B Double Slit Experiment: Dumb question that needs to be asked

[vadadagon]

We don’t need to. Here’s a somewhat handwavy description of how the math works:

The probability of a photon landing at any given point on the screen is calculated by considering all possible paths between the photon source and that point. Each path makes either a positive or a negative contribution to that probability, and we sum all of these to get the total probability (the actual probability is the square of this sum).
When the polarizers are aligned so that any photon can pass through either slit there will be some areas of the screen where the contribution from paths through one slit will be positive while that from the other is negative and they cancel; in others both will have the same sign and they reinforce one another; and we get the alternating regions of high and low probability that make an interference pattern.

But when we arrange the polarizers so that any given photon can only have gone through one slit or the other, then we only have the contribution from the path through that one slit. There’s no opposite sign contribution from the other path to cancel or reinforce it, so no interference pattern. (Actually there is a bit of pattern, usually called a “diffraction pattern”, that comes from having some paths through the left-hand side of the single slit and others through the right-hand side).

Feynman’s non-serious layman-friendly book “QED: The strange theory of light and matter” is worth reading. It is no substitute for learning the math, but it goes into more interesting examples of this “contributions from all paths” model.

I am an amateur and in no way do I understand the math or physics at play.

However, it is a field I find fascinating. Has there been any double slit experiments with different types of filters? Say a medium which will slow down the photons? Does slowing down photons equally thru each slit affect the pattern? I would imagine slowing one and not the other would change the wave pattern. Just interested in knowing what other filters were tried.

 

[Nugatory]

I am an amateur and in no way do I understand the math or physics at play.

Get hold of that Feynman book I mentioned earlier in this thread - it is written for a no-math audience. (The same general model is also online, somewhat less layman-friendly, in the Feynman lectures, someone reading this can provide a link).

Has there been any double slit experiments with different types of filters? Say a medium which will slow down the photons? Does slowing down photons equally thru each slit affect the pattern? I would imagine slowing one and not the other would change the wave pattern.

The easiest way of producing this effect is to slide the barrier a bit sideways so the lengths of the two paths, and hence the travel times and phases at the screen are different. And yes, the interference pattern changes exactly as expected when we calculate the slightly different contributions from the now slightly different paths.

 

[vadadagon]

Get hold of that Feynman book I mentioned earlier in this thread - it is written for a no-math audience. (The same general model is also online, somewhat less layman-friendly, in the Feynman lectures, someone reading this can provide a link).

The easiest way of producing this effect is to slide the barrier a bit sideways so the lengths of the two paths, and hence the travel times and phases at the screen are different. And yes, the interference pattern changes exactly as expected when we calculate the slightly different contributions from the now slightly different paths.

Once again thank you for the book reference. I'll add that to the other book you referred to. In fact, I've already downloaded a copy of each from Kindle. I'll have some light reading tonight.

I'll let you know how it goes (or whether I'll be lost in the woods like Hansel and Gretel).

 

[PeroK]

Once again thank you for the book reference. I'll add that to the other book you referred to. In fact, I've already downloaded a copy of each from Kindle. I'll have some light reading tonight.

I'll let you know how it goes (or whether I'll be lost in the woods like Hansel and Gretel).

On another thread, someone posted a link to David Tong's notes on QM. These are proper undergraduate notes, so they may get a bit advanced for you. If you want to learn the real thing, then these may be worth studying:

http://www.damtp.cam.ac.uk/user/tong/qm/qm1.pdf

 

[vadadagon]

On another thread, someone posted a link to David Tong's notes on QM. These are proper undergraduate notes, so they may get a bit advanced for you. If you want to learn the real thing, then these may be worth studying:

http://www.damtp.cam.ac.uk/user/tong/qm/qm1.pdf

Thanks. Currently half way thru "Where does the weirdness Go?" by David Lindley. It's all pretty straight forward so far. Zero math and all logic. The Copenhagen interpretation, the Bohr interpretation, the EPR proposal and currently David Bohm's interpretation.

