
Is Quantum Theory About Reality or What We Know? - pmcpinto
http://nautil.us//blog/is-quantum-theory-about-reality-or-what-we-know
======
badosu
I hope Pilot Wave Theory [0][1] gets more recognition and future work is able
to extend it to account for relativity, there's hope that we actually can find
a deterministic approach to Quantum Mechanics.

Here's a amusing video of an analogous macroscopic experiment, droplets
oscillating and interacting with each other at stable states:
[https://www.youtube.com/watch?v=JUI_DtzXdw4](https://www.youtube.com/watch?v=JUI_DtzXdw4)

[0]:
[https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory](https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory)

[1]:
[https://en.wikipedia.org/wiki/Pilot_wave](https://en.wikipedia.org/wiki/Pilot_wave)

~~~
rtpg
I've been fairly interested in pilot wave thoery, and the more I delve into
it, the less I feel it's useful.

My understanding is that the statistical equivalent of the pilot wave theory
is "when you flip a coin, there is not a 50/50 chance, but a 100% chance of
getting the result, because you got the result". Hyper-determinism.

The programming equivalent is "all functions are built with lookup tables,
with precomputed values". Great, but I still want to predict the values of the
lookup tables to describe the function.

From the wiki description of the "guiding function" (basically, the universe's
step function):

>The main fact to notice is that this velocity field depends on the actual
positions of all of the N particles in the universe.

This feels totally infalsifiable! "in the exact state of the universe you get
the result you get". None of the results end up being usable because there's
always the "out" of "the universe has changed". You end up going into hyper-
determinism, which is almost all about negating free will.

Even if the pilot wave theory is the "real thing", the other model of quantum
mechanics ends up being more useful because at least you can make some
predictions on the state of the universe.

(Would love to have someone explain to me how we can apply Pilot Wave Theory
in a "local-ish" fashion, people worked hard on this stuff so I imagine
there's some use)

~~~
amazingman
> You end up going into hyper-determinism, which is almost all about negating
> free will.

You seem to think this is a bad thing. Why?

~~~
eli_gottlieb
I would say: because a totally deterministic, nonstochastic universe becomes
subject to paradox theorems about Laplace's Demon.[1] You need at least a
little bit of stochasticity to make prediction and unpredictability work out.

[1] -- [https://rjlipton.wordpress.com/2014/08/08/laplaces-
demon/](https://rjlipton.wordpress.com/2014/08/08/laplaces-demon/)

~~~
bryondowd
Most of the disproofs here don't actually seem to disprove the demon's
existence, just limit it's ability to answer trick questions.

"Suppose that there is a device that can predict the future. Ask that device
what you will do in the evening. Without loss of generality, consider that
there are only two options: (1) watch TV or (2) listen to the radio. After the
device gives a response, for example, (1) watch TV, you instead listen to the
radio on purpose. The device would, therefore, be wrong. No matter what the
device says, we are free to choose the other option. This implies that
Laplace’s demon cannot exist."

This doesn't prove that the demon can't exist, just that it can't answer the
question. If it knows what you will do in any case of its response, and it
knows what its response will be, it knows what you will do, generally. It just
can't tell you what you are going to do. Sort of a difference between being
omniscient versus being omnipotent.

In other words, the demon knows that its response will be X, and your actions
will be Y, it's response just can't be truthful. It's all still perfectly
deterministic.

Free will is just a convenient illusion based on the fact that predicting the
universe faster than realtime would almost certainly require more resources
than the universe contains. At best, we can limit our scope for imperfect
predictions based on imperfect knowledge and limited processing ability. The
universe is likely deterministic, but there's no way to act on that in a
meaningful way, so for the purposes of human existence, we may as well act as
if we have free will.

~~~
prmph
Free will can exist; we just are not able to model it or reason about it with
the constructs we have

~~~
dogma1138
Free will is an outcome of a universe which is not purely deterministic if you
have a proof that the universe is then essentially there is no freewill.

Freewill goes beyond the concept of your brain deciding what to have for
dinner.

People are actually analogous to some physical system it's much harder to
predict the actions of an individual whilst predicting group actions on a
larger scale is easier since the various individual inputs are effectively
canceled out.

