Saturday, January 09, 2016

The Many-Worlds Interpretation is a realist interpretation of the universe, but not a realist interpretation of the world.

The Many Worlds Interpretation (MWI) is a deterministic, realist interpretation of quantum mechanics (QM). MWI starts with two postulates

  1. The universe is described by a quantum state, which is an element of a kind of vector space known as Hilbert space.
  2. The quantum state evolves through time in accordance with the Schrödinger equation, with some particular Hamiltonian.
From the point of view of MWI, the quantum state of the universe (also known as the Universal Wave Function) is the thing that is "real". It evolves in a locally deterministic way.

What happened to the "other worlds"? Why is it even called the "many worlds" interpretation? Other worlds are not postulated by MWI, rather they arise naturally from an understanding of the behavior of the system based on just the two things we do postulate. And what about "decoherence"?

In MWI, decoherence is said to occur when the phase angle between components of the quantum state are sufficiently orthogonal that, for practical purposes, they do not exhibit interference. The fact that this occurs is a consequence of the underlying math. This happens naturally when information about quantum interactions (e.g. the result of a quantum experiment) spreads into the environment through further interactions (e.g. because the result is displayed on the measurement instrument, and photons from the instrument's display reach the experimenter's eyes, the walls, etc). Once that occurs, we can analyze the orthogonal components of the quantum state in isolation. These orthogonal components can be interpreted as independent worlds, or alternative futures of the world, each representing the future following one possible outcome of the interaction (e.g. measured result).

In practice the phase angles are never completely orthogonal, because the spread of information into the environment is limited by the speed of light; there are sufficiently distant regions of the universe where the components may interact. So the meaning of decoherence is interpretational: it depends on what we mean by "sufficiently orthogonal" and what the "practical purposes" are. If we are only interested in what happens in our experimental laboratory, the behavior of distant reaches of the universe in the distant future can be treated as irrelevant.

This is no different from saying that, for sufficiently small velocities, mechanical systems obey classical rather than relativistic behavior. What is "sufficiently small"? It depends on the context. Nature does not care what we mean by sufficiently small, it always obeys the relativistic rules. But the concept of classical behavior allows us to simplify our calculations (at the expense of introducing a small inaccuracy) to improve our understanding of the system.

So it is for decoherence. It is not a term that is rigidly defined in the theory, but (like "non-relativistic velocity") is rather a concept for a simplifying assumption that we use to understand the behavior of the quantum state. Decoherence cannot properly be said to occur at some particular time, like quantum collapse in the Copenhagen interpretation. It is not an "event that happens", but rather a change in the way we interpret the meaning of the quantum state from one time to another.

One accepts MWI at the expense of rejecting objective reality as we know it. When we open the box to see whether Schrödinger's cat is alive or dead, we become entangled with the cat's quantum state. If we see that the cat is alive (as we hope), we cannot say that the cat's status of being alive is a fundamentally true fact about the universe. Rather in the quantum state of the universe, there are nearly orthogonal components that can be interpreted as two versions of our world, one in which we observe the cat being dead, and one in which we observe a living cat. One of them feels more real, somehow, but each component describes a version of us who thinks it is he who is observing the true world. MWI doesn't designate one of these components as somehow more real than the other, and thus we can think of them as separate worlds, or futures.
[MWI] predicts that we will think and claim, that we do not observe superpositions at all, even when our own states are highly indefinite, and that we are simply mistaken in the belief that we see a particular outcome or other. That is, it preserves unitary [deterministic] QM – at the expense of a skepticism that "makes Descartes’s demon and other brain-in-the-vat stories look like wildly optimistic appraisals of our epistemic situation" [The Ashgate Companion to Contemporary Philosophy of Physics page 43]
This is like Einstein's principle of relativity in another way, too. In MWI, the meaning of the world is relative to the observer. If you ask whether the cat is alive or dead as a property of the universe, the simple answer is that the cat is in an indefinite state. To give a more definite answer we would need to know which (mostly) orthogonal component of the quantum state you're asking about. Which world did you mean?

17 comments:

Brian Slesinsky said...

