# The observer effect

1. Jan 20, 2014

### eXmag

Hey guys, Ive been trying to find out an answer to this question I had today but couldn't find it. The question is regarding the measurement problem and the observer effect for the double slit experiment. Say I have 2 electron detectors labeled A and B respectively. Detector A is located at the slit openings to observe which slit a single electron goes through and detector B is located on the screen to observe where the electron hits. In this experiment, electrons are fired one by one. If I, the observer, only watches detector B and never observes detector A while detector A is on, does the wave interference collapse or will I see the wave pattern?

2. Jan 20, 2014

### StevieTNZ

There will be no interference as it is possible to obtain which-way information through observing detector A.

3. Jan 20, 2014

### eXmag

So if detector A is on it will quantum observe which slit the electron passed through therefore collapsing the wave function and I will observe no interference from detector B? Is that correct? Even though i'm never observing which slit the electron actually went through at detector A. I'm only observing the result on the screen from detector B.

Last edited: Jan 20, 2014
4. Jan 20, 2014

### StevieTNZ

Well, technically the electron becomes entangled with detector A and no collapse of the wave function occurs. That is the matter of the measurement problem. HOWEVER, due to the fact that in principle we can obtain which way information (by checking detector A), we don't get interference.

However we could get a bit more technical and say there is interference even with detector A present; its just hidden. If we somehow managed to erase which-way information, we will find interference fringes again by doing correlations. For more information Google Quantum Eraser experiment. Note that the experiments done thus far are not done with erasing 'data' from a macroscopic apparatus. I quote data as its the potential data that resides in detector A if the electron went through that slit it is monitoring. I say potential data because merely the electron becomes entangled with the apparatus and is still in superposition of going through both slits.

I hope that made some sense. I'm not the best at expressing what I want to say in writing to make it clear what I'm trying to get across.

5. Jan 21, 2014

### Staff: Mentor

This is a semantic difficulty sometimes experienced at the beginning level from reading populist accounts or beginner level texts that concentrate more on showing how to solve problems than explaining carefully the terminology.

In QM an observation does not involve a conscious observer. Its anything that leaves a 'mark' here in the commonsense classical macro world of everyday experience that is assumed, just like we normally assume, to exist independent of us and conscious observers.

In fact in modern times it its often assumed to have occurred once decoherence has taken place - but no need to go into that now - you can do a search on it if you are interested and post some follow up queries if stuff still isn't clear.

Also it needs to be said some populist accounts, of the what I call new age touchy feely variety, that want to push a mystical kind of agenda say, without giving all the facts, namely what I said above, that a conscious observer is required. It isn't - simple as that.

Thanks
Bill

6. Jun 15, 2014

### mitchellharper

non-conscious observer

Bill

I find common ground with your answer that an observer need not be conscious, otherwise nothing would ever have occurred in the universe, and everything would be in a never-ending state of superposition.

But one thing disturbs me. In a double slit experiment with 'no' observers, would not one expect quantum fluctuations or particle interference to act as an unconscious observer, so that if the experiment were to run for long enough, a departure from the expected interference pattern would gradually make itself manifest? This would be easy enough to test - has anyone tested it?

mitch

7. Jun 15, 2014

### UltrafastPED

Yes, this has been tested ... in 1908, by G. I. Taylor; you can read about it here:

http://spiff.rit.edu/classes/phys314/lectures/dual3/dual3.html

And no, the interference pattern did not disappear.

I've done lots of interference experiments with electrons; the interference/diffraction patterns remain for as long as you maintain the experimental conditions. For example, a pattern was maintained for 48 hours or more when my automatic camera system continued to record electron diffraction images.

OTOH, if there is a significant enough change (such as a high energy laser pulse on the material), then the image changes rapidly. In my case this was measured to occur in less than half a picosecond.

Last edited: Jun 15, 2014
8. Jun 15, 2014

### Staff: Mentor

Can't say I really understood your reasoning, but UltrafastPED gave the experimental answer.

But just to follow on what he said.

In modern times an observation is associated with decoherence.

An observation is considered to have occurred once decoherence has occurred
http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

It doesn't solve the measurement problem but allows an unambiguous definition of what an observation is without an arbitrary division on classical and quantum.

And the theory of that doesn't depend on time frame in experiments like the double slit.

Thanks
Bill

Last edited by a moderator: May 6, 2017
9. Jun 15, 2014

### mitchellharper

Thank you Bill and UltrafastPED for your prompt answers.

Bill, what I was trying to get at is that if decoherence can occur through naturally occurring events (as one would reasonably expect) then experiments to which UltrafastPED referred should show such deocherence.

