
Quantum observers with knowledge of quantum mechanics break reality - petters
https://arstechnica.com/science/2018/09/quantum-observers-with-knowledge-of-quantum-mechanics-break-reality/
======
rotskoff
This write up not only oversimplifies, but it totally neglects one of the more
interesting motivations of the original paper [1]. The authors are probing
whether or not quantum mechanics is consistent with a single-world
interpretation---that is, whether or not there is a unique reality. Formally,
the claim is that there is no physical theory that is (1) consistent with QM,
(2) consistent with a single-world interpretation, and (3) logically self-
consistent.

1\.
[https://arxiv.org/pdf/1604.07422.pdf](https://arxiv.org/pdf/1604.07422.pdf)

~~~
nessus42
Yes, I'm not sure why this is a Nature article. As far as I understand things,
very few physicists who care about which interpretation of QM is correct would
choose the Copenhagen Interpretation in this day and age. The Copenhagen
Interpretation is not even a fully-fledged interpretation, because it uses
undefined, unscientific terms like "measurement" to determine when a
probability wave collapses.

As I understand things, most physicists don't give much thought to which
interpretation is correct, since any experiments to distinguish between the
various interpretations are virtually impossible to do. And most physicists
don't care about distinctions for which there will be no experimental
evidence.

Among the physicists who _do_ care about the different QM interpretations, it
is my understanding that most would go with the Everett (AKA "Many Worlds)
Interpretation these days. All other interpretations that I know of are hugely
problematic, but there are no significant problems at all with the Everett
Interpretation. The only problem is that many people consider it to be
"creepy". But not liking the best theory because it is "creepy" isn't very
good science, if you ask me.

Regarding there being no single-world interpretation that is logically self-
consistent, I'm not convinced about this: The Bohm Interpretation, for
instance, is experimentally indistinguishable from the Everett Interpretation.
I.e., no matter what incredible technology and powers of QM experimentation we
might develop in the future, we will never be able to do an experiment, even
in theory, that tells us which of these interpretations is the right one.

Consequently, it would seem that the Bohm Interpretation is logically self-
consistent. The problem with the Bohm Interpretation is that it's very ad hoc
and violates Occam's Razor. It only exists in order to calm our feelings about
the universe being "creepy".

~~~
ummonk
Although I go with the Everett interpretation (which I like to think of as the
"universal wave function" interpretation because it is really just a
minimalist theory assuming that the laws of wavefunction evolution always
apply), and the many worlds aspect is just a byproduct of the fact that a wave
function left to its own devices would decohere into a bunch of practically
non-interacting "worlds".

It should be noted, however, that the Everett interpretation does have one
issue: it's not clear why probabilities should work the way they do. There are
different approaches to deriving the laws of probabilities under Everettian
physics, but things very easily get metaphysical once you try to go down that
road.

As you point out, the Bohm Interpretation works as a single world
interpretation, although it relies on reifying particles embedded in waves to
essentially select a single world, which is rather ad-hoc. However, it does
give us the probabilities for free, assuming any reasonable initial setup for
the particles.

~~~
platz
Right, with many worlds, if there is any probability of something quantum
happening, with say a billion billion to one odds, it will happen with 100%
certainty somewhere. And if you are that observer, how do you say it was
unlucky/lucky, sine it must have happened

~~~
beagle3
It's no worse than a single world interpretation: These things happen all the
time, the branch of math that deals with them is called Large Deviation Theory
(and it's closely related to information theory).

