The dual slit experiment seems like...well...BS!

It seems like it would be fairly obvious that the act of observation would be performed by an observer. I am not going to say something that is a complete mouthful to replace it...

They added ain't to the dictionary, and everyone use to say it was not a word from it not being in the dictionary. That is no longer true, and it is now considered English. The word "you" isn't bad English, because they originally used "thou". The English language is constantly evolving. In this case, I really do not know what word you think is supposed to be used in place of "observer" here. I am not going to type out, "a scientist that makes any kind of measurement", every time I would use the word "observer". You never know; there may be non-scientist out there doing that.

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I've seen enough of your posts to realise that further discussion would be futile.

 

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Did you read the Feynman lecture I linked to above? If not, please do so. It should clear up this confusion.

 

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I have never given more than cursory study to the Fiber Bundle formalism.
https://en.wikipedia.org/wiki/Fiber_bundle
From what study I did do of it I concluded that it would not add to my personal understanding of physics. According to what I have read FB formalism has added greatly to the deeper understanding vector, spinor and tensor fields.

To the extent that quantum theory can be said to be understandable some level of familiarity with linear algebra is essential. Not everyone has the inclination to gain such familiarity. This is fine but if you intend to expound upon the topic you should attempt to use terminology that cleaves as closely as possible to the actual content of the theory. Terms such as Observer, Observation and a function Collapse where born out of the lack of understanding of the early pioneers of quantum theory, I do not say this to criticize them but it past time to delete such terms from the lexicon.

My personal laziness with regards to Fiber Bundles is not a virtue and I can't understand why anyone would portray obstinate refusal to use clearest language possible as such.

 

What are the shapes / modes of the outer electron shells of the atoms that form the hard edges of the "double slit" at any particular time? These electrons can all absorb, reflect or retransmit photons in the same or in a different direction while they are occupying the outside orbital states of atomic nuclei, can't they? Once a steady state has been set up (photon flux, thermodynamic states of the two slits), the fringes would be observable as reasonably steady over the long term, but produce chaotic or even random trajectories for the photons over shorter time intervals, yes?

I don't perceive there is a real problem here. I may be ignorant, but I never have.

What does quantum physics inform us about this experiment that could not be surmised by this consideration? How thick are the slits, and does whatever material they are made of or their temperatures or the energy of the photons make any difference to the appearance of the fringes? Why or why not? These seem to be variables I would be interested in, but somehow are seldom or never discussed in connection with the double slit experiment.

 

I think it's a good point you raise here. I posted a link to a recent example of this experiment. I suppose with some digging you could probably get the email address of the people who set up the experiment and ask them. I think it would be worthwhile.

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I just looked over it, and I didn't see anything mention "observer". I am at a loss as to what you referring about to my confusion. Besides, Feynman is dead, so he didn't have any say over what is currently a word or not in physics. Plus, I think he might have been one of the crazy ones that thought that God was playing tricks on them, and He didn't want them to be able to figure out particles by changing the result of the experiment on them (true story).

Why do I always have to be the one that is confused here? WTF? Too many cowboys, and not enough Indians...

 

"You may be thinking: “Don’t use such a bright source! Turn the brightness down! The light waves will then be weaker and will not disturb the electrons so much. Surely, by making the light dimmer and dimmer, eventually the wave will be weak enough that it will have a negligible effect.” O.K. Let’s try it. The first thing we observe is that the flashes of light scattered from the electrons as they pass by does not get weaker. It is always the same-sized flash. The only thing that happens as the light is made dimmer is that sometimes we hear a “click” from the detector but see no flash at all. The electron has gone by without being “seen.” What we are observing is that light also acts like electrons, we knew that it was “wavy,” but now we find that it is also “lumpy.” It always arrives—or is scattered—in lumps that we call “photons.” As we turn down the intensity of the light source we do not change the size of the photons, only the rate at which they are emitted. That explains why, when our source is dim, some electrons get by without being seen. There did not happen to be a photon around at the time the electron went through

This is all a little discouraging. If it is true that whenever we “see” the electron we see the same-sized flash, then those electrons we see are always the disturbed ones." -Feynman

Look at this passage for instance, he uses the word "observe". Then he just uses the word "see" in scare quotes. The thing about this was that Feynman got into quantum mechanics so early, people may have not have been using the terminology of "observer" yet. He didn't know what to call it, so he just uses "see" in scare quotes. They didn't actually "see" it. He is just talking about what the detector picked up.

 

As a professional quantum physicist, I'm going to go on the record and say that there is nothing wrong with with the "observation/observer" terminology, although both terms are kind of dated. In my experience, "measurement/measurement device" are more common, presumably to avoid the misconception that it makes any difference whether an actual scientist is watching the output meter. Similarly, physicists do still talk about "collapse of the wavefunction" as a convenient shorthand for what happens when you perform a strong measurement. Our understanding of measurement has expanded a lot since the original, collapse-based formulation, and many modern applications require a more nuanced approach, built around decoherence through coupling to a noisy environment. But this approach is a generalization of observation-based collapse, not a replacement of it.