This is like modeling say a glass of water, modeling each individual molecule
is nearly impossible because you get to the point of not being able to measure
them especially when you trying to measure or predict the sub atomic make up
of each molecule, but modeling the entire system is easy.

------
danesparza
If you're interested in the subject matter, I highly recommend Richard
Feynman's 'Six Easy Pieces' series (available on Youtube here:
[https://www.youtube.com/playlist?list=PLBxHpsmcxyNghmwd6MJBy...](https://www.youtube.com/playlist?list=PLBxHpsmcxyNghmwd6MJByuhwMr-
WkfoB8) ).

Feynman was a key member of the Manhattan project (Wikipedia link here:
[https://en.wikipedia.org/wiki/Manhattan_Project](https://en.wikipedia.org/wiki/Manhattan_Project)
). I trust him to know what he's talking about. :-)

------
slackerjack
I really enjoyed the article.

Every time I read about Quantum Theory it reminds me about a story I read
online some time ago. I can't fully remember it:

I think it was about a guy who made a Quantum Computer to answer any question
in the universe by asking it to different dimension of himself. I think he
uses it to win the lottery but in the end he mistrusts himself to answer the
question "how to live a happy life" and ends up living an unhappy life.

I really want to re-read it. Does anyone know where I can find this story?

I spend weeks now to find it ... with no luck.

~~~
paviva
[https://physicsnapkins.wordpress.com/2013/05/20/all-paths-
to...](https://physicsnapkins.wordpress.com/2013/05/20/all-paths-to-
happiness/)

~~~
slackerjack
YES, That is it! Thank you!

------
whack
" _Measuring a system will generally change its state from one in which each
possible outcome has some non-zero probability to one in which only one
outcome occurs. That’s a lot like what happens when, in the die game, you
learn that the die does, in fact, show a six. It seems strange to think that
the world would change simply because you measured something. But if it is
just your knowledge that changes, things don’t seem so strange._ "

This is the only interpretation of quantum theory that makes sense to me. The
idea that an objective reality of the world has changed, through our act of
observation, leads to absurd conclusions such as Schrodinger's cat being
simultaneously alive and dead at the same time. I believe this is the same
point that Schrodinger tried to make with his thought experiment as well.

~~~
wu-ikkyu
So the most logical interpretation of quantum physics is that _reality is
subjective_ , correct?

~~~
philipov
No, the most logical interpretation of quantum mechanics is that you can't
measure things without using particle interactions, and particle interactions
change what they interact with.

~~~
danharaj
I don't think that's enough. Particle interactions are unitary, but
measurement looks non-unitary from the point of view of the observer.

Reality is intersubjective, but not subjectivist.

~~~
philipov
I think confusion arises when people hear 'measurement' and jump to thinking
"wait, what's so special about measurement? Would the world not exist if we
weren't there to measure it?" This is due to an imprecise understanding of
measurement, and should be corrected by pointing out all particle interactions
are measurements. Our classical world exists without superposition because
there are enough particles interacting with each other that everything is
constantly being measured.

~~~
singham
I cannot agree with this more. Universe has been existing without intelligent
observers for billions of years. It stands to reason that universe is
constantly being shaped as particles interact.

------
singham
Anyone who thinks that QM is making stuff up, please watch this video. Here
Rubidium atoms (mind you atoms and not electrons or photos) are made to
interfere with each other.

[https://www.youtube.com/watch?v=a_w6AGe_fIo](https://www.youtube.com/watch?v=a_w6AGe_fIo)

Quantum mechanics is a foundation of physics, chemistry and materials science.
Still, there is an ongoing debate about the emergence of the classical,
macroscopic world from the well-understood microscopic world of quantum
mechanics. We contribute to this discourse by demonstrating quantum
superposition of massive particles at the distance (0.5 m) and time scales (2
s) of everyday life, thereby advancing the state-of-the-art of atom de Broglie
wave interferometry by nearly two orders of magnitude [1]. In addition to
testing a central tenet of quantum mechanics, we pave the way for new
precision tests of gravity, including the possible observation of
gravitational waves and tests of the equivalence principle. In related
experimental work, we demonstrate that entangled clusters of approximately
1000 atoms can be used to achieve 10-fold improvement in the sensitivity of
quantum sensors based on atomic transitions; the levels of performance
achieved could not have been realized with any competing (non-entangled)
method [2].

------
roycoding
This is closely related to QBism (quantum Bayesianism)

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

[https://www.quantamagazine.org/20150604-quantum-
bayesianism-...](https://www.quantamagazine.org/20150604-quantum-bayesianism-
qbism/)

------
fiatjaf
"Physicists know how to use quantum theory—your phone and computer give plenty
of evidence of that."

What?

