Dalo

The double slit experiment and Superposition

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The double slit experiment and Superposition
The double slit experiment takes a special position in physics in that it is used as a "crucial experiment" both by classical and quantum Physics. While the first, classical physics, shows an understandable historical preference for light, quantum Physicists, also very understandably, very often use electrons to get their point across.


When electrons or photons are ejected in the direction of the two slits, some strange things happen:
1) with one slit open we see the expected pattern of a single agglomerate of bright points on the screen at a location facing the slit.
2) when both slits are open we get the interference pattern discovered by Young in the 19th century. The same pattern is created whether we shoot electrons/photons continuously, or one by one. This has always been interpreted as the duality of the nature of light and matter. Instead of waves or particles, physicists prefer to speak of wave-like and particle-like properties of light or electrons.
Still, the strangeness really starts when we want to know through which slit electrons go through each time. This is the point where myths are created. Apparently, observing the precise trajectory of electrons destroys the interference pattern. When we do that, when we can indicate through which slit an electron (or photon) has gone, it appears unambiguously that the electron has taken either one or the other slit on its way to the screen. The trouble is that, when unobserved, electrons seem to go through both slits at the same time.
Since the only factor that has been changed is the fact or absence of observation, it seems logical to conclude that electrons/photons behave differently depending on whether an observer is present or not.

I present here an explanation which is I think much more down to earth.

This experiment has often, for obvious pedagogical reasons, been presented with diagrams or video images that, almost without exception, ignore the factual dimensions of the phenomena concerned.
For instance, observation becomes a camera placed between the slits and the screen, and directed at one of the slits to catch the moment where an electron goes through it, without taking into account that such a camera could not possibly exist, for now, or find a place in the minuscule space taken by the experiment.
When we know the dimensions of an electron we quickly realize that such a setup must be understood as an allegory without any real signification.
I will therefore use a much more realistic model to explain the role of observation in double slit experiments.
Instead of a camera, a vapor chamber is usually used, through which electrons can leave a clear path showing where they started and where they ended, eliminating any ambiguity as to which slit they went through.
The known results of such a setting have always been interpreted as the confirmation of the mystery: the use of a vapor chamber as observation instrument destroys the interference pattern, which comes back with a vengeance as soon as only a screen is used to welcome the impacting electrons.

What happens then when an electron goes through a slit? Obviously, in the case where there is only one slit, the question is trivial. The electron will either bounce off the separating wall, or it will go through the slit. leaving its mark through the vapor chamber.

The question that needs now to be answered is why the interference pattern disappears when "observed", meaning when there is a device in place that can record through which slit the electrons have gone through.

Here again we must take leave of the pedagogical caricatures of the double slit experiment, and look at how it can effectively be performed.
To record the movement of the electrons, we will use again a vapor chamber.
The results will be as can be expected from classical physics: each electron will go through one slit and one only. There can be no ambiguity over so called superposition. The tracks left by the electrons are clear enough.
The great mystery is how we can get a simple distributed pattern with two distinct groups of spots each representing the slit through the electrons went, while when we are not using the vapor chamber, and therefore not observing the electrons, the same interference pattern reappears.

My answer will seem rather unorthodox: the simple pattern is in all cases an extrapolation of the tracks left in the vapor chamber. It never really appears on the screen as such, simply because a vapor chamber is itself the screen, and offers no place for an external screen placed as in other experiments.

The myth of the double slit experiment is therefore in many aspects the result of the modes of presentation of the experiment.

We have either the interference pattern as it appears on a screen standing directly in the path of the electrons. Or a simple classic pattern showing the paths taken by each electron. When these patterns are extrapolated to another image, we get the classic pattern all the videos and diagrams are so fond of.
You could look a long time for a video showing in real time the creation of the classical pattern, while the real images of photons or electrons creating the interference pattern can be seen overall.

Edited by Dalo

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A perfect example of the caricatural (re)presentation of the double slit experiment is given in
https://youtu.be/A9tKncAdlHQ

To his defense I must point out that Jim Al Khalili, whose reputation as a scientist and a popularizer of science is deservedly beyond reproach, is here only presenting a universally accepted interpretation of the phenomenon.
 

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7 hours ago, Dalo said:

Instead of a camera, a vapor chamber is usually used, through which electrons can leave a clear path showing where they started and where they ended, eliminating any ambiguity as to which slit they went through.
The known results of such a setting have always been interpreted as the confirmation of the mystery: the use of a vapor chamber as observation instrument destroys the interference pattern, which comes back with a vengeance as soon as only a screen is used to welcome the impacting electrons.

