Double slit experiment, expensive gear?

[rolnor]

TL;DR Summary

I have an idea how to run the double slit experiment in a new way

I have an idéa how to run the double slit experiment that could give new insight to whats hapening. As I understand, when the photons are observed by someone the wawefunction colapses and the photons become particles, this can be seen as the interference pattern dissapears in the experiment. This is said to be a consequence of that a consiousness is observing the system. This observation is made via some detector. I wonder, what kind of detector is used? Is this very expensive gear? I am a organic chemist with a strong interest in physics, I would love to set up this system in my lab. Sorry if my english is non-perfect, I am from Sweden.

 

[pbuk]

As I understand, when the photons are observed by someone the wawefunction colapses and the photons become particles, this can be seen as the interference pattern dissapears in the experiment.

This is not correct.

1. Wave function collapse does not require observation by "someone".
2. Photons do not "become particles".

This is said to be a consequence of that a consiousness is observing the system.

Said by whom? Please provide a reference.

This observation is made via some detector. I wonder, what kind of detector is used? Is this very expensive gear?
...
I would love to set up this system in my lab.

Many videos of DIY or home-based double slit experiments can be found on the internet.

I am a organic chemist with a strong interest in physics.

Did you learn your organic chemistry only by throwing a few chemicals into a pot and seeing what happens, or did you study from any text books or courses? If you have a strong interest in physics then you should follow a text book or course.

 

[rolnor]

This is not correct.

1. Wavefunction collapse does not require observation by "someone".
2. Photons do not "become particles".

Said by whom? Please provide a reference.Many videos of DIY or home-based double slit experiments can be found on the internet.Did you learn your organic chemistry only by throwing a few chemicals into a pot and seeing what happens, or did you study from any text books or courses? If you have a strong interest in physics then you should follow a text book or course.

Have you eaten something bad? Stomach pain? It was a straightforward question, if can you answer it, please do so. I can do without insults.

 

[weirdoguy]

It was a straightforward question

The thing is that what you wrote about double slit experiment is not correct. So I guess I would start with learning about it from proper sources

Did you learn your organic chemistry only by throwing a few chemicals into a pot and seeing what happens, or did you study from any text books or courses? If you have a strong interest in physics then you should follow a text book or course.

This could've been phrased a little nicer. Even for me, and I have a thick skin when it comes to that, PF has become too harsh/passive-aggressive/whatever you want to call it. Don't go that way people

 

[PeterDonis]

It was a straightforward question

Based on incorrect premises, as @pbuk pointed out.

I can do without insults.

While the last part of @pbuk's post could have been phrased better, he was not insulting you. He was pointing out that when you learned organic chemistry, you took the time to do it the right way: by learning from good sources, like textbooks and peer-reviewed papers. But what you say about QM in your OP does not look like that: it looks like what someone would say who had only read some pop science articles about QM and who doesn't have a good understanding of what QM actually says.

In other words, you have not taken the time to understand QM the way you took the time to understand organic chemistry. So before even trying to do what you say you want to do--investigate the double slit experiment--you should first take the time to learn QM the proper way, the way you learned organic chemistry. We can help you with that, but it looks like you first need to unlearn what you think you know about QM and start fresh.

 

[PeroK]

Is this very expensive gear? I am a organic chemist with a strong interest in physics, I would love to set up this system in my lab.

It doesn't have to be expensive:

 

[rolnor]

This is not correct.

1. Wave function collapse does not require observation by "someone".
2. Photons do not "become particles".

Said by whom? Please provide a reference.Many videos of DIY or home-based double slit experiments can be found on the internet.Did you learn your organic chemistry only by throwing a few chemicals into a pot and seeing what happens, or did you study from any text books or courses? If you have a strong interest in physics then you should follow a text book or course.

Thanx! Now its at least partly a discussion, not insults only. Maybe you can try to understand what I mean? give me some slak? Even be a little friendly? A photon can behave as a particle and a wave. In the experiment this changes when a detector is placed to observe the photons. This is described as the wawefunction collapses. Then you get no interference pattern, the photons behave like particles not waves. This is the base for a lot of speculation and theories. Is this all incorrect? If it is incorrect, can you tell me how it really is, then? I am sure you can do that with less then a hundred words. I found this site, the detector is 5000usd. Maybe its the wrong detector. I will check youtube for DIY stuff, thanx.
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=5255

 

[rolnor]

Based on incorrect premises, as @pbuk pointed out.While the last part of @pbuk's post could have been phrased better, he was not insulting you. He was pointing out that when you learned organic chemistry, you took the time to do it the right way: by learning from good sources, like textbooks and peer-reviewed papers. But what you say about QM in your OP does not look like that: it looks like what someone would say who had only read some pop science articles about QM and who doesn't have a good understanding of what QM actually says.

