Problem with the two slit experiment, Observing later

Good Elf,

I regret that my software doesn't allow me to read several of the links you heve been kind enough to provide. As things are .. I have to leave it to others to continue this thread.

Best wishes,

-C2.

I understand your consternation on this matter of Young's experiment. The problem could become a philosophical one and that is up to many on how they chose to deal with this problem.

I will go with R.Feynman and his explanation until we improve the technology in the future.

Think about this if you will, we are all made up of atoms, etc. When you use a detector to make the measurement it is made of the same components as you and me. It could be said that when you look at the results of the experiment you have collapsed the wavefunction and thus cause the results of the previous twin slit experiment.

Just another perspective and you know in QM there is a measurement problem plus how we define what consciousness plays in such a case in the results of the test.

Good Elf has much to say about this and I respect his comments as you do I hope.

This was Einsteins problem with QM in the past and many others as of today. Yet QM is responsible for about one third of the technology of today in the world because of its predictive power.

Hope it has given you something constructive to think about.

ciao_
yquantum

typos to be expected as well as grammar due to a fast tea time.....

Hi Yquantum and Confused2 et al,

I am not sure what the problem is with C2 and his software but I have noticed that when I sometimes mention various pages they become unavailable and no longer accessible. You can never anticipate that and with this forum it cannot be changed after the event.

I am reluctant to drop this subject at this important "crossroad", just when IMHO I was about to make a very important point. I realize that I can be a "drag" at times because of the length of these posts. I would ask any who have followed so far to follow just a little longer. I promise you you will get a deep insight into quantum mechanics you have never thought possible.

I am sure most are aware of the treatment of Quantum Mechanics in
QED: The Strange Theory of Light and Matter. (Alix G. Mautner Memorial Lectures) Richard Phillips Feynman
Even if you have not read this treatment but have some savvy regards technical literature you will be able to use this to gain some deep insight. To accompany this treatment I will also advise these accompanying documents and windows and mac software (where applicable)...
"Teaching Feynman's Sum Over Paths Quantum Theory," Edwin F. Taylor, Stamatis Vokos, John M. O'Meara, and Nora S. Thornber
This document explains the essentials of the software I about to reference and refers back to the treatment by Feynman.

A page that has all of Edwin F. Taylor downloads on it is here...
http://www.eftaylor.com/download.html
The software is found here...
cT Executor and files (~522K) Windows
You must read the readme because this is an unusual program to run. I would also suggest that you unzip it into its own folder rather than into the default.
readme.txt

For additional details please look here to see a reference from "Surely you are Joking Mr Feynman"... "Feynman's Ants"...
Feynman's Ants
With this software you can easily simulate a double slit interference experiment and you then need to understand that what is seen on the slit is the square of the amplitude of the state vector. It is a projection.

The additional Gedanken Aspects of this exercise are taken from the frontpiece of the Book in parts called: "DEMYSTIFYING QUANTUM MECHANICS: A WORKBOOK" Edwin F. Taylor, Department of Physics, Emeritus, Massachusetts Institute of Technology
http://www.eftaylor.com/download.html#quantum

I will now outline what Professor EM Taylor enumerates as the fundamental differences between Quantum Mechanics and Classical Physics...
Old Idea (a) vs New Idea ( B )... eleven points in all...

