Double Slit Experiment: Dumb question that needs to be asked

[Quarinteen]

TL;DR

When conducting the double slit experiment, does the pattern change from an interference to particle pattern with human eyes?

So the double slit experiment. If I understand correctly when electrons are shot through 2 slits and no one is monitoring, measuring or watching they create an interference pattern if they are being measured they create a 1 to 1 pattern. I keep seeing it be said that the mere act of monitoring changes the pattern created. Here is my question and I am sorry if this is obvious but is the 1 to 1 pattern created when the monitoring instrument is the human eye? If not what are they monitoring / measuring with? Is it giving off some field that could be interacting with the electrons? I would imagine with how small the electron is it would not take much of an em field to have an effect on a single electron. I would also think if it were the mere act of observation then when a person is looking at it there should not be an interference pattern created right?

 

[vanhees71]

The point is that if you want to know, through which slit each of the electrons come, you need to measure this, and this leads to a perturbation of the state of the electron in such a way that its coherence is destroyed and thus there is no interference pattern. You can either have full "which-way information" and have no interference pattern collecting enough electrons ("particle aspect") or you don't have which-way information at all and have a clear interference pattern ("wave aspect").

The human eye is of course irrelevant for this. The point is that you need to scatter light on the electron in such a way that the detection of this light with your eye can give you sufficient spatial resolution to know, from where the electron came from. For this you need light with sufficiently small wave length. But now you need at least one photon to be scattered at the electron, and the shorter the wave length the more you destroy the coherence of the electron. So the more accurately you want to localize the electron when it goes through the slit the more you destroy the interference pattern.

 

[Ndrw Lyll]

Monitor, measure and observe her all very bad words because this is not what is happening. Whatever you use see which slit the quantum passes through will itself physically interact with the quantum and destroy its quantum nature. People of trying to get round this with so-called “weak measurement” which you should look up.

 

[DrChinese]

Whatever you use see which slit the quantum passes through will itself physically interact with the quantum and destroy its quantum nature.

This is often repeated, but it is completely untrue. There is no requirement that there be an interaction between the particle being observed and another particle (or not, if not observed). To switch from an interference pattern to no interference pattern, it is enough that you could determine which-slit information. It doesn’t matter if you don’t actually learn that. And as @vanhees71 correctly stated, the human eye is not a factor in any scenario.

The technique is best demonstrated with photons. Place polarizers at each slit. When the polarizers are aligned parallel, there is interference. When aligned perpendicular, there is none. This has been demonstrated experimentally:

https://sciencedemonstrations.fas.h...-demonstrations/files/single_photon_paper.pdf

 

[Ndrw Lyll]

When the polarisers are parallel, there is no interaction, observation, or measurement of the quantum. The whole wave passes unscathed and you cannot determine anything, even if you wanted to. When the polarisers are crossed they interact with the wave and either absorb it or not. In the case where the wave is not absorbed it is rendered classical by its interaction with the polarisers and there is no interference.

 

[DrChinese]

When the polarisers are parallel, there is no interaction, observation, or measurement of the quantum. The whole wave passes unscathed and you cannot determine anything, even if you wanted to. When the polarisers are crossed they interact with the wave and either absorb it or not. In the case where the wave is not absorbed it is rendered classical by its interaction with the polarisers and there is no interference.

Nope. There is interaction with the polarizers in both cases, and in both cases the same intensity is produced (there is no more or no less absorption). And both cases are completely quantum. The quantum rule is that the relationship between the 2 polarizers determines whether there is interference or not. And in fact that relationship can be set from 0 to 90 degrees apart (i.e. including anything in between) and the result is interference produced according to the cos^2(theta) rule.

And in fact, as shown by the reference I provided, it is possible to "erase" the information you get in the perpendicular (crossed) case by insertion of an additional polarizer set at 45 degrees. The interference pattern is restored. That can't happen in a classical scenario. In the classical scenario, there is no way to restore interference for single photons that went through a single slit. That only happens in the quantum scenario.

 

[PeroK]

TL;DR Summary: When conducting the double slit experiment, does the pattern change from an interference to particle pattern with human eyes?

So the double slit experiment. If I understand correctly when electrons are shot through 2 slits and no one is monitoring, measuring or watching they create an interference pattern if they are being measured they create a 1 to 1 pattern. I keep seeing it be said that the mere act of monitoring changes the pattern created. Here is my question and I am sorry if this is obvious but is the 1 to 1 pattern created when the monitoring instrument is the human eye? If not what are they monitoring / measuring with? Is it giving off some field that could be interacting with the electrons? I would imagine with how small the electron is it would not take much of an em field to have an effect on a single electron. I would also think if it were the mere act of observation then when a person is looking at it there should not be an interference pattern created right?