In other words, you have not taken the time to understand QM the way you took the time to understand organic chemistry. So before even trying to do what you say you want to do--investigate the double slit experiment--you should first take the time to learn QM the proper way, the way you learned organic chemistry. We can help you with that, but it looks like you first need to unlearn what you think you know about QM and start fresh.

Thanx, yes, I am using you as educators a little, is that so wrong? I dont know all the physics lingo, but I am a fast learner. I have studied physic textbooks but not as much as chemistry. Can you be a little nice to me? Am I asking to much here?

 

[PeroK]

Thanx, yes, I am using you as educators a little, is that so wrong? I dont know all the physics lingo, but I am a fast learner. I have studied physic textbooks but not as much as chemistry. Can you be a little nice to me? Am I asking to much here?

If you study QM from "popular science" sources, you will hear a lot about wave-particle duality; wavefunction collapse; conscious observers; a particle being in two places at once; etc.

If you study QM from an undergraduate textbook, you won't hear much about these things- except perhaps a lot about wavefunctions.

I would recommend you learn QM.from a mainstream undergraduate textbook.

 

[rolnor]

Thanx, can you recommend a good textbook? If you watch the first few minutes of this video, do you consider this as total BS? Over and over I see this, that a consiousnes makes the wave functio collapse, you see the photons as particles, no interference pattern is present. Is this all a lie? What I like to do is to have a camera "observe" the screen, does that also show that the photons start behave like partikles, not wawes? If so, there is no need for consiousnes, a camera is not a consiousnes. You can watch this on a monitor outside the room with the experiment, perhaps with a 1ns delay, then you are observing a recording, not directly whats going on, will the wavefunction collapse then? I am curious.

 

[PeroK]

Thanx, can you recommend a good textbook? If you watch the first few minutes of this video, do you consider this as total BS? Over and over I see this, that a consiousnes makes the wave functio collapse, you see the photons as particles, no interference pattern is present. Is this all a lie? What I like to do is to have a camera "observe" the screen, does that also show that the photons start behave like partikles, not wawes? If so, there is no need for consiousnes, a camera is not a consiousnes. You can watch this on a monitor outside the room with the experiment, perhaps with a 1ns delay, then you are observing a recording, not directly whats going on, will the wavefunction collapse then? I am curious.

You can't directly observe light without absorbing it. Alhough there are clever ways to do the experiment to get round this.

When you detect light at one slit it has the same effect as closing one slit. You get a single slit interference pattern. And, the critical point is that the double-slit pattern is not the sum of two single slit patterns.

The QM explanation for this is that light behaves according to complex probability amplitudes and not classical real, positive probabilities.

There is the difference between the mainstream theory (complex probability amplitudes), rather than wave-particle duality. That is the contrast between the mathematics of the theory and the heuristic popular explanations that are necessary to an audience that is unwilling to study the relevant mathematics.

This is now your choice: study the full mathematical theory of QM. Or, be satisfied by popular explanations.

 

[rolnor]

Hm... I want to shoot the popular theories dead, and I might do that by reading the math. But you here on this forum must be a little frustrated, even angry, that these idéas are so widespread, no? Popular science is important because it gives many people their view of how things are, how the world is built, constructed? I find these idéas about the consiousnes as a nessecity for the wavefunction to collapse provoking, I think its not true, its to far fetched. Thats why I post my thoughts here. In chemistry we have many people beleive that life started from a "primordial soup", if you have water and some chemicals and subject this to a little lightning you eventually get life, cells. This i complete BS ,it will not happen, nobody has observed any of this in any experiment, no complex molecules large enough, ordered enough, is formed. Just because you can get some traces of racemic amino-acids this way it can never form life as we se it now on earth, even if you wait a quadrillion years.. I beleive in evolution theory but life must have some special origin, it does not form by chance at the conditions on early earth.

 

[PeroK]

Hm... I want to shoot the popular theories dead, and I might do that by reading the math. But you here on this forum must be a little frustrated, even angry, that these idéas are so widespread, no?

If you read through previous threads, you will find a lot of frustration. This site does not promote popular science, but mainly science as an academic subject.

