Double Slit Experiment: Does Observing Affect Interference?

[mizpah12]

The detctor

However, there is a difference between asking a million questions versus the inability to comprehend the answers you were given AND being ignorant of experimental evidence. The former is perfectly valid, the latter is perfectly annoying.

You WILL note and remember that physicists, by definition, are employed to ask and find answers to many questions. If we already know everything, I would be unemployed. However, unlike what is happening here, we simply cannot ignore the experimental observations that are staring right in our faces, AND, we cannot plead ignorance of an observation simply because no one has told us before. Do that a few times and your credibility goes to zero and no one would listen to you, much less give you research funds.

What's my point in all of this? That this "double-slit" phenomenon is just but ONE of a gazillion example of the principle of SUPERPOSITION as applied in quantum mechanics. It means that there are TONS of other observations, ranging from molecular bonding in chemistry, to band structure in solid state physics, to the coherence gap due to the supercurrent in a SQUID experiment, etc.. etc. You cannot simply propose an "explantion" for the double-slit while being ignorant of all of these other experimental observations because they are ALL based on the IDENTICAL principle. Look up anything that says Schrodinger Cat-type states, and you have all the identical principle being illustrate in so many different ways. It is why I asked for where such a "scattering" off the slit walls would occur in a SQUID experiment that exhibits the IDENTICAL interference pattern.

You cannot learn or challenge physics simply by knowing the field in bits and pieces. It doesn't work that way in today's age. Every areas of physics are interrelated. A change in one will unravel in another. If you think that the your explanation of the double-slit ONLY affects the double slit, then you are missing a lot of physics.

Zz.

ZapperZ
Considering all aspects of the experiment, I’d like to ask the following (dumb?) question:
Regarding just the detector; whether the particle went through both slits or not, what happens at the detector, that causes the particle to change configuration from a “waveform” to a “particle”?
That is, what is it about the detector that causes the configuration to change?
My speculation is that something happens at the string level.
That the frequency or geometry of the particle string determines it’s configuration as “wave” or “particle”; and that something happens to the particle string when it contacts the detector.
Mizpah12

 

[ZapperZ]

ZapperZ
Considering all aspects of the experiment, I’d like to ask the following (dumb?) question:
Regarding just the detector; whether the particle went through both slits or not, what happens at the detector, that causes the particle to change configuration from a “waveform” to a “particle”?
That is, what is it about the detector that causes the configuration to change?
My speculation is that something happens at the string level.
That the frequency or geometry of the particle string determines it’s configuration as “wave” or “particle”; and that something happens to the particle string when it contacts the detector.
Mizpah12

The "detector" is also governed by the same rules that we apply on the slits. The nature of the detection is still the same. We just happen to apply a "energy-absorption-causes-a-black-spot-at-a-location" method. But I don't have to. I could, for example measure the current instead, or measure an energy absorption, etc.. etc. Different superposition experiments measure different properties that are in superposition. In the standard 2-slit, it is the superposition of paths. But I could easily design an experiment that shows a superposition of supercurrents, superposition of locations, superposition of momentum, etc... Each one of those has different detection scheme.

Zz.

 

[mizpah12]

Double slit experiment with a modification

ive been pondering this also. and how the observer addition affects results. But what I am confused about. Did the experiment change at all?

Take this for example. I'll try to be articulate.

Since it is a double slit. There is some sort of paper or whatever for the slits to be put in.

Now if you look way down to electron size. The paper would have a width that is much larger then the electron. Such that the electron could be reacting with the sides of the slits. If not colliding with the edge of the slit. The proton/electrons that make up the paper's edge. Their charge effects the electron's path and makes that sort of "effect" of interference.

Perhaps i don't understand the other explanation of how the electron affects itself. Like I've done the quantum physics which explain where the electrons are depending on the valance shell and such. Which seems like they are using the same principle. But it doesn't make sense. Like regardless of all the different positions its likely in. Its not in 4 pieces. Its just one electron-1piece.

