Feynman's double-slit experiment brought to life

Mar 13, 2013
Feynman's double-slit experiment brought to life

(Phys.org) —The precise methodology of Richard Feynman's famous double-slit thought-experiment – a cornerstone of quantum mechanics that showed how electrons behave as both a particle and a wave – has been followed in full for the very first time.

Although the particle-wave duality of electrons has been demonstrated in a number of different ways since Feynman popularised the idea in 1965, none of the experiments have managed to fully replicate the methodology set out in Volume 3 of Feynman's famous Lectures on Physics.

"The technology to do this experiment has been around for about two decades; however, to do a nice data recording of electrons takes some serious effort and has taken us three years," said lead author of the study Professor Herman Batelaan from the University of Nebraska-Lincoln.

"Previous double-slit experiments have successfully demonstrated the mysterious properties of electrons, but none have done so using Feynman's methodology, specifically the opening and closing of both slits at will and the ability to detect electrons one at a time.

"Akira Tonomura's brilliant experiment used a thin, charged wire to split electrons and bring them back together again, instead of two slits in a wall which was proposed by Feynman. To the best of my knowledge, the experiments by Guilio Pozzi were the first to use nano-fabricated slits in a wall; however, the slits were covered up by stuffing them with material so could not be open and closed automatically."

In their experiments, which have been published today in the New Journal of Physics, Batelaan and his team, along with colleagues at the Perimeter Institute of Theoretical Physics, created a modern representation of Feynman's experiment by directing an , capable of firing individual electrons, at a wall made of a gold-coated silicon membrane.

The wall had two 62-nm-wide slits in it with a centre-to-centre separation of 272 nm. A 4.5 µm wide and 10 µm tall moveable mask, controlled by a piezoelectric actuator, was placed behind the wall and slid back and forth to cover the slits.

"We've created an experiment where both slits can be mechanically opened and closed at will and, most importantly, combined this with the capability of detecting one electron at a time.

"It is our task to turn every stone when it comes to the most fundamental experiments that one can do. We have done exactly that with Feynman's famous thought-experiment and have been able to illustrate the key feature of ," continued Batelaan.

Feynman's double-slit experiment

In Feynman's double-slit thought-experiment, a specific material is randomly directed at a wall which has two small slits that can be opened and closed at will – some of the material gets blocked and some passes through the slits, depending on which ones are open.

Based on the pattern that is detected beyond the wall on a backstop – which is fitted with a detector – one can discern whether the material coming through behaves as either a wave or particle.

When particles are fired at the wall with both slits open, they are more likely to hit the backstop in one particular area, whereas waves interfere with each other and hit the backstop at a number of different points with differing strength, creating what is known as an interference pattern.

In 1965, Feynman popularised that electrons – historically thought to be particles – would actually produce the pattern of a wave in the double-split experiment.

Unlike sound waves and water waves, Feynman highlighted that when are fired at the wall one at a time, an interference pattern is still produced. He went on to say that this phenomenon "has in it the heart of quantum physics [but] in reality, it contains the only mystery."

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More information: "Controlled double-slit electron diffraction" Roger Bach et al 2013 New J. Phys. 15 033018 iopscience.iop.org/1367-2630/15/3/033018/article

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1.7 / 5 (12) Mar 14, 2013
Well done guys. Not sure this advances our understanding though?
Surely the electrons act as waves while in flight anyway-regardless of the slits state. The experiment is just too insensitive to measure the edge diffraction effects at both sides of a single slit.
1.7 / 5 (12) Mar 14, 2013
With a single slit the resulting pattern is that expected of a wave going through the single slit, relatively little interference pattern results-get over it already.
The interested part is the disapearance of wave behavior when you break coherence of the two superimposed interfering waveforms (from each slit) by 'making a measurement'. The particle may still continue on as a wave just not the same wave as you started with-so inteference disapears.
1.8 / 5 (13) Mar 14, 2013
Its like breaking the coherence of a laser beam by shining it through a fine grained diffuser material. It now acts like a randomised beam without the interference pattern phenomenon- even if you do it one photon at a time.
Thats another thought experiment- its just so obvious no one would bother to write a stream of articles about it.
3.7 / 5 (12) Mar 14, 2013
Surely the electrons act as waves while in flight anyway

Well, no - if they did then you could have 'partial' impacts, where only part of that 'wave' were to interact with something while another part moves on. But that doesn't happen.

