Double Slit Experiment with Electrons

[diemilio]

Hello,

I have a questions regarding the double slit experiments using an electron beam.

If I understand correctly, in the back measuring plate you will see little tiny dots of electrons colliding; however, the pattern exhibited will be similar to that of the one seen in a wave-like experiment rather than just two lines of dots. My question is, what happens in the wall in between the slits? Do you see electrons colliding there? or do the electrodes start showing a wave-like behavior even before going through the slits?

Thanks,

diemilio

 

[phinds]

Hello,

I have a questions regarding the double slit experiments using an electron beam.

If I understand correctly, in the back measuring plate you will see little tiny dots of electrons colliding; however, the pattern exhibited will be similar to that of the one seen in a wave-like experiment rather than just two lines of dots. My question is, what happens in the wall in between the slits? Do you see electrons colliding there? or do the electrodes start showing a wave-like behavior even before going through the slits?

Thanks,

diemilio

If you were to put a detector in the wall between the slits, you would just see dots as electrons impinged on the detector. There would be no reason for a wave pattern since you are not MEASURING a wave pattern. Remember, quantum objects (such as electrons) exhibit wave behavior if you measure wave behavior and particle behavior if you measure particle behavior. You are, perhaps, trying to figure out whether the electron is a particle or a wave. It is neither one. It is a quantum object.

 

[diemilio]

If you were to put a detector in the wall between the slits, you would just see dots as electrons impinged on the detector. There would be no reason for a wave pattern since you are not MEASURING a wave pattern. Remember, quantum objects (such as electrons) exhibit wave behavior if you measure wave behavior and particle behavior if you measure particle behavior. You are, perhaps, trying to figure out whether the electron is a particle or a wave. It is neither one. It is a quantum object.

Thank you very much for your reply!

I see. So basically, if I want to know what happens in-between the slits I would have to take a measurement and that would collapse the wave-function before crossing the slits therefore there would be no interference pattern? Is this a correct assessment?

I am new to this concept so I really appreciate your help :)

 

[phinds]

Thank you very much for your reply!

I see. So basically, if I want to know what happens in-between the slits I would have to take a measurement and that would collapse the wave-function before crossing the slits therefore there would be no interference pattern? Is this a correct assessment?

Yes, although I'm not big on "collapsing the wave function" but then I'm not a physicists. I look at it as simply a function of what you are measuring. If you measure particle-like behavior (such as by sticking a phosphor detector or CCD) in front of the electron, then you SEE particle-like behavior.

I am new to this concept so I really appreciate your help :)

That's what we're here for.

 

[bhobba]

I see. So basically, if I want to know what happens in-between the slits I would have to take a measurement and that would collapse the wave-function before crossing the slits therefore there would be no interference pattern? Is this a correct assessment?

It makes no difference if detectors are placed in the wall between the slits - you still get interference. It only makes a difference if a detector is placed in the slits themselves - then interference effects disappear.

The key thing is if you know which slit it went through. Detectors between the slits has no effect on that - it detects electrons that didn't go through either slit but banged into the wall between the slits.

Thanks
Bill

 

[LachyP]

Hi, and really good question,
Just remember that because of wave-particle duality electrons are neither particles nor waves at any given time, but a mixture of both. Although this is a definitive factor, we have to think of probability waves when it comes to this. So, because of this, the electron has a possibility of ending up anywhere on the back measuring plate. In fact, the electron has the highest possibility of ending up in the area of the back plate directly in between the two slits. So I guess you could say that the electrons are behaving more like waves in this situation, but I would say a small amount of particle behaviour also exists. I really hope this helps :)

 

[phinds]

Just remember that because of wave-particle duality ...

