your answer? Clearly it must be either impossible or very hard for the universes to interact with one another for many worlds to work, but I'm not entirely sure why the multiverse is allowed to grow in mass/energy like that. Could you explain a bit? $\endgroup$ – LordVoid Jul 19 '16 at 15:26
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To answer your first and second questions, yes.

I think where it may be a point of debate to some is the entire concept of "universes" being spawned. Universes may not necessarily be "spawned" so much as already being in existence, essentially a universe for each discrete outcome of any event, past, present, future, each existing per its own space-time. Assume that there is an upper limit to the amount of particles/matter in every universe, from this then there should also be an upper limit to the maximum number of interactions (observable events let's say) taking that entropy always increases for each universe. That should lead to the conclusion that there is an upper limit to the maximum number of universes that exist and there is nothing in our current understanding of big bang/inflationary theory that limits the amount of matter/energy that could have been created for a number of universes, as long as it isn't infinite which makes no sense as you stated earlier.

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