1 "Kyle Taylor" |
Is Particle/Wave Riddle solved ? | dinsdag 12 augustus 2003 11:55 |

2 |
Re: Is Particle/Wave Riddle solved ? | maandag 18 augustus 2003 22:35 |

3 "Uncle Al" |
Re: Is Particle/Wave Riddle solved ? | dinsdag 19 augustus 2003 2:40 |

4 "Jeffery" |
Re: Is Particle/Wave Riddle solved ? | woensdag 20 augustus 2003 19:28 |

5 "Arkadiusz Jadczyk" |
Re: Is Particle/Wave Riddle solved ? | woensdag 20 augustus 2003 19:31 |

6 "Doug Sweetser" |
Re: Is Particle/Wave Riddle solved ? | zaterdag 23 augustus 2003 8:50 |

7 "Hendrik van Hees" |
Re: Is Particle/Wave Riddle solved ? | dinsdag 26 augustus 2003 23:11 |

8 "Nicolaas Vroom" |
Re: Is Particle/Wave Riddle solved ? | vrijdag 12 september 2003 1:19 |

9 "Mike Mowbray" |
Re: Is Particle/Wave Riddle solved ? | vrijdag 12 september 2003 21:58 |

10 "Hendrik van Hees" |
Re: Is Particle/Wave Riddle solved ? | dinsdag 16 september 2003 2:22 |

11 "Nicolaas Vroom" |
Re: Is Particle/Wave Riddle solved ? | woensdag 17 september 2003 1:03 |

12 "Mike Mowbray" |
Re: Is Particle/Wave Riddle solved ? | donderdag 18 september 2003 23:45 |

13 "scerir" |
Re: Is Particle/Wave Riddle solved ? | woensdag 24 september 2003 6:31 |

14 "Lubos Motl" |
Re: Is Particle/Wave Riddle solved ? | vrijdag 26 september 2003 3:21 |

15 "Ed Keane III" |
Re: Is Particle/Wave Riddle solved ? | vrijdag 26 september 2003 23:59 |

16 "Nicolaas Vroom" |
Re: Is Particle/Wave Riddle solved ? | Maandag 15 september 2003 14:41:45 |

The last message was not posted for publication in sci.physics.research

Hi

I just read and learned about the dual slit experiment where a
mysterious wave function can cause the light to become particle or
wave (depending if there is measuring device). Now does anyone know
the origin of this mysterious wave function (that can cause quantum
interference) that can determine in advance if a measuring device is
used, much akin to time forwarding scan effect or sorta? Can anyone
give or point out a site where summary of the different theories of
why it occurs is given or share it here?

Thanks.

Kyle

> | I just read and learned about the dual slit experiment where a mysterious wave function can cause the light to become particle or wave (depending if there is measuring device). Now does anyone know the origin of this mysterious wave function (that can cause quantum interference) that can determine in advance if a measuring device is used, much akin to time forwarding scan effect or sorta? |

Nothing need be determined in advance. There are two cases, which I will describe in an approximate fashion:

1) the slits are passive, and do nothing but provide two allowed paths to the screen. Only the screen does measurement(s). Collapse occurs after the object has passed through the slits; the screen therefore collapses an object in a superposition of two possible paths, and so will show an interference pattern.

2) the slits are detectors. Here collapse occurs while the object passes through the slits; the screen therefore can only measure objects which have taken only one of the paths, as so the screen sees no interference pattern.

These objects are, at all times, described in part by a wavelike function. Some of these wavelike functions might be so localised in space that they seem particle-like. If the wavelike function also corresponds to a single quantum excitation, then you might imagine some scheme for counting the object in the same way as counting marbles.

"Particle" usually implies two criteria: localised in space, and countable.

-- The wavelike functions associated with an object might be well localised in space, sufficiently satisfying the first "particle" criteria for many purposes.

