Quantum double slit experiment

[Joseph14]

Is there anything inaccurate information portrayed in this video?

Um this clip is from "What the Bleep Do We Know" http://en.wikipedia.org/wiki/What_the_Bleep_Do_We_Know!? which is in general viewed as pseudoscience.

I don't like the way they show particles splitting in two and the observer eyeball thing is ridiculous.

 

[arkajad]

You also mention "probability amplitudes" interfering with each other to produce the photon wave effects. But "probability amplitudes" usually refers to the Schrodinger wave equation and you cannot write such an equation for a photon since it has zero rest mass http://en.wikipedia.org/wiki/Photon. If photon behavior was as easily solved as you indicate people would stop writing books about it (and they haven't).

Regards,

CaPhysics

You can write such an equation. See http://www.cft.edu.pl/~birula/publ/APPPwf.pdf" .

 

[Cthugha]

Maybe I am missing something here but you state that [in paraphrase] "we have zero evidence that photons split in two (subdivide)." But the post you are refuting says "When a single photon went through the beam splitter it was detected on one path or the other, never on both." So what exactly are you refuting?

I am refuting that so called single-photon interference experiments can be considered as a proof that photons indeed travel both paths in such an experiment. A more general treatment in terms of probability amplitudes (where the issue of what the photon does in between is completelly avoided) is sufficient to explain the experiments. We do not have any evidence for what exactly happens "in between".

You also mention "probability amplitudes" interfering with each other to produce the photon wave effects. But "probability amplitudes" usually refers to the Schrodinger wave equation and you cannot write such an equation for a photon since it has zero rest mass http://en.wikipedia.org/wiki/Photon. If photon behavior was as easily solved as you indicate people would stop writing books about it (and they haven't).

Quantum optics treats probability amplitudes more in a - well - Feynman-path-integral-like way. Take all possible emission and detection events leading to the same experimental results, add them and square them. Of course the missing mass of the photon needs to be considered. However, I feel that giving a long introduction into the details of that treatment is out of the scope of a forum discussion, but if you would like to know more details, you can find them in any introductory or advanced book on quantum optics (Mandel/Wolf, Meystre/Sargent or also in Schleich's book I think). A very short and intuitive, but of course not complete, approach is also given in Roy Glauber's Nobel speech and some of his later conference proceedings, for example in "Quantum optics and heavy ion physics".

 

[arkajad]

Quantum optics treats probability amplitudes more in a - well - Feynman-path-integral-like way.

Quantum optics has more than one face, for instance:

The Quantum Trajectory Approach to Problems in Quantum Optics

Principal Investigator Howard Carmichael
Co-Principal Investigator(s)
Recipient Organization University of Oregon Eugene

Summary
A new approach to the physics of open quantum systems emitting photons to the environment is developed using a quantum trajectories method. The averages from this ensemble of trajectories reproduce the results of conventional quantum mechanics. The approach has the advantage that it can simulate real time signals and throws new light on the problems of quantum measurement. Another strength of the trajectory approach is that it is a wavefunction based method, which avoids the problems of the more traditional density matrix techniques. The method will be applied to problems in cavity quantum electrodynamics and the interaction of atoms with nonclassical light. The ultimate hope is a self- consistent theory of quantum measurements in optics.

 

[Cthugha]

Indeed, I was just talking about the most mainstream and common treatment.

Do you know whether that Quantum trajectory approach thesis (I suppose it is a thesis of one of his students) is somehow connected to Carmichael's 1991 lecture note book "An open systems approach to quantum optics"? He already used quantum trajectories a lot in the final chapters of these notes.

 

[arkajad]

From his review for the conference in 1997:

Quantum Jumps Revisited: An Overview of Quantum Trajectory
Theory

H. J. Carmichael

Abstract: The quantum trajectory theory of photon scattering in quantum optics is reviewed. Two features of the theory which bear closely on issues of interpretation in quantum mechanics are emphasized: (1) there exist different unravellings of a scattering process which reveal complementary aspects of the dynamics in the interaction region, and (2) through the making of records via a stochastic implementation of a formalized quantum jump a self-consistent interface between a quantum evolution (in Hilbert space) and a classical evolution for the records (time series of real numbers) is achieved.

See also: H. M. Wiseman, http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/IOP/J-Opt-B/8-205.pdf"

Carmichael's actual interests can be seen from his http://www.physics.auckland.ac.nz/uoa/home/about/our-staff/professor-howard-carmichael/" .

 

[CaPhysics]

You can write such an equation. See http://www.cft.edu.pl/~birula/publ/APPPwf.pdf" .

OK, good find. But journals are many and articles accepted are many squared. Are you sure these represent mainstream opinion? And I note some qualifications in both articles.
The Feynman approach seems to be better founded.

 

[arkajad]

OK, good find. But journals are many and articles accepted are many squared. Are you sure these represent mainstream opinion?