One of the corollaries/interpretations of Sanov's theorem is that, generally
speaking, when faced with an astonishingly improbable outcome (e.g. flipping
9,000 heads and 1.000 tails out of 10,000 independent coin flips), no
statistical test can differentiate between "that improbable occurence with a
fair coin" and "an unfair coin" \- the fair coin, when it does something
improbable, with have (with overwhelming probability) a specific tilted
distribution that looks unfair.

~~~
platz
But in single interpretation only one outcome happens on a trial. The full
distribution does not manifest on a single draw. In MWI all the possibilities
occur, which is different

~~~
Sharlin
_Somebody_ usually wins the lottery. That doesn't change the fact that it's
really unlikely for any given player to win the jackpot.

~~~
platz
I guess you win. MWI is really not different from classical probability in any
way.

------
deckar01
I know they are trying to dumb it down, but I think they are leaving out some
fundamental elements of the puzzle here. Bob interprets the message
incorrectly sometimes and no reason is offered for why or how often. They say
the observers get contradicting answers sometimes, but that revelation seems
anticlimactic considering I have no reason the believe Bob's observer should
observe outcomes matching Alice's observer any more often than Bob himself can
"guess" the result.

I might need to dig into the original paper:
[https://www.nature.com/articles/s41467-018-05739-8](https://www.nature.com/articles/s41467-018-05739-8)

~~~
roywiggins
On first glance it seems like they're building a sort of Godel sentence- a
self-referential experiment that forces things to end up looking
contradictory.

~~~
Severian
I was thinking something along similar lines. Hawking had a lecture[1]
regarding Godel, which indicate incomplete or inconsistent models of systems.
What we may think as inconsistent is probably an incomplete model. Or as
Hawking addressed in the lecture: " According to the positivist philosophy of
science, a physical theory is a mathematical model. So if there are
mathematical results that can not be proved, there are physical problems that
can not be predicted."

[1] [http://www.hawking.org.uk/godel-and-the-end-of-
physics.html](http://www.hawking.org.uk/godel-and-the-end-of-physics.html)

------
pantalaimon
> There is a theory which states that if ever anyone discovers exactly what
> the Universe is for and why it is here, it will instantly disappear and be
> replaced by something even more bizarre and inexplicable.

> There is another theory which states that this has already happened.

\- Douglas Adams, The Restaurant at the End of the Universe

------
dschuetz
Here's another write-up of the same subject:
[https://arstechnica.com/science/2018/09/quantum-observers-
wi...](https://arstechnica.com/science/2018/09/quantum-observers-with-
knowledge-of-quantum-mechanics-break-reality/#p3)

"Quantum observers with knowledge of quantum mechanics break reality"

~~~
lostmsu
This article's comments section links to reviews of the paper, and, basically,
the second reviewer nails it.

In one of the outcomes authors examine, which gives the rise to the apparent
contradiction, measurements do no commute, which means, according to QM it is
impossible to get that set of results in real world.

~~~
yorwba
The reviews are here [https://static-
content.springer.com/esm/art%3A10.1038%2Fs414...](https://static-
content.springer.com/esm/art%3A10.1038%2Fs41467-018-05739-8/MediaObjects/41467_2018_5739_MOESM1_ESM.pdf)

The authors' final response to referee #2:

 _We thank the referee for the clarification of their earlier comment, which
we indeed misunderstood. It is of course correct that the observable of the
measurement leading to z does not commute with the one corresponding to the
measurement of w . This also means that the values z and w cannot be defined
simultaneously (in the sense of “at the same time”). To explain how our
argument avoids this problem, note first that the measurements take place at
different times (as pointed out in the reply above). Concretely, z is measured
at time n :10 and w is measured at time n :30. Note also that, in all
statements we are using, we always explicitly specify the time at which a
variable is supposed to have a given value. Whenever we are talking about z ,
the corresponding time lies (strictly) before n :30, whereas we only talk
about what value w has at times after n :30. We hence never need to assume
that z and w are available at the same time. To clarify this in the
manuscript, we have changed the labelling of all statements. They now have a
superscript which indicates at what time a statement has been made by an
agent._

~~~
lostmsu
Disclaimer: in no way I am an expert in physics. I don't event have any degree
in it.

Plainly speaking, this argument does not make sense, because there's no such
thing as simultaneity.

The way I see it, the commuting argument tells what you, an observer to the
entire system, can or can not see at some fixed point in spacetime, that is
casually descended from the points where observations occur.