How "clear" is this path? i.e. how big are the tracks in a cloud chamber, and what is the slit separation one uses in an electron double-slit experiment, when the electron has sufficient energy not to be deflected much by the material in the cloud chamber?

 

(Moved out of philosophy because this is not philosophy)

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7 hours ago, Dalo said:

I will therefore use a much more realistic model to explain the role of observation in double slit experiments.

I'm not sure this is very realistic. Both in the sense that I don't think anyone has done it this way, and also because the use of a cloud chamber would drastically change the momentum of the electron (and even absorb it completely) and hence obviously change the result.

Surely, a more realistic model would be to describe the experiment as it is actually done? For example, the "which slit" observation can be done without affecting the electron passing through the slit. 

7 hours ago, Dalo said:

My answer will seem rather unorthodox: the simple pattern is in all cases an extrapolation of the tracks left in the vapor chamber.

But you said the tracks in the cloud chamber reflect the classical paths of the electron. How can an extrapolation of these classical paths produce a non-classical result?

When the electrons go through the cloud chamber, the paths will be the same as those popular illustrations that show how ping-pong balls or grains of sand would go through the slits: two piles/peaks behind each slit. No interference pattern.

7 hours ago, Dalo said:

The myth of the double slit experiment is therefore in many aspects the result of the modes of presentation of the experiment.

It would probably be better to address the actual physics or real experiment designs rather than criticising pop-sci representations (which we all know are not accurate).

Edited by Strange

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8 hours ago, Strange said:

I'm not sure this is very realistic. Both in the sense that I don't think anyone has done it this way, and also because the use of a cloud chamber would drastically change the momentum of the electron (and even absorb it completely) and hence obviously change the result.

The issue of momentum is correct. Concerning the use of a vapor chamber I am surprised you have never heard of it.

8 hours ago, swansont said:

(Moved out of philosophy because this is not philosophy)

This is most definitely also a fundamental philosophical issue.

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16 hours ago, Dalo said:

What happens then when an electron goes through a slit?

This is the only question I can find in all that screed.

 

So why should the electron go through the slit? W

hy should it be moving at all?

You say it 'bounces off the wall' otherwise. Why? Electrons can pass through or along solid matter. So what sort of wall is this?

16 hours ago, Dalo said:

The question that needs now to be answered is why the interference pattern disappears when "observed", meaning when there is a device in place that can record through which slit the electrons have gone through.

 

There is no interference pattern in the slits themselves. The pattern appears (perhaps over time depending upon the rate of electron production) in the region beyond the slits.

One of the few good things about Al Khalilli's offering are the white lines radiating outwards showing the alternate high and low density regions and making the comment that these do not denote waves.

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1 minute ago, studiot said:

One of the few good things about Al Khalilli's offering are the white lines radiating outwards showing the alternate high and low density regions and making the comment that these do not denote waves.

I would be interested in your description of the double slit experiment. Maybe then I will know what to answer you.

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17 minutes ago, Dalo said:

I would be interested in your description of the double slit experiment. Maybe then I will know what to answer you.

Which double slit experiment?

My point was to illustrate that electrons are quite different from photons.

And you didn't answer my question, which was about electrons since your only question involved them.

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44 minutes ago, Dalo said:

Concerning the use of a vapor chamber I am surprised you have never heard of it.

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

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I am taking a time out to review my sources and my notes. I will therefore not be answering to objections right away. That does not mean that others cannot come up with new points.

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3 hours ago, Dalo said:

 This is most definitely also a fundamental philosophical issue.

You have asked a physics question, not a philosophy one. And physics questions have been asked of you, which you need to answer.

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Where did I say that my post was an objection?

I am trying to get you to think about the subject you put so much effort into preparing an Opening Post for.

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There are two important points I need to investigate again:

- the use of a vapor chamber;

- the use of electrons instead of photons.

As I said, I will look into it again and come back to the thread.

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3 minutes ago, Dalo said:

There are two important points I need to investigate again:

- the use of a vapor chamber;

- the use of electrons instead of photons.

As I said, I will look into it again and come back to the thread.

When you look again, find out why (or how if you like) the electrons pass through the slit.

This is important because it adds something to the electron version that is not there in the slit experiment for light or water waves.

Edited by studiot

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5 minutes ago, Dalo said:

There are two important points I need to investigate again:

- the use of a vapor chamber;

- the use of electrons instead of photons.

As I said, I will look into it again and come back to the thread.

You might want to consider the fact that the experiment has also been done with atoms, and even large molecules. (With the same results.)

