Light, Waves, Particles etc

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Re: Light, Waves, Particles etc

#61  Postby mizvekov » Oct 02, 2011 5:10 pm

zaybu wrote: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
Hmm, thanks...
Had never seen any active opposition on this issue, guess I'll have a check to see if there is anything to it.
Anyway, thanks for mentioning it :)
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Re: Light, Waves, Particles etc

#62  Postby twistor59 » Oct 03, 2011 12:39 pm

zaybu wrote:
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.



Looking at the electroweak theory, there are two types of scenario:
1) A case where a W-interaction takes place as an internal line, like this.
Muon_Decay_j.jpg
Muon_Decay_j.jpg (9.01 KiB) Viewed 742 times


Here, the phrase “a virtual W- is produced, which subsequently decays to electron and antineutrino” is often used. These are the cases where I’m claiming that the description of the internal line as a “virtual W particle” can be a bit misleading.

The second type of scenario is
2) A case where W particles are produced in the out-state. In this case the W’s are real and their properties can be measured.
WProdn.jpg
WProdn.jpg (9.27 KiB) Viewed 742 times

(the thing that came out as a question mark was meant to be a gamma, i.e a virtual photon).

Same for the Higgs. We have our EWSB models where the Higgs interacts with other particles giving them mass, but these are “virtual Higgs” interactions. That’s why we need to go to all the expense of trying to produce a real Higgs, which will appear in an out-state.

The situation with quarks and gluons is a bit trickier. I don’t have a working knowledge of QCD, but the impression I get is that you can’t do perturbative S-matrix calculations in the usual way because the in and out-states would have to contain free quarks and/or free gluons and this is not possible. Instead of free quarks/gluons what you see in the lab is hadronization/jets. There are, however, a set of tricks that you can use to apply perturbation theory to compute jet cross sections.

Although you can’t isolate a quark or gluon, you can do experiments to estimate their properties. They are real particles, by any reasonable criteria for the definition of “real”.
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Re: Light, Waves, Particles etc

#63  Postby zaybu » Oct 03, 2011 6:07 pm

So do we agree now that at higher energies (smaller scale), we have particles and nothing but particles?
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Re: Light, Waves, Particles etc

#64  Postby twistor59 » Oct 04, 2011 7:06 am

zaybu wrote:So do we agree now that at higher energies (smaller scale), we have particles and nothing but particles?


:lol: I'm not totally sure exactly what that statement means, but did I write something that makes you think I changed my mind ?

The question we were discussing is whether the "particle exchange" represented by the internal lines of Feynman diagrams actually occurs or not. I claim that there is no evidence that it does occur - the "particle exchange" is merely an element of an approximation scheme - the perturbation series. It has no more "reality" than an image charge in electrostatics or a Faraday force line.

How would you define "particle" ?
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Re: Light, Waves, Particles etc

#65  Postby zaybu » Oct 04, 2011 1:27 pm

twistor59 wrote:
zaybu wrote:So do we agree now that at higher energies (smaller scale), we have particles and nothing but particles?


:lol: I'm not totally sure exactly what that statement means, but did I write something that makes you think I changed my mind ?


This particular one: "That’s why we need to go to all the expense of trying to produce a real Higgs, which will appear in an out-state." :grin:

We can't see virtual particles -- incidentally, that's why they are called "virtual" -- but as you say, we are trying our greatest efforts to see them as "real" particles. And when we do confirm their existence as real particles, that gives more support to the model, which is that particles are exchanged when they interact.
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Re: Light, Waves, Particles etc

#66  Postby twistor59 » Oct 04, 2011 8:38 pm

zaybu wrote:
twistor59 wrote:
zaybu wrote:So do we agree now that at higher energies (smaller scale), we have particles and nothing but particles?


:lol: I'm not totally sure exactly what that statement means, but did I write something that makes you think I changed my mind ?


This particular one: "That’s why we need to go to all the expense of trying to produce a real Higgs, which will appear in an out-state." :grin:

We can't see virtual particles -- incidentally, that's why they are called "virtual" -- but as you say, we are trying our greatest efforts to see them as "real" particles. And when we do confirm their existence as real particles, that gives more support to the model, which is that particles are exchanged when they interact.


