Light, Waves, Particles etc

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Light, Waves, Particles etc

#1  Postby twistor59 » Sep 14, 2011 7:14 pm

Continued off topic discussion from this thread.

The topic in question revolves around whether it is appropriate that we should say that light is composed of particles. My position is this:

1 I fully accept that modern quantum field theory provides the best means for modelling interactions involving the electromagnetic field at a fundamental level.

2 Mode excitations can be induced in the electromagnetic field and these are discrete and countable, and referred to as "photons".

3 Photons do not have the same localizability properties as, say, electrons, and for this reason, I hesitate to call them particles, but prefer the term "field quanta".

Incidentally, it is not correct to say that the two slit experiment such as illustrated in the youtube video proves the particulate nature of light. Exactly the same "discrete flash" behaviour is obtained by treating the incident wave as a classical field, but the valence electrons in the detector according to quantum rules. (Note this does not mean that I don't think light is a quantum field, I most certainly do !). So Einstein was right, but for the wrong reason.

Real evidence for the discrete behaviour in light comes from photon antibunching experiments and scattering processes such as the Compton effect.

I believe Einstein thought that light was composed of "light atoms" which had point sources a bit like the Coulomb field, but summed up to produce the classical wave. That picture certainly wasn't right.
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Re: Light, Waves, Particles etc

#2  Postby zaybu » Sep 14, 2011 8:10 pm

I don't see the problem of "localizability". I have no idea from what this comes. It certainly not a topic in QFT. I have nine textbooks on my desk, and none even mention that concept. All particles are subject to the Heisenberg Uncertainty Principle. It's just that fermions obey different statistics than bosons. The only time you will need to consider waves is when you are dealing with a large number of photons. In that case, the wave picture gives you adequate results. But when you deal with one on one: one photon with one electron, or two electrons exchanging a photon, then the particle picture is the only one that makes sense. Either that or you might as well throw Feynman's diagrams under the bus.
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Re: Light, Waves, Particles etc

#3  Postby Grace » Sep 14, 2011 8:31 pm

Hmm... I don't think particles can travel the speed of light. I thought light was radioactive atoms like radioactive hydrogen that gives off its own light.
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Re: Light, Waves, Particles etc

#4  Postby mraltair » Sep 14, 2011 8:42 pm

:coffee:

For my level of understanding, it's both wave and particle. Hopefully I'll learn something in this thread. :thumbup:

Actually I definitely will.
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Re: Light, Waves, Particles etc

#5  Postby Teuton » Sep 15, 2011 12:07 am

Some think or thought that particles are ontologically reducible to wave packets.

"Schrödinger attempted to interpret corpuscles and particularly electrons, as wave packets. Although his formulae are entirely correct, his interpretation cannot be maintained, since on the one hand…the wave packets must in course of time become dissipated, and on the other hand the description of the interaction of two electrons as a collision of two wave packets in ordinary three-dimensional space lands us in grave difficulties."

(Born, Max. Atomic Physics. 8th ed. 1969. Reprint, Mineola, NY: Dover, 1989. p. 95)
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Re: Light, Waves, Particles etc

#6  Postby Teuton » Sep 15, 2011 12:09 am

mraltair wrote:
For my level of understanding, it's both wave and particle.


I doubt that's possible.
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Re: Light, Waves, Particles etc

#7  Postby eric8476 » Sep 15, 2011 1:41 am

twistor59 wrote:Continued off topic discussion from this thread.

The topic in question revolves around whether it is appropriate that we should say that light is composed of particles. My position is this:

1 I fully accept that modern quantum field theory provides the best means for modelling interactions involving the electromagnetic field at a fundamental level.

2 Mode excitations can be induced in the electromagnetic field and these are discrete and countable, and referred to as "photons".

3 Photons do not have the same localizability properties as, say, electrons, and for this reason, I hesitate to call them particles, but prefer the term "field quanta".

