Light, Waves, Particles etc

Study matter and its motion through spacetime...

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Re: Light, Waves, Particles etc

#41  Postby mizvekov » Sep 25, 2011 6:43 pm

twistor59 wrote:Is energy conserved for the entire universe ?

Noether => time symmetry ~ energy conservation

Universe not time symmetric (no timelike Killing vector).

Thanks for the answer, but the topic is already dead in the thread that originated this, and where cavarka came from with it. There I tried to hold this position, but I had to give up on the account of the shitstorm I was receiving from Oldskeptic and hackenslash.
You gave a very concise answer, but I think this simple syllogism will still be managed to be misrepresented by those two.
But anyway, since you also said you don't have all the free time in the world, I recommend you stay as far away from there as you can :)
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Re: Light, Waves, Particles etc

#42  Postby cavarka9 » Sep 25, 2011 7:18 pm

mizvekov wrote:
twistor59 wrote:Is energy conserved for the entire universe ?

Noether => time symmetry ~ energy conservation

Universe not time symmetric (no timelike Killing vector).

Thanks for the answer, but the topic is already dead in the thread that originated this, and where cavarka came from with it. There I tried to hold this position, but I had to give up on the account of the shitstorm I was receiving from Oldskeptic and hackenslash.
You gave a very concise answer, but I think this simple syllogism will still be managed to be misrepresented by those two.
But anyway, since you also said you don't have all the free time in the world, I recommend you stay as far away from there as you can :)

Actually I do not remember reading your debate, :),where exactly?. There was recently I think a thread was posted which asked about conservation of energy, and I read it earlier from sciam anyway also, the universe is expanding, but no, I cant formulate it.
What I am interested in and the reason I asked was to consider that if light were a particle, how does one explain it disappearing, i.e. energy not being conserved. Does QM helps us in explaining the expansion we observe.
Thanks for that.

http://www.rationalskepticism.org/physi ... ml#p960547
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Re: Light, Waves, Particles etc

#43  Postby zaybu » Sep 25, 2011 7:59 pm

twistor59 wrote:
zaybu wrote:

But the "something happens" is not as nebulous as your diagram implies, it's more like: SEE ATTACHMENT

By considering all possible interactions that we can get very precise agreement between theory and experimental observation in QED.


No, those diagrams are only pictorial representations of a bunch of integrals you need to compute the scattering amplitude of the incoming and outgoing electrons. They're only a way of organising the mathematics. You only have to do it this way because you're using perturbation theory. We're only doing perturbation theory because we don't know how to set up and solve the equations of QED exactly.

The scattering amplitude is given by the sum of the contributions of all the diagrams to a given order. Each diagram (in configuration space) requires you to integrate over all the spacetime locations of each vertex. Only the sum has meaning. The electron never really emits a virtual photon.

Let me ask a question:

The diagram (a) in the picture represents a contribution of order α2, (where α is the fine structure constant), diagram (b) of order α4 etc...


Your objection is counter science. We build a model, and on that basis, we make calculations, which allows us to make predictions. If these predictions are verified experimentally, then we say our model is close to what is real. Your objection is: it's an approximate calculation, therefore the pictures (model) doesn't represent reality.

???

Einstein's E=mc2 also comes from an approximation. We nevertheless see that as something describing a real phenomenon. Most of the stuff we get from theory were reached through approximations. Very, very few results were ever obtained through an exact solution, and science would be the poorer if we demand that only exact solutions are valid.

The fact is: each of the Feynman diagrams requires a precise formulation base on the number of straight lines, vertices, wiggling lines, incoming and outcoming lines. If those interactions don't represent the real McCoy, I wonder what does, certainly not the wave model!

