B Variations of the Double Slit?

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My first post, and my apologies to any real physicists lurking around. I am a lay person.
Senario:
* Detectors are turned on, thus interacting with the photons as they pass through the slits;
* Recorders are turned off, and no record of which way data exists to be observed.

Do we get an interference pattern or a wave collapse?
 

vanhees71

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It's not clear to me, how your experimental setup is. In QT it is important to state the complete setup. Otherwise it's impossible to answer your question.

The vanishing of the interference pattern has nothing to do with turning detectors on or off but with the manipulations of the photons close to the slits to gain which-way information.

The most simple way to gain which-way information is the following: Let the photon have momentum in ##z## direction and use photons that are polarized in ##x## direction. The double slit is in the ##xy## plane. Now put a quarter-wave plate in each slit, the one in slit 1 oriented with an angle of ##+45^{\circ}## and the other one in slit 2 perpendicularly oriented to the first, i.e., with an angle ##-45^{\circ}## relative to the ##x## axis.

Then if the photon goes through slit 1 it's left-circular polarized after the slit, and if it goes through slit 2 it's right-circular polarized, i.e., due to the quarter-wave plates in the slits you know with certainty through which slit the photon came by measuring its polarization state after it went through the double-slit. Now the polarization states of the photons running through slit 1 or 2 are preprendicular to each other, i.e., there is no interference term when you calculate the probability for the detection of the photon at a certain point on the screen behind the slits, which means you don't see the double-slit interference pattern.

This example makes it clear that the gain of precise which-way information inevitable destroys the double-slit interference pattern since, if you want interference, there must be an interference term in the probability amplitude for photons going through slit 1 or slit 2 respectively, and this implies that you cannot know in principle through which slit each individual photon went.
 

Nugatory

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Do we get an interference pattern or a wave collapse?
If it is possible to find out, even in principle, which slit the particle went through then there will be no interference pattern.

This might be a bit less mysterious if we consider that "possible to find out" really means "something, no matter how small, changed"; in this case the particle interacted with the detector. This interaction had to change the state of the detector in some small way (or it wouldn't be a detector) so the overall state of the entire system is "particle went through this slit not the other". So no interference.
 
Has this been tested? Bear in mind, I am not asking if which way data was recorded and then erased. I know that has been tested. I am asking - specifically - if the detector interactions alone collapsed the wave function, without the which way data ever having been recorded or come into existence.
 

Nugatory

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I am asking - specifically - if the detector interactions alone collapsed the wave function, without the which way data ever having been recorded or come into existence.
Careful.... it hasn't been recorded, but it has come into existence. Something had to change in the detector, maybe just some atoms being in different positions than if they had not not interacted with the passing particles, but if nothing at all changed it wouldn't be detecting. Just the presence of that tiny change counts as which-way information - the state of the world is that the particle went through that slit and not the other. Without that change, there would be interference.

This is actually the quantum mechanical prediction; that is, if you're expecting something else to happen, you're expecting that QM is wrong and some other as-yet-undiscovered theory provides a better description of how the universe works. However, this discovery about what QM predicts came late in the development of the theory, and by then many misunderstandings about collapse had made it into the popular imagination. David Lindley's book "Where does the weirdness go?" is a good layman-friendly explanation of the modern understanding of collapse. (Googling for "quantum decoherence" will find some of the original papers - they're linked from the Wikipedia article - but you may find the math a bit daunting).

Some of the confusion among laypeople is because for historical reasons we use the words "observation" and "detection" to describe what collapses the wave function. "Interaction" would be better, and "thermodynamically irreversible interaction" might be even better. Everything around you is continuously interacting with everything else, so in effect anything that is not completely isolated is a "detector" collapsing the wave functions of everything around it. That's why we don't see any quantum weirdnesses as we go about our day - but it is also means that everything we see and do is another test of the proposition that interaction is enough to collapse the wave function.
 
