

Quantum Physics Revealed As Non-Mysterious - hhm
http://www.overcomingbias.com/2008/06/quantum-physics.html

======
hugh
Y'know, I'm a physicist, specializing in quantumy stuff, and I suspect that
some form of many-worlds is probably true.

But I don't really like the preachy "Many Worlds is just the way it is, and
anyone who thinks otherwise is a fool" tone of this series of articles.
They're actually pretty well written (I'd rate them above David Deutsch's
Fabric of Reality, frinstance -- though I don't consider that very high
praise), but I recommend taking them with a grain of salt.

Example: _We have embarrassed our Earth long enough by failing to see the
obvious. So for the honor of my Earth, I write as if the existence of many-
worlds were an established fact, because it is. The only question now is how
long it will take for the people of this world to update._ \--- Sheesh, that
kind of thing just doesn't belong in any scientific debate.

The domain name bothers me as well. Is it supposed to imply that anybody who
doesn't agree with everything written in overcomingbias has simply done a poor
job in overcoming their biases?

~~~
DaniFong
I don't really like them either. They're written well enough for one to
basically follow along the sentences, and the tone is such that it seems
they're really understanding something I'm not. But I looked pretty deeply
into these questions. I read the original papers of Einstein Podolsky and
Rosen, of Bohr, of Born. I read much of John Bell's original work, and I'm
pretty sure that there are results being assumed here (and elsewhere) that
haven't really been proven, or were even originally stated.

For example, it's commonly asserted that Bell's theorem rules out any
deterministic quantum mechanical description of reality with local hidden
variables. But this was, to my knowledge, never shown explicitly by Bell -- he
simply ruled out a particular class of local hidden variable theories. There
exist theories, consistent with experiment, which are local and deterministic:
for what it's worth, a toy example is just a classical computer simulating
everything in our universe we've yet seen.

We don't know where the Born probabilities come from, but we also don't really
understand the implications of many body entanglement. We don't even have a
consistent _definition_ of entanglement for three or more particles. We don't
know, we haven't been able to calculate, but I have reason to suspect, that
measurement consistent with Born probabilities could be _entirely_ explained
by the deterministic axioms already part of Quantum Mechanics. We haven't
really gone as far as we could go with them. And already people are treating
Many-Worlds as axiomatic. I don't really buy it.

~~~
yummyfajitas
>for what it's worth, a toy example is just a classical computer simulating
everything in our universe we've yet seen.

I think the state of the classical computer qualifies as a nonlocal hidden
variable.

To use the language of formal logic, the classical computer is a model for
quantum theory, i.e. (classical computer) |- QM. Within the QM theory, the
computer qualifies as a non-local hidden variable, even though within the
classical theory the computer is embedded in, it is local.

Another toy example is Bohmian mechanics on _configuration space_ : the theory
is just a local PDE + local particle. But that's non-local in physical space.

>We don't even have a consistent definition of entanglement for three or more
particles. We don't know, we haven't been able to calculate, but I have reason
to suspect, that measurement consistent with Born probabilities could be
entirely explained by the deterministic axioms already part of Quantum
Mechanics.

Decoherence makes sense on the macroscale (1000+ particles), although it's
true that 40 particles is iffy. Different classical states (i.e., experimental
apparatus has light on vs light off) are separated by a distance sqrt(number
of particles) in configuration space, and don't interact.

As for explaining measurement with Born probabilities, that's reasonable. My
co-conspirators and I currently have a physical, macroscale model where we
show this to be true (no citation yet, but I'd be happy to explain more via
email). But you still need _some_ ontology.

All deterministic QM can show is that the probabilities work out correctly;
i.e., the born probability of (measurement 1 says spin up, measurement 2 says
spin down) = 0.

