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dhd40
2008-Oct-28, 04:57 PM
From:
http://en.wikipedia.org/wiki/Double-slit_experiment#cite_ref-20



when electrons are fired at the target screen in bursts, it is easy to account for the interference pattern that results by assuming that electrons that travel in pairs are interfering with each other because they arrive at the screen at the same time, but when a laboratory apparatus was developed that could reliably fire single electrons at the screen, the emergence of an interference pattern suggested that each electron was interfering with itself; and, therefore, in some sense the electron had to be going through both slits. (my bold)

Now my question: It should be easy to place two detectors very close behind the slits (too close to allow interference) in a way that each detector “clicks” only when a photon (or electron, etc) travelled through its associated slit.
What will happen?
a) both detectors will click simultaneously (the photon travelled through both slits)
b) only one of them (inpredictably) will click (the photon passed only through one of the slits)
c) something else will happen (e.g. neither d1 nor d2 will click)
d) no answer to that silly question
e) this has been discussed before (please give a link)

PraedSt
2008-Oct-28, 05:14 PM
b) is the answer. If you try and find out which way the electron goes, you lose the interference.

e) is probably also true :)

grant hutchison
2008-Oct-28, 05:17 PM
b)
And you won't see an interference pattern.

By measuring the position of the electron at the slits, you lose the possibility of the interference pattern. Stop looking at the slits, and the interference pattern will appear.

Grant Hutchison

Edit: I overlapped with PraedSt.

dhd40
2008-Oct-28, 07:59 PM
b) is the answer. If you try and find out which way the electron goes, you lose the interference.

e) is probably also true :)

I should have said it in my post: Iīm NOT asking whether interference occurs or not. The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...

dhd40
2008-Oct-28, 08:04 PM
b)
And you won't see an interference pattern.

By measuring the position of the electron at the slits, you lose the possibility of the interference pattern. Stop looking at the slits, and the interference pattern will appear.

Grant Hutchison

Edit: I overlapped with PraedSt.

Same comment as to PraedStīs answer: Iīm not asking whether an interference pattern will show up or not (it will not, of course)

01101001
2008-Oct-28, 08:10 PM
Now my question: It should be easy to place two detectors very close behind the slits (too close to allow interference) in a way that each detector “clicks” only when a photon (or electron, etc) travelled through its associated slit.

If it should be easy, maybe you can propose a design, or cite one, that would satisfy those goals of signaling at the right time and also not interfering.

PraedSt
2008-Oct-28, 08:10 PM
The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...

Yes dhd. You'll prove one or the other, but not both. Does that help?

Fazor
2008-Oct-28, 08:11 PM
The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...
Yes it is possible, and the test would show that the photon only enters one of the slits.

At least, that's how I've always understood it.

alainprice
2008-Oct-28, 08:13 PM
I should have said it in my post: Iīm NOT asking whether interference occurs or not. The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...

You got your answer already.

If you measure which slit the electron passed through without blocking or impeding either slit, you measure one click for one electron through one slit. As a consequence of what you have done, the interference pattern is lost and the electrons act as single particles, bullet like.

grant hutchison
2008-Oct-28, 08:14 PM
Same comment as to PraedStīs answer: Iīm not asking whether an interference pattern will show up or not (it will not, of course)Well, in that case, you got your answer plus bonus material. :)

Grant Hutchison

Fazor
2008-Oct-28, 08:17 PM
Well, in that case, you got your answer plus bonus material. :)
Grant Hutchison

I thought that's what you always got here at BAUT?

PraedSt
2008-Oct-28, 08:24 PM
I thought that's what you always got here at BAUT?

We aim to please it seems..

dhd40
2008-Oct-28, 08:41 PM
If it should be easy, maybe you can propose a design, or cite one, that would satisfy those goals of signaling at the right time and also not interfering.

Maybe, "easy" is a little bit overdone. On the other hand, a double-slit experiment using a 550 nm - laser would use slit widths of approximately 50 ĩm, and a slit distance of appr. 250 ĩm (donīt ask me where I got these figures from, I found it in my files handwritten). I would be really surprised if it wasnīt possible to move two detectors close enough to eachother so that their distance is <250 ĩm (the detectors themselves could be 1m long, 1 km, or ...)

BTW, vapour deposition of light-sensitive material could be one option?

But even if itīs not "easy" to "do" it experimentally, there shouldt be a theoretical QM-answer to my question

dhd40
2008-Oct-28, 08:43 PM
Yes dhd. You'll prove one or the other, but not both. Does that help?

Not really. Do you say that all options are equally valid?

dhd40
2008-Oct-28, 08:47 PM
Yes it is possible, and the test would show that the photon only enters one of the slits.

At least, that's how I've always understood it.

Thatīs a clear statement. But itīs not in agreement with Wiki:

and, therefore, in some sense the electron had to be going through both slits.

Unless "in some sense" has a different meaning

Ken G
2008-Oct-28, 08:48 PM
But even if itīs not "easy" to "do" it experimentally, there shouldt be a theoretical QM-answer to my questionThere is, and you have gotten it several times now-- the concept of "particle" is applicable here. That concept means that a particle cannot be detected in two places at once, nor can it be detected as having two separate trajectories. Whether or not that means it cannot have two separate trajectories when it is not detected is more a philosophical issue of whether one requires reality to be that which is actually detectable, or that which is merely conceptualizable. Probably your question opens into a whole series of other questions that center on this latter issue.

dhd40
2008-Oct-28, 08:52 PM
If you measure which slit the electron passed through without blocking or impeding either slit, you measure one click for one electron through one slit.

Where do you know this from? Any literature/references about such experiments?


As a consequence of what you have done, the interference pattern is lost and the electrons act as single particles, bullet like.

Again, Iīm not questioning this at all. But itīs not my point.

PraedSt
2008-Oct-28, 08:57 PM
Sorry dhd, don't think I've understood you. We've answered your multiple choice in the OP. Is there another question?

dhd40
2008-Oct-28, 09:25 PM
There is, and you have gotten it several times now-- the concept of "particle" is applicable here. That concept means that a particle cannot be detected in two places at once, nor can it be detected as having two separate trajectories.

I fully agree with this. But then it should be so easy (sorry
01101001) to experimentally prove that the "idea" of a photon (or electron, or fulleren, ..) passing both slits must be wrong. Has this kind of experiment ever been done? Or is the outcome so trivially evident that itīs not worth to give it a try?


Whether or not that means it cannot have two separate trajectories when it is not detected is more a philosophical issue of whether one requires reality to be that which is actually detectable, or that which is merely conceptualizable.

True. But honestly, I was mostly interested in knowing whether an experiment as described in my OP has ever been done. Just to see if Wikiīs (and probably othersī) statement


in some sense the electron had to be going through both slits

is "real" or "conceptual". To me, an answer to this (your) interesting "real-or-conceptual" aspect is extremely interesting

a1call
2008-Oct-28, 09:26 PM
Thatīs a clear statement. But itīs not in agreement with Wiki:


Unless "in some sense" has a different meaning

It indeed does have a different meaning. What the statement expresses is:

Although you can detect one electron passing through only one slit, if you don't setup a detector then there will be interference patterns which indicate that "in some sense" the electron has passed through both slits and has interfered with itself. This is because the interference pattern has characteristics which are a factor of slit separation. In other words if the electron had not passed through both slits (when you were not detecting) then interference patterns could not have formed (again emphasising "when you are not looking", because when you look/detect, there is no interference).

I have a followup question if I may:

*- What would happen if we put detectors (clickers which are not recorded) on each slit and put a timer on the electron firing and leave and lock the room. Come back the next day. The only recording will be of the electron hits and the resulting patterns if any.
**- Would interference patterns collapse even though there has been no conscious being "observing"?

PraedSt
2008-Oct-28, 09:34 PM
I have a followup question if I may:

*- What would happen if we put detectors (clickers which are not recorded) on each slit and put a timer on the electron firing and leave and lock the room. Come back the next day. The only recording will be of the electron hits and the resulting patterns if any.
**- Would interference patterns collapse even though there has been no conscious being "observing"?

Heh. That's like the: 'does a tree falling in the forest make a noise if there's no-one to hear it?' question.
Answer's yes to both.

Ken G
2008-Oct-28, 09:35 PM
I fully agree with this. But then it should be so easy (sorry
01101001) to experimentally prove that the "idea" of a photon (or electron, or fulleren, ..) passing both slits must be wrong. Has this kind of experiment ever been done? Or is the outcome so trivially evident that itīs not worth to give it a try?It depends on what you will consider a proof that the photon didn't pass through both slits. It is clear that if you track the trajectory of the photon, it will pass through only one slit. However, if you do that, it will also participate in a different pattern on the wall than if you do not track its path. It is common to interpret that as meaning it only passes through a definite slit if you actually track it. Personally, I never say it passes through "both slits"-- I simply say we know nothing about which slit it passed through, or neither, or both, unless we set up an experiment that can detect it. In other words, a question that is never posed to reality is never answered by reality. Reality is too busy with what is real to be bothered with what is hypothetical.