Here's a different way to look at the double-slit experiment.

First, close one slit and fire a large number of electrons at the apparatus. A certain percentage of the electrons get through and you generate a single-slit pattern - corresponding to the one open slit.

Second, close the other slit and repeat the experiment. You get a similar single-slit interference pattern.

Third, repeat the experiment with both slits open. This time you get a double-slit interference pattern, which is not a combination of the two single-slit patterns.

This establishes that electrons in this case do not behave like classical particles.

 

[Ndrw Lyll]

Nope. There is interaction with the polarizers in both cases, and in both cases the same intensity is produced (there is no more or no less absorption). And both cases are completely quantum. The quantum rule is that the relationship between the 2 polarizers determines whether there is interference or not. And in fact that relationship can be set from 0 to 90 degrees apart (i.e. including anything in between) and the result is interference produced according to the cos^2(theta) rule.

And in fact, as shown by the reference I provided, it is possible to "erase" the information you get in the perpendicular (crossed) case by insertion of an additional polarizer set at 45 degrees. The interference pattern is restored. That can't happen in a classical scenario. In the classical scenario, there is no way to restore interference for single photons that went through a single slit. That only happens in the quantum scenario.

Ok I get it now - thanks.

 

[DennisN]

[...] pattern created when the monitoring instrument is the human eye?

Just a reminder if anyone would try this:
Do not place yourself (and your eyes) where the screen would be, choose a better position.

 

[sillyputty]

To switch from an interference pattern to no interference pattern, it is enough that you could determine which-slit information. It doesn’t matter if you don’t actually learn that.

The technique is best demonstrated with photons. Place polarizers at each slit. When the polarizers are aligned parallel, there is interference.

This does not seem right. Why is it being stated as "if you could determine which slit". What does nature care if we can or cannot determine? I also don't see how the polarizers make the situation be one where we can determine which slit a photon went through. I do understand how some kind of detector, such as a disturbance to other particles which the travelling particle causes, makes the situation be one where we can determine which slit the particle went through. But in that case why state it as "we can determine" and not as "there was a disturbance" ?

 

[PeterDonis]

Why is it being stated as "if you could determine which slit". What does nature care if we can or cannot determine?

"Determine" is a process of nature. By "if you could determine which slit" is simply meant "if there is something physical that interacts differently with the particle depending on which slit it goes through". @DrChinese gave a specific example for photons.

I also don't see how the polarizers make the situation be one where we can determine which slit a photon went through.

The difference in angle of the polarizers determines how differently they interact with the photon depending on which slit it goes through.

why state it as "we can determine" and not as "there was a disturbance" ?

They mean the same thing in this context.

 

[Nugatory]

I also don't see how the polarizers make the situation be one where we can determine which slit a photon went through

After a photon passes through a polarizer it will be polarized along the same axis as the polarizer.
If we have a horizontal polarizer at one slit and a vertical polarizer at the other then all the vertically polarized photons reaching the screen will have come through the slit with the vertical polarizer, and likewise for the other slit and the horizontally polarized photons.
If both polarizers are aligned in the same direction, then all the photons that reach the screen will be polarized in that direction, and they could have passed through either polarizer and hence either slit.

 

[sillyputty]

If we have a horizontal polarizer at one slit and a vertical polarizer at the other then all the vertically polarized photons reaching the screen will have come through the slit with the vertical polarizer, and likewise for the other slit and the horizontally polarized photons.

Right, but you are just saying that 'photon type A came from here, not from there'. I am asking: for a given photon that arrives at the screen, how could you determine that this given photon is type A and not the other type? Presumably there is some simple way. What is that way?

 

[vanhees71]

It's the polarization of the photon. If you have polarization filters with an angle of $90^{\circ}$ in relative orientation, then there's a 100% entanglement between polarization and through which slit each photon came. The polarization states are orthogonal to each other, so that there's no interference term between the photon going through slit 1 or slit 2, i.e., there's no double-slit interference pattern.

If you put filters in the same orientation you cannot distinguish a photon coming from slit 1 from one coming from slit 2, and you get a double-slit interference pattern with "maximal contrast".

At either other angle between the orientations of the polarizers you get some thing in between, i.e., a double-slit interference pattern with more or less contrast.

