
Double Slit Experiment No Mystery - fremden
https://billwadge.wordpress.com/2019/10/25/double-slit-experiment-no-mystery/
======
headmelted
This misses whole swathes of the double slit experiment.

It doesn't cover the Observer problem, which means it doesn't acknowledge
duality, which (you would have to assume) means it doesn't even recognise
delayed-choice quantum erasers.

At the risk of getting downvoted to oblivion I feel like the author needs to
study up on quite a bit more of this research as there's a much larger body of
evidence than that which is countered in the article.

~~~
missosoup
The author of TFA has little understanding of what they're talking about and
misses all of the nuance of the double slit experiment and its relationship to
other QM phenomena like the Dirac three polarizer experiment[1] and delayed
choice quantum eraser experiment[2]. The three polariser experiment in
particular makes it almost impossible to argue that light is a particle even
if it had hidden state, unless that hidden state was propagating faster than
light.

TFA reads like /r/iamverysmart content or a medium effort troll post. I don't
understand how this ended up on HN frontpage.

[1]
[http://www.informationphilosopher.com/solutions/experiments/...](http://www.informationphilosopher.com/solutions/experiments/dirac_3-polarizers/)

[2] [https://en.wikipedia.org/wiki/Delayed-
choice_quantum_eraser](https://en.wikipedia.org/wiki/Delayed-
choice_quantum_eraser)

~~~
grumpy8
I didn't know about Duane's hypothesis even though I read a few books
mentioning the double slit experiment so I'm glad I read the article. In the
end, as much as the wave/particle duality seems mysterious at the moment, I'm
convinced we'll find a quite boring/straightforward explanation for it.

------
z92
Actually the mysteries are deeper. This one can't explain "Delayed Choice
Quantum Erasure" experiments, which is also a double slit experiment. But the
arrangement allows you to detect the path _AFTER_ the photon has traveled
through one of the slits and hit the surface. Now _after_ the photon has hit
the surface you can decide whether to detect its path or not. A delayed
choice. But based on your later choice the interference pattern of the photon
from a time from the past appears, or doesn't.

This is the main reason why some scientists come up with many world
interpretation. This article doesn't even touch the surface of it.

[https://en.wikipedia.org/wiki/Delayed-
choice_quantum_eraser](https://en.wikipedia.org/wiki/Delayed-
choice_quantum_eraser)

~~~
quattrofan
How can a "thought experiment" demonstrate anything?

~~~
eternalny1
Well, from the article on the dual-slit experiment ...

> What many don’t realize is that the double slit experiment (with particles),
> proposed by Feynman in 1963, was for decades only a thought experiment.
> Finally, in 2013, it was successfully performed with electrons.

~~~
z92
The author implied it was a thought experiment for "particles" like electron,
though actually demonstrated from the very beginning for non-particles such as
photons.

That claim still might not be true.

------
fsh
It is very common in physics that the same phenomenon can be explained using
different concepts. Treating particles hitting a regular structure as
diffraction of de Broglie waves gives exactly the same results as the momentum
transfer approach the author prefers. In fact, both can be derived from
standard quantum mechanics and both are widely used, depending on which is
easier in the particular case.

However, scattering particles off objects is only one of many experiments they
can be done. For instance, one will have a hard time explaining how atoms work
without using some kind of wavefunction description of the electrons.
Furthermore, as other commenters have mentioned, there are more sophisticated
versions of the double-slit experiment, such as the delayed-choice quantum
eraser, where the "quantum weirdness" simply cannot be argued away.

Frankly, it is pretty insulting by the author to assume that physicists
intentionally choose the "more complicated" explanation over the "simpler"
one.

~~~
dekhn
+1 for the delayed choice quantum eraser as being the one experiment that
makes it more or less impossible to argue against quantum weirdness.

~~~
aeternum
The delayed choice quantum eraser is often explained as being more 'spooky'
than it actually is.

The interference patterns only appear when you compare coincidences between
the detectors. It's easy to see you could get an interference pattern after
the fact by taking a bell curve (no interference) and subtracting out a sine
curve (interference).

~~~
ddalex
but how do you get the sine curve in the first place? because you're not
subtracting a sine curve, you're subtracting non-coincidences, for which there
is no reason they would be on the sine curve

------
4ad
> What many don’t realize is that the double slit experiment (with particles),
> proposed by Feynman in 1963, was for decades only a thought experiment.
> Finally, in 2013, it was successfully performed with electrons.

Total crackpot, electron diffraction was observed in 1927 by several different
groups of people[1][2] some of them who later in 1937 shared the Nobel Prize
in Physics[3].