"wave particle duality" is a totally dead concept and has been for 80 years. We discourage use of the term here. You WILL see it in pop-sci discussions and even in some physics books for beginners but that is due to a mistaken belief that it is easier on beginners than reality (see post #2)

 

[LachyP]

"wave particle duality" is a totally dead concept and has been for 80 years. We discourage use of the term here. You WILL see it in pop-sci discussions and even in some physics books for beginners but that is due to a mistaken belief that it is easier on beginners than reality (see post #2)

The term has been long out of use but the general concept remains. Although it may be expressed in different manners currently, the underlying meaning is very similar and mostly unchanged to that of wave-particle duality. Simply stating that not only light but also other particles may behave as both particles and waves.

 

[phinds]

The term has been long out of use but the general concept remains. Although it may be expressed in different manners currently, the underlying meaning is very similar and mostly unchanged to that of wave-particle duality. Simply stating that not only light but also other particles may behave as both particles and waves.

Sure, but the problem is that the phrase/concept "wave particle duality" implies that quantum objects ARE particles and waves when in fact they are neither.

 

[LachyP]

Sure, but the problem is that the phrase/concept "wave particle duality" implies that quantum objects ARE particles and waves when in fact they are neither.

I may stand corrected, but the way I understand it, aren't they a mixture of both? I hope I'm not frustrating you at the moment :(

 

[phinds]

I may stand corrected, but the way I understand it, aren't they a mixture of both? I hope I'm not frustrating you at the moment :(

No frustration at all. That's what this forum is FOR, after all

No, they are NOT a "mixture of both", they are neither one. They exhibit many of the characteristics of each, depending on what you are measuring, but that does not MAKE them one or the other or both any more than being yellow makes a banana skin the same as a pineapple's innards. This is why the phrase "wave particle duality" is deprecated outside of pop-sci TV shows and, unfortunately, some beginners lessons where it has held on for 80 years despite being wrong.

 

[LachyP]

No frustration at all. That's what this forum is FOR, after all :)

No, they are NOT a "mixture of both", they are neither one. They exhibit many of the characteristics of each, depending on what you are measuring, but that does not MAKE them one or the other or both any more than being yellow makes a banana skin the same as a pineapple's innards. This is why the phrase "wave particle duality" is deprecated outside of pop-sci TV shows and, unfortunately, some beginners lessons where it has held on for 80 years despite being wrong.

Oh wow, I have been wrong this whole time... This concept you speak of sounds exactly like Niels Bohr's interpretation. A particle will change the way it behaves (as a wave or particle) depending on the way you measure it. Am I correct?

 

[phinds]

Oh wow, I have been wrong this whole time... This concept you speak sounds exactly like Niels Bohr's interpretation. A particle will change the way it behaves (as a wave or particle) depending on the way you measure it. Am I correct?

Basically, but I'm not sure it's correct to say that it will "change the way it behaves" so much as it is to simply say it shows different faces depending on what you measure. It has a consistent behavior that exhibits different characteristics depending on what your experimental apparatus is sensitive to. The fact that flummoxed folks for a long time is that waves and particles are very different things and it was just hard to take that something could exhibit such different behavior depending on how you looked at it with it being something that was pretty much one or the other but sometimes acted weirdly like the other. It was in the 1920's when several major physicists (Dirac, et al) hashed out that quantum objects are in fact neither waves nor particles but a thing in their own right.

 

[LachyP]

Basically, but I'm not sure it's correct to say that it will "change the way it behaves" so much as it is to simply say it shows different faces depending on what you measure. It has a consistent behavior that exhibits different characteristics depending on what your experimental apparatus is sensitive to. The fact that flummoxed folks for a long time is that waves and particles are very different things and it was just hard to take that something could exhibit such different behavior depending on how you looked at it with it being something that was pretty much one or the other but sometimes acted weirdly like the other. It was in the 1920's when several major physicists (Dirac, et al) hashed out that quantum object are in fact neither waves nor particles but a thing in their own right.

Interesting, thank you for that, and with that information in hand, I should probably re-word my previous statement. "A quantum object will display characteristics more like a wave or a particle depending on what your experimental apparatus is sensitive to". Is that more like what you mean?

