Two-slit experiment details please

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Two-slit experiment details please

Postby JustAsking on September 8th, 2018, 2:14 pm 

I'm trying to learn more about the infamous two-slit experiment. Every example I read about or see on youtube starts out with "if we shoot a single particle at the screen". And the conclusion always seems to be that the best explanation is the particle acts like both a particle and a wave. And the interpretation is that there isn't really a particle or a wave, just a set of probabilities of locating the particle in any particular region. Ok fine. But if that's the case, then how is it known that a single particle is being shot at the screen?

I'm sure I'm missing something, but this seems equivalent to: ok take a classical particle that's like a pool ball. Shoot it at the screen... etc. Well that sounds like somebody wants their cake and to eat it too. What am I missing? How is it known that precisely one particle is being shot?
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Re: Two-slit experiment details please

Postby Event Horizon on September 8th, 2018, 7:54 pm 

It is not infamous. I dunno where you got that idea from. It's a simple experiment that's been replicated many thousands of times.
It demonstrates electron duality. From that it was discovered that other particles have duality. Photon duality is well covered in innumerable papers.

It is not necessary to fire just one electron when a stream will exhibit the same duality.

If someone would like to elaborate?
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Re: Two-slit experiment details please

Postby bangstrom on September 8th, 2018, 9:00 pm 

If you fire a stream of electrons at the double slit, the slit separates the the stream into two smaller streams and one stream interferes with the other because of their close proximity. This demonstrates the wavelike nature of electrons. This is easily understood.

The “notorious” or bizarre aspect of the experiment is observed when particles are fired at the slit one at a time and the same interference pattern develops over time. This calls into question the previous explanation because it is difficult to imagine how a particle can also act as a wave, even more difficult to explain, is how a single particle can interfere with itself unless it passes through both sides of the slit at the same time.

To add to the confusion, the interference pattern only forms when the slit is not observed. The interference pattern does not develop if the electron is observed to pass through one side of the slit or the other even if both slits remain open. An electron can either pass through one slit or the other but, if we know "which way" it went, the pattern does not form. Any knowledge of "which way" information destroys interference.
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Re: Two-slit experiment details please

Postby JustAsking on September 9th, 2018, 8:05 am 

Thanks bangstrom, but I don't think you answered my question. I'm asking, in the bizarre case where particles are fired one at a time, how is it known that a single particle is being fired?
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Re: Two-slit experiment details please

Postby Braininvat on September 9th, 2018, 11:00 am 


I think there's a bit of confusion here. The double-slit experiment was not performed with "single photons" - it's very hard to even consider what that would mean. At its heart, it is a thought experiment, and it's not really possible to make a real-life device that tests it.

The first low-intensity experiment (Taylor 1909) was challenging the EM field interpretation of photons - the idea was that if photons were localised concentrations of the EM field, as you lowered the intensity, there would be no photons to interfere with each other, and the diffraction pattern would disappear. When the experiment was low-intensity enough that Taylor couldn't distinguish between photons emitted and photons absorbed, he noted that the diffraction pattern still exists, so the photons couldn't just be localised concentrations of the EM field. Dirac had a different explanation - he considered that each individual photon was capable of interacting with itself.

Later, the experiments were repeated not with light, but rather, electrons. Electrons are a lot more convenient than photons, since they obey the Pauli principle: it makes a lot more sense to say "an electron here, an electron there". And you can emit individual electrons, which was first tested in 1974, and it was found that individual electrons do in fact display the same interference pattern. Later, it was found that the same pattern also appears for atoms and complex molecules (the current "world record" has the experiment done with a molecule with more than 800 atoms, at ~10 000 atomic weights; the experiment gets much harder with bigger "particles", since it requires much more precision). But we'll stick to electrons, since they're quite convenient.

Emission of individual electrons is still quite tricky, but they have a few important properties. They carry a charge, and they have mass. Both of these can be measured, and while this does disturb the electron (change its trajectory), it doesn't absorb it. Photons, on the other hand, will be absorbed by any measurement, which makes them tricky to deal with.

So you can measure with precision to individual electrons how many electrons were emitted from your emittor, and how many were absorbed on the detection surface. More importantly, you can try experimenting with what happens when you measure the electrons on the way between the emittor and the detector - and that's when real quantum phenomena come in.


(clipped from physics MB)
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Re: Two-slit experiment details please

Postby hyksos on September 9th, 2018, 1:20 pm 

What am I missing? How is it known that precisely one particle is being shot?

Light has a frequency and an amplitude. A single frequency is used for these experiments, thus they are using a laser. "Amplitude" is how bright the light is.

In double-slit mock ups in a lab, you are firing a laser with the lowest possible amplitude. In other words these experiments are very very dim laser light.

There is a cut-off point where you know you must be firing less energy than a single photon would carry. (this energy cut-off scales with the frequency) You can make the amplitude of the laser even lower than this limit. When you do so, your laser is firing a single photon at intervals of time. You can make the laser so dim that it is firing one photon every 2 seconds.

The single photon falls upon a collection apparatus called a photodiode avalanche detector. This chip starts from the disturbance of a single photon and ratchets it up through a series of more powerful voltages (fed by an external electricity), until the voltage is high enough that it causes an amplifier to make a "tick" sound. For very dim laser light, the individual "ticks" are caused by a single photon being absorbed.

Optics people call these things SPADs. (Single-photon avalanche diode)

Image

http://www.everyphotoncounts.com/spad.php
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Re: Two-slit experiment details please

Postby JustAsking on September 12th, 2018, 1:18 pm 

Well double interesting. One reply says the double slit hasn't been performed with single photons. The next describes how single photons are emitted, presumably for experiments like the double-slit.

For the photons it seems as though they're being presumed to be waves with frequency and amplitude. Is that assuming what's supposed to be proven?
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Re: Two-slit experiment details please

Postby davidm on September 12th, 2018, 1:29 pm 

JustAsking » September 12th, 2018, 11:18 am wrote:Well double interesting. One reply says the double slit hasn't been performed with single photons.


It has.

For the photons it seems as though they're being presumed to be waves with frequency and amplitude. Is that assuming what's supposed to be proven?


No.
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Re: Two-slit experiment details please

Postby JustAsking on September 21st, 2018, 4:16 pm 

davidm » September 12th, 2018, 12:29 pm wrote:
JustAsking » September 12th, 2018, 11:18 am wrote:Well double interesting. One reply says the double slit hasn't been performed with single photons.


It has.

For the photons it seems as though they're being presumed to be waves with frequency and amplitude. Is that assuming what's supposed to be proven?


No.
hysokos wrote this: Light has a frequency and an amplitude. A single frequency is used for these experiments, thus they are using a laser. "Amplitude" is how bright the light is.

Umm, that's wave talk. So how is that not presuming light is waves.
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