
What’s Going on During Wave Function ‘Collapse’ - howard941
https://www.quantamagazine.org/how-quantum-trajectory-theory-lets-physicists-understand-whats-going-on-during-wave-function-collapse-20190703/
======
dcow
The article feels incomplete. I don’t feel like I have a better understanding
of what’s going on during wave function collapse, rather I just know QTT is a
thing and people are experimenting with it and it’s yielded promising results
when using more sophisticated measurement apparatus to observe an “atom”, like
reversing a quantum jump mid-flight by injecting feedback into the system as
it’s being measured.

~~~
jogundas
I liked some commentary in a similar direction
[https://physics.stackexchange.com/questions/484675/does-
the-...](https://physics.stackexchange.com/questions/484675/does-the-new-
finding-on-reversing-a-quantum-jump-mid-flight-rule-out-any-
inter/484681#484681)

~~~
dcow
Thanks for the link it covers all the details I was looking for. My favorite
part:

> Copenhagen and, say, many worlds just differ on how to treat branches of a
> superposition that have completely decohered.

------
malaxii
Quantum trajectories more or less have the usual Copenhagen interpretation
built in, so they don't really solve the old interpretation problems any more
than decoherence does. Although they can be a nice way to understand some
experiments, in particular how conditional probabilities of a set of
measurement results obtained in sequence occur, I'm not sure if it's good to
present it as some fundamental insight in popular articles.

~~~
jeremyjh
This seems to be the opposite of what the article is saying, or implying. I
read it as saying that there is no need for an interpretation, because there
is no wave function collapse; the particle can be described in precise terms.

~~~
chrischen
It was buried way deep in the article, but:

> But what exactly is this trajectory? One thing is clear: It’s not like a
> classical trajectory, meaning a path taken in space. It’s more like the path
> taken through the abstract space of possible states the system might have,
> which is called Hilbert space.

QTT is not a definite prediction of classical physical trajectory.

~~~
simonh
Although classical physical trajectory is one if the possible states QTT helps
us predict. It’s more than that, not less.

------
d33
I recently read a book by a cosmologist Max Tegmark in which he brought up
another theory - that the function wave function doesn't really collapse, it's
just that the moment of measurement lets us determine our location in the
multiverse. I think this is related:

[https://en.wikipedia.org/wiki/Many-
worlds_interpretation#Int...](https://en.wikipedia.org/wiki/Many-
worlds_interpretation#Interpreting_wavefunction_collapse)

And also an interesting read:

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

------
lisper
> “Quantum trajectory theory makes predictions that are impossible to make
> with the standard formulation,” Devoret said.

That is bullshit. If it were true, the predictions made by QTT would be tested
and if they came out as predicted it would be the biggest news in physics
since the Aspect experiment.

What is "going on" during wave function "collapse" has been known for decades:
entanglement and decoherence. That's all.

I am really getting sick and tired of these sensationalistic headlines.

(I'm also not entirely sober. It is the Fourth of July after all.)

~~~
dwaltrip
> What is "going on" during wave function "collapse" has been known for
> decades: entanglement and decoherence. That's all.

Wasn't it a famous physicist who said "If you think you understand Quantum
Mechanics then you don't understand Quantum Mechanics"?

I'm not defending this article, but I was under the impression that the proper
interpretation of quantum mechanics / QFT is far from a settled matter among
physicists.

~~~
MereInterest
It's not so much that it is settled as everybody knows that it isn't a useful
topic of conversation. All the various interpretations have the same math
behind them, give the same prediction, and have the same observable behavior.
Since there isn't anything to distinguish them, they are all equally valid.

In practice, things tend to go with the "Shut up and calculate"[1] philosophy.

[1]
[https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...](https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#shutup)

~~~
dwaltrip
I've heard that idiom, and I'm sure it has its use. I don't think we should
stop calculating, by any means. However, there are individuals in the field
who think new insights are possible through continued attempts to improve our
interpretations (in addition to continued development of the theory as
expressed through robust math). I don't have a source handy, but I've heard
the physicist Sean Carroll express this idea on his podcast, Mindscape.

Numbers by themselves don't mean anything (for humans at least). The meaning
is found in how the numbers relate to the concepts that we make use of.

~~~
lisper
It is legitimate to ask how our everyday perceptions can be so radically and
fundamentally different from what the math appears to say about the nature of
reality. "Shut up and calculate" is not a legitimate answer.

(For the record, the phrase "Shut up and calculate" was coined by David Mermin
as a pejorative characterization of the Copenhagen interpretation, which
actually says simply that we cannot know anything more than what the math
says, and that's just the way it is, and any attempt at further inquiry is
doomed to fail and therefore should not be attempted. It's a deeply
unscientific attitude.)

