# Double-slit in gas chamber

1. Nov 1, 2009

### Vdtta

Were there any attempts to setup double/single slit experiment inside a gas chamber, bubble chamber or similar, so that particle trajectories would leave visible trails? This seem to be the easiest way to figure out what exactly is going on, but internet search returned no result about any such experiments, so is there anything in particular that makes this setup unpractical or otherwise unapproachable?

2. Nov 1, 2009

### DrChinese

If you know the path (i.e. which slit), there is no interference. There have been a number of attempts to trick Mother Nature on this, and they don't work. No one much bothers to publish details of what everyone already expects. So that is more or less the issue you are facing.

Now that does not mean that these type experiments are not performed - they are every day. All kinds of variations are done as can be thought up. But this is an area in which theory and experiment agree already.

3. Nov 2, 2009

### Vdtta

I would like to believe in telekinesis, that might even explain how Uri Geller bend his spoons, but it's hard for me to realize why is one called a science and the other 'paranormal', so I decided to be skeptic, indiscriminately. I'm sure as a fellow scientist you understand the important role skepticism plays in science and so you will forgive my disbelief.

I'm aware of the experiments you're referring to, as Holmes explains to Watson: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."; It's only that my modus ponens tells me the improbable was eliminated before the impossible.

In other words, my skepticism and curiosity has led me to discover quite a few scientists, research organizations and famous universities have performed experiments in the last 5-10 years that claim the opposite of what you just said. I'm also aware these otherwise very well known and respected sources were labeled as crackpots once they have found themselves in minority, so I do not intend to argue nor suggest one opinion over another, they could be mistaken just the same. I'm also aware of the history of science, history of religion and history of history, and once you process all that you find it hard to believe anything at all, while in the same time you have to accept that anything is possible, it's a funny feeling.

In conclusion, when you change the setup of the experiment it should not be surprising the result of the experiment will change as well, in a very peculiar way indeed, but rather than forming an opinion I choose to be undecided until I have enough evidence to contemplate one, hence my question. I want more conclusive evidence, I'm just more of a skeptic type of person than you, I suppose.

CASE A.)

The particular experiment I was talking about, in a gas or bubble chamber, does not seem to has been performed. Can you explain why?

CASE B.)

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[*** e-gun ***]

Also, I find all the experiments shoot photons, electrons or even the whole molecules with some kind of beam that has some width, some fluctuation that takes care some of these particles will not be stopped at the mask between two slits and can actually pass through.

Code (Text):

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e-gun

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e-gun

But what if we use a real *point source* that shoots molecules or electrons one-by-one and the width of the "beam" is exactly the width of the particle being shot, so they all trace exactly the same line? What if we now swivel this gun left-right, or aim directly through one of the slits, or just one slit? Would there be interference pattern still? Have these experiments been performed?

CASE C.)
Code (Text):

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e-gun

What happens if the beam is not perpendicular to the mask and so a single photon, electron or molecule shot one-by-one can not possibly pass through the both slits in the same time since one slit would be further away? Do we still get interference pattern? Has this experiment been performed?

And again, I do not mean to argue any point, I'm just collecting the evidence.

4. Nov 2, 2009

### DrChinese

Nothing wrong with being skeptical. But what is the real issue here?

We have an experiment which allows you to see both the wave and particle nature of quantum objects from photons to molecules. We have a theory that explains this nicely, and that theory says: if you can know which slit, you don't get interference. If you can't know which slit, you do get interference.

The experiment has been done with other things than gas bubbles being used to detect particle tracks. I doubt you could expect to get much useful information from that form of the experiment because the gas itself would affect the trajectory of the particle so much that it would become a factor. A nice way to determine the which path for photons is to use polarizers. For ions, I think light is used - it is shined perpedicular to the path of the particle.

So my point is that scientists have done what you are asking and the results follow theory. So once again, I appreciate your skepticism but am not sure what you are skeptical of. I am not aware of a single experiment in this area, except the controversial Afshar experiment, which gives any results which would be considered even slightly at variance with quantum theory. (Afshar is stuff for a separate thread, and there have already been several here that have dissected this in detail.)

As to your questions about the path of the particle: first, the little area in the middle in your diagrams is on the order of one wavelength. Therefore your "e-gun" idea of shooting in a particular direction won't work. The entire point is you aim to something where there IS uncertainty as to the path taken. Then you get interference. If you aim at an angle, there will still be angles at which interference is formed. So you get nothing there either. It is not possible to use the distance differential to test much in such case due to the frequency considerations. Even if you could, what would you look for? The inteference will disappear as soon as you know which slit.

If you want some rock solid "evidence" to add to your arsenal, I might suggest this (it is complicated to follow however):

http://arxiv.org/ftp/quant-ph/papers/0112/0112065.pdf

http://arxiv.org/abs/quant-ph/0703126

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5. Nov 2, 2009

### Vdtta

I'm looking for very specific experiments, as described above, very specific. I would like to know whether they have been performed or not. If yes, I would like to know the results, and if not, I would like to know why, other than "We can't be bothered as we already know what the results will be".

Other things you are talking about do not trace the trajectories, it should be obvious how and why that is very different, very much different. Why do you assume gas or liquid medium would have any more effect to wave-particle duality than air or polarizers? We already use many different medium chambers for this very purpose of tracing particle trajectories, quite successfully I might add, and it is not the factor of error, but just one of the many different mediums particle-waves can move through.

http://www.cnufos.com/pages/newsandstories/pics/Bubble_Chamber_sm.jpg [Broken]

No. I'm very, very specific about the experiments I'm looking for, very specif. You can either give me a link to one of the three experiments described above, or you can say that you too are not aware of any such experiments. Additionally you may help me search for them.