To what order must we go in order to represent reality ? (just sticking to QED here - assuming no new physics, quantum gravity etc)


Each order has a smaller and smaller probability to happen. It doesn't mean that they don't happen. Remember QM, and by extension QFT, is a theory about probabilities from the start. Are we to say that then, being just probabilities, it doesn't describe reality?
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Re: Light, Waves, Particles etc

#44  Postby mizvekov » Sep 25, 2011 8:52 pm

cavarka9 wrote:Actually I do not remember reading your debate, :),where exactly?.

http://www.rationalskepticism.org/gener ... 21224.html
You participated there, so I assumed you were talking about that, sorry about that.

cavarka9 wrote:There was recently I think a thread was posted which asked about conservation of energy, and I read it earlier from sciam anyway also, the universe is expanding, but no, I cant formulate it.

I thought that you were talking about the article by Sean Carroll, which was posted on that thread by another member.
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Re: Light, Waves, Particles etc

#45  Postby hackenslash » Sep 25, 2011 10:27 pm

mizvekov wrote:Thanks for the answer, but the topic is already dead in the thread that originated this, and where cavarka came from with it. There I tried to hold this position, but I had to give up on the account of the shitstorm I was receiving from Oldskeptic and hackenslash.
You gave a very concise answer, but I think this simple syllogism will still be managed to be misrepresented by those two.
But anyway, since you also said you don't have all the free time in the world, I recommend you stay as far away from there as you can :)


Except, of course, that this is not what you were arguing*. I was there too, remember.


*Or if it was, then you made an incoherent mess of it.
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Re: Light, Waves, Particles etc

#46  Postby mizvekov » Sep 25, 2011 10:40 pm

hackenslash wrote:Except, of course, that this is not what you were arguing*. I was there too, remember.


*Or if it was, then you made an incoherent mess of it.

Well, any one who would bother to read that thread would be able to judge who was making incoherent statements, and what was the point I was making.
And that you would say this is not unexpected, and one of the reasons that I made a point not to argue with you anymore.
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Re: Light, Waves, Particles etc

#47  Postby cavarka9 » Sep 26, 2011 5:31 am

mizvekov wrote:
cavarka9 wrote:Actually I do not remember reading your debate, :),where exactly?.

http://www.rationalskepticism.org/gener ... 21224.html
You participated there, so I assumed you were talking about that, sorry about that.

cavarka9 wrote:There was recently I think a thread was posted which asked about conservation of energy, and I read it earlier from sciam anyway also, the universe is expanding, but no, I cant formulate it.

I thought that you were talking about the article by Sean Carroll, which was posted on that thread by another member.


I remember that I got sean carroll's article from here and I remember thanking for that, dont know where. I only read that bit as that was interesting wrt to the other thread I was reading at the time. It's ok, just point to the thread next time.

So, could anyone help wrt to conservation of energy not being true and its implication to particle nature.Particles should be conserved, I cannot imagine particles not being conserved, and hence if energy is not conserved then it may not be particles which best represent it. Although it could be wrong, if so I would like to know the reason.
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Re: Light, Waves, Particles etc

#48  Postby twistor59 » Sep 26, 2011 7:30 am

zaybu wrote:
twistor59 wrote:
zaybu wrote:

But the "something happens" is not as nebulous as your diagram implies, it's more like: SEE ATTACHMENT

By considering all possible interactions that we can get very precise agreement between theory and experimental observation in QED.


No, those diagrams are only pictorial representations of a bunch of integrals you need to compute the scattering amplitude of the incoming and outgoing electrons. They're only a way of organising the mathematics. You only have to do it this way because you're using perturbation theory. We're only doing perturbation theory because we don't know how to set up and solve the equations of QED exactly.

The scattering amplitude is given by the sum of the contributions of all the diagrams to a given order. Each diagram (in configuration space) requires you to integrate over all the spacetime locations of each vertex. Only the sum has meaning. The electron never really emits a virtual photon.

Let me ask a question:

The diagram (a) in the picture represents a contribution of order α2, (where α is the fine structure constant), diagram (b) of order α4 etc...


Your objection is counter science. We build a model, and on that basis, we make calculations, which allows us to make predictions. If these predictions are verified experimentally, then we say our model is close to what is real. Your objection is: it's an approximate calculation, therefore the pictures (model) doesn't represent reality.

