
Review of Jean Bricmont's book "Making Sense of Quantum Mechanics" - mathgenius
https://arxiv.org/abs/1709.05442
======
lisper
Almost twenty years ago now I had an "aha" moment about QM courtesy of John
Preskill
([http://www.theory.caltech.edu/people/preskill/](http://www.theory.caltech.edu/people/preskill/))
that made it all come together for me: entanglement and measurement are the
same physical phenomenon. Once you understand that, everything else makes
perfect sense, and QM is no more difficult to understand than relativity. See:

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

for the writeup I did at the time. There's also a Google tech talk:

[https://www.youtube.com/watch?v=dEaecUuEqfc](https://www.youtube.com/watch?v=dEaecUuEqfc)

It's dismaying to me that even 20 years later QM is still presented as a deep
mystery. It's not.

~~~
majewsky
> I had an "aha" moment [...] that made it all come together for me:
> entanglement and measurement are the same physical phenomenon.

Wait... that's not common knowledge amongst physicists?!? I arrived at that
same conclusion in my sixth semester of physics (and third semester of quantum
mechanics lectures), when the notion of weak measurements was introduced.

> QM is no more difficult to understand than relativity.

This is a nice quote when taken out of context, because it makes it sound like
relativity is considered simple. :)

~~~
lisper
> Wait... that's not common knowledge amongst physicists?!?

Not only is it not common knowledge, there are still some who vehemently deny
it is true, insisting that the Copenhagen interpretation or Bohmiam mechanics
are still tenable, and that the "measurement problem" is still a problem. To
this day I regularly fool even card-carrying physicists with the EPRG puzzle.

Yes, it's shocking.

> it sound like relativity is considered simple

Conceptually, it is simple, but even relativity is often presented
incorrectly. It is not that the speed of light is the universal speed _limit_
, it is that it is the universal _reference_. Everything is always moving at
the speed of light through space-time. The only thing that changes is what
component of that velocity is moving through space and what component is
moving through time.

------
jostylr
As a proponent of de Broglie-Bohm theory, or pilot wave theory, (particles
have definite positions and are being guided by the wave function of QM), I am
pleased to hear of the book and enjoyed the review.

The reviewer was actually on my thesis committee and he always had an air of
excitement with regards to this material as well as clarity with regards to
both the philosophy and mathematics.

As someone familiar with the material, I can hear within the book the very
common themes that proponents of dBB often have to say. Without having read
it, I can't comment on the book's clarity of presentation, but it certainly
sounds like all the elements are there and more perspectives are always
welcome.

The issue has always been in getting people to try to accept that both
standard QM and several of its alternatives really do not have an adequate
description of reality in them and that this is a problem. I imagine the
chapter that gives an in-depth discussion about what one wants from science is
the most important novel contribution from the book to the field.

If you are curious about the strangeness of QM, then you should read the
review and probably the book.

Just a note, as I have written elsewhere, that dBB is a theory that has been
proven to have existence and uniqueness of solutions, handles identical
particles in a very natural way, handles spin simply, generalizes to the Dirac
equation, generalizes to QFT settings, has numerous proposals on integrating
with relativity, and, quite recently, has led to some promising directions in
resolving the issue of divergences associated with renormalization.

Oh, and cats are always alive until they are not. There is no both alive and
dead situation.

~~~
akvadrako
Bohm's interpretation is definitely one of the more reasonable ones and I
think it's a fine picture to have in your head. But it suffers from two major
flaws.

1\. There is no need for the particle

It's superfluous structure, which has no effect on the dynamical evolution or
predictions. Only the pilot wave is needed, so that's a straightforward
violation of Occam's razor.

2\. It has FTL influences

They may not be exploitable to us, but the primitive ontology of the theory
requires it. This is seriously against the spirit of special relativity.

In my view, Bohm's theory is only suitable as a visual aid or for those who
are in denial about the consequences of pure unitary dynamics.

~~~
alien_at_work
>so that's a straightforward violation of Occam's razor.

I'm probably missing something but I don't get this objection entirely. I've
long considered Occam's razor just a thing you can say. It doesn't have any
predictive power as far as I can tell. The complex answer is just as likely to
be correct as the simple one [1].

