
The Trouble with Quantum Mechanics - themgt
http://www.nybooks.com/articles/2017/01/19/trouble-with-quantum-mechanics/
======
lisper
This article presents a very inaccurate view of the realist approach. The
universe does not "split" when you make a measurement.

The measurement problem is a solved problem. The solution is that measurement
and entanglement are the same physical phenomenon. Measurement is just
entanglement extended to a macroscopic system through a process called
"decoherence". The net result is that, when you do the math, you recover
classical behavior by taking "slices" of the wave function (the mathematical
operation is called a "trace"). This has been known for decades now, and
provides a coherent and easy-to-understand picture of what is "really" going
on. It is astonishing to me that the physics community still bifurcates into
two camps: those who think this is common knowledge, and those who are
completely unaware of it (or think it's a crazy idea).

References:

[http://www.flownet.com/ron/QM.pdf](http://www.flownet.com/ron/QM.pdf)

[https://www.youtube.com/watch?v=dEaecUuEqfc](https://www.youtube.com/watch?v=dEaecUuEqfc)
(The same content in a video)

[http://blog.rongarret.info/2014/09/are-parallel-universes-
re...](http://blog.rongarret.info/2014/09/are-parallel-universes-real.html)

[http://blog.rongarret.info/2014/10/parallel-universes-and-
ar...](http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-
time.html)

~~~
opaque
Decoherence is an extremely useful and elegant mechanism for understanding
quantum systems, but unfortunately it doesn't solve the measurement problem.
At least not to the satisfaction of most practising physicists. The
entanglement and trace operation does produce classical probabilities in the
observed sub-system. However, it requires you to make a pretty arbitrary
division between the observed system and the wider environment. Why is
everything not just entangled with everything else up and up the chain until
the entire universe is in superposition? The problem of where the quantum
world ends and the classical world begins is still unanswered. This is one of
the greatest open questions in physics, likely requiring a unification of
general relativity and quantum mechanics before it's resolved.

~~~
lisper
> Why is everything not just entangled with everything else up and up the
> chain until the entire universe is in superposition?

It is.

I'm going hazard a guess that your next question is going to be: why do I not
perceive myself to be in a superposition of states? And the answer is that you
are not what you think you are. You think you are a human being, a classical
physical object made of atoms, but you aren't. This is a very good
approximation to the truth, but it is not the truth. The truth is that you
(the thing engaged in this conversation) are a software process running on a
human brain. You are a _classical_ computing process, i.e. a process that can
be emulated by a classical computing model like a Turing machine. The reason
for this is that the kinds of things you do necessarily involves _copying
information_ (e.g. the process of reading this comment involves copying
information from your computer into your brain) and quantum information cannot
be copied. Only classical information can be copied. Your conscious awareness
of the existence of physical processes is an emergent phenomenon of
accumulating memories, i.e. copying information. Because of this you cannot
become consciously aware of your quantum nature, and because of _that_ you
cannot demonstrate the quantum nature of any system (to yourself) unless it is
isolated from you. You could in principle demonstrate the quantum nature of
the rest of the universe if you could somehow isolate yourself from it, but
that presents insurmountable practical difficulties.

> The problem of where the quantum world ends and the classical world begins
> is still unanswered.

Because the question tacitly makes the false assumption that there is a hard
boundary between the two. There isn't.

~~~
opaque
Clearly you have your own unorthodox interpretation of quantum mechanics and
you are quite emotionally attached to it. The argument that the universe is
really X, but your brain just perceives it as Y because of Z can be made for
any X,Y & Z. I don't find it compelling whatever the alleged reason for the
disparity (in this case the no cloning theorem). There's also whole host of
other issues, if everything is QM why has the brain evolved as a classical
information processor, QM is unitary so you need to explain
time/thermodynamics ...

Anyway, if you truly believe you have nailed both the measurement problem and
consciousness in one fell swoop. Then I suggest you seek some like minded
collaborators and a peer reviewed outlet for your ideas.

Returning to your original comment.

> It is astonishing to me that the physics community still bifurcates into two
> camps: those who think this is common knowledge, and those who are
> completely unaware of it (or think it's a crazy idea).

As a former member of the physics community I can tell you this is completely
false. Everyone is aware of decoherence, it's covered toward the end most of
undergrad courses in QM where the density matrix is introduced. I've never met
anyone who thinks it solves the measurement problem.

