

Wave-particle duality is the uncertainty principle in disguise - mherrmann
http://phys.org/news/2014-12-quantum-physics-complicated.html

======
TTPrograms
This article gets the paper totally wrong. Learning that the uncertainty
principle gives rise to phenomena that popular science calls "wave-particle"
duality was taught to me in quantum 101.

From the abstract: "This idea...upper bounds the sum of the interference
visibility and the path distinguishability. Such wave-particle duality
relations (WPDRs) are often thought to be conceptually inequivalent to
Heisenberg's uncertainty principle, although this has been debated."

I'm quite surprised that anyone would think that those relationships wouldn't
be direct consequences of uncertainty. They are directly analogous to position
and momenta quantities.

"Here we show that WPDRs correspond precisely to a modern formulation of the
uncertainty principle"

No surprises here.

~~~
Osmium
> Learning that the uncertainty principle gives rise to phenomena that popular
> science calls "wave-particle" duality was taught to me in quantum 101.

Agreed, just to expand on that: The hand-wavey relationship between wave-
particle duality and the uncertainty principle was taught to me like this...

Imagine you have a sine wave. It has a well-defined momentum, p, given by its
wavelength. However, since it's infinitely periodic in space, it has no well-
defined position, x.

Now consider a wave-packet[1]. This has a very well-defined position, given by
the centre of the packet, but no well-defined wavelength/momentum.

These are the limiting cases. The Heisenberg uncertainty principle, ΔpΔx >=
ħ/2, just quantifies this relationship. If you try and measure x, the
wavefunction ends up more 'packet-like' and you can no longer measure p as
well, and if you try and measure p, the wavefunction ends up more 'wave-like'
and you can no longer measure x as well. That's all there is to it.

It's not exactly rigorous, but I don't think this explanation is too
misleading either (though I'd be happy to learn otherwise!).

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

~~~
monochromatic
Just a minor correction, but a wave packet isn't the limiting case. Something
that approaches a delta function is.

------
HCIdivision17
Perhaps I was mistaken, but I thought this was sort a necessary revelation
when learning about QM. It's been years since I took Atomic Physics, so
perhaps this is something much ... deeper?

Was it a serious idea that upon measurement the particle _literally_ lost it's
wave properties? Like, there were actually two separate approaches to the
math? I know we made some efforts to simplify in class, approximating as
particles and such due to the drastically simplified math, but we all knew
that was happening; the professor was reasonably explicit about it. EDIT: As
in, I remember us going over the evolution of a waveform, and how boundaries
affect the solutions, and how uncertainty causes the particles to have field
distributions that just happen to be the wave solution of the particle.

I'm completely serious about being confused here: QM is super easy to
misunderstand, and I'd love to feel the eureka this article is trying to
convey.

~~~
cynicalkane
The mythical "wave or particle" interpretation of wave-particle duality is
oddly persistent, and is not only pervasive in popular science but I've heard
it repeated by serious physicists also, even though it's unsupported by the
very math they work on.

~~~
wellwell123
It's not clear to me what you think the "wave or particle" interpretation _is_
, nor what you think is wrong with it, nor what you think you've heard
"serious physicists" say.

------
doctoboggan
<armchair physicist>

I love reading about these things. It seems like information is slowly being
raised to the same level as energy and matter (All being different ways to
look at the same phenomenon)

The universe does care about preserving information, and there are physical
limits on it's propagation and consumption. This is similar to formalizing
energy as a concept, which can manifest itself in many different ways
(kinetic, potential, radiation, etc) Once we wrapped it up in a concept we
were able to reason about it in a more abstract way. The same process is now
happening with the concept of information. This is leading to breakthroughs in
computing (ML, pattern recognition, etc) and physics. I am excited for the
future of information theory.

</armchair physicist>

~~~
PeterWhittaker
Take a look at David Deutsch's work on quantum mechanics, information theory,
quantum computing, etc. I especially like
[http://arxiv.org/abs/1405.5563](http://arxiv.org/abs/1405.5563)

Cool stuff.

(Related, possibly, is the concept that QM is simply probability with ranges
from [-1,1] instead of the traditional [0,1]: Negative probabilities imply
things that happen that we cannot measure. Or something like that, it's been a
while.)

(Now if only I could find that wicked cool graphic showing the hierarchy of
mathematics (ZFC or something near the bottom) and physics (QM and GR, sitting
atop currently irreconcilable branches of math)....)

