The Higgs Boson: what are the odds?

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Re: The Higgs Boson: what are the odds?

#21  Postby zaybu » Sep 13, 2011 6:47 pm

twistor59 wrote:
zaybu wrote:
epepke wrote:




Ha, ha, Einstein would disagree -- his explanation of the photoelectric effect went against the thinking of the day since light was thought as waves since Maxwell, and a course in QFT would convince you otherwise. Photons are the "particles" that mediate the electromagnetic force. Without that, you might as well through QED under the bus.


It's the many courses in QFT I've taken which convinces me that this is the case !

I would prefer to refer to photons as "field quanta", and reserve the term "particle" for entities that had position operators. Photons do not have position operators (regardless of what Margaret Hawton may say). In fact I am not alone in having this view. Lamb states:

Photons cannot be localized in any meaningful manner, and they do not behave at all like particles, whether described by a wave function or not.


I wouldn't put it as strongly as Lamb did - they do sometimes behave like particles (Compton scattering), but I don't like attaching the term "particle" to something without a well defined position operator, since it gives a misleading mental picture.



You have to remember that in QFT, position is no longer an operator. To satisfy Relativity, in which time is on an equal footing with space: x → parameter, and is replaced by the field Φ(x) → operator.

When you look at a Lagrangian in the Higgs mechanism, such as:

Lfree = ½∂μh∂μh - m2h2 - ¼ Fμν Fμν + q2φ2AμAμ

The second term refers to a scalar particle with mass equal to 2½m, associated with h (the higgs field) and the fourth term, a vector boson with mass 2½qφ, associated with Aμ( the electromagnetic field).

This is the core of QFT, particles with mass, even though the main actors are the fields.
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Re: The Higgs Boson: what are the odds?

#22  Postby twistor59 » Sep 13, 2011 8:20 pm

zaybu wrote:
twistor59 wrote:
zaybu wrote:


It's the many courses in QFT I've taken which convinces me that this is the case !

I would prefer to refer to photons as "field quanta", and reserve the term "particle" for entities that had position operators. Photons do not have position operators (regardless of what Margaret Hawton may say). In fact I am not alone in having this view. Lamb states:

Photons cannot be localized in any meaningful manner, and they do not behave at all like particles, whether described by a wave function or not.


I wouldn't put it as strongly as Lamb did - they do sometimes behave like particles (Compton scattering), but I don't like attaching the term "particle" to something without a well defined position operator, since it gives a misleading mental picture.



You have to remember that in QFT, position is no longer an operator. To satisfy Relativity, in which time is on an equal footing with space: x → parameter, and is replaced by the field Φ(x) → operator.



I'm quite aware that spacetime position in QFT is not an operator, but rather a parameter. What I'm talking about is a position operator, not x, which can be defined for electrons (for example), but not for photons.
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Re: The Higgs Boson: what are the odds?

#23  Postby zaybu » Sep 13, 2011 8:56 pm

twistor59 wrote:

I'm quite aware that spacetime position in QFT is not an operator, but rather a parameter. What I'm talking about is a position operator, not x, which can be defined for electrons (for example), but not for photons.


You've lost me there, twistor. Which position operator is there in QFT?
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Re: The Higgs Boson: what are the odds?

#24  Postby Sovereign » Sep 14, 2011 1:21 am

If Higgs does not exist, it could be a boon for those who favor selector calculus. It would be interesting to see what new(and old) theories come to the forefront to address this but personally, I think it's too soon to give up. If they don't find Higgs, they may find something altogether unexpected that could be just as good.
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Re: The Higgs Boson: what are the odds?

#25  Postby twistor59 » Sep 14, 2011 7:28 am

zaybu wrote:
twistor59 wrote:

I'm quite aware that spacetime position in QFT is not an operator, but rather a parameter. What I'm talking about is a position operator, not x, which can be defined for electrons (for example), but not for photons.


You've lost me there, twistor. Which position operator is there in QFT?


Position operators like this one. They don't generalize to photons.

Another way to put it is that, for an electron, I can define a wavefunction Ψ such that |Ψ|2d3x represents the probability of finding the particle within the volume d3x. (I'm ignoring spin) You can in principle localise the electron to a volume as small as you like, within the constraints imposed by the uncertainty principle.