-- the excitation associated with an object might be countable. This is satisfied for fermions (e.g. an electron), because the excitation will be either Zero (no object) or One (an object). This might be satisfied for bosons (e.g. photons), but it also might not be -- objects made of bosons might contain an indeterminate number of them.

-- ---------------------------------+--------------------------------- Dr. Paul Kinsler Blackett Laboratory (QOLS) (ph) +44-20-759-47520 (fax) 47714 Imperial College London, Dr.Paul.Kinsler@physics.org SW7 2BW, United Kingdom. http://www.qols.ph.ic.ac.uk/~kinsle/Public Key: http://www.qols.ph.ic.ac.uk/~kinsle/key/work-key-2002a

Aan:

> |
Hi
I just read and learned about the dual slit experiment where a
mysterious wave function can cause the light to become particle or
wave (depending if there is measuring device). Now does anyone know
the origin of this mysterious wave function (that can cause quantum
interference) that can determine in advance if a measuring device is
used, much akin to time forwarding scan effect or sorta? Can anyone
give or point out a site where summary of the different theories of
why it occurs is given or share it here? |

Google quantum double eraser 2220 hits

Single (classical) or double (non-classical) slit behavior obtains depending on how you look behind the slit. The effect (observed classical or non-classical behavior) then *precedes* the cause (classical or non-classical behavior at the slit). Double eraser experiments are particularly interesting. The universe does not tolerate contradictons.

Small stuff is both particulate (quantized) and wave-like (de Broglie). Big stuff (C60, C70) also scores,

http://www.quantum.univie.ac.at/research/c60/

It's rather cute to do molecules with a molecular weights of 720.7 and 840.8 and ponder what happens at the slit. The group wants to do small viruses next. Diffracting living things goes beyond physics to philosophy, wherein it will explode into a voluminous heavy vacuum.

There are molecules like semi-bullvalene that rapidly rearrange to regenerate their structure with atoms scrambled,

http://faculty.juniata.edu/reingold/rsch.html

Semibullvalene is particularly rapid, a real speed demon. Imagine diffracting semibullvalene through a multiple slit formed of alternating Peltier junctions so the temps of adjacent slits can be substantially different. Since rearrangement kinetics are an exponential of temp, and semibullvalene is so fast one can get at least one full reaarrangement during slit traversal time, could one phase the diffraction pattern by altering differential slit temps?

-- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) "Quis custodiet ipsos custodes?" The Net!

Aan:

kyletaylorny@yahoo.com (Kyle Taylor) wrote in message news:
I just read and learned about the dual slit experiment where a
mysterious wave function can cause the light to become particle or
wave (depending if there is measuring device). Now does anyone know
the origin of this mysterious wave function (that can cause quantum
interference) that can determine in advance if a measuring device is
used, much akin to time forwarding scan effect or sorta? Can anyone
give or point out a site where summary of the different theories of
why it occurs is given or share it here? Thanks.
Kyle

>
Hi

There is no riddle and there is nothing mysterious about it. The problem is that people try to force the subatomic world to be similar to the macroscopic of daily life, and in reality it is totally different. Under some circumstance a particle such as photon or electron can be most closely mathematically modeled as a particle, and under other circumstances, it can be most closely mathematically modeled as a wave, but in reality it's totally different from either a macroscopic particle or macroscopic wave. People say it's both a wave and particle, but really it's neither a wave nor a particle. It's a subatomic entity totally unlike anything in the macroscopic environment. Nothing can "can cause the light to become particle or wave" because really it's neither a wave nor a particle, and it's doesn't change from one thing to another thing. You can put it in a situation where the closet mathematical model will be to approximate it as a particle, and then you can put it in another situation where the best mathematical model will be to approximate it as a wave. Of course, in reality, it's not like anything you've ever seen in the macroscopic environment. This does not make it mysterious. It's just different than what you're use to. There is nothing mysterious about it. The whole idea of particle-wave duality comes from clinging to a classical view of the world. Feynman referred to this by saying "Your old-fashioned ideas are no damn good!"

