The detector itself contaminating double slit? How do we know?

[mattjfox]

Just a simple question. How do we know the detector isn't messing up the double slit experiment when a single photon is shot out. When there is no detector we know the interference pattern is formed even with a single photon. If we add detector to see where the photon actually went through a defraction pattern appears. As if the wave form collapsed because it was being detected.

To me my first logical thought was that the detector must be somehow messing up the experiment (plugging the hole) or interfering electromagnetically somehow, because the pattern is showing exactly like there is 1 slit again.

Now I realize this experiment is over 100 years old and has been retested numerous times. I am assuming the detector isn't actually messing up the experiment, but I just want to understand how the detector works and how they know it isn't contaminating the results. I have looked all over the intertubes and can find no simple explanation of this.

Thank you.

 

[atyy]

Yes, of course the detector is messing up the experiment. Feynman made a mistake (I think) when he said the double slit is mysterious.

 

[mattjfox]

Why answer with sarcasm. I just want to know how it is done.

 

[atyy]

It wasn't sarcasm. I do think Feynman made an error.

 

[mattjfox]

Oh... :D

 

[atyy]

You can find a discussion of Feynman's error in the introduction of http://arxiv.org/abs/1301.3274 (published in Rev Mod Phys http://dx.doi.org/10.1103/RevModPhys.85.1693).

In the above article, I am only recommending the introduction as a resource for references on a quite widely held view that the detector is messing up the interference pattern. The article goes on to propose an interpretation of QM, which I am not recommending, at least not for this thread. A major interpretational approach that sides with Feynman is the consistent histories approach, which again, I would say is still research and not consensus.

Anyway, take a look at the discussion around this excerpt from p3 of the above:
"Two systems are introduced for a good reason. Without the guarantee of arbitrarily distant parts within the experiment—so that one can conceive of measurements on one, and draw inferences about the other—what justification would one have to think that changing the conditions of the experiment (from one slit closed to both slits open) should not make a deep conceptual difference to its analysis? Without such a guarantee for underwriting a belief that some matter of fact stays constant in the consideration of two experiments, one—it might seem— would be quite justified in responding, “Of course, you change an experiment, and you get a different probability distribution arising from it. So what?”"

I added the bolding above, since I think that's what the question in the OP was. Take also a look at footnote 4 (Ballentine! Maybe some of his many admirers here can add insight. I'm not a big Ballentine fan, but it looks like he got it right here.)

 

[bhobba]

Just a simple question. How do we know the detector isn't messing up the double slit experiment when a single photon is shot out. When there is no detector we know the interference pattern is formed even with a single photon. If we add detector to see where the photon actually went through a defraction pattern appears. As if the wave form collapsed because it was being detected.

With photons that experiment can't be done because photons do not have an actual position or even a well defined path - certainly I don't know a way of detecting it without destroying it. The usual discussion of the double slit experiment with photons strictly speaking is a crock of the proverbial - but is useful as a starting point.

Actually Feynman has a discussion of this sort of stuff in his Lectures On Physics. Many things in beginner treatments of physics are not really true - but are good for starting out and getting a feel for things.

If you are worried about exactness, rather than simply getting a bit of a feel for what's going on, its much better to talk about the double slit with electrons.

QM predicts it doesn't matter what kind of detector you use the same happens. As to whether it's been experimentally verified - blowed if I know - except if it was the case it would be BIG news leading to an immediate Nobel prize.

The other thing to realize, is while popularisations, and even some beginning textbooks, do not point this out; the double slit experiment is not the basis of QM - its simply what some use as a pedagogical aid. The basis of QM actually lies in a generalisation of probabilty theory:
http://www.scottaaronson.com/democritus/lec9.html

From that QM to a large extent follows and because of its conceptual simplicity its pretty hard (but not impossible eg primary state diffusion) to violate without running into some well established experimental fact contradicting it.

In fact QM explains the double slit experiment - not the other way around:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

Its up to experimental guys to decide what might be interesting to check experimentally, but for my money that's a way way long shot.

Thanks
Bill

 

[mattjfox]

Thanks Bill. So you are saying explanations like this one using photon in the link below are complete bs?



He claims that the experimenter left the detector on and simply stopped recording the results and that still left an interference pattern. So it would seem that the recording of the result is what either collapses the photon into a particle or a wave? At least in this example. These types of accounts are all over the net.

Thank you

 

[bhobba]

Thanks Bill. So you are saying explanations like this one using photon in the link below are complete bs?