Won't work? You mean no interference? That's fine with me, as long as we are talking about the actual experiments. Although, how would you explain single-slit experiment then? What the width of the beam has to do with anything? It should be about the wave-length not about the beam-width, right?

The entire point is that I'm interested in experiments, not predictions or assumptions.

I'm looking for interference or no interference. It's all the same to me, as long as we are talking about the actual experimental measurements.

Know? You mean electrical impulses in our brain can influence matter over distance? Midichlorians? Like Jedi? Are you talking about the Force, master Yoda? -- It's not a story the Jedi would tell you. It's a Sith legend... Dark Lord of the Sith, so powerful and so wise... he had such a knowledge of the dark side that he could even keep the ones he cared about from dying.

- He could actually save people from death?
The dark side of the Force is a pathway to many abilities some consider to be unnatural.

- Is it possible to learn this power?
Not from a Jedi.

Please, don't take me wrong, I do not mean to make fun of quantum uncertainty, it's just to make this argument a little bit lighter. In fact, I'm pretty sure that without it there would be no such thing as Free Will aka The Force. -- Anyway, in a single slit experiment, when we are quite certain "which" slit, we still get interference, so I'd say this is not about the human mind, but about the wave-particle duality.

So, DrChinese, I suspect we share the same "religion"?
- "The Chinese character Tao (Dao) means "path" or "way", although in Chinese religion and philosophy it has taken on more abstract meanings. Taoist thought focuses on health, longevity, immortality, wu wei (non-action) and spontaneity.

Taoists believe that man is a microcosm for the universe.

In Taoism, even beyond Chinese folk religion, various rituals, exercises, and substances are said to positively affect one's physical and mental health. They are also intended to align oneself spiritually with cosmic forces, or enable ecstatic spiritual journeys. These concepts seem basic to Taoism in its elite forms. Internal alchemy and spiritual practices are used by some Taoists to improve health and extend life, theoretically even to the point of physical immortality."

Isn't it interesting how quantum mechanics beautifully goes along with the concepts of Taoism and such inspirational movies like Star Wars or The Matrix? Please, let me assure you, I have no trouble with the concept of mind influencing the matter, it's a part of my "religion" too.

I'm aware of all that, but none of those experiments come even close to consider any of the points I brought up in the three cases above. I was very specific and I expect very specific answers. Obviously you too are not aware of any such experiments nor you can explain why they would not be performed, so we should be on the same side here, if for nothing else but to further confirm the theory and expand our knowledge.

What say you, my friend?

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6. Nov 3, 2009

### DrChinese

a. I have told you why the experiment would not be done with a bubble chamber, but I may not have made clear that the other experiments - polarizers or entangled particle versions - are the least obtrusive ways of doing the experiment. A bubble chamber, as you can see from the diagram you posted, prevents the quantum particle from reaching a detector. So there is no way to determine if interference will result or not.

b. If, on the other hand, you reject the reasoning above, it would be incumbent on you to perform the experiment. Generally, scientists spend their time, money and effort on experiments they believe in (or otherwise see benefit in), as opposed to experiments others want.

c. I don't think it is the mind that causes this, sorry if you got that impression. The issue is whether the which path information is knowable in principle. But I guess by logical extension you could say it is the mind of the observer that is involved (I just don't think that is relevant myself).

7. Nov 3, 2009

### Vdtta

And I have told you - gas, liquid or air is not additional obstruction at all, but just the medium where the whole experiment is/would be submerged into. Least obtrusive is still obstructive, but particle-waves can travel through liquid just as they can through the air or any other gas, that does not impact wave particle duality, or perhaps it does since then we would "know".

Maybe we would see particle split in half and go through both slits? Are you not curious what the result would be? You already know, right? I think you are dangerous with all your knowledge, you're collapsing the wave function before we even have a chance to setup the experiment.

- "There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened." (Douglas Adams)

Prevents? What? Where? Huh, uh. Spiral trajectories, as seen on that image, are due to magnetic field so we can identify different particles as they will curve differently depending on their charge and mass.

I can't believe I have to argue this, it's not funny. Particle-waves *CAN* travel through all sorts of propagation mediums. We are already performing all the experiments in a mixture of many different gases that we call air or atmosphere, you know?

You gave no reasoning what so ever. There is no such thing as experiment "will not work", only 'interference' or 'no interference', and both are very valid, useful and informative results.

- What direction has to do with anything?
- How do you explain single-slit interference?
- What width of the beam has to do with anything?

- Is it not possible to move our particle-width-beam 10cm to left or right, then shoot particles at the angle one-by-one and see if there will be interference?

- Single particle would then not be able to pass through both slits in the same time as one slit would be further away and that makes for easy experiment that would tell us a lot, whether it produce interference or not, do you not agree?

Believe in? See benefit? Ok, lalilulelo..

This is how Homer Simpson comments on religion:
- "...and all they ask for is a little bit of blind fate."

What do you think is relevant then? Try to read what you just said, mind is direct logical implication of the word "know" or "knowable", unless... could it be that obtrusion, that polarizer you were talking about? Pick one.

