Layman asks about Quantum "interaction"

[Peter Mole]

TL;DR Summary

Can wave collapse occur simply by gaining "information" about the particle without physical interaction?

I'm a college grad, but not in science or physics. I'm useless on the math. However, I have a solid layman's understanding of double slit experiment as well as the delayed choice quantum eraser . I also have a layman's understanding of quantum physics via reading some mainstream science books and watching documentaries, etc. (kaku, greene)

Is it conventional knowledge that the wave collapses due to only to physical interaction between the subatomic particle and the subatomic means to measure it? In other words, because you need a photon to hit or register the result... is that what's collapsing the wave (showing a particle result instead of an interference pattern)? Or is it just that you've created known "information" and "information" collapses the wave?

Have there been experiments where the particle was interacted with (say using a photon) but the results were not recorded anywhere? In those cases does the wave still collapse?

Are there experiments where the wave collapses without physical interaction with the particle? In other words, are there experiments where simply knowing "information" by non-direct/non-physical interaction, still results in wave collapse?

Is it true that information can never be lost? In the case of the delayed choice quantum eraser, isn't it really a case of the information never being realized? Is this because it wasn't recorded or that it was recorded but then lost again? (I've watched a few videos on it and have an idea how it's setup through entanglement, but I'm still confused).

 

[PeterDonis]

Are you asking about what particular interpretations of QM say about "collapse"? That will depend on the interpretation.

Or are you asking about what predictions QM makes about experimental results, independent of any particular interpretation? In that case, "collapse" can be taken to mean the Von Neumann projection postulate, as described in Rule 7 of this Insights article:

https://www.physicsforums.com/insights/the-7-basic-rules-of-quantum-mechanics/

If the latter is what you are asking about, this thread can be moved to the regular QM forum.

 

[EPR]

You will not gain information without interaction.

 

[Peter Mole]

Are you asking about what particular interpretations of QM say about "collapse"? That will depend on the interpretation.

No, I'm more just asking what makes it collapse. (This is not a bait and switch for consciousness conclusion). Shouldn't subatomic particles be hitting each other all the time just out in nature? Or, maybe you could say aren't the waves being entangled all the time? If it's ONLY a matter of physical interaction and not physical interaction plus "information" learned, then wouldn't the double slit experiment show particles sometimes even if they aren't being measured by a experimental mechanism?

Or are you asking about what predictions QM makes about experimental results, independent of any particular interpretation? In that case, "collapse" can be taken to mean the Von Neumann projection postulate, as described in Rule 7 of this Insights article:

https://www.physicsforums.com/insights/the-7-basic-rules-of-quantum-mechanics/

If the latter is what you are asking about, this thread can be moved to the regular QM forum.

You mean, "#7. For successive, non-destructive projective measurements with discrete results, each measurement with measuring value can be regarded as preparation of a new state whose state vector is the corresponding eigenvector , to be used for the calculation of subsequent time evolution and further measurements."

There's waaay too much for me to unpack there even to understand even the ground rules of the vocabulary being used to describe and explain.

 

[PeterDonis]

I'm more just asking what makes it collapse.

That doesn't help since I still don't know what you mean by "collapse". Do you mean Rule 7 in the Insights article I linked to? Or, at least, will that definition be sufficient for the questions you are asking?

 

[PeterDonis]

There's waaay too much for me to unpack there even to understand even the ground rules of the vocabulary being used to describe and explain.

If that's really the case, your question is probably not answerable since you don't have the background knowledge required to even explain what you are asking about.

However, let me try to rephrase Rule 7 in a way that might be easier for you to get a handle on.

Suppose we are measuring the spin of an electron. We can do this using a device called a Stern-Gerlach device. Schematically, the electron goes in, and comes out in one of two directions, the "up" direction or the "down" direction, corresponding to the two possible results of the spin measurement.

The electron when it goes into the device might have any spin wave function at all. But when it comes out of the device, if we want to correctly predict the results of future spin measurements on that electron, we must assign it the appropriate wave function for the direction it came out: the "up" wave function if it came out in the "up" direction, or the "down" wave function if it came out in the "down" direction. That assignment of a wave function to the electron, for predicting future measurement results, based on the observed result of the measurement, is what Rule 7 is describing.

Informally, the term "collapse" is used to describe the process of assigning the appropriate wave function to a measured system based on the observed result of the measurement. For example, we might say that measuring the electron's spin "collapsed" its wave function, so it is now either "up" or "down" (depending on which measurement result we observed), regardless of what it was before. Doing this is not making any claim about what is "really happening" to the electron; it is just a statement about what wave function we assign to the electron in order to predict future measurement results.

Is that usage of "collapse" the one you have in mind?

 

[PeroK]

Shouldn't subatomic particles be hitting each other all the time just out in nature? Or, maybe you could say aren't the waves being entangled all the time? If it's ONLY a matter of physical interaction and not physical interaction plus "information" learned, then wouldn't the double slit experiment show particles sometimes even if they aren't being measured by a experimental mechanism?

I thought I might have understood the first part of your question, but I don't understand the last part about the double slit. Let me try to find a concrete question that relates to what you are asking.

Let's consider the asteroid belt, with millions of (macroscopic) asteroids floating around. Classically, each has a history and although the specifics are unknown, we can quite safely say that the asteroids formed at some point in the past and have moved about essentially unobserved ever since.

Is your question what does QM have to say about this? In what sense does QM allow for the classical theory of an asteroid belt forming a long time ago, seemingly without specific recorded experiments for every atomic and sub-atomic process that was required for their creation?

 

[Peter Mole]

You will not gain information without interaction.

So unless there's a physical interaction, the wave will not collapse?

With one version of the double slit experiment, if you wait until after the wave passes through the double slits before trying to detect them (but before they hit the screen), the wave will still collapse and create the two bar pattern. (Feel free to correct me if I'm describing this wrong).

So there is no physical interaction until after the "wave" went through the slits. If I understand, the explanation is that the wave went back in time and collapsed before it was interacted with physically by "interaction" that only happened after the wave/particles passed through the slits.

How can interaction AFTER the event cause the wave to collapse in the past? We know the wave collapsed before it went through the slits but there was no interaction until after it when through the slits.