[1]
[http://www.nature.com/nature/journal/v119/n3007/pdf/119890a0...](http://www.nature.com/nature/journal/v119/n3007/pdf/119890a0.pdf)

[2]
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1085484/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1085484/)

[3]
[https://www.nobelprize.org/prizes/physics/1937/summary/](https://www.nobelprize.org/prizes/physics/1937/summary/)

~~~
jeremyjh
Came here to say this, but the 1927 experiment was only the first with
electrons. The first experiment with light was in 1803 by Thomas Young. This
was the foundation of the wave theory of light.

I stopped reading the article at this point.

[http://www.cavendishscience.org/phys/tyoung/tyoung.htm](http://www.cavendishscience.org/phys/tyoung/tyoung.htm)

------
losvedir
Getting some real crackpot vibes here.

How does Duane's hypothesis account for the diffraction pattern going away if
the particular slit the particle goes through is measured?

~~~
empath75
If he had read the article that he linked to, he’d see that duane’s hypothesis
is a special case that can be derived from quantum mechanics.

I don’t believe that you can derive the rest of quantum mechanics from duane’s
hypothesis.

~~~
gus_massa
I agree. The relevant paragraph from
[https://en.wikipedia.org/wiki/Duane's_hypothesis](https://en.wikipedia.org/wiki/Duane's_hypothesis)

> _Quantum mechanics_

> _According to Ballentine, Duane 's proposal of quantum translational
> momentum transfer is no longer needed as a special hypothesis; rather, it is
> predicted as a theorem of quantum mechanics.[22] It is presented in terms of
> quantum mechanics by other present day writers
> also.[23][24][25][26][27][28]_

------
l33tman
I'm a physicist and I'm sorry to say this blogpost is just drivel (I'm sure
the original works linked are perfectly fine).

------
scotty79
Duane doesn't explain double slit experiment. It says momentum exchnge is
quantized if you direct particle at lattice and this gives same pattern as
interference.

If you consider single electron in this interpretation, point like particle
without any associated wave or ability to be in more than one point of space
at the same time ... when it is directed and passes through one of the slits,
it produces the pattern but only if the it potentially could have been
directed at the other slit.

How the hell what could happen with a particle (but didn't) could influence
what actually happened to the particle (quantized, not normal momentum
exchange)?

\--

For me the way to deal with weirdness of double slit experiment is that
electrons and such are just waves, all the time. Fuzzier, sharper but always
waves. The only thing that makes us think that's not the case is that, when
they exchange energy and momentum, it's governed with same equations as if
they were two balls bouncing off of each other.

The truth in my opinion is that electrons don't bounce like tiny balls, but
balls exchange energy and momentum as if they were huge, very sharp electrons.
Macroscopic balls are waves too, but imperceptibly because they are build out
of bazzilion particle waves all tangled together into a very constraining
packet.

~~~
scottlocklin
If you have wave-systems with high enough fidelity (Q-factor in a cavity for
example), they behave a lot like particles in certain energy/time regimes.
People yelling from a mountain top hear echoes.

~~~
scotty79
People have always thought about "atoms" of matter as being primary units of
existance and waves being some second order, compound behavior. That's why it
was so hard to get rid of the idea of ether.

------
mikhailfranco
QM is easy to describe. There is wave-particle duality, but not at the same _'
time'_, only in a strict alternating sequence of propagations and
interactions.

 _Propagation_ is completely wave-like. It is unitary and time reversible,
without loss of information. There is no particle there, and no _' there'_
there, as the propagation is non-local. Waves have complex amplitudes.
Interference happens because that's what waves do. Propagation is described by
the Schrodinger/Dirac equation.

 _Interaction_ is completely particle-like. The interaction is non-unitary and
time-irreversible. The interaction may also be known as _detection_ or
_measurement_.

The wave only contributes the probability of the interaction happening, which
is described by the Born Rule. Amplitudes are complex, so squaring them with
the Born Rule can give negative probabilities, which is why the double-slit
experiment gives many fringes.

The interaction is local, because it creates the _' there'_. Positions and
distances are created by the interaction as a specific spacetime event. Hence
the aphorism _spooky distance at an action_ , meaning that (inter)action is
fundamental, and distance is a strange thing created as a byproduct.

There are _conservation laws_ that balance some incoming and outgoing
properties, _scattering laws_ that determine how the interaction happens for
various _forces_ , and some constraints between the knowable values of
_commuting pairs_ of observable state (particle properties) that are revealed
(entangled) by the interaction. These constraints are also known as
Heisenberg's _Uncertainty Principle_ , but are really just direct and obvious
mathematical properties of wave packets, as described by Fourier Analysis.

After the interaction, there will be propagation of new waves for the outgoing
_particles_ (meaning a bundles of state). The interaction results in an
_entanglement_ (correlation of future possible interactions) between these
distinct outgoing waves.

That's it.