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I very much appreciate your prompt and intuitive answer. The question is an attempt to determine if the wave function collapse occurs due to some physical interaction, resulting from a detector( however small), or if it occurs due to the which way data entering into our reality. I have been following a gentleman by the name of Tom Campbell for some time. He is a physicist, and has proposed an experiment such as I described. He has a theory that is based in V.R., and proposes we live in a non-Newtonian reality made up of information, not material objects. He says his experiment has been published and will be tested in the next year or so. His lectures seem very convincing to me, but as a lay person, I cannot tell if this is just another woo woo take on QM. I would like to get some opinions from you or others who lurk in this form. For your reference, here's a link to the publication of his proposed experiment, and a link to him explaining it. Feedback would be MUCH appreciated. https://www.ijqf.org/archives/4105?fbclid=IwAR1aVFpqj7uK6ZF9LjY9VIEG9NyOzld8g3W8x7MZD0cexVptBbRBdGIbW6Y

 

DrChinese

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I have been following a gentleman by the name of Tom Campbell for some time. He is a physicist, and has proposed an experiment such as I described. He has a theory that is based in V.R., and proposes we live in a non-Newtonian reality made up of information, not material objects. He says his experiment has been published and will be tested in the next year or so. His lectures seem very convincing to me, but as a lay person, I cannot tell if this is just another woo woo take on QM. I would like to get some opinions from you or others who lurk in this form. For your reference, here's a link to the publication of his proposed experiment, and a link to him explaining it. Feedback would be MUCH appreciated. https://www.ijqf.org/archives/4105?fbclid=IwAR1aVFpqj7uK6ZF9LjY9VIEG9NyOzld8g3W8x7MZD0cexVptBbRBdGIbW6Y

Not sure what this has to do with the double slit experiment or quantum physics. If we live in a simulation, presumably that could be accounted for within such. I.e. not sure why the simulation would be good enough to fool us all, and yet leave this one particular loophole.

However, to get back to your original question: YES, which way data has been marked in experiments, and then NOT recorded. There is no interference pattern, just as Nugatory and vanhees71 said above. Here's a reference (please note that this may be a bit difficult to follow given its intended audience):

http://sciencedemonstrations.fas.harvard.edu/files/science-demonstrations/files/single_photon_paper.pdf

Please note that if there is NO interference when the which-slit information is not recorded, there obviously won't be interference when it IS recorded.
 
Not sure what this has to do with the double slit experiment? You might want to have a look at the paper. Thanks again
 

DrChinese

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Not sure what this has to do with the double slit experiment? You might want to have a look at the paper. Thanks again
The paper you reference asks: "Can the theory that reality is a simulation be tested?" That has nothing to do with quantum mechanics, despite the attempt to use the double slit experiment as a component of testing this ridiculous hypothesis*. If we live in a simulation, as I said, there is no particular reason to use elements of quantum theory to "prove" it. See "Last Thursdayism" if you are not already familiar with my explanation.

Quantum theory has many "unusual" elements. What Campbell et al are testing is about as ad hoc as it gets, and certainly far from anything that is going to raise an eyebrow with those familiar with the literature on the subject.

So again, your question has been asked and answered. Read the paper I referenced and you will see that recording is not relevant to the results.

----------------------------------------

*PS On case you think I am being unfairly judgmental: Here is the hypothesis from the paper, which is absurd on the face of it:

"Summing up, to save itself computing work, the system only calculates reality when information becomes available for observation by a player, and to avoid detection by players it maintains a consistent world, but occasionally, conflicts that are unresolvable lead to VR indicators and discontinuities (such as the wave/particle duality). "
 
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Thanks again your input is much appreciated. Are there any others reading this that have a different opinion of Mr. Campbell?
 
Thanks again your input is much appreciated. Are there any others reading this that have a different opinion of Mr. Campbell?
Don't listen to Campbell. He misunderstands some basic ideas, notably the question you asked here.
 