In the double slit experiment, essentially none of the observed photons interact with the barrier atoms at all. If the photon passes through the slit, it basically passes through empty space; otherwise it hits the wall and is absorbed. In principle, there is a very small region along the edge of the slit where the photon might scatter off the barrier without being lost, but the width of this region is negligible compared with the width of the slit, so its effect on the output statistics is also negligible. This is confirmed by experiment, where screens of various thicknesses and materials produce the same patterns, both as each other and as the idealized theory which does not take finite atom size into account.

 

How do you know if the wavefunction collapsed or not without looking at the output meter? How would we know if the results of the experiment changed when someone did look at the output meter?

It does seem rather strange that the blackboard in the back that absorbs the electrons, does not count as an observer (in the sense that it alters the experiment). Would the scientist then know the exact location of an electron where it was absorbed at that moment in time? Or is all the information about the electron after that point lost? It would make it seem like it should really be hard to detect what the actual speed of the electron was in such an experiment.

 

For all we know, looking at the output meter of an experiment like this could be just like Wheeler's delayed choice experiment. This experiment has also been proven. Many different experiments were made similar to his original one. Even the electrons spin is determined by the act of observation. I read about a delayed choice experiment that would split an electron into two pairs. One of the pairs would go in a chamber, and the other pair would be allowed to move on. When they determined the spin of the electron that continued, it would always make the other particle have the opposite spin.

So basically, for all experiments, time is completely irrelevant. The wave function will collapse and the spin of the particle can be changed at any time during the experiment, no matter when the act of observation occurred. So then saying that it doesn't matter if a scientist looks at a screen is like saying Schrodinger's Paradox would never happen, because that is something that doesn't normally happen to cats.

 

What about the photoelectric effect? Let's see..

Typically the double slit experiment is done on a piece of glass like a microscope slide coated in carbon black from a candle and scratched with two single edge razor blades stacked on top of each other. What is the work function of carbon?

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/photoelec.html

4.81 eV, looks like, so that is in the UV range. UV doesn't go through glass, so my guess would be, that wouldn't be a factor.

So, the point of the double slit experiment is that the unpolarized monochromatic photons form those fringe patterns ONLY by interfering with each other, and also that they appear to do so even if only one slit aperture is exposed at a time, meaning that the quantum effect is that they are interfering with other photons whether the other photons are there now or later?

 

Shouldn't someone with one of those fancy femtosecond cameras be capturing a double slit experiment in real time as the interference fringe patterns are built up from interlaced pulses, or has this already been done?

 

That's kind of a separate issue. We can't know for sure what happens in experiments we don't observe, and that's not just true for quantum mechanics. But for reasons like Occam's razor, I think it makes sense to formulate physics in ways that don't depend on whether a scientist is actually watching the apparatus. In the early years of quantum mechanics, the observer was an essential part of the formulation, and there was actually a lot of debate about what really counted as an observer for collapse purposes. The modern formulation works with decoherence and interacting systems, so it allows us to describe the physics of a device without including the experimentalist off to the side writing numbers down.

The blackboard certainly does count as a measurement device, and it collapses the wavefunction. Because it's positioned far from the slits, the electrons have already settled into their interference pattern by the time they get to it, so we see the fringes. If the electrons were somehow able to light up the blackboard and keep going through it, they would not exhibit further interference on the other side.

The scientist would know its location to within the size of the bright spot on the board, which is generally far larger than the electron's compton wavelength. For this reason, the Heisenberg uncertainty principle does not come into play, and there's no reason in principle why it should be hard to detect the electron's velocity with decent precision.

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This is exactly backwards. If only one aperture is exposed at a time, the fringe patterns do not appear; there is only interference when both apertures are open. This by itself doesn't tell us whether the particles are interfering with themselves or each other, but since the experiment keeps working when only one particle is sent through at a time, we know the particles are in fact interfering with themselves and not with each other. Contrary to what Feynman said back in the day, multi-photon interference is a very real phenomenon, and it comes into play in, e.g., the Hong-Ou-Mandel effect. But it does not play a role in the double slit experiment.

It's been done. In fact, you don't need a femtosecond camera; you just need to turn the rate of particle flux way down. When Feynman talked about this in his lectures in the 60's, he was referring to actual experiments.

 

"observation" was a very poor choice of words. That term has mislead many. "Macro interaction" would have been a better choice. Human observations have nothing to do with the "collapse" of the wave function:

For example, balloons were used more than 80 years go (possibly more than 100) to carry stacks of photographic glass* plates into the high atmosphere to get data on the more primary cosmic rays, many with the field of permanent magnets too.

