Wondering if anybody knows

[widdershins]

I'm sure we're all familiar with the famous two-slit experiment showing that photons act as both a particle and a wave.  Then there's the part of the experiment where you try to detect which slit the photon went through, causing light to act only as a particle.  What I want to know is what this detector is.  Obviously to observe the position of the photon it must be interacted with and it is this interaction, not the act of observation which causes the waveform to collapse.  I find myself interested, not in the experiment itself, but in this waveform collapse.  I want to know more about it.  What causes it?  Does it resume after a certain distance or period of time?  Can it be reintroduced prematurely?  Is the waveform back if the photon is reflected?  What about absorbed and re-emitted?

For starters I want to know the basic different ways this waveform can be collapsed as used in the two-slit experiment; what the different types of detectors might be.  Is there a detector which uses electromagnetic fields?  Particles?  Blocking off one slit?  I know there's one which absorbs and re-emits the photon, though that's all I know about it.  Does the particular detector go before or after the slits?  That sort of thing.  A very basic rundown of the different ways to detect which slit the photon (specifically thinking of photons here, not electrons or larger particles or molecules used in similar experiments) might be detected, making the interference pattern disappear.  I especially want to know if there's a reliable way to collapse the waveform and any idea what's going on there and what it means for physics.  Very fascinated with this recently, for some reason.  Thanks.

[Baruch]

The best way to approach something is with a simple experiment, not with a complicated theory.  Quantum theory starts with a simple incandescent light bulb.  In that bulb there will be both photons and thermal electrons.  Also in an old fashioned triode tube.  In one the light is more important, the other has electrons as the point.  If you have a double slit in the light bulb ... then you are in one condition getting both diffracted light and diffracted electric current.  In another condition. the diffraction disappears ... not because of consciousness per se, but because the physical context is different.  One way to detect which slit a photon goes thru, would involve a photo cell to stand in the way of part of one of the slits, vs another photo cell in the way of another slit.  That would be detection by absorbing.  Another way would be by scattering.  There are other detection modes ... including one where you have stimulated emission (which is the one you mentioned).  The fact that the with-detector/without-detector situations are not possible simultaneously ... means when we have a scenario when we shift from one to the other, is like cheating at cards.

It isn't agreed even in principle of the wave function collapses or not in reality, vs in the model.  Under one set of conditions you get interference fringes, in a different circumstance you don't.  Going from one to the other is like asking to eat your cake and have it too.  Is it our idea of the wave function that collapses, or does the wave function collapse?  It is our idea of consistent inconsistency that collapses.  But not all agree, I know a Canadian physics professor that says yes ... the cat is both alive and dead at the same time, and that we all are alive and dead at the same time, it is only a matter of degree.  In the cat in the box experiment, we have artificially set up a condition where it has to be one or the other and not both.

To get specific, the detectors have to be in front of the slits, because we can't tell after the slit, which one it went thru (Edit ... this isn't true, you can do it in front or in back).  But thanks to photon-photon scattering and electron-electron scattering ... and since any given photon is just like another, and any given electron is just like another ... it is like trying to pick out a friendly face in a Chinese train station ;-)  and then we have the photon-electron scattering aka Compton scattering.  So it is much like a billiards table where all the balls are the same color/number.

http://toutestquantique.fr/en/duality/ ... is a computer model that is somewhat misleading, because of the human eye ...

If something is understood well enough to explain to a normal person, then it isn't so complicated ...

https://www.youtube.com/watch?v=KabPQLIXLw4 ... this whole series on elementary Quantum Mechanics ... is as clear as it gets.  This section covers your question.

If we insist that both contexts apply at the same time, this should be confusing, because we are demanding a logical contradiction.  In the math, if you examine it carefully, getting to eat your cake and have it at the same time, is possible for measurement A & B or B & C ... but not A & C.  Consistency given the experimental setup ... simply won't let you get that result.  But if it is possible to get something consistently, then experiment will allow it.

Here is the beginning of these superior series:
https://www.youtube.com/watch?v=Zhq8_63SrvU

Continue on to section 3b to get the rest of the story of wavicles ... and wave function collapse as a common sense thing
https://www.youtube.com/watch?v=1Iy76D5FQuo

[widdershins]

Fascinating stuff, not that I understood much of it.  So, I think I kind of understand an answer to my question.  Please correct me if I'm wrong.