[1]
[http://neuralnetworksanddeeplearning.com/chap3.html](http://neuralnetworksanddeeplearning.com/chap3.html)
Search for "Occam's razor"

~~~
akvadrako
It's not about complex vs simple, but about adding unnecessary assumptions. At
least, that's the strict form of Occam's razor which I'm using here.

If I have two theories:

1\. which says that the Earth orbits the Sun and predicts when seasons and
sunrises occur

2\. the same as 1, plus undetectable pink elephants are floating through space

Then Occam's razor says we should discard the second, because it just adds an
assumption without adding predictive power.

Or more plainly, if my axioms are _a = 2b_ and _a = b^2_ then adding the axiom
_sqrt(a) = b_ creates a strictly inferior theory.

~~~
jostylr
Using just the wave function alone is similar to saying that in specifying the
computation sin(x) where x = 3, you do not need to say x=3 because the value
of sin(3) is part of sin(x) so why do we need to talk about sin(3) by itself?

Of course, we talk about 3 because we have some need of it. In dBB, we have
need of particles because they explain our experience in a fashion that is
readily apparent. At a first approximation, stuff moving around in 3-space is
exactly what we experience. The question then becomes figuring out the motions
of that stuff. If the stuff in the theory moves in the right position in our
experience, then we can keep that theory as a plausible candidate for
predictions.

The particles drive the need for the wave function. In many ways, the wave
function is acting in a fashion like the law of gravity in Newtonian physics.
Fix the law, start with different initial conditions, and we get a different
history. One of those histories might just conform to our experience and we
would then be satisfied.

Without the particles, we have a complex-valued function on configuration
space. Why is there 3-space in our experience? Why do we have a feeling of
some definiteness in the outcome of experiments? The wave function provides
none of that and these are essential ingredients to what we would like to get
from the theory.

~~~
akvadrako
I like your point about needing to localise yourself in the wavefunction to
make sense of your experience. That's true and how to do so is less obvious in
pure unitary dynamics. I think the essential issue is that Bohm's theory makes
a distinction between "actual" things and "real" things. But if you drop that
philosophical distinction then it's clear that the wave-function and _all_
points on it _must_ exist, because otherwise they couldn't influence you.

Those points near you will flow through the universe in a similar way and have
the same interactions with nearby points. So yes, the point particle is a
useful visual aid, but it doesn't work if the other points on the wave don't
exist too.

Your other arguments about 3D space and definite outcomes are weaker. 3D space
emerges from the structure of entanglement and the "feeling" of definite
outcomes is caused by decoherence. In pure wave mechanics the spacetime
locality of interactions is also apparent, while in dBB they are necessarily
non-local. Since spacetime is all about locality, the picture in pure wave
mechanics is superior.

~~~
jostylr
There are two distinct paths to a rebuttal here.

1\. Let's assume the unitary dynamics is completely sufficient for
constructing a theory. In such a theory the outcomes that our life tells us
happens does not happen in the same sense that we have. That is, our
experience is a massive illusion. In contrast, dBB gives us a story that fits
exactly with our experience. This strikes me as sufficient reason to side with
dBB, but certainly it is a personal choice.

As analogy, consider the Library of Babel, perhaps thinned down to only
consistent histories. dBB is the statement that there is one true history in
the library and is a procedure to find it. The unitary theory is saying that
they are all true and you can find the one that speaks to your experience and
read it, but it is not the only action.

Your "actual" vs. "real" statement is equivalent to saying that dBB should
drop the notion that the other books are not true. Just because they are in
the library does not mean that they represent reality.

Your razor argument was aimed at the mathematical extra bit that dBB gives. I
apply the razor to the set of stuff that the theory says happens. The unitary
version says it is all true; infinitely many separate worlds which may
"influence" us, but are not directly observable or able to be experienced. dBB
says there is only one world and this is how it evolves.