If you try to define such a wavefunction for the photon, you enter an area of ongoing heated debate. If you set up the candidate wavefunction, and then try to establish the pdf of the photon position, you realize that you can only do this by looking at what happens to test charges (since these are what establish the photon's presence or absence). But this interaction with test charges at any point x is determined by the value of the EM fields E(x), B(x). However it turns out that E(x), B(x) depend non locally on the candidate wavefunction, so the project fails.

So, for me, photons are less "particle like" than electrons, and I think it would be clearer if we encouraged people to refer to "field quanta", since quantum fields are the primitive entities, not particles. "Particles" belong more to first quantization treatments, and photons seem very reluctant to be treated in this way !
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Re: The Higgs Boson: what are the odds?

#26  Postby zaybu » Sep 14, 2011 11:16 am

twistor59 wrote:
zaybu wrote:
twistor59 wrote:

I'm quite aware that spacetime position in QFT is not an operator, but rather a parameter. What I'm talking about is a position operator, not x, which can be defined for electrons (for example), but not for photons.


You've lost me there, twistor. Which position operator is there in QFT?


Position operators like this one. They don't generalize to photons.

Another way to put it is that, for an electron, I can define a wavefunction Ψ such that |Ψ|2d3x represents the probability of finding the particle within the volume d3x. (I'm ignoring spin) You can in principle localise the electron to a volume as small as you like, within the constraints imposed by the uncertainty principle.

If you try to define such a wavefunction for the photon, you enter an area of ongoing heated debate. If you set up the candidate wavefunction, and then try to establish the pdf of the photon position, you realize that you can only do this by looking at what happens to test charges (since these are what establish the photon's presence or absence). But this interaction with test charges at any point x is determined by the value of the EM fields E(x), B(x). However it turns out that E(x), B(x) depend non locally on the candidate wavefunction, so the project fails.

So, for me, photons are less "particle like" than electrons, and I think it would be clearer if we encouraged people to refer to "field quanta", since quantum fields are the primitive entities, not particles. "Particles" belong more to first quantization treatments, and photons seem very reluctant to be treated in this way !



You're still stuck in QM. QFT goes beyond QM. In QM, x is an observable, and therefore an operator. Ψ is the wavefunction on which any operator, such as x or the momentum p, operates on. And you are correct in stating that:|Ψ|2d3x represents the probability of finding the particle within the volume d3x.

In QFT, there is a dramatic change with these players. X is no longer an operator, it is replaced by a field, which is Ψ in the old theory, which is now THE operator. A lot of people after taking a course in QM get lost when they take a course in QFT. It's a totally different world.

Now experimentally, if you sent one photon at a time through the two-slit experiment, what you see is tiny little grains hitting the screen. The dark and light bands appear only after a very long time. So photons are particles. Describing them by a wave is only good when we are dealing with extremely large numbers of them.

SEE: http://www.youtube.com/watch?v=MbLzh1Y9POQ
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Re: The Higgs Boson: what are the odds?

#27  Postby Jehannum » Sep 14, 2011 1:13 pm

mizvekov wrote:
eric8476 wrote:how can something with no mass have energy? how is a particle associating with energy and how can particles be observable if they have no mass?

Particles with no mass, like the photon, can still interact with ordinary matter, changing their velocity, because they have momentum.
So in simple words, a photon colliding with an atom at rest will make the atom move.


Am I right in thinking all matter is 'at rest' from the point of view of a photon?
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Re: The Higgs Boson: what are the odds?

#28  Postby cavarka9 » Sep 14, 2011 1:41 pm

Jehannum wrote:
mizvekov wrote:
eric8476 wrote:how can something with no mass have energy? how is a particle associating with energy and how can particles be observable if they have no mass?

Particles with no mass, like the photon, can still interact with ordinary matter, changing their velocity, because they have momentum.
So in simple words, a photon colliding with an atom at rest will make the atom move.


Am I right in thinking all matter is 'at rest' from the point of view of a photon?


wrong, if that were the case, then the photon on retracing its path should find the same matter at the same place all the time.
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Re: The Higgs Boson: what are the odds?

#29  Postby twistor59 » Sep 14, 2011 2:12 pm

zaybu wrote:
twistor59 wrote:
zaybu wrote:

You've lost me there, twistor. Which position operator is there in QFT?


Position operators like this one. They don't generalize to photons.

Another way to put it is that, for an electron, I can define a wavefunction Ψ such that |Ψ|2d3x represents the probability of finding the particle within the volume d3x. (I'm ignoring spin) You can in principle localise the electron to a volume as small as you like, within the constraints imposed by the uncertainty principle.