Yes, I've no objections to real Higgses. Assuming the Higgs field exists, you can wiggle it and put a "real" Higgs particle in it.

I'll even concede that the "real" in and out-state particles might be slighly off-shell. However, I can't be happy with assigning "elements of reality" (as Bohm would say) to internal lines in Feynman diagrams. Perturbation theory is just a way of doing an otherwise intractable calculation.

What is a particle ?
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Re: Light, Waves, Particles etc

#67  Postby cavarka9 » Oct 05, 2011 6:11 pm

dont mind me but yes,what is a particle?.
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Re: Light, Waves, Particles etc

#68  Postby hackenslash » Oct 05, 2011 10:10 pm

Probably a behavioural manifestation of the electromagnetic field...
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Re: Light, Waves, Particles etc

#69  Postby twistor59 » Oct 06, 2011 9:34 am

LOL at Hrvoje's post :naughty2:
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Re: Light, Waves, Particles etc

#70  Postby zaybu » Oct 06, 2011 12:48 pm

cavarka9 wrote:dont mind me but yes,what is a particle?.


hackenslash wrote:Probably a behavioural manifestation of the electromagnetic field...


What is a field?
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Re: Light, Waves, Particles etc

#71  Postby cavarka9 » Oct 06, 2011 1:04 pm

zaybu wrote:
cavarka9 wrote:dont mind me but yes,what is a particle?.


hackenslash wrote:Probably a behavioural manifestation of the electromagnetic field...


What is a field?


I was considering that a particle is a structure which has certain properties that it follows,the most important being that it is localized, it must follow conservation of energy, spin, charge and must not do crazy things like vanish without any trace or appear from no where, unless there are perhaps other dimensions. It must also not behave like a wave.

But i get your point, we can at best describe with clarity with the help of math, take the math away and words from human language seem inadequate.

I however think, that for something to be a particle it must be localized and must follow conservation.

While we are at it, might I interest you guys and gals on indistinguishability between particles and their identities.
Isnt a bose-einstein condensate a sort of super atom or perhaps super particle.
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Re: Light, Waves, Particles etc

#72  Postby zaybu » Oct 06, 2011 1:40 pm

twistor59 wrote:

What is a particle ?


At higher energies, the so-called "waves" behave more and more like wave packets, which are closer to particles than waves. That's why the Standard Model is about particles. You get tables listing the particles and their properties - mass, spin, charge, parity, etc. One of the striking differences between QM and QFT textbooks is the absence of the wave model in the latter.
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Re: Light, Waves, Particles etc

#73  Postby Teuton » Oct 06, 2011 7:22 pm

zaybu wrote:
What is a field?


A physical field is a collection of spatiotemporally distributed determinate physical properties which belong to a determinable physical quantity and are possessed by points or regions of spacetime.
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Re: Light, Waves, Particles etc

#74  Postby twistor59 » Oct 06, 2011 7:37 pm

zaybu wrote:
cavarka9 wrote:dont mind me but yes,what is a particle?.


hackenslash wrote:Probably a behavioural manifestation of the electromagnetic field...


What is a field?


It's an operator-valued distribution.

If you want to read about it, I recommend Streater and Wightman.

Edit: after seeing Teuton's response, I should clarify that I'm talking about a quantum field, not a generic field. There are also people who define quantum fields in Teutonic language :o
Last edited by twistor59 on Oct 06, 2011 7:49 pm, edited 1 time in total.
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Re: Light, Waves, Particles etc

#75  Postby twistor59 » Oct 06, 2011 7:45 pm

cavarka9 wrote:
I however think, that for something to be a particle it must be localized and must follow conservation.


Yep, when people think of particles, they usually have some sort of localization in mind.

Conservation, yes, you want the particle to retain some sort of identity. However, with indistinguishable particles you can't say which is which if they have any overlap. Classic analogy is two people with a piece of rope, they simultaneously wiggle their ends, the wiggles travel towards each other, then pass through each other and are received by the other guy. Or do they ? Who's to say that the two wiggles didn't travel towards each other then bounce off each other and back to the originator ? The wiggles are indistinguishable, and so are identical particles in quantum theory.