~~~
justinmchase
> QM is easy to describe.

That's easy for you to say!

~~~
zarmin
I understand QM intuitively. I just don't wanna explain it to you is all.

------
colanderman
> _Duane’s Law says that if a body is periodic in space in a given direction
> then momentum exchanges in that direction are quantized. That means that the
> emerging electron is more likely to go in certain directions than in others,
> depending on how many quanta of sideways momentum are exchanged. Guess what?
> When a stream of electrons is sent through, a ‘diffraction’ pattern
> results._

If this "direction preferentiality" were all that were needed to cause a
diffraction pattern, then we should see it if we run the experiment with one
slit closed for a while, and then the other slit closed for a while.

But that's not what happens! If you send particles through only one slit, you
_do not get a diffraction pattern_! (Actually you do, but the valleys are
spaced far wider than those with both slits open.) No diffraction pattern = no
valleys. Running the experiment again with the other slit closed can't add
those valleys in. This is logically no different than individual electrons
simply "randomly" entering alternate slits, so that can't make a diffraction
pattern either.

What am I missing? It seems like the author is trying to explain double-slit
purely in terms of statistical ensembles of quantum-ish particles rather than
exploiting the full generality of quantum fields, which makes _no sense_
simply by looking at the results of a single- vs. double-slit setup. (e.g.
[https://en.wikipedia.org/wiki/Double-
slit_experiment#/media/...](https://en.wikipedia.org/wiki/Double-
slit_experiment#/media/File:Single_slit_and_double_slit2.jpg))

------
knzhou
Physicist here. This is just self-congratulatory nonsense. Though I shouldn't
be surprised, because I originally signed up for this site to comment against
some other self-congratulatory nonsense.

Quantum mechanics is built on extraordinarily simple rules, in fact simpler
ones than classical mechanics: all you do is sum up the amplitudes for various
paths, and square it to get a probability. The problem is that the objects
obeying these rules thus act _neither_ like particles _nor_ waves. So
obsessing over whether quantum objects are "really" particles or waves is not
useful.

It certainly works sometimes. There are many limits where quantum objects
behave just like particles, or just like waves (there must be, since we must
be able to recover classical mechanics from quantum mechanics). And sometimes
you can _describe_ a quantum calculation in terms of classical wave or
particle intuition, which is what's going on here. The double slit experiment
is in fact so simple that I know of at least 5 ways to derive the result,
using all sorts of different intuition. "Duane's law" is one of them, though I
never knew it had a name, as it pops out rather straightforwardly from the
mathematics.

The reason that we don't try to extend this to cover all of quantum mechanics
is because it doesn't work. Treating the quantum mechanics as if it were
classical underneath leads to all kinds of paradoxes. Bell inequality
violations tell us that the classical variables would have to either influence
each other faster than light, or be pre-set since the beginning of the
universe in a malicious conspiracy. The delayed choice quantum eraser tells us
that if you want to treat the photon as a classical particle, you need to
accept that it can _retroactively change the past_. Yet another failed attempt
is quantum logic: if you insist on treating quantum superpositions as
representing some unknown classical position of a particle, you need
propositions about those positions to _violate the rules of logic themselves_.

Hundreds of experiments probing quantum mechanics have been performed, all
with the same results: committing to a classical ontology in terms of "waves"
or "particles" costs you more than just accepting that quantum objects are
different from either.

Physicists have spent a _lot_ of energy thinking about this. All of the
author's claims were under active debate in the 1920s. He is just completely
ignoring the following century of investigation, and claiming that it never
happened!

~~~
didgeoridoo
> the classical variables would have to either influence each other faster
> than light, or be pre-set since the beginning of the universe in a malicious
> conspiracy

Why is superdeterminism almost universally rejected by working physicists?
From the outside, it feels like the most parsimonious explanation: there is
just a single way things are and will be, set in motion from the very
beginning. It doesn’t feel like a conspiracy of any kind to me.