???


There I must return to my previous example. Does the image charge in the method of images in electrostatics "represent reality" ? No.

A model is just something that allows us to make predictions. In this case, the prediction is about the behaviour of the outgoing particles in response to the incoming particles. Its connection with the physical world is through the in-states and out-states, nothing else.


zaybu wrote:
Einstein's E=mc2 also comes from an approximation. We nevertheless see that as something describing a real phenomenon. Most of the stuff we get from theory were reached through approximations. Very, very few results were ever obtained through an exact solution, and science would be the poorer if we demand that only exact solutions are valid.

The fact is: each of the Feynman diagrams requires a precise formulation base on the number of straight lines, vertices, wiggling lines, incoming and outcoming lines. If those interactions don't represent the real McCoy, I wonder what does, certainly not the wave model!


(Note I've never claimed that the wave model "represents the real McCoy", merely that there are many scenarios, particularly for light, where the wave picture is much more helpful than the particle one.)

The point I'm trying to make is that If we had a model that we could solve exactly, the word "virtual particle" would never have existed. If we could find a way of solving for S in terms of the incoming and outgoing momenta and spins

|pout1, sout1,...poutm, soutm>=S(pout1, sout1, ...poutm, soutm,pin1, sin1, ...pinm, sinm )
|pin1, sin1,...pinm,sinm>

without resorting to this perturbation expansion, then we wouldn't need the approximation scheme that is perturbation theory.

Just because the approximation method is very good, it doesn't mean that the ingredients in the approximation method are physically real.

If you want to, you can do classical mechanics as an approximation using feynman diagrams. If the physical world were classical, would this imply that that the elements of the approximation would have any physical basis ? No, because we know that we can solve the equations exactly in some cases, or use a variety of different approximation techniques.


zaybu wrote:
Each order has a smaller and smaller probability to happen. It doesn't mean that they don't happen. Remember QM, and by extension QFT, is a theory about probabilities from the start. Are we to say that then, being just probabilities, it doesn't describe reality?


No indeed, the probabilities represent the best that can be stated about the system, even in principle. (I'm not a hidden variables person).

Do you see what I'm saying though ? In this perturbation scheme, which works very well, to get the full answer you'd have to add ALL orders:

The answer = (diagrams of order α2) + (diagrams of order α4) + .......

Do you see the problem ?
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Re: Light, Waves, Particles etc

#49  Postby zaybu » Sep 26, 2011 9:09 am

twistor59 wrote:

A model is just something that allows us to make predictions. In this case, the prediction is about the behaviour of the outgoing particles in response to the incoming particles. Its connection with the physical world is through the in-states and out-states, nothing else.

We can say the same about spin. We never see it, just a beam of electrons that separates into two in a Stern-Gerlach experiment. Are you saying that spin is not a reality, just a mathematical gimmick? How about the W's and Z bosons, those also aren't directly observed? What about the gluons and quarks, none of these "particles" are directly observed, are they also mathematical "gimmick"? Why bother pursuing the Higgs boson, since it would be just another mathematical "gimmick"?
zaybu wrote:
Einstein's E=mc2 also comes from an approximation. We nevertheless see that as something describing a real phenomenon. Most of the stuff we get from theory were reached through approximations. Very, very few results were ever obtained through an exact solution, and science would be the poorer if we demand that only exact solutions are valid.

The fact is: each of the Feynman diagrams requires a precise formulation base on the number of straight lines, vertices, wiggling lines, incoming and outcoming lines. If those interactions don't represent the real McCoy, I wonder what does, certainly not the wave model!



The point I'm trying to make is that If we had a model that we could solve exactly, the word "virtual particle" would never have existed. If we could find a way of solving for S in terms of the incoming and outgoing momenta and spins


Are you saying that E=mc2 doesn't reflect reality because it comes from an approximation?



Just because the approximation method is very good, it doesn't mean that the ingredients in the approximation method are physically real.


In this case, the calculation is based on the precision of how the interaction takes place, which assumes the exchange of particles in a very specific way.



