Confusion about Scattering in Quantum Electrodynamics

[physwiz222]

The reason is that textbooks are usually not about open problems but about the basic understanding that works.

Only at finite times.

But it does not invalidate renormalized perturbation theory, which is what the textbooks teach.

Because it is easier to write papers related to theoretical proposals that cannot be tested than to solve problems open for more than 60 yeatrs, where the best minds have tried and not succeeded.

Renormalized Perturbation Theory is only a way to get rid of infinities for asymptotic scattering. It cant describe finite time dynamics. At finite times Relativistic QFT is from what I know inherently nonperturbative no matter the coupling due to Haag’s Theorem. When I say the notion of Particles in interacting QFT is Dubious at best I mean at finite times I should have mentioned.

Anyway its really frustrating to hear scientists and popsci act as if we are so close to a theory of everything but gravity just wont cooperate. In reality we are nowhere near we cant even develop a coherent solidly build description of the Quantum EM field all we can do is map in and out states not the finite time dynamics. Interacting QFT is very very very very poorly understood. The very notion of a particle and perturbation theory at finite times is dubious at best.

 

[physwiz222]

The reason is that textbooks are usually not about open problems but about the basic understanding that works.

Only at finite times.

But it does not invalidate renormalized perturbation theory, which is what the textbooks teach.

Because it is easier to write papers related to theoretical proposals that cannot be tested than to solve problems open for more than 60 yeatrs, where the best minds have tried and not succeeded.

Also the thing is I am not saying books should go in depth with Algebraic QFT just mentioning why we have to limit to asymptotic times because the notion of Perturbation Theory and Particles at finite times is invalid due to Haag’s Theorem. Just maybe a sentence justifying the need for asymptotic times.

 

[A. Neumaier]

Renormalized Perturbation Theory is only a way to get rid of infinities for asymptotic scattering. It cant describe finite time dynamics.

It can't describe the finite-time dynamics of a finite number of particles, since the particle concept fails at finite times. But it can describe the finite-time dynamics of fields in important sectors, for example those related to hydrodynamics. See the reference to Calzetta and Hu given earlier, which you seem to have ignored.

The very notion of a particle and perturbation theory at finite times is dubious at best.

It does not and cannot exist. There is no particle number operator in interacting QFTs. Only fields make sense at finite times.

 

[vanhees71]

The primary observables in QFT are defined by some local operator-correlation functions. These are $N$-point functions, which in principle can be calculated for finite times for a given initial state (statistical operator) using the Schwinger-Keldysh real-time contour technique.

Of course, there's the usual trouble with the UV divergences when calculating these N-point functions, and to my knowledge there's not solution for this renormalization problem for such "off-equilibrium situations". You also have to use some resummation since in contradistinction to the special cases of vacuum QFT for calculating S-matrix elements for scattering processes or equilibrium QFT the strictly order-by-order in hbar or coupling constant expansion-parameter perturbation theory is not described by naive perturbation theory.

One approach is the Kadanoff-Baym $\Phi$-functional method, also known as the two-particle irreducible (2PI) formulation or the Cornwall-Jackiw-Tomboulis approach. There you derive self-consistent approximations for the one-body Green's function and the corresponding self-consistent self-energy, which is given by "skeleton diagrams" derived from the functional approach. Despite the notorious (and to my knowledge unsolved) renormalization problem there are a view studies solving the full Kadanoff-Baym equations for simple truncations and in lower space-time dimensions (to soften the renormalizability problem). One example is a study in simple $\phi^4$ theory in (1+2) dimensions

https://arxiv.org/abs/nucl-th/0401046
https://doi.org/10.1016/j.nuclphysa.2004.07.010

Usually the Kadanoff-Baym equations are used to derive quantum-transport equations, using additional approximations like the gradient expansion ("coarse graining"), leading to transport theories that work with some caveats even for broad resonances rather than "particles". It's further simplified if the spectral functions don't develop too large (collisional) widths. Then you can also apply the quasiparticle approximation, which leads to usual relativistic Boltzmann-Uehling-Uhlenbeck transport equations. For this there's a vast amount of literature related with the study of ultrarelativistic heavy-ion collisions. Some examples are

