
A new experiment hints at surprising hidden mechanics of quantum superpositions - kylnew
https://www.scientificamerican.com/article/quantum-physics-may-be-even-spookier-than-you-think/
======
amelius
Question about the basics. They say:

> And it gets weirder: Measuring which slit such a particle goes through will
> invariably indicate it only goes through one—but then the wavelike
> interference (the “quantumness,” if you will) vanishes. The very act of
> measurement seems to “collapse” the superposition. “We know something fishy
> is going on in a superposition,” says physicist Avshalom Elitzur of the
> Israeli Institute for Advanced Research. “But you’re not allowed to measure
> it. This is what makes quantum mechanics so diabolical.”

But (afaik) measuring means disturbing, because you have to exchange some
energy with the system in order to perform the measurement.

So in the above quote, if you replace "measuring" by "disturbing", then all of
a sudden, the whole paragraph doesn't make any sense ... Can anyone clarify
this?

~~~
gdavisson
>But (afaik) measuring means disturbing, because you have to exchange some
energy with the system in order to perform the measurement.

This isn't necessarily true; QM allows what's called interaction-free
measurement. Say you have a particle that's in a superposition of two states
(or places, or whatever), which I'll call A and B. You do something that'll
interact (and detect) it if it's in A. If you _didn 't_ detect it, that means
you've effectively measured it as being in B.

You can also turn it around, and use a particle in a superposition to see if
an object (that it _would_ interact with if it's in state A) exists; if the
object exists, the possibility of interacting with it will destroy
interference effects between the parts of the particle's superposition,
allowing you to infer the existence of the object. The Elitzur–Vaidman bomb
tester
([https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester](https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester))
is an extreme example of this: it lets you verify that a bomb is "live" (will
explode if interacted with), with only a 50% chance of setting it off.

Another example is the interaction-free version of the quantum Zeno effect.
The quantum Zeno effect is that (under the right circumstances) a particle can
be trapped in a particular state by continuously measuring whether it's still
in the state. In the interaction-free version, you use something that'll
interact if the particle ever leaves the state... and it never does. This has
actually been done; see
[https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-2...](https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-20150415)

~~~
Moodles
Very interesting, thanks.

This is a nice video to expose the weirdness of the double slit experiment:
[https://www.youtube.com/watch?v=DfPeprQ7oGc](https://www.youtube.com/watch?v=DfPeprQ7oGc)

~~~
skipperr
Does anyone know of _actual videos_ of the experiment? All the videos I find
online only show the wave interference pattern, but not the particle-like
pattern shown when measuring/observing the particles.

~~~
spindle
[http://www.physicscentral.com/experiment/askaphysicist/physi...](http://www.physicscentral.com/experiment/askaphysicist/physics-
answer.cfm?uid=20111017091810)

(I refer you to the photos there, which I think are what you want. I haven't
read the text.)

------
jerry40
Does it mean we can create some machine (let's call it timegraph):

\- before some sport event (NBA finals 7th match) we write a question 'who
will win, A (1st team name) or B (2nd team name)?'

\- do 2 experiments with photon and slits:

* (A)

* (B)

\- do not measure their results until tomorrow

\- tomorrow (after the match is over) you measure result of the experiment of
the team that won and destroy results of the another experiment

\- (returning to the previous day): look which experiment shows inteference
and which doesn't

~~~
yorwba
> look which experiment shows inteference and which doesn't

This is also a kind of measurement, and one you can't meaningfully perform
before the other. By the time you know the interference pattern, it's already
too late to decide whether you want to measure the intermediate position.

You might try to measure without looking at the results, but that doesn't
change anything about the fact that the interaction happened. (Superpositions
do not collapse by being observed by a human, they collapse by interacting
with the rest of the world.)

~~~
jerry40
> Superpositions do not collapse by being observed by a human, they collapse
> by interacting with the rest of the world.

But the article says: "Measuring which slit such a particle goes through will
invariably indicate it only goes through one—but then the wavelike
interference (the “quantumness,” if you will) vanishes. The very act of
measurement seems to “collapse” the superposition." "Aharonov’s approach is
called the two-state-vector formalism (TSVF) of quantum mechanics, and
postulates quantum events are in some sense determined by quantum states not
just in the past—but also in the future. "

Doesn't it mean that future measurement 'pushes' superposition into a definite
state in the past?

------
vorg
> The apparent vanishing of particles in one place at one time—and their
> reappearance in other times and places—suggests [...] a particle’s presence
> in one place is somehow “canceled” by its own “counterparticle” in the same
> location. [...] These putative counterparticles should possess negative
> energy and negative mass, allowing them to cancel their counterparts.

I thought negative energies in Quantum Mechanics give rise to senseless
infinities that can't be eliminated. What gives?