~~~
dreamcompiler
Semiconductors couldn't work without quantum mechanics [0]. Heck, _the sun_
couldn't work without quantum mechanics [1].

[0] [https://physics.stackexchange.com/questions/112615/why-is-
it...](https://physics.stackexchange.com/questions/112615/why-is-it-said-that-
without-quantum-mechanics-we-would-not-have-modern-computers)

[1] [https://www.forbes.com/sites/ethansiegel/2015/06/22/its-
the-...](https://www.forbes.com/sites/ethansiegel/2015/06/22/its-the-power-of-
quantum-mechanics-that-allow-the-sun-to-shine/#2014a30143f7)

~~~
dreamcompiler
(This does not make your phone a quantum computer. Your phone is an ordinary
computer that happens to be enabled by quantum mechanics.)

------
eli_gottlieb
I think that "reality vs knowledge" is a false dichotomy. After all, our
knowledge is a classical-level feature _of_ reality, not a core element of
nature's physical ontology. What does nature's physical ontology require, such
that once it all builds up to the classical level, we can know things?

Information as an element of physics is what I'd guess.

------
tzakrajs
As a laymen, it seems to me that quantum theory is similar to the expanding
universe theory in that scientists substitute complexity (cause of expansion,
quantum states) with a token (dark matter/energy, indeterminacy) until you
better understand the underlying systems.

Can anyone tell me why I am incorrect?

~~~
aroberge
The two examples you give are very different. I would argue that you are,
indeed, very incorrect.

The purpose of scientific theories is to be able to make predictions about
physical systems. Fundamental theories are based on some postulates
(assumptions; in mathematics, we would have axioms). Quantum mechanics is
based on a few postulates (which are expressed in very mathematical terms,
like "Physical observables are represented by Hermitian matrices on a Hilbert
Space H").

If the theory makes incorrect predictions, it is deemed to be wrong. If it
makes predictions that cannot be explained using "common sense", then non-
specialist come out of the wood work stating that it is obviously incorrect,
that specialists simply do not understand.

For the expanding universe ... we have a theory (Einstein's Theory of General
Relativity) which has been extremely well tested in all kinds of scenarios,
and has always been found to make correct predictions. Your phone's GPS would
not work if it would use predictions from Newton's theory rather than
Einstein's.

The Theory of General Relativity can be thought as a set of 16 equations of
the form:

(aspect of geometry of space time) = (stress-energy-momentum content of the
universe)

In simplified terms, the left-hand side of these equations relates to the
observed expansion of the universe. The right-hand side of these equations is
what type of "energy/matter" we observe. We find that simply including
baryonic (i.e. "normal") matter that we can see by looking (using telescopes)
at the sky leads to inconsistencies with the observed expansion. If we include
different type of energy/matter that we _cannot_ see (i.e. dark), each obeying
precise equations of state, we can have predictions that are consistent with
what we observe. Thus, we use General Relativity + observations about the
expansion of the universe to make predictions about what is the energy/matter
content of the universe.

What you see as a failure, I see as a success of G.R. as it makes predictions
about what is found in the universe which we did not know before, and had no
way to know simply using the tools we had.

"Fun fact": many years before modern neutrino experiments nailed down the
number of "normal" light neutrino species to 3, using General Relativity and
known nuclear physics, one could use the observations of the ratio of light
elements (He/H, Li/H, etc.) produced in the early universe to make the
prediction that the number of light neutrino species had to be equal to 3.

~~~
AnimalMuppet
> "Fun fact": many years before modern neutrino experiments nailed down the
> number of "normal" light neutrino species to 3, using General Relativity and
> known nuclear physics, one could use the observations of the ratio of light
> elements (He/H, Li/H, etc.) produced in the early universe to make the
> prediction that the number of light neutrino species had to be equal to 3.

Could you give a bit more detail on how the observatsions, plus nuclear
physics, plus general relativity, led to that conclusion?

