# The double slit experiment and Superposition

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9 minutes ago, Mordred said:

I have in fact been planning a separate thread on Bell's Theorem and non-locality. But then, strictly from the logical/epistemological/philosophical perspectives.

I cannot say it often enough, I am not a physicist and will gladly make use of your expertise.

But the Bell's thread will have to wait until I have completed my readings.

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no problem. As mentioned this thread is a different topic.

Now to be fair for this thread let me provide a classical analogy you can relate to in our everyday world.

Lets look specifically at spin first. Spin and phase polarity. Think about the connection in that.

Now superposition. If your measuring the phase itself 120 volt has two phases. But what about 3 phase 480 volt? are they not in superposition but are phase shifted? 3 plane waves vs 1.( in actuality more but this is an oversimplification)

Now under QM and electromagnetism this is far more complex, the above is a very heuristic example but I wanted to direct your attention to wave properties itself. Start looking at constructive and destructive interference of waves. When two waves of the same polarity overlap the amplitude increases. (constructive interference) when not of same polarity you get destrucive interference.

Now apply the interference patterns to the two slit experiment.

Edited by Mordred

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49 minutes ago, Mordred said:

Now apply the interference patterns to the two slit experiment.

I have been trying very hard to avoid this discussion because I do not think it changes anything as to whether a particle (or particle-wave, or wave-particle) comes through one slit or the other. All authors I have consulted agree that observing what is happening shows unambiguously that the particle (whatever its properties), goes either through one slit or the other. The big mystery is the role of observation in the disappearance of the interference pattern. That is what I have tried to tackle in my first post.

Edited by Dalo

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Particle-like (Point like) defined by the Debroglie wavelength.

Here read "There are no Particles there are only fields" by Hobson he specifically deals with the 2 slit experiment.

You will definitely want to watch the video by Sean Caroll. in the last link posted by StringyJunkie 3rd post down of last link.

Throw away the bullet(ball) image of particles.

here is a wave-particle duality image this can be modelled as a planewave

Edited by Mordred

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7 hours ago, Strange said:

Can you provide a reference to it being used with a double slit experiment?

David Bohm: Quantum Theory (1951; edition of 1985)

"To show that this conclusion does not depend on the particular method used to
find through which slit the electron goes, let us consider, for example, the possibility
of setting up a cloud chamber at the detecting screen.
" p.119

I am afraid that it concerns a thought experiment, not a real one. Besides, it was used by Bohm to emphasize his point that it is impossible to know through which slit the electron went, due to the Uncertainty Principle.

"We must remember, however, that the behavior of the electron in the cloud
chamber is also limited by the uncertainty principle."

This is a limitation that the other authors quoted previously did not recognize.

I had used the vapor chamber example from memory, and my example certainly does not stroke with Bohm's interpretation. Still, I think that my argumentation still stands, even if we accept that the vapor chamber is not really a good example.

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Yes this is an older paper far below todays research a lot changes in 60 years in physics lol.

Edited by Mordred

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Bohm also said on the same page:

"an electron is neither a particle nor a wave, but is instead a third kind of object which has
some, but not all, of the properties of both particles and waves.
"

20 minutes ago, Mordred said:

Throw away the bullet(ball) image of particles.

No problem. But It wouldn't resolve the issue of the disappearance of the interference pattern in case of observation.

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That is specifically superposition itself.

A probability function under superposition is a probablistic state. Once measured it is a detetmined state.

Once you intefere with a wavefunction regardless of being probablistic or not, via Observation which causes interference the original wavefunction changes. You cannot measure without interfering.

All measurements cause interference

Edited by Mordred

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1 minute ago, Mordred said:

That is specifically superposition itself.

A probability function under superposition is a probablistic state. Once measured it is a detetmined state.

Once you intefere with a wavefunction regardless of being probablistic or not, via Observation which causes interference the original wavefunction changes. You cannot measure without interfering.

All measurements cause interference

This is an assumption central to the Copenhagen interpretation of Quantum theory. It is not an empirical fact but an attempt to explain empirical observations. I understand if you consider it valid, but this is not a matter of expertise, only of "philosophical" conviction.

Bohm later changed his position, and not everybody agrees with Bohr anymore.

In my first post, I argue that observation does not need to change the outcome of the experiment, but rather that the explanations given are themselves problematic. That is the whole point of the matter:

Does observation/measurement change the outcome? Bohr, von Neumann, Heisenberg and others are convinced that this is the case. My impression is that it is more a petitio principii.