We're not dealing with particles or waves. We're dealing with particle-waves. Particle-waves may share some letters with the words 'particle' and 'wave' but that's where common ground ends.

- particle-waves are not BOTH particle and wave
- partuicle-waves are NEITHER a particle NOR a wave but something entirely different that exhibits particle-LIKE behavior in some circumstances and wave-LIKE behavior in others.
1.7 / 5 (13) Mar 14, 2013
"Well, no - if they did then you could have 'partial' impacts"
Partial impacts or soft/partial measurements as they are usually called are the state of the art in this whole ongoing double slit duality thing. Why do folk need to add extra universes or human observer dependencies: Rather than grasp that the the wavefunction is self coherent in all the space that it can fill, and can interfere with itself. The quandry comes when it interacts with another wavefunction (e.g. in a detector) and the result is what we call a localised particle interaction. And the wavefunction changes in ways we are only just beginning to grasp -by making small changes by partial measurements.
1.6 / 5 (14) Mar 14, 2013
We are now finding that in carefully isolated cold conditions the electrons wavefunction can be split (deconfined) into three parts "spinons, holons and orbitons". Each of which will have a simpler wavefunction with their own unique interactions. Lets leave off the double slit wrangles and move forward to understand wavefunction interactions and why they look like fuzzy Heisenberg-limited 'particle' interactions when we interact with them using suitable detectors.
3.5 / 5 (8) Mar 14, 2013
Partial impacts or soft/partial measurements as they are usually called are the state of the art in this whole ongoing double slit duality thing.

No. The detectors at the other end detect a FULL electron or nothing. There is no in-between or partial detection. The dtection process is quantized.
The interference pattern manifests nevertheless when you superimpose all the results from all the electron runs.
That is why the experiment is so important: you have particle-LIKE behaviour (due to quantized detection) and wave-LIKE behavior (due to interference patterns dependent on multiple slits being open) in the SAME experiment. I.e. with the SAME entity.

The wavefunction fills all space (and multiple slits while passing), but the quantized detection event does not. And it cannot be explained away by a 'interaction with another wave function' because that would also hold for the slits (where the path is NOT quantized).
1.6 / 5 (13) Mar 14, 2013
This misunderstanding is why the wrangle over double slits and duality continue. And multiple universes and observer dependency errors arise.
I agree that the "screen" is a full detector of a single event. But the state of the art in these experiments is now a partial detector before or after the slits but before the final "screen" full detector. These partial detectors make estimates about the particle properties and position and so they only partially decohere the electrons wavefunction as it fills the space between the source and screen while in transit. These partial effects partially effect the interference paterns in a way that is currently being studied with interest.
2.1 / 5 (15) Mar 14, 2013
I suspect that if we had the maths and the understanding of space (beyond a noisy quantum foam) so that we could work out the integral of the wavefunction across space without having to apply a re-normalisation fudge factor we would properly understand wave-particle duality. At the moment we have to say "the answer is one" now "what is the question" for every Quantum interaction.

(Just as we can currently exacly calculate EM radio waves using Maxwells equations and the permitivity and permeability of free space.)
1.9 / 5 (13) Mar 14, 2013
The effects of partial detection have been seen since at least 2005: http://phys.org/news7144.html
We just need another Feynmann or Dirac or Einstein to make coherent sense of it and put this confusion to bed.
Even if the copenhagen interpretation needs updating in the process, as wavefunction interactions dominate our useful physics at the nanometer scale and 'particle phenomena' are just the result of "probability of collision was observerd as =one" like normalising the wavefunction space integral to "one" after an interaction/collision/measurement.
1.4 / 5 (9) Mar 14, 2013
Unlike sound waves and water waves, Feynman highlighted that when electrons are fired at the wall one at a time, an interference pattern is still produced. He went on to say that this phenomenon "has in it the heart of quantum physics [but] in reality, it contains the only mystery."