I’m definitely missing something, but what?

~~~
knzhou
Because if you take superdeterminism seriously, you can't do science.

Sure, it is logically possible that the world runs on classical mechanics, but
that random factors in the light emitted from opposite ends of the observable
universe billions of years ago were delicately set at that point so that
whenever you run an experiment with it, naively assuming that no such
correlations exist, quantum mechanics appears to be true instead.

Unfortunately, if you are consistent with applying this principle you can't
conclude anything at all. If you see the Sun and planets move every day, would
you eventually accept orbital mechanics, or would you prefer the explanation
that "orbital mechanics is false, everything actually moves however it wants
according to random unknown factors, but those factors were arranged just so
that whenever we make observations, standard orbital mechanics _appears_ to be
true"?

Suppose we're betting on horce races, but I win 100 times in a row while you
always lose. Would you accept my explanation that "this was merely
superdetermined to be the case"? Or would you start to suspect that I'm
stealing your money?

Superdeterminism is philosophically equivalent to Cartesian skepticism. It's
just a minor twist on being a brain in a vat. Scientists reject this not
because we can disprove it, but because you can't _do_ anything if it's true.
I mean, you might as well not bother getting out of bed in the morning.

~~~
ncmncm
> _you might as well not bother getting out of bed in the morning._

Sometimes I feel that way.

But I get up, take my meds, and press on.

------
jchook
When I watched a goofy layman’s explanation[1] about the double slit, it
stressed the importance of the effects of observation on the behavior or
nature of the electrons.

When observed to see “which” slit the light passed through, they no longer
created the interference pattern described by Duane’s law. And when left
unobserved (at the slits) the interference pattern emerged even when shooting
“one electron at a time” at the slits.

Does this article or Duane’s Law cover or appreciate this? It seems like it
only describes the interference pattern.

1\.
[https://m.youtube.com/watch?v=Q1YqgPAtzho](https://m.youtube.com/watch?v=Q1YqgPAtzho)

~~~
nabdab
The focus on “observation” in quantum mechanics is dumb and damaging. In
quantum mechanics observation means something very different from what people
assume when you use the word observation.

When you say “if we observe the position of the electron” people think: “we
put an observer who without interring in any active way and he looked at the
electron to see where it is”. What QM actually means is more akin to “if we
take a 200 Ton bulldozer and force the electron into one or two positions and
then test which one it’s in”.

The very issue arising from Observer effects is that you cannot have the
system in a single state where for instance momentum and position are both
observable, and observing either means forcing the system into a state where
the other is “uncertain”, that is to say that it will have a distribution of
outcomes if you use a different 200ton bulldozer to observe this parameter
afterwards. The uncertainty principle basically tells you that the technical
requirements for a “position bulldozer” and a “momentum bulldozer” makes it
fundamentally and axiomatically impossible to make a single bulldozer that
carries out both “observations”.

~~~
Koshkin
Observation is what makes physics matter in the practical sense.

~~~
simonh
Is there anything about the point being made that is inconsistent with that?

~~~
Koshkin
A theory without observables is not science.

~~~
simonh
The point being made is that all observations change the thing being observed,
so you have to be very careful what you measure and how you measure it. That
is all. This is a basic point in science that is often missed. As here,
apparently.

------
scottlocklin
> What many don’t realize is that the double slit experiment (with particles),
> proposed by Feynman in 1963, was for decades only a thought experiment.
> Finally, in 2013, it was successfully performed with electrons. It’s easy to
> see why it took so long: the slits were 62 billionths of a meter apart.

Who is this clown? I did the two slit experiment using a CRT (aka using
electrons) in undergrad experimental physics class in the late 80s, and unless
I had some amazing experience, I'm pretty sure every undergraduate physics
major does the same thing. The '37 Nobel in physics was awarded more or less
for doing this in the 1920s after de Broglie said it would be there.

~~~
kgwgk
Doing it with actual slits and standalone particles is not so easy:

[https://www.sciencedirect.com/science/article/abs/pii/S03043...](https://www.sciencedirect.com/science/article/abs/pii/S0304399112000599)

~~~
scottlocklin
Doing the two slit experiment with electrons and actual slits is absolutely
trivial, and that link doesn't say what you think it means.

~~~
kgwgk
“The general perception is that the electron double-slit experiment has
already been performed. This is true in the sense that Jönsson demonstrated
diffraction from single, double, and multiple (up to five) micro-slits [2],
but he could not observe single particle diffraction, nor close individual
slits. In two separate landmark experiments, individual electron detection was
used to produce interference patterns; however, biprisms were used instead of
double-slits [3, 4]. First, Pozzi recorded the interference patterns at
varying electron beam densities. Then, Tonomura recorded the positions of
individual electron detection events and used them to produce the well known
build-up of an interference pattern. It is interesting to point out that the
build up of a double-slit diffraction pattern has been called 'The most
beautiful experiment in physics' [5, 6], while the build-up for a true double-
slit has, up to now, never been reported.”

