Double-Slit Experiment: Wave or Particle?

[Planck1900]

In the double-slit experiment, as I understand it, a single photon is fired towards a pair of semi transparent mirrors (mirror A and B). If passing through mirror A, the photon will take route A, which leads through slit A. If passing through mirror B, the photon will take route B, which leads through slit B.

This, however, is true only if the path of the photon is detected. If not, the photon existed only as a wave of possibillity, and as such it would pass through both slits, interfere with itself and create an interference pattern. (This is put in very simple terms, neglecting the mathematical implications and complications...)

My question is, how can you fire a single photon if the photon exist only as a wave of possibility until measured/detected?

 

[Matterwave]

When the photon is eventually detected, you detect 1 photon, and so you can infer that you shot out 1 photon. =]

 

[ZapperZ]

In the double-slit experiment, as I understand it, a single photon is fired towards a pair of semi transparent mirrors (mirror A and B). If passing through mirror A, the photon will take route A, which leads through slit A. If passing through mirror B, the photon will take route B, which leads through slit B.

This, however, is true only if the path of the photon is detected. If not, the photon existed only as a wave of possibillity, and as such it would pass through both slits, interfere with itself and create an interference pattern. (This is put in very simple terms, neglecting the mathematical implications and complications...)

My question is, how can you fire a single photon if the photon exist only as wave of possibility until measured/detected?

What about turning the question around and ask, how can there be a "wave" when there is such a thing as a which-way experiment for photons?

http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf

Zz.

 

[Planck1900]

Yes, but when you have an interference pattern (i.e. you didn't determine the path of the photon), what did you shoot? A wave of possibillity?
And why isn't there a pre-slit interference pattern, caused by the semi-transparent mirrors (that would disturb the post-slit dito)?

 

[ZapperZ]

Yes, but when you have an interference pattern (i.e. you didn't determine the path of the photon), what did you shoot? A wave of possibillity?
And why isn't there a pre-slit interference pattern, caused by the semi-transparent mirrors (that would disturb the post-slit dito)?

Again, you don't really need a physical, classical wave, at least in QM, to cause such a phenomenon. The Marcella paper that I've highlighted many times[1] shows how you can get such interference pattern, all using QM. Furthermore, look up Feynman's path integral method. The interference here isn't due to a wave, but rather the "interference" of the probabilities of both path being possible.

Zz.

[1] T.V. Marcella Eur. J. Phys. v.23, p.615 (2002). http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf"

 

[Planck1900]

Ok, so what is being shot away is actually a probability wave that can either remain as such, or become a particle, depending on whether it's detected not? And, again, if so, shouldn't there be two interference patterns; One between the semi-transparent mirrors and the slits, and a second one between the slits and the fluorenscent screen?

 

[f95toli]

No, what is shot away is a photon. And photons are neither (classical) particles nor (classical) waves (also, note that there is no such thing as a wavefunction for a photon, so this is actually not just a "philosophical" point),
(a "correct" technical description of what a photons is goes something like this: "A temporally-spatially localized excitation of the vacuum", this is what is usually used in e.g. papers dealing with single photon detectors)

The "trick" here is to realize that there is no way you can understand QM using classical concepts. Everyone finds this very confusing at first, but After a few years you get used to it.

 

[SpectraCat]

No, what is shot away is a photon. And photons are neither (classical) particles nor (classical) waves (also, note that there is no such thing as a wavefunction for a photon, so this is actually not just a "philosophical" point),

I've actually wondered about this for a while .. does the difference between photons (which don't have wavefunctions), and electrons or other massive particles (which do have wavefunctions), manifest itself in any experimentally detectable way in a double-slit experiment? I am guessing that the answer is "no", but either way it seems to me that the answer to this question might have relevance for questions about the reality of the wavefunction.

The "trick" here is to realize that there is no way you can understand QM using classical concepts. Everyone finds this very confusing at first, but After a few years you get used to it.

Well .. some people never get used to it, but are willing to accept it at least temporarily ... other people aren't even willing to accept it ...

 

[Planck1900]

Ok, I get it. But if in my main question "photon" is replaced with "electron", what is your answer to it then?

 

[ZapperZ]

Ok, I get it. But if in my main question "photon" is replaced with "electron", what is your answer to it then?

You can replace it with electrons, protons, neutrons, buckyballs, 10^11 electrons in a SQUID device, etc... it doesn't matter. As long as they can be described as quantum entities, they have the same general description and phenomena.

Zz.

 

[Planck1900]

You can replace it with electrons, protons, neutrons, buckyballs, 10^11 electrons in a SQUID device, etc... it doesn't matter. As long as they can be described as quantum entities, they have the same general description and phenomena.

Zz.

Of course. I remember now. Electrons, and all quantum entities, reside in transcendent potentia (as possibility waves) beyond space and time, as Heisenberg once noted.

So, again, this would mean that what is being shot in the double-slit experiment is a possible electron (actually being a possibility wave), manifesting its particle nature if, and only if, measured. And if measured (i.e. its path is determined), the electron not only becomes a particle, but also was a particle when shot--retrospectively. Am I right, or have I missed something here?

 

[ZapperZ]

Of course. I remember now. Electrons, and all quantum entities, reside in transcendent potentia (as possibility waves) beyond space and time, as Heisenberg once noted.

Er.. what?

Free electrons or any other free particles, do not "reside" in any potential, by definition.

So, again, this would mean that what is being shot in the double-slit experiment is a possible electron (actually being a possibility wave), manifesting its particle nature if, and only if, measured. And if measured (i.e. its path is determined), the electron not only becomes a particle, but also was a particle when shot--retrospectively. Am I right, or have I missed something here?

Electron interference, neutron interference, buckyball interference, etc. have all been observed. LEED (low energy electron diffraction), for example, is a common tools to study the crystal orientation of materials.

You should read the FAQ thread in the General Physics forum to see why there really is no such thing as "wave-particle duality" here.

Zz.