Popular science is important because it gives many people their view of how things are, how the world is built, constructed? I find these idéas about the consiousnes as a nessecity for the wavefunction to collapse provoking, I think its not true, its to far fetched. Thats why I post my thoughts here.

Its simply not part of QM.

In chemistry we have many people beleive that life started from a "primordial soup", if you have water and some chemicals and subject this to a little lightning you eventually get life, cells. This i complete BS ,it will not happen, nobody has observed any of this in any experiment, no complex molecules large enough, ordered enough, is formed. Just because you can get some traces of racemic amino-acids this way it can never form life as we se it now on earth, even if you wait a quadrillion years.. I beleive in evolution theory but life must have some special origin, it does not form by chance at the conditions on early earth.

If you have one million Earth like planets, each with a billion year window, then something with a very low probability could happen on one of those planets.

In fact, the very low likelihood of abiogenesis is one of the main factors that argues against life being commonplace in the galaxy. If abiogenesis is very rare, then we may be alone as intelligent life in the galaxy.

 

[PeroK]

There are many threads on here about QM textbooks. I like Griffiths, but others disagree.

If you want a challenge you could try Modern QM by JJ Sakurai.

 

[DrChinese]

I find these idéas about the consiousnes as a nessecity for the wavefunction to collapse provoking, I think its not true, its to far fetched.

There is no requirement that consciousness be present to cause collapse (assuming you believe collapse is a physical process).

There are experiments that demonstrate conclusively that the mere possibility of determining an outcome is enough to cause collapse, even if there is no conscious observer. Such has been proven in Double Slit setups such as:

https://sciencedemonstrations.fas.h...-demonstrations/files/single_photon_paper.pdf

In this experiment, no conscious person ever learns the outcome of a which slit observation. Nonetheless, there is collapse if that information is available in the environment.

 

[Nugatory]

But you here on this forum must be a little frustrated, even angry, that these idéas are so widespread, no?

Anyone who allowed this to anger them will have blown a blood vessel and expired long ago, so you're mostly going to be hearing from people who are pretty good at anger management

 

[rolnor]

If you read through previous threads, you will find a lot of frustration. This site does not promote popular science, but mainly science as an academic subject.

Its simply not part of QM.

If you have one million Earth like planets, each with a billion year window, then something with a very low probability could happen on one of those planets.

In fact, the very low likelihood of abiogenesis is one of the main factors that argues against life being commonplace in the galaxy. If abiogenesis is very rare, then we may be alone as intelligent life in the galaxy.

If you look at the chemistry, things tends to go in the wrong way when you wait a long time, it becomes less ordered, smaller molecules. Not the other way. But I have no explanation for life comming to be. Its a complete mystery to me. Many people think its something that science has sorted out, that with some dirty water and some time, you get life. Yes, I see what you mean, my questions about popular science and my ignorans even resulted in something like anger against me at the start. You should note that I was/am critic against popular science, thats why I asked about it.

 

[rolnor]

There is no requirement that consciousness be present to cause collapse (assuming you believe collapse is a physical process).

There are experiments that demonstrate conclusively that the mere possibility of determining an outcome is enough to cause collapse, even if there is no conscious observer. Such has been proven in Double Slit setups such as:

https://sciencedemonstrations.fas.h...-demonstrations/files/single_photon_paper.pdf

In this experiment, no conscious person ever learns the outcome of a which slit observation. Nonetheless, there is collapse if that information is available in the environment.

Thanx, I feel better now. And I was right, the popular science version is incorrect, simply. And widespread. Why do you think there is a collapse when you turn on the detector? Or do you think it is really no collapse att all? I will watch the videolink.

 

[rolnor]

There are many threads on here about QM textbooks. I like Griffiths, but others disagree.

If you want a challenge you could try Modern QM by JJ Sakurai.

Thanx. Is it all math? I dont dislike math but its not easy reading

 

[Nugatory]

Thanx. Is it all math? I dont dislike math but its not easy reading

Unlike classical physics, there's no natural mental mental model based on our daily experience of how things "ought" to work, so it's the model described by the math or nothing at all. If a real textbook with its non-trivial mathematical prerequisites is more than you want to take on, you might try Giancarlo Ghirardi's "Sneaking a look at God's cards" - it's no substitute for the math but it it does avoid most of the pop-sci misrepresentations.