[https://iopscience.iop.org/article/10.1088/1367-2630/15/3/03...](https://iopscience.iop.org/article/10.1088/1367-2630/15/3/033018/meta)

~~~
scottlocklin
I did the experiment in 1989, closing individual slits and everything. There
are entire fields of material science based on photo electron interference
(ARPES, XPS and other photo emission spectroscopies). Quoting some clown who
claims "well you never did it my (irrelevant) way" is not useful here.

~~~
kgwgk
If you think that citing a paper by a physics professor in a peer-reviewed
physics journal is “quoting some clown” you’ll probably understand that I
don’t give much weight to a comment from a random person on HN.

~~~
scottlocklin
You're perfectly free to believe in ghosts, the flat earth or DMT elves as
well. People who know what they're talking about are still going to laugh at
you.

Do you think clowns who publish physics papers (and yes, those guys are
grade-A quacks for writing that paper with such claims) are immune to the
temptations of PR balognia? Do you really think "Ultramicroscopy" is a
reputable physics journal? Had someone actually done something as innovative
as his claims: it would be in Science, Nature and PRL. But I guess you don't
actually know anything about physics and think google substitutes for
knowledge and "muh links" substitute for argument.

~~~
kgwgk
For what it’s worth, one of the clowns who coauthored that Ultramicroscopy
paper is credited with the first observation of the build-up of the
interference pattern from individual electron detections in 1974 (using an
electron biprism, not slits in a plate).

~~~
scottlocklin
I'm sure their motion picture tells us all about it. PR != reality.

~~~
kgwgk
What’s the reality then?

Let’s accept for the sake of the argument that it’s a clownish thing to do in
the 21st century to report that you’re sending electrons one by one (at
intensities low enough so the interaction is necessarily that of independent
particles, not that of a beam) towards a metal plate with two slits (which can
be opened and closed at will) and detecting the arrival of individual
electrons at the other side as independent events. Because that’s a trivial
thing to perform and thousands of students were doing it already in the
eighties.

You say that if it was innovative it would be in the cover of Nature or
something. When would you say it was done for the first time then? Was it
published in a top-tier journal when it was innovative? If not, why not?

~~~
scottlocklin
I'd say you either don't understand the english language or are a bored grad
student with nothing better to do than attempt to defy objective reality on
the internet.

~~~
kgwgk
Maybe there is a misunderstanding. What I originally said is that “Doing it
with actual slits and standalone particles is not so easy”.

Are you claiming that it is absolutely trivial to do it with a double slit and
single electron detections, and you were doing it in 1989, or not?

Probably not, but then I don’t know why did you choose to reply to my comments
in the way you did.

(I don’t know either what do you imagine that I think that the paper I cited
means, or what do you believe that the paper claims that is so innovative that
it would belong in a much more reputable journal.)

------
xwowsersx
This is intellectual arrogance on full display.

------
DoctorOetker
The hallmark for any reinterpretation of QM in terms of more classical ideas
would be that it explains Planck's constant in terms of other constants. If an
explanation of a quantum phenomena succeeds to eliminate one of the
fundamental constants, all physicists will provide a listening ear, no
conspiracy involved...

------
crdrost
With respect I simply disagree with Feynman that the double slit experiment
contains everything... Maybe in suggestive outline?

As an example of how QM is weird, my go-to is a game that I call Betrayal. It
is a challenge for a 3 person team. The teammates go through rounds where they
are relativistically separated and unable to talk with each other; each such
room has a big computer screen and two buttons labeled 0 and 1, and during
each round each teammate has to press one or the other of these buttons; then
their selections are summed up to a number that we call The Sum.

The reason we call this Betrayal is that we are trying to force one of the
teammates to betray the other two. So 25% of the time we run a “control round”
where we ask everyone to make The Sum even, and the team passes the round if
The Sum is even. Or the rest of the time we secretly select one of the three
at random to betray their colleagues: we tell the other two the true goal,
make The Sum odd, and the team passes the round only if it is odd: but we lie
to the traitor and tell them to make The Sum even, so they will work at cross
purposes to the other two.

The key result is that there are 4 cases and classical strategies can only
work for at most 3 of them, so your pass rate is limited to 75%. But quantum
players who share a so-called “GHZ state” can pass 100% of trials in
principle—albeit quantum computers have finite probabilities of simply
failing, but if you could improve that to 90% fidelity then over the course of
100 rounds you will still score visibly ahead of the 75s, occasional 80s, and
rare 85s produced by random chance.

