Problem with the two slit experiment, Observing later

Hi janrinze et al,

From my ealier post .. (for convenience if we could stay with photons for a while that would be nice..)

I suspect 8/ is the bit Einstein didn't like.

We're dividing the 0.1% probability of passing through the pinholes into two parts, saying the phase of the probability is the same at both slits and subsequently doing a vector addition of the probability after travelling via any path to the point of detection (at the screen) to get (statistically) the right answer.

If our emission event looks like an impulse and our detection event looks like an impulse and our probability operation has pretty much null properties in the time domain then I'm not clear about our reason for trying to wrap our photon up into a wavepacket at any stage of the analysis.

As a test I'd be interested to see a suitable experiment* where a wave packet gives the right answer so we can see whether a wavepacket encapsulates any properties that a wavefunction does not.

Best wishes,
-C2.

* Any suggestions?

This post has been edited by Confused2 on May 5 2007, 12:27 AM

Hi Confuesd2, Janrinze, yquantum, Laserlight, Mate, Neil Farbstein,

I like the idea that Confused2 has stated that we try and deal with photons if possible. I understand that electrons have a lot in common with the photons but there are further complications due to the relatively high mass (fermions) and the very short de Broglie Wavelength and "low" speed relative to Light also 1/2 vs full integer spin. Also they are permanently charged. Regarding photons... "ideally" they exhibit a total lack of locality until they are "detected". When detected they lose that non-locality and are fully localized. This is usually referred to as the collapse of the wavefunction in the Copenhagen Interpretation. Alternatively a protective measurement can partially localize the wavefunction and we can know a little more about the path but we cannot know any specific which way information without completely destroying the interference. It is a contradiction to say the "wave" passed through only one slit and then produced an interference pattern.

You can prove this by simply covering one slit and you will see that there will be no interference... indeed there can be no interference. Experimentally this is also verified. Any measurement which localizes the "particle" sufficiently to be capable of passing through only one slit immediately decoheres the wave. Now any wavelike properties that this "particle" exhibits subsequent to this "decoherence" will not be linked to the original source. Photons that then pass through the two slits will act independently and do not interfere since they are not the same source any longer. Naturally I hope we already understand that interference is not a two (or more) photon event but an event that occurs one photon at a time, that is unless the two separate sources are not artificially correlated to an incredible level of precision. To me this seems very obvious but there seems to be a reluctance to believe the results of experiment that one single photon that passes both slits is the source of the interference ... this is simply repeated for as many coherent photons repeat this event. Are there any that doubt this experimentally determined fact?

You could measure some photons on the source side of the slit and then measure the "absorption event" at the screen. Each of these conditions will exhibit spatial wave phenomena and after that single detection the photons will no longer take part in any further coherent interference effects but will exhibit either "ballistic" characteristics from there on as you may expect from the localized "particle", or it "initializes" the "particle" in a Quantum Zeno reset of the state... or both (I suspect both). This interpretation is not the straight Copenhagen Interpretation which takes no account of protective measurements nor Quantum Zeno effect. I suspect that an event that causes the photon to lose the original qubit of information without being absorbed through "scattering" means that the photon starts to spread again from that "detection focus", spreading with the inverse square law, once again "seeking all paths". In one way this occurs when the wave of some photons pass through a double slit. Remember the majority of photons are absorbed on the source side of the slits but the fate of these "collapsed" and scattered/absorbed photons is removed from partaking in the final interference pattern. Only photons that actually pass through the slits and are not "blocked" will take part in the pattern and by definition these photons are totally unobserved.

On hitting the screen any previously detected photons will not be part of the original coherent pattern but will "probably" fall on a pattern that correlates very strongly with the delocalization event in the new time and space coordinates and not with the original source. The kind of scattering (Thompson or Rayleigh Scattering) will depend on "ambient" phenomena in space. This correlates strongly with the environment in which the event is occurring... "free" electron scattering or "confined" electron scattering. It has got to be one or the other as I have previously indicated. It will be usually an electron scattering event and not a sub-nuclear particle scattering event since scattering is strongly wavelength/frequency dependent... The electrons will be associated with atomic shells in atoms 'somewhere".

1) Interference is a single photon phenomenon.
2) Interference patterns do not differ regardless of the fact that one photon an hour passes the slits or a trillion a second... this produces the same pattern.

These facts must be answered. If you want to ignore them then this goes against actual experiment and we will learn nothing from the experiments done in the past. Within the bounds of known experiment ... and we know this pretty well is we at least understand what is happening if not why it is happening.

What we need to do is not address our prejudices in this matter and address the nature of the phenomena. At the same time we have the exact nature of this wave phenomena at the same time as it exhibits "statistical" behavior. There is a third further fact that also must be addressed, and I have said this before and it seems that I am talking to myself on this one...

3) The wave phenomena itself is "perfect" down to the sources being correlated to within a small fraction of a wavelength.