Just to see if Wikiīs (and probably othersī) statement is "real" or "conceptual". It is purely conceptual, to be sure. Indeed, I believe that is what they meant by "in some sense"-- they meant, in a purely conceptual (even philosophical) sense. It should not be taken too literally, expressly because of the kinds of experiments you are talking about.

Ken G
2008-Oct-28, 09:40 PM
I have a followup question if I may:

*- What would happen if we put detectors (clickers which are not recorded) on each slit and put a timer on the electron firing and leave and lock the room. Come back the next day. The only recording will be of the electron hits and the resulting patterns if any.You'll get no interference pattern. It doesn't matter if the information is actually recorded, only that the question of which slit was physically posed.


**- Would interference patterns collapse even though there has been no conscious being "observing"?Yes. Of course, a conscious person has to show up the next day, but science is in no position to assess the role of the observer.

dhd40
2008-Oct-28, 09:41 PM
Sorry dhd, don't think I've understood you.
Donīt worry, thatīs my problem. Itīs sometimes extremely difficult to formulate complex contexts in a non-native language. And I really appreciate the patience of the native-language posters here


We've answered your multiple choice in the OP.

Yes, but I was expecting some reasoning/explanations/citations as well :)

a1call
2008-Oct-28, 09:43 PM
Heh. That's like the: 'does a tree falling in the forest make a noise if there's no-one to hear it?' question.
Answer's yes to both.

*- Then, wouldn't that imply that the "wave function collapse" is the result of interaction with the detecting apparatus rather than actual measurement/observance?

PraedSt
2008-Oct-28, 09:48 PM
*- Then, wouldn't that imply that the "wave function collapse" is the result of interaction with the detecting apparatus rather than actual measurement/observance?

Yes, totally correct. But that's when the observation happens you see; 'observer' doesn't have to mean 'human observer'.

Edit: Whoops! Pressed post by mistake. Is that what you were asking?

PraedSt
2008-Oct-28, 09:56 PM
Yes, but I was expecting some reasoning/explanations/citations as well :)

Ah. Ken's is a good explanation. But if you want more, that could be a problem. :)
The double slit experiment illustrates the wave-particle duality of light. Sometimes light behaves as if it's a wave, sometimes as if it's a particle. But never both together.
This is a very old experiment; but till this day, no-one knows how or why light behaves like this, we just know that it does. Frankly, it's a pain in the b***.

Ken G
2008-Oct-28, 09:58 PM
*- Then, wouldn't that imply that the "wave function collapse" is the result of interaction with the detecting apparatus rather than actual measurement/observance?It depends on how we interpret what the "wave function collapse" is. In Bohr's way of looking at it, which is sometimes called the "Copenhagen interpretation" (though that phrase mixes in some extraneous stuff and can get you bogged down in a hurry), the wave function collapse happens as soon as you cross a conceptual divide between the quantum and classical realms. The classicalness of the apparatus "rubs off", if you will, on the quantum wave function, collapsing it.

However, the problem with this approach is that it does not allow you to treat the classical systems with quantum mechanics, which bothers people who like to imagine quantum mechanics is a fundamental theory. If you include the macroscopic apparatus in the closed system under consideration, then it should not be able to collapse the wavefunction, as it would not obey quantum mechanics to do so. Personally, I just think they are taking quantum mechanics in particular, and science in general, too literally.

alainprice
2008-Oct-28, 10:06 PM
Ah. Ken's is a good explanation. But if you want more, that could be a problem. :)
The double slit experiment illustrates the wave-particle duality of light. Sometimes light behaves as if it's a wave, sometimes as if it's a particle. But never both together.
This is a very old experiment; but till this day, no-one knows how or why light behaves like this, we just know that it does. Frankly, it's a pain in the b***.

I've made the mistake of saying duality means it can't show both at the same time, but this is wrong.

Fire single electrons at a screen until the interference pattern develops. Each detection on the screen is a single particle, an electron, but the overall pattern is for a wave.

Ken G
2008-Oct-28, 10:09 PM
Yes, the way I like to think of the duality is that they are particles that are told where to go by waves. If one takes that picture quite literally, it is called the deBroglie-Bohm interpretation of quantum mechanics. But that requires an extraneous equation that is untestable, and to me unscientific, so I don't view that as something that should be taken literally. Rather, it is a kind of anthropomorphism we use to give us a pedagogical picture of what is happening. Some are dissatisfied with that approach because they want physics to be something more than that, but I'm not really sure why.

alainprice
2008-Oct-28, 10:15 PM
I find it funny that we love to try to narrow down whether it's a particle, or a wave, or something else that looks like A or B. In reality, we do not know the true nature of these fundamental building blocks. What is the 'substance' of an electron? I wish I could answer that.

If we can find this answer, we will likely know why it behaves as it does.

PraedSt
2008-Oct-28, 10:17 PM
Fire single electrons at a screen until the interference pattern develops. Each detection on the screen is a single particle, an electron, but the overall pattern is for a wave.

Yes, definitely true. I'm saying that photons either behave like waves, or behave like particles.
But what say we don't arguing about this particular topic? It's mind-boggling :)

alainprice
2008-Oct-28, 10:20 PM
Agreed!

a1call
2008-Oct-29, 12:21 AM
**- Can a photon be detected at one of the slits without being destroyed?

I assume not so let's stick with Electrons for now. Electrons have charges and hence induce a magnetic field at motion.

I assume this magnetic field can be utilised in some sort of proximity detector that can be used as the at-the-slit-detector in a double slit experiment.

If so such a detector would have the following characteristics:

*- It would require a power supply which could be turned on and off

*- It would have a threshold setting beyond which the detected electrons would be ignored/not-"detected"

Consider the following scenario and it's variations:

*- In a room/lab there is an Electron-double-slit-experiment setup

*- A screen utilized in this setup records/shows interference patterns as expected

*- In this room/lab there are couple of electron detectors which their thresholds are set at say <1mm

I assume:

*- Interference patterns remain while the detectors are across the room even if they are powered on

*- Interference patterns remain even if the detectors are set at 1.9 mm from each slit even if they are powered on

*- Interference patterns remain even if the detectors are set at .9 mm from each slit given they are not powered on

*- Interference patterns are destroyed if the detectors are set at .9 mm from each slit when they are powered on

*- Interference patterns are destroyed if the detectors are set at 1.9 mm from each slit when they are powered on and their treshold is adjusted to <2mm

**- Is any of this correct:liar:, or I have completely missed the boat as usual?

Any enlightment is greatly appreciated.

hhEb09'1
2008-Oct-29, 12:49 AM
*- Interference patterns remain even if the detectors are set at .9 mm from each slit given they are not powered onI'm sure that your exact experiment hasn't been done, but off hand I would say this one would be wrong, under conventional wisdom.

PS: oops, no, I misread that part of your post. Sorry.

a1call
2008-Oct-29, 02:02 AM
Thanks hhEb09'1,
I get lost in my own jumble of statements as well, despite my best efforts at organizing them.

It's probably a reflection of my unfocused mind.

Basically I am wondering if the "wave function collapse" can be correlated in any way with detection device proximity.

BTW I pride myself at being able to perform effective Internet searches mostly thanks to Google and my dabble AT SE (Search Engine) world. As such there seems to be a good deal of ambiguity with related experiment details accessible by general public.

Ken G
2008-Oct-29, 03:19 AM
I'm not exactly what you mean by this proximity "threshold", but I think you have the basic idea right-- no matter what you do, it will appear that reality imposes a kind of conspiracy that anything which can determine which slit the electron went through will destroy the interference, and anything that cannot determine that will not.

a1call
2008-Oct-29, 03:48 AM
Thanks Ken G,

I will take the rare confirmation.
Any marginal grasp of the subject that I achieve is a good thing.:)

dhd40
2008-Oct-29, 03:36 PM
It is clear that if you track the trajectory of the photon, it will pass through only one slit.
And thatīs exactly the problem I have with the kind of experiment I described in my OP. How can the photon "know" that there are detectors behind the slits? It canīt look into itīs own future, or can it? It should be completely free to decide which slit to pass, independent of whether there are detectors or not behind the slits. It will recognize the detectors only after having passed the slit(s). But then itīs too late.


However, if you do that, it will also participate in a different pattern on the wall than if you do not track its path.
No doubt about this


Personally, I never say it passes through "both slits"-- I simply say we know nothing about which slit it passed through, or neither, or both(my bold), unless we set up an experiment that can detect it.

But if we run the experiment with e.g. fullerens, isnīt it then obvious that an individual C60-molecule can never pass through two slits at the same time? Isnīt this the proof that interference doesnīt depend on whether the photon/particle passes through both slits? That "passing through both slits" is purely a conceptual idea which we comfortably use because of our experiences with e.g. water waves?

Maybe, the interference pattern we observe with particles (photons, electrons, etc) has nothing at all in common with water wave interference? Maybe, we are just misled by the fact that these patterns look similar/alike.

StupendousMan
2008-Oct-29, 03:54 PM
And thatīs exactly the problem I have with the kind of experiment I described in my OP. How can the photon "know" that there are detectors behind the slits? It canīt look into itīs own future, or can it? It should be completely free to decide which slit to pass, independent of whether there are detectors or not behind the slits. It will recognize the detectors only after having passed the slit(s). But then itīs too late.



