Double Slit Experiment: Does Observing Affect Interference?

[vincentm]

I stumbled upon this, basically what i want to know is. Does a single electron shot through two slots produce an interference pattern?
When you observe which slot the electron goes through does the act of observing prevent it from producing an interference pattern?

 

[Doc Al]

Does a single electron shot through two slots produce an interference pattern?

A single electron would not make much of a pattern! But if electrons are shot through the double slit one at a time, an interference pattern will develop. (This was demonstrated by Tonomura in 1989.)

When you observe which slot the electron goes through does the act of observing prevent it from producing an interference pattern?

Correct. If your observation is sufficient to unambiguously identify which slit the electron passed through, then no interference pattern will be observed.

 

[simon009988]

something to add here is that quantum mechanics is satistacial in nature. so for example if you could only roll a dice once, you would not be able to see much of the probilistic nature of the die of having 6 equally differnt outcomes.

 

[munky99999]

ive been pondering this also. and how the observer addition affects results. But what I am confused about. Did the experiment change at all?

Take this for example. I'll try to be articulate.

Since it is a double slit. There is some sort of paper or whatever for the slits to be put in.

Now if you look way down to electron size. The paper would have a width that is much larger then the electron. Such that the electron could be reacting with the sides of the slits. If not colliding with the edge of the slit. The proton/electrons that make up the paper's edge. Their charge effects the electron's path and makes that sort of "effect" of interference.

Perhaps i don't understand the other explanation of how the electron affects itself. Like I've done the quantum physics which explain where the electrons are depending on the valance shell and such. Which seems like they are using the same principle. But it doesn't make sense. Like regardless of all the different positions its likely in. Its not in 4 pieces. Its just one electron-1piece.

Anyway I am betting its more simple of a solution and its the experiment equipment which made bad results.

http://img122.imageshack.us/img122/9128/perhaps6or.jpg

 

[ZapperZ]

ive been pondering this also. and how the observer addition affects results. But what I am confused about. Did the experiment change at all?

Take this for example. I'll try to be articulate.

Since it is a double slit. There is some sort of paper or whatever for the slits to be put in.

Now if you look way down to electron size. The paper would have a width that is much larger then the electron. Such that the electron could be reacting with the sides of the slits. If not colliding with the edge of the slit. The proton/electrons that make up the paper's edge. Their charge effects the electron's path and makes that sort of "effect" of interference.

Perhaps i don't understand the other explanation of how the electron affects itself. Like I've done the quantum physics which explain where the electrons are depending on the valance shell and such. Which seems like they are using the same principle. But it doesn't make sense. Like regardless of all the different positions its likely in. Its not in 4 pieces. Its just one electron-1piece.

Anyway I am betting its more simple of a solution and its the experiment equipment which made bad results.

http://img122.imageshack.us/img122/9128/perhaps6or.jpg

Then you have lost your bet. You are ignoring the fact that I could get the same type of interference pattern using a superconducting loop, such as those observed using SQUIDs. There are no "slits" to "scatter" off here, since by definition, the supercurrent maintain long-range coherence. All I have is two separate paths for the supercurrent to flow through. Not only that, I can adjust the phase difference between the two path simply by threading a magnetic field flux around the loop of the two paths!

And I haven't even commented yet on why this scattering scenario would be inconsistent with the observation that the interference pattern changes with slit width and slit separation.

Zz.

 

[munky99999]

ive never heard of this squid thing before. Ill have to go look it up after my vacation.

But my question is. Why would an observer make it act like a particle. When it acts like a wave with no observer. What does the observer bring to the table that would cause this.

I ask this because i don't take the electron interferes with itself explanation. It makes absolutely no sense. Like even if the electron has 4 possible spots to be in. If you take one Planck time interval. It is only in 1 of those spots. The math being not complicated enough to say which. But it should only have 1 position in space.

 

[ZapperZ]

ive never heard of this squid thing before. Ill have to go look it up after my vacation.

But my question is. Why would an observer make it act like a particle. When it acts like a wave with no observer. What does the observer bring to the table that would cause this.

I ask this because i don't take the electron interferes with itself explanation. It makes absolutely no sense. Like even if the electron has 4 possible spots to be in. If you take one Planck time interval. It is only in 1 of those spots. The math being not complicated enough to say which. But it should only have 1 position in space.

