Confusion about Scattering in Quantum Electrodynamics

[Morbert]

This is the general mechanism; nothing is special here. See any derivation of decoherence or Lindblad master equations:

Restricting attention to less than complete information (absolutely necessary for an observer since complete information is never available) always turns the exact unitary evolution into a contractive evolution.

But is this general framing (exact unitary evolution, restriction of attention, and approximate contractive evolution) necessary in algebraic QFT? Fröhlich implies the reverse in some of his writing. In his article on the time-evolution of states He says unitary evolution and nonlinear state collapse "can be understood as an approximation to the fundamental [stochastic] law of evolution of states". I.e. in his AQFT formalism, stochastic state evolution is not arrived at by first sketching some unitary evolution and then coarsening it. He instead seems to start with a principle like Huygen's principle and event algebras implied by it.

 

[A. Neumaier]

But is this general framing (exact unitary evolution, restriction of attention, and approximate contractive evolution) necessary in algebraic QFT? Fröhlich implies the reverse in some of his writing. In his article on the time-evolution of states He says unitary evolution and nonlinear state collapse "can be understood as an approximation to the fundamental [stochastic] law of evolution of states". I.e. in his AQFT formalism, stochastic state evolution is not arrived at by first sketching some unitary evolution and then coarsening it. He instead seems to start with a principle like Huygen's principle and event algebras implied by it.

Well, one can always start anywhere, and build from there.

By definition, QED starts from unitary evolution. If others start at an approximation of QED, it is their freedom. But it doesn't change the fact that the unitary and deterministic evolution of the states is the fundamental one from which the stochastic approximation can be derived. One cannot derive the unitary evolution of QED from its stochastic conic approximation.

 

[WernerQH]

By definition, QED starts from unitary evolution.

Perhaps that's why we can't find a mathematically satisfactory formulation of QED, starting from this "definition".

 

[A. Neumaier]

Perhaps that's why we can't find a mathematically satisfactory formulation of QED, starting from this "definition".

The definition works constructively in 3 spacetime dimensions, and there are no known mathematical reasons (only speculations) why this shouldn't extend to 4 dimensions.

Note also that Fröhlich's approach doesn't construct the contractive variant of QED he is speculating about! Thus your argument also applies to his approach.

 

[WernerQH]

there are no known mathematical reasons (only speculations) why this shouldn't extend to 4 dimensions.

Keep searching. Good luck!

 

[physwiz222]

Your argument is not conclusive as there is no necessity for the Hilbert space of a quantum field theory to be a Fock space. It only follows that the Fock space concept is inadequate to capture interacting QFT.

Note that there are many rigorous constructions of interacting QFTs in 2 and 3 spacetime dimensions, in spite of the validity of Haag's theorem there. But in no case is the finite-time dynamics given in a Fock space.

Has anyone at least attempted naively applying Perturbation Theory to QFT at finite times. I know it wont work because of Haag’s Theorem as the states arent describable as particle states even infinitely many.

What I am asking now is has anyone at least tried to apply Perturbation Theory at finite times by computing the Transition Amplitudes between states at finite time to construct superpositions of the form c0(t)|0>+ck(t)|k>+…cn(t)|n> just like in standard QM.

I know this doesnt work but has anyone at least tried this naive approach and ran into problems. If so could u link some resources like Papers where this approach was tried and failed.

 

[A. Neumaier]

Has anyone at least attempted naively applying Perturbation Theory to QFT at finite times. I know it wont work because of Haag’s Theorem as the states arent describable as particle states even infinitely many.

What I am asking now is has anyone at least tried to apply Perturbation Theory at finite times by computing the Transition Amplitudes between states at finite time to construct superpositions of the form c0(t)|0>+ck(t)|k>+…cn(t)|n> just like in standard QM.

I know this doesnt work but has anyone at least tried this naive approach and ran into problems. If so could u link some resources like Papers where this approach was tried and failed.

This naive approach was tried very early in the history of QFT and discarded since it led to UV divergences; see, e.g., Wikipedia.

Haag's theorem showed rigorously that this naive approach cannot work, i.e., why renormalization is unavoidable if one starts with Fock space.

 

[AndreasC]

This naive approach was tried very early in the history of QFT and discarded since it led to UV divergences; see, e.g., Wikipedia.

Haag's theorem showed rigorously that this naive approach cannot work, i.e., why renormalization is unavoidable.

But wait, I thought divergences related to Haag's theorem are IR, not UV!

 

[A. Neumaier]

But wait, I thought divergences related to Haag's theorem are IR, not UV!