~~~
charlieflowers
I read statements from Feynman that I interpreted as very similar. He said
things like: the question "what is the underlying reality behind all this"
probably "has no meaning." I never understood what he was trying to say.

------
fpsio
Talk by Zlatko Minev on this result:
[https://www.youtube.com/watch?v=JGNL91MhC5A](https://www.youtube.com/watch?v=JGNL91MhC5A)
\- JQI Seminar 4/22/2019 - Zlatko Minev

------
viach
Indeed very interesting, what's going on? Reading the article, the only phrase
where "wave function collapse" occurred is:

> What’s more, this near-complete knowledge of how the system changes smoothly
> over time allows researchers to “rewind the tape” and avoid the apparently
> irreversible “wave function collapse” of the standard quantum formalism.

Please, answer this intriguing question, what's going on?

~~~
simonh
I think in QTT there is no collapse event, just a series of state transitions.
It looks like the concept of a collapse is a product of a less complete
understanding of what is going on.

That’s why the article spends so much time contrasting QTT with the view
offered by Shroedingers equation. With Shroedinger there is no specific state
predicted, so you have to introduce the concept of some event inducing a
specific state to occur. QTT allows continuous prediction of state and even
the ability to prevent or influence state transitions. At least that’s what I
got out if it, although I’ll concede I think that’s a rough attempt at a
summary.

------
kgwgk
Previous discussions:

[https://news.ycombinator.com/item?id=20105091](https://news.ycombinator.com/item?id=20105091)

[https://news.ycombinator.com/item?id=20116267](https://news.ycombinator.com/item?id=20116267)

------
bencollier49
Stopped reading half way through, as it felt like the author didn't understand
what he was writing about. What hope do I have reading the article, then?

~~~
jng
It took me a long time to understand two's complement arithmetic for negative
numbers in binary. Read several explanations on books and magazines (this was
before the internet existed), couldn't understand it. Then one day it clicked.
Awesome trick. I went back to some of the explanations that hadn't helped me
understand it, and I could clearly see the author didn't understand it
themselves. The world is too full of that.

------
Simon_says
Has nobody at Quanta Magazine heard of the Many Worlds Interpretation? I'm
tired of these awful articles from Quanta.

~~~
TheOtherHobbes
This 'awful article' is about experimental tests of a working theory.

Can you provide a reference explaining how MWI might be falsified
experimentally?

~~~
Simon_says
MWI can be differentiated from the Copenhagen Interpretation via experiment as
soon as the Copenhagen Interpretation decides what a "measurement" is, when it
happens, and when it doesn't.

As it is now, the Copenhagen Interpretation is an incomplete theory because it
doesn't give you a formal algorithmic way to know when to apply the
measurement operator. The answer is whatever it needs to be to make the answer
come out right.

To answer your question more directly, MWI could be falsified in a lot of
ways, but one would be if structures larger than a certain size don't self-
interfere.

~~~
kgwgk
Is the MWI more complete in this regard? How does it decide what a "branching
event" is, when it happens, and when it doesn't?

~~~
Simon_says
Yes it is. MWI's only rule is that reality evolves according the the
Schrödinger equation. It's what would get if you took collapse out of the
Copenhagen Interpretation.

The idea of "branching" is a human-legible metaphor for describing the
resultant behavior. But the math doesn't have branching.

~~~
kgwgk
According to you the MWI cannot make any predictions about experimental
outcomes then?

~~~
Simon_says
How did you pick that up from what I said?

~~~
kgwgk
Maybe I misunderstood you. How does the MWI decide what a measurement is, when
it happens, and when it doesn't?

~~~
Simon_says
According to MWI, there's no special privileged operation called "measurement"
like there is in the Copenhagen Interpretation. Of course there's still
scientists looking at stuff, but according to MWI, the scientists and their
machines are made of matter that follows the same law of physics of the matter
they're studying.

~~~
kgwgk
How does the MWI make predictions about experimental outcomes then? Unitary
evolution doesn’t cut it.

~~~
Simon_says
Sure it does. You should read about MWI.

~~~
kgwgk
You do need something more than Schroedinger’s equation to get definite
outcomes and probabilities. Apparently when and how it happens are not
questions so easy to answer, are they?

MWI may be what you would get if you took collapse out of the Copenhagen
Interpretation, but to get the same predictions you have to put the collapse
back in somehow. And the question of what is a measurement cannot be
completely avoided.

~~~
Simon_says
OK, I guess you need Born's Rule, too. But so does every other interpretation,
including the Copenhagen one. My uneducated guess is that there's some
geometric reason for it that hasn't been discovered yet.

> to get the same predictions you have to put the collapse back in somehow.

