# Is destruction of dbl-slit interference pattern wavefunction collapse?

Homework Helper
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I have read in several places (such as this StackExchange reply) that wavefunction collapse is 'non-physical'. The explanations I have read seem clear and understandable to me and have helped me to understand EPR and the implications of Bell's Theorem.

I am trying to reconcile this in my mind with how an interference pattern from a double slit setup is destroyed when a particle detector is activated at one of the slits. Clearly, whatever is happening when the detector is switched on is physical, as we can see its effect. But up to now, my understanding was that the impact of the detector was to collapse the wave function of the particles to an eigenket of the position operator.

However, if wavefunction collapse is non-physical, we should not be able to observe its effects.

Can someone please point out where my thinking has gone wrong? Is the particle detector doing something other than collapsing a wavefunction? Am I misunderstanding what nonphysical means? Is it something else?

Thank you.

## Answers and Replies

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Simon Bridge
Homework Helper
Is destruction of dbl-slit interference pattern wavefunction collapse?
The concept of a "collapse" is a, sort of, narrative fiction that helps us talk about systems described by wavefunctions.

up to now, my understanding was that the impact of the detector was to collapse the wave function of the particles to an eigenket of the position operator.

However, if wavefunction collapse is non-physical, we should not be able to observe its effects.
Whyever not? Probability distributions are non-physical and we can observe their effects.

Maybe you'd prefer if we said that we observe effects that are predicted by math that involves wavefunctions? The wavefunction itself is a bunch of scribbles on a bit of paper of a glowing part of the computer screen ... when we manipulate them we move the squiggles around. Do it right and it seems to be somewhat predictive.

The Mayans used to predict eclipses by moving stones between bowls, or was that Sumerians?[*]
Anyway - they could see that when the High Preist moved the last stone into the final bowl the Sun went out - it worked every time so the stone must be affecting the Sun right?

Perhaps that is what we are doing with our wavefunctions?
Whatever, that's how Richard Feynman used to describe it in his lectures.

These are the rules - they don't have to be physical to be predictive.

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

[*] nope - definitely bowls.

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DrChinese
Gold Member
I am trying to reconcile this in my mind with how an interference pattern from a double slit setup is destroyed when a particle detector is activated at one of the slits. Clearly, whatever is happening when the detector is switched on is physical, as we can see its effect. But up to now, my understanding was that the impact of the detector was to collapse the wave function of the particles to an eigenket of the position operator.

However, if wavefunction collapse is non-physical, we should not be able to observe its effects.

Can someone please point out where my thinking has gone wrong? Is the particle detector doing something other than collapsing a wavefunction? Am I misunderstanding what nonphysical means? Is it something else?
The presence or absence of the interference pattern is not a demonstration of collapse of the wavefunction. There are versions of the double slit in which polarizers are placed in front of each of the 2 slits. When the polarizers are aligned parallel, there IS interference. When they are oriented perpendicularly, there is NO interference. So the light passes through a polarizer either way. Clearly, there is no more collapse one way (physically) than the other.

The thing that controls the interference pattern is "context". When the context allows the slit to be determined (in principle), there is no interference. You can also think of the context as causing the collapse of the wavefunction, but clearly this setup does not yield a description of when/where/what of the collapse itself.

As to whether collapse can be demonstrated to be a physical and irreversible action: at this point the answer is "no". Generally, actions which appear to cause collapse can be "erased" (ie reversed). Any way you look at it, this is a murky area.

andrewkirk
Simon Bridge
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It's commonly represented in lectures in terms of some experiment happening where the wavefunction is said to collapse, but the prof is out of that lab that day and someone records the result, but forgets to pass the note on ... the paper is then lost found, filed, lost again and finally buried in soft peat for three months and recycled as firelighters... and the lecturer asks when the collapse happened.

What happens in terms of QM at the slits has, so far, no consistent treatment (unless you count Feynman's restricted paths idealization) that I am aware of. One can proceed maybe by treating the source+slits as a state preparation device. In which case, things like blocking one slit just prepare the wavefunction in a different position state... which is another kettle of piranhas.

I think the more usual demonstration of purported collapse is in successive Stern-Gerlach experiments.

Homework Helper
Gold Member
Thanks for the replies Simon and Dr Chinese. I am still trying to digest them. In the meantime I have one question, or is it a comment.
Whyever not? Probability distributions are non-physical and we can observe their effects.
Of course it depends on what we mean by cause and effect, but I don't regard abstract objects like probability distributions as having effects. I like the description you use earlier in your post of a 'narrative fiction' we use to help ourselves understand events. I think of probability distributions as highly useful narrative fictions.