Why is it more logical to think a present action changes the past than to say that information gained collapsed the wave without interaction? Neither explanation seems at all logical to me.

 

[PeterDonis]

So unless there's a physical interaction, the wave will not collapse?

We can't answer this question until we know what you mean by "collapse". That word has multiple possible meanings. You need to pick one.

 

[Peter Mole]

If that's really the case, your question is probably not answerable since you don't have the background knowledge required to even explain what you are asking about.

However, let me try to rephrase Rule 7 in a way that might be easier for you to get a handle on.

Suppose we are measuring the spin of an electron. We can do this using a device called a Stern-Gerlach device. Schematically, the electron goes in, and comes out in one of two directions, the "up" direction or the "down" direction, corresponding to the two possible results of the spin measurement.

Thanks for your reply and in helping me to understand.

It would help me, for now, to keep it in terms of the double slit experiment. Perhaps you could help me understand what a "which way" device is and how it works? Is the fired particle/wave going through something before it goes through the slit in order for it to be detected?

Informally, the term "collapse" is used to describe the process of assigning the appropriate wave function to a measured system based on the observed result of the measurement.

I really have spent about an hour now just trying to answer this and type out a response to your description, but it's probably best if I just define my terms as you've been asking me to do... You should know that I have read and re-read your post many times now and I do appreciate your efforts.

So let me just take a stab at defining what I thought was meant by collapse using what might be a flawed understanding of the double slit experiment.

Using the double slit experiment as an example, when a single electron is fired at the slits, this electron is still in super position. In this state, the electron can go through either slit, both slits, or neither, and so it acts like a wave, creating the interference pattern. However, if a which way detector is used to determine which slit the electron goes through, this action causes the electron to leave the super position state and collapse into a physical particle state instead. The electron has collapsed to this state and now acts like a physical particle. And this is why we see double bars instead of the interference pattern.

I'm very interested to have you correct my understanding. I don't have the math or a mastery of the vocabulary you use to describe your understanding of what's happening during the double slit experiment. However, I do have a conceptual understanding enough to see that something bizarre is happening. Something has changed the nature of the electron from one state to another such that it is acting like a physical particle in one state whereas before it was "collapsed" it was not acting like a physical particle but was acting like a wave. If you're unable to think conceptionally without shoehorning my question into your framing, then my question is probably not one that is answerable by you becuase you are unable to separate yourself enough from your academic background to even understand what I'm asking conceptually.

 

[PeroK]

So let me just take a stab at defining what I thought was meant by collapse using what might be a flawed understanding of the double slit experiment.

Using the double slit experiment as an example, when a single electron is fired at the slits, this electron is still in super position. In this state, the electron can go through either slit, both slits, or neither, and so it acts like a wave, creating the interference pattern. However, if a which way detector is used to determine which slit the electron goes through, this action causes the electron to leave the super position state and collapse into a physical particle state instead. The electron has collapsed to this state and now acts like a physical particle. And this is why we see double bars instead of the interference pattern.

This is very much a "pop-science" take on the experiment. The QM description is that an electron is a particle and behaves as a single particle, its wave-function evolving under the SDE (Schroedinger Equation) unless (Copenhagen interpretation) a measurement/interaction with a macroscopic measuring apparatus disrupts this evolution. QM itself, ironically, has no wave-particle duality; QM explains the apparent wave-particle duality that classical physics could not.

In the Copenhagen interpretation it makes no sense to ask which slit the electron went through - unless you measure this. Copenhagen demands you talk about only what is measured.

An intermediate measurement of an electron's position effectively resets the initial conditions for evolution under the SDE: wave function collapse.

The final measurement at the screen is less wave-function collapse than decoherence through absorption into the macroscopic system of trillions of atoms that is the macroscopic screen. After that measurement there is no free electron to analyse further.

 

[Peter Mole]

I thought I might have understood the first part of your question, but I don't understand the last part about the double slit. Let me try to find a concrete question that relates to what you are asking.

I appreciate the effort, but I'm not yet sure if your example is addressing my question.

Let's consider the asteroid belt, with millions of (macroscopic) asteroids floating around. Classically, each has a history and although the specifics are unknown, we can quite safely say that the asteroids formed at some point in the past and have moved about essentially unobserved ever since.

To my mind it is "safe to say" that macroscopic asteroids formed at some point and have been moving about, but then again, to my mind it's safe to say that a single electron was formed at some point and has been moving about, but I'm not sure quantum physics would agree with the later and maybe not the former.

I also noticed you used the word "observed" which might be quite a loaded term. I can accept these asteroids were never "observed," in the common sense, but your sentence structure also suggests they might have been observed during their forming. I don't think that's what you meant, but then why posit they have been "unobserved" "ever since"? Please clarify.

Is your question what does QM have to say about this?

No, but while we're here, what does QM have to say about this?

 

[PeroK]

No, but while we're here, what does QM have to say about this?

I don't know. That might be heavily interpretation dependent.

 

[PeterDonis]

Using the double slit experiment as an example, when a single electron is fired at the slits, this electron is still in super position. In this state, the electron can go through either slit, both slits, or neither, and so it acts like a wave, creating the interference pattern. However, if a which way detector is used to determine which slit the electron goes through, this action causes the electron to leave the super position state and collapse into a physical particle state instead. The electron has collapsed to this state and now acts like a physical particle. And this is why we see double bars instead of the interference pattern.

There are some corrections that could be made here, but I think they would take us too far afield, and this does help to clarify what you mean by collapse. Let me try to rephrase this in simpler terms:

When we run an experiment on some quantum object, the possibilities for what gets recorded afterwards depend on how the experiment is set up. For example:

(1) In a double slit with no "which way" information, what gets recorded is the pattern of dots on the detector screen, and that's it.

(2) In a double slit with "which way" information, what gets recorded is the pattern of dots on the detector screen, plus the output of the "which way" detector that tells which slit each individual electron went through.

You are basically saying that "collapse" happens when information that gets recorded is obtained. So in #1 above, "collapse" happens at the detector screen; but in #2 above, "collapse" happens at the "which way" detectors.