Incidentally, it is not correct to say that the two slit experiment such as illustrated in the youtube video proves the particulate nature of light. Exactly the same "discrete flash" behaviour is obtained by treating the incident wave as a classical field, but the valence electrons in the detector according to quantum rules. (Note this does not mean that I don't think light is a quantum field, I most certainly do !). So Einstein was right, but for the wrong reason.

Real evidence for the discrete behaviour in light comes from photon antibunching experiments and scattering processes such as the Compton effect.

I believe Einstein thought that light was composed of "light atoms" which had point sources a bit like the Coulomb field, but summed up to produce the classical wave. That picture certainly wasn't right.


i wonder about a photon being matter or energy also. they show up on the double slit experiment detector indicating a structured form leaving a signature. is it that our measurements are not capable of determining the mass amount of photons, possible.

i noticed something, see the you tube video of the experiment posted by twistor59 in the first post on this thread. at about 40 seconds into the video the detector screen looks close to empty, after 40 seconds the detector screen fills up fast. is this some form of the compton effect? is there something to the behavior of the photons in quantity? to those reading this, add my name to the list of credits after receiving your nobel prize for answers to this ("corny" is my middle name, hehehe).

p.s. - did the issue of viewing electrons and photons in the double slit experiment get solved?

p.p.s. - @zaybu or others, when beaming a ray of light in the double slit experiment, do the detectors pick up the particles? and if not does that indicate that forces are not particles or does it indicate that detectors are not capable of picking up the particles? if the detectors do not pick up the particles, then that goes to particles permiating through the solid detectors and that could explain quantum tunneling better, correct?
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Re: Light, Waves, Particles etc

#8  Postby twistor59 » Sep 15, 2011 6:59 am

zaybu wrote:I don't see the problem of "localizability". I have no idea from what this comes. It certainly not a topic in QFT. I have nine textbooks on my desk, and none even mention that concept.


Again I'm not claiming anything fundamentally controversial or new here, simply that I, personally, think that "field quanta" is better suited to photons than "particle", since particle implies a certain localizability.

The localizability issue not discussed in most textbooks on QFT, which are simply trying to give particle physicists the tools for calculating scattering amplitudes, no that's true. However, it is discussed in the research literature:

http://www.cft.edu.pl/~birula/publ/CQO7.pdf
http://prl.aps.org/abstract/PRL/v100/i18/e183603
http://www.lois-space.net/Workshops/Vaxjo080616-18/Presentations/tamburini.pdf
http://fau.academia.edu/MEHogan/Papers/280637/Gauge-Invariant_Photon_Wave_Function_and_Diffraction
http://arxiv.org/abs/quant-ph/0604169
http://www.nist.gov/pml/div684/fcdc/upload/preprint.pdf

zaybu wrote:All particles are subject to the Heisenberg Uncertainty Principle. It's just that fermions obey different statistics than bosons. The only time you will need to consider waves is when you are dealing with a large number of photons. In that case, the wave picture gives you adequate results. But when you deal with one on one: one photon with one electron, or two electrons exchanging a photon, then the particle picture is the only one that makes sense. Either that or you might as well throw Feynman's diagrams under the bus.


Imagine I have an ideal experimental setup - an optical cavity, and I put a single quantum of monochromatic light in there. Would you really want to call that a "particle" ?

Incidentally, do you think that two electrons really exchange photons when paticipating in the electromagnetic interaction ?
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Re: Light, Waves, Particles etc

#9  Postby zaybu » Sep 15, 2011 1:30 pm


This paper talks about "first quantization" to describe photons. Just elemerntary stuff.



Couldn't access this paper, requires registration.




Basic elementary QED. The stuff on radioastronomy is interesting.



Here the author relates to electric and magnetic fields. This work is appropriate when large number of photons are involved. What's amusing is that it treats gauge transformation as if it was something new when in reality it was established in the 1950's by Yang, who based his work on Landau back in the 1930's.