Anyway I am betting its more simple of a solution and its the experiment equipment which made bad results.

http://img122.imageshack.us/img122/9128/perhaps6or.jpg

Munky
Along time ago in another place there was an article about the double slit experiment with a modification.
Opposite spins were applied to the electron at the slits.
The explanation went something like this:
You are running down a tunnel as fast as you can. You come to a “Y”. On the right is a blue door, to the left is a red door. You push through the blue door. Suddenly you’re in a small room and stop yourself by hitting the wall with your hands. You’re all alone and notice a pair of blue hand prints on the wall, from fresh paint on the door. Then you notice that on the left are two red hand prints. Although you know you went down the tunnel once and just went through the blue door, you also remember going through the red door.
As offensive as this is to our understanding of what’s real; repeatable controlled scientific experiments prove that, that’s the way it is in the world of the small.
Mizpah

 

[mizpah12]

Thanks, good info

The "detector" is also governed by the same rules that we apply on the slits. The nature of the detection is still the same. We just happen to apply a "energy-absorption-causes-a-black-spot-at-a-location" method. But I don't have to. I could, for example measure the current instead, or measure an energy absorption, etc.. etc. Different superposition experiments measure different properties that are in superposition. In the standard 2-slit, it is the superposition of paths. But I could easily design an experiment that shows a superposition of supercurrents, superposition of locations, superposition of momentum, etc... Each one of those has different detection scheme.

Zz.

ZapperZ
Very interesting, I didn’t know that. Thanks for the insight.
As I understand the experiment, every time an electron is fired through the slits they hit the target detector in a different place building up the familiar interference pattern. I would expect that on rare occasions there would even be a complete miss, where the electron went off to someplace off target.
This must mean that the electron is still moving as a wave, after going through the slits. But when it strikes the target detector it acts as a single particle.
What is it about the target strike that causes this change?
You did answer: “energy-absorption-causes-a-black-spot-at-a-location” .
In my minds eye I see that a wave would be spread out over the surface of the target, yet it results in a spot.
I certainly hope that it doesn’t appear that I’m playing some game to see if I can trip up the professor and cause embarrassment.
I think that this is an important aspect of the experiment that has been overlooked.
Mizpah

 

[DaveC426913]

... a wave would be spread out over the surface of the target, yet it results in a spot...

The wave is not a "wave of photon", it is a "wave of probability" - the probability (i.e. "fraction of 1") that the photon will be found there. If you added up every fraction of every point of every crest and trough of the interference patern, it would total 1.

Thus, it is still a particle that hits the screen. The pattern represents WHERE it hits.

 

[ZapperZ]

What is it about the target strike that causes this change?
You did answer: “energy-absorption-causes-a-black-spot-at-a-location” .
In my minds eye I see that a wave would be spread out over the surface of the target, yet it results in a spot.

Then you need to explain to me the mechanism that causes that "spot".

I can explain it as a photon being absorbed by the CCD at that location. You would have a ton of problems trying to explain why when you have a wave being spread out over the surface, you have just one spot. And not only that, you have to explain what mechanism causes that spot.

Zz.

 

[alfredblase]

This must mean that the electron is still moving as a wave, after going through the slits. But when it strikes the target detector it acts as a single particle.
What is it about the target strike that causes this change?

There is no change in how we describe the electron, other than to say it has transferred some of its energy. The electron is behaving as both a wave and particlea at ALL times, as can been seen over succesive electron registering. The electron acts like a wave AND a particle, that's what the double slit experiment is telling us. that's the whole point. Probably that's also why we find it so confusing conceptually.

 

[Rade]

You do not see the picture when you look very closely at the individual pixels on the screen. It is only when all the pixels are together and you look back, only THEN can you see the pattern. It is THIS pattern that differs whether the photon, electron, neutron, buckyball, etc. passes through only slit at a time, or the superposition of BOTH slits.Zz.

Yes, clearly, thank you. But my comment to the post of "DaveC426913" was to the question of just a "single" photon (or, let us give it mass and consider a single electron)--in this case no "pattern" is possible at either the slit or screen--the QM pattern derives from the probability function of many entities, not a "single" entity. Thus I see no difference between the macroscopic example given by DaveC426913 of a single human walking through two doors at once, and my example of a single photon or electron moving through two slits at the same time--both phenomenon are impossible. Or, are you in fact saying that QM predicts that a "single" photon or electron (just one, not two) does form "two" simultaneous dots on the screen ?

