Possible explanation for the wave-particle duality ?

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Cthugha

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In his Nobel prize lecture, Roy Glauber commented on that point. He published a more focused version of that remark in Nucl. Phys. A 774 (2006) 3-13 (free ArXiv version here: http://arxiv.org/abs/nucl-th/0604021).

Allow me to quote it:
"When you read the first chapter of Dirac’s famous textbook in quantum mechanics [8], however, you are confronted with a very clear statement that rings in everyone’s memory. Dirac is talking about the intensity fringes in the Michelson interferometer, and he says,

"Every photon then interferes only with itself. Interference between two different
photons never occurs."

Now that simple statement, which has been treated as scripture, is absolute nonsense."

He goes on to explain it in detail, but it is very obvious that Dirac was all-out wrong. Reading ancient textbooks which were written before the first laser was invented is a bad idea when trying to learn about quantum optics. Dirac's book is especially bad in that respect.
 

DevilsAvocado

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Now that simple statement, which has been treated as scripture, is absolute nonsense.
OMG :biggrin:

I need a break... I had enough of "physics-fighting" for today... can't handle one more head/bottomless failure... :rofl:
 

atyy

Science Advisor
13,547
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In his Nobel prize lecture, Roy Glauber commented on that point. He published a more focused version of that remark in Nucl. Phys. A 774 (2006) 3-13 (free ArXiv version here: http://arxiv.org/abs/nucl-th/0604021).

Allow me to quote it:
"When you read the first chapter of Dirac’s famous textbook in quantum mechanics [8], however, you are confronted with a very clear statement that rings in everyone’s memory. Dirac is talking about the intensity fringes in the Michelson interferometer, and he says,

"Every photon then interferes only with itself. Interference between two different
photons never occurs."

Now that simple statement, which has been treated as scripture, is absolute nonsense."

He goes on to explain it in detail, but it is very obvious that Dirac was all-out wrong. Reading ancient textbooks which were written before the first laser was invented is a bad idea when trying to learn about quantum optics. Dirac's book is especially bad in that respect.
Presumably the problem is with "only with itself" and "Interference between two different
photons never occurs." I assume it is still ok to say that a single photon interferes with itself? Something like http://falling-walls.com/videos/Alain-Aspect--1216 "single-photon interference"?
 

Cthugha

Science Advisor
1,889
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OMG :biggrin:

I need a break... I had enough of "physics-fighting" for today... can't handle one more head/bottomless failure... :rofl:
When giving talks at major conferences in order to promote my own results, I sometimes added the Dirac quote and the Glauber quote in order to get people interested. At that point you usually hear some laughs and people are indeed interested. In the end it turned out that many of the people in the audience indeed started thinking about that. However, most did that instead of listening to the stuff I tried to get across. :/

Anyhow, it is surprising how much influence such a phrase in an old textbook can have just because it is catchy. However, for the notes: In a simple standard double slit with a simple light source, Dirac is right. However, as soon as you discuss entangled light or other complicated things, Dirac is not a good reference anymore. That should not come as a surprise. All of that stuff was investigated way later. I would not expect deep insights into QM from reading Newton's books either.

atyy said:
I assume it is still ok to say that a single photon interferes with itself?
Sure, within a coherence volume single photon interference happens as expected.
 

DevilsAvocado

Gold Member
751
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When giving talks at major conferences in order to promote my own results, I sometimes added the Dirac quote and the Glauber quote in order to get people interested. At that point you usually hear some laughs and people are indeed interested. In the end it turned out that many of the people in the audience indeed started thinking about that. However, most did that instead of listening to the stuff I tried to get across. :/
It is a dangerous quote, it can vaporize any pompous reasoning, trust me! :approve:

Anyhow, it is surprising how much influence such a phrase in an old textbook can have just because it is catchy.
Yeah, and the worst thing – it makes perfect sense – you don't even have to check it.

However, for the notes: In a simple standard double slit with a simple light source, Dirac is right.
Phew, you've just saved my day... almost...

However, as soon as you discuss entangled light or other complicated things, Dirac is not a good reference anymore. That should not come as a surprise. All of that stuff was investigated way later. I would not expect deep insights into QM from reading Newton's books either.
Nothing surprises me anymore – not even my own "extraterrestrial brilliance" – here comes the final punch! :biggrin:

[Script draft for sequel "Dumb and Dumber To", Nov 2014 release]
In post #42 I quoted this paper:

[PLAIN said:
http://pra.aps.org/abstract/PRA/v49/i5/p4243_1]-->[/PLAIN] [Broken] *Two-photon interference* <-- in a standard Mach-Zehnder interferometer

A pair of light quanta with different colors (155.9-nm difference in center wavelength) generated from parametric down-conversion was injected collinearly into one input port of a Mach-Zehnder interferometer. Coincidence interference behavior was studied over a wide range of optical path differences of the interferometer. A measurement of 75% interference visibility with oscillation of the pump frequency for a large optical path difference of the interferometer (43 cm) is the signature of a quantum two-photon entangled state, which reflects both particle and wave nature of the light quanta in one experiment.