If this is a reasonable statement of how you understand "collapse", then I have two additional comments:

First, there is a correction to be made. In #2 above, the way I take you to be using the term "collapse", "collapse" would happen at both the "which way" detectors and the detector screen where the dots appear. In between those two points, nothing about the electron is being recorded, so we cannot say it is a "classical particle". In this particular experiment, it is true that there is nothing of interest in between that affects its behavior; but that is not true of all experiments. We could even make it not true of this one by putting something in between the slits and the detector screen--for example, what if there were a second set of slits in between, with no "which way" detectors? So for your usage of the term "collapse", we have to apply it everywhere that information that gets recorded is obtained.

There is also a caveat about that term "recorded". In the scenarios we've been discussing, "recorded" means a record is made that humans can read afterwards. But that doesn't have to be true. It is perfectly possible for information to be "recorded" in this sense by macroscopic objects that provide no way for humans to "read" anything. In fact, this possibility is what allows macroscopic objects to behave classically despite the fact that they are made of quantum objects that, in isolation, don't behave classically: information is constantly being "recorded" by the macroscopic objects, and so we can consider "collapse" to be happening all the time with them, hence we never observe them exhibiting things like quantum interference. For example, this is true of the asteroid belt in the example @PeroK used; they behave classically, and hence "collapse" under the above definition is constantly happening with them, but those constant "collapses" don't record any human readable information (unless we want to count the classical positions and velocities of the asteroids as human readable information, but that leaves an awful lot out, and also it's not as though we humans set the asteroids up as an experiment to record their positions and velocities).

My second comment is that the way I take you to be using the term "collapse" in the above is basically the same as Rule 7 in the Insights article I linked to earlier. So if that is what you are interested in, this thread can be moved to the regular QM forum, since that usage of the term "collapse" does not depend on any particular QM interpretation.

However, if you want to accept the above meaning of "collapse" for purposes of this thread, you should be aware that this meaning of "collapse" does not refer to any "real" process that may or may not be happening, despite the way I described things like the asteroids above. This meaning of "collapse" refers solely to something we humans do with our models of objects. It does not refer to any "real" process going on with the objects themselves. Some of the things you say make me think that might not be what you have in mind regarding "collapse"; for example:

Something has changed the nature of the electron from one state to another such that it is acting like a physical particle in one state whereas before it was "collapsed" it was not acting like a physical particle but was acting like a wave.

Here you seem to be talking about "collapse" as something changing with the electron itself, not something that is purely within our human models of the electron. If you are thinking of "collapse" as something that is "really happening" to the electron, then you are not talking about Rule 7 in the Insights article; you are talking about something that, according to some interpretations of QM, does not even exist. Not all QM interpretations even accept the existence of "collapse" as a real process happening to things like the electron.

So you still need to give some more feedback in order for us to know (a) how to respond to your questions, and (b) what forum this thread belongs in.

 

[PeterDonis]

If you're unable to think conceptionally without shoehorning my question into your framing, then my question is probably not one that is answerable by you becuase you are unable to separate yourself enough from your academic background to even understand what I'm asking conceptually.

Please do not display this attitude. You are the one that has a conceptual issue to resolve at this point, not us. I described what that conceptual issue is in my previous post just now (post #14). If we are unable to respond to your questions without further input from you, that is not because we are "unable to separate" from our "academic background". It is because you have not yet resolved conceptual ambiguities that are present in your questions, whether you realized they were there before or not.

 

[atyy]

Is it conventional knowledge that the wave collapses due to only to physical interaction between the subatomic particle and the subatomic means to measure it?

In quantum mechanics there are two fundamentally different types of interaction:
(i) interaction of quantum systems
(ii) measurement

The collapse happens only in situations that permit successive measurements.

Obviously, one may ask - why isn't the measurement an interaction between quantum systems - isn't the observer also a quantum system? This is not resolved within quantum mechanics, where one must decide subjectively when a measurement occurs. This is not a problem for producing accurate predictions for all practical purposes to date. But it is a problem that has led many, including Einstein and Dirac, to suggest that quantum mechanics is incomplete.

 

[Peter Mole]

This is very much a "pop-science" take on the experiment. The QM description is that an electron is a particle and behaves as a single particle, its wave-function evolving under the SDE (Schroedinger Equation)..."

Well hold up there a sec. Before being measured, That particle is certainly not behaving as a single particle would, unless a single particle can create a wave pattern, which it can't. Right? Put another way, single particles tend to have definite properties, but particles in super position do not.

"...unless (Copenhagen interpretation) a measurement/interaction with a macroscopic measuring apparatus disrupts this evolution. QM itself, ironically, has no wave-particle duality; QM explains the apparent wave-particle duality that classical physics could not."

Or in other words, QW explains the state of the electron before measurement and classic physics explains after measurement? Or, QW explains before the wave collapses, and classic physics after?

It's starting to feel very much that you completely understand exactly what I mean by collapse and that you understood from the moment of my first post and yet you are trying very hard pretend you don't. Even though I'm not fully understanding your POV, it's very clear you're trying to reframe my question or divert me from asking it not because you don't understand my question or what I mean by collapse, but because you think my question is invalid. That's fine. But, I'd like to understand WHY my question is invalid.

In the Copenhagen interpretation it makes no sense to ask which slit the electron went through - unless you measure this.

Well, as I'm sure you've heard, someone did think it made sense to ask, and when they did the came up with the idea to measure it, and when they did, the resulting pattern switched from interference to a double bar, a result that stunned the scientific community. If it didn't make sense to ask, they would never have set up the detectors to measure it. Whether or not it makes sense to you or Copenhagen to ask isn't really of concern to me or to those who did the experiment.

Anyway, I'm not necessarily asking what state the particle was in prior to being measured. I'm just trying to understand how in went from a QW state to a classic particle. It's a change in state that I've been calling the collapse of the wave function.

 

[Peter Mole]

No, but while we're here, what does QM have to say about this?

I don't know. That might be heavily interpretation dependent.

I'm very confused why you would interpret , re-direct, and otherwise speculate that I was asking something I wasn't only to tell me you don't know the answer to a question I wasn't asking.