Electrons and protons and photons, and all objects in the universe, don't care whether you understand them. They act in a certain manner, whether or not it makes sense to you.

If you do not accept this simple fact, you should stop thinking about quantum mechanics, because it will just frustrate you.

Ken G
2008-Oct-29, 06:22 PM
And thatīs exactly the problem I have with the kind of experiment I described in my OP. How can the photon "know" that there are detectors behind the slits? It "knows" about the detectors because it is the very purpose of a detector to make a photon know about it. That is how detectors work. If the photon didn't know the detector was there, the detector could not detect the photon. This is a crucial point in quantum mechanics-- our fingerprints are all over the means we use to learn about the universe. That is the logical basis of the "Copenhagen interpretation", and although other interpretations are possible, that one wouldn't be were this not the case.

It canīt look into itīs own future, or can it?There is no need to do that for double-slit experiments, no-- the presence of the detector affects the photon because that's how the detector does its job. When you bring in entanglement of multiple particles, things get a lot more subtle, but then you are detecting correlations between particles, not just particles, so it's a higher level of complexity.

It should be completely free to decide which slit to pass, independent of whether there are detectors or not behind the slits.Detectors alter the interferences in the particle wave function, that's how they work-- and that makes the particle no longer "completely free". A particle is completely free if it is not detected, period.


But if we run the experiment with e.g. fullerens, isnīt it then obvious that an individual C60-molecule can never pass through two slits at the same time?No, that isn't obvious at all. But again, it is not necessary to take the picture so seriously-- you can get an interference pattern, and there's not much more to say without putting in words like "in some sense".

Isnīt this the proof that interference doesnīt depend on whether the photon/particle passes through both slits?No, you could get interference between fullerenes, if you can get their deBroglie wavelength large enough. If you can't, then the structure of the fullerene is becoming important-- it's not a point particle any more, it is multiple point particles. So that does depend on the nature of the particle in question.


That "passing through both slits" is purely a conceptual idea which we comfortably use because of our experiences with e.g. water waves?You are not required to imagine it goes through both slits, you are only required to be able to predict the pattern.


Maybe, the interference pattern we observe with particles (photons, electrons, etc) has nothing at all in common with water wave interference? Maybe, we are just misled by the fact that these patterns look similar/alike.The similarity is in the mathematics of the wave function. Any other similarity is philosophy.

a1call
2008-Oct-30, 01:31 PM
Probabilistic Measurements & Double Slit Experiments

Hi all. I have some follow up questions.

*- It is conceivable that a double slit experiment for electrons can be setup in such a way that detector A's triggering would be an indication that the probabilities are that an electron has passed through slit A with 90% certainty and through slit B with 10% certainty.

Likewise, detector B's triggering would be an indication that the probabilities are that an electron has passed through slit B with 90% certainty and through slit A with 10% certainty.

**- How "Black & White" is the emergence of interference fringes?
**- Would such a setup leave the fringes unaffected or would they fade and be be less defined (i.e. statistically with a less correlation factor to well defined fringes).
**- Has any such experiment been ever conducted?

Thanks in advance.

Ken G
2008-Oct-30, 03:26 PM
[QUOTE=a1call;1353934**- Would such a setup leave the fringes unaffected or would they fade and be be less defined (i.e. statistically with a less correlation factor to well defined fringes).[/quote]It depends on how the 90/10 detection is done. If the particle is detected, but the instrument does not faithfully record the result, the interference would be completely gone. But if the instrument actually allows for some genuine uncertainty as to which slit the particle went through, then you would get a combined pattern that would look like some trace of the interference pattern remaining. That is because the function of a detector like that would have a different effect on the particle. I don't know if it's been done, but pretty much everything you can imagine predicting with quantum mechanics comes out correct.

a1call
2008-Oct-30, 03:41 PM
Thanks Ken G,

I thought so.
It is both amazing and sad. Sad, because every effort at making these effects more tangible is doomed to fail.

dhd40
2008-Oct-30, 05:56 PM
It "knows" about the detectors because it is the very purpose of a detector to make a photon know about it. That is how detectors work. If the photon didn't know the detector was there, the detector could not detect the photon.
Do you say the photon knows about the detector(s) before it arrives at the detector(s) ? :confused:


Detectors alter the interferences in the particle wave function, that's how they work-- and that makes the particle no longer "completely free"
Yes, but only after the photon hit the detector ??


A particle is completely free if it is not detected, period.
Thatīs what I was thinking, because it hasnīt been detected before, or inside, the slits. Only after it has passed the slits. It isnīt detected unless it has left the slits. But then itīs too late, because it had to decide earlier which slit to pass


No, that isn't obvious at all. But again, it is not necessary to take the picture so seriously-- you can get an interference pattern, and there's not much more to say without putting in words like "in some sense".
No, you could get interference between fullerenes, if you can get their deBroglie wavelength large enough.
Even if no two fullerenes ares passing the slit(s) at the same time?


You are not required to imagine it goes through both slits, you are only required to be able to predict the pattern

Yeah, thatīs QM as we know it today. Itīs totally dissatisfying, but we have to face it, or so it seems :sad:

ETA: Ken G, please feel free to stop commenting on my questions/remarks. I can imagine that it must be frustrating to see how people donīt understand "obvious facts".

dhd40
2008-Oct-30, 06:07 PM
Electrons and protons and photons, and all objects in the universe, don't care whether you understand them. They act in a certain manner, whether or not it makes sense to you.

If you do not accept this simple fact, you should stop thinking about quantum mechanics, because it will just frustrate you.

Youīre so right! But I canīt stop thinking about QM. Every single cell inside me refuses to stop thinking about things I donīt understand. I simply canīt help to do. And there a so many of them. Alas.
But then Iīm happy itīs just like this. Otherwise, I would stop thinking. Oooh nooo, no way :wall:

a1call
2008-Oct-30, 06:46 PM
One thing that did help me accept the behavior without "understanding" it which is probably the best any of us can do was the following well done animation:

http://www.youtube.com/watch?v=DfPeprQ7oGc

Ken G
2008-Oct-30, 07:15 PM
Do you say the photon knows about the detector(s) before it arrives at the detector(s) ? :confused:No, I say that anything that happens to the photon after encountering the detector, which is what we are talking about (an interference pattern on the wall), knows about the detector. The point is, you must always carefully specify the question that is being physically posed to the system. If you are asking which slit the particle went through, it requires changing the photon to ask that in a physically meaningful way.

But what you really want to know is, how can it possibly change which slit the particle went through to later ask which slit it went through? But that's an illogical question-- how can it change the slit if the slit isn't determined until asked? This is the point-- reality does not answer questions that are not posed to it, so asking if posing the question changes the answer incorrectly assumes that reality had already answered a question that had not been posed to it.


Yes, but only after the photon hit the detector ??Yes, only after the photon hit the detector. But the point is, once again, that if you are asking the photon a question about what happens to it after it hits the detector, like what interference pattern it should make, then there's no problem with that being determined after it encounters the detector. If you ask a question before it encounters the detector, you have to actually physically pose that question, with some kind of apparatus, and you'll get an answer that knows nothing about the detector. The only way you run into trouble is if you think reality answers hypothetical questions. It does not, it answers the questions actually posed to it, and quantum mechanics tells us the answers reality will give-- at the exact time the question is physically posed.


But then itīs too late, because it had to decide earlier which slit to pass.In case it's not yet clear, this is precisely the kind of reasoning that you can not apply to quantum mechanics. The photon never "decides" anything until the question is physically posed to it. Now, don't confuse that with a conscious observer, that's another matter that gets quite philosophical. It suffices that the question was posed by some kind of physical encounter-- the observer can be hypothetical, but not the apparatus. It's either there or it isn't, and that controls the subsequent behavior.



Even if no two fullerenes ares passing the slit(s) at the same time?
Yes, the existence of multiple objects at the same time never affects the interference patterns, unless the objects directly interact in some way. Quantum mechanics is a linear theory, so you can superimpose any number of systems.


I can imagine that it must be frustrating to see how people donīt understand "obvious facts".It's not frustrating, these are the amazing things about quantum mechanics and the more people who can explain them to others, the better.

Ken G
2008-Oct-30, 07:36 PM
One thing that did help me accept the behavior without "understanding" it which is probably the best any of us can do was the following well done animation:

http://www.youtube.com/watch?v=DfPeprQ7oGc
Yes, that's a nice video because it gives a physical picture you can imagine and remember. However, I caution there are several factual errors in that video. The most glaring is that they pretend you always get marble-like behavior when you have just one slit, or when you know which slit the electron goes through. That's a terrible misconception to instill-- if the electron is going to act like a wave through two slits, it is also going to act like a wave through just one slit. The marble-like behavior is something completely different from wavelike diffraction, it has to do with when you can ignore the diffraction that is always there, entirely based on the deBroglie wavelength associated with the electron. If the deBroglie wavelength is small, you get a marblelike negligibility of diffraction, but if it's large, you get wavelike diffraction (one slit or two). So the whole business should be comparing single slit vs. double slit diffraction, never marbles-- the latter is just wrong.