- "When you have eliminated the impossible, whatever remains, however improbable, must be the truth." (Sherlock Holmes)

8. Nov 3, 2009

### cesiumfrog

Quoting fiction to support an argument, eh?

OP is starting to sound kind of arrogant. If you want confirmation of the result, find it yourself: either http://scholar.google.com" [Broken] (PLA250p230 sounded like a starting place, and I'm sure somebody's observed trajectories around an x-ray diffraction crystal) or organise your own experiment. Scientists aren't asking you for your faith, and nor is your whim their responsibility. Since there's a fair consensus on the theory here (following naturally from simple and well scrutinised principles), would you justify picking this particular target for your skepticism, or are you really just generally unclear on QM basics?

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9. Nov 3, 2009

### hamster143

It won't work in the bubble chamber, because interactions between the electron and the gas in the chamber would be sufficient to destroy interference.

It won't work with the electron gun, because electrons are pointlike and they don't have width.

10. Nov 3, 2009

### DrChinese

As we have told you, a photon going through a gas will interact with the gas. Or not, if it is transparent.

If it does interact, it doesn't hit the screen and you can't see interference because the screen is blank. If it doesn't, you haven't traced anything.

On the other hand, with entangled particles, you can do anything you want to Alice and then look at Bob. But guess what! Nature knows that trick too. No interference will be seen.

So I guess we just keep coming back to that dumb ol' theory, Quantum Mechanics, which keeps on ticking. Not sure what you are hoping to prove by assailing scientists who have worked hard to understand both theory and experiment. I suspect you are probably a bit too embarassed at this point to admit your idea makes no sense, and will instead thrash out at me as you have in previous posts. I do not engage folks who are disrespectful, regardless of why you do it, so I will not be responding further. As a final note, I think you should reconsider both your unwarranted tone and your fondness for quoting fiction as a counter to scientific reasoning.

11. Nov 4, 2009

### Vdtta

Wrong. Photons interact with the medium even if it is very transparent like air, water or glass, and as long as they are not absorbed or reflected out of it, they continue to propagate.

Wrong. In all the experiments photons, electrons or whole molecules already do interact with the air and we can see interference regardless.

http://en.wikipedia.org/wiki/Bubble_chamber
- "It is normally made by filling a large cylinder with a liquid heated to just below its boiling point."

http://en.wikipedia.org/wiki/Cloud_chamber
- "Cloud chamber is a sealed environment containing a supercooled, supersaturated water or alcohol vapour."

http://www.cnufos.com/pages/newsandstories/pics/Bubble_Chamber_sm.jpg [Broken]

Obviously these particle interact with the chamber medium, yet they can still propagate through it exactly as we expect them to, and we can still trace the trajectories, and even though all these chamber mediums are actually quite transparent they really do not need to be if we decide to use electrons, neutrons or molecules instead of photons. In short, you could not be more wrong, and this is the 3rd time now.

There is nothing embarrassing about asking questions, my dear. Not being able to articulate response and blatantly ignoring what was said to you should more appropriately make you blush. You should be asking questions yourself, because if you keep pretending that you know you will never learn. But, if asking questions is indeed something I should be embarrassed about, then so be it.

a.) DrChinese: "The little area in the middle in your diagrams is on the order of one wavelength. Therefore your "e-gun" idea of shooting in a particular direction won't work."

- What direction has to do with anything?
- How do you explain single-slit interference?
- What width of the beam has to do with anything?

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How about polarizers and entangled particles, do they destroy interference?

There is no such thing as experiment "will not work", only 'interference' or 'no interference', and both are very valid, useful and informative results. The whole point of many of these experiments is exactly this, to try and trick Mother Nature, as DrChinese put it. Theoretical predictions need experimental evidence, that's what science is all about - EXPERIMENTAL EVIDENCE. What other experiments should they be performing, the same ones?

According to QM and many experiments, electrons, neutrons or even the whole molecules are not only particle like, but they are also wave like, hence interference patter even if we shoot them one-by-one. This is exactly what these experiments are all about and what is called 'wave-particle duality'. And so, it should work very much the same, because this should be about electron WAVE-LENGTH and not about the electron BEAM-WIDTH. It should work, and that's the whole point.

Would you not like to know what experiments have to say about it?

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12. Nov 4, 2009

### hamster143

Any method that allows you to localize the particle to one of two slits will impart sufficient transverse momentum to destroy the interference pattern.

13. Nov 4, 2009

### Cthugha

That is wrong. Interaction in the sense meaningful in this branch of physics means irreversible interactions (those "leaving a trace"). The kind of elementary excitation of the em-field is different in vacuum and material (for example you have to adopt the polariton picture in solilds), but this is not an interaction in the strong sense as it is reversible.

As I said before, this understanding is wrong. It is well known that those interactions decrease the visibility of the interference pattern (see for example Phys. Rev. Lett. 90, 160401 (2003) by Hornberger et al. for the dependence of fringe visibility on the gas density in fullerene interference experiments). Going from vacuum to air or a bubble chamber is the same as changing the density of the medium. This dependence has been given in the paper above. Checking this again using different gases/liquids is approximately as exciting as checking the influence of the color of the shirt the experimental physicist is wearing on the double slit pattern.

If you want to talk about waves you also have to talk about coherence lengths. For double-slit experiments the important question is, whether the coherence length of the light (or whatever) exceeds the slit separation or not. This is why you do not see an interference pattern using sunlight and a double slit with reasonable slit separation. As coherence length is of course limited by the beam width this quantity is most important.