~~~
aroberge
Look at the very short section titled Big Bang nucleosynthesis on
[https://en.wikipedia.org/wiki/Cosmic_neutrino_background](https://en.wikipedia.org/wiki/Cosmic_neutrino_background).
Based only on the relative abundance of He-4 and D (H-2), one gets an estimate
of 3.14 for the number of neutrino species. You can find more detail on
[http://darkuniverse.uni-
hd.de/pub/Main/WinterSchool08Slides/...](http://darkuniverse.uni-
hd.de/pub/Main/WinterSchool08Slides/CosmologicalNeutrinos.pdf). Basically, the
more light neutrino species there are, the faster the expansion occurred in
the early universe, and the less time there is for making other nucleus from
protons and neutrons. After a while, the matter is to diluted for fusion to
occur and the ratio of various nuclei is "frozen" .... until stars are formed
much later on.

------
yters
Doesn't Bell's inequality address this question?

~~~
millstone
It addresses a related but slightly different question.

You flip a coin: there is a 50/50 chance of heads or tails. These
probabilities are due to our incomplete knowledge: the coin knows which face
is up, and it's only because we don't know that we assign probabilities.

Now if we have a particle that is 50/50 spin up or spin down, we may think
that it is like a coin: the spin is already determined, we just don't know
which it is. This is where Bell's Inequality comes in and shuts that idea
down. Until the particle is observed, it simply does not have a spin, in the
same way that a coin has an up face.

So if a pre-observed particle does not have a spin, what does it actually
have? This is the core of the dispute in the article. There's two possible
answers:

1\. A wavefunction (psi), aka a quantum state. This is the psi-ontic view
which argues the wavefunction is a real and physical thing that evolves in
time regardless of any observations. MWI and Bohm are both psi-ontic theories.

2\. Nothing. This is the psi-epistemic view which holds that it makes no sense
to talk about a particle's reality in-between observations. In this view, only
measurements and their results are real; the wavefunction is a state of
knowledge. Copenhagen, Consistent Histories, and the minimalist ensemble
interpretations are psi-epistemic.

~~~
TheOtherHobbes
The argument about what "a real and physical wavefunction" means boils down to
this question:

"Is there a science of the wavefunction beyond QM?"

Can you ask more questions about it, break it down into constituent processes,
maybe re-engineer it?

Or is it opaque and closed and there is no possible way to do further science
on it?

My money is on option 1, because it's hard to imagine an organising principle
you can't possibly do any science on.

Wavefunction science may be weird and like nothing ever seen before -
especially given non-locality and possibly non-realism. But it's a good rule
of thumb in science that when nature becomes counterintuitive you're on the
right track. If the answer was obvious and familiar you wouldn't need original
and creative thinking to find it.

Option 2 seems like an admission of total defeat. It would be the first time
in science where we hit a wall and didn't even try to understand what it was
made of.

That doesn't mean it can't be correct. But it does mean we shouldn't give up
without trying for quite a while longer. And if we do give up, it should only
be after discovering some kind of meta-Bell theorem which _proves_ there's no
way to go further.

~~~
yters
If I understand it all correctly, the article describes an experiment to test
your preferred option, but so far the evidence is in favor of option 2?

------
catpolice
Oh Hey, I know the author of this! In fact, I think I introduced him to that
Spekkens paper. Small world.

------
erikpukinskis
Seems like it changes reality. The presence of a measurement changes the
interference pattern for all observers, no?

If the waveform collapse only happened in my reality then if I observed a
field, I would expect others to see an interference pattern, even if I only
saw a clear field.

If other people can observe an interference pattern while I observe a clear
field, then that's a multiverse. That means you need a whole new universe for
every observation. Seems like poor compression. Although I guess if it's an
append-only store then it works out ok.

~~~
gumby
The article describes an alternative interpretation.

