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## Main Question or Discussion Point

Can two photons interfere with each other, or is interference strictly limited to a photon (et cetera) "interfering with itself"?

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Can two photons interfere with each other, or is interference strictly limited to a photon (et cetera) "interfering with itself"?

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Of course they can, dispersion-fenomena like the rainbow, or all known diffraction patterns are a proof of that.εllipse said:Can two photons interfere with each other,

marlon

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unlike the previous two posters, I have another opinion. Photons do not interfere with each other.

Example: Double slit experiment

Here, one can show that a photon so to say "interferes with itself". You can read the Marcella paper, where the interference pattern is explained quantum mechanically as single photon interference (it's a very good paper! ):

Thomas Marcella, "Quantum interference with slits", European Journal of Physics 23 (2002), pp. 615-621

Try to download it here: http://www.iop.org/EJ/abstract/0143-0807/23/6/303

@Marlon:

Diffraction can be explained by single photon interference, it's not a result of two photons interacting with each other (see example above).

@mmwave:

In the Michelson interferometer two photons also do not interact.

Another example is the Mach Zehnder interferometer, where single photons show "interference". There's a nice paper on it:

“Photon Quantum Mechanics and Beam Splitters,” C.H. Holbrow, E.J. Galvez, and M.E. Parks, American Journal of Physics 70, 260-265 (2002)

You can download it here for free:

http://departments.colgate.edu/physics/faculty/EGalvez/articles/ajpbs02.pdf

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I do not think you got my point. The observed diffractionpatterns all arise because of the combination of diffraction and interference. that was my pointEdgardo said:@Marlon:

Diffraction can be explained by single photon interference, it's not a result of two photons interacting with each other (see example above).

marlon

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Actually we need to make something clear here. Interference is a wave like property while a photon is a the mere particle-interpretation of a wave. So saying that two photons interfere with each other is incorrect since you are combining two concepts that are different in nature. It is like trying to express or explain 24 in terms of the colour red.Edgardo said:unlike the previous two posters, I have another opinion. Photons do not interfere with each other.

To first order, photons do not interact with each other but in higher order perturbationtheory photons DO mutually interact (indirectly though) via a process called hadronization.

marlon

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The post by mmwave for example shows that there's this misconception.

But let's ask εllipse what exactly he means with his question.

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

side note for Marlon:

Classically, where you have electromagnetic waves, I agree with you Marlon that there might be a difference between the terms "diffraction" and "interference".

For the double slit we have:

a) Two infinite narrow slits => only interference pattern

b) Two finite narrow slits => diffraction (in the classical sense) pattern also occurs, which can be explained by single slit with finite width

However, there might be misunderstanding: When I mention interference I am talking about quantum interference where you add probability amplitudes if indistinguishable ways occur.

Then one explains the double slit interference pattern not as a combination of diffraction and classical interference but

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Hmm, I don't really know; I'm confused on this, so that's why I'm asking .Edgardo said:But let's ask εllipse what exactly he means with his question.

In my physics class, my teacher teacher mentioned a little about the wave properties of light, and I assumed it was similar to how two out of phase sound waves can have destructive interference, but in reading about quantum mechanics I've learned that the wave properties of photons, and everything else, are waves or probablity, and then you have the Feynman interpretation, where one particle takes all paths and "interferes with itself"; is that right? So I was asking if two particles will interfere with each other or if the interference is limitted to one particle interfering with itself. Maybe I don't know enough of what I'm talking about to ask the question right.

Let me try asking another way, and hopefully it'll make sense. Can two light waves which are out of phase destructively interfere and produce no light? And is this a different question from asking, "Can two photons which are out of phase destructively interfere and produce no photons?"

Why is quantum mechanics so much more difficult for me to get a handle on than relativity?

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Hello εllipse,

I'd like to know in which grade you are, then we know how far we can go with our explanations.

It only becomes a problem when you turn down the laser intensity and you see single photons behind the double slit:

http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html [Broken]

http://www.colorado.edu/physics/2000/schroedinger/two-slit3.html [Broken]

It's like the photon says: "Heh, there's another photon, but I don't care. I only interfere with myself and only listen to MY propability amplitude."

The quantum effects seem so weird, but that's what makes quantum theory so interesting.

And a question to you: Do you know the double slit experiment, and what problem comes up when you see single photon dots on the screen?

This experiment really helped me to understand what is meant by "the photon interferes with itself".

I'd like to know in which grade you are, then we know how far we can go with our explanations.