 

[Peter Mole]

If you're unable to think conceptionally without shoehorning my question into your framing, then my question is probably not one that is answerable by you becuase you are unable to separate yourself enough from your academic background to even understand what I'm asking conceptually.

Please do not display this attitude. You are the one that has a conceptual issue to resolve at this point, not us. I described what that conceptual issue is in my previous post just now (post #14). If we are unable to respond to your questions without further input from you, that is not because we are "unable to separate" from our "academic background". It is because you have not yet resolved conceptual ambiguities that are present in your questions, whether you realized they were there before or not.

First who's "us"?

That's real question. You're the only one responding to me.

It feels like you are speaking for some kind of audience instead of speaking to me, some random person who's sincerely asking specific questions he sincerely doesn't understand.

I'm trying real hard not to interpret your attitude as condescending. If this is what your are doing I ask you to please stop displaying it. I truly do feel you are having trouble conceptualizing what I'm saying. Either that or you are being coy.

 

[PeterDonis]

Before being measured, That particle is certainly not behaving as a single particle would, unless a single particle can create a wave pattern, which it can't. Right?

Put another way, single particles tend to have definite properties, but particles in super position do not.

QW explains the state of the electron before measurement and classic physics explains after measurement? Or, QW explains before the wave collapses, and classic physics after?

All of these statements are wrong.

You said in the OP to this thread that you have a solid layman's understanding of the QM experiments you are asking about, but your understanding does not appear to actually be solid.

I'd like to understand WHY my question is invalid.

I don't think you'll be able to unless you forget everything you think you know about QM and start from scratch.

 

[PeterDonis]

who's "us"?

Everyone who is responding to you in this thread.

You're the only one responding to me.

Huh? By my count, four different people (including me) other than you have posted responses to you in this thread, two of them (including me) in multiple posts.

I'm trying real hard not to interpret your attitude as condescending. If this is what your are doing

It isn't.

I truly do feel you are having trouble conceptualizing what I'm saying.

No, that is not what is going on here. What is going on here is that you are having trouble conceptualizing what we are saying, because you have an erroneous understanding of how QM works. As I said at the end of my last post, I think the best thing for you to do at this point is to forget everything you think you know about QM and start from scratch.

Either that or you are being coy.

Not at all. I am trying to convey to you something that I expect will come as a surprise to you, but it needs to be conveyed if this discussion is going to go anywhere.

 

[PeterDonis]

t's starting to feel very much that you completely understand exactly what I mean by collapse and that you understood from the moment of my first post and yet you are trying very hard pretend you don't.

Just to be clear, that is not what is going on here. What is going on here, as I said in my previous post, is that you have an incorrect understanding of how QM works, and because of that incorrect understanding, you don't appear to actually mean any single consistent thing by "collapse". You appear to be shifting between different meanings without realizing it, and we can't always tell which meaning you intend (or indeed whether you intend any of the generally known meanings for "collapse" in either the basic math of QM or various interpretations of it) since you don't realize yourself that you are shifting meanings.

 

[PeterDonis]

I'm very confused why you would interpret

By "interpretation", @PeroK meant "one of the multiple interpretations of QM that appear in the scientific literature". He did not mean his own personal interpretation of anything. What he was trying to convey is that the term "collapse" can have different meanings, depending on which of those interpretations of QM you adopt (and all of those meanings are different from the meaning that corresponds to Rule 7 in the Insights article I linked to before). And, as I said in my previous post just now, you don't appear to be using the term "collapse" consistently with any single one of those recognized meanings, and you might not even be using it to mean any of its recognized meanings. We can't tell.

 

[Peter Mole]

There are some corrections that could be made here, but I think they would take us too far afield, and this does help to clarify what you mean by collapse. Let me try to rephrase this in simpler terms:

When we run an experiment on some quantum object, the possibilities for what gets recorded afterwards depend on how the experiment is set up. For example:

(1) In a double slit with no "which way" information, what gets recorded is the pattern of dots on the detector screen, and that's it.

But it's it more than a pattern of dots? Isn't the pattern an interference pattern? Isn't that distinction critical to the experiment?

(2) In a double slit with "which way" information,

lol. Why are you doing this? Why are you insisting in talking about this experiment in such a deliberately vague manner when I have specifically formed my question around how the interaction between the detectors and and the QW works?

So when you said "with "which way" information", you mean a double slit experiment where which-way detectors are utilized. Right? I just want to make sure we're talking about the same thing.

...what gets recorded is the pattern of dots on the detector screen,

By which you again mean the interference pattern, right?

.. plus the output of the "which way" detector that tells which slit each individual electron went through.

Well here I'm confused. If the detector screen first shows the interference pattern, but then that detector screen is replaced with a fresh screen and the which way detectors are added to the experiment, does the new detector screen show the double bar pattern on top of the interference pattern or just the double bar pattern?

You are basically saying that "collapse" happens when information that gets recorded is obtained. So in #1 above, "collapse" happens at the detector screen; but in #2 above, "collapse" happens at the "which way" detectors.

Not exactly. You asked me what I meant by collapse. My use of the term collapse simply refers to the collapse of the QW state of the electron into the classical state of the electron where it has properties. When I say "collapse" I'm describing a transition from one state to another. From a state of superposition to a classical physics state. Could it be that the collapse is "caused by information getting recorded" or is the collapse caused simply by physical interaction of another subatomic particle like a photon? I don't know. I'm trying to wrap my head around that.

If this is a reasonable statement of how you understand "collapse", then I have two additional comments:

First, there is a correction to be made. In #2 above, the way I take you to be using the term "collapse", "collapse" would happen at both the "which way" detectors and the detector screen where the dots appear.

Yes, that's fair. I hadn't given it much thought that the screen dectector is also collapsing the wave (as I use the term) but yes, that would have to be so of course. Again, still confused if you're saying the screen was or was not refreshed.