 

[ZapperZ]

Yes, clearly, thank you. But my comment to the post of "DaveC426913" was to the question of just a "single" photon (or, let us give it mass and consider a single electron)--in this case no "pattern" is possible at either the slit or screen--the QM pattern derives from the probability function of many entities, not a "single" entity. Thus I see no difference between the macroscopic example given by DaveC426913 of a single human walking through two doors at once, and my example of a single photon or electron moving through two slits at the same time--both phenomenon are impossible. Or, are you in fact saying that QM predicts that a "single" photon or electron (just one, not two) does form "two" simultaneous dots on the screen ?

Again, if those two examples are no different, then QM isn't strange, and the behavior of "quantum objects" should be very familiar.

However, last time I checked, even when I'm not looking, a human being has never produced ALL of the phenomena that electrons have. So your claim of them being no different rings hollow, because it has no experimental support.

Zz.

 

[Rade]

Again, if those two examples are no different, then QM isn't strange, and the behavior of "quantum objects" should be very familiar.However, last time I checked, even when I'm not looking, a human being has never produced ALL of the phenomena that electrons have. So your claim of them being no different rings hollow, because it has no experimental support.Zz.

Thank you--but what experimental support exists that a "single" photon or electron forms two dots on the screen ? (here I do not talk about 2 or more electrons moving through slits, just 1). Seems to me this phenomenon has the same level of experimental support as observation of one human walking through two doors at the same time. Thus does not your claim ring as hollow as mine ?

 

[-Job-]

If it seems to behave both as a wave and a particle, then why do we either see it as a wave or as a particle at different times? It should be a particle and a wave at all times.
Suppose we have a tank of water divided at the middle by some panel with two slits. On one side is a small object (capable of going through the slits) floating in the surface of the water. If you generate a wave in the direction of the slits, then the wave will displace the object and carry it through the slits. Because the wave is what determines where the object goes and the wave is subject to interference at the region of the slits, the final destination of the object should show an interference pattern.
There should two "physical" components for any photon, a very small particle (or just some concentration of energy) and some wave which should be generated by the same system that emits the photon.
The wave could be something like a ripple, like a tiny gravity wave or something. I feel like we're skipping some hypothesis and just going straight for quantum mechanics and superposition. But so far i don't understand why we do so. Zappers, explain that to me again.

 

[ZapperZ]

Thank you--but what experimental support exists that a "single" photon or electron forms two dots on the screen ? (here I do not talk about 2 or more electrons moving through slits, just 1). Seems to me this phenomenon has the same level of experimental support as observation of one human walking through two doors at the same time. Thus does not your claim ring as hollow as mine ?

Where did I say that a single electron or photon froms two dots on the scrreen? Where did QM imply such a thing?

I don't need to justify something I never claim.

Zz.

 

[alfredblase]

It should be a particle and a wave at all times.

No no no! An electron is not a "particle AND a wave" or "a particle XOR a wave". It's just that it behaves LIKE both at the same time, at all times, and the double-slit experiment very clearly and inequivocably supports this idea.

Imagine its an animal recently discovered. Its as graceful as a cat and yet barks like a dog. Its odd and unfamiliar, but its not magical or unphysical.

 

[Doc Al]

But, suppose you are a tangible "single" photon, bobbing up and down as you move at speed of light, -- then demonstrate how QM predicts that this "single" photon passes through both doorways ? As argued by Doc Al on this thread--a "single" photon cannot produce a QM "pattern". Thus I see no difference between microscopic and macroscopic aspects of reality from your example--but I am here to learn.

I think you missed my point. A single particle being detected (photon, electron) cannot produce any pattern--classical, quantum, whatever. A single "dot" does not a pattern make. It is only after multiple runs, after many particles get detected, that a pattern emerges. And the pattern that emerges for the double slit experiment clearly demonstrates QM interference--even if there was never more than a single particle in the apparatus at any time.

