
A wave of experiments is probing the root of quantum weirdness - jonbaer
http://www.nature.com/news/quantum-physics-what-is-really-real-1.17585
======
fizx
I'm going to try to explain this in programming terms :)

There's a longstanding interpretation that the wave-particle duality is
actually caused by (e.g. electrons) being particles riding invisible waves.
This interpretation (Bohmian mechanics) has been largely ignored as "valid but
uninteresting."

A key reason why this isn't historically that appealing is that the state of
the "invisible waves" in the neighborhood of a single particle is
mathematically dependent on the instantaneous state of _every other particle
in the universe._

If you assume the universe is a simulation, then the programmer would be a
freshman computer science student who unnecessarily made stepping forward in
time quadratic!

The oil-droplet experiments are an accidental existence proof that you can get
behavior similar to quantum behavior in particle+wave systems without the
quadratic global update rule.

Is it enough? There's still tons of open questions, and possibly (likely) yet
another dead end.

~~~
justifier
where is the contradiction here?

my understanding of 'natural' physics is as a relation between particle(s) and
every other particle in the universe

and our physics calculations trying to understand and predict those natural
events wave away those relations valued insignificant to the desired
probability threshold?

contrived but applicable example, "assume you are on a frictionless plane with
zero wind resistance"

or in one of the most amazing results of applied physics in recent news :

"Fred Jansen, Rosetta's mission manager at ESA, said officials predicted a 70
to 75 percent probability of a successful landing by Philae before the
mission's launch in 2004. But that number assumed the comet was a rounded
body, not the oddly-shaped world found by Rosetta."

~~~
dghf
> my understanding of 'natural' physics is as a relation between particle(s)
> and every other particle in the universe

The key word in the parent comment is _instantaneous._ This means that pilot-
wave theory is not compatible with relativity, if I understand correctly.

~~~
cordite
Is it instantaneous or is it the known or observable state propagated at the
speed of light?

Perhaps unrelated, but they have already shown that entangled particles
propagate state at the speed of light. So we probably wont have any Ender's
game ansible communication devices any time soon.

~~~
dghf
AFAIUI, under pilot-wave theory it's instantaneous. It's how the theory gets
round Bell's Theorem, which only forbids _local_ hidden variables.

(Again, I am an interested layman, not a scientist, so this comment may be
nonsense.)

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Spooky23
Is there a good resource out there for laymen to explain "quantum stuff" in a
meaningful way?

When I read articles that are using words like "spooky" and "weird" as terms
of art, and using Schrödinger's cat as a way to clarify a topic, I get nothing
out of it. And I'd like to be able to explain to my mom what this stuff is.

~~~
amit_m
"QED: The Strange Theory of Light and Matter" by Richard Feynman:
[http://www.amazon.com/QED-Strange-Theory-Light-
Matter/dp/069...](http://www.amazon.com/QED-Strange-Theory-Light-
Matter/dp/0691024170)

I read this book in high-school and learned about as much as I did in 4
undergrad physics courses about quantum mechanics. There's no mathematics in
this book, so it doesn't teach you how to calculate anything (for this, a
physics degree helps), but the principles are presented very nicely.

~~~
tim333
I second that and after if you are keen it can be good to look at the Feynman
Lectures vol III, Chapter 5 where he talks about spin. He deliberately leaves
spin out of QED to keep things simple. Also if curious there's the Feynman
lectures on Gravitation where he tries to figure out quantum gravity and fails
like everyone else. One thing I like about Feynman is his talks are based on
experiments - if you fire electrons at two slits this happens (QED) or if you
send them past a magnet that happens (Ch 5). This is in contrast to most
popular science books which go on about spooky cats or most academic
treatments which launch into a bunch of abstract maths without looking that
much at what's going on physically. I find it much easier to get my head
around the real experiment approach.

------
andyl
What is the role of the observer? What is the observer? Why does observation
cause the wave function to collapse?

~~~
stephengillie
What if we don't have a human observe? What if all data is only ever
interpreted by canines and bovines (dogs and cows)? Will the wave function
still collapse?

How would you isolate an experiment from the potentially-wave-function-
collapsing influence of human consciousness, and yet still produce a usable
contribution to science?

~~~
TheOtherHobbes
I'm not sure why this was down voted - it's a perfectly valid question.

There are serious philosophical issues with the _idea_ of an observer,
conscious or otherwise, as a useful concept.

Entanglement isn't really the issue, because it doesn't solve the problem.
What does it mean to be entangled with a complex state in a state of some form
of consciousness?