[/quote]
In between those two points, nothing about the electron is being recorded, so we cannot say it is a "classical particle".[/quote]

I'm sorry. I'm using layman's terms here. Would it be better to say it's acting like a classical particle? Are you suggesting the electron has now reverted back to a QW? Are you saying that the electron is now some sort of limited QW? Are you saying it's just not known? Are you saying it's an illegal question to ask what state it's in? I feel like you're fairly criticizing my vocabulary but not replacing it with a better definition. If the electron is not acting as a classic particle then why doesn't it form an interference pattern but in a more narrow field (because it went through the slit and then became a QW again). It's the fact that it forms a double bar instead of the interference pattern the very think that leads us to believe the electron is acting like a classical particle instead of a particle in super position?

I'm trying to fine a term for the electron after it's moved (collapsed) from superposition to a particle with known properties. I've called this a "classical particle." What do you call it? Or again are you saying it's ONLY a classical particle at the moment it's recorded, but then goes back to something else?

In this particular experiment, it is true that there is nothing of interest in between that affects its behavior; but that is not true of all experiments. We could even make it not true of this one by putting something in between the slits and the detector screen--for example, what if there were a second set of slits in between, with no "which way" detectors? So for your usage of the term "collapse", we have to apply it everywhere that information that gets recorded is obtained.

Again, I'm admitting my ignorance here and sincerely trying to understand. It's unclear to me why you want to change the basic experiment rather than just talk about it as is, but I'll try to play along. So sure, I can accept another set of slits placed as you describe would effect the particles behavior. However, I'm confused why that second set of slits would collapse the wave or why you would consider that second set of slits to be recording anything. Effecting behavior? Sure. How? I'm not sure. Your point? Even less sure.

Setting up the double slit experiment without detectors, my description (right or wrong) would be that the wave collapses when the electron hits he screen detector and when it does it forms and interference pattern. Hitting the screen is the only time information is recorded. However, the double slits to effect the behavior of the electron but this effect does not collapse the wave, nor does it count as a recording. And this is part of my confusion regarding physical interaction that collapses the wave verses physical interaction that collapses the wave and also provides information or a "measurement" if you will. I very much allow I'm not understanding some of these terms.

There is also a caveat about that term "recorded". In the scenarios we've been discussing, "recorded" means a record is made that humans can read afterwards. But that doesn't have to be true. It is perfectly possible for information to be "recorded" in this sense by macroscopic objects that provide no way for humans to "read" anything.

Are there quantum experiments where the wave is collapsed without a record being made that humans can read? Are there quantum experiments where the wave is collapsed without physical interaction?

It makes logical sense to me that macroscopic objects don't need to be "recorded" or "measured" for them to act classically, but because macroscopic objects are still made of quantum objects, I guess the idea is they would also need to be "recorded" or "measured" for their wave to collapse, so to speak. Is that right? Am I getting any of this right? Is what Einstein meant when he asked, 'Do you really believe the moon is not there when you are not looking at it?'

Although it seems an odd question, how do you know that it's perfectly possible for information to be recorded by macroscopic objects? If information about macroscopic objects is not recorded, does that mean they are existing in a QW state? These questions are well beyond my focus in my OP, but still very interesting. I'm not sure why you are taking me to these arguments or how they relate to my question, but while we are here I'm just trying to understand the assumptions on which they are based.

To be clear, I'm not and have not been suggesting that human "reading" the recorded information or measurement or whatever is causing the collapse. I even went out of my way to say I wasn't trying to make the consciousness argument.

What I AM and what I HAVE BEEN asking is if physical interaction is what is required to collapse the wave?

In fact, this possibility is what allows macroscopic objects to behave classically despite the fact that they are made of quantum objects that, in isolation, don't behave classically:

This is part of what I simply don't understand about what's being described by QT. Do quantum objects always exist as QW except in moments when they are measured? After being measured can they go back to being in a QW state? Once the wave has collapsed, is it a one way transition from a QW state? Does this have anything to do with the law or concept of information never being lost? Therefore it can't return to a QW state? It's getting hard to contain this conversation. lol.

information is constantly being "recorded" by the macroscopic objects,

because they are constantly engaged in physical interaction on a quantum level?

and so we can consider "collapse" to be happening all the time with them, hence we never observe them exhibiting things like quantum interference. For example, this is true of the asteroid belt in the example @PeroK used; they behave classically, and hence "collapse" under the above definition is constantly happening with them, but those constant "collapses" don't record any human readable information (unless we want to count the classical positions and velocities of the asteroids as human readable information, but that leaves an awful lot out, and also it's not as though we humans set the asteroids up as an experiment to record their positions and velocities).

My second comment is that the way I take you to be using the term "collapse" in the above is basically the same as Rule 7 in the Insights article I linked to earlier. So if that is what you are interested in, this thread can be moved to the regular QM forum, since that usage of the term "collapse" does not depend on any particular QM interpretation.

I still don't know if you completely understand what I mean by collapse without expanding on it, but I do appreciate your long response. I truly have already spent hours of time on this thread myself. If the issue is moving me to another QM forum or subforum, do whatever you think is best. I just hope I can still engage in this kind of conversation for a little longer until I get a better understanding of these concepts.

However, if you want to accept the above meaning of "collapse" for purposes of this thread, you should be aware that this meaning of "collapse" does not refer to any "real" process that may or may not be happening, despite the way I described things like the asteroids above. This meaning of "collapse" refers solely to something we humans do with our models of objects. It does not refer to any "real" process going on with the objects themselves. Some of the things you say make me think that might not be what you have in mind regarding "collapse"; for example:

Here you seem to be talking about "collapse" as something changing with the electron itself, not something that is purely within our human models of the electron. If you are thinking of "collapse" as something that is "really happening" to the electron, then you are not talking about Rule 7 in the Insights article; you are talking about something that, according to some interpretations of QM, does not even exist. Not all QM interpretations even accept the existence of "collapse" as a real process happening to things like the electron.

So you still need to give some more feedback in order for us to know (a) how to respond to your questions, and (b) what forum this thread belongs in.

I don't know what to tell you. lol. When I say collapse I'm describing the transition from one state (super position) to another. Maybe you can help me clear this up by answering some of my question above. Is an electron always in a QW state except for and only for the moment when we measure it and after that it goes back? If QW state isn't "real" then I guess I'd be describing the collapse as a movement from the not-real to the real, but I'm not sure if I see the QW state as not-real to begin with. I do think the collapse changes the behavior of the electron from wave like to more classical behavior. My vocabulary is off, but I think the double slit experiment is showing a change in the behavior of the electron.