If we think of a brain as an evolved device for making decisions, it should be unsurprising that it only takes into account the version of the universe that it observes.

Neal Gafter said...

Agreed. We should also expect that the brain should (have evolved to) behave as if it lives in the world, which acts as if nondeterministic, even if the underlying universe (which we cannot observe) is deterministic, as MWI posits.

carmel Ka said...

Hello Neal,
I was following your replies to :
http://breakingthefreewillillusion.com/quantum-probability-no-freewill/#more-3030
since I wasn blocked by the author Trick Slattery when trying to explain him why local variables (the ones that causes X how he likes in his comments)is refuted by Bell theorem and inequalities:
I debated him in here as Sile:
http://breakingthefreewillillusion.com/ontic-probability-doesnt-exist/
and you continued right after me.
have you been blocked by him also? have you debated him off-line?
Regards
Sile

Neal Gafter said...

Yes, I have been blocked by him also, which is part of the reason I posted this.

I am having an email discussion with Trick now in which I have demonstrated (by counterexample) why his formulation of "deterministic" does not imply "could not have been otherwise" - basically, because in his definition of deterministic causes are necessary causes but not sufficient causes for the events that they cause. See https://en.wikipedia.org/wiki/Causality . He seems to believe he has proven that the former implies the latter; see his "proofs" at http://breakingthefreewillillusion.com/necessary-sufficient-causality/ and http://breakingthefreewillillusion.com/otherwise-causal-contradiction/ .

Unfortunately for him this undermines his entire book and most of what he has been writing recently, so he's fighting tooth and nail for a way out.

carmel Ka said...

Thank you for reply !
Interesting exchange? can you post this email exchange so everyone can benefit of this rebuttal?
You also mentioned about:
"If one were to construct a counterexample one would use a different sequence of causal events in the two world histories to demonstrate “could have been otherwise”. If a variable is the input to some event in one world history (i.e. a cause), and not the input to some event in another world history (i.e. not a cause), then your “logical” reasoning doesn’t apply. Even if the variable is a cause in both world histories, it may be input to *different events* in the two world histories, in which case there is no self-contradiction."
so can you also post a counterexample about this rebuttal?
Thanks!
P.S
I think you are allowed to publish a rebuttal on Amazon (if you are a purchaser), or he can still block you in there?

Neal Gafter said...

I'd prefer to complete the discussion with Trick before publishing. At this point he claims that I've "sneaked in an acausal event", even though there are only two events in my entire counterexample and he cannot identify either of them as acausal. His proof consists of the fact that he has already proven that every (necessary) cause is a sufficient cause, and since my counterexample contradicts that, I must have sneaked in an acausal event somewhere!

carmel Ka said...

Looking forward to hear about his own(Trick) admitting the defeat:)
Take care he is good on tricks/shifting the goal posts(as you noticed during exchanges) and please consider it publishing on the Amazon also(if he as admin for that book does not have the rights/privileges to block his opponents like he did on his blog few times),

...and keep up the good debate!:)
Cheers,
Silviu

carmel Ka said...

Greetings Neal,
are you done with the Trick debate and ready to post it?
Regards,
Silviu

Neal Gafter said...

Still going strong.

carmel Ka said...

Greetings Neal,

the "war" is over with Trik?:)

Nice day!

Neal Gafter said...

We are both going to take a week to gather our thoughts and resume the discussion.

carmel Ka said...

hello Neal,

I hope I found you well, greetings!
It looks like your opponent/Trick just wrote an article after your debate with him:
http://breakingthefreewillillusion.com/must-lead-to-causality/#more-3168
Have you reached a conclusion about ? or still going on...
Looking forward to read from you!
Cheers!

Neal Gafter said...

Preparing a post now.

carmel Ka said...

Where did you post it? on his blog or do you make a new article :)?

Neal Gafter said...

I am giving Trick a chance to proofread the definitions before posting. Currently he's telling me they're all wrong, but not citing the "right" definitions.

carmel Ka said...

Hello Neal,

have you posted your article somewhere pls?:)
Cheers,

Neal Gafter said...

http://gafter.blogspot.com/2016/02/necessary-and-sufficient-causes.html