Y. B. Ivanov, J. Knoll, H. v. Hees and D. N. Voskresensky,
Soft Modes, Resonances and Quantum Transport, Phys.
Atom. Nucl. 64, 652 (2001),
https://arxiv.org/abs/nucl-th/0005075Y. B. Ivanov, J. Knoll and D. N. Voskresensky, Self-consistent
approximations to non-equilibrium many-body theory, Nucl.
Phys. A 657, 413 (1999),
https://arxiv.org/abs/hep-ph/9807351

Y. B. Ivanov, J. Knoll and D. Voskresensky, Resonance
transport and kinetic entropy, Nucl. Phys. A 672, 313 (2000),
https://doi.org/10.1016/S0375-9474(99)00559-X

Y. B. Ivanov, J. Knoll and D. N. Voskresensky, Resonance
Transport and Kinetic Entropy, Nucl. Phys. A 672, 313
(2000), https://arxiv.org/abs/nucl-th/9905028

Y. Ivanov, J. Knoll and D. Voskresensky, Selfconsistent
approach to off-shell transport, Phys. Atom. Nucl. 66, 1902
(2003), https://doi.org/10.1134/1.1619502

J. Knoll, Y. B. Ivanov and D. Voskresensky, Exact
Conservation Laws for the Gradient Expanded
Kadanoff-Baym Equations, Ann. Phys. (NY) 293, 126 (2001),
https://arxiv.org/abs/nucl-th/0102044

W. Cassing, From Kadanoff-Baym dynamics to off-shell
parton transport, Eur. Phys. J. ST 168, 3 (2009),
https://doi.org/10.1140/epjst/e2009-00959-x

and many more.

 

[physwiz222]

The primary observables in QFT are defined by some local operator-correlation functions. These are $N$-point functions, which in principle can be calculated for finite times for a given initial state (statistical operator) using the Schwinger-Keldysh real-time contour technique.

Of course, there's the usual trouble with the UV divergences when calculating these N-point functions, and to my knowledge there's not solution for this renormalization problem for such "off-equilibrium situations". You also have to use some resummation since in contradistinction to the special cases of vacuum QFT for calculating S-matrix elements for scattering processes or equilibrium QFT the strictly order-by-order in hbar or coupling constant expansion-parameter perturbation theory is not described by naive perturbation theory.

One approach is the Kadanoff-Baym $\Phi$-functional method, also known as the two-particle irreducible (2PI) formulation or the Cornwall-Jackiw-Tomboulis approach. There you derive self-consistent approximations for the one-body Green's function and the corresponding self-consistent self-energy, which is given by "skeleton diagrams" derived from the functional approach. Despite the notorious (and to my knowledge unsolved) renormalization problem there are a view studies solving the full Kadanoff-Baym equations for simple truncations and in lower space-time dimensions (to soften the renormalizability problem). One example is a study in simple $\phi^4$ theory in (1+2) dimensions

https://arxiv.org/abs/nucl-th/0401046
https://doi.org/10.1016/j.nuclphysa.2004.07.010

Usually the Kadanoff-Baym equations are used to derive quantum-transport equations, using additional approximations like the gradient expansion ("coarse graining"), leading to transport theories that work with some caveats even for broad resonances rather than "particles". It's further simplified if the spectral functions don't develop too large (collisional) widths. Then you can also apply the quasiparticle approximation, which leads to usual relativistic Boltzmann-Uehling-Uhlenbeck transport equations. For this there's a vast amount of literature related with the study of ultrarelativistic heavy-ion collisions. Some examples are