Maybe your feedback will help me better understand what I mean and what you're asking of me.

If I don't hear from you further, at least know I've appreciated the time you've taken to try to explain.

 

[Peter Mole]

All of these statements are wrong.

You said in the OP to this thread that you have a solid layman's understanding of the QM experiments you are asking about, but your understanding does not appear to actually be solid.

lol. Fair point. Let me change it to I thought I had a solid layman's understanding of the QM experiments I was asking about.

I don't think you'll be able to unless you forget everything you think you know about QM and start from scratch.

Okay, let's do it. For starters, it might help for you to explain to me why my assumptions are wrong instead of just saying they are wrong. Specifically what was wrong with the comments I made above that you listed. Are we arguing about what "single particle" is or just how it is or isn't different from a particle in a QW state or are you saying there's no change in the particle except in it's behavior? I'm just not sure where I've gone wrong.

 

[Peter Mole]

Huh? By my count, four different people (including me) other than you have posted responses to you in this thread, two of them (including me) in multiple posts.

Yes, I did mix you and peroK up as one person. I don't think I've responded to the other two, but might later. Anyway, I stand corrected.

I am trying to convey to you something that I expect will come as a surprise to you, but it needs to be conveyed if this discussion is going to go anywhere.

Okay okay, I'm still here trying to get it so long as you and everyone else has enough patience to try to get it across. (although I might need to come back tomorrow)

 

[Peter Mole]

By "interpretation", @PeroK meant "one of the multiple interpretations of QM that appear in the scientific literature". He did not mean his own personal interpretation of anything. What he was trying to convey is that the term "collapse" can have different meanings, depending on which of those interpretations of QM you adopt (and all of those meanings are different from the meaning that corresponds to Rule 7 in the Insights article I linked to before). And, as I said in my previous post just now, you don't appear to be using the term "collapse" consistently with any single one of those recognized meanings, and you might not even be using it to mean any of its recognized meanings. We can't tell.

Rather than respond to this, I'll repeat my definition and then see if any previous back & forth has already cleared things up.

By collapse, I mean the change in state from a quantum particle being in super position to being measured. The problem is, I may not be using the same vocabulary. When I say the electron is in super position I mean it's existing a state of probability. I mean it's existing as a wave function. I mean it's existing in a state that, when going through a double slit will create an interference pattern on a detector screen. I mean it's existing in a state that may or many not be "real". I mean it's existing in a state that may or may not be of this world. I don't know. But it's in a quantum state of super position.

Right or wrong, I'm describing the collapse as a change from super position to ... some other state. A state of a more limited wave? A state more liken to a particle acting like a classical particle? A state that has measurable properties? A state that is measured or observed or recorded or interacted with physically, or recorded by or not by humans... whatever.

I've been describing the collapse of the larger more wave like state into a smaller more definable state. Is the problem that this kind of collapse just doesn't happen? Is the electron ALWAYS in a QW state except for the very moment it's being measured? Is that what I'm misunderstanding? If the electron is not changing from one state to another, then is it just it's behavior that's changing? It doesn't seem to always be in super position, right?

Throw me a bone here. I've been on this thread for hours making a good faith effort to understand.

 

[PeterDonis]

Isn't the pattern an interference pattern?

In case #1, where there is no "which way" detector, yes.

Isn't that distinction critical to the experiment?

The difference in results between cases #1 and #2 is certainly a crucial experimental fact, yes.

Why are you doing this?

Because, as I've said, I think you have an incorrect understanding of QM, so incorrect that you would be better off forgetting everything you think you know and starting from scratch. So at this point I am assuming that you have done that and that you know nothing about how QM works, theoretically. All I am assuming is that you know the experimental setup and the experimental results.

Why are you insisting in talking about this experiment in such a deliberately vague manner when I have specifically formed my question around how the interaction between the detectors and and the QW works?

Because "the interaction between the detectors and the QW" is the wrong way to look at it. We need to lay the proper groundwork before we can see how to look at it the right way.

So when you said "with "which way" information", you mean a double slit experiment where which-way detectors are utilized. Right?

Yes.

By which you again mean the interference pattern, right?

In case #1 (no which-way detectors), the dots on the detector screen form an interference pattern. In case #2 (with which-way detectors), they don't.

If the detector screen first shows the interference pattern, but then that detector screen is replaced with a fresh screen and the which way detectors are added to the experiment

Yes. Or, if you like, there are two separate experiments, one with no which-way detectors, the other with which-way detectors, and we compare what the two detector screens show after both experiments are done.

My use of the term collapse simply refers to the collapse of the QW state of the electron into the classical state of the electron where it has properties.

And what I, and others, are telling you is that this is word salad. It doesn't mean anything. You think it means something, but you are wrong. What you say here is not any of the recognized meanings for the term "collapse" in QM; it's not the basic meaning in Rule 7 in the Insights article I linked to, and it's not the meaning in any of the QM interpretations in the literature. So if this is what you mean by collapse, we either close this thread right here, or you forget this meaning of collapse, and everything else you think you know about QM, and we try to help you reach a better understanding. Your call.

Would it be better to say it's acting like a classical particle?

No.

Are you suggesting the electron has now reverted back to a QW?

No.

Are you saying that the electron is now some sort of limited QW?

No.

Are you saying it's just not known?

No.

Are you saying it's an illegal question to ask what state it's in?

No.

I feel like you're fairly criticizing my vocabulary but not replacing it with a better definition.

I'm trying to get you to stop guessing, and to stop asking questions based on your current incorrect understanding of QM. I'm trying to get you to forget everything you think you know about QM as a theory and start from scratch. I can't replace what you currently think you know with anything better unless you first forget what you think you know.

Most of the rest of your post is you either guessing or asking questions based on your current incorrect understanding of QM, so I don't have any useful response to most of it. But there are a few additional points that are worth responding to; I'll do that in a follow-up post or posts since this one is getting long.