The paper you reference asks: "Can the theory that reality is a simulation be tested?" That has nothing to do with quantum mechanics, despite the attempt to use the double slit experiment as a component of testing this ridiculous hypothesis*. If we live in a simulation, as I said, there is no particular reason to use elements of quantum theory to "prove" it. See "Last Thursdayism" if you are not already familiar with my explanation.

Quantum theory has many "unusual" elements. What Campbell et al are testing is about as ad hoc as it gets, and certainly far from anything that is going to raise an eyebrow with those familiar with the literature on the subject.

So again, your question has been asked and answered. Read the paper I referenced and you will see that recording is not relevant to the results.

----------------------------------------

*PS On case you think I am being unfairly judgmental: Here is the hypothesis from the paper, which is absurd on the face of it:

"Summing up, to save itself computing work, the system only calculates reality when information becomes available for observation by a player, and to avoid detection by players it maintains a consistent world, but occasionally, conflicts that are unresolvable lead to VR indicators and discontinuities (such as the wave/particle duality). "
Unfairly judgmental? No. Absurd? Yes. Is the many-worlds interpretation any less absurd? How about the entire universe arising from nothing? Cats that are both dead and alive? I could go on. Absurdities are often presented and accepted for discussion in respected scientific circles when examining boundaries of the unknown. The absurdness of the ideas may lessen upon closer examination. While I'm not defending Mr. Campbell's theory with my limited knowledge, I have paid enough attention to his ideas that I understand his references to VR, players, or theorized system's computational efforts at maintaining a consistent world are not given as literal interpretations, but are offered as metaphor's describing the ordered system he theorizes.
 
Don't listen to Campbell. He misunderstands some basic ideas, notably the question you asked here.
"If you think you understand quantum mechanics, you don't understand quantum mechanics." Feynman-
 

DrChinese

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Unfairly judgmental? No. Absurd? Yes. Is the many-worlds interpretation any less absurd? How about the entire universe arising from nothing? Cats that are both dead and alive? I could go on. Absurdities are often presented and accepted for discussion in respected scientific circles when examining boundaries of the unknown. The absurdness of the ideas may lessen upon closer examination. While I'm not defending Mr. Campbell's theory with my limited knowledge, I have paid enough attention to his ideas that I understand his references to VR, players, or theorized system's computational efforts at maintaining a consistent world are not given as literal interpretations, but are offered as metaphor's describing the ordered system he theorizes.
You are off the mark by comparing the work of Campbell et al with other physicists on the basis of absurdity. Everitt (MWI) and others were attempting to resolve known and difficult outstanding issues in science. To varying degrees, they have. That is far far from the ad hoc hypothesis presented in your reference (that we live in a simulation, and there are specific inconsistencies in it because of computational restrictions of our creators).
And again, I point out the paper is really about simulations and not much about the stated physics. The "physics" answer is that recording which slit information does not change the outcome. It is enough that the information is knowable in principle.

Please note that the reference itself does not meet PF guidelines anyway. So I would request that our discussions focus more on the physics of the double slit, and less on defending Campbell's conjectures.
 
You are off the mark by comparing the work of Campbell et al with other physicists on the basis of absurdity. Everitt (MWI) and others were attempting to resolve known and difficult outstanding issues in science. To varying degrees, they have. That is far far from the ad hoc hypothesis presented in your reference (that we live in a simulation, and there are specific inconsistencies in it because of computational restrictions of our creators).
And again, I point out the paper is really about simulations and not much about the stated physics. The "physics" answer is that recording which slit information does not change the outcome. It is enough that the information is knowable in principle.

Please note that the reference itself does not meet PF guidelines anyway. So I would request that our discussions focus more on the physics of the double slit, and less on defending Campbell's conjectures.
I assume you are a moderator? I certainly respect that. I am new to this community. May I ask specifically what guidelines are not being met? I mean no disrespect, but is this forum only open to discussion or theory from select physicists?
 