This part is just not true. There's no collapse and no formal "measurement" in
MWI. You seem like you might have some misconceptions about MWI. I would urge
you to read more about it rather than discussing it with amateurs on HN.

~~~
kgwgk
> OK, I guess you need Born's Rule, too. But so does every other
> interpretation, including the Copenhagen one.

Sure (note that some people claim that Born's rule can be derived from other
considerations but the issue remains highly controversial). And at what point
do you need Born's Rule? (hint: m9t)

I've read enough about the subject to know that there is not "one" MWI but
many different variants (that's of course also the case for the Copenhagen
Interpretation, which is even less well-defined). DeWitt (who introduced the
"many-worlds" label, by the way) seemed to believe that branches are more
"real" than Everet did, for example. And I don't know where do you put the
"consistent histories" formulation (some people say it's many-worlds, some
people say it's neo-Copenhagen).

Anyway, you have not yet explained to me how do you get predictions about
experimental outcomes.

This is what happens in standard QM:

1) You have a wave function describing the system of interest (psi), we assume
it was prepared in that state.

2) You can calculate the possible outocomes of a measurement, say +1 with
probability 50% and -1 with probability 50%.

3) You perform the experiment, the outcome is +1

4) Immediately after the measurment you describe the system with the wave
function psi+ that is the eigentstate corresponding to the outcome +1 (for -1
it would be psi-).

5) You can use the wave function psi+ to make predictions for new experiments,
for example repeating the measurement before the state has time to evolve
(according to Schroedinger's equation) you will get +1 again.

If you think the MWI gives the same predictions, this is my best guess of how
you may think it works:

0) There is a universal wave function describing the whole UNIVERSE (PSI)

1b) Some mathemagical operations can be used to extract a wave function that
describes the physical universe (the branch of the UNIVERSE that we are in,
however you want to call "our world" out of "many worlds"). Let's say we
decompose it as psi for the system (which in "our world" has been prepared in
that state, so it's described as in the standard formulation) and psi' for the
rest of our universe (not the UNIVERSE, mind you).

2) You can calculate the possible outocomes of a measurement, say +1 with
probability 50% and -1 with probability 50%.

3b) You perform the experiment, but you don't really do perform an experiment
because the only thing that happens is that the UNIVERSE evolves unitarily and
PSI follows Schroedinger's equation in a completely deterministic fashion.
However, somehow _you_ do get a random outcome. You do also get the other
outcome, but that's in another branch (or whatever). In your branch, you get
an outcome with is either +1 or -1 as predicted. Remarkably, if you repeat the
experiment the frequency of the different outcomes will match the predicted
probabilities (unless you're unlucky, because there are other branches where
you don't get the expected frequencies and your predictions will fail).
Anyway, say that in your branch you got the result +1.

4b) You do the mathemagical operations of step (1b) again, but in this case to
get the wave function describing your branch you don't say "in my world I have
a system prepared as such and such", you say "in my world I had a system
prepared as such and such and then I did a measurement and I got +1 as
outcome". This gives you the wave function for your universe, that can be
factored as psi+ (because you got the result +1) for the system of interest
and psi'' for the rest of our universe.

5) You can use the wave function psi+ to make predictions for new experiments,
for example repeating the measurement before the state has time to evolve
(according to Schroedinger's equation) you will get +1 again.

As far as I can imagine, to make predictions about "your universe" you have to
include the outcomes of the previous measurements to get the right description
for the system (i.e. you need to recover the "collapsed" wave function psi,
even though PSI doesn't collapse). That's what I mean by putting the collapse
back in somehow.

------
phkahler
I always say _nothing_ is going on during wave function collapse. You can't
tell the difference between a particle whose wave function has collapsed and
one that hasn't. Since there is no change it seems nothing actually happened
;-)

~~~
antonvs
> You can't tell the difference between a particle whose wave function has
> collapsed and one that hasn't.

Could you expand on this? For example, in the double slit experiment, it's
easy to observe the effects of "uncollapsed" (superposition) behavior.

Perhaps you're saying that we can't observe a particle without collapse
occurring (from a Copenhagen perspective), so we never directly observe an
uncollapsed particle. But we can observe the distinct effects that uncollapsed
particles have, which seems to undermine your claim.

~~~
monocasa
The double slit experiment has been replicated in fully classical macroscopic
systems.
[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97.154101)

~~~
Iv
Except that a droplet is not a single particle. We have known for a long time
that a droplet is divisible and can spread as a wave.

Quantum theory states the existence of quantums: indivisible parts of matter
and energy.

~~~
monocasa
The interaction in that experiment doesn't have the droplet dividing or
dissolving into the wave. For the purposes of that experiment, the droplet is
a quantum unit.

------
konschubert
Nobody really cared about this question when I was studying physics. It drove
me crazy.

The amount of handwaving was insane.

