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A wave function does not collapse upon detection?

physics.stackexchange.com

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Re: A wave function does not collapse upon detection?

#51

Post by CuriouslyC »

russellbeattie wrote: post #22

Does anyone have a link to a real world example of the double slit experiment where the photons are observed at the slits , causing the wave function of the light to collapse so the light acts as a particle beyond the slit, ending up in two piles on the detector screen, rather than the wave pattern. Every time I ask this, some do-gooder will not read a word I wrote and send me a video of the basic slit experiment and…

I don't have the link handy but IIRC the result of the experiment when you have minimally invasive detectors at the slits is that depending on the distance of the slits from the final plate and the separation of the slits you'll see either two broad smears, or one wide smear with just a few interference patterns in the middle.

Re: A wave function does not collapse upon detection?

#52

Post by russellbeattie »

tome wrote: post #30
russellbeattie wrote: post #22

Does anyone have a link to a real world example of the double slit experiment where the photons are observed at the slits , causing the wave function of the light to collapse so the light acts as a particle beyond the slit, ending up in two piles on the detector screen, rather than the wave pattern. Every time I ask this, some do-gooder will not read a word I wrote and send me a video of the basic slit experiment and…

If you're trying to say there's something that seem suspicious about such claims then I agree with you. I don't know what it means to "observe" a photon at a slit without completely invalidating the experiment.

I'm just stating the double slit experiment as explained in that PBS video and others and curious about seeing an actual picture.

My only suspicion is that no one has done the actual experiment in a long time.

Re: A wave function does not collapse upon detection?

#53

Post by russellbeattie »

wrycoder wrote: post #37
russellbeattie wrote: post #22

Does anyone have a link to a real world example of the double slit experiment where the photons are observed at the slits , causing the wave function of the light to collapse so the light acts as a particle beyond the slit, ending up in two piles on the detector screen, rather than the wave pattern. Every time I ask this, some do-gooder will not read a word I wrote and send me a video of the basic slit experiment and…

You are asking that the screen be moved back to the slits - less than a slit-slit distance away. The wave aspect of the EM field remains downstream of the slits. The photon aspect is only exhibited at the screen. There may be no such images, because it’s not interesting to observe two bright spots. There has to be enough path length difference to allow the phase difference causing the interference. Think about the cl…

> There may be no such images, because it’s not interesting to observe two bright spots.

*Seriously?* There are an unlimited amount of uninteresting pictures on the internet, but no one has bothered to take a picture of a fundamental claim of quantum mechanics because it's so trivial as to be boring?

Really??

Re: A wave function does not collapse upon detection?

#54

Post by russellbeattie »

CuriouslyC wrote: post #51
russellbeattie wrote: post #22

Does anyone have a link to a real world example of the double slit experiment where the photons are observed at the slits , causing the wave function of the light to collapse so the light acts as a particle beyond the slit, ending up in two piles on the detector screen, rather than the wave pattern. Every time I ask this, some do-gooder will not read a word I wrote and send me a video of the basic slit experiment and…

I don't have the link handy but IIRC the result of the experiment when you have minimally invasive detectors at the slits is that depending on the distance of the slits from the final plate and the separation of the slits you'll see either two broad smears, or one wide smear with just a few interference patterns in the middle.

I've searched for hours so if you actually have a link, I would appreciate it.

Re: A wave function does not collapse upon detection?

#55

Post by tome »

russellbeattie wrote: post #52
tome wrote: post #30

Earlier quoted context omitted.

If you're trying to say there's something that seem suspicious about such claims then I agree with you. I don't know what it means to "observe" a photon at a slit without completely invalidating the experiment.

I'm just stating the double slit experiment as explained in that PBS video and others and curious about seeing an actual picture. My only suspicion is that no one has done the actual experiment in a long time.

Ah, well the video is about electrons, and it's somewhat more plausible to "observe" them going through one slit or the other, since they will emit an electromagnetic field. There do seem to be such recent experiments, for example https://phys.org/news/2011-01-which-way-detector-mystery-dou...

Re: A wave function does not collapse upon detection?

#56

Post by tsimionescu »

tome wrote: post #28
tsimionescu wrote: post #26

Earlier quoted context omitted.

No, it doesn't. Decoherence explains why self-interference of a single wave-function doesn't happen after interaction with the environment. But it doesn't explain two important things. 1, which is very practical, is that it doesn't explain why the wave function always decoheres in the same way. In experiments, you can always chose the basis of measurement, and get some definite results in that particular basis, while…

> In experiments, you can always chose the basis of measurement, and get some definite results in that particular basis, while the properties in another basis remain indefinite. Can you give some examples of such bases? Position is one, for sure. In "Quantum Mechanics 101" we are told that momentum is another, but I'm sceptical that there is a real experiment that can measure momentum without also measuring position.