Also, "the other points on the wavefunction" strike me as a wrong way to state
it. The wavefunction is a function of the configurations. Those points are
what they are. You can use the values of the wavefunction to decide the
relevance of those points to existence, but it is part of the definition of
the theory as to what is considered real or not. In dBB, there is one real
configuration that changes in time being guided by a wavefunction that exists
in some sense, but whose existence is only deduced from the motion of the
particles. Saying more about it being really real beyond that is questionable;
one simply says this is what the objects in the theory are and what they do
with a clear mapping of what one needs to check in order to see if it
corresponds to our experiments/experiences.

2\. There is still the question of whether or not one can get a full theory
that works without adding anything else. It can be done with just a little
addition, say, adding a mass density formed by suitably integrating out
positions. This makes it well-defined and one can do an analysis. It also
gives 3-space a role.

This somewhat undercuts the razor since it is an extra thing though perhaps
less of an effort than the trajectory equations.

To not have something additional is not something I have ever seen done in a
convincing way.

a) The assertion is that a 3-space structure with particles moving around in
that structure arises as an illusion even though nothing of the kind is
actually happening in the theory. It should be appreciated that this is an
extraordinary claim and one has to be super careful in doing an analysis to
show that it happens.

The contrast with dBB is immediate. There are particles moving in 3-space.
That's the starting point. By analyzing those motions, we see that there is
some function guiding the particles and that function is a function on the set
of all possible configurations. There is a natural and easy progression to see
why our experience is the way it is. It is a very natural story.

In the unitary view, we are supposed to have this appearance of particles in
3-space in our experience, but that is not what actually exists in the theory.
The only object in the theory is the wavefunction.

One might be able to argue that if one sets up operators based on 3-space and
use that structure in generating the dynamics then we can read off the 3-space
and particles, but ideally, one would be able to extract the illusion of
3-space (it is an illusion if the unitary dynamics is all there is) given the
abstract Hilbert space and Hamiltonian. In some cases, such as free particles,
it is provable that the base space cannot be extracted out uniquely to N
particles in 3-space vs. one particle in 3N space.

Whether it can be done or not, it should be appreciated how straightforward
the dBB theory is in contrast to the unitary dynamics.

b) Probabilities. What is it a probability of? That's the first question. If
only the wavefunction exists and it is a unitary evolution, then there is no
randomness, there is no probability. And the role of the unitary part of the
unitary evolution becomes extremely obscure as well.

Moving on, pretending that the outcomes of experiments can be looked at in
this theory and the history looked at, then we would want histories that give
the right probabilities according to standard QM. But since all the outcomes
are considered to happen, then there are histories with, say, the cat always
ending up dead. One gets the wrong statistics in those "worlds". These may be
thinly represented in some sense, but it is not clear from a purely unitary
dynamics what that sense could mean. In terms of existence, they do exist and
thus this theory predicts that wrong statistics definitely occur though our
personal histories might not observe it.

c) Who are we? In a unitary dynamics viewpoint, what is an individual in this
story? In dBB, it is roughly a collection of particles moving and shaped in
roughly an individual way, more or less. It is not hard to see us. But in
unitary dynamics, with constant spreading, more and more of us exist and
spread out. So to even talk about my experience becomes rather outside of the
theory.

d) Imagine a wavefunction that is non-zero everywhere on configuration space.
Does that mean that everything, including the future and past of our own
personal experience, is happening right now? It is not only saying that all of
the histories in the Library of Babel are true, but every single page is
happening simultaneously. This includes memories of eggs reassembling and all
sorts of other strangeness. That is to say, predictive power seems to
disappear entirely.

The free dynamics tend to spread out the wavefunction and thus either that
spread out nature needs to be accepted and understood or one has to argue that
the interactions are keeping it tightly confined.

\---

A good starting point with a theory is to say what one cares about in the
theory getting right. For dBB, this is the position of the particles.
Everything else is needed to understand and account for the observed motions.

In unitary wave dynamics, all we have is a single wavefunction that evolves.
Why do we care about this; how do we model our experience with the
wavefunction? That's the question that needs to be clearly and unambiguously
answered.

\--- Locality. The wavefunction is a non-local kind of object. That is just
what it is. That is where the non-locality of dBB comes from. Indeed, taking
the wavefunction seriously is largely what many object to in dBB. Bell proved
that nature has some kind of nonlocality assuming that experiments have
results when we think they happen. That's the choice. Either deny experimental
results or accept non-locality.