Read the FAQ on wave-particle "duality". You are ALREADY assuming that your classical description of a "particle" for an electron is valid. If such a thing is true, we would not have chemistry!

http://www.chem.wisc.edu/courses/115/fall02/PDFfiles/BondingWeinhold.pdf

Zz.

 

[munky99999]

im not assuming the classical is right. I either have an incorrect understanding of QM and wave/particle duality; as there has been an even newer understanding than what i have learnt(textbook was ancient)

Lets scale things up. I am considered a wave in a way.

If I walk i have speed. I bounce up and down when i walk. which gives me the wavelength and period. So in a way I am a wave. But just because I act as a wave. Doesnt make me immaterial. I'm still solid as ever.

Same thing could be said for the wave which you might see at a hockey game or some sport. The people(electrons) exhibit the properties of a wave. While clearly are particle.

 

[ZapperZ]

im not assuming the classical is right. I either have an incorrect understanding of QM and wave/particle duality; as there has been an even newer understanding than what i have learnt(textbook was ancient)

Lets scale things up. I am considered a wave in a way.

If I walk i have speed. I bounce up and down when i walk. which gives me the wavelength and period. So in a way I am a wave. But just because I act as a wave. Doesnt make me immaterial. I'm still solid as ever.

And you think you can do this with an electron? Can you please show me how you are able to track down the trajectory of an electron the very same way you are following the trajectory of your bouncing self? Can you show me your equivalence of a long-range phase coherence that an electron can condense to? Can you show me your bonding-antibonding band similar to the one that an electron in a valence band can form?

You seem to be forgetting that for every analogy that you can come up with, I can come up with several in which there are NO equivalence whatsoever with your classical experience. YOU are not a quantum object. You will have an impossible time to convince me that every single part of your body is in a phase coherence so much so that you can exhibit quantum properties. If we can see quantum properties THAT easily, it wouldn't have been so difficult to accept!

Same thing could be said for the wave which you might see at a hockey game or some sport. The people(electrons) exhibit the properties of a wave. While clearly are particle.

And again, you think this is the SAME thing? Horrors!

Zz.

 

[munky99999]

so your basically saying that my analogy is wrong because the size I am using isn't quantum. But that's what i don't get. How can the rules change when going quantum.

I don't know specifics. but here is my understanding of this.

They did the double slit and found that it had an interference pattern. But then they added an observer and it then followed the particle pattern.

What I am asking. How or in what way does the observer affect the results? I am betting that it doesn't have an effect(as observer's definition means non-interference) and that equipment simply changed for the better. So in the experiment with the observer they had a better electron gun that didnt spread or something. Or as i explained poorly above with the picture.

But quantum mechanics if its the same. would say that each electron goes thru both slits, none, or just 1. and when the both slits occurs that's when its screwed up and becomes interfered, which it would interfere with itself. but when they use an observer this option doesn't happen. Sooooo. Usually when that would happen. It would probably mean the observer is interfering somehow. Or. the experiment with the observer is the true correct result. and the non-observer experiment was flawed.

 

[koantum]

A single electron would not make much of a pattern!

What do you mean not much? A single electron leaves a single mark at the backdrop. The inference pattern is the pattern of marks left by many electrons.

 

[penguinraider]

so your basically saying that my analogy is wrong because the size I am using isn't quantum. But that's what i don't get. How can the rules change when going quantum.

Youre using Newtonian ideals, which should be crushed, stabbed and burned before going into QM. A book which might help you understand QM better is "In search of Schrodingers cat" by John gribbon. Really simplifies stuff. As much as QM can make sense or be simplified.

 

[Doc Al]

A single electron would not make much of a pattern!

What do you mean not much? A single electron leaves a single mark at the backdrop. The inference pattern is the pattern of marks left by many electrons.

Exactly my point. A single electron leaves a single mark--not much of a pattern in a single mark. It takes many electrons to display a pattern.

 

[Schrodinger's Dog]

You'd be able to tell though whether the photon hit the the screen without interference though or had been interfered with even with a single photon wouldn't you? So you'd know if a single photon had "interfered with itself" wouldn't you? Which I believe they say it does unless you have detected which hole it goes through then the single photon is not interfered with or have I got this wrong?

 

[DaveC426913]

If I walk i have speed. I bounce up and down when i walk. which gives me the wavelength and period. So in a way I am a wave. But just because I act as a wave. Doesnt make me immaterial. I'm still solid as ever.

Caveat: While I'll grant this as an analogy for the moment, it is critical that you realize that the subatomic world has no macroscopic equivalent. These analogies will ALWAYS fall apart.