It all comes down to whether you want a classical view of the world or a indeterminate view of the world and who is right. In other words do we live in a deterministic universe or an indeterminate universe and the implications of living in one or the other as well as if any of this is real or not. Anyway, all this simply to say Thank you! I'll look at the QM1.pdf you provided when I'm done with Lindley and Feynman (or maybe I'll read it before since it seems to only be 20 pages).

 

[PeterDonis]

It all comes down to whether you want a classical view of the world or a indeterminate view of the world

Not the way you put it here. We already know that a classical view of the world can't be right; classical physics doesn't work. But the alternative to "classical" is not "indeterminate", it's "quantum"--at least, that's the only other alternative we currently have.

In other words do we live in a deterministic universe or an indeterminate universe and the implications of living in one or the other

As far as our current knowledge goes, this depends on which QM interpretation you choose. Some (like the MWI) are deterministic; others (like "physical collapse" interpretations) are indeterminate. Since all QM interpretations make the same predictions for experimental results, we have no way of resolving this question at our current state of knowledge.

as well as if any of this is real or not.

This is a question of philosophy, not physics.

 

[vadadagon]

Not the way you put it here. We already know that a classical view of the world can't be right; classical physics doesn't work. But the alternative to "classical" is not "indeterminate", it's "quantum"--at least, that's the only other alternative we currently have.As far as our current knowledge goes, this depends on which QM interpretation you choose. Some (like the MWI) are deterministic; others (like "physical collapse" interpretations) are indeterminate. Since all QM interpretations make the same predictions for experimental results, we have no way of resolving this question at our current state of knowledge.This is a question of philosophy, not physics.

I'm not even sure it is worth replying. I came here to honestly learn and perhaps be educated and your responses honestly come across as demeaning to me, which makes me want to avoid replying.

You clearly are educated and have a degree in Physics and I am neither educated in nor do I have a degree in Physics. I don't question your knowledge, but I am certainly not interested in engaging with you at any level.

 

[PeterDonis]

I came here to honestly learn and perhaps be educated and your responses honestly come across as demeaning to me

I'm not sure why. I am giving you honest feedback about what you are saying. That's what you came here for, isn't it?

You clearly are educated and have a degree in Physics and I am neither educated in nor do I have a degree in Physics.

Yes, which means I might possibly know things about the subject under discussion that you don't, and I am trying to tell you some of them in my posts.

You are of course free to ignore what I say and not respond at all. That's entirely up to you. But it seems a bit odd for you to say you want to learn but yet ignore what people who you admit know more about the subject than you do are telling you.

 

[vadadagon]

I'm not sure why. I am giving you honest feedback about what you are saying. That's what you came here for, isn't it?Yes, which means I might possibly know things about the subject under discussion that you don't, and I am trying to tell you some of them in my posts.

You are of course free to ignore what I say and not respond at all. That's entirely up to you. But it seems a bit odd for you to say you want to learn but yet ignore what people who you admit know more about the subject than you do are telling you.

Your responses are not feedback, they are criticisms which are different. A Feedback provides a constructive response, a criticism is simply pointing out a problem without providing any solutions. Criticisms are not conducive to fomenting a discussion but encourage an argument.

"Not the way you put it here. We already know that a classical view of the world can't be right; classical physics doesn't work. But the alternative to "classical" is not "indeterminate", it's "quantum"--at least, that's the only other alternative we currently have."

I never stated that the alternative to Classical is Indeterminate and it is an assumption you made. I don't use specific wording and sure I did say "It all comes down to whether you want a classical view of the world or a indeterminate view of the world"

However, you never stopped to ask what I meant by classical and I didn't mean Classical Physics but classical in the deterministic way. Classical in the sense that if I measure the weight of an object Today, and 100 others also measures the weight of the same unchanged object a minute or an hour later the weight of the will remain the same. Sure there is going to be a margin of error (as there is with any measurement or calculation made), but the error is negligible.

Should I have used Deterministic instead in my statement? Absolutely, but I didn't and for that you response wasn't to provide guidance or advice. No instead it was to point out that I was wrong, but never really bothering to explain why I was wrong, or provide any resources or link or explanations and therefore a criticism and not a feedback.

"You are of course free to ignore what I say and not respond at all. That's entirely up to you. But it seems a bit odd for you to say you want to learn but yet ignore what people who you admit know more about the subject than you do are telling you."