 

[sillyputty]

It's the polarization of the photon. If you have polarization filters with an angle of $90^{\circ}$ in relative orientation, then there's a 100% entanglement between polarization and through which slit each photon came. The polarization states are orthogonal to each other, so that there's no interference term between the photon going through slit 1 or slit 2, i.e., there's no double-slit interference pattern.

If you put filters in the same orientation you cannot distinguish a photon coming from slit 1 from one coming from slit 2, and you get a double-slit interference pattern with "maximal contrast".

At either other angle between the orientations of the polarizers you get some thing in between, i.e., a double-slit interference pattern with more or less contrast.

Yes I understand that, it's all very straight-forward. My question is not being answered. For a given photon behind the slits, how can you determine through which slit that photon came? It is repeatedly said that the interference pattern corresponds to when you can determine through which slit a particle came. I am familiar with experiments which confirm that, but this does not seem one which confirms that. Maybe you could have a detector behind the slits which is sensitive to the type of polarisation. That would confirm it, and would make this very simple and answered. What is the way that we can determine which slit a given photon came from?

I think PeterDonis came closest to answering the question, by specifying that the claim "you can determine" does not mean what it says; rather, it means that there is some physical interaction (as I originally offered) which the particle undergoes, and which is tied to a particular slit. That makes sense. But then I would like to know this: suppose that we moved the screen right behind the slits. At the moment before a given photon is measured behind a slit, that photon underwent an interaction at the filter (so the claim goes). I would like to know this: How was that photon changed exactly? What interaction did that specified photon experience? From what state to what state, did that photon go?

 

[Nugatory]

answered. For a given photon behind the slits, how can you determine through which slit that photon came?

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.

 

[Lord Jestocost]

For a given photon behind the slits, how can you determine through which slit that photon came? It is repeatedly said that the interference pattern corresponds to when you can determine through which slit a particle came.

There seems to be some confusion regarding the double-slit experiment. Maybe, the following quote by Časlav Brukner (in "Elegance and Enigma, The Quantum Interviews", ed. Maximilian Schlosshauer, p. 66) might be of help:

“To a large extent, the various debates about the interpretation of quantum mechanics can be seen as debates about what quantum physics refers to. Does it directly refer to reality – or to our information, on the basis of which we construct reality? I find very suggestive the role information played in the early debates on the meaning of quantum mechanics, most notably in the Bohr–Einstein dialogue. No matter how sophisticated the claim was that it should be possible to both observe the interference fringes and identify which of the two slits the particle goes through, it was invariably found that the flawed mechanism lurking behind this claim can’t, in fact, violate the following principle: any increase of partial information about the particle’s path will always mean a corresponding loss in visibility of the interference pattern, and vice versa. Most importantly, it is not relevant whether we read out that information. All that is necessary is for the information to be present somewhere in the universe.” [Bold by LJ]

 

[Nugatory]

At the moment before a given photon is measured behind a slit, that photon underwent an interaction at the filter (so the claim goes). I would like to know this: How was that photon changed exactly? What interaction did that specified photon experience? From what state to what state, did that photon go?

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.

 

[Dale]

It is repeatedly said that the interference pattern corresponds to when you can determine through which slit a particle came.

I think you have this backwards. The interference pattern corresponds to when you cannot determine through which slit a particle came.

This does not seem right. Why is it being stated as "if you could determine which slit". What does nature care if we can or cannot determine?

What does nature care if it seems right to you?

We can make all sorts of alterations to the light path, such as placing detectors or polarizers or other things in the path. Some subset of alterations will enable us to determine which path a given photon took. Some subset of alterations will remove the two-slit interference pattern. QM predicts that those subsets of alterations are the same. Experiments have confirmed those predictions.

 

[sillyputty]

What does nature care if it seems right to you?

So you are confirming that the outcome depends on whether you can or cannot determine the path.

This is physics. Nature does not care about what you know, or can know. It's not about you.

 

[Dale]

Nature does not care about what you know, or can know

What is your basis for this claim? It has been experimentally demonstrated to be false.

 

[sillyputty]

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.

Thank you, So what you are saying is that there was NO interaction undergone by that photon which I asked about. So, why did you claim that it's not about the path being knowable, but rather "There is something physical that interacts differently with the particle depending on which slit it goes through" ?

 

[sillyputty]

What is your basis for this claim? It has been experimentally demonstrated to be false.

My basis for that claim is that you have no basis for yours. It's that simple. You are making a claim that nature cares about whether you can determine the path. I am denying that your claim is true.