With photons - strictly speaking - yes. Since a photon travels at the speed of light there is no frame where it is at rest so can't have a position.

But that's nothing new - many things in beginner treatments are like that eg its the same with the so called wave particle duality see our FAQ:
https://www.physicsforums.com/showthread.php?t=511178

I think what Zapper said sums it up pretty well:
'We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.'

Same here - to get across the weirdness with laymen liberties are taken. You see it all the time in beginning texts on all sorts of issues. Another well known crock is the rubber sheet analogy to curved space-time - its nothing like that at all.

Thanks
Bill

 

[mattjfox]

Oh come on.. and this one?

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm



These are real experiments using photons and detectors... now you are bordering on sounding like a conspiracy theorist saying these are fake experiments just to confuse laymen.

 

[mattjfox]

Anyone care to provide a real answer instead of saying particle wave duality is just to confuse newbs?

 

[Cthugha]

Oh come on.. and this one?

[nonsense link removed]

These are real experiments using photons and detectors... now you are bordering on sounding like a conspiracy theorist saying these are fake experiments just to confuse laymen.

Bottomlayer is a well known crackpot site. Please avoid linking to it. So is Thomas Campbell whose youtube video you linked. If these were real experiments, there would be peer reviewed publications showing them. Anybody can claim sensational stuff and put it on Youtube (and make money from getting a lot of clicks). This is why discussions in these forums need to be based on peer reviewed publications. Peer review assures a minimum quality.

You will not find peer reviewed publications claiming any influence of turning detectors on or off. There is a reason for that.

 

[bhobba]

Anyone care to provide a real answer instead of saying particle wave duality is just to confuse newbs?

Mate its in the FAQ for a reason.

Around here you get the truth - warts and all.

BTW its not to confuse newbies - liberties are often taken for pedagogical reasons.

Just this morning I was reacquainting myself with Zees book on Quantum Field Theory. He talks about quantizing a mattress connected by springs. No one seriously believes a quantum field is like that but it helps to start with. In fact its an interesting exercise I went through to derive the equations without such an aid. I wouldn't like to have done that though without going through the mattress analogy.

Thanks
Bill

 

[mattjfox]

Ok then here is a link to the peer reviewed paper from the University of Maryland.

http://xxx.lanl.gov/pdf/quant-ph/9903047.pdf

 

[mattjfox]

Bill I appreciate the truth and that's all I want, but I am getting a little upset that this so called "fake" information would be circling out there...

 

[mattjfox]

Ok then here is a link to the peer reviewed paper from the University of Maryland.

http://xxx.lanl.gov/pdf/quant-ph/9903047.pdf

Actually to be honest I don't know if it was peer reviewed.

 

[Cthugha]

Actually to be honest I don't know if it was peer reviewed.

Yes, it is and it is correct, but it does not claim any influence of any conscious observers or detectors being turned on or off. In fact, the "choice" is done randomly in terms of whether a photon gets transmitted or reflected at a beam splitter.

In a nutshell the key to this experiment lies in understanding that you never see an interference pattern directly. You need to filter your detections. The "choice" is now whether you keep the necessary information to perform filtering in a way that leaves you with an interference pattern in the filtered state or not.

What is not happening is that the detections change afterwards. It is not the case that an interference pattern appears out of nowhere which has not existed before.

 

[bhobba]

Bill I appreciate the truth and that's all I want, but I am getting a little upset that this so called "fake" information would be circling out there...

Don't be - like I said liberties are taken for pedagogical reasons - its nothing to worry about.

I am formally trained in applied math. When I came across analysis (that's calculus done rigorously without the the hand-wavy arguments like if delta x is small you can ignore delta x squared etc etc) I learned many of the things from calculus were wrong - but you would have to have rocks in your head to teach it correctly from the start.

And interestingly once you go through that you return to the hand-wavey explanation - its easier - but incorrect.

Thanks
Bill

 

[mattjfox]

Yes, it is and it is correct, but it does not claim any influence of any conscious observers or detectors being turned on or off. In fact, the "choice" is done randomly in terms of whether a photon gets transmitted or reflected at a beam splitter.

In a nutshell the key to this experiment lies in understanding that you never see an interference pattern directly. You need to filter your detections. The "choice" is now whether you keep the necessary information to perform filtering in a way that leaves you with an interference pattern in the filtered state or not.