I haven't ignored the people which have provided me information and books to read as well as the very link I was thanking PeroK for providing and I thanked Nugatory for his suggested reading. I have also liked DrChinese for a link he provided to a lecture I am currently reading. So you assertion that I have ignored people which have provided me with information is incorrect.

If you didn't want a response to your previous post, I sincerely apologize. However your statement "You are of course free to ignore what I say and not respond at all" seemed to me a request and since it was up to me this is my response.

If you care to provide any links for materials or books for me to read, please feel free. However, I won't be replying to any of your future posts.

 

[PeterDonis]

Your responses are not feedback, they are criticisms which are different.

Sorry, I don't buy your hairsplitting distinction here. But as I have already said, you are free to ignore what I or anyone else says.

I never stated that the alternative to Classical is Indeterminate

How else am I supposed to interpret this:

It all comes down to whether you want a classical view of the world or a indeterminate view of the world

Those are your words. I can't read your mind. I can only respond to what you post.

you never stopped to ask what I meant by classical and I didn't mean Classical Physics but classical in the deterministic way.

Then you are using the word "classical" in a way nobody else in the physics literature does. You can't expect to be understood if you do that. If you're going to use a commonly used term, you need to use it with its commonly used meaning--or else you need to explicitly give the non-standard meaning you are using.

Not to mention that equating "classical" with "deterministic" implies that QM is not, which, as I said, is interpretation dependent.

Classical in the sense that if I measure the weight of an object Today, and 100 others also measures the weight of the same unchanged object a minute or an hour later the weight of the will remain the same.

Assuming the object has not changed (taken on any mass or ejected any mass), sure. And this remains true in QM. Does that make QM "classical" by your definition?

Should I have used Deterministic instead in my statement? Absolutely, but I didn't

And then you expect me to read your mind and respond to what you meant, instead of what you said. That is not a realistic expectation. Again, I can only respond to what you post.

you assertion that I have ignored people which have provided me with information is incorrect.

You have thanked people for providing you with references, yes. But I was not providing you with references. I was telling you things about QM that it did not seem to me that you were aware of. I have told you at least one more such thing in this post.

 

[PeterDonis]

@vadadagon please use the PF quote feature to quote things that you are responding to. There are "Quote" and "Reply" buttons at the bottom right of each post; the same buttons also pop up if you highlight a portion of a post. I find that to be the easiest way to quote: highlight, then click "Reply" and the text that was highlighted appears in the response window already quoted.

 

[kered rettop]

As I said, with the polarizers with $90^{\circ}$ relative orientation mounted in the slits the polarization of the photon tells you through which slit it came. So if you measure the polarization, e.g., by using another polarization filter in, e.g., the H-orientation, you know, from where the photon came, i.e., if it's transmitted, it's H-polarized and thus came necessarily from slit 1, if it's absoberd it's V-polarized and thus came necessarily from slit 2.

And that's precisely what's meant by "you can determine" through which slit it came by doing this polarization measurement. Whether you really do this measurement or not, you'll not have double-slit interference patterns.

You can only have the double-slit interference patterns (with full contrast), if it is impossible to gain which-way information by any possible measurement on the photons.

I think the phrase "any possible measurement" is problematical here. With crossed polarisations, it is perfectly possible to rotate the polarisations back to parallel, whereupon the interference comes back. On the face of it, "rotate and detect" would fall into the category of "any possible measurement". It should be excluded, of course, but I'm wondering what a clearer or more precise expression of the criterion would be?

 

[weirdoguy]

your responses honestly come across as demeaning to me

What is demeaning? Saying that what you write is wrong? That is just stating a fact, and there is nothing wrong with being wrong, unless you are not willing to learn proper knowledge.

 

[kered rettop]

The photon started in a state that is mathematically described as a superposition of vertically and horizontally polarized. When it interacts with a vertically oriented polarizer it is either absorbed (we would say that the filter blocked it) or its state becomes polarized along the vertical axis.

