meopemuk said:
This is a big difference between out philosophies. I don't think that "particles are constructed from the Hamiltonian". In my opinion, particles are given to us a priori. The Hamiltonian is an operator, which we write down to describe the interaction between particles.
I didn't say ''particles'' are constructed, but ''operators'' are constructed. Of course, the particles are given.
meopemuk said:
I was probably not very clear on this point before, but I would like to stress it here. I don't consider "dressing transformation" as a desirable way to perform calculations in QFT.
In the present case you don't need any dressing transformation since everything is manifestly finite and as well-defined as the underlying nonrelativistic dynamics with H_full from which the simplified model was derived. So there is no reason not to solve the exercise.
I mentioned dressing only because you wanted to get rid of the interaction terms with less than two annihilators or creators. This is possible without changing the validity of the model only if you do it via a unitary transform, i.e., using a dressing transformation.
But I agree it is an undesirable way of doing the calculations. And the original exercise doesn't need such a detour - you'll encounter nothing unphysical.
meopemuk said:
The desirable way is to define the Hamiltonian, so that there are no unphysical self-interactions
In the present case, the self-interactions are not unphysical but generated by the projection to the main degrees of freedom, which simplifies a complex space-time problem to a simple quantum dot.
meopemuk said:
With this good Hamiltonian there can be no difference between "bare" and "physical" particles. I believe that this is the only appropriate form of the Hamiltonian in QFT.
You _want_ that, but the derivation proves that one gets something different.
meopemuk said:
Unfortunately, Hamiltonians in existing QFT theories do not obey this principle.
No effective Hamiltonians in solid state physics obeys this principle. It is not appropriate for this kind of problems.
meopemuk said:
Now you suggest to consider a theory, which is formulated in this inappropriate non-transparent self-interacting way from the beginning.
No. I suggest to consider a toy problem derived a well-defined microscopic nonrelativistic Hamiltonian for dot+particles that satisfies your requirement. I motivated the reduced Hamiltonian by explicitly deriving everything from the underlying full theory. Nothing in this derivation is inappropriate, and it is fully transparent.
meopemuk said:
So, you invite me to do the cleanup myself. Yes, I can do that following the procedure outlined in the book. Then I would obtain a well-defined Hamiltonian H for physical particles without self-interactions.
You don't need the cleanup if you use instead my well-defined Hamiltonian H for physical particles with self-interactions. Your dressing would just replace my physical particles (which are identical with the microscopic particles) by effective particles.
meopemuk said:
I believe that this is the true Hamiltonian, which can be used in routine quantum mechanical calculations without any tricks and renormalizations.
You don't need any tricks. The resulting renormalizations are precisely the same energy shifts that you'd get when you'd solve the anharmonic oscillator by perturbation theory.
meopemuk said:
For example, if we diagonalize the Hamiltonian H we obtain energies and wave functions of stationary states, that can be compared with experiments. If |\psi(0> is an initial state vector, then exp(iHt)|\psi(0> is the state vector evolved to time t. This is how I understand the title of this thread "Time evolution is quantum field theories".
One gets exactly the same dynamics, whether one works in the representation with the free particles or in the representation with the dressed particles. There is no more difference than the difference between working in the position or the momentum representation.
meopemuk said:
Now you are saying that my understanding is wrong and there should be a different approach to the time evolution - the one based on Wightman functions. In order to "build my intuition" you suggest to immerse into all these calculations with self-interacting "bare" particles, non-trivial vacuum, renormalization, etc. These calculations are meaningless, in my opinion.
You call it meaningless - against a long and successful tradition of using it. I could teach you how to assign meaning to what you consider meaningless. But only if you do the exercise. With the effort you spent in discussing all that you'd have already solved it, and we could progress...