Anyway, the whole discussion about validity of interpretations does not make
any sense to me, as long as they represent the same underlying math.

~~~
millstone
Here the argument is about commuting observables and isn't relativistic. For
example, position and momentum are non-commuting observables. If you measure
both, and then ask "what is the position of this particle," you have to know
whether the position was measured first (in which case the position of the
particle is still known), or whether the momentum was measured, so now the
position is no longer known.

~~~
lostmsu
I think mistreatment of causality might still be another explanation for the
result.

In particular, in the iterative process, that authors suggested to use to get
w=ok, z=ok, each iteration must be connected to the previous in some QM
encodable way, eg as { |continue>, |halt> }, that must be set in the last
step, and measured in the first, therefore combining all participants into a
single entangled QM system.

It is not immediately obvious, that this addition would not affect the outcome
of experiment, and hence should at very least be included in the paper.

------
AnimalMuppet
Once one person has flipped the coin, it seems wrong to me to say that an
observer outside the box sees a superposition of heads and tails, simply
because the person outside the box doesn't know the result of the flip. I
don't think that a human's ignorance is equivalent to a waveform
superposition.

This is true even if you replace the coin with the results of a quantum random
number generator. The problem isn't that the coin is a macroscopic object (and
therefore doesn't really do quantum superposition). The problem is that, once
observed, there is no uncertainty in the output of the quantum random number
generator. There's only human ignorance (for everyone except the one person in
the innermost box).

And as I understand it, experiment agrees with me. You take an entangled pair
of photons, observe one, and you _know_ what the same observation on the other
photon will produce, even if the person making the observation on that photon
doesn't.

It could be that the write-up is creating confusion, or that I am confused,
but something feels off here (and not just in a "that's too weird to be true"
way).

------
dschuetz
I don't think that it _breaks_ quantum mechanics. It shows once again that the
results of the double slit experiments are still not completely understood.
That idea in the article only means that the Copenhagen Interpretation is
being challenged. It doesn't make all the discoveries in particle physics
collapse.

------
roywiggins
Hold on, what on Earth do they mean by a "quantum message"?

~~~
roywiggins
Ah: "In each round n = 0, 1, 2, … of the experiment, agent F¯¯¯¯ tosses a coin
and, depending on the outcome r, polarises a spin particle S in a particular
direction. Agent F then measures the vertical polarisation z of S"

[https://www.nature.com/articles/s41467-018-05739-8](https://www.nature.com/articles/s41467-018-05739-8)

~~~
jhanschoo
Is the point of this so that the whole situation in boxes remain an unobserved
quantum state rather than classical?

------
laretluval
This writeup is incomprehensible. Does anyone know of a better one?

~~~
lisper
I'm working on it (definitely room for improvement there), but the original
paper is long and it will probably take me the better part of the day to work
through it (and I also have to get some work done). In the meantime, the same
conclusion can be reached by a completely different argument:

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

Same content in a video:

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

UPDATE: Just found this:

[https://arstechnica.com/science/2018/09/quantum-observers-
wi...](https://arstechnica.com/science/2018/09/quantum-observers-with-
knowledge-of-quantum-mechanics-break-reality/)

By way of:

[https://news.ycombinator.com/item?id=18025438](https://news.ycombinator.com/item?id=18025438)

~~~
sctb
Thanks! We've updated this submission to the Ars article from
[https://www.nature.com/articles/d41586-018-06749-8](https://www.nature.com/articles/d41586-018-06749-8).