Even for position, you can measure less than a point-like position. For example, you can check if a particle is inside this box or outside it. After this measurement, the particle will either be inside the box or outside it, but it will not be at some fixed position inside the box (or some fixed position outside it). For example, if it's determined to be outside the box, it will definitely not interact with any particle that was inside the box at the same time, but it may interact with any particle anywhere else in the entire universe - and according to MWI, it WILL interact with every other particle everywhere else in the universe, in some part of state-space.

Re: A wave function does not collapse upon detection?

#57

Post by tsimionescu »

CuriouslyC wrote: post #50
tsimionescu wrote: post #26

Earlier quoted context omitted.

No, it doesn't. Decoherence explains why self-interference of a single wave-function doesn't happen after interaction with the environment. But it doesn't explain two important things. 1, which is very practical, is that it doesn't explain why the wave function always decoheres in the same way. In experiments, you can always chose the basis of measurement, and get some definite results in that particular basis, while…

The many worlds interpretation works better with quantized probability. Imagine a huge computer running a monte carlo simulation with a probabilistic current state, and a set of transition states (the wave function). With quantized probability and a universe with finite "memory" the amplitude of the wave function basically maps to the amount of system memory being utilized for a portion of the simulated state space.

Well, quantized probability doesn't really make sense, there are extraordinarily improbable events happening every second. It's easy to create an event that has less probability than many things that have never been observed (flip 10,000 coins - whatever result you get has probability 1/2^10k).

You could perhaps create a version of QM with a quantized amplitude for the wave function in some basis.

It's also important to remember that the Schrodinger equation of a system has an infinity of solutions - for any solution, any linear function of that solution is also a solution. You need to choose a particular basis of measurement - choose a decomposition of the solutions - to be able to apply the Born rule and get from the wave function to a probability distribution.

Re: A wave function does not collapse upon detection?

#58

Post by ijidak »

criddell wrote: post #46
ijidak wrote: post #13

Earlier quoted context omitted.

Your answer is the honest answer. We don't know. The physical reality of wave functions is unknown. It could be an approximation of something yet deeper. The only thing we know is that the math works.

Is there a leading theory as to what the physical reality is?

This Wikipedia article references a few different views.

https://en.m.wikipedia.org/wiki/Wave_function_collapse#:~:te....

I'm no expert on QM. So not sure if there is an overall leading interpretation, currently.

Re: A wave function does not collapse upon detection?

#59

Post by russellbeattie »

tome wrote: post #55
russellbeattie wrote: post #52

Earlier quoted context omitted.

I'm just stating the double slit experiment as explained in that PBS video and others and curious about seeing an actual picture. My only suspicion is that no one has done the actual experiment in a long time.

Ah, well the video is about electrons, and it's somewhat more plausible to "observe" them going through one slit or the other, since they will emit an electromagnetic field. There do seem to be such recent experiments, for example https://phys.org/news/2011-01-which-way-detector-mystery-dou...

That's a great link! Thanks!

Derp, sorry... Electrons and photons are completely different types of particles. I'm getting things mixed up. And I had just skimmed that video again! I need to pay attention.

Still, I would have loved for the authors of that paper to get a pic of the setup and results on the computer monitor next to it or something.

Also, isn't this stuff fundamental to theoretical quantum encryption? If we try to observe information as it flows by, we always alter it in detectable ways because of the observation effect? I think?

(In some ways it's absurd that here we are, everyday people, chatting away trying to get a mental hold on the weird, abstract yet fundamental way the universe works, as dreamed up by literal scientific geniuses a century ago. But in reality, getting a general grasp of this stuff is knowledge you need to have in so many fields, from suburban electrician to anyone working in high tech. Crazy.)

Re: A wave function does not collapse upon detection?

#60

Post by pdonis »

8note wrote: post #16
pdonis wrote: post #9

Earlier quoted context omitted.

> If you measure the exact position of an electron, the wave function collapses to a point. Note, though, that in reality you can never measure the exact position of an electron (or anything else); the best you can do is to measure that the electron is inside some finite-sized (possibly very small) region. The "exact position" measurement is an idealization that can be useful for pedagogy but can't be realized in any…

spin? things go one direction or the other in a magnetic field

As moonchild posted, I was only saying that you can't measure the position of an electron exactly. I was not saying that you can't measure any properties of electrons at all.

The more general statement is that you can't measure exact values for any observable with a continuous spectrum; you can only measure that a quantum system is within some finite range of values. Spin has a discrete spectrum, so you can measure exact values for spin.

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