
What’s the Difference Between Copenhagen and Everett? - kgwgk
http://www.math.columbia.edu/~woit/wordpress/?p=11202
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etatoby
I highly recommend Eliezer Yudkowsky's Quantum Physics Sequence (of articles):

[https://wiki.lesswrong.com/wiki/The_Quantum_Physics_Sequence](https://wiki.lesswrong.com/wiki/The_Quantum_Physics_Sequence)

According to Everett / Decoherence interpretation, as explained by Eliezer,
atomic or subatomic particles are not _physically real_ —let alone macroscopic
objects made with them.

What's _physically real_ is the single _amplitude distribution_ over the
infinite-dimensional mathematical space of all possible _joint configurations_
of all the particles in the universe. This distribution—as well as boggling
the mind—is complex-valued, continuous, differentiable, and evolving with time
according to the well known QM equations. Crucially, it may be _more or less
factorizable_ over any given subset of its infinitely many dimensions.

"You" and "me" are just two (more or less) factored subspaces of The amplitude
distribution.

Therefore, the question of where do the Born probabilities come from (an open
question in QM) is shown to be inextricably linked to an understanding of the
physical basis of consciousness (another big open question.) The Born
probabilities may represent the "amount of consciousness" (of "you", the
observer) that is split among newly factored subspaces, every time "you" get
further fragmented, aka. decohered, aka. "entangled" with new blobs of
amplitude.

I cannot recommend Eliezer's series of articles highly enough.

~~~
danharaj
You will not gain much understanding of quantum mechanics from that series of
blog posts and will accumulate a lot of woo.

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ganzuul
I've found Sean Carroll's podcasts since starting to study and think about
this subject more seriously. I can definitely recommend it.

Realism vs. subjectivism might be a false dichotomy in the quantum realm,
hiding alternative narratives. For example, perhaps the measurement problem is
able to give us a notion of true objects, as opposed nebulous clouds of atoms,
where objects are strongly entangled internally and until they are entangled
with each other they almost exist in separate realities. Background to this
idea is back holes being the fastest scramblers of information, my limited
understanding, and wild imagination.

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ggggtez
I'm becoming more of a fan of Everett. Wave collapse feels like nonsense, and
requires arbitrary notions of "observers". I don't fully grasp the
ramifications of the Many Worlds interpretation, but if it means reality is
deterministic, I'll take it.

~~~
TheOtherHobbes
MWI feels like nonsense to me.

The problem is that it's unbounded. It's not just about specific measured
quantum events, because that begs the question about how and where and why
universes branch off.

Without that foundation, it becomes about _all possible_ quantum events.
Literally. Any moving particle that has a chance to decay may decay at any
time, spawning a universe. And some of those spawned interactions will lead to
other spawned interactions...

Somehow you have to get the Born Rule from a number of possible interactions
that is so big it reduces the number of particles in the universe to an
insignificant rounding error.

Assuming this number is even countable (up for debate) there must be universes
in which quantum physics doesn't appear to work as it does here.

You can't just assume that somehow you get the right answer with a nice
converging integral. Because the complete list of possible MWI universes
includes a very small - but not zero - number where consistently unlikely
interactions are the norm, so they operate with the "wrong" laws - for example
neutrons that refuse to decay, or gluons that stay permanently glued.

Which being the case, we don't know whether _this_ universe is a probabilistic
freak and the "real" laws are different to the ones we see here. Maybe our
Born Rule is the freak, and most universes operate differently. With MWI,
there's no way to tell.

And so on for other issues. MWI is best left to science fiction plot lines. I
really can't see it making sense as either falsifiable science or consistent
philosophy.

~~~
eigenspace
This is a very common misunderstanding based on the sloppy way people tend to
explain Many Worlds. In the Many Worlds picture, branching of wave functions
isn’t built into the basic postulates of QM. Not every quantum event results
in branching.

Branching an emergent property of large systems. Consider the following:

* I have a qbit in a superposition for 0 and 1

* I as a large, hot, wet macroscopic system interact with that qbit in such a way that my future behaviour depends on the state of that qbit (say if I measure it to be 1 I shoot myself and if I get 0 I go for a walk)

If I try to measure the qbit, I become entangled with it. According to unitary
evolution, my wave function would be an entangled superposition of the two
events. However, for macroscopic systems entanglement and superpositions are
basically meaningless. I couldn’t do an interferometry experiment on my body
that would determine if I’m in a entangled state so a many worlds person would
say that the wave function of be unvierse has effectively branched into two
parts that never very little overlap or influence on each other.

——————————————————

Maybe this analogy will help: If I have a cup of water and we look at it on a
macroscopic scale, there’s a very clear difference between the liquid and the
air above it. There’s a division. However, if you zoom in, that division
becomes very fuzzy. There’s a region near the waters surface where air
molecules and water molecules freely mix and interact and there’s actually a
somewhat smooth crossover between the air and water. So the phase separation
between a cup of water and the air above it isn’t a fundamental part of the
rules of physics. It’s just an emergent behaviour like the branching of the
wave function.

~~~
millstone
Right. The problem of interpretation arises when the qbit has value 0 with
probability 75%, and value 1 with probability 25%. In what sense can the
universe be said to have branched into two, yet the probabilities are not
evenly divided? What can probability even mean if both branches are in some
sense realized - is one branch 3x as "real" as the other?

~~~
eigenspace
Again, I’d say you’re misunderstanding the point. The branches aren’t a real
thing in quantum mechanics. They’re just a particular way of looking at the
wave function of the universe. In the evolution of the wave function, it
sometimes will ‘split’ into two parts that are essentially independent of each
other and may have different amplitudes associated with them. These amplitudes
may be shown to correspond to the (square root of the) probability of finding
yourself in one branch or another.

However, if you knew about the entire wavefunction as an outside godly
observer to the universe might, then there’d be no point in speaking of
‘branches’.

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pkamb
A lot of bikes and less of a smell.