Y. B. Ivanov, J. Knoll, H. v. Hees and D. N. Voskresensky,
Soft Modes, Resonances and Quantum Transport, Phys.
Atom. Nucl. 64, 652 (2001),
https://arxiv.org/abs/nucl-th/0005075Y. B. Ivanov, J. Knoll and D. N. Voskresensky, Self-consistent
approximations to non-equilibrium many-body theory, Nucl.
Phys. A 657, 413 (1999),
https://arxiv.org/abs/hep-ph/9807351

Y. B. Ivanov, J. Knoll and D. Voskresensky, Resonance
transport and kinetic entropy, Nucl. Phys. A 672, 313 (2000),
https://doi.org/10.1016/S0375-9474(99)00559-X

Y. B. Ivanov, J. Knoll and D. N. Voskresensky, Resonance
Transport and Kinetic Entropy, Nucl. Phys. A 672, 313
(2000), https://arxiv.org/abs/nucl-th/9905028

Y. Ivanov, J. Knoll and D. Voskresensky, Selfconsistent
approach to off-shell transport, Phys. Atom. Nucl. 66, 1902
(2003), https://doi.org/10.1134/1.1619502

J. Knoll, Y. B. Ivanov and D. Voskresensky, Exact
Conservation Laws for the Gradient Expanded
Kadanoff-Baym Equations, Ann. Phys. (NY) 293, 126 (2001),
https://arxiv.org/abs/nucl-th/0102044

W. Cassing, From Kadanoff-Baym dynamics to off-shell
parton transport, Eur. Phys. J. ST 168, 3 (2009),
https://doi.org/10.1140/epjst/e2009-00959-x

and many more.

Interesting anyway which observables do the N Point Functions exactly allow u to compute and do they encode the finite time dynamics of all possible observables of the theory.

 

[physwiz222]

It can't describe the finite-time dynamics of a finite number of particles, since the particle concept fails at finite times. But it can describe the finite-time dynamics of fields in important sectors, for example those related to hydrodynamics. See the reference to Calzetta and Hu given earlier, which you seem to have ignored.It does not and cannot exist. There is no particle number operator in interacting QFTs. Only fields make sense at finite times.

I didnt ignore but these techniques in Nonequilibrium QFT are very different from “standard” perturbation theory for renormalizing scattering amplitudes. When I refer to renormalized Perturbation theory I am referring to standard QFT perturbation theory for S Matrix elements.

 

[vanhees71]

Interesting anyway which observables do the N Point Functions exactly allow u to compute and do they encode the finite time dynamics of all possible observables of the theory.

You can, in principle, describe observables like densities of charges, the energy-momentum tensor, etc. It's pretty much the quantum version of a continuum theory like hydrodynamics.

 

[A. Neumaier]

I didnt ignore but these techniques in Nonequilibrium QFT are very different from “standard” perturbation theory for renormalizing scattering amplitudes.

Only in computational details, but not in the basic goals (need to avoid infinities) and techniques (use of N-point functions).

When I refer to renormalized Perturbation theory

When I refer to renormalized perturbation theory I refer to what physicists themselves call renormalized perturbation theory. You'd do well to extend your horizon!

I am referring to standard QFT perturbation theory for S Matrix elements.

You cannot complain that there is no finite time version of that, since S-matrix elements are by definition asymptotic objects!

 

[physwiz222]

Only in computational details, but not in the basic goals (need to avoid infinities) and techniques (use of N-point functions).

When I refer to renormalized perturbation theory I refer to what physicists themselves call renormalized perturbation theory. You'd do well to extend your horizon!

You cannot complain that there is no finite time version of that, since S-matrix elements are by definition asymptotic objects!

Anyway I think if Mainstream Particle Physics is to progress and get out of the 50 year stagnation it should abandon or at least move attempt to move beyond the S matrix approach to mapping in and out states and asymptotic time calculations and focus on computing correlation functions at finite times to describe finite time dynamics of scattering and many other processes like cosmological phenomena. I think the field should address the foundational issues as well as find ways to address the implications of haag’s theorem.

