
Not even wrong: Why does nobody like pilot-wave theory? [pdf] - fanf2
http://www.tcm.phy.cam.ac.uk/~mdt26/PWT/lectures/bohm7.pdf
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AgentME
My understanding of QM is only surface-level, so there may be subtleties I'm
missing, but this presentation's arguments against the many-worlds
interpretation seem pretty weak.

Page 28 grudgingly alludes to the idea that MWI could be simpler than other
theories ("Objection based on surprisingly common misconception that standard
QM defined solely by Schrödinger's equation ... It is only within a many-
worlds framework that this view could begin to make sense"), but later
introduces MWI as a contender beginning with a nonsense news headline and then
characterizes it as intuitively bizarre for proposing a large multiverse, as
if the size for its proposed multiverse was an obvious point against it. It
seems to me there's an implied misuse of Occam's razor: "Entities are not to
be multiplied without necessity" is most properly applied to the number of
rules in a theory, not the amount of matter a theory predicts. (If you apply
it based on the amount of matter a theory predicts, then Occam's razor surely
should rule out theories that dim clusters of light in the sky correspond to
trillions of galaxies like our own, and should prefer theories that predict
that they're illusions, reflections, or something otherwise insignificant that
fits the observations.)

Page 39 and 40 both quote paragraphs from books that seem more convincing to
me than the presentation's seemingly hand-waving refutations. Page 40's
refutation appears to do nothing but insist upon PWT's "epiphenomenal
'pointer'", naming our branch as real and explaining away the many deep
interactions of other branches as only being "mathematical significant" rather
than having "ontological significance". ... Maybe this is a bad time to
mention my preference for the Mathematical universe hypothesis ("which posits
that all computable mathematical structures (in Gödel's sense) exist"), which
seems to make the idea of something having mathematical significance but not
ontological significance meaningless.

~~~
millstone
The "nonsense news headline" and followup discussion are less critiquing MWI
than poking fun at David Deutsch. This is because Deutsch is a prominent
critic of PWT.

The arguments against MWI from Occam's Razor are indeed weak. The stronger
critiques are the difficulty of deriving the Born rule, or indeed quantum
randomness at all, from the deterministic MWI. This is presumably what the
authors hint at in their "doubtful it [pure Hamiltonian evolution] makes sense
even there (in MWI)".

~~~
AgentME
Why do you say that quantum randomness is a problem in MWI? I thought it was
one of the neat things of MWI that it doesn't require randomness. There are no
random events: instead you have a branch for each outcome. The only randomness
is which branch you find yourself in as an individual.

~~~
XorNot
It's a problem because it closes off further investigation - same as the
anthropic principle. It's an answer for everything, but doesn't tell us
anything about where to look next.

So if it's right, it needs to rigourously exclude everything else.

~~~
AgentME
MWI doesn't mean anything and everything can happen because there's infinite
universes. The equations of quantum mechanics describe the ways that worlds
can branch apart and the proportions of those branches.

Sorry if I'm underestimating your familiarity with MWI here; if I am, szemet
has a much better response. I'm just a little concerned that some people in
the thread don't realize MWI refers to something much more specific and
grounded than what-if-multiple-universes, and I wanted to dispel that notion.

~~~
kybernetikos
I hear a lot that MWI implies that everything that can happen does happen.
What I find frustrating is that the obvious next interesting question then is
'well what _can_ happen?', and people don't seem to talk too much about that.

~~~
AgentME
The short answer is the Schrödinger equation.

If you want a more layman's explanation of what sort of things would cause a
branch and how the branches would differ: one simple example is that when an
excited atom emits a photon, it doesn't emit it in a random direction, instead
there's a superposition of it emitting the photon across all paths away from
itself. In many of these paths, the photon will interact with particles
differently than happens with other paths, and the paths will decohere into
different branches of the world with separate consequences from the photon
hitting in a different place in each branch. In one branch, the photon could
hit a particle in the air and affect its temperature and velocity by the
tiniest amount. In another branch, the photon could instead hit a receptor in
a person's eye and immediately trigger a reaction that would not have
otherwise occurred.

Small differences between the branches could build up into bigger differences
over time, especially when you consider the sheer number of concurrent
interactions that are creating overlapping superpositions which decohere in
many different ways. It could be that there's enough branches that most things
that could have reasonably happened by chance do happen in some branch.