~~~
lisper
> Clearly you have your own unorthodox interpretation of quantum mechanics

None of these ideas are original with me. All of them can be found in the
literature. My only contribution (if I've made a contribution at all) is
pedagogical.

> and you are quite emotionally attached to it.

Yes, I am quite emotionally attached to the truth. And yes, it does annoy when
people promulgate the myth that QM is hard to understand or contains
intractable mysteries when I know it isn't true. It _particularly_ annoys me
when people say this and simultaneously ignore the incontestable fact that
entanglement and measurement are the same thing, in the same sense that space
and time are the same and matter and energy are the same. Yes, all of these
things are weird, and yet all of these things are true, and none of them are
intractable mysteries or even hard to understand.

> I suggest you seek some like minded collaborators and a peer reviewed outlet
> for your ideas.

Like I said, these aren't my ideas. But I did submit my paper to Physics Today
back when I first wrote it in 2001. It was rejected on the grounds that
everything in it was already common knowledge.

> As a former member of the physics community I can tell you this is
> completely false. Everyone is aware of decoherence

But obviously not everyone is aware of its implications. Your challenging me
on this is manifest evidence of that. And for 25 years I have been stumping
card-carrying physicists with the EPRG thought experiment. Heck, it took me
ten years to find _anyone_ in the physics community who knew the answer! (I
even had a chance to pose the question to Freeman Dyson, and _he_ didn't know
the answer!)

------
wolfgke
Relevant for those who are interested in the topic: Two rather unusual
interpretations if quantum mechanics:

\- De Brogle-Bohm theory:
[https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory](https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory)

\- Gerard 't Hooft - The Cellular Automaton Interpretation of Quantum
Mechanics: [https://arxiv.org/abs/1405.1548](https://arxiv.org/abs/1405.1548)
(see also
[https://en.wikipedia.org/wiki/Superdeterminism](https://en.wikipedia.org/wiki/Superdeterminism))

~~~
demonshalo
the Automaton interpretation sounds like something I would enjoy reading. But
considering that I do not have the time this week nor enough knowledge in the
subject, could you perhaps provide me with a quick summary as to what it is
about?

~~~
tps5
My understanding is this: Hooft is an "instrumentalist" as described in this
article. He does not believe that quantum mechanics describes "what's really
going on" and is interested in classical models that explain why QM makes such
good predictions.

"Superdeterminism" is a very interesting perspective that Hooft feels could
resolve this dilemma. Others feel that superdeterminism is not falsifiable, so
there's no point in discussing it.

I came across this interesting article on stackexchange from a few years ago:
[http://physics.stackexchange.com/questions/34217/why-do-
peop...](http://physics.stackexchange.com/questions/34217/why-do-people-
categorically-dismiss-some-simple-quantum-models)

The first response is from Peter Shor, presumably the discoverer of Shor's
algoithm. Kinda cool that such intelligent people with the highest academic
credentials use a public forum.

~~~
miltondts
Actually the kind of superdeterministic theories proposed by t'Hooft are
falsifiable. From [0] "If engineers ever succeed in making such quantum
computers, it seems to me that the CAT is falsified; no classical theory can
explain quantum mechanics." By "such quantum computers" he means computers
that can run Shor's algorithm. "...but factoring a number with millions of
digits into its prime factors will not be possible – unless fundamentally
improved classical algorithms turn out to exist."

[0] - [https://arxiv.org/abs/1405.1548](https://arxiv.org/abs/1405.1548)

------
MichailP
Let me give example why physics is hard, using most simple example.

Ever encountered term observer (like observer in inertial frame)?

Well, it turns out, observer in physics means whole laboratory with
established procedures of measuring time and length, not observer as you would
mean in everyday life. And established procedure, what is that? Say, how do
you measure time when certain event happened? It turns out, you can measure
time only locally, and for this you need to have synchronized clocks spread
out in your lab in points of interest. Some more philosophically inclined
physicist would say that time means synchronized clocks. And how do you
synchronize clocks? Well, it has to do with speed of light but I will stop
here.

And this is for the most simple term. Imagine something more complicated. Now
imagine hundreds of people writing papers and books with only partial
understanding, and signal to noise ratio :-)

End of rant :-)

------
runn1ng
I know that Eliezer Yudkowsky is _very_ controversial and some of his
statements cannot be taken seriously, but I really liked his sequence of
articles on QM. It made it a little bit more accessible to me, who knows very
little about it.

I cannot judge how accurate it is; but from googling around, it doesn't seem
he does any strong errors. Also he makes some very strong statements in the
end which I found preposterous (mainly, that Bayes reasoning is better than
scientific reasoning, and that Bayes reasoning must lead to Many Worlds
theory).

The sequence is here

[http://lesswrong.com/lw/r5/the_quantum_physics_sequence/](http://lesswrong.com/lw/r5/the_quantum_physics_sequence/)

