Re: [Phys-L] quantum entanglement

[John Denker <jsd@av8n.com>]

On 3/21/19 12:32 PM, Antti Savinainen via Phys-l wrote:

> This might interest some of you:
> https://www.technologyreview.com/s/613092/a-quantum-experiment-suggests-theres-no-such-thing-as-objective-reality/
>
> There is a link to the original manuscript at the end of the MIT Review
> piece.

Yeah, there have been a flurry of papers recently about
the quantum measurement process, entanglement, and various
so-called "paradoxes".  Here's a theory paper in the same
vein:
  https://www.nature.com/articles/s41467-018-05739-8

IMHO the hype is unwarranted.  Note that the MIT Technology
Review carries a lot of clickbait articles, and has done
so for decades.  Certain people send me clippings and get
offended when I don't accept them as the Word of God.

As for entanglement in particular, the recent results don't
tell us much we didn't already know.  In particular, I am
not ready to give up on "objective reality".  AFAICT quantum
mechanics is not wrong or even incomplete.  Many parts of it
are somewhat peculiar.  The parts that involve entanglement,
EPR, Bell inequalities etc. are verrrrry peculiar.  They are
inconsistent with naïve classical intuition, but we already
knew that.

Entanglement a very recondite bit of physics.  AFAICT it has
no practical applications.  Even things that are intensely
quantum-mechanical, including lasers, semiconductors, atoms,
etc., can be understood just fine without worrying about
entanglement or spooky correlations at a distance.

That being said, if you would still like some background on
these topics, I recommend the lecture by David Mermin, who
(a) knows what he's talking about, and (b) has a gift for
explaining things to a general audience.  I suggest skipping
the part where the organizer spends 9+ minutes introducing
the speaker.  (IMHO as a general rule that sort of biographical
background might sometimes serve as a way of enticing people
to come to the lecture, but for people who are already in the
room such introductions are just a waste of time.)  Mermin's
lecture begins here:
  https://youtu.be/ta09WXiUqcQ?t=555

If you're in a hurry you can skip his prefatory discussion
of the history and the famous physics personalities involved,
and start with the actual physics:
  https://youtu.be/ta09WXiUqcQ?t=1342

Note:  When Mermin talks about "1 color" and "2 color" he
means something like "first color" and "second color" or
perhaps "top color" and "bottom color".  (He does not mean
that there are 1-color particles each having a single color
as distinct from 2-color particles each having a pair of
colors.)

=======================

I would add the following:

1) A key question that is often not addressed head-on has
to do with the relationship between quantum mechanics and
classical physics.  I say that there is only one set of
rules:  quantum mechanics describes the universe we live
in.  QM predicts that the classical description is a good
approximation in certain limits.  You can say that QM
contains, predicts, and explains classical physics ...
whereas classical physics tells us nothing about QM.

2) There are no rules for quantum "measurement" apart from
the usual QM equations of motion.  I mention this because
a lot of people get into trouble by making ill-founded
assumptions about measuring devices.  Such devices are
complicated, but that doesn't make them exempt from the
rules.

3) Following Mermin's lecture there is a question about the
role of an anthropomorphic "observer", sometimes referred to
as "Wigner's friend".  As a general rule, anything involving
anthropomorphic observers is misleading and worse than useless.
This applies to "observers" in special relativity as well as
in quantum mechanics.

Instead, the relevant QM concept is /irreversibility/.  What
we think of as a measurement, especially in the limit where
it looks like a classical measurement, must be irreversible.
This in turn requires dissipation;  not necessarily during
the measurement but at least in preparation.
 -- For example, a ballistic pendulum must be prepared by
  using friction to zero out any previous motion;  otherwise
  the outcome would be determined as much by history as by
  the thing you are trying to measure.
 -- Similarly, but somewhat less obviously, a voltmeter
  absolutely must contain a resistor, and the properties of
  the resistor are essential to understanding how the QM
  laws (including the uncertainty principle) are enforced.

