Further S matrix clarifications

  • Context: Undergrad 
  • Thread starter Thread starter Silviu
  • Start date Start date
  • Tags Tags
    Matrix S matrix
Click For Summary

Discussion Overview

The discussion revolves around clarifications regarding the S-matrix in quantum field theory, particularly its relation to creation and annihilation operators and their evolution under the interaction Hamiltonian. Participants explore the implications of these operators acting on the vacuum state and the interpretation of the S-matrix in specific contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the creation and annihilation operators at ##\pm\infty## are not the same due to their evolution with the full interaction Hamiltonian, suggesting they have different values over time.
  • Another participant expresses confusion about the S-matrix's role in the expression $$=$$, questioning whether this indicates that the S-matrix does nothing in this context.
  • There is a discussion about the necessity of terms like ##e^{iHt}## to evolve the operators between different times, raising questions about the use of the S-matrix versus the usual time evolution operator.
  • One participant points out that Schwartz mentions the dependence of the operators on ##e^{iHt}## and suggests that the derivation may be elaborated in later chapters, but notes a lack of explanation regarding the absence of the S-matrix in certain contexts.
  • A reference to a specific page in Schwartz's text is provided for further clarification on the topic.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the interpretation and application of the S-matrix, with no consensus reached on its implications or the necessity of certain terms in the evolution of operators.

Contextual Notes

Participants reference different texts and interpretations, indicating potential variations in definitions and approaches to the S-matrix and vacuum states, which may affect their understanding of the topic.

Silviu
Messages
612
Reaction score
11
Hello! I attached a SS of the part of my book that I am confused about. So there they write the initial and final states in term of creation and annihilation operators, acting on the (not free) vacuum i.e. ##|\Omega>##. So first thing, the value of the creation (annihilation) operators at ##\pm\infty## are not the same, right? They evolve in time with the full interaction hamiltonian, so differently from the free case they have different values in time. Second thing, the last line, if I understand it well, and plugging in the values for the initial and final states, would be $$<f|S|i>=<f|i>$$ Is this right? It looks like the S matrix does nothing here. What am I doing wrong? Thank you!
 

Attachments

  • qft.png
    qft.png
    27.9 KB · Views: 493
Physics news on Phys.org
A few sentences down Schwartz says the creation/annihilation operators are rotated by the interacting hamiltonian and so are different at different times t and t'.

The use of the ##S## in ##<f|i>## here is a bit confusing, compare to Srednicki.
 
bolbteppa said:
A few sentences down Schwartz says the creation/annihilation operators are rotated by the interacting hamiltonian and so are different at different times t and t'.

The use of the ##S## in ##<f|i>## here is a bit confusing, compare to Srednicki.
Thank you for this! I am sorry I honestly didn't have time to go further yet. However, the creation/annihilation operators, don't they evolve with the full interaction hamiltonian? Shouldn't we have some terms of the form ##e^{iHt}## in between them, to evolve them between the 2 times? Why do we use the S-matrix and not the usual time evolution operator? Edit: I read the notes from the link you mentioned. He doesn't even mention the S matrix there... Also even if he uses the interaction theory he uses for the vacuum ##|0>## and not ##|\Omega>##. Is his ##|0>## the ground state of the interaction theory? And what happens to the S matrix?
 
Last edited:
Schwartz mentioned the dependence of the ##a(t)##'s on ##e^{iHt}## on the same page and goes over the whole derivation in the next few chapters, e.g. I think he brings up ##|\Omega >## more in the next chapter, though I do not see where he explained why ##S## does not appear, he should have mentioned something like ##S|in > = |out>##, ##<out|in > = <in|S|in>## somewhere, something analogous to slide 18 here.
 
Look at page 56 in Schwartz.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 1 ·
Replies
1
Views
893
  • · Replies 9 ·
Replies
9
Views
3K