If you want to, you can do classical mechanics as an approximation using feynman diagrams. If the physical world were classical, would this imply that that the elements of the approximation would have any physical basis ? No, because we know that we can solve the equations exactly in some cases, or use a variety of different approximation techniques.

There is a similarity between perturbation in classical physics and in QFT, but the two are very far apart.

For instance, we use the Hamiltonian in both Classical physics and QFT, but they represent altogether different things. In QFT, H is an operator on the the Hilbert states. It is also the operator that appears in the time evolution operator, making unitarity one of the fundamental requisite of QM. None of that is to be found in classical physics. There are similarities, but also differences.


zaybu wrote:
Each order has a smaller and smaller probability to happen. It doesn't mean that they don't happen. Remember QM, and by extension QFT, is a theory about probabilities from the start. Are we to say that then, being just probabilities, it doesn't describe reality?


No indeed, the probabilities represent the best that can be stated about the system, even in principle. (I'm not a hidden variables person).

Do you see what I'm saying though ? In this perturbation scheme, which works very well, to get the full answer you'd have to add ALL orders:

The answer = (diagrams of order α2) + (diagrams of order α4) + .......

Do you see the problem ?

The mathematics is always based on a model. When Einstein predicted that light from a star would deflect as it passes near the sun, he based that calculations on a model: that space-time is curved. Do we ever observe this curvature? No, we don't, what we do see is that light does bend, and so we accept the model as such.

Each term in the perturbation series is based on a model: that particles are exchanged -- in QED, it is the photons; in QCD, it is the gluons; in the electro-weak theory, the W's and Z bosons. And this model is the best there is to explain what's going on. It's not just a fluke or a mathematical gimmick. The evidence supporting that model is overwhelming.

It turns out that this approximation method gives an insight into nature better than any other mathematical "gimmick". Besides, no one has ever solved that equation exactly, perhaps that is an indication that the approximation method is the only real deal, and looking for an exact solution is like chasing ghosts.
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Re: Light, Waves, Particles etc

#50  Postby twistor59 » Sep 29, 2011 7:40 am

zaybu wrote:
twistor59 wrote:
A model is just something that allows us to make predictions. In this case, the prediction is about the behaviour of the outgoing particles in response to the incoming particles. Its connection with the physical world is through the in-states and out-states, nothing else.

We can say the same about spin. We never see it, just a beam of electrons that separates into two in a Stern-Gerlach experiment. Are you saying that spin is not a reality, just a mathematical gimmick? How about the W's and Z bosons, those also aren't directly observed? What about the gluons and quarks, none of these "particles" are directly observed, are they also mathematical "gimmick"? Why bother pursuing the Higgs boson, since it would be just another mathematical "gimmick"?


You, not me, introduced the word "gimmick", which implies a trivialization. I've used "calculational device".

No, spin is a property of real particles, which we can measure in the laboratory. I repeat: real particles are represented in the In and Out states of the scattering process. They are the ONLY things which are measurable. The virtual particles in the perturbation expansion cannot be measured. What can be measured is the scattering cross section given, to a very good approximation, by summing the contributions of the diagrams at successively increasing order. What cannot be measured are the individual contributions to the cross section.

Feynman in an interview with Charles Weiner:
Feynman: I probably made diagrams to help me think about perturbation expressions. It was probably not any specific invention but just a sort of shorthand with which I was helping myself think, which gradually developed into specific rules for some diagrams.
Weiner: For helping you think physically ? In other words, you wher seeing in physical-
Feynman: No, mathematical expressions. Mathematical expressions. A diagram to help write down the mathematical expressions.



zaybu wrote:
Einstein's E=mc2 also comes from an approximation. We nevertheless see that as something describing a real phenomenon. Most of the stuff we get from theory were reached through approximations. Very, very few results were ever obtained through an exact solution, and science would be the poorer if we demand that only exact solutions are valid.