We can use this to understand the role played by Schrödinger's
cat and by Wigner's friend:  they serve as the essential
/dissipative/ element.  They serve this purpose rather poorly,
because they are too complicated to be understood in QM terms.
Instead, I recommend using something much simpler, such as a
50 Ω resistor, which can (with some effort) be understood down
to the last detail, quantum mechanically.

4) Let's talk about /spooky correlations at a distance/.

Suppose there are two particles, A and B, in an entangled state.
Then the simplest way to describe Mermin's Gedankenexperiment
is to say that measurements on one particle change the state(*)
of the other particle.  That would be routine if the particles
were side by side, or were otherwise able to communicate.  On
the other hand, it is very peculiar when the two particles are
too far apart to permit communication on any relevant timescale,
i.e. when they are separated by a spacelike interval in spacetime.
That is to say, the situation is spooky because it violates some
of our classical intuitions about /relativistic causality/ and
about /locality/.

At first glance, such violations would seem to be very serious,
because spacelike intervals are not well-ordered with respect
to past and future.  There will be situations where (A=past,
B=future) in one reference frame while (B=past, A=future) in
another.  This looks like a mechanism for time travel.

On the third hand, we have to be very careful about this.  We
must not call it "action" at a distance, because you cannot
use measurements on one particle to "act" on the other particle
in any way.  You cannot "communicate" any useful information.
All you can do is arrange for a /correlation/ to exist.  An
outcome that you cannot control at one location will be correlated
with an outcome that you cannot control at the other location.
There is no mechanism for time travel.

One way to rationalize this involves a head-spinning relaxation
of our notions of locality and causality.  Perhaps there can be
correlations at a distance, even when there can be no action or
even useful communication at a distance.  Nobody has suggested
a plausible mechanistic explanation for such a thing.  Perhaps a
mechanism exists, but it won't be simple;  the Bell inequalities
rule out a wide class of "hidden variable" mechanisms.

Given a choice between giving up on objective reality and relaxing
my notion of relativistic causality, I prefer the latter.

It must be emphasized that we don't need a mechanism.  Galileo
did not have a mechanistic explanation for "how" a free particle
kept moving with constant speed and direction (the first law of
motion), nor any mechanistic explanation for "how" a particle in
a gravitational field exhibited a constant rate-of-change of the
vertical component of its velocity.  He emphasized that physics
needs to say /what happens/.  It might or might not explain how
it happens.  The fundamental laws almost never say why it happens.
Galileo divorced physics from metaphysics and philosophy.  This
is considered Day One of modern science.  Newton, having gone to
school on Galileo, when asked for a mechanism replied "hypotheses
non fingo".

Some people try to make QM seem less peculiar by introducing
"interpretations", such as the many-worlds interpretation.
IMHO this is a waste of time.  My rule is:
	*That which interprets least interprets best.*
That is to say, the equations are right, and do not require
interpretation or mechanistic explanation.  If the equations
make a prediction that you find peculiar, that's a problem
with your intuition;  it's not a problem with the equations.

Along the same lines, arguments based directly on conservation
laws are valid /independent of mechanism/.  We don't need to
know "how" or "why" the conservation laws are enforced.  In
contrast, there are some corners of physics where we do have
a microscopic mechanistic explanation, which is nice ... but
it's not a necessity.  Often the mechanistic description is
imperfect, e.g. electromagnetic field lines, which have some
value, even though they are sometimes misleading and must be
used with care.  Similarly, treating the t dimension of special
relativity on the same footing as x, y, and z is profoundly
interesting and useful, even though the timelike dimension is
not exactly the same as the others.  It's more the same than
it is different, and this leads to valuable geometrical and
trigonometric insights.

Footnote:
(*) Classically we imagine that A has a state of its own, perhaps
as a function of 3 variables, and B has a state of its own,
as a function of another 3 variables.  However, QM denies this.
It insists there is only one wavefunction, describing A and B
jointly, as a single function of 6 variables.

Classical notions of locality and causality suggest that separation
of variables "should" be possible, but QM says that's not possible
in general;  it's only possible in the classical limit.  Cases where
it's not possible are called /entangled/ states, or simply /cat/
states, in honor of Schrödinger's cat.