 

[PeterDonis]

Setting up the double slit experiment without detectors, my description (right or wrong) would be that the wave collapses when the electron hits he screen detector and when it does it forms and interference pattern.

A single electron does not form an interference pattern. It just makes a single dot somewhere on the detector screen. The interference pattern only shows up after many electrons have all passed through the apparatus, one by one, and made their dots on the detector screen: the interference pattern is formed by the many dots the many electrons made.

If we just talk about the 7 basic rules of QM, as described in the Insights article I linked to, Rule 7, which can be called "collapse", could in principle be applied to each individual electron when it hits the detector screen, but there wouldn't be much point since the electron gets absorbed by the screen and we can't make any further measurements on it. But in any case, that "collapse" is just a mathematical procedure; the 7 basic rules don't say anything about what "really happens" to the electron. They just tell you how to predict the results of experiments. So if you're just using the 7 basic rules, it's pointless to ask whether the electron "changes state" when it hits the screen, or whether it stops being a quantum wave and starts being a classical particle, or any of that. The 7 basic rules simply don't say anything about any of that.

If you insist on trying to say things about "what really happens" to the electron, then any question you ask will have multiple possible answers which are inconsistent with each other, since there are multiple interpretations of QM in the literature and they are inconsistent with each other. There are interpretations that say that "collapse" is a real physical process that happens to the electron when it hits the screen (but none of them say that real process turns the electron from a quantum wave into a classical particle). There are other interpretations, like the many worlds interpretation, that say there is no such thing as "collapse" at all.

 

[PeterDonis]

my confusion regarding physical interaction that collapses the wave verses physical interaction that collapses the wave and also provides information or a "measurement" if you will

Let me rephrase and respond to this question in a way that avoids all the issues with the term "collapse".

Go back to Rule 7 in the Insights article, or rather my simplified version of it in post #6 of this thread. Basically, we have this mathematical procedure which says, when a measurement result is known, you replace whatever wave function you assigned to the measured system before the measurement, with whatever wave function corresponds to the measurement result. So, for example, if you measured the spin of an electron, and the result was spin "up", you would replace whatever wave function you assigned to the electron before the measurement with the "spin up" wave function.

Let's apply this to the double slit, the original version with no "which way" detectors. We send an electron through the apparatus, and we observe where on the detector screen the electron makes a dot. That is the measurement result. So once we observe that result, we replace the wave function we assigned to the electron before the measurement, with a wave function that describes an electron localized in space at the location of the dot on the detector screen. Remember, this is just a mathematical procedure; we aren't saying anything about what the "real" state of the electron is, or how (or even whether) it changed when it hit the screen. We're just applying a mathematical procedure.

Now consider the second version of the double slit, where we include "which way" detectors. Now we have to apply the Rule 7 mathematical procedure twice for each electron. We apply it once when one of the "which way" detectors registers (only one will register for each electron), and we apply it a second time when the electron makes a dot on the detector screen.

The first question that naturally comes to mind, which I didn't address in post #3 of this thread is: what tells us to apply the Rule 7 procedure when the "which way" detector registers? The answer is simple: because we have to to make correct predictions about what we will see on the detector screen! If we don't assign a new wave function to the electron based on the "which way" detector registering, we will incorrectly predict that the dots on the detector screen will end up forming an interference pattern. So the general rule for when to apply the Rule 7 procedure is to do it wherever it has to be done to make correct experimental predictions.

Of course this is very unsatisfying to people (and it seems like you're one of them--don't worry, you're in good company) who expect QM to tell them from first principles when something like "collapse" is supposed to happen. Many, perhaps most, physicists would agree that the fact that QM cannot tell us from first principles when to apply Rule 7 means that QM, as a physical theory, is incomplete. But in practical terms, the rule given above works quite well. In fact, in practical terms, asking when the theory says you're "supposed" to apply the Rule 7 procedure is looking at things backwards. What actually happens is, experimenters run experiments with quantum systems, and those experiments help us to figure out where the Rule 7 procedure needs to be applied. And we hope that by doing enough such experiments we will come to a better understanding of what is actually going on "behind the scenes" and we will hopefully be able to come up with a more complete theory of quantum systems, one that will tell us from first principles when the Rule 7 procedure should be applied, instead of us having to put that in by hand in each individual case.

So one answer to the question of "which physical interactions trigger the Rule 7 procedure, and which don't?" is that we don't know from first principles how to determine that. All we have are the results of experiments and the knowledge of where we have to apply the Rule 7 procedure in order to properly predict those results. Hopefully in time we will come up with something better. But in the meantime, there simply is no single answer to the question; there is the practical answer ("do it whenever it has to be done to make correct predictions"), and there are answers given by various QM interpretations, which are inconsistent with each other. If you think this is an unsatisfactory state of affairs, you're certainly not alone; but that's the state of affairs.

 

[PeterDonis]

Throw me a bone here.

I don't know if you will consider it a "bone" or not, but let me point out some things for you to ponder as you give my last few posts a careful reading.

Note that, when I describe the Rule 7 procedure, I say that we replace the wave function we assigned to the electron before the measurement with a new wave function that corresponds to the measurement result. Note carefully that the electron is described by a wave function before and after; there is no change from "wave" to "particle" or from a wave function to something else. The electron is always described by a wave function.

Note also that I say the electron is described by a wave function. I do not say the electron is the wave function. I do not say the electron is a wave, or a particle, or changes from wave to particle. I do not say anything about what the electron is. I only say how we describe it. (To say anything about what the electron is would require adopting a particular interpretation of QM, and different interpretations say different and mutually inconsistent things about what the electron is.)

I have not said much about the details of these wave functions, but for the double slit experiment the following additional details might be helpful:

For cases #1 (no which-way detectors) and #2 (with which-way detectors), before the electron reaches the slits, it is described by a wave function that is, roughly speaking, spread out equally in space in the transverse direction (i.e., parallel to the first screen with the slits in it) and moving, roughly, in the direction towards the slits (i.e., perpendicular to the screen with the slits in it). The technical term for this wave function is a "plane wave", because its wave fronts are planes transverse to the direction of motion.

For case #1, after the electron has passed the slits but before it hits the detector screen, it is described by a wave function that, roughly speaking, is the sum of two diffracted waves, one spreading out in circular wave fronts from each slit.