Jeffery Winkler

Aan:

On 12 Aug 2003 06:55:30 -0400, kyletaylorny@yahoo.com (Kyle Taylor) wrote:

> | Now does anyone know the origin of this mysterious wave function (that can cause quantum interference) that can determine in advance if a measuring device is used, |

The wave function does not have to know anything "in advance". When it meets a detector, it starts to behave differently (its norm decreases, because the detector is a "sink"). When the norm decreases "enough" - then something may "happen" - for instance the wave function "collapses" (that is is being operated upon by an "operator"), and detector "clicks". But it may also happen that detector does not click, and the wave function travels to the next detector.

The devil is in the details.

ark P.S. What I wrote above is one scenario - one that I like and one that I know. Other physicists will offer different scenarios. To see how wave function collapses and to listen to detector clicks you can play with Java applet here:

http://www.cassiopaea.org/quantum_future/applets/eeqt.html

ark

-- Arkadiusz Jadczyk http://www.cassiopaea.org/quantum_future/homepage.htm --

Your note reminds me of one of my pet peeves about explanations of quantum mechanics: that measuring devices _do_ something to a system. Measuring systems collect information about a system. In the case of the dual slit experiments, what is being studied is a coherent, quantum source. If you understand what a coherent source of quantum events happens to be, then you will understand how different ways of looking at a coherent source change what one can see about that source.

Looking at the world with one eye closed is different than with both eyes open, but the viewer does not change the world. This topic has been discussed in SPR recently. An advance groups search in Google should find the thread. I argued there that it is the properties of the complex numbers that give rise to the behavior seen for a coherent source viewed with two slits (even has an ASCII picture).

doug

quaternions.com

Aan:

> |

I fear you read a misleading book. Unfortunately, the most popular books about quantum theory use an old predecessor version of the now established quantum theory, namely the so called "wave-particle dualism theory" by Einstein and de Broglie, developed from 1900 (Planck's radiation formula) over Einstein's famous article from 1905 about light quanta to de Broglie's thesis about "matter waves" (if I remember right, that was 1923).

Nowadays we have a much more clear concept at hand, namely quantum theory which was found 1925 by Heisenberg, Born and Jordan in its "matrix mechanics" version and 1926 by Schrödinger in it's "wave mechanics" version.

The best formulation of the quantum theory (QT) is that by Dirac (1926-1927) which is independent from special representations. It is an abstract mathematical formalism which, as far as we know, enables us to describe the behaviour of nature, from the smallest known entities ("elementary particles") to the bulk matter surrounding us. In other words: Today, there are no experimental evidences that the QT might be wrong.

The price we have to pay for this success is an abstract mathematical picture of the world, at least compared to Newtonian mechanics. Nevertheless the physical concept underlying QT is not that difficult: The QT only takes into account the fact that all we can learn about objects (say the possition of an electron) is due to measurements. For these measurements we have to make the electron interacting with the measurement apparatus, such that we can read off the position of the electron from it. For instance you may think about an detector which is placed on a certain position and which registers the electron.

So far this is not different from classical physics, but now we must take into account the observation that matter appears to be "atomistic". Especially the electric charge is always an integer number of a smallest charge, the charge of a proton (for positively charged matter) or an electron (for negatively charged matter). NB: The charge of an electron is exactly the negative of that of a proton.

The interaction of the electron with the measurement device, necessary to determine its positition, is (mainly) due to electromagnetic interactions. Thus to have this interaction for position measurement we can use light (electromagnetic waves). To create this light we need other moving charges which are at least as strong as the electron's charge itself since there are no smaller (free) charges in nature.