I would imagine from the point of view of classical physics that if we fired, say tennis balls, at a target, given fixed conditions (wind, humidity, temperature variations etc) we should expect them to hit in the same spot each time.

This would not be what QM would predict. We would end up with a less sharply defined target area.

In a double slit experiment, factors external to those introduced by the experimenter could be expected to leave a 'mark', as you suggest, on the results of the experiment regardless of whether there was no intervention (an interference pattern), or some observational interference by the experimenter. Over time, one could expect these 'marks' to add to the blur of the initial patterns observed on the screen.

If this gradual blurring of the patterns is not observed, what conclusions can we draw concerning the existence of these events which are expected to leave their 'marks', and those effects generated by experimental observers which are virtually instantaneous.

mitch

10. Jun 15, 2014

### Staff: Mentor

Scratching my head here.

You seem to be saying that factors unknown to the experimenter may affect the result, but since none are observed you are reaching the conclusion something must be amiss, rather than the obvious conclusion that those factors do not matter.

Such reasoning doesn't make sense to me.

My conclusion that the experimental result is exactly what QM predicts is the external factors are negligible.

Thanks
Bill

11. Jun 15, 2014

### Staff: Mentor

On that point you need to become familiar with chaos. Since its not possible to know initial conditions exactly its not possible to know the results exactly. Whether such is of concern depends entirely on the application.

Thanks
Bill

12. Jun 16, 2014

### mitchellharper

mitchellharper

Bill

Above you stated:

'In QM an observation does not involve a conscious observer. Its anything that leaves a 'mark' here in the commonsense classical macro world of everyday experience that is assumed, just like we normally assume, to exist independent of us and conscious observers.'

So we would expect if an experiment ran for long enough, such unconscious effects - ie those not based on the experiment being conducted, would leave their 'mark' on the experiment without the intervention of a conscious observer.

Yet as UltrafastPED points out - and you seem to agree with him when you say:

'My conclusion that the experimental result is exactly what QM predicts is the external factors are negligible.'

- there is no experimental evidence for these external factors. No 'marks' left by unconscious macro or micro effects have been detected.

So are we left with the conclusion that experiments point to the fact that the conscious observer is necessary for the reduction of a wave function to a single eigenstate?

mitch

Last edited: Jun 16, 2014
13. Jun 16, 2014

### Staff: Mentor

Interesting use of 'we' here - I think its more 'I' since you are the only one I have ever heard raise it.

I would not expect that at all, nor do I think any physicist or applied mathematician would either, maybe philosophers, but they aren't usually known for their grasp such things, any more than I would expect all the atoms in the room would all of a sudden move in the same direction and levitate me. If you wait long enough it must happen, but calculations show the mean time for that to happen is so long it wont happen in any reasonable time frame eg the life of the universe. Same here. Indeed its possible that some random event may muck up the experiment but experiment shows it doesn't happen indicating its like me levitating.

If you believe otherwise show us your calculations - if you can prove it and its at variance with experiment you have an instant Nobel prize.

First QM does not say that observation reduces a wave function to an eigenstate - but that is another issue. Still the fact you say that suggests you have not gone particularly far in your QM education.

But to the substance of your statement, for want of a better way of expressing it - diplomatically your logic is erroneous, but being less diplomatic - it is simply nonsense.

First you make a hand wavy claim ie if you wait long enough some random event must make its presence known, without any calculation showing the time frame, then you draw the conclusion because of this conciousness is required.

If you want to go further I need to see your calculations showing such random occurrences will likely occur in a time frame the experiments have been done over and hence the theoretical predictions have other factors than those usually thought preventing it ie conciousness.

Thanks
Bill

Last edited: Jun 16, 2014
14. Jun 18, 2014

### mitchellharper

Be gentle with me Bill, I'm only an ignorant biochemist/geneticist.

It seems I've upset you somewhat, so can we reload and try and clear up any misunderstanding.

First, can I ask you to explain what you meant when you stated:

'In QM an observation does not involve a conscious observer. Its anything that leaves a 'mark' here in the commonsense classical macro world of everyday experience that is assumed, just like we normally assume, to exist independent of us and conscious observers.'

mitch

15. Jun 18, 2014

### Staff: Mentor

There's an old and famous rhetorical question: "Do you believe that the moon is there even when nobody is looking at it?" I do, and I expect that you do as well. That's just commonsense, applied to the world around us; things exist whether we know about them or not.

Now, suppose that the needle on a dial on my lab equipment twitches. The same commonsense argument works; the needle twitched whether someone was watching it or not. If the needle twitches only when a photon passes through one slit in a double-slit experiment, then we can say that the photon was observed when the needle twitched.

