I Confusion about Scattering in Quantum Electrodynamics

[weirdoguy]

Particle physicists arent worried about foundational issues

But mathematical physicists are, and I think you should look at what they do in this context. Sorry, but I don't think you've done enough research on this issues to have such a strong opinions.

 

[physwiz222]

But mathematical physicists are, and I think you should look at what they do in this context. Sorry, but I don't think you've done enough research on this issues to have such a strong opinions.

I know mathematical physicists are I just think Particle Physicists should also focus on these issues in their field of study.

 

[physwiz222]

There is no stagnation in HEP physics. One should realize that the finding that there's nothing beyond the standard model in the realm we observe with our current experiments is also progress. That's why the LHC including the existing detectors got recently upgraded and new detectors were built (already with the first direct measurement of collider-produced neutrinos). It may well be that after all something new is found with these new instruments.

Well the standard model cant be the full story what about Gravity, Dark Matter, Dark Energy. Anyway no real progress in 50 years definitely counts as stagnation. Anyways I still stand by what I say that the S Matrix isnt the best way if doing physics for HEP and it should move towards a dynamical approach like the relativistic kinetic theory one you mention, which should be mainstream.

 

[PeterDonis]

no real progress in 50 years definitely counts as stagnation

If you think there is something the HEP community should be doing that they're not, what is it? And wouldn't your efforts be better spent at going and doing it?

 

[physwiz222]

If you think there is something the HEP community should be doing that they're not, what is it? And wouldn't your efforts be better spent at going and doing it?

I already said look at my long essay on fixing the stagnation in the above comments, abandon or go beyond the S Matrix approach and focus on Finite time dynamics and correlation functions to describe scattering and many other kinds of phenomena like Relativistic Hydrodynamics, Transport, Nonequilibrium Phenomena, Quark Gluon Plasma, Bound States, basically adopt an approach similar to Many Body theory. Understand how particles and fields evolve in time in terms of how correlation functions and densities evolve rather than just S Matrix elements. Develop better Perturbative Methods similar to many body theory and Nonperturbative approaches.

 

[PeterDonis]

I already said look at my long essay on fixing the stagnation in the above comments...

Yes, so if you have all these wonderful suggestions, why aren't you doing them?

 

[Vanadium 50]

I mean the 50 year period of stagnation in particle physics happening now.

Except of course

• Charm quarks
• Bottom quarks
• Top quarks
• W and Z Bososns
• Higgs Boson
• Exactly 3 generations
• Neutrino masses and mixing
• Precision QCD
• Precision Electroweak
• Atomic Parity Violation
• CP-violation in heavy quarks
• Quark-gluon plasma
• Hadronic molecules
• Particle cosmology

If this is "stagnation", I hope we get more of it.

 

[physwiz222]

Except of course

• Charm quarks
• Bottom quarks
• Top quarks
• W and Z Bososns
• Higgs Boson
• Exactly 3 generations
• Neutrino masses and mixing
• Precision QCD
• Precision Electroweak
• Atomic Parity Violation
• CP-violation in heavy quarks
• Quark-gluon plasma
• Hadronic molecules
• Particle cosmology

If this is "stagnation", I hope we get more of it.

Anyway I still stand by my points even if particle physics isnt stagnating per se it which is debatable i still say it would be a huge benefit to move beyond the S Matrix and focus on time dependent observables like charge and current density of the fields, the stress energy tensor, and correlation functions at Finite times at least for Scattering. This alone would be a major improvement. I don’t think this black box way of only finding probabilities between in and out states is the right way of describing the fundamental theory of nature as it ignores conceptually understanding and is deeply unsatisfactory after being able to describe and visualize finite time evolution of the wavefunction in standard QM and the Electric and Magnetic Fields in E&M its just a big let down. I mean if we want a theory of everything we should be able to at least describe and understand finite time dynamics.

 

[vanhees71]

As I repeatedly said, all this is used, where it is needed, i.e., in many-body theory.

 

[physwiz222]

As I repeatedly said, all this is used, where it is needed, i.e., in many-body theory.

I know what I propose is that this should be the approach for all of QFT period High energy included. i know this is the approach to many body theory I am saying This also should be used in HEP.

 

[vanhees71]

I don't know, what you think you might achieve with such an approach. In HEP experiments one measures cross sections, and that's what's described by the usual S-matrix approach. I don't know, what else you expect to be observable by calculating some transient states. For which quantities? And given the quantities, how do you think are they related to observables?