Further the wavelength of the light, used to determine the position of the electron, must be at least of the same order of magnitude as the position resolution we want to have for the electron. On the other hand Maxwell's theory of electromagnetism tells us that light carries a momentum which is the larger the smaller its wavelength is. The interaction of the light with the electron thus gives the electron a momentum which is the larger the more precise we like to know its position. It is impossible to determine this momentum transfer to the electron, i.e., after the (precise) measurement of the electron's position we know very little about its momentum.

One can also think about the measurement of the momentum of the electron. It comes out that the more precise we like to determine the momentum of the particle the less precise we know its position.

This example of the Heisenberg uncertainty relation (Delta x Delta p>\hbar/2) shows that, due to the atomistic nature of matter, that it is impossible to make the disturbance of the measured object by the necessary interaction with the measurement apparatus, arbitrarily small. This means that not all possible observables of an object are sharply determined the same time. For instance, as we have argued above, it makes no sense to say an electron (or any other object) has the same time a precisely determined position and momentum. Only one of those "incompatible" observables can be determined precisely. The other observable is then necessarily undetermined.

The QT describes precisely the outcome of experiments in the "atomistic" world and what we can say about observables of objects which are not determined precisely, because another observable, which is incompatible with it, is measured precisely.

Finally we look on the double slit experiment. First we have to get clear, how this experiment is done: Let's assume we put the double slit somewhere and shine on it with laser light. Then one findes an interference pattern on the wall opposite to the slits.

Now we look on this experiment from the point of view of quantum theory: The laser sends out an electromagnetic wave with a precisely determined frequency. From the point of view of quantum theory electromagnetic waves are described by light quanta, i.e., if we dim the laser light as much as we can, we have only one light quantum coming out of it. This light quantum (also called photon) has a precisely determined momentum. As we discussed above for the electron, its position is completely unknown. Thus it is impossible to know through which of the slits it will go. Neither do we know where on the wall it will appear. The only thing what we can calculate from the principles of quantum theory is the probability to find the photon on a certain position on the wall.

To test this prediction of quantum theory, we simply have to do our one-photon experiment a large number of times and to count, how many photons appear on a certain position on the wall. It comes out that we obtain exactly the same interference pattern as appears due to Maxwell's classical theory of light.

The interesting thing is know, what happens if we look at the double slit through which of the slits the photons come. For this we have to detect the photons at the slits. Quantum theory tells us that the more precise we like to know through which slit the photons go, the less sharp is the contrast of the interference pattern.

To give finally a short answer: The modern quantum theory describes nature in a consistent way. No wave-particle dualism is needed, and physical entities like "elementary particles" or "electromagnetic waves" are neither particles or waves in the classical sense but described by an abstract mathematical formalism as "quanta".

For a good introduction read the first chapter of

J. Schwinger, Quantum Mechanics, an formalism for atomistic measurements, Springer

It does not use any mathematics, but is a precise description why quantum theory is necessarily as it is, because matter appears to be "atomistic". The further chapters of the book then develop the mathematics of quantum theory from these physical considerations. It's not an easy book, but a very good one for physicists who already learnt quantum theory in the introductury lecture.

-- Hendrik van Hees Fakultät für Physik Phone: +49 521/106-6221 Universität Bielefeld Fax: +49 521/106-2961 Universitätsstraße 25 http://theory.gsi.de/~vanhees/ D-33615 Bielefeld

Aan: "physics_research"

> | Now we look on this experiment from the point of view of quantum theory: The laser sends out an electromagnetic wave with a precisely determined frequency. From the point of view of quantum theory electromagnetic waves are described by light quanta, i.e., if we dim the laser light as much as we can, we have only one light quantum coming out of it. This light quantum (also called photon) has a precisely determined momentum. As we discussed above for the electron, its position is completely unknown. Thus it is impossible to know through which of the slits it will go. |

The question is:
is it allowed to say that each photon goes through both slits ?

And:
is it allowed to say that each photon interferes with it self ?

> | Neither do we know where on the wall it will appear. The only thing what we can calculate from the principles of quantum theory is the probability to find the photon on a certain position on the wall. |

Sorry a lot of good stuff deleted.