 

[ZapperZ]

.. and to add to what Doc Al has said, it is why the interference pattern that we observe is called a "single-particle" interference, not a two-two particle, three-particle, etc. interference. Single-photon interference is dominant, and distinctly different than 2-photon interference[1], etc. which are all higher order effects[2]. In fact, Dirac didn't think 2-photon interference was even possible (it is, but a very weak event).

Zz.

[1] L. Mandel, Rev. Mod. Phys. v.71, p.274 (1999).
[2] T.B. Pittman et al. Phys. Rev. Lett. v.77, p.1917 (1996).

 

[jtbell]

demonstrate how QM predicts that this "single" photon passes through both doorways ?

QM is agnostic about what a photon "really" does while it is en route from the source to the detector. It predicts different probability distributions for the arrival of the photon at various locations, depending on whether one slit is open or two slits. It makes no statement about whether the photon "really" goes through only one slit, or somehow through both slits at once. Such a statement is an inference by us, in the absence of a direct observation. Many physicists would say that we should avoid making such inferences altogether.

 

[DaveC426913]

Yes, it's not the photon itself that makes a pattern, it's the distribution of photons (or electrons, or atoms - it works for all of them).

- A photon is released and creates a single hit on the screen at point X. No pattern (with only one sample, how could there be?)
- Five seconds pass.
- A second photon is released and creates a single dot on the screen. You've got two dots separated by some arbitrary distance.
- etc. etc.
- After a few dozen photons hit the screen, you notice that the hits are not completely unifromaly distributed on the screen - they are slightly more concentrated in vertical bars, with gaps of no hits between each.
- As you let more and more photons through, one at a time, the clarity of this pattern gets more pronounced.

 

[Pickels]

Question: If the detector is capable of breaking the wave why isn't the screen? After all it too records which slit the electron passes (even if it's all of them). I imagine this is because the electron at that point has already gone through superposition and at that stage of the experiment we are dealing with the electron that has interfered with itself, in the case of no detector(?).

 

[masudr]

The slits act as position detectors, and the resulting wavefunctions are delta functions at the positions of the slit. The screen acts as a momentum detector in the direction perpendicular to motion.

See the Marcella paper that ZapperZ has referred to countless times.

 

[Rade]

I think you missed my point. A single particle being detected (photon, electron) cannot produce any pattern--classical, quantum, whatever. A single "dot" does not a pattern make. It is only after multiple runs, after many particles get detected, that a pattern emerges. And the pattern that emerges for the double slit experiment clearly demonstrates QM interference--even if there was never more than a single particle in the apparatus at any time.

Thank you--just so I understand--does this then mean that if I took 100,000 individual humans (and had them move up and down to simulate a wave) and asked them to randomly walk toward two doors, move through one randomly, and make a dot on a screen on the other side of the doors, that the dot pattern thus produced would be identical to the dot pattern observed for 100,000 photons or electrons in a double slit experiment ?

 

[DaveC426913]

...does this then mean that if I took 100,000 individual humans ... the dot pattern thus produced would be identical to the dot pattern observed for 100,000 photons or electrons in a double slit experiment ?

No. Humans cannot pass through both slits and then interfere with themselves.

You can only get the interference when the objects are given the opportunity to pass through both slits (that does not mean either/or, it means *both*).

 

[Doc Al]

Thank you--just so I understand--does this then mean that if I took 100,000 individual humans (and had them move up and down to simulate a wave) and asked them to randomly walk toward two doors, move through one randomly, and make a dot on a screen on the other side of the doors, that the dot pattern thus produced would be identical to the dot pattern observed for 100,000 photons or electrons in a double slit experiment ?

I'm at a loss as to why you would think this. What makes you think that a human (a huge macroscopic, incoherent collection of particles) would behave like a single microscopic particle such as an electron or photon? The humans would behave classically, just as one would expect. For one thing, their behavior is hardly random! And they clearly can be seen going through one door or the other, not both.

And why do you think that "moving up and down" has anything to do with the de Broglie wavelength that describes the wavelike behavior of particles? No reason to think that electrons are "moving up and down" as they travel to the slits.