You still need a way to actually pick a configuration based on that
probability distribution. The universe as we (you, me, even pg) know it is a
point in configuration space, not a wavefunction. I'm happy with both MW and
Bohm for that purpose.

~~~
DaniFong
I concede that by this definition of locality, a simulator doesn't quite
count. But it _is_ a physical model both consistent with our quantum
experiments and special relativity, and it's local in _something_. And if it's
local physics in _something_ we're talking about, why is it 3-space that we're
treating as the _real embedding_.

> Decoherence makes sense on the macroscale (1000+ particles), although it's
> true that 40 particles is iffy. Different classical states (i.e.,
> experimental apparatus has light on vs light off) are separated by a
> distance sqrt(number of particles) in configuration space, and don't
> interact.

> As for explaining measurement with Born probabilities, that's reasonable. My
> co-conspirators and I currently have a physical, macroscale model where we
> show this to be true (no citation yet, but I'd be happy to explain more via
> email). But you still need some ontology.

Please do. I looked for a while at decoherence and others, and the mechanism
behind the _processes_ kept fading from view. It's was like thermodynamics,
where we can say something about the equilibrium states eventually reached,
but we're having a hard time explaining the processes by which it reaches one
state or another, and by those processes, the reasoning in other parts of
physics break down. Like microscopic <-> macroscopic reversibility.

But the people in nonequilbrium statistical mechanics have made a lot of
progress in reconciling microscopic reversibility and macroscopic apparent
irreversibility. Is such a thing possible for quantum measurement, or more
generally, the quantum classical transition, as well? Might there be a
reversible description -- Schrodinger's all the way down, so to speak?

Finally, I'm not so sure that MW, decoherence, Bohmian mechanics, etc. are
truly equivalent. In other words, I expect that one might start getting
different answers.

And there's reason to believe that they're incomplete descriptions. If you
take one measurement, you'll notice it takes time. And the microphysics of QM
says that it's time evolution should be unitary. So, halfway done, if we stop
the clock, when we're doing a measurement, what do we find? Or rather, what
would our laws tell us we'd find?

The more popular interpretation seem to tell me 'don't ask this question.' But
it seems there's something important hidden here.

~~~
yummyfajitas
>Might there be a reversible description -- Schrodinger's all the way down, so
to speak?

Basically, what I've got is a model of a particle interacting with a
measurement apparatus (a BEC, to make the calculations simple). You can reduce
the many body schrodinger equation to a mean field model on reduced
configuration space: (particle coordinate X BEC coordinate). So yes, it is
schrodinger (actually madelung) all the way down.

Measurements (of position) correspond to the particle making a splash in the
BEC (1). Splashes at different locations correspond to different measured
outcomes. Once the difference between splash sizes is macroscopic ( (number of
particles) * splash profile =O(1) ), the measurement is complete.

By "complete', I mean that if you pick a random BEC configuration (N BEC
particle locations), and you can determine with statistical significance (i.e.
99.9999% sure) where the splash is.

Before this occurs, you've just got two overlapping probability distributions
in configuration space. Picking random BEC configurations won't tell you
(statistically significantly) the particle location.

The process is continuous, but it doesn't look that way to us since it is also
very fast, i.e. t = O(1/number of particles in observation apparatus).

~~~
DaniFong
Yes, this is exactly the sort of example I was looking for! Do you have any
further details? Maybe I should work it out myself, to see if our results
match up.

I suspect that there are all sorts of examples like this, accessible in theory
and models of measurement in experiment, where things basically match up with
with Born probabilities. Though I expect in some degenerate case they won't,
just as, as the fluctuation theorem in nonequilibrium shows us, sometimes the
second law is inaccurate, because whatever we call 'entropy' decreases.

------
lg
This bothers me because it pretends that it's railing against an unenlightened
status quo, and it isn't. Philosophers of physics considered, and largely
rejected, the many-worlds interpretation because, if quantum events result in
every possible outcome occurring in some world, and our consciousness ends up
in one of them by some chancy process, then there's no reason why we observe
some events with higher probabilities than others. But we do, and many-worlds
doesn't explain that. The author talks around this by saying that, yes, these
probabilities are very mysterious, indeed. Well, there are other
interpretations under which they're not. Such as a) the many flavors of GRW (a
collapse theory), b) the bohmian interpretation (waves + particles), and c)
the many-minds interpretation (anti-realist and therefore the most out-there
of the three). Those are all pretty much consistent with QM, or there's a good
reason to believe that they'll be beaten into a form that is so consistent
(e.g. tumulka's relativized version of GRW-flash, but that's outside the scope
of a HN comment).