What I think people mean when they say 'wavefunction collapse is nonphysical' is that there is no observation that can be made to determine whether wavefunction collapse has already occurred prior to making the observation.

Is that what you think people like Dr Lubos in the above link mean when they talk about nonphysicality?

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Simon Bridge
Homework Helper
No, I think they mean non-physical in the same sense that a brick is physical.
When I meet this in the classroom I usually tell students, as part of their ongoing learning, that what you can measure is real, everything else is just maths.

(I know there are theoreticians who will argue with me about this one - relax guys I don't leave it there.)

bhobba
Mentor
Is that what you think people like Dr Lubos in the above link mean when they talk about nonphysicality?
I agree with everything Dr Chinese and Simon Bridge said but one thing they haven't really emphasized, and I think is strongly related to the above issue, is different interpretations have different takes on the reality of the wavefunction and if collapse even occurs.

I hold basically to the shut up and calculate interpretation aka known as the ensemble interpretation. There are slight variations - but basically it and other similar views are generally collected under the shut up and calculate umbrella. Its the interpretation Einstein held to (yes - despite what some articles will tell you Einstein actuality believed in QM - he merely thought it incomplete - not incorrect - interesting hey?)

Anyway in the ensemble interpretation a wavefunction refers to a conceptual ensemble (ie large collection) of things that are prepared in such a way to be in the same state. What an observation does is simply picks one of them. No collapse occurs because nothing real or physical happens. Here is a bit more detail:
http://en.wikipedia.org/wiki/Ensemble_interpretation

Another thing to realize is QM is a model - most notably a mathematical model - used to make predictions. Models often contain things you cant point to out there as existing. For example throw a dice - if its unbiased each head comes up with a probability of 1/6. Physically this means if you do this a very large number of times the proportion will be 1/6th for each outcome. Probabilities are a conceptual device we use in our models - it doesn't exist out there in any real sense. The same with wavefunctions in QM - it a device we use to make physical predictions.

Thanks
Bill

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bhobba
Mentor
(I know there are theoreticians who will argue with me about this one - relax guys I don't leave it there.)
As a person more in the mathematical physicist mold that's good to hear.

To the OP there is a good natured bit of bantering between those of a more experimental bent and those of a more theoretical bent. Guys like me whose background is math and not physics are most definitely of the theoretical bent.

Its the same bit of good natured bantering that happens with pure and applied mathematicians. I did a degree in applied math and lectures often had such good natured comments interspersed. Acquaintances who did pure math (not at the place I went to which was only into mathematical modeling and other applied math pursuits) told me it was reciprocated.

Thanks
Bill

Simon Bridge
Homework Helper
Interpretations of QM
All the interpretations have advantages and disadvantages. It's best not to fall in love with any one of them.

iirc the ensemble interpretation has an issue with non-locality?
Not sure it is that great for 2-slit-1-particle interference either.

On the other note:

The banter between theoreticians and experimentalists is a bit like the banter between Discworld heroes and wizards.

I can certainly see why theoreticians feel there is more to the mathematical models than "just maths" but it's like I tell my boss - "my paypacket certainly looks good in the equations, but the equations won't buy me dinner."

bhobba
Mentor
All the interpretations have advantages and disadvantages. It's best not to fall in love with any one of them.
Well said

iirc the ensemble interpretation has an issue with non-locality? Not sure it is that great for 2-slit-1-particle interference either.
It can resolve all issues - but sometimes only by some unnatural devices. Originally as proposed by Einstein the ensemble was an ensemble of the outcomes of the observation. But Kochen-Specker put that one to rest - it can't have the property prior to observation. This means the ensemble is the outcome and observational apparatus combined and you assume no property prior to observation - which is a bit weird. Actually in his 1970 article on it Ballentine isn't too careful about this point:
http://www.scribd.com/doc/134354426/Ballentine-Ensemble-Interpretation-1970

Somewhere in there he assumes some sub-quantum process selects it. But in his textbook - QM - A Modern Development - he moved away from that position to the ensemble including the observational apparatus. I personally get around it by decoherence transforming the pure state into an imporoper mixed state and assuming its a proper one.

I can certainly see why theoreticians feel there is more to the mathematical models than "just maths" but it's like I tell my boss - "my paypacket certainly looks good in the equations, but the equations won't buy me dinner."
Good one.

But actually its worse than that - many like myself believe the math is the reality - usually just in a descriptive sense - but some like Penrose believe it in an actual sense. I used to side with Penrose but after coming across some interesting stuff by Murray Gell-Mann on why theories are beautiful shied away from Penrose and just think is descriptive:

Thanks
Bill

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Simon Bridge