Sniping slits in double slit experiment?

[Jabbu]

Why can we not be more precise when shooting stuff through the double slit, so that we know if it goes in the direction of one slit or the other? By the way, is there any significant difference in workings between single and double slit interference?

 

[Dale]

You can, but then it doesn't interfere.

 

[Jabbu]

You can, but then it doesn't interfere.

Do you know what keyword would help me find some more information about those "slit-sniping" experiments?

 

[Dale]

You want to look for "which way" double slit experiments.

 

[Jabbu]

You want to look for "which way" double slit experiments.

I've seen those, but all of them we inserting some kind of detectors in one of the slits, rather than knowing which direction each photon went from the point of emission.

 

[fluidistic]

The way I've been taught QM, there is no such thing as a trajectory of a particle.
Therefore I fail to see any sense in trying to determinate through which slit did the particle pass.

 

[Jabbu]

The way I've been taught QM, there is no such thing as a trajectory of a particle.
Therefore I fail to see any sense in trying to determinate through which slit did the particle pass.

Not determine, but cause it. Lenses. I think we can produce very focused laser beams with precision range far below the size of the slits. So like they do in surgery or integrated circuit manufacturing procedures, we should be able to point our photons precisely in the direction where we want them to go, one slit or the other. Is there any technological limit here I am not aware of?

 

[fluidistic]

Not determine, but cause it. Lenses. I think we can produce very focused laser beams with precision range far below the size of the slits. So like they do in surgery or integrated circuit manufacturing procedures, we should be able to point our photons precisely in the direction where we want them to go, one slit or the other. Is there any technological limit here I am not aware of?

Well if the slits are really large compared to the wavelength of the EM waves, the interference pattern disappears gradually as you enlarge the slits. So I am guessing that if we describe this effect at a quantum level, maybe we could replace the words EM waves by photons but I'm not 100% sure.
I didn't learn the quantum mechanics of photons, these things are massless so I guess that they don't satisfy the Schrödinger's equation. I don't know how to describe them.

 

[Dale]

I've seen those, but all of them we inserting some kind of detectors in one of the slits, rather than knowing which direction each photon went from the point of emission.

That is a single slit experiment.

 

[DrChinese]

Why can we not be more precise when shooting stuff through the double slit, so that we know if it goes in the direction of one slit or the other? By the way, is there any significant difference in workings between single and double slit interference?

Jabbu, you can't keep asking the same question over and over. You asked and it was answered by DaleSpam. I realize that you don't want to read and understand any more than the basics, but this will be necessary for you to get to the next level. I am trying to give you some helpful information, because at the current rate you will have managed to get more threads closed in a week than anyone in recent history here.

The "mystery" of the double slit is that the particle goes through both slits, not just one. You can set it up so that you know which slit the particle goes through. And this can even be done WITHOUT pointing it to one slit or the other, and without blocking a specific slit (polarizers are used). When this happens, there is no interference.

When there is no possibility of knowing the which-slit answer, there is interference. And only then. Molecules as large as buckyballs exhibit quantum interference (C60). Interference is explained by quantum concepts, but not classical ones (which fall apart when particle behavior is exhibited).

 

[Jabbu]

You can set it up so that you know which slit the particle goes through. And this can even be done WITHOUT pointing it to one slit or the other, and without blocking a specific slit (polarizers are used). When this happens, there is no interference.

I only know about experiments where detectors are placed in the slits. Can you point me where I can read more about those experiments which point particles to one slit or the other?

 

[Dale]

Again, that is a single slit experiment.

You certainly can do any number of things to aim the source so that you illuminate only a single slit. But then, by definition, it is not a double slit experiment. Is that not perfectly clear?

 

[Jabbu]

Again, that is a single slit experiment.

I'm asking whether there is a double slit experiment in which particles are pointed towards one slit or the other.

DrChinese said:
- "And this can even be done WITHOUT pointing it to one slit or the other..."

It's about a double slit experiment, suggesting they can be done by pointing particles to one slit or the other. I'd like to know about such double slit experiments because I never heard about them before. I thought no one has done anything like that, that's why I asked in the OP why not. But if someone has actually done it, then I'd like to read about it.

 

[Dale]

I'd like to know about such double slit experiments because I never heard about them before.

You have heard about them. Everyone else calls them single slit experiments.

You cannot illuminate a single slit and call your experiment a double slit experiment. It is a contradiction in terms.

