Question about the 'double slit' experiment

[bwanaaa]

When 2 slits are close to each other and many electrons are fired at them concurrently, an interference paterns results. My understanding of the paradox is that when the same number of electrons are fired one at a time through the apparatus, the same interference pattern results - the salient question being 'how does the electron interfere with itself'. The next page of the paradox is that 'observing the electron' eliminates the interference pattern.

My question has to do with the field generated by a moving electron. Is that not sufficient to generate an interference effect upon the electron as the electron moves through the slit? In oher words, the presence of a second slit nearby where the electron passes, may deflect thefield it is generating. So many descriptions of the double slit experiment neglect the fact that the electron is a charged particle-they always use the analogy of balls, marbles, etc.

Secondly, when one 'observes an electron' isnt this necessarily done with a field of some sort? If that is the case, then we are disturbing the field generated by the electron and eliminating the interference effect.

Can the double slit experiment be reproduced with neutrons? and if so, please provide a reference to the data. Thanks.

 

[Born2bwire]

Originally posted by: bwanaaa
When 2 slits are close to each other and many electrons are fired at them concurrently, an interference paterns results. My understanding of the paradox is that when the same number of electrons are fired one at a time through the apparatus, the same interference pattern results - the salient question being 'how does the electron interfere with itself'. The next page of the paradox is that 'observing the electron' eliminates the interference pattern.

My question has to do with the field generated by a moving electron. Is that not sufficient to generate an interference effect upon the electron as the electron moves through the slit? In oher words, the presence of a second slit nearby where the electron passes, may deflect thefield it is generating. So many descriptions of the double slit experiment neglect the fact that the electron is a charged particle-they always use the analogy of balls, marbles, etc.

Secondly, when one 'observes an electron' isnt this necessarily done with a field of some sort? If that is the case, then we are disturbing the field generated by the electron and eliminating the interference effect.

Can the double slit experiment be reproduced with neutrons? and if so, please provide a reference to the data. Thanks.

The slit experiment can be done easily with photons. In fact, it is a simple matter to code up a Yee algorithm code to demonstrate the single and multi-slit Young experiments using electromagnetic waves. So no, the field generated by an electron does not interact with the electron. A moving electron produces a changing electric field in time. Thus, a moving electron produces a magnetic field. If you look at the superposition of electrons moving in a common direction, like a current carrying wire, the resulting field is proportional to I/r and is in the phi direction where the direction of the electrons is in the z direction. There is no magnetic field generated at position B that will be present at position A further along the path of the electron. So you can see in a general matter how the electron never sees it's own magnetic field.

The behavior of the double-slit experiment is a matter of quantum mechanics. The particle ends up having two possible paths to follow through, and thus has a superposition of wave functions. These wave functions interfere with each other and the resulting wave function produces the interference pattern despite the fact that there is only one possible path the particle can actually travel. When ever you perform a detection on a particle, you collapse its wave function around the result of the detection. Hence, if you place detectors in the slots, you detect which slot the particle will travel through. But, you now have collapsed the wave function, forcing it to only uphold the possibility that the particle goes through slot A. This causes the inteference to disappear since there is no longer a wave function contribution from slot B.

There really is not an exact physical explanation of this because the behavior lies in the realm of the quantum world. Our way of describing quantum behavior is primarily through probabilities. So there isn't any real way for us to describe what is happening physically, since we cannot determine that to any amount of exact certainty. We can describe the physics that dictates the model and give an interpretation from that. Basically what I mean is, we only have a mathematical description, not the physical process. Physically, we say that the particle goes through only one slot. One way of interpreting the math of the quantum mechanics, is that the particle goes through both slots and interferes with itself.

 

[f95toli]

About the first question: No, it is not and even if it was you could eliminanate that error by simply using an insulator to make the double slit.

Second question: No, there are many ways to do this but it is usually done by absorbing the electron somehow. One obvios way of doing this is to simply coat a screen with phosphorus which emits light when it is hit by electrons; that is how a Cathode Ray Tube works (i.e. an old fashion analog oscilloscope, CRT monitor or TV). The second, more sensitive way of doing it is to use an avanche effect of some sort; something like a CCD detector similar to what you use in a digital camera but with electrons instead of light (a single electron or photon creates an "avalanche" many electrons which in turn generates an electric signal).