------
mathgenius
> Denials of an objective reality are, in the opinion of the author, and also
> of the reviewer, aberrations of human thought, plain and simple.

I found this sentence rather disappointing, although I can sympathize. It
really does seem like there is an objective reality out there, and humanity
has certainly benefited greatly from our collective power over the natural
world.

However, if this "reality" is mathematical in nature, and mathematics is
incomplete, then I don't see what is the big deal with giving up the idea of
an objective reality. The founders of QM seemed to be much more open to these
ideas, eg. Heisenberg and Bohr.

~~~
jamesrcole
Why do you think incompleteness means non-objectivity?

One reason I ask is that, as I understand it, incompleteness concerns the
ability to prove certain characteristics rather than meaning a system
has/hasn't got those characteristics.

~~~
mathgenius
You can go on adding new axioms to your incomplete system, in an attempt to
complete it. This to me looks like measurement processes in quantum mechanics.
Mathematicians gave up on the idea of the "one true" mathematics, but it seems
physicists still hope for the "one true" reality.

Maybe this is a facile analogy. I'm still pondering if there are deeper things
to be said about this.

------
Synaesthesia
I actually found this Amazon review of the book to be particularly
enlightening
[https://www.amazon.com/gp/aw/review/3319258877/R36M2YLHIMTBL...](https://www.amazon.com/gp/aw/review/3319258877/R36M2YLHIMTBL1/ref=cm_cr_dp_mb_rvw_2?ie=UTF8&cursor=2)

------
fermigier
Why did the author publish with Springer, at such a high price, and not some
more mainstream, and cheaper, publisher ?

Also, it seems Bricmont has an upcoming title, still with Springer, but at a
more reasonable price, and probably more mainstream (judging only by the
cover):

[https://www.amazon.fr/Quantum-Sense-Nonsense-Jean-
Bricmont/d...](https://www.amazon.fr/Quantum-Sense-Nonsense-Jean-
Bricmont/dp/3319652702)

~~~
gumby
> Why did the author publish with Springer, at such a high price, and not some
> more mainstream, and cheaper, publisher ?

All the summary is is an answer to your question: 'This is an invited review
of Jean Bricmont's book "Making Sense of Quantum Mechanics."'

~~~
fermigier
By "the author" I meant Bricmont, sorry for the confusion.

~~~
gumby
It makes sense for an academic book to be published in an academic press. A
mainstream press probably wouldn't be interested; the academic press would
have the relationships with the academic bookshops which is more likely to
reach the likely audience.

------
fernly
[https://www.amazon.com/Making-Sense-Quantum-Mechanics-
Bricmo...](https://www.amazon.com/Making-Sense-Quantum-Mechanics-Bricmont-
ebook/dp/B01AJHYA7M)

$55 to buy, $30 to rent.

~~~
mathgenius
You can also find it on libgen, if you are so inclined.

~~~
wensheng
thank you

------
ivan_ah
Nice. The book seems like a good review of foundations. It's good to see the
topic discussed by someone expert, because otherwise QM education is left in
the hands of gurus and popular science who totally butcher the subject.

See this excerpt for my attempt to explain QM from first principles:
[https://minireference.com/static/excerpts/noBSguide2LA_previ...](https://minireference.com/static/excerpts/noBSguide2LA_preview.pdf#page=125)
(content warning: contains math equations)

------
Luc
Meanwhile, Lubos Motl sure doesn't like it!

[https://www.amazon.com/gp/customer-
reviews/R3NWAU0FI5RP3L/re...](https://www.amazon.com/gp/customer-
reviews/R3NWAU0FI5RP3L/ref=cm_cr_arp_d_rvw_ttl?ie=UTF8&ASIN=3319258877)

(the whole thread of comments under the review is quite good! Sort by oldest
first)

------
ivan_ah
related article on interpretations:
[https://www.newscientist.com/article/2097199-seven-ways-
to-s...](https://www.newscientist.com/article/2097199-seven-ways-to-skin-
schrodingers-cat/)