The key is that quantum systems come out with these strange global
correlations that cannot be explained classically. You also cannot see them
locally; nobody knows the round succeeded until everyone comes back together
to compare what bits everyone committed to.

------
braythwayt
1\. It’s very nice to be introduced to Landé and Duane.

2\. After so much emphasis on reason and experiment and evidence, it is
jarring to switch gears to a conspiracy theory about scientists concealing or
ignoring theories in order to make themselves seem “superhuman.”

3\. It’s very nice to be introduced to Landé and Duane.

~~~
mandukya
You'd be surprised how much scientists hush and push dissenting voices which
don't suite a particular theory which one cabal wants to be true. Sean Caroll
has been advocating strongly in public to go back and take a look at
measurement problem but nobody wants to do it. He speaks more about this on
his podcast (mindscape) and also on Joe Rogans podcast ( I mean the fact that
he had to bring it up in Joe Rogans podcast to let the public know what's up)
is pretty bizzare and in indicative of the kind of schemes that go on in
academia.

~~~
knzhou
This kind of conspiratorial thinking is precisely the problem. There is a
constant avalanche of papers being published on interpretations of quantum
mechanics, in my mind, too many. To believe that Sean Carroll is the only
person thinking about this is frankly insulting.

This is comparable to hearing one C++ developer speak, then getting the
impression that they are the only programmer in the world who uses a strongly
typed language, and that there's a conspiracy to cover up strong typing.

------
jules
> Admittedly there’s still a bit of mystery left. How does periodicity in
> space quantize momentum (I think if it as a sort of resonance phenomenon).
> But it’s hardly an impenetrable, soul searing mystery that makes you
> question reality itself.

I don't find this to be true. What's left is the key issue. The entire mystery
is left. The old QM worked exactly like that explanation: you take a classical
explanation and postulate that some quantity in it is quantised. This is ad-
hoc and not a general approach. A major breakthrough in the new QM was an
explanation why things are quantised. And yes, it's a resonance phenomenon.
Resonance of what? Probability amplitude waves. Once you have that explanation
you can do away with the classical+quantised part of the explanation, because
it becomes superfluous.

------
dooglius
The problem/mystery is more that the double-slit experiment and other quantum
phenomena are always presented in a hand-wavy way where it's not clear how it
derives from the mathematical formalisms.

------
olliej
The author has gotten confused the bit about the double slit experiment that
is “mysterious”.

The mysterious bit isn’t what happens when you transmit a stream of particles.

It’s what happens when you only send one at a time. If the movement was purely
changes to particle momentum you would get a uniform spread across your
detector.

But that’s not what happens. What happens is you get a diffraction, that only
makes sense if your particle goes through both slits.

------
pontifier
Single slit diffraction and double slit diffraction are very different. This
difference is elegantly and simply explained by the wave model. Without it,
you'd expect the double slit experiment to create a simple sum of the pattern
from each of the slits, but what you get is a completely new pattern that
depends on the spacing between the slits, and the wavelength of the thing
passing through.

------
jl2718
Reading things like this article and replies, it seems apparent that physics
research, or at least teaching, often blurs the distinction between
experiments in thought or otherwise explaining a theory, and those that
motivated theory by empirically disproving previously-held beliefs.

Also I’ve noticed a strange form of entropy in physics that tends to show more
evidence for models that have the largest number of equivalent formulations
based on different assumptions. You could say that this is a result of how
physicists work to prove their own theories, but perhaps the gods work the
same way.

Waves of probability amplitudes with quantized action is a nice abstraction
that works so far, but so was continuum mechanics. Wave-like observations
could easily be a simple consequence of the central limit theorem in the same
way.

------
ksaj
Has the double slit experiment been done and verified in a vacuum? I'm able to
produce the superficial results with a simple neon laser, but it always struck
me that outside a vacuum, there is a lot available to skew those results.

~~~
gus_massa
> _Has the double slit experiment been done and verified in a vacuum?_

99.9999999% sure yes, but I can't find a source with the actual experiment.
The versions with electrons are made in vacuum, but I'm not sure they use the
double slit pattern.

You can reproduce the experiment with light in air. It's expected that you
will get the same result in vacuum. An actual experiment is nice, but the
setup in more complicated, so it's not done in a normal lab class.

It should be easy to setup for someone with access to a vacuum pump, and some
free time (1 week top). It would be nice to see how the lines move when the
refraction index of the air that is almost 1 changes to the refraction index
of vacuum that is exactly 1.

It's hard to convince someone of doing the actual experiment, because it looks
a lot of work without an insightful result. Perhaps you are lucky and someone
already did this. I guess this is the case, but I can't find it. Perhaps you
can convince someone like Cody of "Cody's lab" to make the experiment and
upload a video.