"Mainstream opinion" is of no value if it is wrong - which, as history shows, sometimes happens. The only thing that counts is whether the paper is correct or wrong. Don't rely on someone's opinion or on referees - opinions are often biased. Always check it yourself.

But if you want to have another example, because, for instance, you do not trust Europe, here are some, on the page of http://oco.uoregon.edu/raymer-group/publiations" , for instance this:

"“Photon wave functions, wave-packet quantization of light, and coherence theory,” Brian J. Smith and M. G. Raymer, New J. Phys. 9, 414 (2007)"

You will find many more references there.

 

[bdavlin]

So is the act of human observation enough to alter the behavior of the particle? Or is it because we're using energy to observe it and therefore altering what is being observed?

 

[arkajad]

So is the act of human observation enough to alter the behavior of the particle? Or is it because we're using energy to observe it and therefore altering what is being observed?

It is the presence of the measuring devices that changes the evolution of the quantum state vector. Human observation is not important. It can animal observation or no observation at all. Just registration.

 

[A. Neumaier]

OK, good find. But journals are many and articles accepted are many squared. Are you sure these represent mainstream opinion? And I note some qualifications in both articles.
The Feynman approach seems to be better founded.

Birula's papers give the view that is consistent with experiment and matches what is done in quantum optics once you do it seriously. See also Chapter B2: Photons and Electrons of my theoretical physics FAQ at http://arnold-neumaier.at/physfaq/physics-faq.html#B2

 

[chris2112]

It is the presence of the measuring devices that changes the evolution of the quantum state vector. Human observation is not important. It can animal observation or no observation at all. Just registration.

So- just to clarify- someone could setup a detector to detect which slit the particles are going through, never check the data, and see no interference pattern?

 

[A. Neumaier]

So- just to clarify- someone could setup a detector to detect which slit the particles are going through, never check the data, and see no interference pattern?

No. The detector impairs the interference patterns, no matter whether anyone looks at the data it produces.

 

[morrobay]

No. The detector impairs the interference patterns, no matter whether anyone looks at the data it produces.

Is my understanding of your above statement correct :
That the double slit interference pattern is destroyed based on classical physics alone.
During the detection process using photons or other particles the electrons path is altered
causing the interference pattern to vanish for physical reasons only.
And that you are refuting the QM explanation that it is the act of observer knowledge on which
slit the electron passed that inhibits the interference pattern.
The reason I want to confirm this is because you seem to have an extensive
background in QM

 

[A. Neumaier]

Is my understanding of your above statement correct :
That the double slit interference pattern is destroyed based on classical physics alone.
During the detection process using photons or other particles the electrons path is altered causing the interference pattern to vanish for physical reasons only.
And that you are refuting the QM explanation that it is the act of observer knowledge on which slit the electron passed that inhibits the interference pattern.

No. I am refuting nothing, just giving the explanation a more precise, objective meaning.

The pattern is destroyed by the (quantum) interaction with a macroscopic detector. This is loosely called a measurement (or, even more loosely, an act of observation). But it requires no doer or seer, not even a recording, but just the presence of the interaction.

In general, as long as a quantum phenomenon or explanation looks weird to you, you can be sure that you haven't understood what's going on. Understanding drives out all weirdness.

 

[zketrouble]

Regretfully I find some problems in this post. First, "wave function collapse" is normally equivalent to "collapse of the state vector Ψ" and |ψ|2 gives you the probability of finding an electron in a specific location whereas a photon is subject to Maxwell's equations and the probability of a photon in a location is proportional to the square of the radiation energy density at that location. So I think this post has confused computational ψ waves with real-world radiation energy waves. Not the first time this has happened.

A second problem is the easy identification of photon with particle. There is scant evidence for the photon as particle aside from the lazy assumption that anything traversing space and terminating at a point must be a particle. As to why this "particle" should exhibit all sorts of wave behavior before its "particle" termination, many don't want to be bothered with that.

So the sensible answer as to why a photon can pass through both slits is that it is a wave and a wave, unlike a particle, can subdivide (and later rejoin and interfere).

The real question is why an electron can do the same thing. That is 1) tied up with the wave nature of the moving electron and 2) the subject of a different thread.

Regards,

PP

This doesn't take into account another important discovery of science: a photon can bounce into an electron and cause it to move out of its way. Thus, this wave has momentum, and for it to have momentum, by definition it has mass. A photon isn't "just a wave" nor "just a particle." Photons express wave-particle duality; they express both wave-like and particle characteristics. I think it is a little foolish for scientists to spend so much time assessing whether it should be called particle or a wave, understanding how it works is more important. And it seems to work in both ways so far.

 

[A. Neumaier]

This doesn't take into account another important discovery of science: a photon can bounce into an electron and cause it to move out of its way. Thus, this wave has momentum, and for it to have momentum, by definition it has mass.

By convention, the mass of a particle always refers to its rest mass (unless one specifically says otherwise). Thus the spatial momentum p carried by a photon doesn't add to its mass m, but only to its energy E=c*sqrt{(mc)^2+p^2}.