If you try to define such a wavefunction for the photon, you enter an area of ongoing heated debate. If you set up the candidate wavefunction, and then try to establish the pdf of the photon position, you realize that you can only do this by looking at what happens to test charges (since these are what establish the photon's presence or absence). But this interaction with test charges at any point x is determined by the value of the EM fields E(x), B(x). However it turns out that E(x), B(x) depend non locally on the candidate wavefunction, so the project fails.

So, for me, photons are less "particle like" than electrons, and I think it would be clearer if we encouraged people to refer to "field quanta", since quantum fields are the primitive entities, not particles. "Particles" belong more to first quantization treatments, and photons seem very reluctant to be treated in this way !



You're still stuck in QM. QFT goes beyond QM. In QM, x is an observable, and therefore an operator. Ψ is the wavefunction on which any operator, such as x or the momentum p, operates on. And you are correct in stating that:|Ψ|2d3x represents the probability of finding the particle within the volume d3x.

In QFT, there is a dramatic change with these players. X is no longer an operator, it is replaced by a field, which is Ψ in the old theory, which is now THE operator. A lot of people after taking a course in QM get lost when they take a course in QFT. It's a totally different world.

Now experimentally, if you sent one photon at a time through the two-slit experiment, what you see is tiny little grains hitting the screen. The dark and light bands appear only after a very long time. So photons are particles. Describing them by a wave is only good when we are dealing with extremely large numbers of them.

SEE: http://www.youtube.com/watch?v=MbLzh1Y9POQ


Reread the last sentence (highlighted above) in my last post. I'm expressing the opinion that the term "particle" is best associated with first quantization i.e. quantum mechanics, and this is even less appropriate (some would say impossible) for photons, whereas it is for electrons. Photons are less localizable than electrons.
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Re: The Higgs Boson: what are the odds?

#30  Postby zaybu » Sep 14, 2011 3:18 pm

twistor59 wrote:
Reread the last sentence (highlighted above) in my last post. I'm expressing the opinion that the term "particle" is best associated with first quantization i.e. quantum mechanics, and this is even less appropriate (some would say impossible) for photons, whereas it is for electrons. Photons are less localizable than electrons.


Fine, but you can't stop just at QM, which was developped in the 1920-30's. A lot was said about QM in those days, which most of it being plain wrong, including statements by Bohr, Heisenberg, and yes, even Einstein. Only a thorough study of QFT, which was developped between 1930-1970's, will make those ideas go away. The most revolutionary idea was not from Einstein but from Yukawa who demonstrated that ''forces'' are really the exchange of particles. What was thought as matter and forces, we know now that it's really just particles, and all of these can be identified by their mass, spin, parity, and what types of charge they carry. All particles of ''matter'' have half-integral spin (fermions), all particles of ''forces'' have integral spin (bosons).It is really Yukawa's idea that made unification a possible idea in physics. If you look at that youtube video, there's no question that light is made of particles, spin-1 bosons known as photons.
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Re: The Higgs Boson: what are the odds?

#31  Postby twistor59 » Sep 14, 2011 6:44 pm

zaybu wrote:
twistor59 wrote:
Reread the last sentence (highlighted above) in my last post. I'm expressing the opinion that the term "particle" is best associated with first quantization i.e. quantum mechanics, and this is even less appropriate (some would say impossible) for photons, whereas it is for electrons. Photons are less localizable than electrons.


Fine, but you can't stop just at QM, which was developped in the 1920-30's. A lot was said about QM in those days, which most of it being plain wrong, including statements by Bohr, Heisenberg, and yes, even Einstein. Only a thorough study of QFT, which was developped between 1930-1970's, will make those ideas go away. The most revolutionary idea was not from Einstein but from Yukawa who demonstrated that ''forces'' are really the exchange of particles. What was thought as matter and forces, we know now that it's really just particles, and all of these can be identified by their mass, spin, parity, and what types of charge they carry. All particles of ''matter'' have half-integral spin (fermions), all particles of ''forces'' have integral spin (bosons).It is really Yukawa's idea that made unification a possible idea in physics. If you look at that youtube video, there's no question that light is made of particles, spin-1 bosons known as photons.


Since this is getting a bit off topic from your OP, I think we should continue this discussion in another thread, so I'll open one...

Edit off topic continued here.
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Re: The Higgs Boson: what are the odds?

#32  Postby zaybu » Sep 16, 2011 7:45 pm

Thomas Dorigo will be giving a talk on this topic. See his blog at : Explaining The Higgs At TEDx
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