What you CAN do, though, is talk about the number of particles. Particles are countable.
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Re: Light, Waves, Particles etc

#76  Postby twistor59 » Oct 06, 2011 7:57 pm

zaybu wrote:
twistor59 wrote:

What is a particle ?


At higher energies, the so-called "waves" behave more and more like wave packets, which are closer to particles than waves. That's why the Standard Model is about particles. You get tables listing the particles and their properties - mass, spin, charge, parity, etc. One of the striking differences between QM and QFT textbooks is the absence of the wave model in the latter.


Yes indeed, there are tables listing the mass, spin etc of particles. Won't find any listing the properties of "virtual particles" though, since they're just bookmarkers for an approximation scheme. :naughty2:
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Re: Light, Waves, Particles etc

#77  Postby Teuton » Oct 06, 2011 8:02 pm

twistor59 wrote:
Edit: after seeing Teuton's response, I should clarify that I'm talking about a quantum field, not a generic field. There are also people who define quantum fields in Teutonic language :o


What is the essential physical difference between "classical" fields and quantum fields?
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Re: Light, Waves, Particles etc

#78  Postby mizvekov » Oct 06, 2011 8:03 pm

cavarka9 wrote:I was considering that a particle is a structure which has certain properties that it follows,the most important being that it is localized, it must follow conservation of energy, spin, charge and must not do crazy things like vanish without any trace or appear from no where, unless there are perhaps other dimensions. It must also not behave like a wave.

That's the classical definition of particle, and if that's the only definition that you find acceptable, then in QM and QFT there are no such particles.
There is another 'thing' in QFT that is also called 'particle', and these things under certain circumstances (high energies) behave approximately like the classical particles. These particles can be though of as ripples in the quantum field, and they are described in terms of harmonic oscillators.
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Re: Light, Waves, Particles etc

#79  Postby twistor59 » Oct 06, 2011 8:13 pm

Teuton wrote:
twistor59 wrote:
Edit: after seeing Teuton's response, I should clarify that I'm talking about a quantum field, not a generic field. There are also people who define quantum fields in Teutonic language :o


What is the essential physical difference between "classical" fields and quantum fields?


A classical field assigns a value of some measurable quantity to each point in spacetime.

A quantum field assigns an operator to each point in spacetime (this is the heuristic version - the more mathematically rigorous version involves operator valued distributions as I alluded to earlier). This operator acts on "states" which live in an abstract (Hilbert) space and does stuff like changing the number of quanta.
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Re: Light, Waves, Particles etc

#80  Postby Teuton » Oct 06, 2011 8:15 pm

Teuton wrote:
What is the essential physical difference between "classical" fields and quantum fields?


"In very loose terms, the operator valuedness of quantum fields means that to each space-time point (x,t) a field value φ(x,t) is assigned which is an operator. This is the fundamental difference to classical fields because an operator valued quantum field φ(x,t) does not by itself correspond to definite values of a physical quantity like the strength of the electromagnetic field. On this background, Teller has argued in Teller 1995 that the field interpretation of QFT is inappropriate since the alleged fields in QFT are not to be interpreted as physical fields with definite values of some sort which are assigned to space-time points, like in the case of the classical electromagnetic field. Rather, quantum fields are what Teller calls ‘determinables’ (p. 95), as it becomes manifest by the fact that quantum fields are described by mappings from space-time points to operators. Operators are mathematical entities which are defined by how they act on something. They do not represent definite values of quantities but they specify what can be measured, therefore Teller's expression ‘determinables’. (Below it will be discussed why this talk in terms of a field at a point has to be refined using the notion of a smeared field φ(f).)"

(http://plato.stanford.edu/entries/quantum-field-theory/)

"[Operators] do not represent definite values of quantities but they specify what can be measured[.]"

Does this mean that operators represent the range of all possible determinate properties of a determinable quantity that can be possessed by a spacetime point? But how could an actual physical field be a collection of merely possible properties? Is it possible at all for there to be indeterminate physical properties in physical reality? For example, is it possible to have a mass without having a determinate mass?
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