 

[rolnor]

Unlike classical physics, there's no natural mental mental model based on our daily experience of how things "ought" to work, so it's the model described by the math or nothing at all. If a real textbook with it's non-trivial mathematical prerequisites is more than you want to take on, you might try Giancarlo Ghirardi's "Sneaking a look at God's cards" - it's no substitute for the math but it it does avoid most of the pop-sci misrepresentations.

Thanx! I read about decoherence. Is it maybe something here that the cat actually makes this system "less QM"? You mix a mechanical world with a "QM world" and this makes the whole system "less QM" If you have my camera inside the box you make it even "less QM?" Its a "observer" a "meassuring device"? I will check that book, it sounds exactly what I need. Sorry for my ignorance.
https://en.wikipedia.org/wiki/Quantum_decoherence

 

[rolnor]

Thanx! I read about decoherence. Is it maybe something here that the cat actually makes this system "less QM"? You mix a mechanical world with a "QM world" and this makes the whole system "less QM" If you have my camera inside the box you make it even "less QM?" Its a "observer" a "meassuring device"? I will check that book, it sounds exactly what I need. Sorry for my ignorance.
https://en.wikipedia.org/wiki/Quantum_decoherence

Its cheap!

 

[rolnor]

Thanx! I read about decoherence. Is it maybe something here that the cat actually makes this system "less QM"? You mix a mechanical world with a "QM world" and this makes the whole system "less QM" If you have my camera inside the box you make it even "less QM?" Its a "observer" a "meassuring device"? I will check that book, it sounds exactly what I need. Sorry for my ignorance.
https://en.wikipedia.org/wiki/Quantum_decoherence

I think I got something, the cat is really just a way of measuring, its a "observer"? Like a Geiger counter? Its not important that it is a animal? A Geiger counter would also be a "mechanical world" device?

 

[PeterDonis]

Now its at least partly a discussion, not insults only. Maybe you can try to understand what I mean? give me some slak? Even be a little friendly?

@roinor, you need to stop with these comments. This attitude is not constructive.

 

[rolnor]

You are being friendly now so I stop.

 

[PeterDonis]

You are being friendly now so I stop.

If you think someone is violating the PF rules against insulting other members, the correct response is to use the Report button, not to respond in kind in the thread. Nor is it correct to remark when you think someone has become "friendly" again.

 

[PeterDonis]

I read about decoherence. Is it maybe something here that the cat actually makes this system "less QM"?

No. Decoherence is still a quantum process.

 

[rolnor]

No. Decoherence is still a quantum process.

But decoherence makes QM information being "lost" somehow?

 

[rolnor]

If you think someone is violating the PF rules against insulting other members, the correct response is to use the Report button, not to respond in kind in the thread. Nor is it correct to remark when you think someone has become "friendly" again.

OK. I am not saying anyone is being unfriendly anymore, quite the opposite.

 

[PeterDonis]

decoherence makes QM information being "lost" somehow?

It makes the phase information that would allow you to measure interference effects inaccessible, by spreading it among a very large number of untrackable degrees of freedom, usually referred to in the decoherence literature as "the environment". However, it doesn't actually have to be an external "environment". For example, a large macroscopic object like, say, a cat (a la Schrodinger) can serve as its own "environment": any information about interference between, say, two atoms in the cat is quickly spread among all of the cat's degrees of freedom and is thus inaccessible. That's what allows us to say that the cat is either alive or dead, and that when it dies, the death is irreversible--because the information about interference between "dead" and "alive" states of the cat is inaccessible.

 

[rolnor]

No. Decoherence is still a quantum process.

But it makes a QM system less coherent. And the "QM quality", the coherence, of the system gets lost, it "leaks" into the environment. Is the cat such a environment? it makes the system less coherent? The cat that dies is a form of measurement?

 

[PeterDonis]

it makes a QM system less coherent.

Sort of.

Is the cat such a environment?

See my post #30 above.

 

[rolnor]

Thanks, I feel that I know what I need to know now to dismiss the cat in the box. One thing, is it not theoretically possible to track all these "freedoms"? Is it practicaly impossible or theoretically impossible? Its a huge difference. And if the object is smaller, like 500 atoms, would that amount of freedoms be trackable? Is there a sharp limit to the size were it can not be tracked, when decoherence occur?

 

[PeterDonis]

is it not theoretically possible to track all these "freedoms"?