> Hendrik van Hees wrote:

>> Now we look on this experiment from the point of view of quantum >> theory: The laser sends out an electromagnetic wave with a >> precisely determined frequency. From the point of view of >> quantum theory electromagnetic waves are described by light >> quanta, i.e., if we dim the laser light as much as we can, we >> have only one light quantum coming out of it. This light quantum >> (also called photon) has a precisely determined momentum. As >> we discussed above for the electron, its position is completely >> unknown. Thus it is impossible to know through which of the >> slits it will go.

> The question is: > is it allowed to say that each photon goes through both slits?

Not really. If we model a laser beam as a coherent state, the photon number of the state is indeterminate. When people talk about the number of photons in a coherent state, they usually mean implicitly some kind of time-average, e.g: how many photons per second on average (i.e: after integrating over a long time).

Descriptions of so-called "single-photon-at-a-time" experiments can be a bit naive/misleading. Diming the laser light just means that the expectation value of photon/sec is decreased - to the point where the human eye+brain can resolve individual flashes on a detector screen. But the photon number of the (dimmed) coherent state nevertheless remains indeterminate.

Therefore, it doesn't really make sense to speak of "each photon".

Also, phrases like "goes through a slit" don't really make much sense in a QM context. The term "goes through" implies a notion of translation and hence momentum, while "a slit" implies a notion of position. But we know from QM that we cannot meaningfully attribute exact properties of momentum and position to a quantum state simultaneously.

> And: > is it allowed to say that each photon interferes with it self?

It's better to think of the double-slit as a *filter*, which takes one QM state and gives you another. I.e: it's like an operator on the Hilbert space. In this case, it's convolving two (approx) delta functions of position with the original state, to yield a different state on the other side of the double slits.

In contrast, a detector is like a mapping from a QM state to a number, i.e: you give it a state and it gives you back a number, representing position in this case. So filters and detectors are very different things.

BTW, I really like the comments of Willis Lamb (Nobel Prize winner for Lamb shift) regarding QM and photons. (Thanks to Arnold Neumaier for the link). As these sorts of issues come up repeatedly on spr, I append a copy of Lamb's comments below, FYI.

- MikeM.

/Begin comments by Willis E. Lamb from http://www.aro.army.mil/phys/proceed.html

[...] Over a period of over fifty years, I have come to a number of conclusions about quantum mechanics. I will enumerate some of them as eight numbered statements:

1. Anyone wanting to discuss a quantum mechanical problem had better understand and learn to apply quantum mechanics to that problem. Quantum mechanics is able to provide answers to various kinds of questions. At the most primitive level, a system is a well defined and highly isolated dynamical entity with a certain number of degrees of freedom. Initially it is in a state described by a known wave function. There are methods for dealing with the case in which the state is not so well known, but there is a price to be paid for ignorance. All disturbances of the system have to be considered. Measurements not only involve disturbances, but add to the number of degrees of freedom of the combined system plus measurement system.

2. To talk of the "quantum theory of the universe" is to make a bad play on words.

3. Reliance on Bohr's Correspondence Principle, Heisenberg's Uncertainty Principle or Bohr's Principle of Complementarity, or Wave Particle-Duality are sure signs of misunderstanding of quantum mechanics.

4. Commonly held notions about wave function collapse or wave packet reduction have no validity whatsoever.

5. The ideas of Dirac on the measurement of observables were very naive. Von Neumann had similar ideas, as expressed in Chapter III of his 1932 book. Neither of them gave any indication of how a measurement was to be made. Furthermore, any such measurements would be very disruptive and the results would be of no use for later studies of the system of interest. In Chapter VI, on the last two pages of his book, Von Neumann did outline a model for a meter for making position measurements, but he never made any use of it. The next reference to this work came only 33 years later in a short note of Arthurs and Kelly.