Also, the video underscores the common misconception that observing the electron is some kind of mystical interaction. It's not, it is a perfectly physical interaction that can be understood physically, and if it is not a physical interaction, it will not have any effect on the electron. In other words, the observation is specifically set up to have an effect, that's the whole point of an observation-- the effect is to get the electron to answer a question it would not otherwise be called on to answer at all. That's an effect, it's not mystical. Mysterious, yes, but mystical, no.

dhd40
2008-Oct-30, 09:55 PM
No, I say that anything that happens to the photon after encountering the detector, which is what we are talking about (an interference pattern on the wall)

Iīm sorry. But thatīs exactly what Iīm NOT talking about. In my OP experiment I donīt even look at the wall to see whether there is an interference pattern or not. There canīt be an interference pattern, the photon has been sucked up by one of the detectors (or both, or none?).


The point is, you must always carefully specify the question that is being physically posed to the system.

The question I posed to the system was simply which of the two detectors (d1, d2) would click. d1, or d2, or both, or none?


If you ask a question before it encounters the detector, you have to actually physically pose that question, with some kind of apparatus

Isnīt the double slit between the source (emitter) and the detectors (not the wall/screen where the interference pattern might show up!) such a physical question? At least, that is my understanding


The photon never "decides" anything until the question is physically posed to it. ..... It suffices that the question was posed by some kind of physical encounter .... It's either there or it isn't, and that controls the subsequent behavior.

Again, I thought the double slit is such a physical encounter which really poses a physical question, which should give the click-answer d1, or d2, or ...
I really hope that we do not talk at cross purposes


It's not frustrating, these are the amazing things about quantum mechanics and the more people who can explain them to others, the better.

Thanks

stutefish
2008-Oct-30, 10:59 PM
If the slit is "clicking", it's a detector, and it's giving you the physical question Ken describes.

If the slit isn't clicking, it's just a gap in a board, and it isn't detecing anything, and no physical question has been asked of the electron yet.

Ken G
2008-Oct-31, 12:40 AM
The question I posed to the system was simply which of the two detectors (d1, d2) would click. d1, or d2, or both, or none? That question was answered a long time ago.
Isnīt the double slit between the source (emitter) and the detectors (not the wall/screen where the interference pattern might show up!) such a physical question?Of course. The detector can detect which slit the particle went through, after or before it goes through. That's up to the setup. The issue is, if it detects which slit the particle goes through after it goes through, was that answer somehow specified prior to the detection? The answer is no, that is not determined prior to the detection, even if the detection comes after the slit. Once again: reality never answers any questions before they are physically posed. This is simply true-- can you think of a case where reality answers a question that is not posed?

Again, I thought the double slit is such a physical encounter which really poses a physical question, which should give the click-answer d1, or d2, or... As I said, the simple experiment you describe does pose that question, and does answer it. Your problem is that you imagine the question is answered prior to its being posed, and find a contradiction as a result.

dhd40
2008-Oct-31, 09:35 PM
The issue is, if it detects which slit the particle goes through after it goes through, was that answer somehow specified prior to the detection? The answer is no
Maybe, this is a strange idea (but youīre responsible for this because of your question):
What if warped spacetime is "reality" also in the quantum world? Does anything forbid GRīs spacetime to also act there? As some kind of micro or quantum spacetime. While GR spacetime tells masses how to move (e.g. planets around the Sun), quantum spacetime tells "quantum masses" (photons, electrons, etc) how to move. And also quantum spacetime would be told by quantum masses how to warp.
Therefore, the setup of any experiment would determine the structure of the quantum spacetime fabric surrounding its experimental setup. And this would, of course, look different for a one-slit or a double-slit experiment. And therefore it would be only "natural" that the outcome of these experiments is different (no interference, interference). And it would also be "natural" that any modification of the experimental setup, e.g., shutters, mirrors, detectors, etc would change the quantum spacetime fabric, and consequently the result of the experiment.
This could explain why a photon "knows" about the number of slits "in advance", or whether there are detectors behind the slit(s) or not. Just because the quantum spacetime fabric before and after the slit(s) is different, depending on whether there is only one open slit or there are two of them. Maybe, even the double-slit interference pattern isnīt the result of interfering photons (electrons, etc) but of interfering spacetime fabric which tells the photon where to go?
OMG, I stop here to avoid posting more insane ideas.

BTW: Iīve read about Bohmīs Quantum Potential. But instead of interpreting this in terms of fluctuations in an underlying ether, it could simply be an "extension" of GRīs spacetime into the quantum world.



Once again: reality never answers any questions before they are physically posed. This is simply true-- can you think of a case where reality answers a question that is not posed?

Desperately seeking for an example :)

Ken G
2008-Nov-01, 01:43 AM
Maybe, this is a strange idea (but youīre responsible for this because of your question):
What if warped spacetime is "reality" also in the quantum world?The problem with this picture is not that there's anything wrong about it, it's just not terribly specific. It sounds like the flavor of what you are saying is not that much different from the standard description of a wave function and the constraints on it. Just substitute your "quantum spacetime" with the "wave function", and your "warp" with the standard "contraints on the wave function imposed by the environment" and it's more or less all the same ideas. The main difference is that the standard language comes with instructions on how to make quantitative predictions-- and its success at doing so suggests it cannot be improved by replacing a specific picture with a vague one. On the other hand, if your goal is not to make quantitative predictions, I see nothing wrong in using the language you have chosen to give yourself a sense of why things behave the way they do.

To me, the key in all this is that the apparatus determines the way reality plays out-- reality is not something separate from the apparatus that is merely "read" by the apparatus, instead reality includes the apparatus. As long as you use a picture that involves that basic truth, you can't go too far wrong. Now, some efforts, like Bohm's stuff, attempt to have their cake and eat it too-- in that the reality exists separately from the apparatus that reads it, and the particle knows which slit it will go through before it encounters either the slit or the detector. But the price they pay to accomplish that is the theory includes ad hoc elements that are both contrived and untestable, and to me do not sound much different from the equally untestable approach of simply saying "God did it". (Claims are made that Bohm theory is testable, or that God's will is testable for that matter, but I've never seen a scrap of scientific evidence that supports either of those claims.)

dhd40
2008-Nov-01, 08:04 PM
It sounds like the flavor of what you are saying is not that much different from the standard description of a wave function and the constraints on it. .... The main difference is that the standard language comes with instructions on how to make quantitative predictions-- and its success at doing so suggests it cannot be improved by replacing a specific picture with a vague one.

I will never be able to make quantitative predictions from my vague idea. Thatīs far beyond my competency. Anyhow, to me it looked appealing to think that GR could also govern the microcosmos.


To me, the key in all this is that the apparatus determines the way reality plays out-- reality is not something separate from the apparatus that is merely "read" by the apparatus, instead reality includes the apparatus.

Now, thatīs what I always took for granted. Itīs the simplest thing to understand. I canīt imagine that anybody looks at it in a different way

tommac
2008-Nov-01, 08:22 PM
I should have said it in my post: Iīm NOT asking whether interference occurs or not. The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...

From my understanding, once the electron is detected it is destroyed.
In this case you detect in in one slit or the other at that point it is destroyed.

I also think according to Bhor that the more you try to observe a photon the more it acts like a macroscopic object.
In this case I think it does pass through both slits but possibly not at exactly the same time. The instant that the photon is detected it is destroyed so it can only be detected in one slit and not the other even though there is evidence that it passes through both because of the interference patterns that shows interference with itself.

Len Moran
2008-Nov-01, 10:38 PM
To me, the key in all this is that the apparatus determines the way reality plays out-- reality is not something separate from the apparatus that is merely "read" by the apparatus, instead reality includes the apparatus.


Now, thatīs what I always took for granted. Itīs the simplest thing to understand. I canīt imagine that anybody looks at it in a different way


On that basis there would be an acute shortage of physical realists around, which I hardly think is the case. Just to reinforce what I think is the important point here - the apparatus includes the questions and the notion of an observer, rendering physics at the quantum level as being an inescapable holistic form of empirical enquiry (but practiced in an objective manner). The underlying reality that lay outside of our involvement (such as in the two slit experiment) is by definition mind independent and hence inaccessible - a notion that is unpalatable to many scientists and perhaps not appreciated by a large number of non scientists.

Ken G
2008-Nov-01, 11:46 PM
On that basis there would be an acute shortage of physical realists around, which I hardly think is the case.This gets to a very subtle and important issue-- just what is realism in science, anyway? A naive definition is basically that reality would be the same whether it was being observed or not. But that's a scientifically meaningless statement, because of the crucial role of observation in science. A less naive but unsuccessful approach is to say scientific realism means if you imagine two identical experimental setups, and one inserts an additional observation not present in the other, then the outcomes will be the same of any other observation-- the presence of the additional observation has a negligible effect. The two-slit experiment puts the lie to that type of realism. Neither does including observations on entangled particles restore the realism, because to include the additional observation you then have to look at correlations, and when you do that you again find the realistic expectation fails. There simply are no "fly on the wall" observations unless they are observing something already established about the system, and that's no observation at all. Ergo, the observation is simply part of the reality.