Back on earth the plates were developed and a few with the more interesting tracks on one 2D plate were measured by physics graduate students, but there were far too many tracks in the plates compared to graduate students willing to do this tedious work. Some unlucky graduates student do this work still, more for their education than the data produced.

I. e. the human observation may and does take place sometimes 100 years after the Macro interactions. The tracks in the 100 years old developed photography plates do not suddenly appear when some graduate student is the first human to look at the plate and see tracks.

* Glass was used mainly to avoid film shrinking / spatial distortions during development and drying. A track might be, and usually was, on more than one plate. No reason why it would follow the 2D surface of the plate. This made extracting the information difficult – Graduate student work.

 

Right. Ready for something 'out of left field'? http://www.perceptions.couk.com/uef/radiation.html#interference
[Need to move up a bit to beginning of sub-topic that starts with:
NEXT
The famous `two-slit experiment' (as within the experiment above)]
I think it's crap but was intrigued by the matter-of-fact presentation. Anyone here has ready access to either reference given (annoyingly with nothing more specific than the books - not even which edition)? Tried twice to contact that gent for details but no response (no surprise - in a way). Maybe someone else will have better luck.

 

At first I was a little disappointed that you found this page interesting, because its first few sections are so full of blatanly wrong claims. A few examples:
-Atoms always emit photons in pairs
-Quantum mechanics cannot explain radiation from an accelerating charge
-Time dilation scales with speed relative to a preferred ether frame
-Time dilation at high altitudes depends not on gravity, but on air pressure
-Electrons are bound states of the electromagnetic field

But when I got to the two-slit part, I was also impressed by how deadpan the presentation is. For those who don't want to read through it: the author claims that in the double-slit experiment, if you block one slit and then unblock it without turning the light off, the interference pattern will not reappear. Instead, you have to turn the light off and back on to "reset" the interference pattern. The author laments the fact that physics textbooks never cover this aspect of the full experiment, and uses it as a point in favor of his pet model. Having done the double-slit experiment myself, I can confirm that this is a baldfaced lie, but I also lack access to his cited references.

 

Yep - his UEF theory is crazy and maybe I should have specifically made that point in #36, but my interest was strictly in the double-slit 'revelation' part.

Thanks for the confirmation - which I take it means you have performed the experiment as per author's prescription? There is believable BS out there, some more skilled at the art than others. In that case, I could not figure what was hoped to be gained by outright lying, but who knows exactly what state of mind is involved there.

 

The vacuum has certain values of permittivity and permeability......suggesting something non zero.....this non zero may be perturbed at quantum level by photons and electrons, and by the presence of detector.......so far no explanation involves the possible impact this non zero can have on this experiment, who knows this is the real culprit......

 

I know that some physicist would argue that we cannot use logic or common sense or Occam's Razor at all when it comes to quantum mechanics. As crazy as it might sound, quantum mechanics deals with a lot of craziness, so you cannot really count on anything dealing with it to turn out like you wold expect it too. Theoretically, a Schrodinger Cat experiment should make a cat exist and not exist at the same time, according to Einstein and other colleagues. That has not changed the fact that particles behave in this manner, as far as we know it. Basically, there is no scientific evidence that shows or proves that it doesn't have anything to do with the literal observer. It is a variable that has not been able to be removed from the experiment to show otherwise.

I am not saying that everyone believes that it only has to do with a literal observer when they say "observer", but I have read pop physics books written by a Ph.D. that claim it could be a true part of it. I may have gone completely mad starting to believe it myself, but that does not imply that an observer has to be a person. Then I cannot prove that the observer has to be more literal (no one has been able too). I do use the term rather loosely, but my belief that it could be taken in a more literal sense has no change in the meaning of the context of which it is used. I just happen to believe in the effects from acts of observation more religiously than most people do.

The only way to make logical sense out of quantum mechanics was for me to develop my own pseudo-scientific theories about the unification of quantum mechanics and relativity as being the description of the hidden variables. In this way, quantum mechanics actually makes sense, unless I just went insane from reading about the craziness that occurs in quantum mechanics too much. I even read over the same books twice, keeping my idea in mind for everything that it said, and I eventually started to believe that it started making sense. One of the biggest problems right now is finding a way to teach quantum mechanics in a way that actually does make sense. That is one of the main reason's why I decided to start using using forums. I felt, if there was any real understanding to be had there, then the word should be able to get out about it.

Personally, I believe that an observer can be the same in any sense of the word used the same in relativity, any person or device which makes an observation. Then in most context it is used it just refers to the device making the measurement. Then there are some circles this extends to the person making the observation themselves. I guess it can sound that way sometimes when I use the word, but I also try to use it in a context where it being a person or device is irrelevant to what I am saying.