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Just now, Strange said:

You might want to consider the fact that the experiment has also been done with atoms, and even large molecules. (With the same results.)

I know about the bucky-balls. I am also wondering if I should have used atoms instead of electrons. But I want to make sure.

"There was never any drawn-out controversy about whether electrons or any other 
constituents of matter were other than particle-like. Individual electrons produce 
scintillations on a phosphor screen—that is how TV works. But electrons also 
exhibit diffraction effects, which indicates that they too have wavelike attributes. 
An analog of the double-slit experiment using electrons instead of light is  
technically difficult, but has been done. " 
 Sly Blinder "Introduction to  Quantum Mechanics in Chemistry, Materials Science and Biology", 2004, p.23

Hopefully more references to come.

 

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1 hour ago, Dalo said:

using electrons instead of light is  
technically difficult, but has been done.

 

So why is it technically difficult?

That is what my comments are about.

 

 

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1 minute ago, studiot said:

 

So why is it technically difficult?

That is what my comments are about.

 

 

I am afraid I would not know. But I would very much like to.

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Well, if I turn on a light switch the light will flood out in all directions and fill up all available space.

It doesn't need pushing it just goes there.

In particular, no further apparatus is needed, though additional apparatus is sometimes used for particular purposes.

Further it will flood through any gaps in obstructions and try to fill the space beyond.

 

In (your) previous threads about the double slit there was a demonstration of water waves approaching and passing a double slit.

They are the same.

Once generated they just keep going and spreading.

 

They are also the same in another respect.

The generated wave is a plane wave and the waves emerging from the slits are circular or spherical.

 

Finally neither the water nor the light carries a charge.

 

But the electron does. 

This is fortuitous since a free electrons does not readily jump through gaps.

It wanders about randomly until it is snapped up by nearby matter particles.

The fact that it is charged allows us to accelerate and direct it so that we can send it through the slit.

But this requires extra apparatus which is normally ignored when discussing the slits.

So the electron has to be forced through. The photon just wanders through naturally.

 

 

As you with me thus far?

 

 

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I think I am Socrates!

:)

"When the pin is withdrawn, the screen becomes a mobile detection system 
which is sensitive to the momentum p = h/\ of the particles hitting the screen. 
It recoils when a particle arrives and, by measuring this recoil accurately, we 
can measure the vertical momentum of the particle detected at the screen and 
hence identify the slit from which the particle came. For example, near the 
centre of the screen, a particle from the upper slit has a downward momentum 
of pd/2D and a particle from the lower slit has an upward momentum of 
pd/2D. In general, the difference in vertical momenta of particles from the 
two slits is approximately Ap ps pd/D. Thus, if the momentum of the recoiling 
screen is measured with an accuracy of 
pd 
' D ' 
A.17) 
we can identify the slit from which each particle emerges. When this is the case, 
a wave-like passage through both slits is not possible and an interference 
pattern should not build up. " 
 A.C. Phillips  Introduction to Quantum Mechanics, 2014, p.14

[the equations are jumbled, but are irrelevant here. Also this concerns another device than a vapor chamber, but the principle is the same.]

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Following the previous quote is also a very interesting variation on the double slit experiment:

"Finally, some readers may find it instructively disturbing to consider a 
further variation of the two-slit experiment which was pointed out by Wheeler 
in 1978. In this variation, we imagine a situation in which the choice of the 
experimental arrangement in Fig. 1.7 is delayed until after the particle has 
passed the slits. We could, for example, insert the pin and fix the position of 
the screen just before each particle arrives at the screen. In this case an 
interference pattern builds up, which is characteristic of wave-like particles 
which pass through both slits. Alternatively, just before each particle arrives 
at the screen, we could withdraw the pin so as to allow the screen to recoil and 
determine the slit from which the particle comes. In this case, no interference 
pattern builds up. 
Thus, a delayed choice of the experimental arrangement seems to influence 
the behaviour of the particle at an earlier time.
 
" (Phillips, op.cit, my emphasis)

**************************

There is also a reference to a "real experiment" [sic], instead of a thought experiment. I will give you the abstract here, but I am afraid that it won't tell you much. At least, I did not feel any wiser after reading it. If anyone on this forum does have access to the full article and would be willing to write a short review, it would be very much appreciated.