14. Nov 4, 2009

### Vdtta

That does not make my statement wrong, what is your point anyway? I'm saying that photons, electrons, neutrons and molecules have their wave-particle duality regardless of the changes in the medium and regardless of how their energy is modified along the way. What are you trying to say?

Finally something tangible.

Can you provide a link to that paper?
What densities have they tried, what gas?

So, increased density decreases fringe visibility, why would that be? Does increased density make particle-waves more like particles and less like waves? I suppose you are talking about scattering, diffraction and refraction, but that does not necessarily apply directly to density, it should not impact wave-particle duality nor it can be applied to different particle and medium combinations equally.

Different gases/liquids coupled with different temperatures and pressures can change propagation properties in regards to different particles quite a bit. Until you actually know how and why density, in that particular case, was responsible for decreased fringe visibility your insistence to not perform any more experiments is disturbing.

How is coherence length limited by the beam width?

What does beam width have to do with anything if we shoot particles one-by-one?

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15. Nov 4, 2009

Staff Emeritus
No, Dr. Chinese is correct, and it is you who are wrong. Some of the best evidence is in the picture you posted. You see these spirals? The fact that it's a spiral and not a circle is because the particle being tracked is losing momentum and energy to the medium.

This is sounding less and less like a search for knowledge and more and more like someone pushing a particular position. I hope the thread does not continue to go in that direction.

16. Nov 4, 2009

### Cthugha

I am trying to say that the wave-particle duality pretty much goes down the drain if "their energy is modified along the way".

I was just referring to the first paragraph of my posting again.

I can just give you the link to the PRL page, where you need a subscription to have a look at it. I do not know, whether there is version of this paper somewhere on Arxiv.
The gas they used is methane in a pressure range from (almost) 0 to $$2.5 \times 10^{-9}$$ bar. Here the fringe visibility already dropped to about 3%.

http://prola.aps.org/abstract/PRL/v90/i16/e160401

If you want to treat a particle as a wave the interesting thing to look at is the phase. So coherence time (will the phase of the wavefunction still be correlated to the phase now in x seconds?) and coherence length (is the phase of the wavefunction over there still correlated with the phase of the wavefunction here) become important. Incoherent interactions (incoherent scattering and such stuff) pretty much spoil this fixed phase relationship and the particle stops behaving as a wave. Now if I increase the density of my medium I also increase the probability that a single particle undergoes such an incoherent interaction. So the ratio of particles, which behave wavelike goes down and so does the fringe visibility.

Sure, you can try different materials, but that is more or less engineering, not physics.

Coherence is a measure of indistinguishability of probability amplitudes leading to the same result. In the case of a double slit this means the probability amplitudes to reach a certain spot on a screen by going through slit A or B. A small beam width corresponds to a small uncertainty in the position, where the particle will hit the double slit and therefore a small degree of coherence. You can see that pretty well from all the experiments with single photon sources and entangled photon sources. The more "single-particle-like" a photon gets (low uncertainty in direction, position and time of emission) the lower the degree of coherence of the corresponding light field will be.

17. Nov 4, 2009

### Vdtta

Dr. Chinese said photons do not interact with transparent medium.

And you're now explaining that is in fact correct by talking about the loss of momentum of electrons(?) Photons, electrons, neutrons and molecules all interact differently with different mediums, but they all DO INTERACT with it regardless of the transparency and regardless of loss or no loss of the momentum, which is different story with different particles and different mediums, different temperatures, densities and pressures, but none of it, however, should have impact on wave-particle duality, which is what I'm talking about.

I do have serious doubts about human mind influencing matter over distance, unintentionally that is, but the only position I'm pushing is that new and different experiments should be performed.

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Ok, let me first say that I'm very pleased you could join as I greatly appreciate your directness and specificity. You helped to narrow down what seem to be the main problem of this argument. Thank you. -- Now, I want to investigate why and how can wave-particle duality go down the drain if their energy is modified along the way.

According to QM particle-waves are actually pretty much just waves and not particles at all, and that they only APPEAR as particles when "wave function collapses", or something like that, right?

Having that in mind, and forgetting about double slit experiment for a moment, I do not see how and why scattering, diffraction or refraction would make any impact on this fundamental wave-like property of all the matter as those kinds of interactions are specific only to waves, which actually confirms the persistent wave-like nature of all this quanta, regardless of energy modification it experiences along the way.

The best example for this would probably be the light propagation as studied in optics were all the experiments pretty much depend on this invariance of wave-like nature of photons, so the light when passing through one medium to another or passing through different densities of the same medium will retain it's wave-like properties regardless of energy modification along the way.

In conclusion, some particular double-slit experiment seem to confirm what you just said, but everything else, the theory itself and all the other experiments, do not support the idea of wave-particle duality being dependent on scattering, diffraction, refraction or anything else.

Ok, thank you. I could not access that, but here is something similar.

Matter-Wave Decoherence due to a Gas Environment in an Atom Interferometer
http://www.atomwave.org/otherarticles/gasd_PRL.pdf [Broken]

Consider what is really going on there. If the particle energy aka wave-length was changed, than it should not be surprising the same separation distance between two slits will have different impact on fringe visibility, but that does not mean the wave became a particle, it only means the wave-length was modified and so we need to use different distance between slits.

And so, we only need to figure out how much energy is lost and what is the new particle wave-length just before the collision with the slits, and then we just adjust separation between the slits accordingly and our fringe visibility should come back. How's this?