Also, if you were to treat the humans as coherent particles, the associated de Broglie wavelength would be incredibly small (due to the high momentum of walking humans): way, way smaller than any doorway that you could walk through. (The slits and their separation must be on the order of the particle's wavelength in order to clearly display interference.) So I see no feasible way to even imagine such an experiment that could demonstrate interference of human-sized "particles".

 

[Michael_E]

QM is agnostic about what a photon "really" does while it is en route from the source to the detector. It predicts different probability distributions for the arrival of the photon at various locations, depending on whether one slit is open or two slits. It makes no statement about whether the photon "really" goes through only one slit, or somehow through both slits at once. Such a statement is an inference by us, in the absence of a direct observation. Many physicists would say that we should avoid making such inferences altogether.

This statement seems to parallel the attitude that Feyman takes in his lectures and popular book about QED. He just shrugs his shoulders and says, " We don't know why and can't come up with a scenario to figure it out". Then goes on to discuss how they came up with a calculations that work to determine the probability of where the effect of a photon will be detected.

BUT this attitude seems to be kind of in opposition to the way that science works, that being to make a supposition or inference that can be tested, test it and then change your suppositions based on that testing and do it all over again. If you decide not to make any suppositions that can be tested, you would get high centered and be unable to proceed in your understanding wouldn't you ?

In the case of these interference experiments and the idea of superposition and the interference of probablities, (whatever that means), what is the thinking in regard to where that superposition begins and how it ends in a single effect ? Does the superposition of different possible locations for an effect start at the moment that the photon is generated and then end once the photon is absorbed with little ability to tell what is happening in between ? Can you even say anything about what a photon "actually" is ? If any of you have a good source for reading on this I'd appreciate it, as most of the stuff on the web seems to be bogus, too simplistic or inconclusive.

Michael E.

 

[jtbell]

BUT this attitude seems to be kind of in opposition to the way that science works, that being to make a supposition or inference that can be tested, test it and then change your suppositions based on that testing and do it all over again.

The key phrase here is "can be tested," of course. According to QM there is no way to test whether the particle "actually" goes through slit A, slit B or both, without destroying the two-slit interference pattern. As far as I understand, experiments along this line bear this out so far. Someone who comes up with a way to do this and maintain the two-slit pattern would probably be high on the list for a Nobel Prize.

 

[masudr]

And why do you think that "moving up and down" has anything to do with the de Broglie wavelength that describes the wavelike behavior of particles? No reason to think that electrons are "moving up and down" as they travel to the slits.

Just to re-emphasise this point:

Firstly, the "wave" associated with any coherent object is NOT an undulating wave of the stuff the matter is made up of. This is a wave that exists not in the space that we inhabit but instead the state space of the system.

Secondly, these waves are not necessarily all perfectly sinusoidal by any means, in fact the best way to understand them are kets (or vectors) living in some complex vector space. While the "wave/particle" discussion is a good pedagogical tool sometimes, the real physics is contained in the full mathematical description. If you are familiar with that, then you willl realize why humans bobbing up and down have nothing to do with photons through 2 slits.

And finally, "observation" itself appears to be merely interaction with a large macroscopic object. And since humans are just that, there's no reason to expect human sized objects to be in a superposition of being at both slits and hence no reason to expect an interference pattern.

 

[Rade]

No. Humans cannot pass through both slits and then interfere with themselves. You can only get the interference when the objects are given the opportunity to pass through both slits (that does not mean either/or, it means *both*).

OK, thanks. So are you are saying that a "single" electron with mass can move though "both" slits at the same time (not either, or) because it is a wave function, and not a single particle ? But, if so, then why is it that this electron does not produce two dots on the screen, one associated with each slit that it moves through--since from above statement by ZZapper it was made clear that QM does not predict that a single electron moving through slits will produce two dots ?

 

[mizpah12]

follow the energy!

There is no change in how we describe the electron, other than to say it has transferred some of its energy.

Alfredblase
I think You nailed it. So obvious. I should have seen it myself!
Follow the energy! Like a detective follows the money.
Is the following awkward statement true?
“Directed” energy causes the particle to become a wave. When that wave strikes a “solid” barrier (no holes or slits) it releases the energy at the barrier and becomes a particle.
Mizpah