The reason why innovation is so hard in the field is because most is just scattering amplitudes at infinite times so when all you have is a hammer everything looks like a nail. I think Particle Physics should take an approach similar to Many Body Theory for condensed matter. Theres a reason condensed matter is a popular thriving field while particle physics is stagnating. Also when I refer to Particle Physics I mean Mainstream High Energy Physics I know some people do other things that are non asymptotic but those are non mainstream. I know the difficulties of this new approach but it should be attempted this is what Particle Physics needs a focus on Finite time dynamics and Correlation Functions of Fields, not String Theory or Supersymmetry.

I think this instrumentalist approach of just predicting numbers in a collider is not a good way of doing physics. Even if the equations describe “what we can say about Nature” We should still try and understand how they describe systems, particles, and fields evolving dynamically even if limited. The main issue with an instrumentalist approach is why do we even make physics theories. We dont invent physics as fodder for experimentalists because they are bored. Also when we have confirmed our theories what next do we declare it useless. This is why I think an instrumentalist view in particle physics isnt a good approach. Who cares if our colliders cant probe those timescales our instruments also cant probe the evolution of the wavefunction in standard QM yet its important and computed and simulated. We also can't probe the inside of a black hole yet physicists are interested.

I know the mathematical difficulties associated with anything other than asymptotic states in High Energy QFT but more physicists should at least attempt it and give it a shot and focus on New Nonperturbative Methods and some new nontraditional perturbative methods as if we are to describe finite time dynamics we will likely have to move past or at least expand and modify perturbation theory as well as describe nonperturbative phenomena. I apologize if this is a lengthy essay.

 

[weirdoguy]

I think the field should address the foundational issues as well as find ways to address the implications of haag’s theorem.

Um, and why in the world you think it doesn't?

 

[vanhees71]

Anyway I think if Mainstream Particle Physics is to progress and get out of the 50 year stagnation it should abandon or at least move attempt to move beyond the S matrix approach to mapping in and out states and asymptotic time calculations and focus on computing correlation functions at finite times to describe finite time dynamics of scattering and many other processes like cosmological phenomena. I think the field should address the foundational issues as well as find ways to address the implications of haag’s theorem.

What do you mean by "stagnation"? If there's one field in contemporary theoretical physics it's the Standard Model of elementary particles based on relativistic QFT and the S-matrix.

What's often forgotten is that there are also successful applications of relativistic many-body QFT in the field of relativistic heavy-ion collisions, including the evaluation of the cross-over pseudo-critical temperature for the confinement-deconfinement/chiral cross-over transition at vanishing baryochemical potential with thermal lattice QCD, but also the derivation of (partially off-shell) quantum-transport equations as well as the derivation of various versions of viscous relativistic hydrodynamics from it.

Last but not least one must not forget the vast applications of relativstic QFT in quantum optics and AMO physics.

The reason why innovation is so hard in the field is because most is just scattering amplitudes at infinite times so when all you have is a hammer everything looks like a nail. I think Particle Physics should take an approach similar to Many Body Theory for condensed matter. Theres a reason condensed matter is a popular thriving field while particle physics is stagnating. Also when I refer to Particle Physics I mean Mainstream High Energy Physics I know some people do other things that are non asymptotic but those are non mainstream. I know the difficulties of this new approach but it should be attempted this is what Particle Physics needs a focus on Finite time dynamics and Correlation Functions of Fields, not String Theory or Supersymmetry.

As I said, that's precisely what's done. QFT is more a metatheory of almost all of physics rather than being limited to the single field of relativistic HEP physics, and there almost always all you need is the S-matrix approach (with the important exception of neutrino-oscillations, where you need to take into account the finite distance between source and detector making use of the wave-packet ansatz for asymptotic states).

I think this instrumentalist approach of just predicting numbers in a collider is not a good way of doing physics. Even if the equations describe “what we can say about Nature” We should still try and understand how they describe systems, particles, and fields evolving dynamically even if limited. The main issue with an instrumentalist approach is why do we even make physics theories. We dont invent physics as fodder for experimentalists because they are bored. Also when we have confirmed our theories what next do we declare it useless. This is why I think an instrumentalist view in particle physics isnt a good approach. Who cares if our colliders cant probe those timescales our instruments also cant probe the evolution of the wavefunction in standard QM yet its important and computed and simulated. We also can't probe the inside of a black hole yet physicists are interested.