What is not happening is that the detections change afterwards. It is not the case that an interference pattern appears out of nowhere which has not existed before.

Let me paraphrase this...

You are basically saying the detections don't change afterwards, however, if you keep the information to perform filtering in a specified way you have an interference pattern if you don't filter information in that way you do not... that is bizarre.

 

[atyy]

Anyone care to provide a real answer instead of saying particle wave duality is just to confuse newbs?

Wave-particle duality is fine in its naive form for the double slit interference of a single particle. Here we imagine that the particle is a quantum wave, not too different from a classical light wave, and that it passes through the slit like a light wave. For a light wave, the interference pattern is different for the single slit and the double slit, and it is no different for the quantum wave. The only difference is that the meaning of a quantum wave (squared) gives the probability for the particle to be detected at a particular position. So that's the naive wave-particle duality - a quantum wave determines a position, which is a particle like quantity.

For two particles, wave-particle duality still holds, but not in a naive form. For two particles, we cannot imagine that the wave is propagating in ordinary space. Instead it propagates it an abstract space called a Hilbert space. We still have wave-particle duality, because the number of dimensions of the Hilbert space is determined by the number of particles present in ordinary space.

 

[bhobba]

Yes Atty is correct - you can often get away with a wave particle duality 'picture' - but not always. Even for single particles it has issues with the delayed choice experiment:
http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment
'Any explanation of what goes on in a specific individual observation of one photon has to take into account the whole experimental apparatus of the complete quantum state consisting of both photons, and it can only make sense after all information concerning complementary variables has been recorded. Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely'

For what's going on in the double slit please read the link I gave before that gives the QM explanation:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

Now for the kicker. Even that is not really correct:
http://arxiv.org/pdf/1009.2408.pdf

Its a really annoying thing about physics isn't it?

Or maybe its just that all sorts of things can be viewed at various levels of sophistication and we chose the level appropriate to our needs.

Seriously its just a fact of life - although an initially perplexing one.

Think about it

Thanks
Bill

 

[Cthugha]

Let me paraphrase this...

You are basically saying the detections don't change afterwards, however, if you keep the information to perform filtering in a specified way you have an interference pattern if you don't filter information in that way you do not... that is bizarre.

Why is that bizarre? Some of the photons you detect will show an interference pattern. Some other ophotons will show a different interference pattern. The superposition of all of these patterns will give no pattern at all. If you somehow manage to identifyone "subgroup" of photons which belongs to one pattern, you will be able to see it.

You can get interference patterns via filtering in a fully classical setting, too. Shine white light on a double slit. You will get a pattern similar to the one shown here:
https://www.physicsforums.com/showthread.php?t=218869

If your detector is just sensitive to intensity, you will not see much of an interference pattern. Think of a true greyscale image of the image shown there. If your detector is color-sensitive, you will be able to see that you get a different pattern for each color and each of them has good visibility. If you keep information which color corresponds to each detected photon you can reconstruct all of these patterns. Otherwise you will not be able to reconstruct the patterns. What is more bizarre? Saying that you kept track of colors or saying that performing color filtering afterwards actually changes the photon detection events that happened a while ago?

 

[mattjfox]

Sorry I guess I can be dense sometimes.. these were really just not the answers I was anticipating at all. I didn't think you would in essence agree that the detectors were messing up the experiment or that there were no peer reviewed papers with detectors not recording etc... I really thought these were experiments that had been concluded thousands of times. I mean there are sooo many books/web sites explaining it this way.

I guess it makes sense that that they are explaining the theory that they are trying to extend. Not necessarily an experiment that gave rise to the theory. As a graduate student we never conducted experiments and then came up with a theory, it was always the other way around.

 

[bhobba]

I didn't think you would in essence agree that the detectors were messing up the experiment

I don't think detectors messing up the experiment is the correct way of viewing it.

You mentioned

As a graduate student we never conducted experiments and then came up with a theory, it was always the other way around.

If you pass on what area that was then maybe we can point you in the direction of some literature. Mostly it depends on your level of mathematical sophistication.

If you have done a bit of calculus I highly recommend Lenny Susskinds lectures:
http://theoreticalminimum.com/

Thanks
Bill

 

[atyy]

I don't think detectors messing up the experiment is the correct way of viewing it.

Why not? With a detector blocking one path, it's a single slit experiment. Without the detector, it's a double slit experiment. The interference patterns are different for single slit and double slit experiments.