I am very glad that you have joined the discussion because you have finally brought it to the main point.

I only do not think that the issue can be solved with technical arguments. It is a deeply philosophical/metaphysical issue.

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If you want emphirical fact simply look at "action under QFT. You cannot transmit less than a quanta of action. In order to have information exchange via interaction object to detector you require action.

Name one detection device whether biological or otherwise that doesn't follow this rule. If all particles are field excitations then under the modern mathematics (including all Feyman diagrams) Action requires a quanta under Compton wavelengths for timelike interactions (massless particle interactions)

Debroglie wavelength (massive particle interactions) spacelike.

As they are waveforms described by wavefunctions they have a wavelength connected to its field. (Particle itself is a misnomer term we keep more for historical reasons) then they follow the laws of physics (classical and quantum) with all regards to wavelengths.

IF you want a particular Path integral following any of the common interactions (including those related to " Observable as defined under math all primary terms in physics have a mathematical definition.

Edited by Mordred

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21 minutes ago, Dalo said:

Bohm later changed his position

I think I confused him with de Broglie who indeed changed twice of position concerning his pilot wave theory.

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Yes I recall some of those papers, some are still around today (though very little compared to 10 years back.) least from when I was interested in it.

Here is the thing its mathematics also use Operators which which models the observable interactions.

( Don't trust pop media descriptives) Lol little secret it doesn't matter where a math treatment originates, some methodologies often get incorporated into other theories. This includes lessons from pilot wave

Edited by Mordred

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The two examples given above, that of the screen with the pin, and the photographic plate, both show that the "particle" goes either through one or the other slit. That is what matters. That and the fact that there is only an interference pattern when there is no observation.

The problem then becomes: does the "particle" go through one or the other slit only when it is observed? Bohr says yes, because it in fact goes through both slits. But then, according to Bohr again, only observables "exist". Which observables? The fact that the particle goes through a single slit can also be called an observable. In fact, it is the fact that the particle goes through both slits that is never observed!

***

8 minutes ago, Mordred said:

( Don't trust pop media descriptives)

Do the references I have given fall under this category?

Edited by Dalo

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22 minutes ago, Dalo said:

Do the references I have given fall under this category?

Ok lets describe my view point, I don't follow the metaphysics debates. I focus on what is going on under the math. Under what the models truly state under the math. I rarely see any metaphysics paper truly address what the math shows.

Here is a key detail QM  and QFT operators  are not the same. In QFT the fields are the operators. Changes a lot of the arguments or should.

Edited by Mordred

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4 hours ago, Dalo said:

Bohm also said on the same page:

"an electron is neither a particle nor a wave, but is instead a third kind of object which has
some, but not all, of the properties of both particles and waves.
"

I don't think anyone would disagree with that.

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6 hours ago, Dalo said:

David Bohm: Quantum Theory (1951; edition of 1985)

"To show that this conclusion does not depend on the particular method used to
find through which slit the electron goes, let us consider, for example, the possibility
of setting up a cloud chamber at the detecting screen.
" p.119

I am afraid that it concerns a thought experiment, not a real one. Besides, it was used by Bohm to emphasize his point that it is impossible to know through which slit the electron went, due to the Uncertainty Principle.

"We must remember, however, that the behavior of the electron in the cloud
chamber is also limited by the uncertainty principle."

This is a limitation that the other authors quoted previously did not recognize.

I had used the vapor chamber example from memory, and my example certainly does not stroke with Bohm's interpretation. Still, I think that my argumentation still stands, even if we accept that the vapor chamber is not really a good example.

This ties back to the questions I asked, and which you still have not answered. Physics, instead of hand-waving.

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1 hour ago, Strange said:

I don't think anyone would disagree with that.

Yes we all agree this, but it should be remembered that both particles and waves are idealisations.

We can use them in analysis because one characteristic or another has a dominant role, so we can ignore others.

But in the real world I don't think there is an perfect example of either.

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6 hours ago, Mordred said:

All measurements cause interference

Does the burden of proof actually lie on those who believe that measurements can be made without interference?**

Do they have to produce an example of a measurement that can be made without interference ?(interference in the sense of "affect" rather than the possibly related "interference pattern")

** I  think I have heard that measurements can be made indirectly  but am not clear on this.

Edited by geordief

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35 minutes ago, geordief said:

Does the burden of proof actually lie on those who believe that measurements can be made without interference?**

Do they have to produce an example of a measurement that can be made without interference ?(interference in the sense of "affect" rather than the possibly related "interference pattern")

** I  think I have heard that measurements can be made indirectly  but am not clear on this.

It's a good, and quite subtle, point.