 

[phinds]

Interesting, thank you for that, and with that information in hand, I should probably re-word my previous statement. "A quantum object will display characteristics more like a wave or a particle depending on what your experimental apparatus is sensitive to". Is that more like what you mean?

Yep.

 

[LachyP]

Yep.

OK thank you SO much for that, I really appreciate it :)

 

[jerromyjon]

If you try to measure the path an electron takes it behaves like a particle. If you try to measure where it could wind up it acts like a wave. The results defy classical physical logic, because even if 1 single electron at a time traverses the slits it would have to interfere with itself to arrive where it does more often than not.

 

[bhobba]

The term has been long out of use but the general concept remains.

Its the other way around.

It still in use, unfortunately, and the general concept no longer holds, and hasn't since Dirac came up with his transformation theory late 1926.

Thanks
Bill

 

[LachyP]

Its the other way around.

It still in use, unfortunately, and the general concept no longer holds, and hasn't since Dirac came up with his transformation theory late 1926.

Thanks
Bill

Yes, I have been very mistaken in this thread...

 

[diemilio]

Hi,

Thanks everyone for the discussion; I have learned a lot from this. I still have one question related to my original post:

So if I put a detector in-between the slits (or any other detector that gives information of where the electrons went through) is it true that I won't see the wave distribution pattern in the back plate anymore? Is it true that I will now see only two definite stripes that will clearly show that the electrons took a definitive path rather than interfering with themselves?

If this is the case, does anyone have a link to a video or a series of images that show this behavior? I've seen many experimental results that show the interference pattern, but for the two stripes I have only seen drawn diagrams, no experimental results.

Thanks!

 

[phinds]

Hi,

Thanks everyone for the discussion; I have learned a lot from this. I still have one question related to my original post:

So if I put a detector in-between the slits (or any other detector that gives information of where the electrons went through) is it true that I won't see the wave distribution pattern in the back plate anymore? Is it true that I will now see only two definite stripes that will clearly show that the electrons took a definitive path rather than interfering with themselves?

No, the fact that you are detecting those electrons that hit between the slits has nothing to do with the electrons that go through the slits.

 

[jerromyjon]

detector that gives information of where the electrons went through

http://en.wikipedia.org/wiki/Quantum_eraser_experiment

 

[diemilio]

No, the fact that you are detecting those electrons that hit between the slits has nothing to do with the electrons that go through the slits.

OK, understood. What if I put detectors in the slits as bhobba mentioned? what is it about doing that that's cause the pattern to disappear? I'm a very skeptical person, and I have seen this concept being mentioned everywhere, but never seen an actual experiment of it. I tried looking it up in YouTube, but all I could find were videos that show the interference pattern, I guess print those detectors in is a much more complicated thing to do, so I was wondering if anyone has a reference to experimental data.

Thanks again!

 

[Nugatory]

I tried looking it up in YouTube, but all I could find were videos that show the interference pattern

Not surprising, as YouTube is in the entertainment business, not the physics teaching business...

You might start with this paper and the papers it cites: http://arxiv.org/pdf/1210.6243v1.pdf

 

[bhobba]

OK, understood. What if I put detectors in the slits as bhobba mentioned? what is it about doing that that's cause the pattern to disappear?

It, due to interaction with the measuring device, changes the wave-function so interference can not occur. For interference to occur behind the slits must be a superposition of going through both slits - that can't happen if a detector is present.

Although the math is possibly a bit beyond you hopefully the following will give the gist:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

See equation 9. It only has that form due to the symmetry of the situation ie as the text says because you can't favour one slit over the other. Put a detector in there and it changes - its no longer symmetrical - it goes through one slit or the other and there is no interference.

Thanks
Bill

 

[diemilio]

Not surprising, as YouTube is in the entertainment business, not the physics teaching business...