1a/ Light consists of waves.
1b/ Light is energy “hunks” called photons.
2a/ Photons reflect the from front surface and back surface of a sheet of glass.
2b/ Photons are scattered by electrons throughout the glass
3a/ A photon or electron moves along a straight-line path from source to detector.
3b/A photon or electron explores ALL paths between source and detector.
4a/ The photon “quantum stopwatch” hand rotates as the photon explores each path.
4b/ Photon clock hand remains stationary as photon explores path. Initial position of clock hand depends on initial emission time from source. (Electron “quantum stopwatch” hand DOES rotate as electron explores path.)
5a/ A “path” (explored by photon or electron) means a trajectory in space.
5b/ A “path” means a trajectory in space PLUS location of particle on that trajectory at each time. In relativitytalk, a path is a “worldline in the spacetime diagram.”
6a/ A free electron (no gravity or electrical force) has a constant kinetic energy along the path.
8b/ A free electron exploring many paths may have different kinetic energies along different paths and along different portions of a single path.
9a/ An electron moves in an atom under a force provided by the electromagnetic field of the nucleus.
9b/ An electron explores all paths in an atom, exchanging virtual photons with the nucleus and with itself (NO field!).
10a/ Photons do not interact with one another.
10b/ Photons tend to “clump” as a result of the statistics that they follow Bose-Einstein statistics). Feynman calls this a “polarization” effect.
11a/ Electrons repel one another because of their charge.
11b/ Yes, but also electrons avoid one another as a result of the statistics that they
follow (Fermi-Dirac statistics). Feynman calls this a “polarization” effect.

Given all the above we have a fundamental difference of opinion between what we are calling a "particle" and what we describe as a "wave". It all goes back to the interpretation of the "path"...

Here is the nub... the photon is an object that "seeks out all paths" and it sums along various boundaries to "project" into a particle interaction phenomena. We are unable to examine the space in which the photon travels because it is "hidden" but what we can be certain of is it touches "everywhere" in the accessible Universe to a greater or lesser effect... exploring all paths. In the end is "appears" that it travels in straight lines, but that is a pure illusion developed from the sum of many paths and taking into account the relative phases. The "confinement" along that path depends on factors outside the software package leading to the various "modes" within the packet. The software does not account for spin in all its forms... neither particle angular momentum (spin) nor orbital angular momentum that bosons all can carry. This would have the effect of confining the lateral spread of a single photon. Some of this was beyond Feynman's lifetime to comment upon and I am sure that if he were alive today he would have made some additional comments on his theory by now. It is left to us to make those comments in his place.

Where the photon is physically when it is between source in destination according to Feynman is not an simply an "unknown" it is simply "unknowable" from our point of view. Remember that all intercepted photons on the way do not contribute to any interference patterns. The subtle difference is it is not a "probability" that is propagating it is an electromagnetic wave with phase and the wavefront of a single photon "particle" is a lot larger than we usually give it credit for, capable of folding onto itself and "mingling" everywhere and passing through both slits at once in Young's Experiment. These simple experiments do not allow for more complex behavior of a packet when confronting real path folding or to tunneling around or near objects.

You may say it is localized into about 1/2 a wavelength in the direction of propagation but it is surely spreading "laterally" in all orthogonal directions as it maps out all space. The effect of objects around the direct path of a particular photon is an interference effect but not a destructive one if it is able to reach its target. This single photon packet is like an expanding shell confined to the wavefront. It is not an aging packet since it travels at the speed of light time is entirely frozen. As shown by these experiments you can do above with the program, you can see that electrons are similar but their wavepackets "evolve" in time (the Group Velocity and the Phase Velocity are different, the group velocity is below the speed of light but the phase velocity is above the speed of light). They are similar to "light" constrained to traveling in a circle and though the electron de Broglie wave is non-stationary in time the interior components (a photon) are stationary in time since they are also traveling at the speed of light and we only recognize that the electron is moving in space as well as in time as a "whole"... the supraluminal aspects are confined to the evanescent region as always and conforms totally with conventional theory.

Virtual Photons exchanges are associated with unobservable internal "forces, and are needed to "balance the books" in the face of conventional classical behavior to cancel out so called accelerations as seen from outside the system. I have spoken of these before. Bose-Einstein and Fermi-Dirac statistics define the "Polarization" effects noted by Feynman.