That should not be quite so, not according to theory.

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e-gun

"~~~" = singe particle with some wave-length

In theory, particle-wave should be going directly between the two slits and if the distance between the slits and the their width is at appropriate relation to the particle wave-length then it should pass through both slits, even if this wave-particle traces the exactly the same path each and every time, going right between the two slits. In fact, to properly model the prediction of the theory we should really make sure this is how experiment is done and this random left-right fluctuation of the path or the beam-width you're talking about would only contribute to the noise of the fringe pattern and so we should actually avoid it.

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18. Nov 5, 2009

### Cthugha

It is not exactly that easy, but let me comment on that later on.

Fine, that is the field I work in. So let me stick to optics for the next few sentences.

A wave does not have that many parameters: Wave vector, amplitude, wavelength and phase. If you want the wave picture to be a good picture you need some stable long range behaviour. This is mostly determined by the phase. If there are long range phase correlations, the wave picture holds and wave effects are dominant. If there are no phase correlations, you do not see any wave effects. The time scale over which there are phase correlations can be easily tested by interferometry, for example using a Michelson interferometer. If you have interactions like scattering or diffraction, there are two basic possibilities: these interactions can be coherent or incoherent. Incoherent processes introduce a random phase shift. Therefore the long range phase correlations vanish and the wave-like behaviour breaks down. Coherent interactions introduce a well defined phase shift and the long range phase correlations persist and the wave picture remains valid. Common scattering processes between light and atoms are incoherent and wave like behaviour gets lost along the way.

This is pretty much what distinguishes particle-like behaviour from wave-like behaviour: the range or timescale over which a fixed phase relationship is lost.

19. Nov 5, 2009

### vissarion.eu

What if in double slit experiment there is not interference pattern but diffraction pattern? - Just a thought. Diffraction in young experiment also have some rings if light going through small hole, so there just diffraction frings combination from both slits and just measuring device with turned little bit down contrast, so that it would be more similar to seems as black and white frings, but actualy just more similar to 2x strength frings and 1x strength frings and those 1x strenght frings due to lowered down contrast of sensability - those 1x apears as black or almost black, so like wavelength configuration in double slit experiments have important role and maybe over seems inocent tricks making thing that diffraction is interference? Sorry for offtop.
Edit: BTW, physics teacher say ones that to them try to show interference or diffraction or Niuton rings, but unsucessfuly - nothing seen like predict theory, more similar to corpuscular theory. So maybe each time electron or photon going through one of slits and electrons which stroking in center don't going though at all and so all this interference pattern maybe is just two separated difraction patterns combined together and themis almost imposible to recognise from each over, like I was reading in some textbook that there was try to get some rays or electrons interference pattern in crystal perhaps or with some over matterial so it was very similar to difraction pattern - so there for physicists in past was hard to drow line between interference pattern and difraction pattern. Also measuring device probably nessasary consist of kind computer inside so computer counting those photons/electrons with some cycles and if photon don't shoot into cycle then no spot or say maybe scheme is structorized in such way that there of milions sensitive sensors connected to milions transistors, and per each cycle each transistor is cheked if have signal so to check all transistors connected to sensors need some time or milions cycles for cpu inside measuring device of double slit experiment, so skipped cycles is skiped wave(s) pressure points. and if such device working at say 1 kHz then million points...hmm... 1 point per 10 seconds. It faster measuring device testing and not physical laws - just my opinion.

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20. Nov 5, 2009

### Vdtta

Yes, let's simplify, that's what I'm trying to do and is the best method to make things perfectly clear. Since you mentioned the 'amplitude' let me get back to my last diagram for a few corrections and some new questions. But first, let me say that I find it strange amplitude is very rarely mentioned in relation to EM waves, as if it is constant for all EM waves or has some invariable relation to the wave-length, but is that really so? Can you always tell what is the amplitude of EM wave if I tell you its wave-length?

Code (Text):

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e-gun

"~~~" = singe particle with some wave-length

Ok now, this length "~~~" more appropriately corresponds to amplitude since that is how EM waves propagate as transverse waves, and so the wave-length is perpendicular, while amplitude is parallel to the mask/screen in that diagram.

The other thing is that I distorted the picture so it almost makes sense, but in reality, in some typical setup, amplitudes and wave-lengths are in nanometers while the slit separation and slit widths are in millimeters. It should be really like this:

Code (Text):