Here the author stresses that Maxwell equations pertain to a single photon. Cute.




Another work that tries to relate the Maxwell equation directly to the Schroedinger equation.

..................................................................................................................................................

To be honest, twistor, if you want to study QFT, Weinberg is THE book. All that stuff you linked to is passé.




zaybu wrote:All particles are subject to the Heisenberg Uncertainty Principle. It's just that fermions obey different statistics than bosons. The only time you will need to consider waves is when you are dealing with a large number of photons. In that case, the wave picture gives you adequate results. But when you deal with one on one: one photon with one electron, or two electrons exchanging a photon, then the particle picture is the only one that makes sense. Either that or you might as well throw Feynman's diagrams under the bus.


Imagine I have an ideal experimental setup - an optical cavity, and I put a single quantum of monochromatic light in there. Would you really want to call that a "particle" ?


Yes, if it's a "single" quantum, it will behave like a particle. The hard trick is to produce a "single" quantum one at a time. The youtube video I linked you to did just that. And the presence of tiny grains appearing one by one on the screen testifies to the particle nature of the photon.

Incidentally, do you think that two electrons really exchange photons when paticipating in the electromagnetic interaction ?


That's what QED says. Do you have a better theory?

Also, we get the W+, W-, Z boson mediating the weak force, and gluons for the strong force. Are you saying that the Standard model is wrong?
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Re: Light, Waves, Particles etc

#10  Postby cavarka9 » Sep 15, 2011 6:02 pm

havent read qft by anyone yet :dopey: .But do have the book of weinberg, ebook that is. :)

My question is, since e and e+ do indeed annihilate to produce a photon, and we know that e and positron are made of quarks. shouldnt we expect photon to have an internal structure?.
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Re: Light, Waves, Particles etc

#11  Postby Zwaarddijk » Sep 15, 2011 6:43 pm

Grace wrote:Hmm... I don't think particles can travel the speed of light. I thought light was radioactive atoms like radioactive hydrogen that gives off its own light.


uh what? Light can be generated by radioactive atoms, yes, but light does not need that. Light consists of photons - a kind of particle that mostly travels at the speed of light on account of not having any mass. But uh, light gives off light sounds about par for the course when talking to you.
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Re: Light, Waves, Particles etc

#12  Postby locutus7 » Sep 15, 2011 7:11 pm

Light is the absence of darkness. The photon is the absence of the anti-photon. Wave goodbye (*_*)
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Re: Light, Waves, Particles etc

#13  Postby zaybu » Sep 15, 2011 7:30 pm

cavarka9 wrote:havent read qft by anyone yet :dopey: .But do have the book of weinberg, ebook that is. :)

My question is, since e and e+ do indeed annihilate to produce a photon, and we know that e and positron are made of quarks. shouldnt we expect photon to have an internal structure?.


Electrons are thought to be elementary, so far anyway. You may be thinking of protons, which are made up of three quarks.

But an electron and a positron can annihilate into a photon, which in turn could give rise to other particles if the energy is sufficiently large.

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Re: Light, Waves, Particles etc

#14  Postby twistor59 » Sep 15, 2011 7:56 pm

zaybu wrote:

This paper talks about "first quantization" to describe photons. Just elemerntary stuff.


If you think that applying first quantization to photons is "elementary", then you have a different definition of elementary to the one I have !



zaybu wrote:
To be honest, twistor, if you want to study QFT, Weinberg is THE book. All that stuff you linked to is passé.


That's a rather off-hand dismissal of some interesting and controversial discussions, whereas in fact the quantum field theory you are presenting as the shizzle forms the basis of first year postgraduate theoretical physics courses.



zaybu wrote:
twistor59 wrote:
zaybu wrote:All particles are subject to the Heisenberg Uncertainty Principle. It's just that fermions obey different statistics than bosons. The only time you will need to consider waves is when you are dealing with a large number of photons. In that case, the wave picture gives you adequate results. But when you deal with one on one: one photon with one electron, or two electrons exchanging a photon, then the particle picture is the only one that makes sense. Either that or you might as well throw Feynman's diagrams under the bus.