Getting back to the Delayed Choice Quantum Eraser, Feynman's explanation that the non-contemporaneous nature of emission and absorption of photons is not explained by an extended period over which the photon was emitted. The DCQE shows that the absorption of the entangled photon into the screen at one point in our time and then to affect that event with a random "which way" determination later in time on its twin is insufficient to account for the anomaly. This anomaly would be many thousands of emission periods "delayed" and could not account for a local explanation for this phenomena. Thus a non-local influence must be the reason this has occurred and philosophically indicates something about the dimensional nature of "Quantum Space" versus our "spacetime". For the pair of photons at least the "event" is a "sheet" thrown over the Universe and is "immobile" in time even though it takes "our time" to put it there. The significance of the electron and its sub-light velocity can be explained by de Broglie behavior as the velocity of the particle approaches some "zero" of velocity. As I have noted before this is related to a broad interpretation of Einsteins Special Theory of Relativity ... the other end of the scale. As to point 5b above I would like to add that the "thickness" of a worldline varies with velocity due to de Broglie's Hypothesis and this is relative to some "stationary state" that we are not normally able to experience. It would also be the case if a particle was deprived of all its mass. It is suggestive that this "winding" around the lightcone is an end of the state experience when a particle has been accelerated to almost the speed of light... just the reciprocal state from our point of view.

Together with the information presented above many may find this phenomena very illuminating. It gives a real meaning to the Emitter-Absorber Theory that Feynman proposed and QED needs to incorporate some aspects of this to completely describe the way events fit into our universe. To many these ideas of Feynman's are too different to those methods they have learned in University Classes to really be understood easily. I do not think that is the case though, it makes too much sense. These advanced potential and the retarded potentials merge only when viewed from outside the venue of our universe where the added dimensionality allows for the existence of negative frequencies that can affect our Universe "retrospectively". We cannot know them from our point of view but we may be able to see single "transactions" in our Universe that violate causality under exceptional conditions. How do we know they are non-causal? That will be probably difficult to detect.

I welcome any interest in this idea. All legitimate questions are accepted.

Cheers

This post has been edited by Good Elf on Oct 15 2006, 03:07 PM

Hi Good Elf, y et al,

Struggling as usual .. I can't get the software to do anything (windows ME)

I suspect the following will clarify matters a lot.. a real experiment.. Hopefully it introduces the same 'philosophy' in a much simpler context.

Michaelson-Morley .. but all we need is the apparatus..
http://www.ligo-wa.caltech.edu/teachers_co...s/IFO_9t12.html

We will still get interference if the legs are of wildly unequal lengths. (YES/NO)

Good Elf and myself have established that a photon can only intefere with itself. (dispute welcome).. meanwhile..

With 'legs' of length 1 mile and 1 foot.
High intensity .. we get interference (YES/NO)

If we reduce the intensity of the laser to the point where photons are arriving one at a time ..
Interference .. (YES/NO)

--------------------------------------------
I vote yes to all three. It rather undermines the notion of 'the speed of light'.
If we have different answers then clearly our explanations are bound to conflict. Let us first try to agree on the result of the experiment.

-C2.

Hi Yquantum and Confused2 et al,

Umm... yes... we were not discussing this experiment. This is not set up to process DCQE using entangled photons. The implications are always the same here... it is a test of Special Relativity. OK ... I will treat this as "interference" and examine the idea of a boson state and the way a single photon is only capable of interfering with itself.

Yes... I am not sure that we are worried about this result?... are we?

Yes again but once again this experiment is only showing interference fringes of apparently unentangled photons. What it is that you want to demonstrate here... something about quantum physics or Special Relativity? If what you are trying to point out is that photons have to travel the length of a tube (one mile or whatever) and the same photon has to travel one foot to interfere so how can the one photon interfere with itself and produce interference fringes? Well my explanation would be each photon interferes with itself so each photon must travel partially through both paths (you cannot know "which way"). The test for special relativity is in the idea that the speed of light may be different in both arms due to an "aether wind". Of course there is no "aether wind". Lets ignore that part of this experiment for now.