This is what was told in the conventional way, nowadays we know it mechanism which could explain how it works, and then it is not mystery anymore.
1.7 / 5 (11) Mar 14, 2013
The whole point of wave particle duality is it isn't as simple as waves on a pond. And while I also like reasoning by analogy to get a handle on physical processes, you need to know where your analogy breaks down or you end up chasing imaginary rabits down multiple universes. Five minutes on wikipedia will dispell the simplistic it's all sorted myth. Start here :http://en.wikiped...inciples
1.4 / 5 (9) Mar 14, 2013
The whole point of wave particle duality is it isn't as simple as waves on a pond
I beg to disagree.. Instead of it, the whole basis of particle wave duality actually boils down to the physical wave concept, if you try to think about it in consequential way... The abstract wave is indeed massless pure harmonic artifact, but the physical wave is always connected with some intrinsic nonlinearity, which makes it an obstacle for another waves, i.e. the particle. Every water surface deformed with wave has larger specific area than the surrounding water and as such it behaves like sorta sparse blob for another waves.

We therefore cannot separate the particle aspect even from ripples at the water surface - the high energy density of vacuum waves just makes their autofocusing solitonic character more pronounced, but the principle remain. After all, the correspondence principle requires the seamless connection of classical mechanics with quantum mechanics principles.
1.4 / 5 (9) Mar 14, 2013
BTW You proponents of mainstream physics are apparently focused to differences instead of connections of reality. This is not a healthy introductory strategy in search of generalized theory. Such an approach indeed helps the occupation of physicists, but it slows down the unification of physics. You should decide, whether you want to become an experts in some particular field of physics, or if you're really interested about development of TOE. You cannot sit on two chairs at the same moment.
3.2 / 5 (9) Mar 14, 2013
You cannot sit on two chairs at the same moment.

I thought first Heisenburg and then Pauli had worked that one out. Heisenburg theorized that I could sit in both chairs at once, and Pauli theorized that only one of us could sit in this chair or that chair at the same moment.
1.4 / 5 (9) Mar 14, 2013
Anyons manage to switch the chairs fast, but they can exist in geometrically frustrated systems only, where the number of spatial dimensions remains limited. After all, the neverending wobbling (zitterbewegung) of quantum particles indicates, their natural occurrence resides in higher number of dimensions, then we can manage to observe by now. The shadow of 3D rod smoothly rotating around all three directions projected at the 2D plane appears 1)random wobbling and 2) much smaller in average too.
5 / 5 (4) Mar 14, 2013
VeleriaT you needed be squabbling here in the comments, why aren't you writing paper to prove how idiotic all current physics theorists are compared to yourself.
1 / 5 (6) Mar 16, 2013
I prefer the many worlds interpretations of QM. It does away with the 'indeterministic' silliness. Each time an interaction takes place the universe is split.

With 2 chairs in a room you may be looking at the chair to your left and see it empty, but in an alternate world you're looking at the chair to your right and it is the one that is empty.
1 / 5 (6) Mar 16, 2013
If an observer were to walk into the room you're in, he'd find you in one of the two chairs. Until he opened the door to your room your position would be in a probabilistic state evenly distributed across the two chairs (wavefunction), the opening of the door destroys the probabilistic distribution (wave-function collapse).

If we reverse the process we encounter a problem. The observer sees you in 1 of the chairs, walks backwards out of the room, closes the door, and you go from being 100% in one chair into a state where you're 50% in 1 chair and 50% in the other (don't envision a butt cheek on each, the chairs are distanced across the room from one another).
With the many worlds interpretation this problem is resolved, you are 100% in one chair in one world, and 100% in the other chair in another world.
1 / 5 (6) Mar 16, 2013
The many worlds interpretation does away with the wave-function collapse. When the door opens and the observer looks, the timeline is split. In one world the observer finds you sitting on the left chair, in the other world he sees you sitting on the right one. The wave-function was correct, you're on both chairs.

Without the many worlds interpretation time reversal does not work. The wave function predicts equal probability of you sitting in either chair, once measurement is taken you're only found occupying one of them. The single outcome (collapsed wave function) cannot be reversed back into a probabilistic state (wave function).

With the many worlds interpretation time reversal does work. The wave function shows 2 predictions (sitting in left and right chairs). The splitting timeline allows both outcomes to be real. The wave function does not collapse into a single state. Sitting in right chair and sitting in left chair (the wave function predictions) are both true.
1 / 5 (6) Mar 16, 2013
I believe that outcomes are 100% determinable. I think there are hidden variables involved which affect the quantum experiments.

Chaos theory says that small events (such as a butterfly flapping its wings) can have huge results (like a hurricane). It is hard to prove that a little butterfly flying about set a series of events into motion which eventually led to a hurricane, but it is possible.