~~~
danharaj
One of the issues with engaging with Yudkowsky's posts is how verbose his
arguments are. Fortunately, for something as well formalized as quantum
mechanics, there's a succinct counter-example: A formalism of quantum
mechanics that makes perfect sense with neither the idea of wavefunction
collapse _nor_ some abundance of ontologically dubious and extravagant
entities (alternate universes you can never observe). That formalism is
relational quantum mechanics.

Accessible overview: [https://plato.stanford.edu/entries/qm-
relational/](https://plato.stanford.edu/entries/qm-relational/) In particular,
this overview addresses the fundamental difference between Everett's theory
and Rovelli's theory.

Original paper: [https://arxiv.org/abs/quant-
ph/9609002](https://arxiv.org/abs/quant-ph/9609002)

A fluffy paper by the same author about QFT and its relational nature:
[https://arxiv.org/abs/hep-th/9910131](https://arxiv.org/abs/hep-th/9910131)

In short: The way you can evade the difficulties MWI addresses completely by
changing one premise of the problem: that physical systems have states
independent of their observers. In relational quantum mechanics, there is no
universal wavefunction because there is no external observer of the universe.
Call it the zero worlds interpretation. How parsimonious!

There is an element of Yudkowsky's sequence that deals with relative
configuration spaces, but this is a subtly different concept and is really
just a discussion from a clever guy unequipped with the right concepts about
the difference between, say, affine spaces and vector spaces or torsors and
groups. The idea that there is a configuration space independent of the
observer is the assumption he misses.

In general I find Yudkowsky's extreme certainty in his own arguments to range
from amusing to obnoxious. It's not hard to find places where uncertainty
creeps in. It doesn't come from mistakes in his reasoning because what he
considers he is usually meticulous about considering, but from what he doesn't
consider.

------
thisrod
This is a really good description of the measurement problem. It takes all the
sides of the debate seriously, which is hard for most physicists to do. If you
think about quantum mechanics all day every day, you're almost bound to settle
on one interpretation as your working model, and it becomes hard to take the
others seriously.

------
gpsx
Weinberg mentions the quote by Eugene Wigner regarding the importance of
consciousness in the section on instrumentalists. However, I see it is being
fundamental to realist's interpretation also. I subscribe the the realist
approach, which I view as meaning nothing special happens when a human makes
an observation - it is just quantum mechanics as usual. This has interesting
implications. The key is that the observer is not outside the system, he is a
part of it. His wave function becomes entangled with the system when the
measurement is made. But what makes this difficult to analyze is that we don't
know what it means for the observer to say "I see the result was *". What
physical process allows the observer to be aware of the measurement?

We can make a simplifying and unsatisfying assumption - the observer has a
register in his head which records the result of the measurement. Here, it is
easy to imagine this register is part of the wave function just like the
object being measured. As mentioned above, the wave function for this register
is entangled with the object being measured so the measurement and register
value are correlated. In our model this corresponds to the person
independently thinking each of the possible results, which some people call
multiple universes.

But this simple register is not what goes on inside the brain. Not knowing
this makes it difficult to accurately model the measurement process.

EDIT: for clarity

~~~
evanb
In the realist interpretation, even without people, you wind up in entangled
states after measurement of a spin (where by "measurement" I mean: allow a
giant classical system to interact with a spin and tilt a pointer on some
gauge to / wind up with some register holding + or -). It's perfectly possible
to formulate the realist ideas without any consciousness or insistence that
the measurements register in a brain or observer.

~~~
gpsx
Yes, I agree you can formulate the realist ideas, because the observer is
nothing special at all. And I also think this is how the world works. But from
an experimental verification point of view I think you really need a model for
what the brain does to say you understand how a measurement is made. What is a
theory without verification?

Granted his quote used the word formulate, but I interpret the comment as
referring to not just writing some rules but also having some justification
for them.

------
EGreg
What is wrong with the Pilot Wave Theory intepretation? I'm ok with FTL
information transfer, and so Bell's inequalities are satisfied.