The fact is: each of the Feynman diagrams requires a precise formulation base on the number of straight lines, vertices, wiggling lines, incoming and outcoming lines. If those interactions don't represent the real McCoy, I wonder what does, certainly not the wave model!




zaybu wrote:
twistor59 wrote:
The point I'm trying to make is that If we had a model that we could solve exactly, the word "virtual particle" would never have existed. If we could find a way of solving for S in terms of the incoming and outgoing momenta and spins


Are you saying that E=mc2 doesn't reflect reality because it comes from an approximation?


I'm saying that the output of the calculation (the scattering cross section) connects with what can be measured, but that there is no reason to suppose that the "internals" of the calculation do. In fact they most definitely cannot.


zaybu wrote:
twistor59 wrote:
Just because the approximation method is very good, it doesn't mean that the ingredients in the approximation method are physically real.


In this case, the calculation is based on the precision of how the interaction takes place, which assumes the exchange of particles in a very specific way.



No, "assumes the exchange of particles" is an interpretation of the mathematics. The calculation is based on the perturbation expansion involving the QED vertex terms ΨbarγμΨAμ and the free particle electron and photon propagators.


zaybu wrote:
twistor59 wrote:
If you want to, you can do classical mechanics as an approximation using feynman diagrams. If the physical world were classical, would this imply that that the elements of the approximation would have any physical basis ? No, because we know that we can solve the equations exactly in some cases, or use a variety of different approximation techniques.

There is a similarity between perturbation in classical physics and in QFT, but the two are very far apart.

For instance, we use the Hamiltonian in both Classical physics and QFT, but they represent altogether different things. In QFT, H is an operator on the the Hilbert states. It is also the operator that appears in the time evolution operator, making unitarity one of the fundamental requisite of QM. None of that is to be found in classical physics. There are similarities, but also differences.



That's not relevant to the argument. I presented the classical mechanics argument to counter your stance that just because an approximation gives highly accurate results, the internals of the approximation must have some basis in reality.
Give me a square integrable function and I can expand it in terms of sinusoids, or if I feel so inclined, I can expand it in Walsh functions. Which is the "real" one ? They're just different ways to do the approximation.





zaybu wrote:
Each order has a smaller and smaller probability to happen. It doesn't mean that they don't happen. Remember QM, and by extension QFT, is a theory about probabilities from the start. Are we to say that then, being just probabilities, it doesn't describe reality?



twistor59 wrote:No indeed, the probabilities represent the best that can be stated about the system, even in principle. (I'm not a hidden variables person).

Do you see what I'm saying though ? In this perturbation scheme, which works very well, to get the full answer you'd have to add ALL orders:

The answer = (diagrams of order α2) + (diagrams of order α4) + .......

Do you see the problem ?


zaybu wrote:
The mathematics is always based on a model. When Einstein predicted that light from a star would deflect as it passes near the sun, he based that calculations on a model: that space-time is curved. Do we ever observe this curvature? No, we don't, what we do see is that light does bend, and so we accept the model as such.


Do we ever observe this curvature ? Yes of course, we can measure the components of the metric tensor, which uniquely determine the curvature. We just need rods and clocks. The curvature is thus a measurable quantity. Virtual particles are not, even in principle, measurable. For this reason, I am reluctant to attach terms such as "real" or "physical" to them.

zaybu wrote:
Each term in the perturbation series is based on a model: that particles are exchanged -- in QED, it is the photons; in QCD, it is the gluons; in the electro-weak theory, the W's and Z bosons. And this model is the best there is to explain what's going on. It's not just a fluke or a mathematical gimmick. The evidence supporting that model is overwhelming.


Again - I never introduced the word "fluke" or "gimmick" to the conversation. QED perturbation theory is a model par excellence. However it only makes contact with measurements though the in and out states. You can't, with any certainty, ascribe elements of reality to the internals of the calculation, although this helps with visualization and, as Feynman says, organizing the mathematics. It also helps sell books with popularizations of physics.

zaybu wrote:
It turns out that this approximation method gives an insight into nature better than any other mathematical "gimmick". Besides, no one has ever solved that equation exactly, perhaps that is an indication that the approximation method is the only real deal, and looking for an exact solution is like chasing ghosts.