------
skywhopper
I found this article impossible to make sense of. But what I did gather
reminds me of the simultaneity paradox in relativity where it's possible for
different observers of two events to perceive their relative order in time
differently. So that observer 1 may see event A happen before event B,
observer 2 may see event B happen before event A, and observer 3 may see
events A and B happen at the same time.

~~~
romaniv
This is different, because relativity can actually be observed directly.
Different interpretations of QM cannot.

------
scotty79
I think people trying to interpret quantum mechanics are mislead by saying
that probability wave upon observation collapses to precise value that upon
next measurement stays exactly the same. That's unrealistic because no
measurement is of perfect precission. Next measurement will give slightly
different value. So we really can't tell if we have one exact value or still a
probability wave just reshaped to very narrow thing by interaction needed to
do the measurement. We can make probability wave wider and fuzzier again just
by scattering particle we measure of some other unmeasured particle.

That's how I understand quantum mechanics wave-particle duality. That there's
no duality. There are just waves that interact and reshape each other through
that interactions. And the only reason that we think of them sometimes as sort
of little billiard balls is because annoyingly equations that govern evolution
of narrow and sharp probability waves look (not accidentally) exactly the same
as primary school level math that governs billard balls.

~~~
sixo
If you make 3 subsequent measurements of different but related variables (like
a vector component wrt to 3 different axes) the 3rd's probability distribution
will depend on the value you observed in the 2nd observation in a way that
cannot be replicated without the 2nd.

Meaning it's not just matter of poor precision. See the Stern-gorlach
experiment.

~~~
Thespian2
A simple version of this (not the exact experiment) is to take three
polarizing filters. Put one down, and see it reduces the amount of light
passing through. Put a second one down, in line with the first, then rotate it
to 90 degrees off. You will observe the light dim until the second filter is
completely dark, blocking all the light which passed the first filter.
Finally, take the third filter, and place it _between_ the first two, at 45
degrees off, and suddenly light will pass through all three.

This very clearly demonstrates that each filter reshapes the wave's
polarization, not to a single orientation, but to a narrow band.

~~~
posterboy
The inaccuracy persists, e.g. in your comment - "the wave" I can image along
literal hand waving. I'm being cynic because this polarization filter
experiment had me dumbfounded.

Blocking the way between the second and third filter will show no light, as if
the third filter wasn't there. It appears as if there is a wave but polarized
in a way that doesn't reflect from the hand and the third filter would
polarize it again. I believe that's also what you are saying. Only, what is
"the wave"?

> a narrow band.

Are you agreeing with Scotty79 above?

What band?

But, anyway, I don't see the analogy to Stern-Gerlach (which I only skimmed
quickly on wikipedia).

------
leoh
From the original paper:

>Analysing the experiment under this presumption, we find that one agent, upon
observing a particular measurement outcome, must conclude that another agent
has predicted the opposite outcome with certainty. The agents’ conclusions,
although all derived within quantum theory, are thus inconsistent. This
indicates that quantum theory cannot be extrapolated to complex systems, at
least not in a straightforward manner.

It really sounds to me that what they're saying is that some phenomena can be
described multiple ways. And in particular, it is likely to explain phenomena
consistently as X in one setting and Y in another.

This seems akin to the concept of two strings (A and B) being hashed to the
same value; the creator of the hash had one value (A) and an observer has a
rainbow table that believes the value is different (B) and both are right, but
the observer is not able to understand the creator's intention.

Is this analogy roughly correct? And if so, why is this so darn surprising?

------
program_whiz
Here's my writeup, this is a first-attempt so please offer constructive
criticism if wrong:

2 labs (L1 and L2), these are like the box of shrodingers cat, they are in
quantum uncertainty.

2 lab workers (Alice, Jane)

2 outside observers (Bob, Frank)

Time is denoted as T0, T1, ...

T0 Alice in Lab1 randomly selects heads / tails

T1 Alice uses heads / tails to setup a quantum particle S

T2 Jane in Lab2 reads the state of S, infers heads / tails and stores this
information in a new particle Z

T3 Frank reads the state of Lab2 to infer Z, S, heads/tails and determines
pass / fail (pass == was heads), lets call this variable W_frank

T4 Bob reads the state of Lab1 to infer S, heads / tails, and then makes his
own pass / fail (pass == was heads) and that's variable W_bob

Since these are separate readings on untangled particles by Bob and Frank,
they can get disparate readings W_frank != W_bob

This is a problem since we "collapsed" the state of W_frank -> Z -> S -> Coin,
but this doesn't necessarily imply that we can known with certainty that W_bob
will match (i.e. forward collapsed Coin -> S -> W_bob).