The "instrumentalist approach" is all there is within objective natural sciences. You may speculate what's the philosophical meaning of the description in terms of QFT, it doesn't add anything to the fact that all we can objectively say about Nature is, how she behaves in a given situation. After all these are all mathematical models, no more no less.

I know the mathematical difficulties associated with anything other than asymptotic states in High Energy QFT but more physicists should at least attempt it and give it a shot and focus on New Nonperturbative Methods and some new nontraditional perturbative methods as if we are to describe finite time dynamics we will likely have to move past or at least expand and modify perturbation theory as well as describe nonperturbative phenomena. I apologize if this is a lengthy essay.

That's also what's done. 20 years ago the Kadanoff-Baym approach of real-time off-equilibrium many-body QFT became applied in the relativistic domain, leading to the above mentioned derivations of transport and hydrodynamic description. Somewhat later, given the difficuluties of this Phi-derivable 2PI approach with (gauge) symmetries also the 1PI (Dyson Schwinger) approach has been worked out in more detail, and is very successful in collaboration with lattice QFT. Last but not least in more recent years the functional renormalization group has become vigorously and still is applied in the field of relativsitic heavy-ion collisions.

There is indeed much more than the S-matrix approach in the application of relativistic QFT than you might be aware of!

 

[physwiz222]

What do you mean by "stagnation"? If there's one field in contemporary theoretical physics it's the Standard Model of elementary particles based on relativistic QFT and the S-matrix.

What's often forgotten is that there are also successful applications of relativistic many-body QFT in the field of relativistic heavy-ion collisions, including the evaluation of the cross-over pseudo-critical temperature for the confinement-deconfinement/chiral cross-over transition at vanishing baryochemical potential with thermal lattice QCD, but also the derivation of (partially off-shell) quantum-transport equations as well as the derivation of various versions of viscous relativistic hydrodynamics from it.

Last but not least one must not forget the vast applications of relativstic QFT in quantum optics and AMO physics.

As I said, that's precisely what's done. QFT is more a metatheory of almost all of physics rather than being limited to the single field of relativistic HEP physics, and there almost always all you need is the S-matrix approach (with the important exception of neutrino-oscillations, where you need to take into account the finite distance between source and detector making use of the wave-packet ansatz for asymptotic states).

The "instrumentalist approach" is all there is within objective natural sciences. You may speculate what's the philosophical meaning of the description in terms of QFT, it doesn't add anything to the fact that all we can objectively say about Nature is, how she behaves in a given situation. After all these are all mathematical models, no more no less.

That's also what's done. 20 years ago the Kadanoff-Baym approach of real-time off-equilibrium many-body QFT became applied in the relativistic domain, leading to the above mentioned derivations of transport and hydrodynamic description. Somewhat later, given the difficuluties of this Phi-derivable 2PI approach with (gauge) symmetries also the 1PI (Dyson Schwinger) approach has been worked out in more detail, and is very successful in collaboration with lattice QFT. Last but not least in more recent years the functional renormalization group has become vigorously and still is applied in the field of relativsitic heavy-ion collisions.

There is indeed much more than the S-matrix approach in the application of relativistic QFT than you might be aware of!

Interesting reply. just one thing when I say instrumentalist approach is an issue I dont mean philosophy of physics type things but rather that we should focus on how the system evolves according to the equations rather than just predicting some numbers and nothing more. I am not talking about this in the sense of philosophy of Physics but rather that things like how systems evolve and interact are important and the most important thing to use a theory for is conceptual understanding of the phenomena. Predictions are important dont get me wrong but the ultimate reason why we invent a theory is to understand nature in some way even if its limited. Just wanted to clarify what I meant by “instrumentalism is not the best way of doing physics”.