But my point here would be, this doesn't ruin scientific realism, not at all. It only ruins a nonscientific version we should have realized wouldn't work anyway-- the version where the observation is not part of the reality being observed. If we instead state that realism means you can have a reality if there is no observation, and you can have a reality if there is an observation, and the two are not the same but are distinguished solely by the presence of the observation, then there is no problem at all in applying realistic principles. Note also that by "an observation", I mean that in the general sense of an interaction capable of establishing certain information. It matters not if anyone is around to see the tree fall, if the interaction occurs.


The underlying reality that lay outside of our involvement (such as in the two slit experiment) is by definition mind independent and hence inaccessible - a notion that is unpalatable to many scientists and perhaps not appreciated by a large number of non scientists.The question I would pose here is, is there any such thing as an "underlying reality", and is it realism to assert that there is? Why can't realism just be a recognition of the actual reality? It seems to me the postulate of an "underlying" reality is not realism, but rather "underlying realism", if you will.

Len Moran
2008-Nov-02, 09:00 AM
The question I would pose here is, is there any such thing as an "underlying reality", and is it realism to assert that there is? Why can't realism just be a recognition of the actual reality? It seems to me the postulate of an "underlying" reality is not realism, but rather "underlying realism", if you will.


I do consider there is an underlying reality that can be pointed to as a "notion" (and nothing more than that) and that it can be contrasted with actual reality simply in terms of recognizing the role of the observer in physics in the manner you describe. I know that you don't see a huge distinction between the macro and micro scales in this sense, but I have always found it easier to make that distinction (in terms of the essential requirement of the observer). But the notion I talk about is completely at odds with what a physical realist would consider it as being - the notion pointed to by quantum mechanics is of an underlying reality that cannot ever be objectively accessed - even if some theory replaces quantum mechanics, the experiments will always invoke the notion of an observer.

As I have said, I find it easier to think of this notion at the quantum level where it is clear to see that classical objectivity is compromised. You I know consider that objectivity is also compromised at the classical level, but I always tend to come back (perhaps simplistically) to thinking of a cannon ball and an electron. The cannon ball can always be localized independently of us as having definite macroscopic attributes, the electron cannot. That (to me) seems to make a clear distinction between objectivity at the classical level and objectivity at the quantum level which gives rise (for me), at the quantum level, a much clearer conception of an "underlying reality" outside of our involvement, but very importantly, a reality that can never, ever, be objectively accessed.

But I agree with what I think are your sentiments here - the term "underlying reality" perhaps does give rise to something that is tangible, lurking inaccessibly in the background. I don't think of it like that, I just think of at as having a form that does not invoke any macroscopic notions whatsoever, but in that sense I suppose even using such words as "form" is a little contradictory.

Ken G
2008-Nov-02, 09:57 AM
But I agree with what I think are your sentiments here - the term "underlying reality" perhaps does give rise to something that is tangible, lurking inaccessibly in the background. I don't think of it like that, I just think of at as having a form that does not invoke any macroscopic notions whatsoever, but in that sense I suppose even using such words as "form" is a little contradictory.I don't have any problems with a reality that invokes no macroscopic notions, whereever that "really" applies. All I'm saying is, to have that kind of reality playing out somewhere in the universe, it has to be somewhere that there are no macroscopic couplings, for real. In other words, I have no problem with thinking hypothetically about reality-- what would have happened (statistically) had there been a macroscopic coupling, what would have happened had there not been. But we must not mix the two-- that's not realism. It's not realism to say that both those realities somehow coexist at the same place and time-- it is realism to assert it is always one or the other. And yes, only one of them is objectively accessible, and that is also the only one playing out whereever there is science being applied. The other is just as real, but we should not expect the same physics to apply, because the reality is different. Indeed, in some cases, we cannot even say that any type of physics applies, because we have no way to assert that meaningfully.

So what I'm saying is, the mistake of the realists is not that they want reality to exist whether it is being observed or not, it is that they want reality to be the same whether it is observed or not. Science only works on one kind of reality, the kind that our fingerprints are all over, the kind that is objectively accessible (whether classically or quantum mechanically). All we know about the other aspects of reality are how they project onto this one kind-- nothing more. I think we agree about that latter issue-- the exception I take is with the idea that somehow reality is playing out at the two levels at once, at the same place and time, like an accessible "classical" reality and an underlying "quantum" reality. But reality is just one thing at any one place and time-- and all we need track is where and when the classical couplings come into play (they will have to at some point or we can't even talk about it, we'd have no language to do so).

This is a very Copenhagen way of thinking. Heisenberg has been accused of being anti-realist, but I would say it is the "underlying quantum reality" people who are being anti-realist, because they either pretend that the reality is the same in the presence of classical couplings (as in the Bohm approach), or they embed those couplings in an untestable and inaccessible "many worlds" milieu that they imagine is deeper or more fundamental, a reality surrounding and encompassing our own concept of experience. That doesn't sound like realism to me, it sounds like extrapolation outside the proper realm of science-- a return to rationalist natural philosophy, despite all the advantages of splitting science apart along empirical lines.

To me, the key concept here is that of projection. Where I agree with the Copenhagen interpretation is that the projection necessarily must replace quantum concepts with classical language-- there are no quantum concepts that are independent of classical language. That's just plain true, they are demonstrably right about that. But I think the projection goes even deeper, and occurs as much classically as quantum mechanically. The projection is from what one might call an irreducible reality into an algorithmic one. What I mean by irreducible is simply that reality is whatever happens, it needs no reason to happen and there's nothing else that could have happened other than what did. That's the reality. But we can make no progress understanding irreducible reality, so we must instead take a projection onto algorithmic reality-- the reality that follows rules, where things happen for a reason, where other things could have happened if we had asked different questions and set up the experiment differently to see how those imaginary rules would have played out. This is the reality that obeys "laws" of physics, and we just made it up, because that was our goal all along. We always had to make that projection, or our brains would be of no use-- and that was true long before quantum mechanics was even invented.

Of course, the deeper question which this view begs is, why does this work at all? Why does projecting an irreducible reality onto an algorithmic version preserve any of the salient features of the true reality? This is a complete mystery, I don't think anyone has the slightest clue beyond saying that our brains evolved to do precisely that. It seems to work mind-bogglingly well, I agree that is amazing. We expected to be amazed by reality, and instead we are amazed at how well we understand reality. But still, despite all our successes, we still make a grave error of hubris to require therefore that the reality must actually be the algorithmic one, that reality actually does "obey laws", at it most fundamental level. This is the classic case of reversing the proper logic-- we invent theory to explain reality, not to constrain reality.

dhd40
2008-Nov-02, 11:19 AM
From my understanding, once the electron is detected it is destroyed.
In this case you detect in in one slit or the other at that point it is destroyed.

Most probably, this is in agreement with Ken Gīs argumentation:


It's not realism to say that both those realities somehow coexist at the same place and time-- it is realism to assert it is always one or the other …. reality is just one thing at any one place and time

I slowly start to understand Kenīs view of reality, and I must say it sounds damned good in my ears

Len Moran
2008-Nov-03, 11:08 AM
I don't have any problems with a reality that invokes no macroscopic notions, whereever that "really" applies. All I'm saying is, to have that kind of reality playing out somewhere in the universe, it has to be somewhere that there are no macroscopic couplings, for real. In other words, I have no problem with thinking hypothetically about reality-- what would have happened (statistically) had there been a macroscopic coupling, what would have happened had there not been. But we must not mix the two-- that's not realism. It's not realism to say that both those realities somehow coexist at the same place and time-- it is realism to assert it is always one or the other. And yes, only one of them is objectively accessible, and that is also the only one playing out whereever there is science being applied. The other is just as real, but we should not expect the same physics to apply, because the reality is different. Indeed, in some cases, we cannot even say that any type of physics applies, because we have no way to assert that meaningfully.


Thanks for that very informative reply, it is a perspective that has made me think about the way I look at this question of an inaccessible underlying reality and how it relates to our reality. But in the meantime I would like to clarify for myself your perceived role of macroscopic couplings, in particular in terms of any distinction you make between reality with and without an observer at the macroscopic level.

As far as I understand, decoherence theory allows for macroscopic couplings throughout the universe - the moon is there whether we look at it or not. On that basis a passive undisturbing observation on our part (if such a thing is possible) plays very little (or no) part in the reality of the universe, what we see is what there is. There may be an underlying irreducible reality, but that becomes completely consumed by macroscopic couplings that do not include us.

Now generally at the classical level, it "appears" to be the case that we don't really affect the nature of the reality we observe and experiment with. But I think you will argue that this is not so, in which case are you saying that even at the classical level, the macroscopic couplings that consume the irreducible reality must also be physically affected by our involvement? So in this sense how do I make a distinction between the universe with and without observation. Is the universe different when we don't look at it?