Now. You are a tangible object, bobbing up and down as you walk. You come upon two doorways, separated by a gap. Demonstrate how you pass through both of them.

 

[Doc Al]

You'd be able to tell though whether the photon hit the the screen without interference though or had been interfered with even with a single photon wouldn't you? So you'd know if a single photon had "interfered with itself" wouldn't you? Which I believe they say it does unless you have detected which hole it goes through then the single photon is not interfered with or have I got this wrong?

I'm not clear on what you are saying. If your experiment consists of passing a single photon through a double slit, you will get a single mark on the screen, whether or not you observed the slit that it passed through. Can't tell much from a single mark.

But it's certainly true that the interference that takes place as photons pass through the double slits is not due to photon-photon interactions. You can set up the experiment such that at any given time only a single photon is passing through the slits. (You can also do this with electrons, as I mentioned earlier.) After many photons have passed, the pattern displayed will demonstrate the typical double-slit interference. But if you conspire to observe through which slit the photons pass (essentially localizing it), the interference pattern will be lost.

 

[Schrodinger's Dog]

if it's not photon - photon interaction then it must be the photon itself which causes the interference, it interferes with itself,at least I'm lead to believe(I have been wrong in the past and could so easily be so again) is it just me or are you unhappy with this. I can't help thinking that there's something simple there but because of the nature of light itself it won't let us see it, maybe I just need to do a bit more digging or research?

Could I not as easlily say if it behaves like a wave unless we view it directly then it behaves more like a particle and the interference is lost is merely just the act of the detector itself causing or making the wave travel through that slit and not both(affecting it in some way, that forces the path, it is still a wave but it has been diverted repolarized or whatever you want to call it by the detector into that path and that path alone. If this is so then maybe it is simply an effect of the interaction and not some mysterious inderterminency? Am I way off here, I usually am?

Since wherever you place the detector the light hits the screen in the same place because of the same effective interaction? I think I need to dig deeper maybe

 

[munky99999]

"In search of Schrodingers cat" by John gribbon.

amazon.com doesn't have it. and i don't have the time to goto the bookstore. I am goin to mexico in a couple days.

Youre using Newtonian ideals, which should be crushed, stabbed and burned before going into QM.

Im using a sort of mix.

Im sure you've done the valance shell stuff with QM and the atoms. With the probably places which 1 electron might be. Where you have a couple little balloon type areas where the electron will most likely be. I agree with this. But the sense I am getting is that your trying to say that the electron isn't in just 1 of these spots. but rather all of them.

Say I were omnipotent(im not saying I am not) and i made us quantum size and froze time. We went up to the atom. What would we see? I am guessing that the electron is in one of those areas that QM would suggest. But not in all of them.

Unlesssss. when they say there is 2 electrons in the 1st valance shell. its really just 1 that's split up in a sort of weird type thing trying to be claimed here.

 

[masudr]

Youre using Newtonian ideals, which should be crushed, stabbed and burned before going into QM.

Be careful there. Try and retain a lot of the Hamiltonian formalism (and the Lagrangian stuff too for later work) and QM won't appear as if it's been pulled out of thin air when you first come to it.

 

[ZapperZ]

Im sure you've done the valance shell stuff with QM and the atoms. With the probably places which 1 electron might be. Where you have a couple little balloon type areas where the electron will most likely be. I agree with this. But the sense I am getting is that your trying to say that the electron isn't in just 1 of these spots. but rather all of them.

Say I were omnipotent(im not saying I am not) and i made us quantum size and froze time. We went up to the atom. What would we see? I am guessing that the electron is in one of those areas that QM would suggest. But not in all of them.

Unlesssss. when they say there is 2 electrons in the 1st valance shell. its really just 1 that's split up in a sort of weird type thing trying to be claimed here.

But here's the problem with what you are saying. IF the electron is just at one place at any given time, and all those "balloon" types are simply where the electron are whizzing around, you will get RADIATION coming out of an atom in the ground state. An electron whizzing around THAT fast in such a way that it could maintain chemical bonds would radiate SO MUCH ENERGY, you'd have cancer already by now.

And we haven't even tackle yet how you propose to explain that the s-orbital, for example, has ZERO angular momentum. Is this typical of something whizzing around in a spherically symmetry volume?

Zz.