"The neutron interferometer as a device for illustrating the strange behavior of quantum systems

Daniel M. Greenberger

Rev. Mod. Phys. 55, 875 (1983) – Published 1 October 1983

The neutron interferometer is a unique instrument that allows one to construct a neutron wave packet of macroscopic size, divide it into two components separated by centimeters, and then coherently recombine them. A number of experiments clearly showing the difference between quantum and classical theory have been performed with it, which are suitable for presentation in elementary quantum courses. This article presents a simple mathematical model of the interferometer, which can be used to illustrate clearly many of the surprising features of quantum systems. For example, one can describe an experiment to determine which component beam the neutron takes (an analog of the two-slit electron experiment). One can then trace in detail the loss of coherence of the wave function, rather than merely invoke the usual "handwaving" uncertainty arguments. The author discusses the effect of gravity on the neutron beam [the classic COW (Colella, Overhauser, and Werner) experiment], including a simple analysis in an accelerated reference frame, and its relation to the equivalence principle, the red shift, and the twin paradox. Also described are the effect of rotation of the neutron by 360° to change its phase, the effect on the wave function of measuring the absence of the particle from a beam ("Dicke's paradox"), and a realizable version of Wheeler's "delayed-choice" experiments, as well as their relation to the problem of "Schrödinger's cat." The treatment is suitable for bright undergraduates and first-year graduate students."

Edited by Dalo

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56 minutes ago, Strange said:

Yep. Non-locality goes across space and time. 

You say it like it is a fact. The debate is still ongoing.

*************************************

We find a very interesting strategy in

Claude Cohen-Tannoudji (et al),Quantum Mechanics, Volume 1, 1991, ch.1.

It consists in declaring impossible the knowledge of which slit the photon went through:

"we can imagine placing detectors  (photomultipliers) behind [each slit]... But, obviously, the photons detected in this way are absorbed and do not reach the screen. " (p.13)

So, it is better to use only one detector behind one slit, which leaves what happens with the other one in a theoretical limbo.

Edited by Dalo

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I am still looking for where I read about the double slit experiment and the vapor chamber. Meanwhile, here is another equivalent setting. This time from:

Bohmian Mechanics : The Physics and Mathematics of Quantum Theory by Detlef Dürr & Stefan Teufel, 2009, p.8

"

A Red Herring: The Double Slit Experiment
This is a quantum mechanical experiment which is often cited as conflicting with
the idea that there can be particles with trajectories. One sends a particle (i.e., a wave
packet ψ) through a double slit. Behind the slit at some distance is a photographic
plate. When the particle arrives at the plate it leaves a black spot at its place of arrival.
Nothing yet speaks against the idea that the particle moves on a trajectory. But
now repeat the experiment. The next particle marks a different spot of the photographic
plate. Repeating this a great many times the spots begin to show a pattern.
They trace out the points of constructive interference of the wave packet ψ which,
when passing the two slits, shows the typical Huygens interference of two spherical
waves emerging from each slit. Suppose the wave packet reaches the photographic
plate after a time T. Then the spots show the |ψ(T)|2 distribution,4 in the sense that
this is their empirical distribution. Analyzing this using Bohmian mechanics, i.e.,
analyzing Schr¨odinger’s equation and the guiding equation (1.4), one immediately
understands why the experiment produces the result it does. It is clear that in each
run the particle goes either through the upper or through the lower slit.
"

The authors continue:

"So is logic false? Is the particle idea nonsense?
No, the argument is a red herring, since
[Close slit 1 and open slit 2.], [close slit 2 and open slit 1.], and [Both slits are open.] are physically distinct." [original emphasis]

In other words, it is a non-problem. Which is not very helpful. What is important here is that the experiment is another logical equivalent of the version with the vapor chamber.

Edited by Dalo

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2 hours ago, Strange said:

Yep. Non-locality goes across space and time. 

 

I can confirm it is accurate by simply giving an appropriate scenario.

Take an object in 4d spacetime, with the usual coordinates (ct,x,y,z) now change the time coordinate to a coordinate of the 4 beyond the range of 4d causality defined by the speed of information exchange c.

A good example is Bells experiment. When particles are initially entangled, they establish a correlation. This is a past event non local to when one of the particles gets measured. Local event from particle to detector. This is the meaning when you hear the statement " Entangled particle correlations must be treated with non locality". 

Locality being defined by speed of information exchange under time dependant ( within a given time slice) equations. Under 4d past events can also be non local even if its the same spatial coordinates.

Far too many people fail to see the 4d side of the experiment and simply see the 3d they are familiar with so equate locality as being 3d when its defined under 4d not 3d.

Lol granted they rarely understand what a correlation function is to begin with.

Anyways were talking about a different experiment in this thread feel free to ask questions on this in a seperate thread.

Edited by Mordred

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