 

[Nugatory]

Why not? With a detector blocking one path, it's a single slit experiment. Without the detector, it's a double slit experiment. The interference patterns are different for single slit and double slit experiments.

That I'll buy, but I still don't like thinking of it as the detector "messing up the experiment". I prefer thinking that we have two quantum systems (one particle+detector+screen and the other particle+screen). They're different systems so they evolve differently, but each one is still best considered as a complete system. The problem with the "messes up the experiment" interpretation is that tempts us towards thinking in classical terms in which the detector, the screen, and the particle are naturally analyzed separately. That's OK for this problem, but it causes great confusion when it also leads people to think of the photon as a little bullet that can be discussed/considered in isolation.

 

[atyy]

That I'll buy, but I still don't like thinking of it as the detector "messing up the experiment". I prefer thinking that we have two quantum systems (one particle+detector+screen and the other particle+screen). They're different systems so they evolve differently, but each one is still best considered as a complete system. The problem with the "messes up the experiment" interpretation is that tempts us towards thinking in classical terms in which the detector, the screen, and the particle are naturally analyzed separately. That's OK for this problem, but it causes great confusion when it also leads people to think of the photon as a little bullet that can be discussed/considered in isolation.

Let me paraphrase to see if I understand. The "detector messes up the experiment" is ok as a short hand for the single and double slit experiments being simply different experiments. However, we should also stress that in neither experiment does the particle take a classical trajectory to the screen, ie. in quantum mechanics, the particle is not a classical particle with simultaneously well defined position and momentum.

 

[mattjfox]

Ok... when I said "messes up the experiment", I simply meant to say that in the 2 slit the outcome became different than what the experimenters were supposedly expecting.

Meaning the experimenters expected to see an interference pattern after they added the detector to the experiment. They didn't think the interference would go away when a detector was measuring the holes to see which hole the photon/electron went through. The purpose of adding the detector was simply just to see which way the single photon went.

Ultimately though some came to believe that the detector itself changed the duality of the particle into a lump rather than a wave.

To me the first thought I had was that the procedure itself of adding a detector must have somehow accidentally altered the result of the interference somehow.

It seems you all agree with this statement. That it did somehow alter the result and thus not showing an interference pattern.

Everything I was finding online though from non-peer review sources was saying that the detection itself as an observer changed the particle duality, which I am hearing here for the first time is non-sense and non-verifiable. Which actually makes sense because I could find no documentation on the methods or precedures of these detectors in any peer reviewed scholarly article on dual slit experiment using detectors.

 

[DrChinese]

I didn't think you would in essence agree that the detectors were messing up the experiment ...

As bhobba says, that is not really a good way to view things. A better way is as follows: If you know, or could know, which path was taken, there will be NO interference.

This is easily seen by placing polarizers in front of BOTH slits. When the polarizers are parallel, there IS interference. When the polarizers are aligned perpendicular, there is NO interference. The only variable here is the angle difference between the polarizers, so obviously if it only goes one way it shouldn't make any difference at all.

The polarizers allow you to potentially determine the which-slit information, when they are perpendicular. It does not matter whether you actually seek to gain that information.

 

[bhobba]

Why not? With a detector blocking one path, it's a single slit experiment. Without the detector, it's a double slit experiment. The interference patterns are different for single slit and double slit experiments.

Its 'mucking up' I am concerned with - I would prefer looking at the total experimental set-up.

Thanks
Bill

 

[bhobba]

That I'll buy, but I still don't like thinking of it as the detector "messing up the experiment". I prefer thinking that we have two quantum systems (one particle+detector+screen and the other particle+screen).

Yea - that's all. A bit of semantic clearness in expressing this stuff helps in understanding it IMHO.

Thanks
Bill

 

[mattjfox]

I would still call that messing up the experiment. Excuse my laymen terminology... but these guys don't create experiments without expected results... they were expecting to see a single photon going through that's why they put the detector up to see where it went through. When they did that the wave form collapsed and they didn't see what they wanted to see, thus a messed up experiment and indication that they were missing something in the quantum theory...

 

[DrChinese]

I prefer thinking that we have two quantum systems (one particle+detector+screen and the other particle+screen). They're different systems so they evolve differently, but each one is still best considered as a complete system.

This is really to mattjfox:

In my example (post #29) there is ONE quantum system: particle+2 detectors+screen. The 2 detectors are physically alike. When oriented relative to each other one way (parallel), there IS interference. When oriented relative to each other another way (crossed), there is NO interference.