 

[Schrodinger's Dog]

if it's not photon - photon interaction then it must be the photon itself which causes the interference, it interferes with itself,at least I'm lead to believe(I have been wrong in the past and could so easily be so again) is it just me or are you unhappy with this. I can't help thinking that there's something simple there but because of the nature of light itself it won't let us see it, maybe I just need to do a bit more digging or research?

Could I not as easlily say if it behaves like a wave unless we view it directly then it behaves more like a particle and the interference is lost is merely just the act of the detector itself causing or making the wave travel through that slit and not both(affecting it in some way, that forces the path, it is still a wave but it has been diverted repolarized or whatever you want to call it by the detector into that path and that path alone. If this is so then maybe it is simply an effect of the interaction and not some mysterious inderterminency? Am I way off here, I usually am?

Since wherever you place the detector the light hits the screen in the same place because of the same effective interaction? I think I need to dig deeper maybe

I should really expalin this as it's not clear what I mean, I mean there is no destruction of the wave just a changing of its waveform, the energy involved in detecting it changes the wave so that now it behaves as a wave after the interaction but with some particle like constraint, so that the waves at the second slit dissapear, that means the the wave itself is directed through the slit only because of the x factor of detection however you want to define it, the wavelike properties that pass through both slits are destroyed but the wave is kind of recreated as a wave at a point beyond the slit where as it was more wavelike until this "collision"(kind of a reaction to certain polarizations causing interference to disapear, trying to infer the creation of this polarization to destroy the wave, but not if you see what I mean, yeah it's pretty silly) sounds stupid and I agree, but it was just a thought, if this new "wave" progresses it will make a point as if it was a particle will it not? But essentialy it has had it's energy diverted or directed in an odd way. I now realize this is pretty silly but when I was writing it it sounded like sense, I then realized later the whole idea was pretty unconvincing, but please forgive the thought

I can't be the first person to grasp at straws to get some understanding of this mind boggling wierdness, especially when you bring electrons or other suposedly more particle like matter into it.

 

[munky99999]

thats just it. There are things of QM that QM explains which makes complete sense. But other things which create a "wierdness" that I am calling out on.


Like I understand that electron is particle and wave. as the same goes for me in a way. But the explanation that QM gives forth which happens to break down anytime that an observer is used. doesn't make sense and I am calling them out.

How does the observer affect the results. Or explain why the original hypothesis was wrong.

 

[Schrodinger's Dog]

Yeah I doubt even they are very happy with the Copnehagen interpretation, it's all very well but it's dishearteningly vague. Science is about being able to look at results and extrapolate theory from them, when youre forced into doing it the other way round it's kind of annoying. Whilst it's great when you get the results from a theory you expect(I would imagine) When someone says you can never get any results, you throw your hands in the air and say pah this can't be the answer, we're just not looking hard enough or we're missing something? At least that's the way I think, I'd rather ask a million wrong questions than none at all

 

[Doc Al]

thats just it. There are things of QM that QM explains which makes complete sense. But other things which create a "wierdness" that I am calling out on.

Sounds like you are challenging QM to explain itself to you in classically understandable terms. Lots of luck!

Like I understand that electron is particle and wave.

You do? Those are just semi-classical analogies, neither of which quite works to "explain" things--at least not in the classical manner. I could say, yes, the electron is a "particle", but particles don't act the way you think they do.

as the same goes for me in a way. But the explanation that QM gives forth which happens to break down anytime that an observer is used. doesn't make sense and I am calling them out.

How does the observer affect the results. Or explain why the original hypothesis was wrong.

Can you see that the experimental setup required to "observe" the position of the photon (or electron) as it passes through the slits is physically different than that required to let the particles pass without observation?

That "source + slits + screen" is physically different than "source + slits + detectors + screen"? And that the detectors must interact with and thus influence the behavior of the particles they are "observing"? On the scale of bowling balls, this influence is negligible. But on the scale of electrons and photons, it is not.

 

[ZapperZ]

Yeah I doubt even they are very happy with the Copnehagen interpretation, it's all very well but it's dishearteningly vague. Science is about being able to look at results and extrapolate theory from them, when youre forced into doing it the other way round it's kind of annoying. Whilst it's great when you get the results from a theory you expect(I would imagine) When someone says you can never get any results, you throw your hands in the air and say pah this can't be the answer, we're just not looking hard enough or we're missing something? At least that's the way I think, I'd rather ask a million wrong questions than none at all

However, there is a difference between asking a million questions versus the inability to comprehend the answers you were given AND being ignorant of experimental evidence. The former is perfectly valid, the latter is perfectly annoying.