For case #2, after the electron has passed through one slit or the other and the which-way detector has registered, it is described by a wave function that, roughly speaking, is a single diffracted wave spreading out in circular wave fronts from the slit whose which-way detector registered. (In other words, it is one or the other of the individual diffracted wave functions whose sum is the wave function for case #1 after the slits, above.)

For cases #1 and #2, after the electron has hit the screen and a dot has been observed, it is described by a wave function that is sharply localized in space at the position of the dot. Since the electron is absorbed by the screen at this point, this wave function doesn't really serve any purpose in predicting further measurement results.

 

[PeroK]

By collapse, I mean the change in state from a quantum particle being in super position to being measured. The problem is, I may not be using the same vocabulary. When I say the electron is in super position I mean it's existing a state of probability. I mean it's existing as a wave function. I mean it's existing in a state that, when going through a double slit will create an interference pattern on a detector screen. I mean it's existing in a state that may or many not be "real". I mean it's existing in a state that may or may not be of this world. I don't know. But it's in a quantum state of super position.

Throw me a bone here. I've been on this thread for hours making a good faith effort to understand.

In terms of having a meaningful discussion you probably need to start at the beginning and develop a more precise understanding of the basics of QM.

A lot of your misconceptions are common to those who have learned QM from popular science sources - the authors do the best they can, but it's not easy without real exercises for the student. In particular, what you have learned about QM has not demanded that you develop a precise understanding of the material and how to apply it to particular problems or scenarios.

What does QM have to say about an electron?

1) It's a stable elementary particle with a given mass, unit charge and total spin of 1/2. You can look that up in any table of the standard model of particle physics.

2) It's dynamic properties (position, momentum, energy, angular momentum, spin state etc.) are described by its wave-function. These dynamic properties are also known as the state of the electron.

Your understanding of things like state, superposition and where probabilities (not to mention probability amplitudes) fit in is not solid. I suggest you address these misconceptions in a methodical way.

 

[Peter Mole]

Why are you insisting in talking about this experiment in such a deliberately vague manner when I have specifically formed my question around how the interaction between the detectors and and the QW works?

Because "the interaction between the detectors and the QW" is the wrong way to look at it. We need to lay the proper groundwork before we can see how to look at it the right way.

When I said vague I was referring to you calling the results "a pattern of dots" instead of calling it an interference pattern or calling it "a double slit with "which way" information" instead of saying which-way detectors were used. As you'll recall, my question was about physical interaction, so describing the experiment as "information" without implying physical interaction makes it confusing.

Beyond that, I'll take your statement in good faith, but your comment sounds very much like what someone might say who's trying to convince me to believe in god or someone who's trying to convince me to believe in the ether.

...at this point I am assuming that you have done that and that you know nothing about how QM works, theoretically.

lol. Alright, I'm hanging on by the mane at this point.

All I am assuming is that you know the experimental setup and the experimental results.

Alright, then maybe try to at least start with explaining things within the confines of the experiment I thought I understood before introducing hypothetical elements to said experiment?

"the interaction between the detectors and the QW" is the wrong way to look at it. We need to lay the proper groundwork before we can see how to look at it the right way.

Ok.

Or, if you like, there are two separate experiments, one with no which-way detectors, the other with which-way detectors, and we compare what the two detector screens show after both experiments are done.

Okay, thanks for the clarification.

And what I, and others, are telling you is that this is word salad. It doesn't mean anything. You think it means something, but you are wrong. What you say here is not any of the recognized meanings for the term "collapse" in QM; it's not the basic meaning in Rule 7 in the Insights article I linked to, and it's not the meaning in any of the QM interpretations in the literature.

Primarily I have used the word "collapse" to describe a change in the behavior or state of the electron. It would be helpful to me if at this point you could also say that this description of what's happening is also wrong. I'm eager to understand what you describe is happening with the double slit experiment without this flawed definition of "collapse" and I will try to be open minded and let go of preconceived notions.

So if this is what you mean by collapse, we either close this thread right here, or you forget this meaning of collapse, and everything else you think you know about QM, and we try to help you reach a better understanding. Your call.

I'm a man of average intelligence with a high school understanding of physics paired with an intellectual interest that has led me to watch many documentaries and a few books on the top of QT. Any time you're too frustrated with my lack of understanding you're certainly free to ignore me or even kick me off this forum if that's in your power.

I can put aside this idea of collapse and just try to listen. I can accept my wrong thinking about something is getting in the way of my understanding. That's an experience I've had many times in my life. However, I won't be told not to think critically or not to form my own conclusions about any topic and I won't be convinced view my thinking is wrong before the effort of a conversation or debate bares it out. If you can't accept that, then by all means end the conversation now.

 

[PeroK]

Primarily I have used the word "collapse" to describe a change in the behavior or state of the electron.

There's only really the state. So, let's delete "behavior" and say "collapse" is a change of state of the electron.
Now, technically, the electron's state is continuously evolving with time, under the SDE. This is also called unitary evolution. So, let's call "collapse" a non-unitary change of state. I.e. upon measurement.

The only aspect of the electron's state we are interested in here is where we are likely to find it. And, ultimately, where it is likely to impact the final screen.

If we run the double-slit without a which-slit measurement, then the electron impacts the screen according to a certain probability distribution: a "double-slit interference pattern".

If we make an intermediate measurement of position after the slits, then (Copenhagen) that causes a collapse of the electron's wave-function. In this case the electron impacts the screen according to a different probability distribution: the appropriate single slit pattern.

Note that in this second case the electron still exhibits single-slit diffraction, which is a non-classical phenomenon.

Note also that the electron only follows a different probability distribution in the second case because we caused it to interact (indirectly) with an intermediate measusing apparatus. In a strict QM sense it does not behave like a wave in one case and a particle in the other. That's where popular science leads you astray.

If you insist: both cases show wave-like behaviour: double-slit interference and single slit diffraction respectively.

 

[PeterDonis]

I can put aside this idea of collapse and just try to listen.

Then read my posts #29, #30, and #31 and ask further questions after you have considered them carefully.