The point being is that the idea that anything is "messed up" is just not correct, period. This is a quantum system, and quantum rules apply. And it has nothing to do with the physical observation being the source of some alteration to the setup. Nothing need be observed at all, as a matter of fact, as this setup demonstrates. Just the relative orientation of the polarizers changes. And you can vary to any degree in between too, with predictable results.

 

[mattjfox]

That's because those are the results you are expecting after years of doing this. I am referring to when scientists first conducted the experiment and were surprised by the results.

Also I don't really get the validity of the polarization experiments. When you turn the lens 90 degrees from the other lens aren't you are essentially just blocking light from going through one of the holes. If I am understanding that correctly I am not sure how this method validates anything except recreating a 1 slit experiment where you get a blob of light instead of interference pattern again.

 

[atyy]

I would still call that messing up the experiment. Excuse my laymen terminology... but these guys don't create experiments without expected results... they were expecting to see a single photon going through that's why they put the detector up to see where it went through. When they did that the wave form collapsed and they didn't see what they wanted to see, thus a messed up experiment and indication that they were missing something in the quantum theory...

That's because those are the results you are expecting after years of doing this. I am referring to when scientists first conducted the experiment and were surprised by the results.

Also I don't really get the validity of the polarization experiments. When you turn the lens 90 degrees from the other lens aren't you are essentially just blocking light from going through one of the holes. If I am understanding that correctly I am not sure how this method validates anything except recreating a 1 slit experiment where you get a blob of light instead of interference pattern again.

I'm not quite sure you got the technical bits about "collapse" quite correct. But anyway, let's say it's obvious now that single and double slit experiments give different results. Was there anything deeply wrong with say Feynman's surprise?

My thought is that "messed up" is correct in the sense that Feyman did get something deeply wrong. He meant to use the double slit to show that quantum mechanics is mysterious. Now that we can simply explain the double slit and single slits as different experiments, does that mean there is nothing mysterious about quantum mechanics? I think no - Feynman did not identify the true source of mystery, and that was a deep mistake.

The true source of mystery is not the double slit but in
(1) the classical/quantum split in the Copenhagen interpretation ('the measurement problem')
(2) the fact that any realistic solution to the classical/quamtum split must be nonlocal ('Bell test')

So the deep error is that Feynman, when commenting on the double slit "In this chapter we shall tackle immediately the basic element of the mysterious behavior in its most strange form. We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by “explaining” how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics." was wrong in that the double slit is not the "only mystery", if it is a mystery at all. Rather it is the Bell test that encapsulates what we consider the mystery of quantum mechanics.

The Feynamn quote is from http://www.feynmanlectures.caltech.edu/III_01.html#Ch1-S8. Bolding above is mine. The Feynman lectures are of course superb, even though they have a few errors.

Here's Dr Chinese introduction to the Bell test, which I think Feynman should have presented, not the double slit. http://www.drchinese.com/Bells_Theorem.htm

Here is an excellent, but slightly technical introduction to the measurement problem. http://www.tau.ac.il/~quantum/Vaidman/IQM/BellAM.pdf

Another excellent, slightly less technical write-up on these issues. http://www.nature.com/news/physics-bell-s-theorem-still-reverberates-1.15435

I think another misleading quote from Feynman is "But, when one does not try to tell which way the electron goes, when there is nothing in the experiment to disturb the electrons, then one may not say that an electron goes either through hole 1 or hole 2. If one does say that, and starts to make any deductions from the statement, he will make errors in the analysis. This is the logical tightrope on which we must walk if we wish to describe nature successfully.". This is correct within the Copenhagen interpretation of quantum mechanics, which I certainly use. However, it can mislead one into thinking that it is necessarily true in all physical theories that explain the double slit. It is not true, for example, in Bohmian mechanics, which can successfully explain the double slit experiement with particles that have definite trajectories. http://scienceblogs.com/principles/2011/06/03/watching-photons-interfere-obs/

But before you go further in the foundations, may I suggest: learn how to do some actual quantum mechanical calculations in the Copenhagen interpretation. I haven't read all of these, but a quick glance seems to indicate that these are good introductions:

http://www.feynmanlectures.caltech.edu/III_toc.html (Feynman, of course, in spite of the occasional error)

http://arxiv.org/abs/1007.4184

http://www.youtube.com/playlist?list=PL84C10A9CB1D13841

http://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2013/other/