IR divergences appear only in theories containing a massless field (such as QED). But Haag's theorem applies to all interacting fields, hence also to massive scalar ##\Phi^4## QFT, which has no IR divergences.

The problem exposed by Haag's theorem are the superselection sectors that appear when the infinite volume limit is taken. Though the latter is a long-distance (IR) phenomenon, the associated divergence can come from both UV and IR aspects.

 

[A. Neumaier]

IR divergences appear only in theories containing a massless field (such as QED). But Haag's theorem applies to all interacting fields, hence also to massive scalar ##\Phi^4## QFT, which has no IR divergences.

The problem exposed by Haag's theorem are the superselection sectors that appear when the infinite volume limit is taken. Though the latter is a long-distance (IR) phenomenon, the associated divergence can come from both UV and IR aspects.

See also https://physics.stackexchange.com/questions/778763

 

[AndreasC]

The problem exposed by Haag's theorem are the superselection sectors that appear when the infinite volume limit is taken. Though the latter is a long-distance (IR) phenomenon, the associated divergence can come from both UV and IR aspects.

Ah ok this is what I had in mind, thanks for the clarification.

 

[vanhees71]

Also the IR divergences are dealt with, at least in gauge theories, by using the correct asymptotic states, which also correct the naive "naked-particle picture" of the "charged" particles in such a theory. The true asymptotic states of, e.g., an electron in QED is an "infraparticle state", which describes the "bare electron + it's own electromagnetic field around it" (speaking in a hand-waving heuristic way, of course).

What's done FAPP is anyway to use some regularization procedure. To get rid of the formal trouble with the non-existence of the interaction picture a la Haag, you can introduce a quantization volume, e.g., a cube imposing periodic spatial boundary conditions on the fields and then taking the infinite-volume limit only at the end.

 

[physwiz222]

This naive approach was tried very early in the history of QFT and discarded since it led to UV divergences; see, e.g., Wikipedia.

Haag's theorem showed rigorously that this naive approach cannot work, i.e., why renormalization is unavoidable if one starts with Fock space.

Why cant Renormalization then be used to remove the divergences and make this naive approach work then. You say this naive approach fails due to divergences. Isnt that what Renormalization was invented for to fix such divergences. Is QFT at finite times not renormalizable is that why Renormalization cant make this naive approach work.

 

[A. Neumaier]

Why cant Renormalization then be used to remove the divergences and make this naive approach work then. You say this naive approach fails due to divergences. Isnt that what Renormalization was invented for to fix such divergences. Is QFT at finite times not renormalizable is that why Renormalization cant make this naive approach work.

Textbook renormalization is done only at the asymptotic level of the S-matrix (involving the limit ##t\to\pm\infty##) , where it works for massive theories (only). The reason is that the asymptotic states of a massive QFT are free particle states, hence describable by a free QFT in a Fock space.

For theories with massless particles, the asymptotic states of a QFT are no longer free particle states but infraparticle states. Therefore the asymptotic state space defining the S-matrix is not a Fock space, and infrared problems are not cured by renormalization, not even at asymptotic times. They must be handled differently, conventionally by coherent state methods (in addition to renormalization, which handle the UV issues); see post #34. Very recently, Duch and Dybalski wrote a survey paper about what is known about curing the IR problem at a more rigorous level.

Renormalization at finite times can currently be done with the CTP (closed time path) formalism; though not for few-particle states, but only for states describing the hydrodynamic or kinetic many-body regime. See post #59.

 

[physwiz222]

Textbook renormalization is done only at the asymptotic level of the S-matrix (involving the limit ##t\to\pm\infty##) , where it works for massive theories (only). The reason is that the asymptotic states of a massive QFT are free particle states, hence describable by a free QFT in a Fock space.

For theories with massless particles, the asymptotic states of a QFT are no longer free particle states but infraparticle states. Therefore the asymptotic state space defining the S-matrix is not a Fock space, and infrared problems are not cured by renormalization, not even at asymptotic times. They must be handled differently, conventionally by coherent state methods; see post #34. Very recently, Duch and Dybalski wrote a survey paper about what is known about curing the IR problem at a more rigorous level.

Renormalization at finite times can currently be done with the CTP (closed time path) formalism; though not for few-particle states, but only for states describing the hydrodynamic or kinetic many-body regime. See post #59.

So essentially the reason this naive approach fails is a combination of divergences which we dont know how to deal with at finite time and Haag’s Theorem prohibiting describing the interacting states as superpositions. With all this in mind just out of curiosity is Relativistic QED at finite times nonrenormalizable or we simply dont know how to apply renormalization.