A polarising beam splitter - a stack of Brewster angle plates - splits the beam without absorbing one of the components. This gives us the opportunity to run all four experiments at the same time: HH, HV, VH and VV.
Of course each one shows the same pattern as it would if it were the only case. Edit: It's tidier than having half the photons in the |absorbed> state!

 

[PeterDonis]

On the face of it, "rotate and detect" would fall into the category of "any possible measurement".

The "rotate" part is not a measurement, it's just another polarization operation, which, if done at the right angle, removes all of the which-way information.

It should be excluded

No, it doesn't need to be.

I'm wondering what a clearer or more precise expression of the criterion would be?

The criterion is simple: which-way information removes the interference pattern. Since there is a continuous range of how much which-way information you can have, from zero (parallel polarizations) to complete (perpendicular polarizations), there is a continuous range of how much interference you will observe (from 100% to zero).

 

[vanhees71]

I think the phrase "any possible measurement" is problematical here. With crossed polarisations, it is perfectly possible to rotate the polarisations back to parallel, whereupon the interference comes back. On the face of it, "rotate and detect" would fall into the category of "any possible measurement". It should be excluded, of course, but I'm wondering what a clearer or more precise expression of the criterion would be?

Sure if you put another wave plate in front of the other slit such as to change the polarization state of that photons to the same as those arising from the other slit, you again make the photons indistinguishable and thus you restore the interference pattern, and it's again impossible to get which-way information about the photon, i.e., then there's again no measurement which lets you know through which slit the photon came. I don't know why the phrase "any possible measurement" should be problematical here.

 

[vanhees71]

The "rotate" part is not a measurement, it's just another polarization operation, which, if done at the right angle, removes all of the which-way information.No, it doesn't need to be.The criterion is simple: which-way information removes the interference pattern. Since there is a continuous range of how much which-way information you can have, from zero (parallel polarizations) to complete (perpendicular polarizations), there is a continuous range of how much interference you will observe (from 100% to zero).

Indeed, if you choose to know only something about which way the photon went, i.e., if you are satisfied with only having one path more likely than the other, you'll find an interference pattern with lower contrast.

I also agree that anything that's done to the photon before registering it with a detector/photoplate is part of the "preparation procedure" rather than a measurement. A measurement by definition is when an experimental result is "irreversibly stored" somehow.

 

[Lord Jestocost]

One should always keep “Feynman’s Rules for interference” in mind when analyzing experiments such as the double slit experiment in terms of the quantum mechanical formalism.

“The implication is that actually measuring “which path" a particle takes is not required: the mere possibility in principle of measuring this – that is to say, the existence of anything in the environmental state from which such information could conceivably be drawn – is already enough. This is the content of “Feynman's rules for interference":

I – if two or more fundamentally indistinguishable processes can lead to the same final event (e.g., a particle appearing at a given point on a screen), then add the complex amplitudes for these processes, and take the absolute square of the result to find the probability of the event.

II – if processes are distinguishable, even in principle, then take the absolute squares of their amplitudes individually, and add the resulting probabilities to find the total probability of the event.

This language can still confuse people – what exactly do we mean by “in principle"? For instance, as Einstein repeatedly pointed out, it is certainly possible to measure which slit a particle passes through. The question is whether that possibility exists after the particle has reached the screen. If the experiment was built in such a way that once the particle reaches the screen, there is no longer any conceivable way to measure which slit it followed, then interference occurs; otherwise, it does not.“

From “Quantum Measurements: a modern view for quantum optics Experimentalists“ by Aephraim M. Steinberg (https://arxiv.org/abs/1406.5535)

 

[Morbert]

I also agree that anything that's done to the photon before registering it with a detector/photoplate is part of the "preparation procedure" rather than a measurement. A measurement by definition is when an experimental result is "irreversibly stored" somehow.

The issue is that introduces an interpretation dependency, not strictly enforced by the formalism, insofar as the formalism doesn't stop us from interpreting the initial state as the preparation procedure, and modeling passage through the slits etc with some time-evolution. Perhaps a more interpretation free approach: Instead of saying "You can only have the double-slit interference patterns (with full contrast), if it is impossible to gain which-way information by any possible measurement on the photons." it might be better to say "Measurements which yield which-way information will not contribute to a full-contrast interference pattern."