The individual photon is in both paths and it is capable of interfering with itself. This interference occurs at the beam splitter (on the way through)from the laser. Maybe some more interference will occur on the way back, especially if the geometry changes. The result of that interference at the beam splitter is witnessed on the screen when any of the photons get there as a fringe. The exact fringing is defined by the geometry (length of arms in the tube)... the packet contains spatial interference fringes (alternate maxima and minima) that are "fixed in space" as the packet moves bye through the instrument (see idea 4b above) with its "clock stopped" exhibiting only "spatial phase". It just so happens that all the photons have this one monotonous pattern regardless of "which way" they travel. The one packet actually simultaneously travels to both ends of the instrument before it makes another "pass" of the beam splitter. I am not worried that the wavefront passes the same point in space twice, and at different times, because of the action of "folding" due to the mirrors at the ends.

Is that what you mean... The wavefront of a single photon may "spread out" over a large portion of the instrument but that does not affect the packet in any material way since the energy processes are "conservative". With a single photon the absorption of the photon will decide what will happen to the final "measurement"... if this occurs on the screen then you will see a flash, if not an absorbed photon elsewhere will not play any part in the "observed" experiment.

The action of the beam splitter is the interesting part of the experiment and that is where all the interference actually occurs for all the photons. Regardless of the interpretation you place on it a photon as a particle may go one way or another but you cannot know which. The interference pattern indicates the "real" aspect of the experiment as it propagates as a wave and the particle aspect can only be tested through "destruction" interaction at the screen. If you determined "which way" by some means this photon "particle" actually passed you will no longer produce an interference fringe for that photon (would not be able to assist in building up a "fringe picture" on the screen over time). I would strenuously object to the possibility that multiple photons are able to interfere with each other in different boson states (from different 1/2 (split) phase wavelength wavefronts) or even "between" each other. The photons in the one wavefront are all bosons traveling together and if there is one or a million they all behave identically and share some boson properties with each other. They do not share any properties with photons on any other wavefront at any other time. I stress again... They are only interfering with themselves... individually. They may be "bounced around a bit" but each photon stays on the one wavefront they were "born" on and only "consorts" with itself.

Is this what you believe?... or not! Test of "character" here... do we Believe it or not? I for one believe it!

Cheers

Hi Good Elf,

The main point of introducing this experiment was to examine the nature of a 'wavefront' and how it applies in the case of a single photon. Clearly if the wavefront were an expanding shell then we would only see interference (of a single photon) if the 'legs' of the MM experiment were (almost) precisely equal. If the wavefront is not a shell that expands with time at 'c' (and to me it doesn't seem to be) .. then I don't think it's a wavefront... it is a 'something else'. I am not convinced that your explanation has fully encapsulated just how much of a 'something else' a photon really is.

My own rather painful analysis using Feynman's graphical 'sum over paths' method (+) does not allow me to make any prediction about interference until the paths actually arrive at the screen .. I don't see anything happening in the beam splitter except we establish two paths with a probability of 50-50. I hope you agree that the alternative paths bear a striking similarity to the double slit experiment but without the complication of entanglement.

On a good day my 'method' will predict where a photon is most likely to hit the screen and where interference will occur.. can your explanation do the same?

-C2.

(+) Feynman's graphical 'sum over paths' method is described in his book 'QED' .. I will describe it if anyone isn't already familiar with it.

This post has been edited by Confused2 on Oct 16 2006, 10:59 AM

Hi Confused2,

The individual photon is indeed an expanding shell traveling at the speed of light but the geometry of the space can be altered by the position of all the objects within that space even out to infinity (according to Feynman's interpretation).. The actual picture is fully three dimensional but side lobes are suppressed. If it was possible to "track" that photon "core" around the place, that is where it will finally end up, but do not be too sure certain arrangements of "harmonic" reflective surfaces can result in the position of the photon being found in a quite unusual place through phase cancellation and reinforcement in periodic ways. The "shell" of a single wavefront "could" extend way beyond the instrument and far out into the Universe "exploring" all possible paths. Obviously a pencil thin laser beam has suppressed side lobes and has significantly reduced the possibility of finding the photon in some "remote locale" but the possibility does exist and though the probability is low there will be finite possibilities that some stray photons will go somewhere "interesting" far beyond the "ray" interpretation and related more to resonances of the space the photon has found itself in.