I think similar effects are causing a photon to land on a particular spot on the screen in the double-slit. If we knew all of the details surrounding the experiment we'd predict exactly where that photon would land. It is the hidden details ('variables'), IMO, which leave us to say where the photon will 'probably' land.

Until the day that we are able to predict the landing point for every individual photon, I will prefer the 'many worlds interpretation' of QM (where each photon lands on every predicted point in a different world).
1 / 5 (8) Mar 16, 2013
why aren't you writing paper to prove how idiotic all current physics theorists are
www.... waste of paper?
3 / 5 (4) Mar 17, 2013
Where in the wave is the charge carried? Where is the energy carried? Where is the mass carried?

"Surely the electrons act as waves while in flight anyway-regardless of the slits state." - Eyenstein

The problem is always one of locality.

Waves are distributed through space, while particles are detected at well defined, points, or at least tiny volumes.

The problem with the wave view is that the wave function describing the evolution of an electron's state can be arbitrarily shaped by experiment.

Say for example that one places an electron in a mirrored box so that it can not escape and keeps it there for a time long enough for it's location in the box to become equally distributed.

One can then open a hole in the box at regular intervals to produce an arbitrarly shaped waveform.

3 / 5 (4) Mar 17, 2013
The waveform could for example be a series of pulses which themselves can be further directed by mirrors or other devices and shaped into any pattern desired, as long as the apparatus doing the shaping has no contact with the outside world.

Given that the output waveform can essentially be arbitrarily constructed the shape of the waveform can play no part in the mechanics of observing the electron.

The waveform can shape the probability of observing the waveform. But it can not actually take any role in the underlying mechanics of observation.

So what you are left with for the observation is a wavefuction who's wave shape plays no part in detection.

This is why the modern view of the problem rejects electrons as consisting as waves.

3.4 / 5 (5) Mar 17, 2013
In the modern view, electrons are particles, but to accommodate the apparent wave nature of these things, two assumptions are made.

1. A particles wave function is the complex sum of it's components.

2. The components of a particles wave function represent all possible events (and therefor all possible paths) between the current time and the time of the particle's creation.

Properties 1 and 2 combine to produce the particles wave function, and create the condition where if a particle has a choice of being in n different states then it in effect is in all n states at the same time as long as the fact remains unconfirmed by measurement.

This axiomatic approach to quantum mechanics is entirely unsatisfying because...

1. It does not mesh with what is seen in the macroscopic world.

2. By design it avoids any discussion of the mechanism of local interaction but it does answer the questions of where.

Where is charge? Where you observe it.

I remain perplexed.
1 / 5 (6) Mar 17, 2013
You should tell us here what you know, not what you don't know.
It does not mesh with what is seen in the macroscopic world
You just don't know about it.
5 / 5 (2) Mar 17, 2013
I know that here in the macroscopic world apples do not defract through doorways or exist both in the freezer and crisper of my refrigerator at the same time.

Are things different on your planet?

Your link of course, gives hope to the Newtonian Materialists, but can not explain quantum phenomenon due to the fact that the experiments waves are artificially generated by the apparatus and can not be sustained without the input of energy into the system.

Bohm pilot waves are no more real than particle waves are. Both are artificial constructs.

1 / 5 (1) Mar 18, 2013
An excerpt from comment in Physics World on the same article by FUBB about wrong credit and how we create myths, legends & "objective"history. This experiment was in textbooks before Feynman Lectures.

fubb Mar 15, 2013 - Nice experiment, wrong credits

"...Feynman Lectures is... just a personal variation ...of what was already a textbook commonplace. The same basic discussion can be found in Chapt 1, Sect 3 of Dirac's Principles of Quantum Mechanics, 4th edit (1958), which ...pre-dates the Feynman Lectures. It probably goes back to the 1st edit (1930), ...likely the ultimate source of all such discussions. ...experimental demonstration of the interference of electrons, C. Davisson and G. P. Thomson won a Nobel prize for that in 1937. The details were different, but the experiment was... equivalent...

... who wrote ... this story is based, were obviously unaware of the historical background... It would be a shame if they succeed in establishing yet another myth about Feynman."
not rated yet Apr 15, 2013
Where did the wavicles go?
1 / 5 (1) Apr 15, 2013

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