How much consciousness is needed to make a difference? Why should adult human
consciousness be the benchmark, and not semi-consciousness, or distraction, or
unconsciousness, or toddler-level pre-verbal awareness, or jellyfish
consciousness, or LSD-induced hallucination?

If consciousness is necessary, why doesn't reality stop working when we fall
asleep? Why do plants live in a consistent physical reality even though they
don't know what a Hamiltonian is?

If physics had code smells, this idea of consciousness might not pass the
sniff test.

Of course it's possible it's still the right answer. But if it is, it's
interesting there's _no useful explanation of anything_ in the concept yet.
Nor is there any formalism to support it. (Obviously there's a formalism for
wave function collapse. But so far as I know, there's nothing in the math that
says "And these variables are where the observer works his/her/its magic.")

Really it's just something that might be true, maybe, because we have no idea
what's going on and it's as good a guess as any other.

------
Xcelerate
I think it may ultimately turn out that the best explanation for quantum
mechanics is simply the set of postulates we already have, _minus_ the Born
rule. It's well known that a quantum system continues to evolve unitarily
(deterministically) via the propagation operator _regardless_ of whether any
subsystems have collapsed. How can you have a discontinuous subsystem within a
larger continuous system? You can't.

The explanation for the _appearance_ of collapse lies in the phenomenon of
decoherence, which basically says that subsystems tend to quickly evolve into
something resembling an eigenstate. This evolution must necessarily occur on
an incredibly short timescale. It might be possible to design an experiment
that would test the assumption that collapse is instantaneous.

I think the best definition of "collapse" is that it is the moment in time in
which a particular system can no longer be described (to good approximation)
as the direct product of two subsystems (see
[http://en.wikipedia.org/wiki/Separable_state](http://en.wikipedia.org/wiki/Separable_state)).
The concept of a "good approximation" is of course subjective, but it can
always be objectively metricized (totally made that word up) by using some
kind of error term.

Epigrammatically, collapse is not so much a physical process as it is a
characterization of the _capability_ to represent a quantum state in a
specific mathematical form.

(Of course, this doesn't preclude you from categorizing physical processes as
"collapse events"; it just means that collapse isn't a fundamental phenomenon
so much as it is an emergent one. Kind of like quasiparticles.)

So what about the randomness? I think it's better to refer to it as
unpredictability. The difference is subtle but crucial. True randomness
(assuming it exists) is the result of absolute indeterminism. On the other
hand, if eigenstate selection is merely "unpredictable", then that implies
collapse _is_ in fact a deterministic process (specifically e^(-iHt) applied
to Ψ over some time interval that we've decided to call a "measurement");
however, we're unable to extract enough information from the environment to
make exact predictions because _we ourselves_ constitute the required missing
information. In other words, the information necessary for absolute predictive
capability is trapped in the subsystem constituting the measuring environment,
and it becomes lost when that subsystem becomes entangled with the subsystem
being measured. And there's not really any way to prevent that from occurring,
because entanglement _must_ occur in order to learn anything about a system.

This even applies classically. The only difference is that classical
entanglement occurs between localized physical boundaries instead of between
subspace boundaries in an abstract Hilbert space.

To somewhat reify this, assume (for the sake of argument) that a classical
description of physics is enough to describe a human. Then perform a large MD
simulation of all the atoms inside a physics lab, including those of a
physicist. The evolution of this simulated system is provably deterministic.
Yet the physicist _appears_ to have free will, and it _appears_ like he is
deciding which measurements to perform on his environment. But he's just an
arbitrary collection of atoms that we've labeled "human", and he obeys the
same time-transformation rules that the unlabeled atoms in the system obey.
Mathematically, it's simply impossible for him to predict everything that
occurs within the virtual system -- not because of indeterminism -- but
because he isn't so much "choosing" what to measure as he is "appearing to
choose". There's a limit to the amount of information any system can obtain
about itself (well, maybe there's some fractals that are exceptions, but
generally speaking, it holds true.)