~~~
throwaway344
I found the diagram you mentioned:

[http://24.media.tumblr.com/tumblr_lil7n0uiEl1qfjvexo1_500.pn...](http://24.media.tumblr.com/tumblr_lil7n0uiEl1qfjvexo1_500.png)

~~~
halo
The original source of this diagram: [http://arxiv.org/abs/gr-
qc/9704009](http://arxiv.org/abs/gr-qc/9704009)

------
nilkn
I'm no theoretical physicist, but I thought wave-particle duality was solved a
long time ago by quantum field theory. Feynman, for instance, was able to
formulate electromagnetism fully in terms of particles, using his path
integral formulation. Since this formulation computes probability amplitudes,
it seems to fit well with the uncertainty principle. This approach gave rise
to QED, the most accurate physical theory ever developed.

In other words, without more detail, and not being a physicist myself, I can't
tell what's actually new here.

~~~
brute
You are right, quantum field theory is perfectly capable of explaining both
phenomena (but not in terms of particles but rather in terms of descrete field
modes). The idea that it is connected to the uncertainty principle is even
older.

Reading this article made my kind of angry, as it implies that wave-particle
duality was not understood until now. Like here:

> _" has proved that two peculiar features of the quantum world previously
> considered distinct are different manifestations of the same thing"_

I frequently encounter this _mysterious_ yet-to-be-understood duality in
esoteric discussions (with all kinds of wrong conclusions of course). I think
that ultimatly, whenever we teach someone about the paradox behavior of
particles/waves, the very next sentence should be "but this is perfectly
understood and not paradox at all". While we are at it, instead of saying

> _" this is what happens until you sneak a look at which slit a particle goes
> through"_

we should say, "this is what happens unless you bombard the particle with
photons, which changes everything, of course".

~~~
judk
Richard Feynman said that only about 6 people understand quantum
mechanics."perfectly understood" is an overstatement.

~~~
ikeboy
I understand it perfectly. I'm not sure what an eigenvalue is, though, but I'm
sticking to my claim of understanding.

------
ajkjk
[http://arxiv.org/pdf/1403.4687v2.pdf](http://arxiv.org/pdf/1403.4687v2.pdf)
is the actual preprint.

I'm finding it pretty hard to parse. It seems to be a reformulation of what we
already know about non-commuting observables in terms of entropy/ignorance. I
might need to read some of the cited papers to understand exactly if that's
significant and why.

------
jwmerrill
I know that many people feel that you have to write like this to get anyone to
pay attention (maybe they're right), but after enough years in physics, the
breathlessness of this kind of reporting really starts to grate.

> international team of researchers has proved

> made the breakthrough

> discovered the 'Rosetta Stone'

I haven't read this particular article carefully, but I think it's safe to
assume that words like "reformulated," "clarified," "extended," or "embedded
in a new framework" would be more appropriate. Which is fine. New and better
explanations are useful and important.

But despite what you may read, we don't find a new Rosetta Stone for quantum
physics every year.

I get much more excited about science writing where the reader can come away
with a better understanding of an actual concept, rather than just a sense
that someone somewhere is doing something hard and smart.

~~~
jwmerrill
To put it another way, I think you are much more likely to absorb powerful
ways of thinking while being entertained if you read [https://what-
if.xkcd.com/](https://what-if.xkcd.com/) than if you read
[http://phys.org](http://phys.org)

What If shows by example that you can apply quantitative reasoning to the most
outlandish situations and actually draw interesting distinctions and
conclusions.

Phys.org style reporting urges you to accept by argument from authority that
the universe is mostly made of dark energy, and gravity is made out of
particles traveling backwards in time through 11 dimensions or whatever it was
I read last month I can't quite remember. These ideas come from actual serious
research, but they aren't powerful when presented this way because you can't
apply this style of reasoning successfully unless you have tons and tons of
training working in the specific areas they report on.

------
noobermin
[http://arxiv.org/pdf/1403.4687v2.pdf](http://arxiv.org/pdf/1403.4687v2.pdf)

------
SO2Bohr-ed
Feynman on the "work" of gravity theorists: "something correct that is obvious
and self-evident, but worked out by a long and difficult analysis, and
presented as an important discovery"

To be fair, this case is the fault of the journalist(s), not necessarily the
research itself.

------
nathanathan
> The particles pile up behind the slits not in two heaps as classical objects
> would, but in a stripy pattern like you'd expect for waves interfering. At
> least this is what happens until you sneak a look at which slit a particle
> goes through - do that and the interference pattern vanishes.

Could any of the physicists here verify whether it is the case that detecting
the slit the particle passes through destroys the interference pattern. I had
the impression that the path of the photons could be determined without
destroying the pattern.

~~~
TTPrograms
Detecting which slit does destroy the interference pattern, yes.

------
shoo
Tangential comment: if you are interested in quantum mechanics then you may be
interested to read about "Bohmian Mechanics".

