# Splitting in MWI

## Main Question or Discussion Point

In MWI does everytime something splits it splits into an infinite number of worlds?

Can something continue to split forever?

If MWI is true does this mean for example that every proton in effect consists of an infinite number of indistinguishable protons and that every person consists of an infinite number of identical copies of themselves that go on to differ through decoherence?

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A universe splits proportional to the amount of possibilities which arise. Since infinity is abstract and since infinity cannot never be produced in the lab, it is unlikely a universe does split into an infinite amount of other universe-possibilities.

For instance, you flip a coin 10 times, you make 1024 universe-possibilities real!

K^2
That's not really how it works. The splits are not discrete. They are described as discrete for simple projective measurements, but there is no such thing as a simple projective measurement in the real world. In reality, observer's state interacts with observable state, which takes up a finite amount of time. There isn't a specific moment there when you went from state A to state B.

Furthermore, in the example with flipping the coin, the coin's state doesn't consist of just heads or tails. The coin's state is fairly complex, and observer interacts with the whole thing, whether or not the observer actually takes note of things like orientation of the coin and where it landed. Even with experiments on particle spins, where it really is just up or down, you rarely work with a single spin, and what observer really interacts with is the measurement apparatus which is a complex system that can interact with other quantum states of the particle.

A universe splits proportional to the amount of possibilities which arise. Since infinity is abstract and since infinity cannot never be produced in the lab, it is unlikely a universe does split into an infinite amount of other universe-possibilities.
But if a wavefunction has infinite possibilities and each possibility is another 'world' than doesn't that mean that each faux collapse/decoherence event create an infinite number of objects from the parent object? So if MWI is true than a cat turns into an infinite number of cats or postcat configurations (each decoherence event reverberating up from quantum to macroscopic level) and each surviving cat can go on to become additional infinite sets of other cats/post cat configurations indefinitely right?

Hurkyl
Staff Emeritus
Gold Member
In MWI does everytime something splits it splits into an infinite number of worlds?
Do you understand how a quantum state evolves via Schrödinger's equation?

Good; that's exactly what MWI says happens. All this talk of "splitting" is just a way to describe things.

In MWI does everytime something splits it splits into an infinite number of worlds?

Can something continue to split forever?

If MWI is true does this mean for example that every proton in effect consists of an infinite number of indistinguishable protons and that every person consists of an infinite number of identical copies of themselves that go on to differ through decoherence?
Good question! Let me try a simple thought experiment for discussion. Let us say we have light that has passed through a vertical polariser and then pass it through a polarising analyser orientated at 60 degrees from the vertical. The probability of a given photon passing through the second polariser and being detected is 25%. Presumably MWI says the world splits into 4 new worlds, in 3 of which the photons are not detected and 1 in which the photon is detected. Now if we orientate the second polariser at 89 degrees the chance of passing through the second polariser is approximately 3 in 10,000 so 10,000 new worlds are required of which a positive detection is made in 3 of them. As we get arbitrarily close to a relative orientation of 90 degrees for the second polariser the number of new worlds get arbitrarily large, but maybe not infinite, because when the polarisers are at exactly 90 degrees there is zero chance of being detected.

Now if it is often stated that that every time a decision is made then a split occurs. So let us say our observer has a choice of three positions (A B and C) that he can turn his analyser to. Let us say our observer rotates the analyser to position C, then 3 new worlds are created, one in which he turned the analyser to C and two others where he turned the analyser to A or B. Even if the observer does not rotate the analyser to a new position, there is the the possibility that he *might* of rotated the analyser to one of the other positions so every instant 3 new worlds are created even if the observer does nothing to allow for the possibility that he might have done something. Now if we have an analyser that is circular and can be turned to any one of infinite number of orientations, then the world has to split into an infinite number of new worlds every instant to allow for that possibility. It would seem that only way to avoid the infinite possibilities would be to somehow quantisize the rotation angle.

So the question is, does the world split only when a quantum particle passes through a measurement device, or does it also split when a sentient being makes a decision?