 

[bhobba]

What might help you is if you see a correct analysis of the single and double slit experiment:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

Usually it is used to motivate the QM formalism, but then they do not go back as show how that new formalism explains it. The above corrects that.

However I am with Dr Chinese - something seems to be holding you back.

Maybe approaching QM a different way will help. Here's what it is from its conceptual core:
http://www.scottaaronson.com/democritus/lec9.html

But all of us have had to come to grips with QM - its a long hard slog with a lot of ah-ha moments over even the simplest of things. One such moment for me was the realisation an observation does not require an organic observer, then the realisation that the formalism was about observations that occur in a common sense classical world. For me, the foundations then fell into place.

Hopefully you can sort out exactly what your primary issue is and come to grips with it.

Thanks
Bill

 

[Jabbu]

You have heard about them. Everyone else calls them single slit experiments.

You cannot illuminate a single slit and call your experiment a double slit experiment. It is a contradiction in terms.

They could point every odd photon in the direction of one slit and every even photon in the direction of the other slit. They could aim at only one of the two slits and see if the pattern matches single or double slit fringes. Has anyone done anything like that?

 

[bhobba]

It's about a double slit experiment, suggesting they can be done by pointing particles to one slit or the other

Look at the explanation of the double slit in the paper I linked to.

We have a centrally located source whose position is known so the momentum of what it emits is unknown. Since it has a definite kinetic energy it means its direction is unknown. It can go through either slit. This means its state at the screen is the sum of the state of it going through both slits (a superposition) so you get interference.

If you point it at a particular slit it can't go through both slits so you do not get interference.

Thanks
Bill

 

[bhobba]

They could point every odd photon in the direction of one slit and every even photon in the direction of the other slit. They could aim at only one of the two slits and see if the pattern matches single or double slit fringes. Has anyone done anything like that?

There is no need to do it.

Its obvious what will happen.

If you point it at one slit the state behind the slits is not a superposition of going through both slits and you will not get interference.

Thanks
Bill

 

[atyy]

The strict answer is that you cannot aim an electron to go through one slit or the other. Basically, this is because a single electron is a wave. So even if there is only one slit, you will still get an interference pattern.

Now let's think about classical light for the moment. Athough light is a wave, it can be treated to very good approximation as a ray in some circumstances, when the width of the slit is large compared to the wavelength of the light wave. Similarly, if the slits and the spacing between them is much larger than the electron's wavelength, then we can aim the electron at one of the slits.

 

[bhobba]

The strict answer is that you cannot aim an electron to go through one slit or the other.

You can.

Imagine a long tube that exits close to one slit - and its easy to see such is really the single slit experiment.

Thanks
Bill

 

[Jabbu]

We have a centrally located source whose position is known so the momentum of what it emits is unknown. Since it has a definite kinetic energy it means its direction is unknown. It can go through either slit. This means its state at the screen is the sum of the state of it going through both slits (a superposition) so you get interference.

Can we not direct photons with lenses in precise direction of one slit and away from the other?

 

[Jabbu]

You can.

Imagine a long tube that exits close to one slit - and its easy to see such is really the single slit experiment.

Thanks
Bill

Great. It's much less intrusive way to know "which way" than placing a detector in one of the slits, isn't it? So why not try it, and can you really be sure no one has?

 

[bhobba]

Can we not direct photons with lenses in precise direction of one slit and away from the other?

I will leave if such is actually possible to experimental guys.

But if you direct it to one slit you - BY DEFINITION OF WHAT DIRECTING IT AT ONE SLIT MEANS - it goes through one slit and you do not get interference.

You have logically excluded what you are trying to achieve.

Thanks
Bill

 

[Jabbu]

I will leave if such is actually possible to experimental guys.

But if you direct it to one slit you - BY DEFINITION OF WHAT DIRECTING IT AT ONE SLIT MEANS - it goes through one slit and you do not get interference.

You have logically excluded what you are trying to achieve.

Thanks
Bill

We could simulate random dispersion by deliberately slightly changing direction for each photon so that overall we cover both slits and area around them, but still knowing which photon went which way. How could photons know the difference? Is that not worth trying out? How do you know photons wouldn't go through both slits even if we aim them at only one slit?

 

[atyy]

You can.

Imagine a long tube that exits close to one slit - and its easy to see such is really the single slit experiment.