DOI: 10.1103/PhysRevA.49.4243
And a few posts later I (and my dear friend Dirac), claimed it completely impossible! Do I get a prize for this achievement? Ig Nobel maybe?? :blushing:

:rofl:


P.S: Thanks for the laughs Cthugha, and here's a cake on your "1,500 posts day".

cake1.jpg
 
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He goes on to explain it in detail, but it is very obvious that Dirac was all-out wrong. Reading ancient textbooks which were written before the first laser was invented is a bad idea when trying to learn about quantum optics. Dirac's book is especially bad in that respect.
As a person that learned QM from Dirac's and Von-Neumann's books I fully concur.

Both are full of stuff that from our vantage point are downright WRONG.

Both are classics and should be in the library of anyone seriously interested in QM - but do not learn QM from them or you will run into trouble and need to unlearn stuff - just like I did.

Ballentine leaves them for dead.

Thanks
Bill
 
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I have been thinking recently and I have a few questions. I wonder if the following thought experiment was valid :

We establish two sources of "perfectly inelastic" particles facing directly to each other, so that they generate particles moving exactly towards the opposite direction than the other. We generate these particles with the same exact speed that we determine beforehand, and so they cant be generated with an arbitrary precision towards the above direction but some % of the particles will still have to move the way we want it, so that they collide and bounce back exactly in the direction where they came from. What I'd like to establish is the this:

We generate two particles (lets call them Alice and Bob) in 'x' and '-x' directions. After they collided, they are going to switch directions, so if Alice moved in 'x' direction, after the collision it will move to '-x', and vice versa.
While the particles are coming back to their sources, we replace one's generator with a detector, so that we can determine when the desired collision has happened. This means that for ex. when the detector of Alice detects that Alice has come back, Bob must be also at its origin, and this means we know both the place and speed of Bob at the same time with an infinite precision. What I'd like to know is that if we let Bob pass through a double slit, would we still see it interfering with itself ?

And another thing, can time be arbitrarily short ?
 
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And another thing, can time be arbitrarily short ?
We don't know. We currently assume it is - but we do not know.

Your collision thought experiment will not work - collide quantum objects and all sorts of strange things happen such as new particles spewing forth and scattering.

Thanks
Bill
 
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About slit experiment video in post 50.
As delta X becomes smaller and smaller the delta P becomes larger and larger.
But why does it do so in direction exactly perpendicular to slit? Why not in random directions to form a circular spot on screen?
 
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probert84 - your "answer" is more confusing than your question.

I just think of the "mass" of a wave = E/c^2 and the dimension of the wave is the entire wave front in which the entire energy, E, is spread out over the ENTIRE wavefront (I.e., exists simultaneously - no "probability" involved.)

In truly empty space where there is no mass and no energy, this is immediately disrupted by any particle or energy which imparts mass, curves or displaces space and changes the physics of that region.
 
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About slit experiment video in post 50.
As delta X becomes smaller and smaller the delta P becomes larger and larger.
But why does it do so in direction exactly perpendicular to slit? Why not in random directions to form a circular spot on screen?


The slit is vertical so you are only restricting the light path horizontally. Vertically the slit stays the same height, i.e. no shortening and in a sense it's still in the classical domain..
 
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But why does it do so in direction exactly perpendicular to slit? Why not in random directions to form a circular spot on screen?
Because being a vertical slit the position is not localized vertically - only horizontally.

Thanks
Bill
 
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We don't know. We currently assume it is - but we do not know.

Your collision thought experiment will not work - collide quantum objects and all sorts of strange things happen such as new particles spewing forth and scattering.

Thanks
Bill
Why do we assume that about time ?

So there is no perfectly inelastic collision at all in the quantum world ?
 
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Why do we assume that about time ?
Why not?

Its made in all areas of physics because its common-sensical, works and allows the powerful methods of calculus to be employed. If you want to develop a version that doesn't assume it go ahead - feel free. And make quantitative predictions with it that experimentally distinguish it from the standard theory. A Nobel prize awaits if you can. But you wont have calculus to help you.

So there is no perfectly inelastic collision at all in the quantum world ?
Collisions in QM are not amenable to such a classification because they are not particles in a classical sense. When, for example, two photons collide, a positron and electron can come out - and that's not all that can happen - its quite complicated. Its described by Feynman diagrams and that mathematically difficult area of QFT.

Thanks
Bill
 
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Because that would explain why energy is quantized.
Energy is not always quantized.

But aside from that your logic escapes me.

First, before going any further, exactly what do you think energy is and why is it conserved?

Modern physics knows the answer to that, and when you do you realize a statement like you made is nonsensical, but before going any further lets pin down what you think it is.

Thanks
Bill
 
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Under which circumstance is it not quantized ?

My definition of energy would be something like this:

<<Personal speculation deleted>>
 
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Under which circumstance is it not quantized ?
Typically the energy of bound states are quantized, but the energy of free states are not quantized. So, for example, a hydrogen atom has a whole series of quantized energy levels, but once you add enough energy to separate the electron from the proton (ionization) the energy is no longer quantized.

My definition of energy would be something like this:

<<Personal speculation deleted>>
With that, this thread is closed.
 

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