Imagine I have an ideal experimental setup - an optical cavity, and I put a single quantum of monochromatic light in there. Would you really want to call that a "particle" ?


Yes, if it's a "single" quantum, it will behave like a particle. The hard trick is to produce a "single" quantum one at a time. The youtube video I linked you to did just that. And the presence of tiny grains appearing one by one on the screen testifies to the particle nature of the photon.


Did you read what I wrote above in the first post of this thread ? You get exactly the same discrete behaviour (flashes building up the interference pattern) if you treat the incident field as purely classical ! The flashes do not provide a proof of the quantum nature of the electromagnetic field - however other things do provide such proof.


zaybu wrote:
twistor59 wrote:
Incidentally, do you think that two electrons really exchange photons when paticipating in the electromagnetic interaction ?


That's what QED says. Do you have a better theory?


QED says nothing of the sort. It merely says that one way (perturbation theory) of computing the effects of the interaction is to treat the system as if virtual quanta are exchanged. However this is merely a calculational device. If you could solve the nonlinear interaction exactly, there would be no need to invoke virtual particles. The electromagnetic field, however, does allow real excitations, namely the photons that we can measure.
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Re: Light, Waves, Particles etc

#15  Postby cavarka9 » Sep 15, 2011 8:32 pm

zaybu wrote:
cavarka9 wrote:havent read qft by anyone yet :dopey: .But do have the book of weinberg, ebook that is. :)

My question is, since e and e+ do indeed annihilate to produce a photon, and we know that e and positron are made of quarks. shouldnt we expect photon to have an internal structure?.


Electrons are thought to be elementary, so far anyway. You may be thinking of protons, which are made up of three quarks.

But an electron and a positron can annihilate into a photon, which in turn could give rise to other particles if the energy is sufficiently large.

Image

oops, you are right, electrons are leptons. I never liked particle physics.
So, I will change that to proton and anti-proton :grin: .

In any case, the question is shouldnt a photon have a structure?. or a structure of interaction.
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Re: Light, Waves, Particles etc

#16  Postby zaybu » Sep 15, 2011 8:57 pm

twistor59 wrote:
zaybu wrote:

This paper talks about "first quantization" to describe photons. Just elemerntary stuff.


If you think that applying first quantization to photons is "elementary", then you have a different definition of elementary to the one I have !


It's elementary in the sense that there hasn't been any new development in QM since the 1930's. Omitting the advancement in QFT from 1930 to 1970 would be like studying math without studying any math from calculus and on. Your grasp of the subject would be quite limited. How could you discuss renormalization, gauge invariance, higgs mechanism or Wilson loop? Yet these are the backbones of the subject matter.

zaybu wrote:
To be honest, twistor, if you want to study QFT, Weinberg is THE book. All that stuff you linked to is passé.


That's a rather off-hand dismissal of some interesting and controversial discussions, whereas in fact the quantum field theory you are presenting as the shizzle forms the basis of first year postgraduate theoretical physics courses.


No it wasn't. You linked to papers that had very little to do with your concept of "localizability". Just basic papers making a connection between classical physics (Maxwell equations) to QM. I still have no clue with what you mean with your concept. Are there equations that describe that concept? If you do, please let me know.




Did you read what I wrote above in the first post of this thread ? You get exactly the same discrete behaviour (flashes building up the interference pattern) if you treat the incident field as purely classical ! The flashes do not provide a proof of the quantum nature of the electromagnetic field - however other things do provide such proof.


I disagree. The graininess demonstrates that releasing one photon at a time shows their particle nature. No wave model can explain that.


zaybu wrote:
twistor59 wrote:
Incidentally, do you think that two electrons really exchange photons when paticipating in the electromagnetic interaction ?