*** DIAGRAM ALPHA ***

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~~~
~~~
~~~
~~~
e-gun

"~~~" = singe particle with some amplitude

This makes much less sense than the first diagram as we now have to figure out why in the world this wave-particle does not get stopped at the mask between two slits?!? So, here are the questions... having some typical double-slit setup, what are the values of the following sizes, in millimeters:

a.) wave-length?
b.) wave-amplitude?
c.) slits width?
d.) separation distance between slits?

Ok, but let's simplify again. I would like that from now on we talk about only one setup, the most characteristic and the most puzzling one. The one that describes all the peculiarities of wave-particle duality and consequently is the key to unlocking this mystery - the setup from the diagram above where we shoot particles ONE-BY-ONE, right between two slits.

So, what is phase and what is coherence...

Phase is simply a velocity of this wave and coherence is related to the difference in wave properties among all the particle-waves we shot at our double-slit. So, we have wave-direction, wave-velocity, wave-length and wave-amplitude, and so if all the particle-waves we shoot arrive at double-slit with these four attributes the same, we can say our beam is coherent. There should be other kinds of coherence where some of the properties of these waves can be different, but then it should be more appropriately called 'harmonics'.

Anyhow, let's suppose that in our setup we have such point source that can shoot single wave-particles one-by-one, in an arbitrary time interval, where each and every one of these quanta have all four of these wave properties identical. In other words we are producing a coherent beam, or better to say "stream".

So, what coherence has to do with fringe visibility...

It actually has to do with the separation distance between two slits and the slits width, just because we made our double slit setup to suit a certain wave-lengths and amplitudes, some particular wave energy, and if some of these wave-particles get modified due to medium scattering, diffraction or refraction it will simply interact differently with the slits, or not interact at all, hence the noise and loss of fringe visibility due to incoherence.

http://en.wikipedia.org/wiki/Double_slit
- "Decreasing the distance between slits will increase the distance between fringes. Increasing the wavelength will also increase the distance between fringes as long as the slits are wide enough to permit the passage of light of that wavelength.

Yes, but I believe it can be explained in more simple terms, as above.

And now, the big surprise...
http://en.wikipedia.org/wiki/Double_slit
- "Any modification of the apparatus that can determine which slit a photon passes through destroys the interference pattern, illustrating the complementarity principle; that the light can demonstrate both particle and wave characteristics, but not both at the same time.

However, an experiment performed in 1987 produced results that demonstrated that which-path information could be obtained without destroying the possibility of interference. This showed the effect of measurements that disturbed the particles in transit to a lesser degree and thereby influenced the interference pattern only to a comparable extent."

Now, this does confirm, if you are to trust Wikipedia, my initial prediction that "knowing" has nothing to do with wave-particle duality, interference and fringe visibility, but it nevertheless leaves quite a few questions in regards to the diagram and the "most characteristic example" I'm trying to analyze here, DIAGRAM ALPHA. So, whether people decide to trust Wikipedia or not, this is really not what concerns me as it is just a beginning, and the real mystery goes deeper than that, right down the rabbit hole.

21. Nov 5, 2009

### Cthugha

Amplitude and wavelength are unrelated. Amplitude is related to the intensity of the wave.

But it does get stopped at the mask. You will not have any signal in this configuration.

This depends on the kind of particle used. However the one relation, which must be fulfilled is:
$$s \alpha \ll \lambda$$

$$\lambda$$ is the wavelength of the radiation, s is the width of the illuminated area (slit distance) and $$\alpha$$ is the aperture angle (the angle subtended by the illumination source at the plane of the slits) and you need to illuminate the whole slit assembly.

For electrons you have wavelengths of roughly 0.005 nm, slit widths of roughly 300 nm and slit separations of 1000 to 2000 nm (see for example Zeitschrift für Physik 161, 454-474,1961 by C. Jönsson (unfortunately in German, but I am sure there is a translation somewhere out there in the www).

You mean your second picture? But nothing happens there.

No.The phase defines the momentary elongation of the wave at a certain position

Not really. It is about whether there is a fixed phase relationship of the wave function at several positions and times. It is therefore related to the difference in the properties of two fields, not particles.

No, this is not sufficient. The phase could be the same by accident and without having a fixed relationship. However the beam is only coherent if there is such a fixed phase relationship, but it is not necessary that the phases are the same.

This is not what one would consider coherent. A coherent beam is more complicated. At first one must consider that we are discussing fields here. The probability densities to find a particle somewhere are given by the square of the value of the field at some point. If there is only one field present at some spot (like having only one single point-like light source) this is easy. The probability density to detect a particle at this spot is just the absolute square of the value of the field and you can know for sure that the particle originated from this one light source. If you have two fields originating from different spots there (like having two light source or two slits), the problem gets more difficult. The probability density is the square of the sum of both fields, so you get the first field squared, the second field squared and cross terms with the product of these fields. If the phase relationship of these fields is random, these cross terms will cancel out (because the fields are 0 on average), but if there is a fixed phase relationship those cross terms will not cancel out. In the latter case both fields contribute to the probability to detect a particle at this position and you cannot know, where it originated from. These cross terms indicate coherence and imply indistinguishability.

So, let me apply this to your setup. Consider the field originating from your particle source. What happens at the slits? If the particles are emitted very directed into the middle of the mask, this means that the probability density to find the particle at one of the slits will be 0 and the field amplitude at the slits will be 0, too. Accordingly there will be no signal at all.