 

[A. Neumaier]

So essentially the reason this naive approach fails is a combination of divergences which we dont know how to deal with at finite time and Haag’s Theorem prohibiting describing the interacting states as superpositions.

Haag's theorem also shows what is needed to deal with at finite times, namely to give up Fock space. As examples in spacetime dimensions 2 and 3 show, the interaction picture is perfectly well-defined if the Hilbert space is adapted to the field theory rather than being taken a priori as Fock space.

With all this in mind just out of curiosity is Relativistic QED at finite times nonrenormalizable or we simply dont know how to apply renormalization.

We know for hydrodynamic and kinetic studies, where CTP works perturbatively.

We still don't know it in general, since QED has not yet been constructed perturbatively. But neither is there any evidence that it would be impossible. See https://www.physicsoverflow.org/21328/

 

[physwiz222]

I am understanding the OP's question to refer to the fact that QFT texts rarely discuss what form finite time evolution might take in QFT.

In other words why we generally seem to avoid formulating a state ##\rho(T)## at some time ##T## in even a semi-explicit manner and solving for its evolution over some finite time interval: ##U(t-T)\rho(T)##. This is quite unlike non-relativistic QM where this is often sketched, even though in many cases it could not be explicitly computed. Non-relativistic QM texts will cover cases outside of the overlap of asymptotic states.

I would give four initial reasons for this:

1. In QFT it is very difficult to characterise states that are not asymptotic. In non-relativistic QM of N-particles, pure states at any given time have the form ##\psi(x_{1},\ldots,x_{N})##, i.e. some function in ##L^{2}(\mathbb{R}^{3N})##.
   In QFT however we don't even know the Hilbert space for states in general and thus cannot even guess their rough functional form. At asymptotic times we know they are like free field states and thus can be represented as states in a Fock space of a finite number of particle species. At intermediate times it is an open problem of mathematical physics what form the states take, since we know that in general they cannot be understood in terms of particle states.
2. For asymptotic times we have the LSZ formalism that tells us we may compute the overlap between asymptotic states, i.e. the S-matrix, by computing Amputated n-point correlation functions. Since these n-point functions are expressions involving only finite products of local fields, we have further theorems showing they may be computed via Feynman graph techniques perturbatively.
   We have no such calculational recipe in general for finite time evolution.
3. Finite time evolution requires further renormalisations beyond those of asymptotic S-matrix calculations. A finite time propogator ##U(t_{2},t_{1})## requires formulating the theory on constant time slices, which due to the highly singular nature of the fields introduces new singularities. The finite time Green's functions that replace the usual propogator don't have the nice properties that make all-order BPHZ style renormalisation possible. Thus we don't have a general renormalisation prescription.
4. For QED in particular we have to deal with superselection sectors caused by an infrared cloud of photons surrounding a given electron. This means that nonperturbatively the relevant states are not Lorentz irreps and thus we lose all the simplifications such a simple representation theory brings.
   In fact non-perturbatively the evolution is most likely some non-unitary contractive Markovian evolution* about which very little is currently know.
   
   *One may understand this evolution in simpler terms as one involving a "perfect/exact" form of decoherence.

This is just a brief introduction to the issues involved. It's hard to explain in one go, let me know if you have questions.

What about Closed Time Path can that be used tor Scattering at Intermediate Times. Also what about just using the time evolution operator to evolve states Without taking the infinite time limit. Why cant these be used.

 

[physwiz222]

It's true that there are many mathematical problems unsolved in QFT. However, one must also say that in the vacuum QFT the calculation of the time-evolution (the initial-value problem) is pretty useless since there are no observables it depicts.

Things change, when it comes to the question of many-body physics. In relativistic heavy-ion collisions in fact we have to calculate the time evolution to a certain extent. For realistic simulations of the hot and dense fireball the most "fundamental" thing that's feasible computationally is the solution of quantum Boltzmann-Uehling Uhlenbeck equations on both the partonic and the hadronic level. There has a lot of progress over the last few decades. Among other things multi-particle collision terms (beyond the 2->2 level) can now be simulated accurately. Newest developments are mostly centered around "spin transport" and the derivation of hydrodynamics from relativistic kinetic theory. Also there there was tremendous progress over the last few years. Among other things now there's also a stable causal formulation of first-order ("Navier-Stokes") relativistic hydrodynamics as well as systematic approximations of higher orders. Of course also on the hydro level there's a lot of interest in spin and magneto-hydrodynamics.