In ultrarelativistic Heavy Ion Collisions one has a strongly coupled collectively moving many-body system, which is also described by relativistic QFT (mostly QCD), and there it is used to describe the time-evolution of this "fireball" by deriving transport equations and hydrodynamics equations for this collectively moving medium. Here one has some limited experimental information from many observables like the chemical freeze-out (particle abundancies/ratios), the identified-hadron spectra (pT distributions, various anisotropic-flow parameters,...), dilepton and photon spectra (as space-time weighted averages over the complete fireball-evolution time), heavy-quark (D/B-mesons, Quarkonia) drag and diffusion coefficients (with still large uncertainties), jet (quenching),...

 

[physwiz222]

I don't know, what you think you might achieve with such an approach. In HEP experiments one measures cross sections, and that's what's described by the usual S-matrix approach. I don't know, what else you expect to be observable by calculating some transient states. For which quantities? And given the quantities, how do you think are they related to observables?

In ultrarelativistic Heavy Ion Collisions one has a strongly coupled collectively moving many-body system, which is also described by relativistic QFT (mostly QCD), and there it is used to describe the time-evolution of this "fireball" by deriving transport equations and hydrodynamics equations for this collectively moving medium. Here one has some limited experimental information from many observables like the chemical freeze-out (particle abundancies/ratios), the identified-hadron spectra (pT distributions, various anisotropic-flow parameters,...), dilepton and photon spectra (as space-time weighted averages over the complete fireball-evolution time), heavy-quark (D/B-mesons, Quarkonia) drag and diffusion coefficients (with still large uncertainties), jet (quenching),...

What you achieve is understanding how interacting quantum field states evolve in time. The goal of
Physics isnt just to make some predictions and move on its to understand nature. As for what is observable I guess charge density, energy density, current density, correlations between densities. These are important observables in their own right. The goal isnt just to make a prediction but to understand the interactions of fields. Personally I dont subscribe to the instrumentalist view of predictions and nothing more.

Predictions are of course very important but not the ultimate goal of physics. Anyways we do compute time evolution for unobservable things for example wavefunction evolution in regular QM for say the quantum harmonic oscillator. In cosmology you also describe the evolution of the universe and its expansion even though no telescope can probe billions or trillions of years in the future the same way our detectors cant probe the exact finite time dynamics. so this point of “we cant measure it” isnt really valid.

I understand there are great mathematical problems with Relativistic QFT at finite times and that standard perturbation theory is not valid for finite times but this is different than saying “its not important because we cant probe these times”.

 

[Vanadium 50]

Anyway I still stand by my point

Real scientists don't have the luxury of "standing by our points" in the fact of contradictory data. Real scientists have to change our minds,

I see little point in continuing. If a dozen counter-examples doesn't convince you, even if I were to come up with a million it won't convince you either.

However, just to keep the misinformation level low once this thread is inevitably tied off,

1. If you perform scattering experiments, predicting them through the formalism for scattering is natural and the right thing to do. Wishing for it to be otherwise is like wishing you could solve heat flow problems without thermodynamics.
2. There are approaches that do not use the S-matrix. The award-winning work of Nate Isgur and Mark Wise, the award winning work of Lance Dixon and Zvi Bern and collaborators, the award winning work of Ken Wilson, and....

 

[physwiz222]

Well you have some valid points which I agree with just want to say about number one first of all I am not saying to describe scattering without scattering theory. I agree the formalism of scattering is natural for collider experiments 100%.

What i am saying is that because scattering is inherently a time dependent process it would be better to focus on the finite time dynamics of the scattering process rather than just asymptotic in and out states, the s matrix formalism.

And about your thermodynamics analogy what I am advocating is not the equivalent of describing heat flow without thermodynamics but rather describe the dynamical evolution of heat flow rather than initial and final temperature. The point isnt to describe scattering experiments as something else but to focus on the finite time dynamics as its inherently a time dependent process.

I hope you can see my point of view. Also the counterexamples you provided arent really reasons why my views are illogical they just say that the current approach is enough for predictions in colliders which I technically agree with. What I argue is that we should attempt to aim for descriptive power not just predictive and we should go beyond just predicting things to measure in a collider even for scattering and focus on dynamical behaviour. Like I said I dont agree with the instrumentalist approach.

 

[PeterDonis]

because scattering is inherently a time dependent process it would be better to focus on the finite time dynamics of the scattering process rather than just asymptotic in and out states

And if you think that is a good thing to do, go do it. Complaining on an Internet site that other people are not doing it is pointless. That's not how science advances.

we should attempt to aim for descriptive power not just predictive

And again, if you think science should do this, you should go do science that does it. Complaining that other people are not doing something you think they should do is pointless.

This thread has run its course and is now closed.