Thus, in QM an observation is any interaction (for example, between the photon and my lab equipment) between the quantum object and the commonsense real world around us that causes the latter to change.

16. Jun 18, 2014

### Staff: Mentor

Sorry mate. Its just your argument didn't make any sense, to me at least. I tend to get a bit twitchy with that sort of stuff.

No problem we can start again.

The founders of QM battled with the issues of QM and came up with the standard Copenhagen interpretation:
http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

What it said was that QM is a theory about measurements/observations that occur here in an assumed common sense classical world. Normally measurements have some kind of readout, dial, digital display, recording to computer memory, or something like that. I generalise that to 'marks' but it does raise the issue of what exactly is a measurement and not having an exact definition is a blemish with Copenhagen.

QM is the fundamental theory of the world we see around us, so should be able to explain that world. Yet how does a theory that assumes the existence of such a world carry out this feat. This in fact was the issue with Schroedinger's Cat. In Copenhagen its utterly trivial. An observation occurred when the particle detector clicked, flashed or whatever. Everything is classical from that point on. The cat is never alive and dead - it is alive or dead - period. This is a genuine issue - but not what pop-sci stuff will tell you.

A lot of work has been done in addressing that issue and considerable progress has been made, especially in theory of decoherence:
http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

But some issues still remain eg the so called factoring problem.

Thanks
Bill

Last edited by a moderator: May 6, 2017
17. Jun 18, 2014

### Staff: Mentor

Looks like you are either Mitch Raemsch or a follower of his that I used to encounter when I posted on sci.physics.relativity who used to advocate such crankish nonsense.

It's utter nonsense, and you will find the moderators of this forum is much less tolerant of such things than sci.physics.relativity.
https://www.physicsforums.com/showthread.php?t=733933

Continue such and you will, correctly, be banned.

Thanks
Bill

18. Jun 20, 2014

### mitchellharper

Bill

I hope you don't think I am Mitch Raemsch. Nor am I a follower of his. I really am Mitchell Harper - I never hide behind made up names on the net. I am a retired Biochemist/Geneticist and teacher from Victoria, Australia. I am writing a sci fi book and I need to know what are the views of diverse players in the field of quantum physics. I want to base my book on real science, not the Micky Mouse magic that serves in much contemporary sci fi.

Now if I scared you before with an out of this planet conclusion, it was not a reflection of my current understanding of quantum physics and the observer effect. I just want to pin down a point of difference, a seeming contradiction. Somewhere along the path of reasoning there is something that you know that I don't - it is not explicit in our dialogue so far. I want to pursue this until I find out what that bit of knowledge is. I don't believe in fairies at the bottom of the garden - to me the moon exists when I turn my back on it, and I don't accept my senses deceive me.

I do sometimes take a contrary position for the sake of finding out where the forks in a discussion might lie - I'll try to avoid this in the future.

Now, I totally accept the last two posts from you and 'Nugatory' concerning QM interactions not requiring conscious observers.

Can I ask this: you have explained how the macroscopic experimental apparatus can have an effect on quantum systems and cause a change in the world around us - can quantum systems interact with each other and give rise to similar effects. If so, could you give some examples.

mitch

19. Jun 20, 2014

### Staff: Mentor

Mate its that other idiot I thought could be Mitch Raemsch - not you :thumbs::thumbs::thumbs::thumbs:

You are getting into highly technical areas here.

Here is the technical detail:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

It will probably be fairly hard to follow without the required math, but please do read it, you will likely get something from it.

The best I can do at the beginner level is to explain an observation is a specific type of interaction between the thing being observed, the environment, and what is observing it. That type of interaction is called decoherence and is an example of what is known as entanglement. What decoherence does is change a superposition to what's called an improper mixed state. An improper mixed state looks like collapse has occurred - the exact words often used is apparent collapse. Exactly what looks like means is highly technical and tied up with with the difference between a proper mixed state and an improper one. If it was a proper one collapse would have occurred and everything would be sweet. But it isn't - its an improper one. The kicker though is its impossible by any means to tell the difference - which again explains why its called apparent. The issue this solves however is when an observation has occurred - in modern times it after decoherence which is a purely quantum process independent of where you draw the line between quantum and classical.

That's the best I can do without the technicalities. The jig is up, at least for me, to go any further than that.

If you want to go further I suggest the following:
http://theoreticalminimum.com/

It will require a bit of math, but unfortunately its simply not possible to go any deeper without it.

Thanks
Bill

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