On a more fundamental level one can solve toy models, using the real-time formalism of many-body off-equilibrium QFT, leading to Kadanoff-Baym equations (from which the above mentioned transport equations can be derived through gradient expansion of the Wigner representation of the contour Green's functions).

Two recent books about these topics are

G. S. Denicol and D. H. Rischke, Microscopic Foundationsof
Relativistic Fluid Dynamics, Springer, Cham (2021),
https://doi.org/10.1007/978-3-030-82077-0

W. Cassing, Transport Theories for Strongly-Interacting
Systems: Applications to Heavy-Ion Collisions, Springer,
Cham (2021),
https://doi.org/10.1007/978-3-030-80295-0

For the Interacting Vacuum wouldn’t the important observables be the Particle Number Density and Energy Density as functions of position and time.

 

[A. Neumaier]

For the Interacting Vacuum wouldn’t the important observables be the Particle Number Density and Energy Density as functions of position and time.

In a vacuum state there is nothing to be observed. For the interacting vacuum the particle number density is zero.

 

[physwiz222]

In a vacuum state there is nothing to be observed. For the interacting vacuum the particle number density is zero.

How do we know this. This interacting vacuum Ω is nothing like the free vacuum and is a complicated object from what I hear so how do you know the Particle Density n(r,t) or the Energy density is 0.

 

[Demystifier]

How do we know this. This interacting vacuum Ω is nothing like the free vacuum and is a complicated object from what I hear so how do you know the Particle Density n(r,t) or the Energy density is 0.

The interacting vacuum is complicated when expressed in terms of eigenstates of the free Hamiltonian. But it is simple when expressed in terms of eigenstates of the interacting Hamiltonian. In the latter sense, it has zero particles and lowest possible energy.

 

[physwiz222]

The interacting vacuum is complicated when expressed in terms of eigenstates of the free Hamiltonian. But it is simple when expressed in terms of eigenstates of the interacting Hamiltonian. In the latter sense, it has zero particles and lowest possible energy.

Still wont the Interacting Vacuum have a nonzero Particle Number Density or Energy Density

 

[Demystifier]

Still wont the Interacting Vacuum have a nonzero Particle Number Density or Energy Density

How do you define interacting vacuum? How do you define particle number density?

 

[A. Neumaier]

Still wont the Interacting Vacuum have a nonzero Particle Number Density or Energy Density

No. Vacuum means ''nothing'', even in QFT!

 

[Morbert]

In quantum chemistry, if you expand the ground state of the interacting Hamiltonian in an energy eigenbasis of a mean-field approximation, some elements of that basis corresponding to excited states will have nonzero amplitudes. (A method called Configuration Interaction)

Is it similar here? If you expand the ground state of the interacting Hamiltonian in the eigenbasis of the free Hamiltonian, will you get some elements of the basis corresponding to > 0 particles having nonzero amplitudes?

 

[A. Neumaier]

In quantum chemistry, if you expand the ground state of the interacting Hamiltonian in an energy eigenbasis of a mean-field approximation, some elements of that basis corresponding to excited states will have nonzero amplitudes. (A method called Configuration Interaction)

Is it similar here? If you expand the ground state of the interacting Hamiltonian in the eigenbasis of the free Hamiltonian, will you get some elements of the basis corresponding to > 0 particles having nonzero amplitudes?

In an interaction relativistic QFT there is no particle number operator, hence no sensible notion of particle number, so the question does not make sense. Moreover, you cannot expand the interacting theory in terms of the free Hamiltonian since they act on disjoint Hilbert spaces.

Only little is left from the particle picture: For theories without massless fields, there are discrete mass shells defining a notion of bound states. But everything with mass ##\ge## twice the minimal mass is in the continuous spectrum of the mass, and can be resolved into particles only asymptotically (for infinite time). For theories like QED with massless fields, the mass spectrum is only continuous and the particle notion is even questionable at infinity.

 

[physwiz222]

In an interaction relativistic QFT there is no particle number operator, hence no sensible notion of particle number, so the question does not make sense. Moreover, you cannot expand the interacting theory in terms of the free Hamiltonian since they act on disjoint Hilbert spaces.

Only little is left from the particle picture: For theories without massless fields, there are discrete mass shells defining a notion of bound states. But everything with mass ##\ge## twice the minimal mass is in the continuous spectrum of the mass, and can be resolved into particles only asymptotically (for infinite time). For theories like QED with massless fields, the mass spectrum is only continuous and the particle notion is even questionable at infinity.

Is there a density operator of some kind for interacting fields where one can talk about the field’s density like is there an equivalent of the electron number density for an interacting field.