This also "obeys" statistical probability but holograms are a particularly important phenomena... just how do people think these things occur? It is not "magic".. What I have said previously is that these represent two differently geared imperatives which are mutually incompatible in the limit (measurables and statistics). What we actually "know" is that a "dark room" with a single source of divergent coherent radiation produces "perfect" wavefront replicas of the entire room that can reconstruct the source and also the entire room to boot. You can't ignore that all source photons carry very significant phase information about "seeking all possible paths" in the "dark room" and this is recorded in the emulsion. This "information" is actually standing wave information that interact inside the emulsion differentially exposing interfering wavefronts "in depth" inside the hologram's emulsion... bright then dark! These standing waves imply solutions to Schrodinger or Dirac's Wave Equation... or at least some kind of wave equation... get out your hand lens and you can see the "waves" in the holograms gelatin, and "they are not going anywhere". These are not progressive waves which would pass through the photographic emulsion exposing the entire body of the gelatin with "statistical" processes. This very same hologram could equally be exposed one photon at a time with an "hour" between individual photons provided the room remains "fixed" in place and "dark" other than the one source of coherent light (perhaps using a single excited quantum dot). The fringing is responding to standing waves in space... Yes or no?? The fringes are equivalent to where light is absorbed or not absorbed in the silver halide crystals... in depth. This will continue to go round in circles until someone comes up with some other consistent theory that tallies with the experiment. In this same "dark room" go place your double slits experiment in there as well and see what the hologram records. Input is welcome.

Cheers

This post has been edited by Good Elf on May 5 2007, 03:11 AM

GE,

I agree with your premise regarding standing waves... but with a caveat. The
coherent photons are propagating/traveling thru the volume of the cavity
of "space" but the standing waves that you refer to are the energy density
signatures of "interfering" waves at fixed superposition points in geometric space.

At any point of crossing, the waves represent superpositions of energy density
within the confines of the cavity but you cannot observe the interference or
overlap of wave energy until it is detected. The energy contained within
propagating coherent photon waves is crossing, but not interfering until
matter absorbs the resultant standing wave EM energy at the point of detection.

My assertion is that standing waves are the summation of EM energy at fixed
superposition points in space. Interference/summation occurs at the point of
detection.


A simple water wave example:

A wave propagating across a long wave tank will move away from the source
toward the end of the tank. If you stick a pencil vertically into the water at any
point in front of the wave you will detect part of the wave energy at that point
even as the rest of the wave energy continues past that point.

If you have 2 synchronous but separated wave sources, each wave will propagate
from its point of origin with its individual wave energy moving away from the
source. At the locations where the 2 separate waves overlap and superpose their
summed energy, a standing wave will form as both wave energies that are
in phase superose at those fixed spatial locations. The individual wave energy
passing thru those wave superposition interference points will continue advancing
and the total standing wave energy at those overlap points will not be observed
unless they are detected due to wave collapse/summation.

Comments, discussion?
LL

This post has been edited by Laserlight on May 5 2007, 04:51 AM

GE,

To elaborate on your example of a holographic emulsion that captures the
inteference pattern of a "scene", we must understand that the scene is
a mosaic of individual wave energy superpositions caused by reflections from the
scene, each with different spatial and time references.

Each physical item that is part of the scene is reflecting its own relative time,
position, and phase information, as it relates to a single coherent photon source.

The hologram pattern that is recorded in the emulsion follows the ISL, as it
relates to the distance from the objects that form the image, as well as throughout
the thickness and physical dimensions of the emulsion.

The interference recorded throughout the emulsion is due to superpositions of
the wave phases at points of interference between the scene wave pattern and
the reference beam. The scene objects that are recorded are the interference
standing waves where the energy of the waves superpose across the
detection/recording emulsion medium.

Comments?
LL

This post has been edited by Laserlight on May 5 2007, 05:32 AM

Hi LL,
Re:- your simple water wave example.
We seem to agree that to know all about a wave DSE all you need to do is work out the wave amplitude and phase at a particular point (we've seen the maths for that several times) and Bob's your uncle. I don't understand why you (elsewhere) need to introduce standing waves and cavities just so you can add two waves from two sources .. can you explain?
Best wishes,
-C2.

Hi GE,

Looking at http://en.wikipedia.org/wiki/Holography we see that a hologram is an interference effect between a reference beam and an 'object' beam. The absolute phase of the source is irrelevent because the effect relies on the phase difference between the reference and object beams as they meet in the recording medium.
In your explanation you use the word 'wavefront' - this is a problem for anyone familiar with the speed of light and the way wavefronts generally expand at it. If we go back to Laserlight's simple wave analysis (sans cavities sans standing waves ) then we start to get the right answers. If we abandon simple wave analysis then we start to get the wrong answers. Could this be trying to tell us something about the nature of a (single) photon?
Best wishes,
-C2.