Ah well, this is an interesting question. QED currently is currently formulated as a perturbative theory from the outset. But even if we were to say that it can only be formulated perturbatively (i.e. no non-perturbative formulation will ever be found), there is a problem:

Scattering amplitude = (contributions from diagrams of order α2) + (contributions from diagrams of order α4) +....

To get the "true" answer, we have to add the contributions of diagrams of all orders. Unfortunately that appears to give infinity.
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Re: Light, Waves, Particles etc

#51  Postby zaybu » Sep 29, 2011 1:32 pm

twistor59 wrote:[

QED perturbation theory is a model par excellence. However it only makes contact with measurements through the in and out states. You can't, with any certainty, ascribe elements of reality to the internals of the calculation, although this helps with visualization and, as Feynman says, organizing the mathematics. It also helps sell books with popularizations of physics.


If you consider only the in and out states as real, then quarks, gluons, W's and Z bosons are never in these in and out states, and by that arguments, they are not real. Ditto with the Higgs boson. We would have to conclude that the majority of the particles in the Standard Model are fictional!


To get the "true" answer, we have to add the contributions of diagrams of all orders. Unfortunately that appears to give infinity.


Each diagram has a probability coefficient, which gets smaller and smaller with higher terms, if the series converge. But that's besides the point. What's important is that the calculation is based on a model. If the calculations then give you predictions that are confirmed by observation, then we take the model to represent reality. That's the way science has always been. I mean, do we ever see a proton or an electron?
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Re: Light, Waves, Particles etc

#52  Postby mizvekov » Oct 01, 2011 7:16 am

twistor59 wrote:QED perturbation theory is a model par excellence. However it only makes contact with measurements through the in and out states. You can't, with any certainty, ascribe elements of reality to the internals of the calculation, although this helps with visualization and, as Feynman says, organizing the mathematics. It also helps sell books with popularizations of physics.
Although certainty is not really a good argument to select what to ascribe reality to. Besides the points you have raised, doing ontology in physics helps people make sense of it, and it helps them use scientific knowledge in their arguments in natural language, without having to recourse to mathematics and model building for simple claims, even though there is always the risk there that the concept will be used beyond it's domain and absurd conclusions be drawn.

Still, doing no ontology at all (the shut up and calculate approach) is hardly advocated by the most prominent physicists, and not really used at all for education. Kids are still going to be taught that atoms exists, or that magnetic fields exist, even if you don't need to assume they exist to calculate stuff. These concepts make a lot of sense to us, they are useful for communicating and some of them are even hard on the mind to deny their existence.

zaybu wrote:Each diagram has a probability coefficient, which gets smaller and smaller with higher terms, if the series converge. But that's besides the point. What's important is that the calculation is based on a model. If the calculations then give you predictions that are confirmed by observation, then we take the model to represent reality. That's the way science has always been. I mean, do we ever see a proton or an electron?

Well, on the other hand you are taking scientific realism to the extreme there. I don't think such a position is even common, and physicists normally think in terms of other criteria (mainly usefulness) to decide what to ascribe reality to.
There are physicists who deny existence of forces because they are unnecessary, others who defend them because it is still useful to think and to teach in terms of forces, and the same is true with many other concepts. In fact, it seems to be very even out between QFT physicists who think particles exist but fields don't, those who think particles don't but fields do, and even those who think neither exist.