That said, experiment is always the ultimate arbitrator of truth, and I wonder
if there might yet be some clever way to tell whether our universe is simply
unpredictable instead of random, despite the possibility that both potential
mechanisms might impose the same limits on predictive capability (in fact,
Colbeck and Renner recently proved that QM is _already_ maximally predictive,
independent of whatever underlying mechanism governs eigenstate selection --
see
[http://www.nature.com/ncomms/journal/v2/n8/abs/ncomms1416.ht...](http://www.nature.com/ncomms/journal/v2/n8/abs/ncomms1416.html))

~~~
derptacious
Great analysis. Is there even a theory of what causes randomness? Where does
it come from? Why should we believe it is discrete from "unpredictability?"
Even in random number generators, the game is about pulling from widely
unpredictable sources to generate entropy - the word "random" should maybe be
a misnomer. I've never believed in anything besides unpredictability in
various scopes of systems. By scopes of systems, I mean, in some contexts of
analysis it makes sense to deem a system temporarily closed to analyze certain
parts of it. For example, the earth is not a closed system, but for some
discussions and analysis it makes sense to simply treat it like one.

I don't know why it is so hard for this description - or paradigm - to
proliferate to the masses and various pop writers. Writers so often are tying
human consciousness to QM experimentation as if it were something special. The
fact of the matter is: in each QM experiment the only things really
interacting with the experiment are the atoms of the measurement apparatuses,
sensors, and whatnot. In the case of the double slit experiment, we could have
them "interpreted" automatically - and say, kill a cat if an interference
pattern is created and not kill it if one is not made. Making the discussion
about consciousness is a distraction from the core issues.

------
ChuckMcM
The exploration of what the wavefunction "means" in terms of the world around
us has been one of the more interesting things to watch. When I took quantum
physics in school (under the generic heading "Modern Physics") psi and quantum
mechanics was very much simply a mathematical treatment of things rather,
unlike thermodynamics which was actually visible and "real". Between the
teleporation work and recent wavefunction work it seems like a lot more layers
of the universe are being revealed.

------
rjdagost
One question I have: if a pilot wave description of quantum mechanics was in
fact the "right" one, would quantum computers be impossible? It's confusing
because many articles claim that the various interpretations of QM yield the
same predictions, and yet the irreducible randomness of the Copenhagen
interpretation seems to be a prerequisite for quantum computing.

~~~
anoncoder
See my comment below. Anderson and Brady seem to think that quantum computing
is impossible. It reduces to analog computing.

------
opneg
Good article on the original PBR paper:
[http://mattleifer.info/2011/11/20/can-the-quantum-state-
be-i...](http://mattleifer.info/2011/11/20/can-the-quantum-state-be-
interpreted-statistically/)

------
snarfy
My own layman theory on this is that time has a wave like nature to it, not
the particles. The idea is to rewrite the wave equation solving for time using
the minkowski metric (space and time are related through relativity).

The results should be the same, but it is a different interpretation.

------
jivos
Someone more knowledgeable about solitons can chime in on this...

A friend of mine modeled soliton interactions. When one soliton passes through
another, information can be exchanged such that colliding solitons contain
bits of each other when they move apart. No matter how far apart these
solitons get, the soliton "children" still "chat" with their "parents". One
soliton can contain multiple elements of other solitons, all of them
interacting at a distance. Their behavior can mimic "spooky action at a
distance". Also, solitons can have wave like behavior or particle like
behavior depending on how they are observed.

------
return0
So we still have little progress in the interpretation problem, but the
planned experiments are going to shed light?

------
ciokan
One of those posts where I read the comments and I realise how dumb I really
am.

------
jitan
Practical quantum field experience -> Meditation, no cat needed.

------
anoncoder
See [http://arxiv.org/abs/1401.4356](http://arxiv.org/abs/1401.4356), a paper
by Ross Anderson and Robert Brady. They've since expanded the theory greatly,
with other papers on arxiv.

Also, check out this fascinating debate about these matters on Scott
Aaronson's blog. Link is
[http://www.scottaaronson.com/blog/?p=1255](http://www.scottaaronson.com/blog/?p=1255)

Brady and Anderson have many posts in this debate, and IMHO, wind up winning.

It's looking more and more like physics took a wrong turn with the Copenhagen
interpretation of quantum mechanics.

~~~
jeffsco
I am biased as a Scott A. fan and as somebody who knows essentially nothing
about QM. However I see no way to conclude that Anderson and Brady even know
what they're talking about, much less that they win the debate on the linked
Scott Aaronson page. I just read through it again to confirm my recollection
(skipping the Motl and Sidles posts fwiw).

~~~
anoncoder
The main reason I felt Scott A. (and company) lost the debate is because he
could not point to a single experiment that actually uses qubits to do a
calculation above a very small threshold. There is much written about
experiments that seem to show entanglement of multi qubit systems, but none of
them actually do any calculations with the qubits.

Brady and Anderson's point is that it's not a qubit until you can calculate
with it. And their theory suggests it will get exponentially harder for each
qubit above 4 (I think), which throws out all of the interesting quantum
algorithms.

I also felt Brady and Anderson silenced every objection. After one or more of
their rebuttals, they were no longer challenged. The most recent post about
Feynman is also interesting.

In any event, it is a fascinating blog post.