 

[vanhees71]

For me setting up the slits including the quarter-wave plates in these slits and letting a single photon go through these is all part of the preparation procedure. The measurement is achieved as soon as somehow the photon is registered at the screen (e.g., leaving a spot on a photoplate or saving the position of the excited pixel of a silicon detector in some computer storage). The state of the photon represents the state of the electromagnetic field before the measurement has been performed. After the measurement the photon is absorbed and the em. field is then prepared in the vacuum state.

My formulation refers to the setup with the two quarter-wave plates in position. Then the photon state is such that "which way" (i.e., whether it came through slit 1 or slit 2) is 100% entangled with its polarization state (L-circular when it came through slit 1, R-circular when it came through slit 2). Then simply let the photon hit the screen. Note that its polarization has not been measured before it hits the screen. Nevertheless the interference pattern is gone (more precisly you see the single-slit interference pattern). You don't need to really determine/measure through which slit the photon came to destroy the interference pattern. It's sufficient to prepare it in a state, where you could make a measurement which with certainty determins through which slit the photon went. Without doing that measurement you still don't know through which slit it came, but nevertheless repeating this same experiment the interference pattern is gone.

 

[kered rettop]

I understand the physics just fine, thanks. If you guys think "any possible measurement" is clear enough then so be it.

 

[sillyputty]

Perhaps a more interpretation free approach: Instead of saying "You can only have the double-slit interference patterns (with full contrast), if it is impossible to gain which-way information by any possible measurement on the photons." it might be better to say "Measurements which yield which-way information will not contribute to a full-contrast interference pattern."

Seems cleaner to get rid of the measurement term and especially the which-way info term. This seems to work:
The interference pattern always occurs but we see it only when it is enhanced enough to notice it.

 

[PeterDonis]

The interference pattern always occurs

But it doesn't. If you have polarizers in orthogonal directions at the two slits, there is no interference pattern at all.

 

[sillyputty]

That's not true.
Not only do you get IP, but you get 2 of them.

 

[PeterDonis]

That's not true.
Not only do you get IP, but you get 2 of them.

What specific experiment do you have in mind? Can you give a reference?

 

[sillyputty]

I'm not sure if it was you or someone else here, who provided the experiment .You were talking about a simple double slit, with 1 slit blocked and you stated that there is still 1 slit diffraction that occurs (the OP was thinking that blocking a slit causes the pattern to be completely gone and change into as if little stones were shot through that one slit, no fringes at all). I know that polarizers over slits (oriented perpendicular to each other), means certain photons are blocked at one slit, and are let to pass at the other. So (unless there is something that makes this special and different) this polarizer experiment would also produce fringes for each slit, albeit perhaps less noticeable (than the prior example) because not all that 'head' to that slit make it through.

 

[Nugatory]

The interference pattern always occurs but we see it only when it is enhanced enough to notice it.

But it doesn't occur at all unless paths through slits are available to the photon, which is to say that there is no which-way information. If that which-way information is available we get only the single-slit diffraction pattern at each open slit.

 

[PeterDonis]

You were talking about a simple double slit, with 1 slit blocked and you stated that there is still 1 slit diffraction that occurs

There was a recent thread about that (and I was not the only one posting clarifications there along the lines you describe), but one of the points made in that thread is that a diffraction pattern (what you get with one slit open, or with both slits open but orthogonal polarizers at each slit, as @Nugatory describes) is different from an interference pattern (what you get with both slits open and no polarizers anywhere).

 

[sillyputty]

But it doesn't occur at all unless paths through slits are available to the photon, which is to say that there is no which-way information. If that which-way information is available we get only the single-slit diffraction pattern at each open slit.

Yes that's what I am saying (single slit x2)

 

[sillyputty]

There was a recent thread about that (and I was not the only one posting clarifications there along the lines you describe), but one of the points made in that thread is that a diffraction pattern (what you get with one slit open, or with both slits open but orthogonal polarizers at each slit, as @Nugatory describes) is different from an interference pattern (what you get with both slits open and no polarizers anywhere).

I see what the snag is. You are thinking of a diffraction pattern as a completely independent separate phenomenon to an IP. I don't think of it as separate.