You might start with this paper and the papers it cites: http://arxiv.org/pdf/1210.6243v1.pdf

Hi Nugatory,

Thank you very much; this is a very good paper. However, it does not show what I want to see. It seems the figures are for either an double-slit interference pattern or a probability distribution for single slits. What I would really want to see is this so-called "collapsing" of the wave function in which, without closing any of the two slits, the back-plate will exhibit something along the lines of the sum of the individual distributions (I think it would be something like |φ₁|²+|φ₂|²). I know phinds does not agree with the term of "collapsing" the wave function, but the final effect seems to be real, because it is mentioned everywhere.

Thanks!

 

[diemilio]

Not surprising, as YouTube is in the entertainment business, not the physics teaching business...

You might start with this paper and the papers it cites: http://arxiv.org/pdf/1210.6243v1.pdf

I am wondering if what I am looking for is something that cannot be done. There's a quoute in that paper from Feynman that says: "you should not try to set up [the experiment]” because “the apparatus would have to be made on an impossibly small scale to show the effects we are interested in.”

Is that quote referring to the type experiment I would like to see? Having a measuring device in the slits that would actually change the pattern is "impossible" because there is no way to take this measurement? Or has this actually been done?

Thanks again!

 

[diemilio]

It, due to interaction with the measuring device, changes the wave-function so interference can not occur. For interference to occur behind the slits must be a superposition of going through both slits - that can't happen if a detector is present.

Although the math is possibly a bit beyond you hopefully the following will give the gist:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

See equation 9. It only has that form due to the symmetry of the situation ie as the text says because you can't favour one slit over the other. Put a detector in there and it changes - its no longer symmetrical - it goes through one slit or the other and there is no interference.

Thanks
Bill

I see. Makes sense. So this means that there is no way to actually do this because of what I just posted related to Feynman's quote; correct?

 

[bhobba]

I see. Makes sense. So this means that there is no way to actually do this because of what I just posted related to Feynman's quote; correct?

There is nothing in principle against making a measurement in the slits.

Its exactly as I said - the electron interacts with the measuring device and changes its wave-function.

To be specific at a technical level decoherence takes place and interference terms disappear:
http://www.ipod.org.uk/reality/reality_decoherence.asp

Thanks
Bill

 

[diemilio]

There is nothing in principle against making a measurement in the slits.

Its exactly as I said - the electron interacts with the measuring device and changes its wave-function.

To be specific at a technical level decoherence takes place and interference terms disappear:
http://www.ipod.org.uk/reality/reality_decoherence.asp

Thanks
Bill

So what you're saying is that the measurement itself in a way "corrupts" the original experiment so the results of the original test change, correct? That I can understand.

 

[bhobba]

So what you're saying is that the measurement itself in a way "corrupts" the original experiment so the results of the original test change, correct? That I can understand.

No.

I am saying that the thing being measured and what you measure it with interacts, becomes entangled (specifically via decoherence), and when that happens interference terms are suppressed.

That might be techno-babble, but unfortunately its the best I can do.

The following will explain it carefully:
https://www.amazon.com/dp/0465036678/?tag=pfamazon01-20

Warning - it requires math. But unfortunately that's the only way to explain it CORRECTLY. You will find plenty of popularisations that purport to explain it but mostly its rubbish. Susskind avoids such issues - but your thinking cap needs to be on.

Thanks
Bill

 

[diemilio]

No.

I am saying that the thing being measured and what you measure it with interacts, becomes entangled (specifically via decoherence), and when that happens interference terms are suppressed.

That might be techno-babble, but unfortunately its the best I can do.

The following will explain it carefully:
https://www.amazon.com/dp/0465036678/?tag=pfamazon01-20

Warning - it requires math. But unfortunately that's the only way to explain it CORRECTLY. You will find plenty of popularisations that purport to explain it but mostly its rubbish. Susskind avoids such issues - but your thinking cap needs to be on.

Thanks
Bill

Thanks Bill, I will check out the reference. I have "OK" math skills, so I think I should be fine. If you happen to know of a paper where they show the experimental results I am looking for I would appreciate it.

Thanks again.