Double-slit Experiment Questions

[RobbieG]

After reading up on particle-wave duality and the double-slit experiment, I came upon this in the article.

 

The most baffling part of this experiment comes when only one photon at a time is fired at the barrier with both slits open. The pattern of interference remains the same, as can be seen if many photons are emitted one at a time and recorded on the same sheet of photographic film. The clear implication is that something with a wavelike nature passes simultaneously through both slits and interferes with itself — even though there is only one photon present. (The experiment works with electrons, atoms, and even some molecules too.)

http://en.wikipedia.org/wiki/Double-slit_experiment

 

 

Would someone care to try and explain this? Or do we just not know enough as of yet to understand why this happens?

[Baby Astronaut]

Just ignore the "baffling" part. Scientists understand it's acting like a wave.

 

Imagine that the photons being fired are more like waves coming out really fast. Much like an ocean wave, it's spread out -- enough so that it hits both slits. Now as the wave reaches the two gaps/slits, the wave hits them, entering both, and so naturally it'll be split into two parts. Again, both "new" waves spread out and so naturally they'll soon meet and do the interference pattern you see in nature from waves colliding in a pond.

 

But I'm yet a newb at double-slit knowledge and thus might be incorrect, however someone will point it out if so.

[vordhosbn]

Yes, actualy it is still behaving somewhat as a paraticle, because it hits one exact spot on the test screen, but when we fire more and more electrons, the dots on the screen begin to resemble an interference pattern. It's like the electrons intefere with eachother... at different time. The really curiuous part, however, comes when we put some device on one of the slits to watch if the electrons pass through slit #1 or slit #2.

The result is, that if we try to measure exactly trough which slit did they pass, the electrons do not form interference pattern anymore, but just usual scattered pattern (like golfballs). As if the act of observation* turns off the wave-like properties of electrons. This is called, i believe, wave function collapse.

 

* Be carefull to understand that 'observation' should be read as 'measurement' and not 'observation' as in "I am looking at it."

[Abstract_Logic]

The logical basis of quantum mechanics is the idea of both/and, as opposed to either/or logic. Either/or logic is classical propositional logic, where things are interpreted in terms of 'black and white'. Quantum logic, on the other hand, is the both/and logic, similar to fuzzy logic, where the 'grayness' is taken into account, in this case it would be wave-particle duality. All quantum entities have both a wave and a particle-like nature. In quantum theory, there is something called the superposition principle, according to which quantum entities can experience more than one possible reality at once. So it is generally said that, in the two-slit experiment, a single photon is the superposition of two equally possible realities, i.e. wave-like and particle-like.

 

Just ignore the "baffling" part. Scientists understand it's acting like a wave.

 

It is true, scientists do understand it acts like a wave, but that doesn't mean to disregard the particle-like nature of the photon, nor does it mean this phenomenon is not baffling. It is called wave-particle duality simply because the photon behaves like a wave at one point, and a particle at another, so it is easy to see how this can be so baffling to scientists who were so used to the Newtonian world view (i.e. the either/or view).

 

The collapse of the wave function is exactly like what vordhosbn explained. The Schrödinger wave function is a mathematical description of each possible reality associated with any quantum system. Once we attempt to measure (observe) a quantum system, the wave function collapses and only one single reality is experienced (either a wave or a particle). So it appears that whatever it is that draws the line between propositional either/or logic and quantum both/and logic, it definitely has to do with the act of measurement.

 

I would recommend you do a wikipedia search for all the terms I have put in bold face. You might also be interested in the Schödinger's cat experiment, which is a thought experiment involving the wave function and the superposition principle.

Edited December 18, 2009 by Abstract_Logic

[toastywombel]

After reading up on particle-wave duality and the double-slit experiment, I came upon this in the article.

 

The most baffling part of this experiment comes when only one photon at a time is fired at the barrier with both slits open. The pattern of interference remains the same, as can be seen if many photons are emitted one at a time and recorded on the same sheet of photographic film. The clear implication is that something with a wavelike nature passes simultaneously through both slits and interferes with itself — even though there is only one photon present. (The experiment works with electrons, atoms, and even some molecules too.)

http://en.wikipedia.org/wiki/Double-slit_experiment

 

 

Would someone care to try and explain this? Or do we just not know enough as of yet to understand why this happens?

 

Check out this video. It is a bit juvenile, but rather informative.

[michel123456]

http://www.youtube.com/scienceforums#p/c/328F3DF13C3BDF90/11/DfPeprQ7oGc

( I could not reach your link, there was a : missing)

 

let me dare to say something: i think scientists do not understand anything of what is going on, meaning that there is no satisfactory explanation that completes with intuition and everyday experience. But they can actually calculate anything of it, and very accurately.