You WILL note and remember that physicists, by definition, are employed to ask and find answers to many questions. If we already know everything, I would be unemployed. However, unlike what is happening here, we simply cannot ignore the experimental observations that are staring right in our faces, AND, we cannot plead ignorance of an observation simply because no one has told us before. Do that a few times and your credibility goes to zero and no one would listen to you, much less give you research funds.

What's my point in all of this? That this "double-slit" phenomenon is just but ONE of a gazillion example of the principle of SUPERPOSITION as applied in quantum mechanics. It means that there are TONS of other observations, ranging from molecular bonding in chemistry, to band structure in solid state physics, to the coherence gap due to the supercurrent in a SQUID experiment, etc.. etc. You cannot simply propose an "explantion" for the double-slit while being ignorant of all of these other experimental observations because they are ALL based on the IDENTICAL principle. Look up anything that says Schrodinger Cat-type states, and you have all the identical principle being illustrate in so many different ways. It is why I asked for where such a "scattering" off the slit walls would occur in a SQUID experiment that exhibits the IDENTICAL interference pattern.

You cannot learn or challenge physics simply by knowing the field in bits and pieces. It doesn't work that way in today's age. Every areas of physics are interrelated. A change in one will unravel in another. If you think that the your explanation of the double-slit ONLY affects the double slit, then you are missing a lot of physics.

Zz.

 

[munky99999]

Can you see that the experimental setup required to "observe" the position of the photon (or electron) as it passes through the slits is physically different than that required to let the particles pass without observation?

That "source + slits + screen" is physically different than "source + slits + detectors + screen"? And that the detectors must interact with and thus influence the behavior of the particles they are "observing"? On the scale of bowling balls, this influence is negligible. But on the scale of electrons and photons, it is not.

Ok there we go. Observer interferes and corrupts data. So the results from the observer experiment is flawed and there is no more wierdness. In that regard. So perhaps a new way. Now there isn't any big contradiction for me.

 

[Schrodinger's Dog]

You misconstrue my points, I understand the wierdness( a contradiction in terms but you know what I mean) and am not looking for a classical explanation, that may never happen. What I'm looking for is something lacking general bewilderment and a shrugging of shoulders because we're doing experiments with boxing gloves and maybe without our spectacles. OK if it really is just so, then fair enough, but without knowing it all I'm not quite ready to lay down my unquestioning agreement to what is an interpritation not a rule book. And I'm not accusing physisists of doing the not answering questions bit, I agree we have a lot to learn, and that's what excites me as a proto or even pseudo scientist. I could think of no worse fate than being born into the world of classical physics in 1810.

Did Dr Al once say the same as I did, cool, I love it when that happens I know I'm not the only one. I've spent a lot of personal time looking into this area which is at the heart of QM interpretation. I agree with Bohr I'm a bit confused, but happily confused, nothing would be worse than having all the answers at least if you plan on making a career of this stuff.

 

[ZapperZ]

However, in this WHOLE thread, there is a clear attempt to either ignore, or simply a lack of understanding of the fact that one CAN actually detect the consequences of such a superposition. Why there is a lack of attention to this fact is very puzzling to me. So it isn't just about "detectors" or "detection". It is a fundamental aspect of our world as far as QM is concerned, the same way the Heisenberg uncertainty principle isn't a "detector accuracy", but something inherent.

Zz.

 

[Schrodinger's Dog]

Bien sur, that's why you're here to clear up the absurd notions and irrelevant questions. It would be such a boring forum if people didn't ask "stupid questions" We do appreciate the time you spend pointing out the obvious to the knowless or layman, you might not believe it sometimes, but we do

 

[Doc Al]

Ok there we go. Observer interferes and corrupts data.

What corruption? A different experiment gives different data. The "observation" of which slit is traversed destroys the coherence of the two amplitudes (one from each slit).

So the results from the observer experiment is flawed and there is no more wierdness. In that regard. So perhaps a new way. Now there isn't any big contradiction for me.

No flaw; an inherent property of the world. But I'm puzzled by your lack of puzzlement. If you know which slit the particles traverse, then the pattern on the screen is the one classically expected: No interference is observed. But the really interesting thing is the interference pattern! Direct evidence of quantum superposition! (Just one of many such examples of classically unexpected behavior, as ZapperZ stated.) This is truly "wierd" stuff, at least from a classical physics perspective.