If the particles are emitted very directed onto one slit, the field amplitude at the slit will be high. However, the amplitude at the other slit will be 0. At the screen you now have a situation like described before. The illuminated slit is a small particle source illuminating the screen. At the screen you have no interference cross terms (all particles originate from one slit) and you will see no interference pattern. If the particles are emitted undirected, the field amplitudes at both slits will be equally large. This is the second situation described before. You have two particle sources and if there is a fixed phase relationship of the fields at the slits, there will be the cross terms mentioned before at the different screen positions. You now see an interference pattern. This is why beam width matters. Only sources, which emit particles randomly enough to illuminate both slits simultaneously will produce interference. Note that this is not a random emission of particles with strongly varying, but well defined direction of emission. The direction of emission of each particle is not clearly defined.

Where is the surprise? Complementarity states that the more path information you have the less interference you will see. This is determined by the Englert-Greenberger duality relation: (path distinguishability)^2 + (fringe visibility)^2 <1

You can get a bit of which-way information and a bit of an interference pattern simultaneously, but not both with full visibility/distinguishability. Being able to know one still decreases what you are able to know about the other.

22. Nov 5, 2009

### Vdtta

Try to support your statements with some reference. You are supposed to know what theory says and what is written in the textbooks, one way or another. Would you believe Dr. Quantum?

If this single particle-wave is not going right between the two slits then it can not go through both slits in the same time. If it goes directly towards one of the slits then it would simply go just through that one slit, as you later explain with probability. If it goes just a little bit to the left or right then it would still get stopped or it would again not be able to go through both slits in the same time.

a.) wave-length = 0.005 nm
c.) slits width = 300 nm
d.) separation distance between slits = 1000 nm

b.) wave-amplitude?

See how they always forget to mention the amplitude?
It doesn't matter what kind of beam as long as we know all four properties.

We are discussing waves here, there is no need to mention fields. If you mean to disagree with what I said you would really need to point out how my statement does not correspond to reality, make sure you are in fact not saying the same thing and also not forget that we are talking about shooting particles ONE-BY-ONE, all having the same initial wave attributes.

You are complicating it without saying anything, and you are forgetting the most important thing again - slits width and slit separation with their relation to wave-length and wave-amplitude. You are talking about the measurement, but that does not even begin to explain what really happens, it only describes an attempt and subsequent failure as expressed in probabilities. That is hardly a theory supposed to explain anything, it's a statistics, which explains the insistence on input randomness, otherwise that is known as Monte Carlo method.

If you mean to be interpreting QM than you should really talk about what is written in textbooks. If the particle is not going right between the two slits, then it can not pass through both slits in the same time and interact with itself, it can not have the equal probability of passing through both slits, but at least you are shooting particles ONE-BY-ONE.

You're forgetting what our friend vissarion.eu said about single-slit diffraction, but at least you are still shooting particles ONE-BY-ONE.

So far you were talking about shooting particles ONE-BY-ONE and now you need a second particle to produce interference. Single particle is supposed to interact with itself, that is the whole point here.

Simultaneously?? We are talking about shooting particles ONE-BY-ONE.

How is one single particle-wave supposed to go through both slits in the same time and interact with itself if all you are doing here is shooting them left-right until two of them randomly happen to pass through two different slits?!?

It begins with the word "HOWEVER", which denotes the previous paragraph as false, never mind.

Last edited by a moderator: Sep 25, 2014
23. Nov 6, 2009

### Cthugha

I do not have sound on my office computer.

You wanted an amplitude in nm. However, the amplitude of an em wave is not some spatial oscillation. The amplitude is given in $$\frac{kg m}{A s^3}$$. You are mixing classical and quantum concepts here, which can only go wrong. A constant amplitude is just defined for an infinitely long wave train containing lots of photons. If you want to fire few or single photons this does not work. The amplitude is not constant and many more problems occur. Amplitude is a measure of the strength of the field and therefore related to intensity. So by changing the "amplitude" you just change the number of particles fired per time. It is not of interest for this experiment.

I disagree strongly. The underlying fields are the only wave-like thing in the double slit experiment, so they are of high importance. I do not know your level of knowledge, but you can read up on it in the Feynman lectures on physics (medium difficulty) Mandel and Wolf's "Optical coherence and quantum optics" (high difficulty, but very good) or Glauber's Nobel speech "100 years of light quanta" (not very complete, but a good first glimpse for beginners). These references also contain all the stuff I will be saying in the next few answers in this post. So let me use more clean language now without using the easy speech sometimes used to not confuse beginners. Particles do NOT have wave properties. All the properties you mention like phase or amplitude are not particle properties, but properties of the underlying field, which constitutes the particle. This difference is most important to understand anything.

NO! You obviously do not get the meaning of what probability densities are. This is not attempt and failure. This is an intrinsic property.

Sorry, but as has been explained several times to you, you will see nothing if a particle is shot at the mask between the slits. It must be shot at both simultaneously. This seems to be strange from the point of view of classical mechanics, but this is the point, where one must depart from it. This is not strange at all as soon as you consider the underlying fields. A particle in qm is not the classical ball known from classical mechanics having a well defined position and such stuff. A particle in qm has more or less just one single important property It interacts quantized. This means, you will only detect 0,1,2,3 or some other integer number of photons, but not 2.45. This is the only meaning a particle still has in qm. It is not some small ball having a well defined width going on some well defined path from here to there. Now let us have a look at the underlying fields. A photon is composed of the underlying em field. For electrons the wave function is a field and so on. Now it is important that the only information you have about the particle if it does not interact is the probability density that it might interact somewhere. This is the absolute square of the wavefunction (for electrons) or the em field.