Yes, that's correct. If you don't use quantum mechanics, then you can compare the wave properties of light with that of sound waves. And there's nothing wrong with this model, it's a wonderful theory which allows you to calculate the pattern behind a double slit, single slit etc..εllipse said:In my physics class, my teacher teacher mentioned a little about the wave properties of light, and I assumed it was similar to how two out of phase sound waves can have destructive interference

It only becomes a problem when you turn down the laser intensity and you see single photons behind the double slit:

http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html [Broken]

http://www.colorado.edu/physics/2000/schroedinger/two-slit3.html [Broken]

Yes, correct. Your "waves of probability" are called "probability amplitudes" in quantum mechanics. And the Feynman interpretation says that the photon interferes with itself.εllipse said:, but in reading about quantum mechanics I've learned that the wave properties of photons, and everything else, are waves or probablity, and then you have the Feynman interpretation, where one particle takes all paths and "interferes with itself"; is that right?

In my understanding it's only the latter, namely interference is limited to one particle with itself.εllipse said:So I was asking if two particles will interfere with each other or if the interference is limitted to one particle interfering with itself. Maybe I don't know enough of what I'm talking about to ask the question right.

Two photons, which are out of phase do not interfere with each other destructively. You have two photons, each of them with a probability amplitude. And these probability amplitudes (or probability "waves) do not interfere with each other.εllipse said:Let me try asking another way, and hopefully it'll make sense. Can two light waves which are out of phase destructively interfere and produce no light? And is this a different question from asking, "Can two photons which are out of phase destructively interfere and produce no photons?"

It's like the photon says: "Heh, there's another photon, but I don't care. I only interfere with myself and only listen to MY propability amplitude."

Hey no problem, that's normal. Everyone finds quantum mechanics difficult at the beginning. To be honest, I only understood in 5th semester university what was meant by "the photon interferes with itself".εllipse said:Why is quantum mechanics so much more difficult for me to get a handle on than relativity?

The quantum effects seem so weird, but that's what makes quantum theory so interesting.

And a question to you: Do you know the double slit experiment, and what problem comes up when you see single photon dots on the screen?

This experiment really helped me to understand what is meant by "the photon interferes with itself".

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There's a book by Richard Feynman with the title "QED: The strange theory of light and matter". It's a book for everyone. And in the first chapter Feynman gives an example of the photon interfering with itself (glassplate, reflectivity changes with thickness). The photon's probability amplitude is illustrated as arrow in this book, just check it out.

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Yes.εllipse said:Can two light waves which are out of phase destructively interfere and produce no light?

Yes.εllipse said:And is this a different question from asking, "Can two photons which are out of phase destructively interfere and produce no photons?"

It's probably the more difficult for everyone. There's just moreεllipse said:Why is quantum mechanics so much more difficult for me to get a handle on than relativity?

stuff to learn to "get a handle on" quantum theory. I don't have

a handle on it yet, being only an intermittent student -- just some

of the basics.

Below are some links to papers and lists of references.

http://student.science.nus.edu.sg/~g0203645/Atomic Molecular and Optical Physics/Quantum effects in one-photon and two-photon interference.pdf

http://www.bu.edu/qil/pdf/PRL-09-02-96.pdf [Broken]

http://physics.umbc.edu/Research/quantum/publications.new.html [Broken]

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Ok, thanks.

**does** "really" happen, but there are times when you can say there are certain things that obviously **don't** "really" happen (like the Bell inequality, which puts a restricts certain ways the world **can't** be (local realism, etc)). So two sources of light won't interfere with each other in the sense of destructive interference and wave patterns? The Michelson-Morely experiment stuff I've read has talked about it as if two photons interfere with each other, but I am assuming that in the context of the quantum mechanics, what's really happening is an individual photon is travelling both paths and interfering with itself?

12th (US)Edgardo said:I'd like to know in which grade you are, then we know how far we can go with our explanations.

Ok, so what you're saying is although this can model reality, it's not at all what really happens. And I know physicists shy away from saying whatYes, that's correct. If you don't use quantum mechanics, then you can compare the wave properties of light with that of sound waves. And there's nothing wrong with this model, it's a wonderful theory which allows you to calculate the pattern behind a double slit, single slit etc..

It only becomes a problem when you turn down the laser intensity and you see single photons behind the double slit:

http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html [Broken]

http://www.colorado.edu/physics/2000/schroedinger/two-slit3.html [Broken]

Yes, I think so. I know about the double slit experiment and have read that it has been performed by sending single particles through at a time, so the conclusion is that there is some way the particles take both paths, because when they end up on the other side it's just a dot, although statistically it makes an interference pattern as more and more dots build up.And a question to you: Do you know the double slit experiment, and what problem comes up when you see single photon dots on the screen?

This experiment really helped me to understand what is meant by "the photon interferes with itself".