By measuring the position of the photon as it travels through the slits we are not "actually" collapsing the waveform.  That is, the nature of the photon is not changed.  We are simply influencing its path to such an extent that we are getting different results, but if we were to add 2 more slits some distance way from the first two so that each photon were to travel through 2 sets of slits, its position detected only at the first set, the interference pattern would reemerge after the second set.  Is this basically correct?

[Baruch]

We can go over specifics one step at a time, if you point out which film and at what minute marker you are talking about.

A setup with two slits followed by two more, is different than just one set of slits.  In either alternative, the presence of a "disturbing" measuring device changes the situation that was in the other setup.  I do believe, that if you had a detector in front of the first set of slits, or between the first and second sets of slits ... then a no-interference fringe set of electrons show up at the front of the second set of slits ... but that is the same as the original situation with ... a detector in front of a single set of slits.  As long as at some point, before the second set of slits ... you know the path of enough individual electrons ... there are no interference fringes after the final set of slits.  The interference fringes don't reconstruct (at least over short distances).  Over a very long distance, an electron that you formerly knew the path for ... you don't any more.  After that distance ... a new set of slits would produce interference fringes.  So we are misunderstanding cause/effect here ... we have set up an illogical situation, and are surprised that nature isn't as illogical as you are ;-).  Heisenberg Uncertainty Principle in action ... if you know the path of the individual electron ... then you know its position and time ... and that causes you to not know about the energy and momentum (electron microscope spectrum) of that much of the electron stream.  But this means knowing about the paths of most of the electrons ... not just one out of a giant storm.  A toll booth, not a speed trap ... on the electron highway.

So you are both wrong and correct ... it depends on whether you allow yourself to forget the path of the individual electron, after having determined it.  It reflects the state of your knowledge.  This is hard for positivist science to accept ... and thus Einstein to accept.  If you accept that one can determine things to arbitrary assurance, and then deterministically know those things later by calculation ... then you won't see waves.  It is a measure of what we can know objectively ... and this offends objectivists.  But unlike a bad interpretation of Schrodinger's Cat ... we aren't causing the outcome, our ability to know is limited, and our own actions interfere with the ability to know even that.  What the electron really is or what it really is doing ... is ultimately unknown.

Now the early modern view, of Newton, that G-d knows every little electron ... if this were true ... that G-d was measuring every little electron everywhere and everywhen ... then nobody would see any waves at all.  So the kind of omniscient G-d imagined by theology, is destroyed by the fact that waves exist ... in that case there would only be the individual evenly random blips, no fringes.  So G-d MUST ignore us, in order for physics to work.  This doesn't surprise an atheist.  And that brings us back to the "all seeing eye" of the French simulation ... voila!

[widdershins]

Essentially knowing that the waveform is not actually "collapsed", that the nature of the photon is not altered, that it STILL travels as both a particle and a wave, even after being measured tells me all I really needed to know there.  The science, itself, is beyond me for now, I'm afraid.  I would need to start with some basics first and work my way up.  But I do have a greater understanding of what's going no now in that I know you don't change the wave/particle into only a particle with the measurement, which is what I was really wanting to understand.  Thanks.

[Baruch]

Essentially knowing that the waveform is not actually "collapsed", that the nature of the photon is not altered, that it STILL travels as both a particle and a wave, even after being measured tells me all I really needed to know there.  The science, itself, is beyond me for now, I'm afraid.  I would need to start with some basics first and work my way up.  But I do have a greater understanding of what's going no now in that I know you don't change the wave/particle into only a particle with the measurement, which is what I was really wanting to understand.  Thanks.

Great.  Don't stop asking questions.  Basically because science is just a model of how things work, and because of the uncertainly principle, and because the electrons are all identical and the photons are all identical ... what is going on is neither wave nor particle ... we don't know what it is.  As Feynman said, if you think you understand QM (aka reality) then you don't.  But we can sometimes model it as a wave, and sometimes as a particle, as circumstance and convenience dictates.  Basically Positivism and Platonism fail.

[widdershins]

I don't think I'm in any danger of believing I actually understand it any time soon, but I get this part, anyway...sort of...well enough that my question is answered.

Essentially I had a fascination with the fact that you could change the fundamental nature of the particle; make it travel as only a particle, shedding its wave characteristics.  Since I now understand this to be not even remotely true and the truth is far more mundane than the human ability to alter subatomic particles in such an extreme way, the fascination is gone.  But I still have a general fascination with particle and high-energy physics.  Once we understand everything about the smallest particle understanding of the larger particles will essentially be automatic.