And anyway the other interpretations DO NOT rule out FTL information transfer.
For example if one entangled electron flies into a black hole then we would be
able to know its spin by measuring the other one even if light from it won't
reach us.

Also there is this:

[https://en.m.wikipedia.org/wiki/Wigner's_friend](https://en.m.wikipedia.org/wiki/Wigner's_friend)

~~~
baytrailcat
>I'm ok with FTL information transfer

Which will lead to the conclusion that Special Relativity (which was
extensively validated) is either wrong or incomplete.

I have seen a revival of Pilot-wave theory here on HN, but the real conclusion
(a real particle pushed around by the enigmatic pilot wave or quantum
potential or whatever) is as bizarre or as satisfying as saying particle is in
a superposition. Also, I haven't seen anyone satisfyingly explain, in a
classical-deterministic sense, the Delayed Choice Quantum Eraser Experiment
(also, see the the Wheeler Thought Experiment)

~~~
goatlover
There is the silicone oil droplet phenomena which is a classical version of
the pilot wave, and it does reproduce some quantum behavior, including the
double slit. That a pilot wave has been discovered (granted at macro scale)
does make one reconsider.

[https://youtu.be/WIyTZDHuarQ](https://youtu.be/WIyTZDHuarQ)

~~~
krastanov
It does not, really. The macroscopic realisation is not particularly
surprising (although it is quite awesome and original). If you put a ball
floating on top of a wave you will observe the predictions from a mathematical
model of that system, which is exactly what the pilot wave theory is, and
there is nothing surprising here. Moreover, the macroscopic model simulates
something which by definition is an unobservable construction in the quantum
model. It does not simulate any inherently quantum behavior (classical waves
is a thing we already knew exists).

~~~
lisivka
Nobody tells that walkers are simulate all quantum behavior and doing that
correctly. However, they helps to understand some of quantum puzzles. For
example, droplets have spin. Can you predict behavior of the classical droplet
spin in compare to the puzzling quantum spin?

~~~
krastanov
But you can do the same with classical setups that mimic some effects from the
typical quantum mechanical formulations. Those classical experiments are
indeed amusing and interesting, but they do not illuminate the "quantum
puzzles", no matter whether they are modeled after pilot wave theory or after
quantum mechanics. And very importantly, those amusing demonstrations do not
scale! Sure, you can mimic with classical contraptions the pilot wave (or the
wave function) of a single particle, but the nice intuitive demonstrations
fail when you try to scale it up to more particles (or anything that would be
exhibiting the interesting, nontrivial quantum behavior).

~~~
lisivka
So, your prediction for walker droplet spin is that walkers, in kind of Stern
and Gerlach experiment, will behave like classical magnets, not like quantum
particles, right?

~~~
krastanov
No, they would behave like a ball floating on top of a wave and given that
there are waves involved there will also be interference patterns. There is
nothing quantum here. Sure, in one particular way it looks like a quantum
particle (to the extent of a cargo cult), but in all the important ways it
does not (entanglement, computational power, generalisation to multiple
particles).

~~~
lisivka
I asking about outcome of Stern and Gerlach experiment. It's binary thing.
Make public prediction, please.

~~~
krastanov
Certainly, but then please first pose/define the question clearly. What do you
call a macroscopic Stern-Gerlach experiment with balls floating on top of the
waves of a fluid? It would be an amusing toy problem to work out if you define
it for me. However, it does not change the main argument: there is nothing
quantum in macroscopic experiments with water waves - interference does not
imply "quantumness".

~~~
lisivka
Lets define experiment as following (excuse me my bad English, please):

\- vibrating bath with non-conductive, non-magnetic, non-paramagnetic, non-
diamagnetic fluid;

\- vibrating bath is wide enough to avoid excessive interference with
reflected waves from bath sides;

\- vibrating bath has regular pattern on top of fluid, without any
irregularities in space of experiment;

\- small _charged_ droplets of fluid on top of bath;

\- north and south poles of a magnet are placed horizontally, without touching
of bath fluid or droplets, e.g. at sides of bath, OR over fluid, OR under
bath;

\- an apparatus creates droplets of same size with _random spin in all 3
dimensions_ ;

\- droplets are forced to walk through the batch, starting at center line
between north and south pole and following that line;

\- without magnetic field applied, droplets must walk straight;

\- an detectors to measure decline of droplet path from center line must be
installed at end of magnetic pole.

I expect that, when magnetic field is applied, droplets will slide completely
to left or completely to the right, like electrons in Stern-Gerlach
experiment.