At the quantum level, things (at least to me) seem to be much more black and white. It is very much the case that we affect the physical nature of the reality we observe, so here the couplings are very much affected by our involvement. There is no question of being able to separate our involvement from the results, and here we are forced to acknowledge that there may be some inaccessible reality that exists outside of our involvement. But the important point here seems to be trying to define this irreducible reality, which of course we can't because we will never be able to access it. But what you would say is that it has to be a hypothetical entity imagined to exist in a situation where no macroscopic coupling can occur and that it can't exist side by side with macroscopic reality, it can only be consumed by macroscopic reality.

So if I understand you correctly, this view certainly does put a new perspective on "realism". In terms of what you say, all of physics is "realist" in the sense that we are investigating the only reality that can exist in any one place at any one time. There is no reality outside of that investigation as existing on its own terms outside of our involvement other than invoking a (I think very valid) hypothetical notion of an inaccessible reality that has no macroscopic couplings. But where does that leave an undisturbing observation of the universe?

Ken G
2008-Nov-03, 02:22 PM
As far as I understand, decoherence theory allows for macroscopic couplings throughout the universe - the moon is there whether we look at it or not.Yes, decoherence is very much what I'm talking about. Decoherence is a required part of the projection we always make from reality to our conceptions of reality, though it is not the only thing that happens in that projection.


On that basis a passive undisturbing observation on our part (if such a thing is possible) plays very little (or no) part in the reality of the universe, what we see is what there is.An observation is "passive" whenever what is being observed has already been established by some other physical coupling. One example would be if you measure the spin of an electron, and then measure the spin again along the same axis-- the second measurement is passive because it represents information that is already there. Passive = superfluous in quantum mechanics, and that is the primary distinction from classical physics-- in the latter, all measurements are treated as superfluous, there is no uncertainty principle.
Nevertheless, even classically a measured reality represents a projection of what is actually there, it's just a projection that is superfluous to the other projections being taken.


There may be an underlying irreducible reality, but that becomes completely consumed by macroscopic couplings that do not include us.I would say that if the macro couplings are really there, then they are part of the only reality that exists there. Realism thus dictates that we recognize their existence-- to the degree that it is meaningless to suggest that there is any other reality there, "underlying" or otherwise. In other words, it is not the macro couplings that cause the projection, they are just part of the reality. We do the projection, but we require the macro couplings to do it.


Now generally at the classical level, it "appears" to be the case that we don't really affect the nature of the reality we observe and experiment with. But I think you will argue that this is not so, in which case are you saying that even at the classical level, the macroscopic couplings that consume the irreducible reality must also be physically affected by our involvement?I would say that macro couplings don't "alter" the classical world because they are always present in the classical world, so there is no other kind of classical world to alter. It is the projections our minds make that do the altering, when we replace the irreducible classical reality with the algorithmic reality of "laws". The same for quantum physics, the laws are just different in the absence of decoherence.


So in this sense how do I make a distinction between the universe with and without observation. Is the universe different when we don't look at it? It is different if it is different-- if there is decoherence introduced by observation that would not otherwise be there. If the decoherence is already there (as with the Moon), then our observation is superfluous to the projection we are making (but it's still a projection of the actual reality).


At the quantum level, things (at least to me) seem to be much more black and white. It is very much the case that we affect the physical nature of the reality we observe, so here the couplings are very much affected by our involvement. But it is not our involvement that alters the reality, it is the decoherence that is part of that reality. That is the genuine "realist" approach.
There is no question of being able to separate our involvement from the results, and here we are forced to acknowledge that there may be some inaccessible reality that exists outside of our involvement.The requirement that there be decoherence clarifies these issues, yes, but they were always there. There was always the difference between reality and how we choose to describe reality.


But what you would say is that it has to be a hypothetical entity imagined to exist in a situation where no macroscopic coupling can occur and that it can't exist side by side with macroscopic reality, it can only be consumed by macroscopic reality. Yes.

In terms of what you say, all of physics is "realist" in the sense that we are investigating the only reality that can exist in any one place at any one time. There is no reality outside of that investigation as existing on its own terms outside of our involvement other than invoking a (I think very valid) hypothetical notion of an inaccessible reality that has no macroscopic couplings. But where does that leave an undisturbing observation of the universe?I think it says that we are forced to accept what we should have understood must be true all along-- that our understanding of reality will always be different from reality itself. The best we can do is characterize how the latter projects onto the former, and we should not be surprised when this causes conundrums when we pretend the two are the same.

ETA: In other words, realism is not pretending those are the same thing, realism is recognizing they are different.

hhEb09'1
2008-Nov-03, 03:15 PM
From my understanding, once the electron is detected it is destroyed.
In this case you detect in in one slit or the other at that point it is destroyed. Most probably, this is in agreement with Ken Gīs argumentation:Depends upon what you mean by destroyed. The experiment has been run where a single particle at a time is released towards the slits and screen. If the setup is such that there is no detection, the pattern on the screen, built up one particle at a time, reveals an interference pattern. If detection does occur of each particle, each particle goes on to impinge the screen, but no interference pattern is built up on the screen.

dhd40
2008-Nov-03, 04:22 PM
Depends upon what you mean by destroyed.

I think what tommac means is that the electron is no longer "available" for interference purposes, and I agree with this view

tommac
2008-Nov-03, 04:42 PM
If the setup is such that there is no detection, the pattern on the screen, built up one particle at a time, reveals an interference pattern. If detection does occur of each particle, each particle goes on to impinge the screen, but no interference pattern is built up on the screen.

Is this possible? How is this experiment done?

I was under the impression that to detect a photon you must absorb it. If it is absorbed it is done and can not continue on to the screen.

Ken G
2008-Nov-03, 11:13 PM
hhEb09'1 is right-- and dhd40 was talking about electrons rather than photons especially because it is easier to detect electrons without destroying them (photons seem more "fragile", not having a rest mass). But if we interpret "destroyed" as meaning "no longer available for interference", then everything being said is consistent. The key issue is that an experiment that answers three questions (where is the particle emitted, which slit does it go through, and where is it absorbed at the wall) is simply a different reality than an experiment that only answers the first and last questions.

hhEb09'1
2008-Nov-04, 04:13 PM
I think what tommac means is that the electron is no longer "available" for interference purposes, and I agree with this viewIt seems clear from tommac's comment below that "destroy" means not available for anything :)

But the electron is not destroyed, and it goes on to impinge upon the screen.

Has anybody ever done a three slit experiment, with a detector in just one slit? What happened?

tommac
2008-Nov-04, 04:17 PM
I was thinking photon didnt catch that we were talking about electrons.

For an electron I think once you detect it because of HUP you have changed the experiment and you no longer get the interference.


It seems clear from tommac's comment below that "destroy" means not available for anything :)

But the electron is not destroyed, and it goes on to impinge upon the screen.

Has anybody ever done a three slit experiment, with a detector in just one slit? What happened?

dhd40
2008-Nov-04, 04:36 PM
Has anybody ever done a three slit experiment, with a detector in just one slit? What happened?

I would expect the "normal" interference pattern if the distance between the two open slits fits the interference boundary conditions

dhd40
2008-Nov-04, 04:39 PM
I was thinking photon didnt catch that we were talking about electrons.

For an electron I think once you detect it because of HUP you have changed the experiment and you no longer get the interference.

HUP = Heisenberg Uncertainty Principle ?

tommac
2008-Nov-04, 04:47 PM
HUP = Heisenberg Uncertainty Principle ?

Yes.

tommac
2008-Nov-04, 04:49 PM
I would expect the "normal" interference pattern if the distance between the two open slits fits the interference boundary conditions

I would expect that you would not have any interference from the slit that had the detector.

One question I do have, in a 2 slit experiment is there a 50/50 distribution ( assuming two detectors ) of which slit the electron went through?

If only one detector do you still get 50% of the photons?

dhd40
2008-Nov-04, 05:14 PM
I would expect that you would not have any interference from the slit that had the detector.

Yes, thatīs implied in what I said


One question I do have, in a 2 slit experiment is there a 50/50 distribution ( assuming two detectors ) of which slit the electron went through?

Thatīs what I would say. Maybe, the answer is easier for photons


If only one detector do you still get 50% of the photons?

How could you know this if you have no information about the total number of photons? (you donīt know how many photons went through the "non-detector-slit"!

hhEb09'1
2008-Nov-04, 05:46 PM
How could you know this if you have no information about the total number of photons? (you donīt know how many photons went through the "non-detector-slit"!Isn't the setup such that the particles can be made to go "one at a time", so that they can be counted when they impinge upon the final screen? Thus, the total number would be known?

dhd40
2008-Nov-05, 11:40 AM
I would expect that you would not have any interference from the slit that had the detector.
Of course. With "open" I meant "without a detector behind the slit". My lazy wording!

dhd40
2008-Nov-05, 11:42 AM
Isn't the setup such that the particles can be made to go "one at a time", so that they can be counted when they impinge upon the final screen? Thus, the total number would be known?