That's what QED says. Do you have a better theory?


QED says nothing of the sort. It merely says that one way (perturbation theory) of computing the effects of the interaction is to treat the system as if virtual quanta are exchanged. However this is merely a calculational device. If you could solve the nonlinear interaction exactly, there would be no need to invoke virtual particles. The electromagnetic field, however, does allow real excitations, namely the photons that we can measure.


QM is based on several assumptions such as operators are observables, or that the probability is the square of the amplitude, etc. The point is that this assumption of exchange particles explains many phenomena. The thousands of Feynman diagrams that explain millions of interactions observed in the many supercolliders across the planet wouldn't make any sense without that assumption. That's why it's called high energy particle physics, and the Standard Model describes the fundamental particles of nature.
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Re: Light, Waves, Particles etc

#17  Postby Grace » Sep 16, 2011 2:57 am

Someone doesn't know how to use idioms correctly...
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Re: Light, Waves, Particles etc

#18  Postby twistor59 » Sep 16, 2011 7:46 am

zaybu wrote:
twistor59 wrote:
zaybu wrote:
This paper talks about "first quantization" to describe photons. Just elemerntary stuff.


If you think that applying first quantization to photons is "elementary", then you have a different definition of elementary to the one I have !


It's elementary in the sense that there hasn't been any new development in QM since the 1930's. Omitting the advancement in QFT from 1930 to 1970 would be like studying math without studying any math from calculus and on. Your grasp of the subject would be quite limited. How could you discuss renormalization, gauge invariance, higgs mechanism or Wilson loop? Yet these are the backbones of the subject matter.



(I'll split the responses to the previous post into separate posts, otherwise the paragraphs are going to be very hard to keep track of.)

But the issue under discussion, which is essentially something along the lines of "what does a photon look like - how should I think of it?" does not need the advances you describe (with the exception, perhaps, of the gauge idea, which has been around since Hermann Weyl, and which is important in identifying the fundamental degrees of freedom). Although you can get by without asking simplistic questions like the one I'm addressing, you cannot suppress the urge to try to answer such questions, and such questions are surprisingly difficult to answer.
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Re: Light, Waves, Particles etc

#19  Postby twistor59 » Sep 16, 2011 7:50 am

zaybu wrote:
twistor59 wrote:
zaybu wrote:
To be honest, twistor, if you want to study QFT, Weinberg is THE book. All that stuff you linked to is passé.


That's a rather off-hand dismissal of some interesting and controversial discussions, whereas in fact the quantum field theory you are presenting as the shizzle forms the basis of first year postgraduate theoretical physics courses.


No it wasn't. You linked to papers that had very little to do with your concept of "localizability". Just basic papers making a connection between classical physics (Maxwell equations) to QM. I still have no clue with what you mean with your concept. Are there equations that describe that concept? If you do, please let me know.


I'll try to explain it a bit more, but it will have to wait since I'm at work now and away for most of the weekend...
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Re: Light, Waves, Particles etc

#20  Postby twistor59 » Sep 16, 2011 8:05 am

zaybu wrote:
twistor59 wrote:

Did you read what I wrote above in the first post of this thread ? You get exactly the same discrete behaviour (flashes building up the interference pattern) if you treat the incident field as purely classical ! The flashes do not provide a proof of the quantum nature of the electromagnetic field - however other things do provide such proof.


I disagree. The graininess demonstrates that releasing one photon at a time shows their particle nature. No wave model can explain that.



That is a common misconception. The analysis provided in Mandel and Wolf demonstrates that the discrete flashes with Poisson statistics can be derived by treating the atoms in the detector quantum mechanically, but the incident EM wave classically. Very surprising, I only realised this relatively recently, but the photoelectric effect does not provide proof of the the quantum nature of light. Photon antibunching is probably the best demonstration of this quantum nature.
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