DrChinese

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I assume you are a moderator? I certainly respect that. I am new to this community. May I ask specifically what guidelines are not being met? I mean no disrespect, but is this forum only open to discussion or theory from select physicists?
I am NOT a moderator. :smile: I can't find the posting guidelines right now, but I can paraphrase. This forum is restricted to discussions of generally accepted physics. References should either support that, and/or be published in a suitable peer-reviewed journal. (There is no "select" group of physicists.) Moderators have the final say, and tend to steer discussions back on track if needed.

The moderators, such as Nugatory, are often lenient with references (such as what you gave) if you stay close to established physics. Please keep in mind that overly speculative works are not really welcome here for discussion - there are other places for those to be discussed. No one is trying to suppress new ideas, but this is not the place for them.

In this case, there are plenty of established experiments that you should be aware of before jumping into the waters of speculative hypotheses. If you stick close to those, you'll find there is plenty to discuss here.
 

Nugatory

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I mean no disrespect, but is this forum only open to discussion or theory from select physicists?
It is open to discussion of any theory that has been published in an appropriate peer-reviewed scientific journal or standard textbook. There’s more detail in the stickied thread at the top of the feedback section: https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/

Predatory and pay-to-publish “journals” have proliferated on the Internet in recent years, so not everything that looks cool and sciency is necessarily an appropriate source. At a quick glance, IJQF has no impact factor and fairly low standards - and @charters has a point in post #11 above.

This thread has being allowed because you started it with a reasonable question about what quantum mechanics does say and what is confirmed by experiment. That’s clearly OK under the rules, but we won’t host an extended discussion of the simulation hypothesis as a fully qualified alternative to quantum mechanics - it’s not and that’s not what we’re here for.
 

Nugatory

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Is the many-worlds interpretation any less absurd?
You’ll find Everett’s dissertation, in which he introduces MWI, here: https://www-tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf. I think it speaks for itself - but do note that Everett develops the concept not by making an outlandish assumption, but by discarding an arbitrary and somewhat problematic one.
How about the entire universe arising from nothing?
Although the big-bang theory is often described that way in non-technical writing, that is not what it says. It does not say that the universe arose from nothing; it describes the evolution of the universe from an initial state very different from the current state and supports this claim with substantial observational evidence. (As for how that initial state came to be? That’s a question for religious scholars).
Cats that are both dead and alive?
That is indeed absurd, but that was Schrodinger’s point. He invented the cat thought experiment to show that something had to be wrong with the then-current (1920s) understanding of quantum mechanics because the math seemed to predict that absurd result. He was not asserting that cats can be both dead and alive, but the exact opposite. (As an aside, the Lindley book I mentioned above is a pretty good explanation of how decoherence resolves that particular problem with the 1920s-vintage understanding of quantum mechanics).

The point of all of this is that it is difficult to judge the relative absurdity of various hypotheses without a good understanding of what our best current theories actually say, and that popular summaries are seldom a good base for that understanding.
 
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You’ll find Everett’s dissertation, in which he introduces MWI, here: https://www-tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf. I think it speaks for itself - but do note that Everett develops the concept not by making an outlandish assumption, but by discarding an arbitrary and somewhat problematic one.
Although the big-bang theory is often described that way in non-technical writing, that is not what it says. It does not say that the universe arose from nothing; it describes the evolution of the universe from an initial state very different from the current state and supports this claim with substantial observational evidence. (As for how that initial state came to be? That’s a question for religious scholars).
That is indeed absurd, but that was Schrodinger’s point. He invented the cat thought experiment to show that something had to be wrong with the then-current (1920s) understanding of quantum mechanics because the math seemed to predict that absurd result. He was not asserting that cats can be both dead and alive, but the exact opposite. (As an aside, the Lindley book I mentioned above is a pretty good explanation of how decoherence resolves that particular problem with the 1920s-vintage understanding of quantum mechanics).