For example, the indirect method of determining which slit a photon went through is to start with an entangled pair - one goes through the double slit apparatus and the other is used as a "probe". By measuring the probe particle, it is possible to know which slit the entangled partner went through, even though you haven't done anything directly to that photon.

By measuring the probe particle, you break the entanglement, and you can consider this to be an interference with the other particle (more accurately, interference with the pair - as they have a common wave function, they are really a single thing). And that is what stops the interference pattern appearing.

The interesting thing is that you can do this measurement of the probe particle (or not) a long way from the rest of the experiment, so that the pattern has already formed on the screen. Even though the pattern has already formed, measuring the probe particle changes what happens. It is hard to see any alternative explanation than temporal non-locality. (Some people describe this as reverse causality, but that is just different words for the same thing.)

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5 minutes ago, Strange said:

It's a good, and quite subtle, point.

For example, the indirect method of determining which slit a photon went through is to start with an entangled pair - one goes through the double slit apparatus and the other is used as a "probe". By measuring the probe particle, it is possible to know which slit the entangled partner went through, even though you haven't done anything directly to that photon.

By measuring the probe particle, you break the entanglement, and you can consider this to be an interference with the other particle (more accurately, interference with the pair - as they have a common wave function, they are really a single thing). And that is what stops the interference pattern appearing.

The interesting thing is that you can do this measurement of the probe particle (or not) a long way from the rest of the experiment, so that the pattern has already formed on the screen. Even though the pattern has already formed, measuring the probe particle changes what happens. It is hard to see any alternative explanation than temporal non-locality. (Some people describe this as reverse causality, but that is just different words for the same thing.)

When I thought it couldn't get any stranger......

The interference pattern doesn't change in "real time " surely?  It doesn't "reverse itself" surely .?That would be madness.

Have you a link that describes this experiment?

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Here: https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser (it is pretty hard to follow, though; I'm not sure I have fully understood it)

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6 hours ago, swansont said:

This ties back to the questions I asked, and which you still have not answered. Physics, instead of hand-waving.

It is not a matter of Physics, or Mathematics, since everybody, me included, agrees on what we can "see" happening at each double slit experiment. The question is how to interpret it.

My argument, and as far as I know, it has never been presented by anyone else, is that the so-called disappearance of the interference pattern does not actually happen, but is a theoretical assumption.

I am willing to change my mind and go with the flow on one simple condition: show me this disappearance of interference pattern in (the video of) a real life experiment. Not an animation, and not a diagram.

Edited by Dalo

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1 hour ago, Dalo said:

It is not a matter of Physics, or Mathematics, since everybody, me included, agrees on what we can "see" happening at each double slit experiment. The question is how to interpret it.

My argument, and as far as I know, it has never been presented by anyone else, is that the so-called disappearance of the interference pattern does not actually happen, but is a theoretical assumption.

I am willing to change my mind and go with the flow on one simple condition: show me this disappearance of interference pattern in (the video of) a real life experiment. Not an animation, and not a diagram.

here's college lab report where they did it

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2 minutes ago, swansont said:

here's college lab report where they did it

I stopped as soon I came across "Mach-Zehnder interferometer". I am willing to speak about this kind of experiments in another thread. Specifically, whether they can be considered as equivalent to a double slit experiment. I am not so sure about that. I wrote about interferometers long ago in another forum and I would have to consult my notes.

To make a long story short, I do not think it is an acceptable alternative for a two slit experiment. Moreover, I do not think that (Mach-Zehnder) interferometers actually show what the theory tells us they show. Just like with double slit experiments, interpretation of the data is crucial. But then interpretation is always crucial, with all scientific experiments.

But I promise you that I would be willing to discuss them in their own thread.

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1 hour ago, Dalo said:

My argument, and as far as I know, it has never been presented by anyone else, is that the so-called disappearance of the interference pattern does not actually happen, but is a theoretical assumption.

How is it a theoretical assumption, when it can be observed in experiments.

Incidentally, "theoretical assumption" doesn't really makes sense. Assumptions are based on little or no evidence, theories are based on large amounts of evidence. You may have meant, "a theoretical conclusion that hasn't been observed." Except that isn't the case. It was a theoretical prediction that has been shown to be correct.