[nessus42]

> If the universe is an isolated quantum system evolving unitarily, how does the MWI help to understand the laws of physics that we observe?

It definitively answers the question of when wave collapse occurs and what causes it.

The answer given by the Everett Interpretation is simply that there is no wave collapse, and what we observe is the result of our brains being in a superposition of multiple states.

[kgwgk]

Consider this simple application of standard QM:

1) we have a one-particle system that has been prepared into a pure state by measuring the spin along the x-axis

2) we are going to measure the spin along the z-axis

3) the quantum state before is a superposition of the |up> and |down> states (in the basis corresponding to the Sz operator)

4) the theory predicts that we are going to find either |up> or |down> with equal probability

5) immediately after the measurement the quantum state will be either |up> or |down>, depending on the outcome

What is the answer given by the Everett Interpretation? What is the description of the initial conditions? What is the prediction of theory? What is description of the end state?

I hope the answer is not just handwaving and mumbling about "superposition".

[nessus42]

The prediction of the Everett Interpretation is that our brains end up in a superposition of states in which one of those states perceives an up particle and one of those states percieves a down particle, but that the original particle is still, in actuality, in a superposition of up and down states. I.e., the superposition has not actually collapsed into either up or down. It's just that our brains are now in a superposition of states too.

I don't understand why this would be difficult to understand.

[galaxyLogic]

> It's just that our brains are now in a superposition of states too.

Are you saying they are "now" in a superposition, meaning they were not before the observation?

If so then how can the act of us observing the thing cause us, our brains to go into super-position? And is it just our brain that goes into super-position? Why not the rest of my body too? And what if I'm holding the hand of another person, does she too go into super-position?

[kgwgk]

> the original particle is still, in actuality, in a superposition of up and down states

If you cannot say that the particle is in a well defined up or down state after the measurement then you cannot say either that it remains in a superposition of up and down states. Because that original state was the result of a previous measurement. You cannot even say that the particle (or you, for that matter) does actually exist. The universe is a superposition of states where it does and states where it doesn't. (Does the universe exist at all?)

The standard interpretation of QM:

- we want to explain the physical world that we experience

- we come up with a theory based on a mathematical description of physical states and the equation describing its evolution

- the theory allows for superpositions of states, incompatible with the physical world where we observe only definite states

- why do we observe only definite states?

- we postulate that when we measure the wave function changes becomes and becomes consistent with the observation

- we also postulate the Born rule to compute the probability of observing each potential outcome

- everything is experimentally verified, there are open questions (what is a "measurement"?) but the theory works fine in practice

- we know that this is not a complete description of the world (gravity!)

The Everett interpretation:

- let's assume that the physical world that we experience is just one aspect of a larger, out-of-reach thing

- what is real is the mathematical description (nevermind that it's incomplete), evolving according to the Schroedinger equation

- why do we observe only definite states?

- because this is the way our brains experience the physical world! (see how easily we solved the issue with measurement?)

- what is the probability of observing each outcome?

- ... (but, hey, did you notice how elegantly did we skipped, I mean, solved the measurement problem?)

- we have a collapse-free interpretation of QM! (but remember that if you want to study the physical world that we experience you have to use the projection postulate, because this is the way our brains experience the physical world)

I find interesting that the MWI is so popular among cosmologist, given that QM doesn't handle cosmological issues well. But of course this interpretation "solves" the problem of why the observed universe is precisely the one that we observe.

Everett's theory is interesting, but not a panacea. Maybe decoherence is the key to explaining "collapse", maybe gravity is the key, maybe the answer is somewhere else...