You won't find any "explanation" of Quantum Mechanics anywhere. Either you accept it and work with it, an it's all fine, either you refuse it and dive into a well of darkness.

Unless........speculations.

[swansont]

let me dare to say something: i think scientists do not understand anything of what is going on, meaning that there is no satisfactory explanation that completes with intuition and everyday experience.

 

I think that scientists deal with things that are not part of everyday experience, and do understand what's going on.

[michel123456]

From Wiki http://en.wikipedia.org/wiki/Double-slit_experiment#cite_note-6

"It is a widespread misunderstanding that, when two slits are open but a detector is added to the experiment to determine which slit a photon has passed through, then the interference pattern no longer forms and the experimental apparatus yields two simple patterns, one from each slit, superposed without interference."

 

& "However, an experiment performed in 1987[7] produced results that demonstrated that which-path information could be obtained without destroying the possibility of interference." from the same article, ^ P. Mittelstaedt, A. Prieur and R. Schieder, Unsharp particle-wave duality in a photon split-beam experiment, Foundations of Physics 17, 891-903 (1987).

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Question:

when they shoot the first single photon through the dubble slit, do they obtain one or 2 spots?

[swansont]

when they shoot the first single photon through the dubble slit, do they obtain one or 2 spots?

 

One.

[michel123456]

So they should conclude that the photon do not pass through both slits at the same time. And consequently, do not interfere with itself. Something different is occuring.

[swansont]

So they should conclude that the photon do not pass through both slits at the same time. And consequently, do not interfere with itself. Something different is occuring.

 

How do you account for the interference pattern that emerges after many single photons are passed through the double slit?

[vordhosbn]

Here's the description of the experiment in question.

http://arxiv.org/pdf/quant-ph/0702188

So what I think i understood till now is, that every single photon (or other elementary particle for that matter) is a wave - a cloud of probabilities until something affects it's energy - then this quantum cloud of probabilities collapses to single point in space and time and manifests itself. Correct?

[michel123456]

How do you account for the interference pattern that emerges after many single photons are passed through the double slit?

 

I don't know.

 

I have never been involded in such a question, but if you want my opinon (who am I?), the link gently given by vordhosbn is not sufficient to me.

I would like a complete technical description of the experiment, including the distance between the light source and the pinholes, the distance between the pinholes to the screen, width of the obstacle containing the pinholes and its material (the reflection coefficient on both sides, a.s.o.),the accuracy of the pinholes, the material of the screen,..., everything. Also the conditions of the experiments, I suppose it was conducted under void conditions and near zero temperature of course).

 

At first sight, I wonder how they manage to avoid interference due to reflection.

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Something like this:

Edited December 21, 2009 by michel123456
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[swansont]

Your reflections violate Snell's law.

[insane_alien]

and is still works even when it gets out to the point where they cannot hit the mask with the slits

[michel123456]

Your reflections violate Snell's law.

 

Snell's law applies in case of specular reflection. If the surface of the screen is not perfect (and if the angle of incidence is not perfectly orthogonal), my sketch applies.

 

and is still works even when it gets out to the point where they cannot hit the mask with the slits

Too bad.

[swansont]

Snell's law applies in case of specular reflection. If the surface of the screen is not perfect (and if the angle of incidence is not perfectly orthogonal), my sketch applies.

 

Your sketch either shows interference, in which the point is moot, or reflection where the angle is not equal to that of the incident light.

[michel123456]

BTW I was wrong when saying that Snell's law applies (only) in case of specular reflection. Snell's law applies always, but it gets more complicated.

 

 

 

Is this the only one experiment showing the complementarity wave-particle?

Edited December 22, 2009 by michel123456

[swansont]

Is this the only one experiment showing the complementarity wave-particle?

 

It's been repeated a number of times, and other particles have been made to interfere (electrons, neutrons, atoms and even molecules)

[michel123456]

Well understood, but is there no other kind of experiment, other configuration?

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Well understood, but is there no other kind of experiment, other configuration?

 

No answer.

Does that mean the entire QM is hanging from this single kind of experiment?

[swansont]

Well understood, but is there no other kind of experiment, other configuration?

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No answer.

Does that mean the entire QM is hanging from this single kind of experiment?

 

Give us a chance to respond.

 

Mach-Zehnder interferometers and Bonse-Hart (aka LLL) interferometers are two others with which I am familiar.

[michel123456]

Sorry for the hurry. Blame the time zones.

Thank you for the answer.

 

In which way do these interferometers support QM?

Edited December 23, 2009 by michel123456

[swansont]

They show the interference I mentioned earlier.

[michel123456]

Interference is one thing.

A single photon (or a single molecule) passing through 2 slits at the same time is another.

[swansont]

Interference is one thing.

A single photon (or a single molecule) passing through 2 slits at the same time is another.

 

You had asked about "experiment showing the complementarity wave-particle"