6. A discussion of measurements on a quantum mechanical system has to be based on a realistic quantum mechanical analysis of that system in interaction with another system (a meter of some kind used for the measurable). It is also necessary to have a good understanding of the borderland between quantum and classical physics.

7. Quantum mechanics is a theory dealing with probabilities, and there is no way around that fact.

8. There is no such thing as a photon. There is a quantum theory of radiation, and conservation laws for energy, momentum and angular momentum are built into it. Only in very simple special cases, hard to realize in practice does it make sense to talk about photons.

I have written a number of papers on the interpretation of quantum mechanics and the theory of radiative processes. Two of the most recent of these are called "Suppose Newton had discovered wave mechanics" in American Journal of Physics, March, 1984, and "Anti-Photon", to be published in Applied Physics B. [...]

[...] I have strong doubts that merely attenuating incoherent pulses will entitle one to think reliably of "one photon" states. This is spelled out in my Anti-photon" paper.

/end Willis Lamb comments.

Aan:

> |
The question is:
is it allowed to say that each photon goes through both slits ? And: is it allowed to say that each photon interferes with it self ? |

I'd not express it in this way. When learning quantum theory, it is important, not to think about things in terms of classical concepts. So you should not think about a photon as a classical particle or a classical wave, but as a quantum. As I explained in my previous posting, in the here considered double-slit experiment, we have prepared photons with a certain momentum (which, of course, is an idealisation, since we can do this only approximatively, because each em. wave has a finite line width). Then it does not make sense to speak of a photon as a classical particle which has a certain position. So we should forget about position at all.

Thus, we have the following picture about the photon: All we can say about the the photon is, what is described by the quantum state. In our case, we know precisely its momentum. Due to Heisenberg's uncertainty principle this excludes necessarily a precise knowledge about its position. The quantum state gives only probability distributions, where to find the photon. This probability distribution can be calculated with help of the quantum theoretical dynamics, where the slit is modelled by boundary conditions (which, of course, is an idealisation again, because it's not a completely microskopical description of the slits, which is impossible, because they consist of macroscopic matter, so that we can make the approximation and treat it as boundary conditions).

Now, we have a prediction about the behaviour of the position of the photon in the double-slit experiment, namely a probability distribution for the place, where the photon will leave its track on the wall, but we have no more information and, due to quantum theory, we cannot have any more.

Thus, to test the prediction from quantum theory, we need to check, whether the probality distribution is the right one. This can only be done by repeating the experiment a lot of times. "Repeating" here means that one has to prepare a lot of photons, which have to be independent of each other, and do the double-slit experiment with them. Then we can count, how many photons appear in a certain region of the wall, and this should give the predicted probability distribution.

Since physics is about measureable facts about objects, this means that
quantum theory describes ensembles of independently prepared systems.
About each individual systems, we precisely know only those
observables, we have prepared in the preparation procedure. The
preparation procedure means to assign a certain quantum state,
described by a ray in the Hilbert space of the system (or,
equivalently, a operator of the form |psi>

First of all, quantum theory tells us, which observables are compatible, i.e., which observables can be determined precisely at the same time: Each observable is described by a self-adjoint operator in Hilbert space, and the possible outcomes of measurements is given by the spectrum of this operator. Tow observables are compatible if and only if the associated operators commute. A set of compatible operators is a set of pairwise commuting operators, describing observables such that, if you determine the values of these observables precisely, the system is prepared in a pure quantum state, i.e., the common (generalised) eigenvector is unique.

To make the things easier, we look only on systems that are prepared in such a pure quantum state.

Now, if we have prepared the system in such a pure quantum state, at time t0, and if we know the Hamilton operator of the system precisely, we can predict its state for any later time. So the state of the system is precisely determined at any time, but it contains only information about the probability of the outcome of further measurements, not more (nor less). An observable has a determined value if and only if the state is an eigenvector of the associated operator. If it is not an eigenvector, we know only the probabilities to measure a certain value of the observable, which has to be an eigenvalue of the associated operator.