I think we talk about detector(s) immediately behind the slit(s), not the final screen (which, of course, is also a detector)

hhEb09'1
2008-Nov-06, 09:00 AM
How could you know this if you have no information about the total number of photons? (you donīt know how many photons went through the "non-detector-slit"!
I think we talk about detector(s) immediately behind the slit(s), not the final screen (which, of course, is also a detector)I meant, N are detected at slit A, M are detected on the final screen, therefore, M-N must have gone through slit B.

dhd40
2008-Nov-06, 01:53 PM
I meant, N are detected at slit A, M are detected on the final screen, therefore, M-N must have gone through slit B.

Oh yes! :doh:

Next question: What happens if the two slits have different widths (but still in a range where "normal" interference could happen). Would the interference pattern look skewed in some way?

Ken G
2008-Nov-06, 02:07 PM
Yes, indeed in the limit as you gradually reduce the second slit away to nothing, you approach the one-slit situation.

Len Moran
2008-Nov-06, 05:54 PM
I think it says that we are forced to accept what we should have understood must be true all along-- that our understanding of reality will always be different from reality itself. The best we can do is characterize how the latter projects onto the former, and we should not be surprised when this causes conundrums when we pretend the two are the same.

ETA: In other words, realism is not pretending those are the same thing, realism is recognizing they are different.

I think your comments have clarified the position (especially with regard to there being one and only reality) that I essentially take from Bernard d'Espagnat on this issue, but I do find it very interesting that you both arrive at the same end result but (seemingly) from different directions.

D'Espaganat sees objective reality at the classical level as invoking counterfactuality - it is assumed that the event that we describe or predict is an event regardless of our presence. Your term "irreducible reality" seems to signifies such a stance - but importantly you see our role as taking on a projection from that irreducible reality and creating an objective reality. So there is a very strong element of modeling this irreducible reality in terms of the laws of physics which stem from us as sentient beings. This scenario says that there is an irreducible reality which we make sense of in terms of an algorithmic notion - rules and laws are invoked that are a function of us as sentient beings. But presumably that irreducible reality still involves the moon orbiting the earth (in some form) with or without our presence (or can we not say anything about this irreducable reality using familiar terms).

So for you, inaccessible (mind independent) reality manifests itself at the classical level in terms of projections, whereas d'Espagnat derives it from the distinctions that can be discerned between objective reality as it is applied at the classical and quantum levels.

At the quantum level, d'Espagnat considers that classical objective reality is not present. Counterfactuality does not hold - quantum mechanics predicts observations, not events. There is no sense of being able to describe something happening at the quantum level without including the observer. So at this level the nature of the reality that "is out there" waiting to be probed by us is inaccessible, contrary to the macroscopic reality that has the moon orbiting the earth.

So from my perspective (based entirely on d'Espagnat) it boils down to the following. I don't know whether you see your perspective fitting into this framework, but as I said, I do find it interesting that you both arrive at the same end result, namely a notion of mind independent reality.

1. Macroscopically, objective reality gives us a notion that things are happening whether we are present or not. This is how science is practiced at this level.
2. At the quantum level we can't say this - classical objectivity is compromised.
3. If at this quantum level we cannot separate our involvement from our reality as demonstrated by experiment then what kind of reality is waiting to be invoked by our probing of the quantum world?
4. The only thing that we can say about this reality is that it is hidden, inaccessible, and that it is there (independently of us) waiting to be probed. Once we probe, that reality transforms to a reality that involves us (by default) to give a macroscopic one and only reality. Probe differently, and we get a different, one and only, reality (such as in the two slit experiment).

What form this mind independent reality takes is a philosophical question and is not the important issue from my perspective. What seems to be important to me is that experimental physics points to an absolute reality as being mind independent and hence inaccessible. When we probe, that probing is not accessing that absolute reality, it is probing a transformed absolute reality that now has us as an essential ingredient whether we want it or not.

Ken G
2008-Nov-06, 08:17 PM
This scenario says that there is an irreducible reality which we make sense of in terms of an algorithmic notion - rules and laws are invoked that are a function of us as sentient beings. But presumably that irreducible reality still involves the moon orbiting the earth (in some form) with or without our presence (or can we not say anything about this irreducable reality using familiar terms).We can indeed say that, because we are not trying to leave realism, we are trying to find it-- science needs a concept of realism or it falls into sophistry. But the point is, we must not make claims about the reality of the Moon that are not actually playing out in, well, reality. If the Moon is there now, it is because it is now involved in processes that determine what is real there. If those processes are classical (as they are for the Moon), then we will not need any further processing to observe that reality, it is pre-packaged for our awareness. If, as holds for quantum systems, those processes are not occuring normally, we will need to artificially include them in the packaging process. That packaging process is part of the reality, if it is really happening-- it is just not necessary for the Moon.

Then we can consider counterfactuality, which classically or quantum mechanically, is an invention of the human brain-- reality is just reality. We need counterfactual conceptualizations to do science, but we must not confuse the reality of the Moon with the counterfactual projection we use whenever we subject the Moon to science. Science isn't the Moon, because we had the Moon before we had science-- that's realism. How much we lose about the real Moon when we do science on it is not entirely clear, but it is surely something (ask the poet). It is just more obvious what we lose in quantum mechanics-- still, we should have always expected that, we just lost track of what we were losing classically because we did not need to come to terms with it.



So for you, inaccessible (mind independent) reality manifests itself at the classical level in terms of projections, whereas d'Espagnat derives it from the distinctions that can be discerned between objective reality as it is applied at the classical and quantum levels.Yes, I would say the only reason the projection is manifested at the classical level is because our instruments of detection are classical objects. As such, whereever there is science going on, there is a classical projection going on.

But one might imagine quantum mechanical brains manipulating quantum mechanical instruments, and the science done by those brains would not require any classical projections. However, they would still be projections, all the same, because they would still be looking for a way to replace irreducible reality with an understandable, predictive, algorithmic, counterfactual version.

That's what science is, so we should never be surprised when defining scientific realism any other way leads to contradictions like the quantum-mechanical EPR paradox, or the classical paradox of determinism. The hypothetical "quantum brain" I referred to would merely run into different paradoxes if it made that same mistake.


At the quantum level, d'Espagnat considers that classical objective reality is not present. Counterfactually does not hold - quantum mechanics predicts observations, not events. That is true-- but I substitute the word "physics" where he has "quantum mechanics" in that last statement. And for the first statement, I would say "classical objective reality is not the same thing as either the true quantum reality, or the true classical reality, but it is more obviously different from the former, as would be natural to expect". In other words, the reality that is being observed is never the reality that is not being observed, because it is really being observed, and it is really being conceptualized by some brain. If these were the same realities, it would require that all classical reality is necessarily objective and understandable, but only circular arguments are capable of asserting that.


There is no sense of being able to describe something happening at the quantum level without including the observer.Right, we have no language to talk about what is happening at that level, being macroscopic entities ourselves. Realism requires that something real be happening at that level, but we have limitations in describing it. We can either ignore those limitations, and choose a version of realism that essentially builds reality in our own image, or we can accept those limitations, and accept that physics applies to a projected version of reality (classically and quantum mechanically).


So at this level the nature of the reality that "is out there" waiting to be probed by us is inaccessible, contrary to the macroscopic reality that has the moon orbiting the earth.Right, except I would say that even that Moon reality is also inaccessible to us. We have simply forgotten about the inaccessibility, because we are so accustomed to confusing algorithmic counterfactual projected reality for actual reality. But the key point is, "realism" as a philosophy can only be applied to the latter concept, not the former, as it is self-contradictory in the former context.

1. Macroscopically, objective reality gives us a notion that things are happening whether we are present or not. This is how science is practiced at this level.If we adopt realism as our starting point, then we assert categorically that something is happening whether we are present or not. Macroscopic experience merely informs us as to how much difference it makes if we are there or not-- and the answer is, not much. But the point is, the key difference, the huge difference, has nothing to do with whether we are there or not-- it has to do if whether or not we are applying the conceptual projection that is the very language of objective observation.

We can easily apply that hypothetically, and often do in science, and it makes no difference if someone is really there (the "tree falling in the woods"), but what does make a difference is that we still use the language of objective observation. That language is simply not applicable to a reality that does not include aspects that make the language mean something, i.e., classical couplings. That's the Copenhagen interpretation, in a nutshell. Where that interpretation runs into problems is only when it seems to run afoul of realism, but that only happens if one confuses the irreducible reality that realism actually applies to, with the algorithmic reality that presents a need for an interpretation of quantum mechanics in the first place.


2. At the quantum level we can't say this - classical objectivity is compromised.Worse, realism is compromised-- if no observation is being made, then the classically posed question is not being answered, so to ask reality to answer it anyway is quintessentially un-realistic. Realism means reality is reality, whether we are there or not, but whether we are there or not is part of the reality! It is only when that last bit is forgotten that we run into paradoxes.


3. If at this quantum level we cannot separate our involvement from our reality as demonstrated by experiment then what kind of reality is waiting to be invoked by our probing of the quantum world? No other kind, that is clear. We are stuck with our limitations-- and we do better to recognize that than to pretend it doesn't matter.