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bob_theslob646
The website is down.

~~~
scottyelich
[http://www.thewebsiteisdown.com/](http://www.thewebsiteisdown.com/)

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cfadvan
This is a horrendous article, clearly written by someone who doesn’t grasp the
basics. I gave up when they conflated how the double-slit experiment
demonstrates wave-particle duality with a demonstration of superposition. I
can’t believe how much of a joke SciAm has become, it makes me sad.

~~~
russdill
You can derive the result by treating the particle like a wave, but that is
clearly a false/incomplete model. There is only one particle, the only thing
it can be interfering with is itself. It's in a superposition of going through
both slits.

~~~
cfadvan
A single photon will register one click on the detector, it’s only with an
ensemble of many photons that an interference pattern emerges.

~~~
roywiggins
You will build up an interference pattern if you fire single photons, one at a
time, spread out over as long a time period as you like.

~~~
axilmar
Since the interference pattern is built over time, how do we know a particle
is actually a wave that interferes with itself and not a particle that selects
a different path from a set each time it is fired?

~~~
ben_w
If it was acting like a particle and selecting a different path each time, the
expectation would be only two peaks, each following line-of-sight to the
source:

    
    
        —————|—|—————
    

What is seen is many peaks, as if it was a wave:

    
    
        —+—+—|—|—+—+—

~~~
TheOtherHobbes
That doesn't solve the problem, because the question is really how the set of
paths available to the particle is constrained.

If you go Full Copenhagen, there are no waves, no particles, and no paths.
There are only evolving probabilities - which act in unintuitively non-local
but predictable ways - and events which appear to sample one possible result
from the current state of a probability density.

The probabilities can be composite, which allows for entanglement and
superposition.

The probability part is fairly well understood, or at least fairly easy to
calculate.

The exact nature of an event/measurement/whatever is still a complete mystery.

But there is nothing in a naive wave/particle/path model that makes it any
less mysterious or easier to understand.

------
oldandtired
What intrigues me about the entire situation is that the scientists are not
standing back and looking at all the assumptions they use.

Simple things like that the belief that there are "virtual" particles
"popping" in and out of existence and they don't affect the path of "real"
particles like photons or electrons.

Simple things like what processes is the matter that exists each side of the
slits undergoing in relation to the particles that pass through the slits and
how each affects the other.

Simple things like what is a photon or other such particle.

Is the particle/wave duality idea hindering or helping the further
understanding of these processes? Is quantum mechanics the best approximation
that we have or are there other ideas that would simply the models in use?

I liken it to a programming project that has gone down one path based on a
series of assumptions and when these assumptions are found to be incorrect or
different from what is actually there, it becomes very difficult to change
direction without doing a complete rebuild.

If one looks at the history of investigations over the last 100 years or so,
one finds that there are a variety of ideas that never gained any traction at
the time of proposal. Yet, today appear to give a handle on some of the
puzzles that are being found. These ideas are ignored because they didn't gain
traction at the time of first proposal.

Mayhaps, it would be worth spending some time to investigate to see if they
have any merit. They may not, but it can't hurt to see if they have.

~~~
narag
_Is the particle /wave duality idea hindering or helping the further
understanding of these processes?_

Is that an idea? I thought that it was an observed fact. Some people believed
that light was a wave, some other people believed that it was particles so
they tried their assumptions and both calculations were correct, as long as
you don't mix them. Maybe it makes more difficult to understand reality, but
sweeping duality under the rug doesn't seem right.

~~~
oldandtired
We observe certain things, the interpretation is the question. I am in no way
saying that duality should be swept under the rug, but rather have the entire
interpretation and information held up to the light to see what other
information can be brought to bear to the problems this duality give rise to.

As I intimated, should the observations that give rise to the interpretation
that there are "virtual" particles that pop in and out of existence be brought
to bear on this particular problem? There are those who would have you believe
that the quantum vacuum is not empty but full of these "virtual" particles.
Does this idea give any explanation to why we see a wave phenomena? Or this
there something else that might give a better handle on the subject matter.

Is how we measure adding additional things that give rise to the effects we
see? There are so many questions that I do not see being asked.

We should not be confusing the evidence we see with the interpretation we come
up with. Too often, the interpretation of the facts is treated as if it is the
fact.

~~~
Nemrod67
One thing kinda like it is the optical mechanics used to describe a mirror,
they use virtual points that are beyond the mirror (if my French Lycée memory
serves me well) but still produce the correct answer.

In a way you could say that wave physics (from the Lycée too) behave like
described by the equations... even though they migh be represented as in a
fluid (ripples on water molecules-meta-material).

Sometimes it's hard to reconsider the model and the perception of reality we
have.

Nice to see you kept commenting after having so many downvotes ;)

~~~
oldandtired
Well being ill in bed does put a crimp in responding. What I find interesting
is the different views that come out when someone does make a comment that
others find not satisfactory. Further grist for the mill in understanding
people and the nature of the universe around should be welcome.

The wonderful thing is that there are so many questions and so few real
answers. It means that we are able to have fun in investigating the universe
around us. The child's "why?" is a wonderful question and should be delighted
in.