Thanks
Bill

Yes, if you block one slit, then it will not go through the slit. But the electron will always go through all open slits.

 

[atyy]

I will leave if such is actually possible to experimental guys.

But if you direct it to one slit you - BY DEFINITION OF WHAT DIRECTING IT AT ONE SLIT MEANS - it goes through one slit and you do not get interference.

You have logically excluded what you are trying to achieve.

Thanks
Bill

In fact even if it goes through one slit there is still interference.

 

[bhobba]

So why not try it, and can you really be sure no one has?

Try what?

If you know the position for sure at one of the slits how can the state behind the slits be a superposition of position at both slits?

Thanks
Bill

 

[bhobba]

In fact even if it goes through one slit there is still interference.

Sure - but not double slit interference.

Thanks
Bill

 

[bhobba]

Yes, if you block one slit, then it will not go through the slit. But the electron will always go through all open slits.

Not if its position is known at one slit. If the tube exits at one slit you have blocked off the path to go through the other slit. If you move the exit back a bit and you have a path of traveling along the slit screen then through the other slit - yes it will go through both slits. But viewing it via Feymann's sum over histories approach the slight variation of that wild path will always cancel. Move the exit further back and that new path will cancel less and less so you get gradually more double slit interference.

Thanks
Bill

 

[bhobba]

We could simulate random dispersion by deliberately slightly changing direction for each photon so that overall we cover both slits and area around them, but still knowing which photon went which way.

If you know which slit it went through you do not get double slit interference. Obviously what you are proposing means we know which slit it went through. If you know which slit it went through the state behind the slit screen is not a superposition of position at both slits which is what you need for double slit interference.

Why this constant semantic game to try and avoid it?

Thanks
Bill

 

[Jabbu]

Not if its position is known at one slit.

Thanks
Bill

Position would not be known generally, only starting position and the direction in which it was emitted, and location where it was detected.

Try what?

If you know the position for sure at one of the slits how can the state behind the slits be a superposition of position at both slits?

That's also true for those experiments where they place a detector in on of the slits, but aiming photons in known direction is much less intrusive way to know "which way", so wouldn't that be a better type of the same experiment?

 

[atyy]

Not if its position is known at one slit. If the tube exits at one slit you have blocked off the path to go through the other slit. If you move the exit back a bit and you have a path of traveling along the slit screen then through the other slit - yes it will go through both slits. But viewing it via Feymann's sum over histories approach the slight variation of that wild path will always cancel. Move the exit further back and that new path will cancel less and less so you get gradually more double slit interference.

Thanks
Bill

Yes, I agree. It's a matter of semantics. This is the same as blocking the other slit - in the sense that Jabbu cannot say 'this is a less intrusive way to know "which way" than placing a detector in one of the slits'.

 

[atyy]

That's also true for those experiments where they place a detector in on of the slits, but aiming photons in known direction is much less intrusive way to know "which way", so wouldn't that be a better type of the same experiment?

The "which way" language is a confusing language, even though you will see it often in the peer-reviewed literature. Electrons do not have any way which is a way governed by equations that are the classical limit of the quantum equations. So we cannot ask "which way" the electron went - unless you measure the electron's position at every point in the electron's trajectory - but these experiments do not do that - they only measure position at a few points.

 

[bhobba]

Position would not be known generally, only starting position and the direction in which it was emitted, and location where it was detected.

If you know direction you know momentum and hence its not spread over space and can't go through both slits. To go through both slits it must be in a superposition of direction.

That's also true for those experiments where they place a detector in on of the slits, but aiming photons in known direction is much less intrusive way to know "which way", so wouldn't that be a better type of the same experiment?

If you place a detector at one slit the state behind the slit screen is not a superposition of position for both slits.

Read the link I gave. See equation 9 and its assumption - 'Assuming the source-slit geometry does not favor one slit is over the other, the state vector is the superposition of position eigenvectors'. But if you have a detector at one slit y1 or y2 in that equation is not a Dirac Delta function - its a big fat zero - so the analysis is as per the single slit.

Thanks
Bill

 

[atyy]

Why can we not be more precise when shooting stuff through the double slit, so that we know if it goes in the direction of one slit or the other? By the way, is there any significant difference in workings between single and double slit interference?

There is no fundamental difference between the single and double slit interference. Even with a single slit you get an interference pattern. The double slit experiment is just a single slit experiment in which the slit has a weird shape.