I don't know that the decoherence literature directly addresses this question. My answer would be no, because to "track" them would require you to be able to control their interaction with other degrees of freedom that you use to measure them, and you can't. But I'm not sure all physicists in the field would agree. This kind of question probably gets into the area of QM interpretations, which should be discussed in the interpretations subforum.

if the object is smaller, like 500 atoms, would that amount of freedoms be trackable?

Experiments have been done showing that at least some kinds of quantum coherence can be maintained with fairly large numbers of atoms by microscopic standards. The double slit experiment has been done with buckyballs, which have roughly 100 atoms. I believe coherence of the magnetic flux in superconductors has been shown to be maintained with roughly a trillion atoms. But there is still a huge gap from there to typical macroscopic objects with something like $10^{25}$ or more atoms.

Is there a sharp limit to the size were it can not be tracked, when decoherence occur?

No sharp limit is known at present. Maintaining coherence gradually becomes more difficult as the number of degrees of freedom increases.

 

[vanhees71]

Thanx! Now its at least partly a discussion, not insults only. Maybe you can try to understand what I mean? give me some slak? Even be a little friendly? A photon can behave as a particle and a wave. In the experiment this changes when a detector is placed to observe the photons. This is described as the wawefunction collapses. Then you get no interference pattern, the photons behave like particles not waves. This is the base for a lot of speculation and theories. Is this all incorrect? If it is incorrect, can you tell me how it really is, then? I am sure you can do that with less then a hundred words. I found this site, the detector is 5000usd. Maybe its the wrong detector. I will check youtube for DIY stuff, thanx.
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=5255

A photon is a photon, and it does never behave like a particle. It has not even a position observable. The naive photon picture, introduced by Einstein in 1905, is long outdated (in 1926 Born and Jordan and somewhat later Dirac gave the correct description in terms of field quantization).

A photon is a asymptotically free single-quantum Fock state of the electromagnetic field. So the first thing you need is a single-photon source. Today that's provided usually by parametric down conversion, i.e., by shooting with a laser on a birefringent crystal, where you can get entangled photon pairs and use the one photon to "herald" the other photon, which you can then let go through a double slit. Finally you need a single-photon detector, which I guess is the most expensive part.

What you will observe is that for any single photon going through the slit (with some probability) you'll register this photon at one spot at the photodetector. The location of this spot cannot be predicted in any way but only the probability. Now it turns out that quantum electrodynamics predicts that the probability distribution is given by the properly normalized classical energy densit, i.e., the interference pattern you expect from classical diffraction theory.

What's meant by "wave-particle duality" in modern terms just means that on the one hand you have a kind of "particle property" for a single-photon Fock state, i.e., it can be registered either as a whole or not at all. It's, however not some kind of localizable massless particle following a trajectory of any kind. You cannot even define a position observable for a photon (as you can for massive "particles", i.e., field quanta of massive fields)! All you can know is the probability distribution for the "registration events" at any place of the photodetector (you can use a pixel detector, so that you get an interference pattern by registering many equally prepared photons). On the other hand the probability distribution is given in terms of field theory and it thus shows interference effects.

The next thing is that you can also try to figure out through which slit of the double slit each photon came. This is only possible, if you somehow mark each photon behind the slit in such a way that you can say with certainty through which slit it came. One clever way to do this is to use the polarization observable of this photon to mark it accordingly, i.e., you aim for a perfect "entanglement" between the photon's polarization state and the "which-way information".

This can be achieved by using linearly polarized incoming photons (say in $x$-direction) (this is easily achieved by using a polarization filter) and mount two quarter-wave plates into the slits one oriented in $+\pi/4$ and the other in $-\pi/4$ orientation. Then any photon going through one slit will be left-circular polarized and any photon going through the other slit will be right-circular polarized, i.e., by measuring the polarization state for the photons going through the slits you could figure out exactly through which slit each photon came (but you cannot predict it beforehand, i.e., you have with 50% probability a photon coming from one and 50% from the other slit). Since now the photons going through the one slit are perfectly polarized in a way that the polarization state is perpendicular to the one of the photon going through the other slit, the interference effect is completely done, i.e., now the partial intensities for photon going through the one or the other slit add, i.e., you completely loose the double-slit interference pattern (the single-slit envelope is still visible though).

This example shows that it is very easy to understand, why gaining 100% "which-way information" (or only the possibility to gain it by measuring the polarization state of the outgoing photons) destroys the double-slit interference pattern completely. Again, this is not understandable within the naive photon picture of 1905 but you need quantum-field theory!