4. The only thing that we can say about this reality is that it is hidden, inaccessible, and that it is there (independently of us) waiting to be probed. Once we probe, that reality transforms to a reality that involves us (by default) to give a macroscopic one and only reality. Probe differently, and we get a different, one and only, reality (such as in the two slit experiment).Yes, that's what I'm saying, and I would stress that probing reality differently does not require any mysterious properties to produce a different reality-- it is already a different reality as soon as you assert there is a different probe at play (by default, as you say). So yes, at the end we both come to a similar point-- we participate in the reality we study. But I see that reality as irreducible, and not being "transformed" by our participation-- the transformation (projection) comes when we conceptualize what is happening via the language of objective observation. If I'm playing a basketball game, or watching a basketball game, these are simply two different realities involving a basketball game-- but how the sportscaster describes the events might seem transformed in some way (like, "this is really embarassing to watch").


What seems to be important to me is that experimental physics points to an absolute reality as being mind independent and hence inaccessible. It could not point to anything else-- the whole basis of experimental physics is built around that rubric, and out comes whatever comes out. We cannot experiment on the participation of our minds, so we experiment on something we are always forced to interpret as mind-independent reality. All we can say is that it seems to work extremely well, so we shall embrace realism.


When we probe, that probing is not accessing that absolute reality, it is probing a transformed absolute reality that now has us as an essential ingredient whether we want it or not.Yes, this is the core conclusion. All I'm adding to that is that this holds just as strongly in classical physics-- we are just more accustomed to ignoring it in that context. There are two transformations happening-- one that pre-processes whatever is happening into a condition suitable for classical detection (a step unnecessary for classical physics but a required element of the second transformation in quantum systems), and a second that performs the classical detection and invokes the language and analysis of objective science, that we have built up around such classical detections. Indeed, it is only the second stage that I view as fundamentally "transformative", and as such we should have always known this process would lead to contradictions if we apply philosophical realism to the algorithmic projection we derive from what is real. The former, being built by us, is just too flimsy a scaffolding to hold up under the weight of such requirements (even classically-- witness the angst over free will and classical determinism).

dhd40
2008-Nov-07, 05:16 PM
Ken G and Len Moran: Why donīt you write a book about REALITY?

I would be among the first to buy it

BTW: Which book written by Bernard d'Espagnat would you recommend to start with (focussing on reality) ?

Len Moran
2008-Nov-07, 07:43 PM
Ken G and Len Moran: Why donīt you write a book about REALITY?

I would be among the first to buy it

BTW: Which book written by Bernard d'Espagnat would you recommend to start with (focussing on reality) ?

Thanks for the compliment, but I don't think I'm up to writing a book:)

But I have gleaned a lot from comparing Ken G's views with those of Bernard d'Espagnat since both viewpoints lead in differing ways to a notion of an absolute reality that is mind independent and hence inaccessible.

I only have one book by d'Espagnat, his most recent - "On Physics and Philosophy". It has essentially been written in a non technical manner, but even so it is definitely not an "everyman's (or women's) easy guide" to the nature of physics and reality, and in fact I do often find it quite hard going - but that could be a reflection of my own shortcomings:) The book follows on from "Veiled Reality" which I have not read, but I gather is a bit more technical. Prior to these books he has written "Conceptual Foundations of Quantum Mechanics" which I know nothing about, but apparently is fairly well known. I think a lot of his other writings are in the French language.

dhd40
2008-Nov-08, 06:10 PM
I only have one book by d'Espagnat, his most recent - "On Physics and Philosophy". It has essentially been written in a non technical manner, but even so it is definitely not an "everyman's (or women's) easy guide" to the nature of physics and reality

Thanks, that sounds good. Iīll give it a try (actually, my wife will, Xmas is lurking :) ). Itīs available at a reasonable (28.-Euro) cost


The book follows on from "Veiled Reality" which I have not read, but I gather is a bit more technical.

And itīs a little bit more expensive (71.- Euro :sad:)

torque of the town
2008-Nov-09, 01:38 PM
Thanks, that sounds good. Iīll give it a try (actually, my wife will, Xmas is lurking :) ). Itīs available at a reasonable (28.-Euro) cost



And itīs a little bit more expensive (71.- Euro :sad:)


Check out Amazon guys from Ģ10.00, I've just placed this book in my Christmas wish list

http://www.amazon.co.uk/Physics-Philosophy-Bernard-dEspagnat/dp/0691119643/ref=sr_1_1?ie=UTF8&s=books&qid=1226237693&sr=1-1

David

dhd40
2008-Nov-09, 05:35 PM
Check out Amazon guys from Ģ10.00, I've just placed this book in my Christmas wish list

http://www.amazon.co.uk/Physics-Philosophy-Bernard-dEspagnat/dp/0691119643/ref=sr_1_1?ie=UTF8&s=books&qid=1226237693&sr=1-1

David

Thanks for the hint. Iīll check the delivery cost to Germany.

Weltraum
2009-Dec-26, 09:31 AM
I think this entire experiment and discussion demonstrate a fundamental problem for us in trying to understand the universe: We can never know how light truly is if we, by attempting to measure it, also interfere with it. We can never hope to know its true speed nor its true structure. And this may apply to all forms of energy we detect and measure.

I am a layman here, but this problem of observation and measurement intrigues me a great deal. It is like Plato's cave, is it not? We do not see reality itself, but rather a filtered, distorted, altered, personal version thereof. If we are to get beyond this, we must find a way to describe and predict the nature of a phenomenon such as light beyond the aid of instrumentation. The question is, how far could, say, mathematics get us in doing this? And just what is the wave function of light? How separate is it truly from the particle nature? This sounds more like two observable phenomena of something we cannot actually fathom in its entirety. That is, there are truths about light that remain unknown to our senses and even our imaginations. The rest is hiding behind a shroud of dark matter or something - we see just a portion. In fact, if we could only see the whole picture, there wouldn't be separate phenomena we call light, X-rays, gamma rays or whatever - there would be one giant unity, of which we've been seeing various aspects, and which we've been misidentifying as separate phenomena.

Is there much new in what I've said? Does it make sense?

tommac
2009-Dec-27, 09:44 PM
b) only one of them (inpredictably) will click (the photon passed only through one of the slits)
... is the answer

However is there is no interference I am not sure what the point of the experiment is?



From:
http://en.wikipedia.org/wiki/Double-slit_experiment#cite_ref-20




Now my question: It should be easy to place two detectors very close behind the slits (too close to allow interference) in a way that each detector “clicks” only when a photon (or electron, etc) travelled through its associated slit.
What will happen?
a) both detectors will click simultaneously (the photon travelled through both slits)
b) only one of them (inpredictably) will click (the photon passed only through one of the slits)
c) something else will happen (e.g. neither d1 nor d2 will click)
d) no answer to that silly question
e) this has been discussed before (please give a link)

Ken G
2009-Dec-28, 03:47 AM
We do not see reality itself, but rather a filtered, distorted, altered, personal version thereof.Yes, that's the problem exactly. Or perhaps it is not a "problem", perhaps it's just our version of the best solution we can find.


If we are to get beyond this, we must find a way to describe and predict the nature of a phenomenon such as light beyond the aid of instrumentation. But to do so would require leaving science, so perhaps it is not the goal to "get around" it. When we eat food, we digest it for our bodies, which involves changing the food. But no one sees that as a problem to get around-- it's just the point of eating. It's only a problem if there is something going wrong and toxic results are occuring. By and large, that does not seem to be happening with science.


In fact, if we could only see the whole picture, there wouldn't be separate phenomena we call light, X-rays, gamma rays or whatever - there would be one giant unity, of which we've been seeing various aspects, and which we've been misidentifying as separate phenomena.
That is very much the spirit of "unification", which is always a goal of science because it involves simplification and gaining power. It's just the ultimate goal. I think what you're asking is, do we unify because things really are one, or because we gain the most from imagining they are? That is an issue that philosophy has debated for a long time, under the heading of "monism" versus the alternatives.

blueshift
2009-Dec-28, 05:45 AM
First off, photons do not get "absorbed" by electrons. They get scattered and when a spectroscope is between a chilled gas and a light source the rainbow the observer sees will have some slits of darkness where photons of those wavelengths will have been scattered that are called "absorption lines". Emission lines are seen by aiming the spectroscope off the limb of the light source.

Secondly, we look at electrons by shining light on them. To measure the speed accurately we must shine a light with a single wavelength and it becomes easy to determine a particle's speed. Measure the wave from crest to crest over time and velocity can be determined. However, the location of the particle has become highly improbable. Just as a nice ocean wave rolling toward shore resembles an incoming light source and is easy to determine its velocity, we cannot tell where the contact lens is that we dropped in the water and gave it the property of riding all the way to shore on the wall of the wave. It can be anywhere along the entire wall of the wave that stretches for miles. So how do we locate a particle whose probability is greatest along the entire crest of the wave? With a localized wave packet, something akin to a periscope that a sub pokes out of the water and is easy to spot. But in creating such a wave packet, one must use several short wavelength waves, all that differ from one another and make it impossible to measure velocity as was done before. So in the second experiment we have destroyed our capability to measure velocity while in the first we destroyed our capability to measure location.

Without knowing the location and the velocity of a particle it becomes impossibel to measure cause and effect, something that doesn't exist at the quantum level. Only probability exists.