(If you're wondering how we could know in theory that the world may branch
like this, it's because branches where particles take different paths but then
later have all of their positions line up together interfere with each other.
See the two-slit experiment.)

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Koshkin
I am sure that laymen like it - because it seems to offer an "intuitive"
picture of the wave-particle dualism.

Problem is, neither QFT nor modern deeper theories such as strings gain
anything from this kind of metaphysical image.

~~~
meri_dian
Equivalently we can say that neither QFT nor string theory gains anything from
the empty hand-waviness of the copehagen interpretation, Everett's many-
world's, etc.

~~~
Barrin92
Those also don't really play a role in the professional life of a physicist,
nowadays you really do stare at computer screens a lot and try to find things
in data, it's not like physicists sit around and discuss many-world theory as
part of their day job.

These are just the popular things that make it out into public discourse

~~~
madhadron
Depends on the physicist. Those working on foundations of quantum mechanics
do. There just aren't many of them.

~~~
svarnypetr
For example the whole group around beyondspacetime.net does that.

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olegkikin
I have an unrelated question to the physics nerds. Is it even possible to
determine whether the universe is deterministic or not from inside the
universe?

Let's imagine a simplified example. I give you two sets of numbers (let's
pretend they are atom coordinates). One set is purely random, another one is
generated with a very simple formula (example: f(n) = SHA256(N + salt)). I
will give you as many of the numbers as you want. Can you determine which set
is which (if you don't know the formula ahead of time)?

~~~
Govindae
Any randomness could in principle be predetermined. God could have rolled the
dice ahead of time.

~~~
Houshalter
It can also be explained by branching. Run a simulation of a universe, and
every time a random bit is required, fork into two separate processes. One
where the "random" bit is 1 and another where the bit is 0. From the inside,
it would seem indistinguishable from true randomness. But the system as a
whole is purely deterministic.

~~~
hasenj
How would forking occur? Forking requires copying data. Does the universe get
cloned infinitely many copies all the time? That's quite an extraordinary
claim.

~~~
hossbeast
Copy-on-Write

~~~
hasenj
That's worse (more complicated).

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thisrod
Pilot waves are not a theory. No experiment can exclude them, unless it
falsifies the entire quantum theory of mechanics. One reason to dislike pilot
waves is that people, speaking as scientists, take them too seriously. This is
not to say that scientists shouldn't talk about them, just that we shouldn't
pretend we're doing science when we do.

In particular, don't pretend that anyone can be scientifically right or wrong
about the human interface to quantum mechanics. Some interfaces cause fewer
errors than others do, and the rest is Vi and Emacs.

Speaking subjectively, there are two reasons that I prefer Everett's relative
state interpretation. (It's the serious version of the approach that these
slides send up as many worlds.)

Relative states make it blatantly obvious why I can't put a contract on my
great-great-grandfather. In other approaches, you have to think about this.

In the pilot wave "theory", there are superpositions, a.k.a. "pilot waves",
and a bunch of other things. In the Everett worldview, by contrast, there are
superpositions, full stop.

Everett does leave some grey areas, roughly speaking the quantum version of,
"Is 7 a random number?" I doubt that any interpretation of quantum mechanics
can clarify that, and Everett at least acknowledged the problem and made it
fairly explicit.

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vbuwivbiu
[http://puredhamma.net/wp-content/uploads/Observing-the-
Avera...](http://puredhamma.net/wp-content/uploads/Observing-the-Average-
Trajectories-of-Single-Photons-in-a-Two-Slit-Interferometer.pdf)

Using weak measurements, average photon trajectories predicted by Bohmian
mechanics observed.

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amai
Has anybody successfully incorporated relativity into pilot wave theory? For
example, how does the Dirac equation emerge from pilot-wave theory?

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EGreg
I love PWT. I think it is the right explanation for what we observe.

Other theories don't rule out nonlocality anyway. And I'd rather have
nonlocality than their non realism!!

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guscost
TL;DR: Postmodernists don't like it because it abandons locality in favor of
determinism. Modernists like it for the exact same reason.

~~~
CPAhem
Also Bohm was unpopular with the US government in the early 1950's - he had to
leave for Brazil because of McCarthyism and anti-communist feeling.