~~~
ksaj
Exactly. Lots of food for thought there.

I _expect_ these results in the open air, since there is so much for the
photons to bounce off of. They're probably even bouncing off the sides of the
actual slits, unless the block is thin and precise enough, and anti-reflective
enough to not allow light bouncing off of it - straight polarized lines only.

This is why I ask such a specific question. I don't deny the effect is
happening at all - I question the influence of other parts not taken care of
in the experiment.

We speak of "noise" (interference) when discussing related observations in
regards to quantum computing. I'm hoping we're at least as stringent about
these experiments. But so far, I've only ever seen them done in open-air
environments, which to me leaves too many nagging questions. I doubt the
quantum effects I can produce in my dusty office are as precise and pure as
they would be in a proper lab, but all the evidence I've seen is that I'm
doing it pretty much precisely the same (sloppy) way.

Electrons do indeed show the same effect and I recall reading once that the
experiment was first proposed using electrons. There are other particle/wave
effects as well, but those two are the easiest to demonstrate with,
apparently.

~~~
gus_massa
> _We speak of "noise" (interference)_

Note that interference here has a very technical meaning that is NOT noise.
(Sorry for the all-caps, but it's very important in these experiments.)

I think you don't understand the important point in the double slit
experiment. Let's try to explain it, but it's a long explanation.

\---

First you must understand the single slit experiment. If the slit is thin
enough you get a nice diffraction patter.

If this spread was caused by dust or something like that, a good guess is that
the image gets a gaussian blur like exp(-x^2).

In a diffraction experiment, the image you get is something like sin²(x)/x².
(You should draw them in Wolfram Alpha or something.)

The main difference is that the first one is always positive, so you get light
everywhere, but it get's dimmer very soon. The second one has a lot of zeros,
so you get strips of light and darkness. The math details are complicated, so
let's ignore them.

You can do this experimentally. In the lab we used two razorblades with some
screws to move them arcuately. There should be plenty of recipes in Internet
to reproduce it. You can try it in more dusty environments, and clean
environment. Sharp razorblades, not sharp razorblades. Wind or not wind. Try
it a few times until you get use to the images it produces. You will notice
that the cleaner environment produce the better results, so I hope you will
get convinced that it's not caused by the dust or something

You can probably buy some single slit from an educational provider. Buy some
of them of different width to compare the results with your homemade version.

It's important to understand how it looks before going to the second step, so
play for some time with the single slit version. Some resources:
[https://en.wikipedia.org/wiki/Diffraction#Single-
slit_diffra...](https://en.wikipedia.org/wiki/Diffraction#Single-
slit_diffraction)

\---

Now, the double slit version looks more difficult to make at home. I think you
should buy one (or a few), and the single slit version with the same width.

The idea is that if it has only one slit you would see the old image. If you
have only the other slit you would see the old image. If the screen is far
away and the slits are very close, you can't notice the difference. So you
would expect to see the same image, but with more bright.

The weird thing is that you don't get the same image. You get almost the same
image but with a lot of additional strips. There are the old dark lines and
many new dark lines. These new dark lines are the interesting one. In this
lines you have something like

. light + light = darkness

Try the experiment in air and try to read the explanations in
[https://en.wikipedia.org/wiki/Double-
slit_experiment](https://en.wikipedia.org/wiki/Double-slit_experiment) and
[https://www.youtube.com/watch?v=Iuv6hY6zsd0](https://www.youtube.com/watch?v=Iuv6hY6zsd0)
. Try again until you scream <all-caps> "The new dark lines are weird!"</all-
caps> If you are not screaming, try the experiments with one and two slits
again and the info again.

You can try with a dusty windy environment, and it should make the lines more
fuzzy. You can try wit a clean environment and get better results. The lines
are not caused by the dust, they are just weird.

. light + light = darkness

(If you are going to buy things, add a diffraction gratting. It's like using
many slits instead of two. The math is more difficult but they create nice
rainbows.
[https://en.wikipedia.org/wiki/Diffraction_grating](https://en.wikipedia.org/wiki/Diffraction_grating)
)

The dark lines are not so surprising when you use light that is usually
approximated a wave. But if you get more advanced equipment you can repeat the
same experiment with electrons or neutrons and get the same weird lines. (When
you understand this, you can scream again.)

------
6gvONxR4sf7o
I actually don't like the double slit experiment because it's so easy to
confuse yourself. Good for learning the math, maybe, but bad for presenting
the "weirdness." Tests of Bell's inequalities are much more interesting and
IMO cut straight to the heart of the weirdness of QM. Likewise with tests of
the quantum Zeno effect. For me, at least, these two experiments go straight
to "okay fine, measurement is weird and let's just treat superposition as its
own thing" while double slit experiments beg for hand waving mysticism.

------
carapace
If you like crackpot "science" check out Rex Research. (Fair warning, not
everything there is BS, but you can find all sorts of weird stuff to gawk at.)

------
tus88
The problem I have with this. Quantization must be a factor of wavelength and
perhaps width of one slit. Yet the interference pattern produced by the double
slit experiment depends on the distance between two slits. Diffraction pattern
is not the same as an interference pattern, something the original researchers
were well aware of.

------
neonate
For those who don't know, Bill Wadge is an esteemed computer scientist who
created the Lucid dataflow language with Ed Ashcroft.

Not that that has anything to do with the double slit experiment, but it's
worth knowing.

------
download13
"Something that is periodic in space"

I could swear we have a name for that. I think it started with a W?

------
sudoaza
No physicist but feel like this should show up in a one slit experiment

------
jhanschoo
The wave/particle duality notion isn't really meant to hold an accurate deep
insight into QM, it's just meant to inform intuitions, and sorta communicate
it to laypeople.

