
The Trouble with Quantum Mechanics (2017) [pdf] - alokrai
http://quantum.phys.unm.edu/466-17/QuantumMechanicsWeinberg.pdf
======
l33tman
The trouble is physicists who work in foundational QM who still utters phrases
like "But the vista of all these parallel histories is deeply unsettling, and
like many other physicists I would prefer a single history." and then they get
dragged down into complicated alternative frameworks instead of the simple one
they don't like. Einstein wasn't and will not be the only one to go down that
path.

Over and over again it has been shown you need to calculate all the possible
histories to arrive at the probability of a specific history to be realized.
And if you need to calculate those histories to match the universe, chances
are high the universe as well has to calculate them.

~~~
perl4ever
On the other hand, certain people like to say that quantum computing is not
about simply computing every possible result of something in parallel
universes, that you can't do _that_ much better than with classical computing.
If that's so, doesn't it imply in some vague way that any parallel dimension
is something less than completely real?

~~~
jacb
Perhaps it's consistent with computation being done in many "parallel
universes," but with heavily restricted communication - in this model it's
almost trivial to parallelize work (flip a quantum coin, do A if heads else
B), what's tough is making sure that you don't get parallelized along with the
work (i.e. that you don't become entangled with the computation) and making
sure there's a way to accumulate the parallel computation into something
that's singly-readable and useful.

------
chmaynard
Source: The NY Review of Books

Posted on the website for UNM Physics 466, Fall 2017.

Course Text: "Physical Mathematics, 2nd Ed." by Kevin Cahill

Kevin Cahill was born in New York, New York. He attended public and Catholic
elementary schools, Regis High School, the University of Notre Dame, and
Harvard University, receiving his PhD in physics in 1967 under the supervision
of Roy Glauber (prix Nobel 2005). Cahill has done research at NBS, LBL, Ecole
Polytechnique, Saclay, Orsay, and Harvard and has taught at Nice, Wesleyan,
LSU, Indiana, Harvard, Fudan, and the University of New Mexico where he is a
professor of physics and astronomy.

------
daltonlp
Just gonna leave this here :)

[http://www.daltonlp.com/de/](http://www.daltonlp.com/de/)

------
jaygray0919
Am reading this article. But the act of reading is changing the rotation of
the words. Probable cause: imperfection in my reading glasses.

------
Kednicma
I'm reminded of the Zen koan where the students cannot tell whether the flag
is moving in the wind or the wind is moving around the flag. The master tells
them that the mind, neither the flag nor the wind, is what is moving.

Both sides of the interpretative coin are needed. We must simultaneously
understand that QM is epistemic and also that QM is contextual. The author
asks,

> How do probabilities get into QM?

Because knowledge of physical systems is always limited and QM is epistemic,
all quantities we manipulate in QM are probabilistic. Further, the Kochen-
Specker Theorem requires that some measurements be not fully determined by
past history, as a matter of living in at least three dimensions of space.
Finally, basic linear logic allows us to replicate most of QM's effects with
macrostates.

While Schrödinger hated this and used his famous cat thought-experiment to try
to refute it, children learn about linear logic and probabilities in
macrostates when they are given random toys or packs of trading cards, and
today we recognize that it really is possible to condition relatively large
differences in macrostate observations on single entangled or otherwise-
prepared microstates.

> In the [Everett] realist approach the history of the world is endlessly
> splitting;

Eh, kind of? But it's also endlessly joining. We wouldn't just be in a "cosmic
history" which is "sufficiently benign" for us, but also we're likely to be in
a likely history. In fact, we're _exactly_ as likely to be on our current path
as our current path is to be randomly picked from amongst the possible paths.
So it turns out that Everett's many-worlds analysis is a little tautological.

> But how can something so nonlocal represent reality?

Finally, a good deep question. There are two parts to the answer, and each are
deep enough to warrant a series of lectures. First, the Kochen-Specker
Theorem, combined with general relativity, leads to the Free Will Theorem:
When we measure particles, we are sending them a query, and they choose how to
respond to the query from among the possible replies. Second, reality isn't
one single fabric, but interlocking systems of geometry and topology. With
apologies to Wheeler, geometry tells reality how to interact, and topology
tells reality how to propagate information. This insight leads to the co-
hygiene principle [0].

The author sketches some possible directions for experimental work; it would
be interesting to read about turning them into proper experiments and seeing
the results.

[0] [https://fexpr.blogspot.com/2016/06/the-co-hygiene-
principle....](https://fexpr.blogspot.com/2016/06/the-co-hygiene-
principle.html)

~~~
meroes
As an non expert, aren't there ontological interpretations too? You say QM is
epistemic, but is that referencing the Copenhagen Interpretation as commonly
taught?

I know probability and unpredictablity don't go away, but MWI is closer to an
ontologic theory no? I mean as far as science can claim to be ontological.

And similarly for nonlocality. Is it _only_ ontological? I feel like many
worlds would say one thing vs copenhagen. And how do non traversible string
theoretic wormholes reprsenting entanglement a la Susskind fit in if the
theory can only be about epistemic knowledge.

~~~
Kednicma
As a fellow non-expert, I use tables like [0] to help keep everything
straight.

Yes, the Copenhagen interpretation says that the wavefunctions we manipulate
are epistemic; they exist in our mind and on the blackboard but not
necessarily IRL. The epistemic toy theory [1] makes this concept very
concrete, although note that that theory has hidden local variables and thus
doesn't correctly implement Bell/KS/Free Will.

Yes, MWI is quite ontic; it's probably the most ontic that a QM interpretation
can be, since it insists that all possibilities exist. Just like modal
realism, MWI is the most extreme of the phrasings in that direction.

Non-locality sadly isn't negotiable anymore. Bell's inequalities have been
violated in laboratories well enough to my satisfaction [2], so theories that
can't handle it need to be fully rejected. I'm not sure what's left; maybe the
transactional interpretation?

[0]
[https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...](https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#Comparisons)

[1] [https://arxiv.org/abs/quant-ph/0401052](https://arxiv.org/abs/quant-
ph/0401052)

[2]
[https://en.wikipedia.org/wiki/Bell_test_experiments](https://en.wikipedia.org/wiki/Bell_test_experiments)

~~~
meroes
You say nonlocality isn't negotiable but I _think_ one of Bell's assumptions
isn't met in MW, namely single outcome of experiment. I think this is shown in
that table as the local dynamics column.

But then without nonlocality nor hidden variables, I truly am at a loss how
quantum computers even work according to MWI. But I know this area gets so
complex so fast.