About the existence of virtual particles, I think it is a very controversial issue. There are those who argue in favor of their existence because it's useful from a pedagogical point of view as there are still some processes which the mental picture of virtual particles is the one which makes the most sense (for example vacuum polarization). There are others who deny their existence on the grounds that they think it's just a mathematical gimmick and that there are many models which, although normally thought of in terms of virtual particles, can be described without using them. See for example the following paper: The Casimir Effect and the Quantum Vacuum where they build a model without any recourse at all to virtual particles.
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Re: Light, Waves, Particles etc

#53  Postby mizvekov » Oct 01, 2011 9:49 am

By the way, I forgot to add to the last post that I think there is another potential source of confusion going on around here.
QFT has a definition of real particles, which is contrasted with virtual particles, and the former are what correspond to ins and outs of feynman diagrams, while the later are the internal arrows.
Under this definition, and this is what I think twistor is using, everyone has to agree that virtual particles are not real, because it closely follows from the definition.
There is also the 'real' definition of what exists in an ontological sense, and that seems to be what zaybu is using, because it follows from his view of scientific realism.

I think there are other interesting ways to think about how 'real' (in the ontological sense) virtual particles are, for example you can view it as a sort of fuzzy, ie it can have 'shades' of realness.
For example, take this passage:
Introduction to Elementary Particles, 1st edition, page 60 wrote:... a virtual particle does not carry the same mass as the corresponding free particle. In fact, a virtual particle can have any mass - whatever the conservation laws require. * In the business, we say that virtual particles do not lie on their mass shell. External lines, by contrast, represent real particles, and these do carry the "correct" mass.
[Actually, the physical distinction between real and virtual particles is not quite as sharp as I have implied. If a photon is emitted on Alpha Centauri, and absorbed in your eye, it is technically a virtual photon, I suppose. However, in general, the farther a virtual particle is from its mass shell the shorter it lives, so a photon from a distant star would have to be extremely close to its "correct" mass; it would have to be very close to "real." As a calculational matter, you would get essentially the same answer if you treated the process as two separate events (emission of a real photon by star, followed by absorption of a real photon by eye). You might say that a real particle is a virtual particle which lasts long enough that we don't care to inquire how it was produced, or how it is eventually absorbed.]

I think the one above is a very interesting view on the matter, but some people find 'shades of realness' unnaceptable in principle (either something exists or ir does not).
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Re: Light, Waves, Particles etc

#54  Postby zaybu » Oct 01, 2011 12:05 pm

mizvekov wrote:By the way, I forgot to add to the last post that I think there is another potential source of confusion going on around here.
QFT has a definition of real particles, which is contrasted with virtual particles, and the former are what correspond to ins and outs of feynman diagrams, while the later are the internal arrows.



If you take that position, as I have already pointed out to Twistor, then quarks, gluons, W's and Z bosons are never in these in and out states, and by that arguments, they are not real. Ditto with the Higgs boson.

I believe that "virtual" in QFT means "cannot be seen while the exchange takes place". It doesn't mean they are fictional. They are as real as any other "particles", and that's the gist of my arguments: at higher energies, what exist are just particles.
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Re: Light, Waves, Particles etc

#55  Postby mizvekov » Oct 01, 2011 12:32 pm

zaybu wrote:If you take that position, as I have already pointed out to Twistor, then quarks, gluons, W's and Z bosons are never in these in and out states, and by that arguments, they are not real. Ditto with the Higgs boson.

I don't take any position, I merely note that there is that definition that most QFT materials seem to endorse, but I also note that it is not equivalent to an ontological claim to reality. It's just the same word, 'real', used in two different meanings.
zaybu wrote:I believe that "virtual" in QFT means cannot be seen. It doesn't mean they are fictional. They are as real as any other "particles", and that's the gist of my arguments: at higher energies, what exist are just particles.

And as that book I quoted made this point, there is another way to claim that a particle is virtual, and that is to say that it does not obey the m2c4 = E2 − p2c2 relationship, or what he says about the particles being "off mass shell". Eg, the particles can violate energy conservation for short periods of time by not having the "correct" mass. As more "incorrect" the mass of the particle is, the more likely it is for it to be destroyed by destructive interference, and so less time it is expected to live.
But the weird thing of this account is that you cannot say that any particles are "real" either, they are only approximately (sometimes to a ridiculously high degree) real.
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Re: Light, Waves, Particles etc