Yes, double slit experiments can be done with neutrons. Search for "neutron interferometry" in Google Scholar. It has also been done using various atoms.
In fact it has even been done using C60.
Here is a link to an abstract
http://scitation.aip.org/getabs/servlet...0004000319000001&idtype=cvips&gifs=yes

 

[pcy]

Hi,

I don't understand the problem.

I see no difficulty with one particle going through both slits and the interfereing with itself.



Peter

 

[Lord Banshee]

Born2bwire said it best

1) object a small magitude ie electrons have wave like properties.

2) my understanding of why they don't when you see which whole they go though is because your are adding energy to the particle (with photons from the light). I do not know really, because the probablity of finding a particle at one spot is 0. ... I am taking modern physics right now and we covered this last week... This stuff is still very new to me

 

[f95toli]

Originally posted by: Lord Banshee
Born2bwire said it best

1) object a small magitude ie electrons have wave like properties.

2) my understanding of why they don't when you see which whole they go though is because your are adding energy to the particle (with photons from the light). I do not know really, because the probablity of finding a particle at one spot is 0. ... I am taking modern physics right now and we covered this last week... This stuff is still very new to me

The is not exactly correct. All objects have "wavelike" properties, it is just that the wavelength is inversely proportial to mass; meaning macroscopic object do not usually behave like waves.
The second statement is also only "sort of correct". The problem is that is much more fundamental than that, you will find the "you-need-at-least-one-photon-to-probe-an-object" explanation in many books but it is not the whole truth.
The "problem" is that in order to measure a property (in this case posistion, which is obviously a good observable) you need to interact with it and couple it to the enviroment which is macroscopic (since WE are macroscopic objects) and contains a large number of degrees of freedom; it is this coupling that causes the wavefunction to "collapse" and become "point-like".
Note that this is true also for Schroedingers famous cat, in a real experiment the cat would never be in a superposition; it would EITHER be dead or alive simply because the
cat is too large to be in a superposition of the states |dead> and |alive>.
It is also worth pointing out that intelligence has nothing to do with it, i.e. an "observer" does not have to be a person, it just have to be something macroscopic; the idea that an "intelligent observer" is causing the collapse is just a popular myth.

We have learned a lot about this over the past 25 years, to a large extent because of newly developed theories which describes the process of decoherence and many experiments on quantum compting. This have by neccesisty transformed the subject from what whas essentially philosophy to an engineering problem; the latter is definitly true for solid-state qubits/quantum computers where the rate of decoherence etc is directly linked to what essentially amounts to electrical engineering, i.e. you encounted very practical questions such as how to design the filters you need to "protect" the qubit from external degrees of freedom.

 

[bwanaaa]

If one has a single slit there is no interference pattern. But if that slit is divided into 2 smaller slits, there is an interference pattern. Has anyone looked at more than 2 slits?

btw, tnx, f95toli for your reference-amazing that a bucky ball could behave like an electron or an elementary particle.

Is there a way to do this experiment with single photons?

 

[Born2bwire]

Originally posted by: bwanaaa
If one has a single slit there is no interference pattern. But if that slit is divided into 2 smaller slits, there is an interference pattern. Has anyone looked at more than 2 slits?

btw, tnx, f95toli for your reference-amazing that a bucky ball could behave like an electron or an elementary particle.

Is there a way to do this experiment with single photons?

Multiple slits will of course still produce interference patterns. A single slit will decompose an incident wave back into a point source at the slit. So with two slits, you have two point sources and the superposition of the two cylindrical waves that result produce your interference pattern. Same thing would happen with multiple slits though I think that the effect would be less pronounced because you have a chance that one pair of slit's interference region will overlap another pair's constructive interference region.

And yes, there are ways to do this experiment with single particle emissions and they have been done. This allowed an excellent confirmation of quantum mechanics. You can find a wonderful movie of this with electrons here.

I believe you could also do this the way that Young originally did. That is, provide a point source by passing light through a pinhole. Then split the light with the edge of a card. You can probably get a better effect using a laser as your light source.