This post has been edited by Confused2 on May 5 2007, 01:14 PM

C2,

Do you by any chance have in mind specific property which could distinguish one from
another?

Anton

The specific property I had in mind was along the lines that one works and the other doesn't. Earlier I proposed dividing a wavepacket between two pinholes .. hmm.. not so good.

Best wishes,
-C2.

Good Elf wrote:

Why would that be a contradiction? It is not that the real world ( "real" conditionally speaking ) has to be logical or comprehensible to our minds. Nor we can say that we know about so called real world enough so nothing can exist if we cannot detect it or observe it with our current level of the technological development.

Yes. Which does not necessary imply that what you said is not the fact indeed.

What would be interesting is to design an experiment with one slit which would "give" an interference. I have a rough idea how that could possibly be done.

Anton

http://en.wikipedia.org/wiki/Diffraction#T..._of_diffraction

"The very heart of the explanation of all diffraction phenomena is interference."
-C2

http://en.wikipedia.org/wiki/Diffraction#T..._of_diffraction

"The very heart of the explanation of all diffraction phenomena is interference."
-C2

I was thinking along a speculation of arranging a set up for the single slit experiment which would have as the result an interference pattern of hits on the screen.

Anton

This post has been edited by Mate on May 5 2007, 02:14 PM

Hi Anton,

See http://www.teachspin.com/instruments/two_s...periments.shtml .. they give the result for each slit too.

-C2.

C2,

I read this article too but that is still not what I have in mind.

What I have in mind is still a rough idea. If I will be able to develop it in the satisfactory manner I will post it. Right now I am not sure where would lead me, if anywhere.

Sorry for mentioning it at all while I am still not sure about it . That was a mistake of thinking aloud, so to speak.

Anton

Hi All,

I am a bit confused about the whole hologram discussion.
creating parts of a hologram with only one photon is completely impossible.
Actually when creating a hologram one needs to keep in mind that the spatial coherence of a laser restricts the size of the space that can be illuminated while still resulting in a distinguishable interference pattern (the hologram).
Spatial coherence implies that each photon emitted is in phase with the previous and following photon. And when intensity is high the result is (almost) identical to one large wavepacket with the physical lenght of the spatial coherence of the laser. The spatial coherence lenght usually stems from the way the laser is built. With gas lasers it is in the order of N times the tube length. No idea what it is with solid-state lasers though but it must be in the order of N times its cavity lenght. With gas lasers N is relatively small.

I agree with C2 that the classical hologram is created by means of direct and indirect illumination of a photographic emulsion. This creates a superposition of all possible paths from the source to the plate. Where possible means from any reflective surface. The result will be a time integrated intensity for the duration of the exposure of the photographic emulsion. The longer the spactial coherence the higher the quality of the hologram. Increasing intensity has the same effect since more photons are available to generate the hologram though insufficient spatial coherence will result in loss of the 'proper' interference patern..

With regard to the DS experiment it looks like LL is implying that the 'spatial coherence' of a single photon is infinite, which is wrong i.m.h.o. This reasoning leads to a misconception that the photon interferes with itself over very long distances and therefore could generate a hologram all by it self (albeit not being able to express that since it can manifest only once on the photographic plate by interacting with the photgraphic emulsion) His reasoning does not explain why there will be any interference at all. Since the photon hits just about anything in the room and should therefore have equal opportunity to be absorbed just about anywhere. The resulting interference pattern should therefore be with an intensity that equals the area of the photographic plate divided by the total area of the rest of the 'room'..

Now back to the DS experiment. There is no difference whether we use photons or electrons in regard to the outcome of the experiment. If we try to measure the momentum this will result in loss of the interference pattern. In terms of spatial coherence there could (should?) be some situation where wave packets from different sources (slits) do not temporally overlap when arriving at a specific point of detection. (we could do such an experiment with computer simulations of wave packets and verify the results in real life.) The thing is that when we talk about wave packets and diffraction the wave packet is non-uniformly spread out over a large area and forms a wavefront. This could imply that this wavefront should be seen as one single event. On detection the wave should not collapse but the wavefront will contract to the place where it actually interacts with something. Thereby creating the localized detection of an amount of energy. Since the spread of the wavepacket over the wave front (by proof of the DS experiment) appears not nescessarily to be contiguous (can be cut up but is still one event) we need some other method of allowing it to be in multiple places at once. A multiverse solution seems adequate but does not really fit in Occam's Razor i.m.h.o. since that would hardly explain the probability distribution (all universes should be infinitsimally small but have equal probability..) So what would fit here without having to resort to extreme or radical ideas? Brings to mind that apparently the information of wavecollapse should propagate instantaneously all over the entire wavefront.


Food for thought..

Jan Rinze.

The hologram records the interactions of a huge number of photons all destructively and constructivly interfring with each other.