The point of all of this is that it is difficult to judge the relative absurdity of various hypotheses without a good understanding of what our best current theories actually say, and that popular summaries are seldom a good base for that understanding.
I very much appreciate your feedback. I consider myself a skeptic. Perhaps I should’ve laid out references to Mr Campbell to begin with. Thank you again
 

Nugatory

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I consider myself a skeptic.
Skepticism is fine, but for every ounce of skepticism you apply to mainsteam interpretations, you should be applying pounds to Campbell.
 
Unfairly judgmental? No. Absurd? Yes. Is the many-worlds interpretation any less absurd? How about the entire universe arising from nothing? Cats that are both dead and alive? I could go on. Absurdities are often presented and accepted for discussion in respected scientific circles when examining boundaries of the unknown. The absurdness of the ideas may lessen upon closer examination. While I'm not defending Mr. Campbell's theory with my limited knowledge, I have paid enough attention to his ideas that I understand his references to VR, players, or theorized system's computational efforts at maintaining a consistent world are not given as literal interpretations, but are offered as metaphor's describing the ordered system he theorizes.
The difference is Campbell's ideas rely on his own personal predictions about the outcomes of experiments which are inconsistent with quantum mechanics. He thinks the answer to the scenario in your original question is we do get interference, but QM clearly predicts we do not. Then he wiggles out of this by saying the *exact* experiment he has suggested has not been done before, so maybe QM will be disproven by it. But this is probably not even true that the experiment has never been done - the result is just too trivial for anyone to bother trying to publish it.

So, weird ideas or not, everyone else is at least trying to interpret the meaning of what physics actually says and shows, while Campbell just changes/misrepresents the physics in order to push his VR story.
 
If I may ask, is anyone here familiar with the International Journal of Quantum Foundations, where this paper was published? Is this a respected source?
 

Nugatory

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But this is probably not even true that the experiment has never been done - the result is just too trivial for anyone to bother trying to publish it.
Probably?
Yes, probably. It's hard to say for sure.

This is for pretty much the same reason that despite centuries of experiments on gravitational attraction since Galileo demonstrated that different masses fall at the same speed, I would find it very difficult to respond to someone who challenged me for experimental evidence that earth's gravity exerts the same force on one kilogram of cold-rolled 1045 steel alloy as on one kilogram of 1018 steel alloy. Yes, we've tried many different materials over the years, and always the gravitational force depends on the mass and not on the composition.... but there's no experiment showing that this general principle applies to these two specific steel alloys. On the one hand no one is going to waste their time performing this experiment and reporting on the results; on the other hand it is very likely that sometime past few decades someone has built something under the assumption that gravity affects both equally, thereby testing the proposition without noticing.
 
Probably?
Nobody is aware of every experiment in history, so I don't know if the idea of " double slit with detectors on, record keeping device off" has been done in a planned way and written up into a paper. But I am pretty confident that at multiple points in the last 100 years somebody somewhere has done this by accident, and when they saw no interference pattern they just said "well, of course" and went on with their day.

Not every minor permutation of an experiment is interesting. After you test how fast a baseball falls due to gravity, you don't ask "what if we drop a baseball while eating a sandwich?" There has to be some plausible motivation to expect a different result, especially if you are talking about publishing a negative result after the fact. Given our understanding of entanglement, the math of pure states and reduced density matrices, and all existing evidence from similar experiments, Campbell's suggestion is on par with the sandwich tweak. It is just abundantly clear this will turn out how QM predicts it will, not how Campbell hopes it will for his VR story.

If I may ask, is anyone here familiar with the International Journal of Quantum Foundations, where this paper was published? Is this a respected source?
In general its a bad idea to substitute "well it got said some random journal" for your own learning and critical thinking. You posted here because on some level you already knew what Campbell is saying can't be true. So don't start appealing to authority now.
 

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