The double slit with single particles now shows the difference between your understanding and the quantum understanding. In qm to have an interference pattern appear, you need the probability density to be nonzero at both slits. This means that the underlying field is nonzero at both slits and therefore the probability density to find an electron at both slits is nonzero as well. This is NOT a statistical spread telling you that half of the electrons took this path and half of the electron took the other path. The path, which one electron took is not defined at all. The position is only well defined as soon as it interacted somewhere wiith something. This is the particle part of the duality. The interaction is localized while the more fundamental fields defining where this interaction can take place are not localized and need to be nonzero at both slits.

Did he say something important an on topic? If yes, I did not get it.

Not two particles. Two fields! Either you get the difference know or we can stop this discussion.

Yes exactly. The beam width defined the extent of the underlying field. The field has to be able to reach both slits. This is not strange at all if you consider what I said before about particle and field properties. Let me quote the famous Roy Glauber (taken from R. Glauber, Quantum optics and Heavy Ion physics):

Last edited: Nov 6, 2009
24. Nov 6, 2009

### Vdtta

Lalilulelo... are you making fun of yourself? Yes, that is what I asked and there is no "however" because spatial oscillations ARE measured in units of distance, one of which is nanometer. However, that is not what amplitude of an EM wave is.

Are you able to provide some typical numbers/ranges or not?

For example, the amplitude for the case you've given before, roughly?

ELECTRONS, ROUGHLY:
a.) wave-length = 0.005 nm
c.) slits width = 300 nm
d.) separation distance between slits = 1000 nm

b.) wave-amplitude?

"Does not work"? I like that kind of argument.

Do you mean to say a single photon does not have wave-like properties?

Fields are not wave-like, oscillation of fields can be wave-like. Yes, of course fields are important, there is nothing else in this world but fields, but you are ruining my reductionism and now we have to skip to the very end of this story.

What is the range of magnetic AND electric filed amplitudes of the visible light?

You can measure the level of my knowledge by the amount of anger you're experiencing right now and the redness in your face when you manage to grasp your incompetence.

Your attempt to argue semantics, for some reason, is amusingly wrong on THREE accounts, beside being completely unnecessary.

First, your logic unit seem to be broken.

1. All men are mortal
2. Socrates is a man
3. (Therefore,) Socrates is mortal

Deductive reasoning
http://en.wikipedia.org/wiki/Deductive_reasoning

Second, amplitude and wavelength is a description of motion, those are WAVE properties, while FIELDS have property like 'potential'. It is only when fields MOVE, only when they describe wave-like motion that their potentials become a part of that description.

Third, the usual semantics is that particles produce fields and not the other way around. Electron is particle and we do not say, though funny enough I agree with you, that it is made of magneto-electric fields, but instead we say it produces fields, as in "magnetic field due to moving charge".

http://en.wikipedia.org/wiki/Field_(physics [Broken])
- "The fact that the electromagnetic field can possess momentum and energy makes it very real... A PARTICLE MAKES A FIELD, and a field acts on another particle, and the field has such familiar properties as energy content and momentum, just as particles can have."

Intrinsic property of the method of measurement, not necessarily the intrinsic property of the entity itself. Probability density is a numerical value used in statistical predictions of complex systems mechanics. It's like saying randomness is an intrinsic property of wind and rain. But that's cool, feel free to keep your current understanding. Why so angry?

Don't be sorry, you are making us laugh.

Now, I'm asking you for the second time to back up your claim, will you?

Being angry only makes your confusion more hilarious. You can't even put two sentences together without contradicting yourself, see what you said in previous quote, the part in bold.

Probability amplitudes associated with different possible histories, eh?

Hahahahaahaaaa! Yeah, that explains everything! The funniest thing is how that statement still refutes all what you were saying about particles, or even a single particle, being shot at both slits simultaneously. -- My friend, have you ever read "The Emperor's New Clothes" by Hans Christian Andersen?

Oh mercy! No worries, I'll wait.

Last edited by a moderator: May 4, 2017
25. Nov 6, 2009

### Cthugha

I just forgot a "not" while tailoring this sentence about 20 times because I already knew you would try to misunderstand it. Mea culpa. It is NOT a spatial oscillation.

The wave function is a probability amplitude. It does not have a unit. Integrated over all space it will be 1.

Wow a troll talking nonsense. If you do not get the difference between there is no constant amplitude and there is no amplitude I cannot help you. A wave packet has no constant amplitude, too. Do you think a wave packet has no wave properties as well.

Sorry to ruin your reductionism. Reality has a tendency to ruin wrong views. And to quote Wikipedia (not a good source, I know): "At lower frequencies the electromagnetic field may be radio waves or infrared light." Your understanding shows wrong again.

Electric field: Between 0 and infinite$$\frac{kg m}{s^3 A}$$
Magnetic field: Between 0 and infinite $$\frac{A}{m}$$

In reality both are usually less than infinity.

Proportional or antiproportional? I suppose it is proportional and as I did not notice any incompetence on my side so far I suppose you have not that much knowledge about physics. Now that gives me a level I can talk on.

It is neither of those.

We were talking quantum optics and particle physics here not some first semester electrical engineering exercises. Have you ever come across an em field describing a photon, which is at rest?

I would never say that an electron is made of the em field. It is not. The photon is. An electron acts according to the undelying probability density, which behaves (surprise) like a field. I never said it is impossible that particles produce fields, which in turn give rise to other particles like the electron does to the em field, which constitutes photons.

Sure, but it does not make its own field.

I quoted a Nobel laureate telling you differently. Sorry I have no higher authority available at the moment to convince you. God does not answer the phone.

I have given you three references in the last post. Glauber's Nobel speech is available for free on the internet. You will have to read it yourself.

And the contradiction is where? One particle consist of the product of two field operators. So if you have the field(s) reach both slits this is the closest to what one could call shooting a particle at both slits.

Sigh - no, it does not refute what I said. And Nobel laureates are not the kind of guys putting extremely funny things in Nobel speeches. Although Glauber indeed is a humorous guy. He cleans up at the IG-Nobel celebration. And yes indeed. It does explain everything.

Well, let us call it a day. I suppose the thread is going to get locked soon anyway.

Last edited by a moderator: May 4, 2017