What we have to learn, and admittedly it is very hard to keep this in mind, is to forget our daily experience with objects in the macroscopic world, where it seems clear that any observable has a certain value. In the quantum world this concept doesn't make any sense.

It is another very interesting question, why our daily experience is that of classical physics, when we believe that the underlying natural laws are quantum laws. The answer is, what is called decoherence, which explains why we never see interference patterns, entanglement for "Schrödinger's cat". A very nice introduction in this topic is

http://arxiv.org/abs/quant-ph/9803052

-- Hendrik van Hees Fakultät für Physik Phone: +49 521/106-6221 Universität Bielefeld Fax: +49 521/106-2961 Universitätsstraße 25 http://theory.gsi.de/~vanhees/ D-33615 Bielefeld

Aan: "physics_research"

> |
Nicolaas Vroom wrote: > The question is: > is it allowed to say that each photon goes through both slits? Descriptions of so-called "single-photon-at-a-time" experiments can be a bit naive/misleading. Diming the laser light just means that the expectation value of photon/sec is decreased - to the point where the human eye+brain can resolve individual flashes on a detector screen. But the photon number of the (dimmed) coherent state nevertheless remains indeterminate. |

For single photon experiments See:

http://ophelia.princeton.edu/~page/single_photon.html

http://www.physics.brown.edu/Studies/Demo/modern/demo/7a5520.htm

old url http://www.optica.tn.tudelft.nl/Education/photons.htm

new url http://www.optica.tn.tudelft.nl/education/photons.asp

> | Therefore, it doesn't really make sense to speak of "each photon". |

The above references speak for them selves.

> | Also, phrases like "goes through a slit" don't really make much sense in a QM context. |

The question is, if you want to explain the " double slit experiment with single photons", if it is allowed to say: that each photon goes through both slits.

> |
The term "goes through" implies a notion
of translation and hence momentum, while "a slit" implies a
notion of position. But we know from QM that we cannot
meaningfully attribute exact properties of momentum and position
to a quantum state simultaneously.
> And: > is it allowed to say that each photon interferes with it self? It's better to think of the double-slit as a *filter*, which takes one QM state and gives you another. |

May be that is true but I think that my proposed interpretation is much simpler. The question is if that is true.

> | 3. Reliance on Bohr's Correspondence Principle, Heisenberg's Uncertainty Principle or Bohr's Principle of Complementarity, or Wave Particle-Duality are sure signs of misunderstanding of quantum mechanics. |

The last part is too strong wording What are the alternatives ?

> | 7. Quantum mechanics is a theory dealing with probabilities, and there is no way around that fact. |

Too general wording.
Probabilities of what ?

Physics deals with theories and experiments.
Probabilities deal with mathematical interpretations of the
results of those experiments, specific the accuracies involved.

Nick.

>> is it allowed to say that each photon goes through both slits?

I answered:

>> Descriptions of so-called "single-photon-at-a-time" experiments >> can be a bit naive/misleading. Diming the laser light just means >> that the expectation value of photon/sec is decreased - to the >> point where the human eye+brain can resolve individual flashes >> on a detector screen. But the photon number of the (dimmed) >> coherent state nevertheless remains indeterminate.

Nicolaas Vroom replied:

> For single photon experiments See:

> http://ophelia.princeton.edu/~page/single_photon.html

> http://www.physics.brown.edu/Studies/Demo/modern/demo/7a5520.htm

> old url http://www.optica.tn.tudelft.nl/Education/photons.htm

> new url http://www.optica.tn.tudelft.nl/education/photons.asp

These were indeed the references I had in mind when I used the phrase "a bit naive/misleading". They ignore the point that photon number of a coherent state remains indeterminate, no matter how much you attentuate it.

>> Also, phrases like "goes through a slit" don't really >> make much sense in a QM context.