I became interested in Bohmian Mechanics after reading an email exchange
between Sheldon Goldstein & Steven Weinberg [1]. It contains a few quite
entertaining quotes, in particular:

    
    
        > And since Bohm’s equations make exactly the same
        > predictions as those of ordinary quantum mechanics,
        > it is not clear what is accomplished by adding the
        > complication of guiding waves, except to restore a
        > sense of sanity to the whole affair.
    

Scott Aaronson has a brief introduction to Bohmian Mechanics at the end of his
notes on decoherence and hidden variables [2]:

    
    
        > Again, the amazing thing about this theory is that it's
        > deterministic: specify the "actual" positions of all the
        > particles in the universe at any one time, and you've specified
        > their "actual" positions at all earlier and later times.
    

Scott's perspective is particularly interesting because he points out some
limitations of Bohmian Mechanics. To paraphrase badly, the deterministic
particle trajectories obtained from Bohmian Mechanics rely upon dealing with
actual particles, position, momentum, in infinite dimensional spaces. It
doesn't give you deterministic behaviour in the finite dimensional spaces that
computer scientists prefer.

Some proponents of Bohmian Mechanics point out that the theoretical
predictions of deterministic particle trajectories for the famous double-slit
experiment agree with recent experimental results.

Theoretical predictions: see Figure 1, page 14 of [3], which is an adaptation
of a figure from [4].

Experimental results: see Figure 3, page 1171 of [5].

Note that [5] carefully frame their experimental results as "Using weak
measurements, however, it is possible to operationally define a set of
trajectories for an ensemble of quantum particles".

For further reading, please see [6] for links to introductory writing about
Bohmian Mechanics.

[1]: [http://inference-review.com/article/on-bohmian-
mechanics](http://inference-review.com/article/on-bohmian-mechanics)

[2]:
[http://www.scottaaronson.com/democritus/lec11.html](http://www.scottaaronson.com/democritus/lec11.html)

[3]: [http://www.mathematik.uni-
muenchen.de/~bohmmech/BohmHome/fil...](http://www.mathematik.uni-
muenchen.de/~bohmmech/BohmHome/files/Introduction.pdf)

[4]: "Quantum interference and the quantum potential" \-- Philippidis,
Dewdney, Hiley -- Il Nuovo Cimento B, 1979; [paywalled article] --
[http://link.springer.com/article/10.1007/BF02743566](http://link.springer.com/article/10.1007/BF02743566)

[5]: "Observing the Average Trajectories of Single Photons in a Two-Slit
Interferometer" \-- Kocsis, Braverman, Ravets, Stevens, Mirin, Shalm,
Steinberg; Science, 2011 -- [PDF]
[http://materias.df.uba.ar/labo5Aa2012c2/files/2012/10/Weak-m...](http://materias.df.uba.ar/labo5Aa2012c2/files/2012/10/Weak-
measurement.pdf)

[6]: [http://www.bohmian-
mechanics.net/whatisbm_introduction.html](http://www.bohmian-
mechanics.net/whatisbm_introduction.html)

~~~
ajkjk
I don't find Bohmian mechanics any more satisfying than regular QM. In an
effort to formulate a 'hidden state' that makes QM make.. sense.. you add a
guiding wave equation that depends on the state of the universe? That's not
better at all.

I'm all for trying to formulate QM in a way that doesn't offend basic
sensibility, but I'm not convinced this is it. I don't think deterministic-
but-nonlocal is better than nondeterministic-but-local.

~~~
jostylr
Standard quantum mechanics only has the wave function. The thing you object to
in Bohmian mechanics is the wave function. Both are nonlocal. You should
object to both.

In particular, standard quantum mechanics gets results of experiments by
collapsing the wave function. That is, it takes something which is universe
spanning and it collapses it to something, with the collapse based on what a
physicist does in a laboratory. That, of course, should seem rather absurd. To
get around that, you have to add something either particle positions, a matter
density, flashes, etc. You need results to experiments in a theory. Without
something (and collapse from experiments is adding the collapse mechanism +
external experiments, whatever that means), you do not get results. So the
theory is a complete failure.

Only many worlds is not non-local (it adds a matter density by integrating the
wave function appropriately), but they escape it by not having results of
experiments at the time that history records there were results. I think. That
theory makes me a little dizzy at times.

Also, the hidden state of BM is not hidden. We have a basic sense that stuff
has positions. That may be an illusion, but Bohmian mechanics says that we can
choose a theory that does not have that be an illusion. The thing that is
hidden, that is deduced from the behavior of stuff which we see, is the wave
function.

What BM explains is why it seems that our world is particle-like and why we
have singular outcomes to experiments. All done naturally. Sure, the nonlocal
nature is a problem that we are still trying to understand, but so what? No
one has got the full story yet and so BM is viable and gives a great intuition
about QM (the standard QM (operators, collapse) is a derivable formalism, like
thermodynamics from Newtonian Mechanics).