 

[A. Neumaier]

Is there a density operator of some kind for interacting fields where one can talk about the field’s density like is there an equivalent of the electron number density for an interacting field.

In QED one has the mass density and the charge density, both scalar-valued, and the vector-valued energy-momentum density. All vanish by definition in the physical vacuum state (what you called the interacting vacuum).

 

[PeterDonis]

Moderator's note: Some posts on a changed topic have been spun off to a separate thread:

https://www.physicsforums.com/threa...tween-quantum-and-classical-behavior.1055703/

 

[physwiz222]

This naive approach was tried very early in the history of QFT and discarded since it led to UV divergences; see, e.g., Wikipedia.

Haag's theorem showed rigorously that this naive approach cannot work, i.e., why renormalization is unavoidable if one starts with Fock space.

If this is so then why is it in Many Body Theory for solids as an example that Green’s Functions and Correlation functions can be computed at finite times as well as perturbative methods for finite time dynamics unlike particle physics if Haag’s Theorem is such a big issue. Is Haag’s theorem only an issue in Relativistic QFT and not in nonrelativistic many body theory. Do Green’s Functions somehow circumvent this restriction that the interacting states cant be expressed as fock states.

 

[A. Neumaier]

If this is so then why is it in Many Body Theory for solids as an example that Green’s Functions and Correlation functions can be computed at finite times as well as perturbative methods for finite time dynamics unlike particle physics if Haag’s Theorem is such a big issue.

Because this is nonrelativistic quantum physics. There the hard problems of QFT are absent. Haag's theorem assumes local quantum physics in Minkowski spacetime.

Is Haag’s theorem only an issue in Relativistic QFT and not in nonrelativistic many body theory.

Yes.

Do Green’s Functions somehow circumvent this restriction that the interacting states cant be expressed as fock states.

Not if constructed in the usual Fock-based way.

 

[physwiz222]

Because this is nonrelativistic quantum physics. There the hard problems of QFT are absent. Haag's theorem assumes local quantum physics in Minkowski spacetime.

Yes.

Not if constructed in the usual Fock-based way.

I do wonder with all this in mind why no textbooks ever bring these issues up with describing finite time dynamics of even simple scattering processes as this seems to be a very open unsolved problem. It seems that the very notion of particles in Interacting QFT is dubious at best and the idea of finite time dynamics even for scattering is inherently Nonperturbative no matter how weak the coupling constant is as Haag’s Theorem essentially invalidates using perturbation theory in the same way as regular QM. I wonder why no books ever bring up these major issues as they seem to be open problems. Also why are people talking about Quantum Gravity and Supersymmetric 10 dimensional Branes when we don't even have a proper coherent theory and understanding of basic interacting Relativistic QFT and can only map between states in the far past and future even for weak couplings forget Bound states.

 

[weirdoguy]

as this seems to be a very open unsolved problem.

Most textbooks don't delve into open unsolved problems, mainly because they are unsolved and open

Also why are people talking about Quantum Gravity and Supersymmetric 10 dimensional Branes when we don't even have a proper coherent theory and understanding of basic interacting Relativistic QFT

So everyone should halt their work until people from QFT community solve their problems? One can work on quantum gravity or branes without using interacting QFT, so there is no issue there. And maybe work on those models will shed some light on Haag.

 

[A. Neumaier]

I do wonder with all this in mind why no textbooks ever bring these issues up with describing finite time dynamics of even simple scattering processes as this seems to be a very open unsolved problem.

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

It seems that the very notion of particles in Interacting QFT is dubious

Only at finite times.

Nonperturbative no matter how weak the coupling constant is as Haag’s Theorem essentially invalidates using perturbation theory in the same way as regular QM.

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

why are people talking about Quantum Gravity and Supersymmetric 10 dimensional Branes when we don't even have a proper coherent theory and understanding of basic interacting Relativistic QFT

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

 

[physwiz222]

Most textbooks don't delve into open unsolved problems, mainly because they are unsolved and open
So everyone should halt their work until people from QFT community solve their problems? One can work on quantum gravity or branes without using interacting QFT, so there is no issue there. And maybe work on those models will shed some light on Haag.

The statement was more hyperbolic the point is that its weird that we are trying to seek a theory of Quantum Gravity but we dont even have a proper solidly built Relativistic Quantum theory for the EM field. It was more of saying its frustrating when people say we are so close to a theory of everything but gravity just wont cooperate when this isnt the case. In reality we are a long ways off as we struggle and stumble to find a solidly built description of the other forces.