 

[physwiz222]

What do you mean by "stagnation"? If there's one field in contemporary theoretical physics it's the Standard Model of elementary particles based on relativistic QFT and the S-matrix.

What's often forgotten is that there are also successful applications of relativistic many-body QFT in the field of relativistic heavy-ion collisions, including the evaluation of the cross-over pseudo-critical temperature for the confinement-deconfinement/chiral cross-over transition at vanishing baryochemical potential with thermal lattice QCD, but also the derivation of (partially off-shell) quantum-transport equations as well as the derivation of various versions of viscous relativistic hydrodynamics from it.

Last but not least one must not forget the vast applications of relativstic QFT in quantum optics and AMO physics.

As I said, that's precisely what's done. QFT is more a metatheory of almost all of physics rather than being limited to the single field of relativistic HEP physics, and there almost always all you need is the S-matrix approach (with the important exception of neutrino-oscillations, where you need to take into account the finite distance between source and detector making use of the wave-packet ansatz for asymptotic states).

The "instrumentalist approach" is all there is within objective natural sciences. You may speculate what's the philosophical meaning of the description in terms of QFT, it doesn't add anything to the fact that all we can objectively say about Nature is, how she behaves in a given situation. After all these are all mathematical models, no more no less.

That's also what's done. 20 years ago the Kadanoff-Baym approach of real-time off-equilibrium many-body QFT became applied in the relativistic domain, leading to the above mentioned derivations of transport and hydrodynamic description. Somewhat later, given the difficuluties of this Phi-derivable 2PI approach with (gauge) symmetries also the 1PI (Dyson Schwinger) approach has been worked out in more detail, and is very successful in collaboration with lattice QFT. Last but not least in more recent years the functional renormalization group has become vigorously and still is applied in the field of relativsitic heavy-ion collisions.

There is indeed much more than the S-matrix approach in the application of relativistic QFT than you might be aware of!

By Stagnation I mean the 50 year period of stagnation in particle physics happening now. Also I know there are applications to QFT beyond high energy particle physics like relativistic hydrodynamics I mentioned this my criticism was the standard S Matrix approach of mainstream particle physics. Also I think this relativistic kinetic/hydrodynamic approach that you bring up is what particle physics needs not string theory and supersymmetry. I think it should be more mainstream. I am aware there is more than the s matrix but I argue it should be the norm not some niche thing.

You also say the S Matrix is all you need for HEP. Technically true but I disagree as it gives a limited view. It cant describe finite time dynamics which I think is important. Also I think this relativistic kinetic/hydrodynamic approach or similar methods is better for High energy particle interactions for finite time dynamics of observables like densities and correlation functions than the standard s matrix approach.

 

[physwiz222]

Um, and why in the world you think it doesn't?

Standard QFT in mainstream high energy physics high energy physics is based on computing scattering amplitudes at infinite time with the S Matrix. All it can really compute is mapping between input and output states. There are indeed other applications like relativistic kinetic theory which Vanhees mentions and I argue they should become mainstream. Particle physicists arent worried about foundational issues their goal is to predict asymptotic scattering to compare with collider results and dont forget string theorists in their la la land of 10 dimensional supersymmetric branes.

 

[vanhees71]

By Stagnation I mean the 50 year period of stagnation in particle physics happening now. Also I know there are applications to QFT beyond high energy particle physics like relativistic hydrodynamics I mentioned this my criticism was the standard S Matrix approach of mainstream particle physics. Also I think this relativistic kinetic/hydrodynamic approach that you bring up is what particle physics needs not string theory and supersymmetry. I think it should be more mainstream. I am aware there is more than the s matrix but I argue it should be the norm not some niche thing.

You also say the S Matrix is all you need for HEP. Technically true but I disagree as it gives a limited view. It cant describe finite time dynamics which I think is important. Also I think this relativistic kinetic/hydrodynamic approach or similar methods is better for High energy particle interactions for finite time dynamics of observables like densities and correlation functions than the standard s matrix approach.

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.

 

[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.