# What is a universal wavefunction in MWI?

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1. Aug 6, 2015

### Nav

I'm alittle confused, is it saying that all the fundamental particles in the universe are really just one wave function?

2. Aug 6, 2015

### stevendaryl

Staff Emeritus
In nonrelativistic quantum mechanics, for any isolated system of $N$ particles, there is a corresponding wave function

$\psi(x_1, y_1, z_1, x_2, y_2, z_2, ..., x_N, y_N, z_N, t)$

The meaning of this quantity is that $|\psi|^2 \delta V^{N}$ is the probability at time $t$ of finding the first particle within a little cube of volume $\delta V$ centered at $x=x_1, y=y_1, z=z_1$ and finding the second particle within a little cube centered on $x=x_2, y=y_2, z=z_2$, etc. In general, there is only one wave function for the entire system of interest, regardless of how many particles are involved. The universal wave function is just the limiting case in which the "system of interest" is the entire universe.

3. Aug 6, 2015

### atyy

When we describe a quantum system of many particles, there is only one wave function in which all the particles are included.

In the standard interpretation of quantum mechanics, quantum mechanics only makes sense for describing user-defined parts of the universe (which have many particles), and not the whole universe. This is a problem, since there are presumably laws of physics that govern the whole universe, and not just parts of it.

Many-worlds is an attempt to extend quantum mechanics to the whole universe.