These photons will still "explore all paths". This cannot be seen but nevertheless is happening. Obviously the "core" which I have formerly described as "an eye of a hurricane" really does not have any true "substance" to it and it is not substantially any different to the rest of the photon.

Feynman construction is really the way it occurs and it shows how phase is the deciding factor. This is not the way it is usually taught. I have indicated the software and it is easy enough for people to test for themselves how these things work. There are few simplifications other than reduction to two dimensions in the example given. The actual construction actually does account for the third dimension in a realistic way since this "model" projects onto this plane and it just so happens that if you do this the result is as given. The constructions do not allow for mirrors but everything has limitations. You cannot do that construction with this simple program you can only model "expanding shells" not "re-entrant" ones. As I said stay with the simple case of Young's Double Slit Experiment and all is well or be prepared for the full analysis.

Naturally photons can appear to behave like "rays" under ideal conditions but beware of this simplistic reduction into absurdity. Light is not made up of "rays". This is what a non-collimated source will do...


http://www.tpub.com/content/neets/14182/css/14182_186.htm
This radiation pattern is not what we observe in most cases in the case of a laser pencil the "lobes" are suppressed. Please note that in the plane perpendicular to this view we would note that most transmitters will develop "nodes" that are periodic in the solenoidal direction. Here is a typical "directed" radiation pattern from an antenna but it could just as easily be a laser pencil....

This represents is a two dimensional pattern of an actual three dimensional pattern with "lobes" and all. Naturally for a laser pencil the "reverse" lobe is also very much attenuated by suppressing this direction with a strongly reflecting mirror. The small side lobes are "always" there and cannot be ignored in the greater picture. Have a look at these radiation patterns which relate to other kinds of "confined" radiation based on a sphere and cavity...
Spherical Harmonics 1 : Wolfram Research
Spherical Harmonics 2 : Wolfram Research

These are the same solutions we have for atomic orbitals or even idealized spherical resonant "chambers" or cavities. To see dynamic versions of these radiation patterns see this transition from one atomic eigen state to another...
Superposition state of the hydrogen
This involves loading and running a short quicktime movie animation of a transition between two states in a "confined space"... an atomic "cavity". The pretty colors represent the complex numbers found in electromagnetic theory. They also represent the complex state of some of the vectors in time in Feynman Constructions. Of course "probability" are just "projections" of these dynamic entities onto an "orthogonal plane" and then squared.

Cheers

Hi Good Elf,

Sorry to keep coming back to this point .. it remains unclear.
Looking at the diagram here (lower part of page) -
http://hyperphysics.phy-astr.gsu.edu/hbase.../michel.html#c2

We see that we can vary either Lf or Lm. We could make Lf = 1000 metres and Lm = 1 metre.
To save messing about I propose making the the total length of the Lf path to be 1000 metre and the total length of the Lm path to be 1 metre.

Assuming we have a laser light source..

A classical analysis..

We turn our laser on ..
After 1/c seconds the light from the Lm path arrives at the screen .. no interference
After 1000/c seconds the light from the Lf path arrives .. we have interference.

-----------------

Not so classical..

While all other paths may be possible it would be helpful if we consider the most probable path for a moment or two..

The beam is split in two by the half-silvered mirror .. 'half' each way.


Since we know a photon can only interfere with itself .. are we not left wondering how 'half' of it takes 1000/c to arrive at the screen and the other 'half' takes 1/c to arrive?

-C2.

Maybe I should confess that the answer I am fishing for is that that QM only predicts the probability of detection .. the classical notion of 'interference' may not be adequate to address this point.
-C2.

Hi C2, GE, Y et all,
I won't claim to know exactly what I'm talking about, but in QED and the sum of all paths, to me it doesn't seem to matter the length the photon travels but more the 'rotations of the clock' for whether or not you will see interference. So this 1/c and 1000/c only makes sense to me if were talking lambda. No?
Thanks,
Ron

Hi Ron,

Yes.. that's my impression too.. divide the path length by lambda and chuck away the integer part .. even though the path lengths are wildly different all we need is frac(lambda_1) and frac(lambda_2) .. not terribly classical!