~~~
hhm
I don't know much about this, but wouldn't it be a lot more probable to be in
a world of those many where the probabilities of events are distributed in a
reasonable way, than to be in another world where the distribution of
probabilities is out of the standard? (that is, there are many possible time
lines, but the distribution of probabilities would indicate that there would
be a lot more time lines for probable events, than for improbable events, so
it's easier for us to be in a probable time line than in an improbable one).
Or am I understanding something wrong?

(Sorry for my English, I'm not a native English speaker)

~~~
lg
The problem with many-worlds is that there aren't more timelines for probable
events. If every outcome occurs in some world, then for some event with two
possible outcomes, there ends up being two of you; say, Lefty and Righty, one
in each world. So there's no sense in saying "I'll probably end up being
Lefty, since his outcome was more reasonable," because you'll end up being
both of them.

~~~
yummyfajitas
I think in MW, there are continuously many (macroscopically identical) lefty
universes and continuously many righty universes.

So one macroscopic state (righty) corresponds to a set of finite volume; the
set of all configurations who's macroscopic state (i.e., ignore quantum
details humans can't see) looks like righty.

So all one can reasonably ask is "what is the probability my universe lives in
that volume?" The obvious answer is the integral of |\psi|^2 over that volume,
but that is a postulate, not something you can derive from the schrodinger
equation.

~~~
lg
I believe that on Everett's approach, when you measure some observable with
two possible outcomes, the total number of worlds increases, like a bacterium
dividing. But on the version you're suggesting, it sounds like we constantly
hop from one already-existing universe to another, because every instant
brings with it a different configuration. But it makes little sense for our
bodies to hop between worlds, since they're parts of worlds, so is it our
'mind' that does it? But that sounds like many-minds, except with many worlds
instead of one, and with a new problem of how we do the hopping.

Unless by configuration, you mean that each world is a world-line that
contains a set of definite outcomes for all quantum events, and so all the
quantum outcomes are predetermined for each world, and we explain
probabilities by recourse to the proportion of some worlds to others. That's
not really many-worlds, that's modal realism (every possible world exists, and
"the actual world" just means "my world"). Which is more David Lewis than Hugh
Everett.

~~~
yummyfajitas
In Everett's approach, I think it is the _macroscopic_ state which splits. But
the macroscopic state is a volume of worlds. So I guess think of the interval
[0,1] as consisting of a single (macroscopic) world. But [0,1] has infinitely
many numbers in it; each one comprises a different "world".

Under evolution, [0,1] splits into [-0.5,0] U (0.5,1], two macroscopic states
but still infinitely many microscopic ones. Every state in [0,0.5] was turned
into a state in [-0.5, 0] and similarly (0.5,1] -> [0.5,1]. (Each real number
corresponds to a world configuration.)

(note: there are quite a few variants of many worlds, and not everyone
realizes they are talking about different theories. I don't know if I'm
describing the most common view. )

My (non-mainstream) mental picture of MW is bohmian mechanics, with each
possible bohmian trajectory corresponding to a different world history.

~~~
lg
>Every state in [0,0.5] was turned into a state in [-0.5, 0] and similarly
(0.5,1] -> [0.5,1].

I hadn't heard this version, but it doesn't make sense to me. So these
microscopic worlds each change when a quantum event occurs? If they can
change, then why not think that there's only one world, which changes when you
measure something? It's a lot simpler and seems to handle everything that this
theory does. Of course, if they don't change, then we're in a microscopic
world with a precise configuration at any given moment, and every observable
has a value, and Einstein was right, and we wouldn't observe Bell's
inequalities. But we do, and anyway, the whole reason we postulate this stuff
is because we think that things _in our world_ (you know, the one I'm sitting
in, microscopic or otherwise) actually evolve according to the wave function.

BTW, if your picture is Bohmian mechanics, then you only need one world, where
every particle has a well-defined position all the time, and these evolve
according to the wave function. These different possible histories are
epistemically possible because we don't know what region of the wave function
the particle currently inhabits. But that's not a continuous infinity of
worlds, microscopic or otherwise, unless you're just using the word "world"
that way.

------
mattmaroon
Wow do I wish I had the time to read all of that. As a former physics nerd, I
enjoy well-written pieces that explain complex principles simply (though I'm
not averse to some calculus).

Maybe someday after I leave startupland.

------
nazgulnarsil
the AI and Bayesian stuff is OB's strong suit. I don't think they've made
their pet QM case very well.