> The question is, if you want to explain the "double slit > experiment with single photons", if it is allowed to say: > that each photon goes through both slits.

Well, I suppose it's "allowed" if you're happy to phrase questions in meaningless ways, or using expressions that are subtly incorrect, or reliant on misconceptions, or which ignore the QM fact of position-momentum uncertainty.

Unfortunately, I must decline to respond to the rest of your comments on Lamb's remarks, since I was merely reproducing them here on spr for general interest.

- MikeM.

Aan:

> |
The question is: is it allowed to say that each photon goes
through both slits? And: is it allowed to say that each photon interferes with it self? |

Can I add: is it allowed to say that each photon goes through one slit, and then - coming back, in time - also through the other slit? s.

On Wed, 24 Sep 2003, scerir wrote:

> | Nick Vroom wrote: |

> > |
The question is: is it allowed to say that each photon goes through
both slits? And: is it allowed to say that each photon interferes with it self? |

Yes, everyone is allowed to say so, because of the freedom of speech. The difference between the Soviet Union and the USA was that in the people in the USA also enjoyed the freedom *after* the speech. ;-)

Both answers are essentially yes, as long as one realizes that the "thing" that goes through both slits and interferes with "itself" is not quite the photon itself but rather its wavefunction whose interpretation requires us to talk about the probabilities.

> | Can I add: is it allowed to say that each photon goes through one slit, and then - coming back, in time - also through the other slit? |

The usual interference pattern is certainly not a result of a complicated motion of the photon that would return and vibrate back and forth. One must really consider the inteference of the two direct paths through two slits.

______________________________________________________________________________ E-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/ phone: work: +1-617/496-8199 home: +1-617/868-4487 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Superstring/M-theory is the language in which God wrote the world.

Lubos Motl

> | On Wed, 24 Sep 2003, scerir wrote: |

> > |
Nick Vroom wrote: |

> > > |
The question is: is it allowed to say that each photon goes through both slits? And: is it allowed to say that each photon interferes with it self? |

> |
Yes, everyone is allowed to say so, because of the freedom of speech. The difference between the Soviet Union and the USA was that in the people in the USA also enjoyed the freedom *after* the speech. ;-) Both answers are essentially yes, as long as one realizes that the "thing" that goes through both slits and interferes with "itself" is not quite the photon itself but rather its wavefunction whose interpretation requires us to talk about the probabilities. |

Do you think it is better not to talk of radiation traveling through space when discussing QM? One could talk about a photon going from A to B or one could talk about a photon that goes from A to a point C that is between A and B. One would not talk about intercepting a photon going from A to B at C.

Do you think that such descriptions have any potential to do more than satisfy some people more than others? Some description might lead one person to a better understanding of the application of QM but could different descriptions, or theories, be analyzed to the point where they disagree on some actual experimental outcome?

-Ed keane III

keane at westelcom.com

Aan: "physics_research"

> |
The question is:
is it allowed to say that each photon goes through both slits ? And: is it allowed to say that each photon interferes with it self ? |

To validate this question you should perform the two slit experiment
with single photons in the following 4 configurations.
For each configuration the DURATION of the experiment is the SAME.
There is no difference in photon emitter.
There is no difference in CCD screen.
The only difference is in slit arrangement.

1. Both slits are open. Duration is roughly 20 hits.

2. Left slit is open.

3. Right slit is open.

4. The whole space between both slits is open.

In fact you create a hole. The purpose is to count all photons.

IMO you can answer the above question affirmative if the number of counts in each case is approx. the same.

For single photon experiments See:

http://ophelia.princeton.edu/~page/single_photon.html

http://www.physics.brown.edu/Studies/Demo/modern/demo/7a5520.htm

old url http://www.optica.tn.tudelft.nl/Education/photons.htm

new url http://www.optica.tn.tudelft.nl/education/photons.asp

Created: 26 September 2001

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