Thanks,

C2.

?
Phase for interference purposes = 2 * pi * frac(lambda_n)

This post has been edited by Confused2 on Oct 17 2006, 02:15 PM

Hi Confused2, Ron, Yquantum et al,

I suppose this is why we are discussing "Problem with the two slit experiment... Observing later". It is clear that what happens later "has already determined the outcome of self-interference". Light with its "stopped clock" is only describing a single instant in its time no matter how far it travels, or "when" it travels. Once it is in the cavity it already "knows"... instantly... that it reached the mirror at the end and interfered with itself ... if you like. The fringes are determined by the shape of the cavity and not due to a photon actually traveling up and down a path... As you have indicated this interpretation should not work. Those fringes have always existed there in that space at that time and they are "immobile". The photon and its wavefronts just paint it out as a totally "static picture" over time. From our point of view "foreknowing" the position of all the moving objects in the Universe and mapping this pattern out "once and for all time". The actual wavefront will twist and turn and wind all over the place, but will never need any "updating" due to something moving between when the photon was emitted and when it was absorbed.

This result can only occur if all parts of the wavefront represent a single instant in time (stopped clock). This is just "Special Relativity"... light travels at C and it never ages so where ever it goes in "our time" is a single instantaneous moment in the photons life.... when it "explored all paths" ... all the way out to the ends of our Universe (or whatever). Some "paths" will actually go there in the general case. Just because in the Delayed Choice Quantum Eraser Experiment one of the matched photons was destroyed in the distant past, the fate of the diffraction pattern determined in that past is actually being decided in the future... our future... not the photon's future which it obviously never has. This is not a debatable point it is the result of practical experimentation. This is no statistics either it happens "every time" with the entangled photons. It is not strictly a classic result in one sense since we are very parochial about how we measure time and the way we view causality but this is one case where how we measure time actually counts for a lot.

Cheers

PS: He he he... I couldn't resist waxing just a little "lyrical" at this point... When all the stars in the Universe are blinking out at the end of time, some wavefronts are just deciding then when and where a particular photon that was "destroyed" in the first few moments after the "Big Bang" met it's fate way way back then. This means that events in the far distant future are affecting the past... the Emitter Absorber Theory in action... Advanced waves propagate back in time to the beginning of the Universe to complete a "great circle of life"... Everything old is new again. Obviously more than just a couple of photons are "entangled".... everything is actually "entangled".

This post has been edited by Good Elf on Oct 17 2006, 03:20 PM

Good_Elf, Ron et al,

Sorry, I'm not convinced we've got the first bit sorted out yet ..

In the odd-legs Michaelson Morley experiment

1/ When we start.. Are we happy that we don't have to wait for the light to go all the way round the long leg before we can detect a photon.. and we still get interference?

2/ If we assume the most likely path is very much more likely than any other then our photons will turn up in a pattern dictated by the probability distribution (as revealed by the phase difference between the two 'main' paths). This answer should (I think) be the same for both the MM and the two slit experiment.

3/ Do we feel strong enough to consider why the photon seems to want to have a wavelength even though we think (or do we?) that no time is passing for the photon travelling at 'c' ?

4/ Even this photons travel at 'c' thing is starting to look like a dodgy assumption.. any takers?

-C2.

Would anyone prefer me to start a new thread for the odd legs MM? I suspect we would have to try to unravel the whole of QED (and QM?) to get a full answer to the question..
-C2.

Hi C2,
I've been on other threads recently and, with Zephir pushing his AWT, you might lose sight of what your intentions are pretty quickly!
Also, if your talking about the $150 version (which is pretty cool btw), you're going to run into noise issues that have to be accounted for very carefully (ie:vector addition and subtraction of problems arising from rotation of the Earth at all hour of the day). I'm sure your familiar with some of the non-null proposals that have been made over the years, which mostly have to due with error correction, so, while it would be a fun exercise, I doubt we'd get any real insight. But, if you'd like, I'd really suggest titling it something more generic than MME or the like.
Just thinking out loud,
Take care,
Ron