 

[vanhees71]

Yes I understand that, it's all very straight-forward. My question is not being answered. For a given photon behind the slits, how can you determine through which slit that photon came? It is repeatedly said that the interference pattern corresponds to when you can determine through which slit a particle came. I am familiar with experiments which confirm that, but this does not seem one which confirms that. Maybe you could have a detector behind the slits which is sensitive to the type of polarisation. That would confirm it, and would make this very simple and answered. What is the way that we can determine which slit a given photon came from?

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.

I think PeterDonis came closest to answering the question, by specifying that the claim "you can determine" does not mean what it says; rather, it means that there is some physical interaction (as I originally offered) which the particle undergoes, and which is tied to a particular slit. That makes sense. But then I would like to know this: suppose that we moved the screen right behind the slits. At the moment before a given photon is measured behind a slit, that photon underwent an interaction at the filter (so the claim goes). I would like to know this: How was that photon changed exactly? What interaction did that specified photon experience? From what state to what state, did that photon go?

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.

 

[DrChinese]

My basis for that claim is that you have no basis for yours [of experimental confirmation]. It's that simple. You are making a claim that nature cares about whether you can determine the path. I am denying that your claim is true.

Here is the reference I presented in post #4, and if you read this and re-read my posts #4 and #6 you should have a better understanding. Nature cares whether you *can* determine the path, not whether you *did* determine the path. @Dale @Nugatory @vanhees71 @PeterDonis et al are correct. It doesn't really make sense to deny things when there is referenced literature showing the opposite.

https://sciencedemonstrations.fas.h...-demonstrations/files/single_photon_paper.pdf

Note figures 8 and 9. Figure 8: Polarizers are parallel, there IS interference. Figure 9: Polarizers are perpendicular, there is NO interference - yet the which-slit marker (a different photon polarization is tied to each slit) is unknown to the observer. This demonstrates conclusively that the observer's complete knowledge is NOT crucial to the outcome; it is enough that the observer *could* have known the slit.

Note that any photon arriving at the screen must go through a polarizer in either configuration. There is no more or no less "disturbance" in Figure 8 versus Figure 9. As is often the case in QM, it is the relationship of one thing to the other that is central to what occurs. In this case, the relative polarizer settings.

 

[sillyputty]

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.

Excellent. Thank you very much. You exactly answered what I was asking for.

 

[Dale]

My basis for that claim is that you have no basis for yours. It's that simple. You are making a claim that nature cares about whether you can determine the path. I am denying that your claim is true.

This is incorrect. The basis for my claim are the many experiments and professional scientific references that support it and contradict your claim.

There is the famous delayed choice quantum eraser experiment
https://doi.org/10.1103/PhysRevLett.84.1

There is the earlier quantum eraser experiment
https://doi.org/10.1038/353507b0

This is now so well understood that there are even articles about incorporating this class of experiments into the undergraduate curriculum
https://site.physics.georgetown.edu/~jkf/publications/stern_gerlach_eraser_ajp_2020.pdf

Here, personal opinion and claims are insufficient. The standard we use is the professional scientific literature.

 

[vanhees71]

Wow! The papers quoted in the Nature article seem to be among the very first early experiments with SPDC entangled photon pairs of this kind. BTW: there's a nice review about the technical side (entangled-photon sources, single-photon detectors, and all that) of the quantum-foundation experiments:

Climério Paulo da Silva Neto, Materializing the Foundations of Quantum Mechanics, Instruments and the First Bell Tests, Springer (2023)
https://doi.org/10.1007/978-3-031-29797-7

 

[mattt]

So you are confirming that the outcome depends on whether you can or cannot determine the path.

This is physics. Nature does not care about what you know, or can know. It's not about you.

This is what happens when you try to translate math into words...

"Whether or not the path can be determined" is a mathematical condition translated into English words, it has nothing to do with humans looking at it or even humans doing anything...

 

[DrChinese]

This is what happens when you try to translate math into words...

"Whether or not the path can be determined" is a mathematical condition translated into English words, it has nothing to do with humans looking at it or even humans doing anything...

So true. Reference 1. below is a purely theoretical treatment - all math! Reference 2. also provides a theoretical treatment, in addition to executing an experimental implementation.

https://arxiv.org/abs/1110.4309 (2011)
1. The Double Slit Experiment With Polarizers

https://arxiv.org/abs/quant-ph/9903047 (2008)
2. A Delayed Choice Quantum Eraser (same as @Dale references above, but not behind paywall)