#56  Postby hackenslash » Oct 01, 2011 1:41 pm

Suddenly, much becomes clear. It seems that our earlier disagreement rests upon what actually constitutes a violation. I would contend that short-term or local violations are no violation at all, because conservation holds overall. I think this is the semantic sticking point we were held up on, and I see no problem now in your earlier contention, as long as it's made clear that what we're talking about is local violations, either in the temporal sense or otherwise. The net energy/mass remains the same overall, yes?
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Re: Light, Waves, Particles etc

#57  Postby zaybu » Oct 01, 2011 2:25 pm

mizvekov wrote:

And as that book I quoted made this point, there is another way to claim that a particle is virtual, and that is to say that it does not obey the m2c4 = E2 − p2c2 relationship, or what he says about the particles being "off mass shell". Eg, the particles can violate energy conservation for short periods of time by not having the "correct" mass. As more "incorrect" the mass of the particle is, the more likely it is for it to be destroyed by destructive interference, and so less time it is expected to live.
But the weird thing of this account is that you cannot say that any particles are "real" either, they are only approximately (sometimes to a ridiculously high degree) real.


You can make an argument that the word "particle" is different in QFT than what it is in classical physics, and I would agree. In QM, a particle behaves more like a wave packet. But even in that prescription, at higher and higher energies ( smaller and smaller distances) the wave packet is almost synonymous with a particle. And hence, we can talk about the mass of a particle, we never speak of the mass of a wave.

NB: String theorists will argue that, at even smaller scale (planck scale), you get strings.
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Re: Light, Waves, Particles etc

#58  Postby mizvekov » Oct 01, 2011 9:29 pm

hackenslash wrote:Suddenly, much becomes clear. It seems that our earlier disagreement rests upon what actually constitutes a violation. I would contend that short-term or local violations are no violation at all, because conservation holds overall. I think this is the semantic sticking point we were held up on, and I see no problem now in your earlier contention, as long as it's made clear that what we're talking about is local violations, either in the temporal sense or otherwise. The net energy/mass remains the same overall, yes?

But that's just QM/QFT. Later in that thread, I was also defending the other issue that the GR field equations don't take energy conservation seriously at all, as the article by Sean Carroll points it out. You would certainly be very out of your element if you talked about conservation of energy in cosmological scales. See for example:
http://johanw.home.xs4all.nl/PhysFAQ/Relativity/GR/energy_gr.html
http://www.physicsforums.com/showthread.php?t=506985
And even in QM/QFT, claiming that energy conservation holds at all times is too strong a claim, since energy conservation (EC) is an emergent property that itself arises from the destructive interference patterns of EC violating processes.
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Re: Light, Waves, Particles etc

#59  Postby hackenslash » Oct 01, 2011 10:28 pm

Thanks. I'll absorb those later.
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Re: Light, Waves, Particles etc

#60  Postby zaybu » Oct 02, 2011 1:17 pm

mizvekov wrote:
hackenslash wrote:Suddenly, much becomes clear. It seems that our earlier disagreement rests upon what actually constitutes a violation. I would contend that short-term or local violations are no violation at all, because conservation holds overall. I think this is the semantic sticking point we were held up on, and I see no problem now in your earlier contention, as long as it's made clear that what we're talking about is local violations, either in the temporal sense or otherwise. The net energy/mass remains the same overall, yes?

But that's just QM/QFT. Later in that thread, I was also defending the other issue that the GR field equations don't take energy conservation seriously at all, as the article by Sean Carroll points it out. You would certainly be very out of your element if you talked about conservation of energy in cosmological scales. See for example:
http://johanw.home.xs4all.nl/PhysFAQ/Relativity/GR/energy_gr.html
http://www.physicsforums.com/showthread.php?t=506985
And even in QM/QFT, claiming that energy conservation holds at all times is too strong a claim, since energy conservation (EC) is an emergent property that itself arises from the destructive interference patterns of EC violating processes.



This is a bit off topic. However, if you're interested in the opposite view, Phillip Gibbs is a great proponent that energy is conserved in GR. See his paper: http://www.prespacetime.com/index.php/p ... File/89/85
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