Basically its just that each lab is in a quantum uncertainty (Quantum Coin is
heads/tails, S is in either state, Z is in either state until measured). And
making the measurement should reveal this, but there's no guarantee both
collapses will result in the same outcome (so in one case the coin was heads,
and the other it was tails).

My conclusions:

1\. There is no randomness, we just suck at measuring still

2\. We are in the "many worlds" but impossible branches can't happen (reality
stays consistent somehow) -- so it won't happen even though its theoretically
possible

3\. The forward collapse does happen, we just haven't done the experiment to
verify it. In other words, the pass/fail result would change to keep things
consistent (the whole system is entangled). Including the memories everyone
would have about it. So maybe this is happening constantly but we just don't
know it because it changes even our memories about it.

4\. This experiment fractures reality, and we realize we all live in a
simulation and that's where "white holes" come from :P

~~~
kgwgk
I think their problem is considering that the labs L1 and L2 are isolated
quantum systems because there is obvious entanglement through the state S.

In the sequence of events that they present at T1 the observer in L1 prepares
S, transfers it to L2, and knowing the current state of L2 predicts what the
observer outside L2 will measure later assuming that L2 is left unperturbed
(the actions of the internal observer do not affect the system).

But this prediction is invalid because the protocol specifies that before the
last observer enters the scene at T4 the system L1/L2 will be perturbed by
another external observer (the observer outside L1 when he does his thing at
T3, as you said he's effectively measuring Z).

~~~
kgwgk
Lubos Motl’s take on the paper:
[https://motls.blogspot.com/2018/09/frauchiger-renner-qm-
is-i...](https://motls.blogspot.com/2018/09/frauchiger-renner-qm-is-
inconsistent.html)

(Tl;dr: “People who still try to prove an inconsistency of quantum mechanics
in 2018 are cretins.“)

------
MrQuincle
The original Maxwell daemon experiment didn't work because there was some
"work" hidden in the formulation.

A few days ago I was reading this
[https://phys.org/news/2018-09-quantum.html](https://phys.org/news/2018-09-quantum.html)
(didn't get on the frontpage), but it's about the difficulty of disappearing
quantum information.

As a layman, I can imagine that decoherence plays a similar role in these
thought experiments. Maybe perfect transfer of quantum information is
impossible.

It would be a nice merge of Schrodinger's cat with Maxwell's daemon. :-D

------
sebringj
Thought this might be interesting about many-worlds
[https://www.hedweb.com/everett/everett.htm#believes](https://www.hedweb.com/everett/everett.htm#believes)
if Feynman and Hawking and others are on board with it. I keep seeing with the
string theorists and cosmologists that seem to be the leaders in the field are
leaning toward many-worlds as the most sensible explanation. It then makes
this article seem less interesting if the majority fall into copenhagen but
the ones actually doing the real work are more in the many-worlds camp than
not.

~~~
visarga
Is Hawking still onboard?

~~~
diminoten
Unfortunately, Hawking passed away in March of this year. :(

~~~
iscrewyou
I think that was the joke.

------
GorgeRonde
> And different researchers tend to draw different conclusions. “Most people
> claim that the experiment shows that their interpretation is the only one
> that is correct.”

Exactly like in the experiment ...

Edit: To be clearer, why not try to explain the way the paper is received with
the very material the paper is about ?

------
Myrmornis
> (Frauchiger has now left academia.)

Tangential: why is that statement in the article?

~~~
kgwgk
Because she's no longer at ETHZ and normally they would add her current
affiliation.

------
jeletonskelly
Our simulation is written in a functional language using lazy evaluation,
obviously. Observing a quantum state forces evaluation and consumes more
memory on the machine running our simulation. If we observe too much the
beings simulating us are going to kill -9 us.

------
juris
The tao that can be named is not the real tao.

------
martin1975
I am happy and sad to hear this.