It's not a quantum experiment, of course, but it can provide insight on nature
of quantum spin.

PS.

Sorry, droplets must be charged, not magnetic. Updated.

~~~
krastanov
The main point of the Stern-Gerlach experiment was that the electrons hitting
the screen were forming two distinct dots instead of a long spread out line,
therefore proving that the angular momentum is quantised. In your example you
will instead have simply a spread-out distribution because there is no
quantisation of the angular momentum of your droplets.

The pilot wave usually refers to the spatial degrees of freedom, especially in
these classical mock-ups with balls on top of waves. They do not properly
addressed internal degrees of freedom like spin.

Unrelated to those macroscopic mock-ups, pilot wave theory actually has
serious problems with the description of anything that is not a spatial degree
of freedom.

You can still use pilot wave theory to describe the quantum behavior of the
coordinates of a particle. But even then, the classical mock-ups we are
discussing will not show anything inherently quantum - it will simply produce
some interference patterns, that can be explained classically.

P.S. side note: An important part in the Stern-Gerlach experiment was that the
magnetic field was not homogeneous, because it is the gradient of the field,
not the field itself that causes the electrons to move.

~~~
lisivka
IMHO, Stern-Gerlach experiment demonstrates interaction of magnet field with
guiding wave mediated via particle, so I expect that magnet will steer
particle-wave into same spots, thus will demonstrate «quantum» behavior of
particle spin at macro level.

~~~
krastanov
Without disrespect I insist that you are wrong about that. The spin degree of
freedom is "internal", unrelated to the position of the particle. The pilot
wave does not influence that spin, and if you have a big ball on top of a
wave, that wave does not care about the angular momentum of the ball. The ball
is big and classical, hence its angular momentum is (practically) not
quantized.

The thread got a bit long, but if you are really interested in learning about
this I would be happy to continue the discussion through email
(stefan.krastanov@yale.edu). You probably also need proof of some kind of
qualification on my part - my online profile does prove that I work at a
respected institute doing research on that topic.

------
brianberns
It seems that the scientific method has led us to a theory that is beautiful,
accurate, and yet utterly incomprehensible. Until we can explain exactly what
happens during a wave function collapse, I think QM will remain that way.

~~~
akvadrako
It doesn't seem that way to me. Decoherence is understood pretty well;
although not every detail is completely worked out it explains our experience
and the subjective appearance of collapse. It's just approximate though and
hence we get quantum interference.

The reason you think it's incomprehensible is that a lot of smart people have
trouble accepting what we called reality is only one of many classical
"worlds". It's a philosophical concern; technically there are no major issues
and it's not that hard to understand.

The reality of QM won't be commonly accepted until we find a way for people to
accept that the universe isn't what they expected or we change their
expectations.

------
coldcode
I had three semesters of QM in grad school and I still have trouble with it.

~~~
jessriedel
Well, the OP author is arguably the greatest living physicist, so it's not
surprising that 3 semesters isn't enough :)

------
nickpeterson
They overcharge you if your license is from two simultaneous states.

------
jayajay
> In this musical analogy, the act of measuring the spin somehow shifts all
> the intensity of the chord to one of the notes, which we then hear on its
> own.

People can accept that measurements and observables are operators which act on
the wave function (which can be expressed as a linear combination of
eigenstates of the operator in question). People can even accept that some
observables cannot be measured simultaneously, because they do not commute,
and cannot share eigenstates.

People cannot accept that particles can _only be found in eigenstates_ of the
observable in question, without some mechanism to explain why this happens. If
a particle's measured value can only be one of the eigenvalues of the
operator, it begs the question "which eigenvalue is it going to show us?".
Alternatively, if nature let us measure the _expectation value_ instead of
"one of the eigenvalues", then quantum mechanics would not be "weird" at all.
Too bad, nature isn't like that, or maybe, be thankful that it is.

> But if the corrections to quantum mechanics represented by the new terms in
> the Lindblad equations (expressed as energies) were as large as one part in
> a hundred million billion of the energy difference of the atomic states used
> in the clock, this precision would have been quite lost. The new terms must
> therefore be even smaller than this.

I always see people (physicists) complaining about String Theory, etc. because
they play around in regimes which are too small for us to actually work with.
We have observed macroscopic effects which are due to a build-up of quantum
mechanical effects (superconductivity, solid state, etc.). It would be
expected that a correction to quantum mechanics would also make macroscopic
predictions...