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Do you mean that two probability waves from the same photon which go through different slits in the double slit experiment will ineterfere, or are you saying that two "light waves" of some other meaning from different sources can interfere?Sherlock said:Yes.εllipse said:Can two light waves which are out of phase destructively interfere and produce no light?

I'm sorry if the answer to this was addressed in the links you provided. It is late here, and I need to get some sleep. I will read through the links in the morning, thanks.

member 11137

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fibre optics, anyone?

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In my opinion what "really" happens is that single photons interfere with themselves.ellipse said:Ok, so what you're saying is although this can model reality, it's not at all what really happens.

But I want to stress that I would never tell somebody that the wavemodel is "wrong", since it makes correct predictions.

What you probably read two electromagnetic waves interfering, NOT two photons interfering with each other. You can use the wave model too.ellipse said:The Michelson-Morely experiment stuff I've read has talked about it as if two photons interfere with each other, but I am assuming that in the context of the quantum mechanics, what's really happening is an individual photon is travelling both paths and interfering with itself?

But then like in the double slit, when you have a weak laser such that you see single photons, you must use quantum mechanics and you have individual photons that interfere with themselves.

For example read the abstract here:

http://prola.aps.org/abstract/PRL/v74/i24/p4763_1

The physicist Paul Kwiat uses a Michelson interferometer to make "quantum mechanical bomb tests"

See also here, figure 6: http://www.lanl.gov/orgs/p/pdfs/pr/pr_97_98/ifm.pdf

where a Michelson interferometer is shown.

I am a little careful here. I did a little search on the internet and read about an experiment where you have twoellipse said:So two sources of light won't interfere with each other in the sense of destructive interference and wave patterns?

But from what I read from another discussion board and a paper [3], it's not to be understood as an interference of two photons:

I typed in google

http://www.madsci.org/posts/archives/dec97/878174535.Ph.r.html

http://www.lns.cornell.edu/spr/1999-02/msg0014357.html

http://www.physicstoday.org/pt/vol-54/iss-8/p62.html [Broken]

Some quotes from them:

[PLAIN said:http://www.physicstoday.org/pt/vol-54/iss-8/p62.html][/PLAIN] [Broken]

Pursuing the theme of fourth-order interference with pairs of photons in the late 1980s, Mandel and his students demonstrated quantum spatial beating, violations of local realism, and phase memory due to quantum entanglement with the vacuum. His group showed that P. A. M. Dirac's well-known statementabout single-photon interference must be modified to assert that, in fourth-order interference, a pair of photons interferes only with the pair itself. He and his students also introduced what became known as the Hong-Ou- Mandel interferometer.

I've given you links above, but always be careful about what you read from other websites. There's much crap on the internet. I only would trust the peer reviewed papers (see [1],[2],[3] at the bottom)."Quantum effects in one-photon and two-photon interference" said:It should be clear from the foregoing that in these experimentsone photon does not interfere with another one; only the two probability amplitudes of the same photon interfere with each other. This has been confirmed more explicitly in interference experiments with a single photon (Grangieret al.,1986) and in experiments with two independent laser beams, in which interference was observed even when then light was so weak that one photon passed through the interferometer and was absorbed by the detector

long before the next photon came along (Pfleegor and Mandel, 1967, 1968).

Besides, it's nice that you already understand the problem with the double slit experiment.

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References

[1] "Interference of Independent Photon Beams", R.L. Pfleegor and L. Mandel, Physical Review Letters, Volume 159, Number 5, 1967

http://prola.aps.org/abstract/PR/v159/i5/p1084_1

[2] "Quantum Theory of Interference of Light from Two Lasers", Jordan and Ghielmetti, Physical Review Letters, Volume 12, Number 22, June 1964

http://prola.aps.org/abstract/PRL/v12/i22/p607_1

[3] "Quantum effects in one-photon and two-photon interference", L. Mandel,

Review of Modern Physics, Volume 71, Number 2, 1999

http://prola.aps.org/abstract/RMP/v71/i2/pS274_1

http://student.science.nus.edu.sg/~g0203645/Atomic Molecular and Optical Physics/Quantum effects in one-photon and two-photon interference.pdf

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I took your question about "light waves" as referring to classicalεllipse said:Do you mean that two probability waves from the same photon which go through different slits in the double slit experiment will ineterfere, or are you saying that two "light waves" of some other meaning from different sources can interfere?

light waves. You can do this experiment at home and see the

regions of constructive and destructive interference.

Get recent textbooks on the classical and quantum theories of light

if you can. I have to do this myself. :-) There are lots of

online sources for good condition used books that will probably

fit your budget.

I don't know what to recommend at this time since I'm still

looking -- maybe some of the science advisers here can let us

know what they think are some good ones.

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