~~~
Koshkin
This is not correct. The duality is the fact that an elementary particle is
point-like but requires wave mechanics to describe its motion. Another related
discovery was that energy waves, too, consist of quanta, which in many
respects behave as particles. Those are valuable physical insights (similar in
its power to the idea that “everything consists of atoms”).

~~~
gus_massa
I disagree with your disagreement, but perhaps we all three agree.

The electrons/photons/whatever must be described quantum field theory, but the
math is too complex to use it unless it's a very simple experiment with a few
particles or with a computer. So it's good to have some simplifications.

In some cases, you can approximate an electron/photon/whatever as a wave. The
math is much simpler and with some training you can to with paper and pencil
or in some simple cases with handwaving.

In other cases, you can approximate an electron/photon/whatever as a particle.
The math is much simpler and with some training you can to with paper and
pencil or in some simple cases with handwaving.

I don't like the name "wave–particle duality" but it is probably too late to
change it. It's not true that an electron/photon/whatever _is_ sometimes a
particle and sometimes _is_ a wave. It is always a weird quantum thing that
sometimes can be _approximated_ as particle and sometimes can be
_approximated_ as wave.

For historical reasons, people discovered first the approximations, named it
as the "wave–particle duality", and later understood what is really happening.

~~~
Koshkin
Except we may never know what's "really happening." Field theory is, well,
"just" a theory, that is to say, an approximation, albeit a better one.

~~~
gus_massa
Yes, perhaps string theory is correct.

------
hos234
[https://en.wikipedia.org/wiki/Escalation_of_commitment](https://en.wikipedia.org/wiki/Escalation_of_commitment)

------
momentmaker
I'd recommend reading the book The Physics of God.

The author goes into a whole different theory on how it's possible for a
photon to be both a wave or a particle depending on whether there is an
Intelligent Observer/sensor.

------
EliRivers
This explanation doesn't seem to leave any room for the experimental
observation that simply knowing which slit the electron went through - not
some physical interaction that disturbs it but simply having that knowledge -
changes the pattern.

~~~
mistercow
How would you know which slit the particle went through without physically
interacting with it?

~~~
EliRivers
You can decide afterwards whether to figure out which way it went or not, and
make your observation then. While the English language isn't great at
describing this sort of thing, it seems fairly safe to suggest that I cannot
choose now to physically interact with an event that happened in the past.

------
ryanthedev
I absolutely agree. We tend to forget that nothing is still in our universe.

Everything to the smallest quanta oscillates. The only way to get something to
stop moving is by freezing it. You need to hit absolute zero to stop movement
completely, I don't believe we have even observed absolute zero.

If you think about it, absolute zero shouldn't be possible. There are an
infinite amount of degrees between 1 and 0. Aren't we only limited by the
sensitivity of our equipment?

~~~
ncmncm
They don't stop at absolute zero, either, because then position and momentum
would both be fixed.

But there is a continuous range of temperature all the way down, and there is
plenty of work at increasingly tiny fractions of the last degree. It is
meaningful to talk about what happens at a billionth of a degree.

