Vacuum diagrams vs. tree diagrams vs. loop diagrams

Click For Summary

Discussion Overview

The discussion centers around the differences between tree diagrams, loop diagrams, and vacuum diagrams in the context of theoretical physics, particularly in quantum field theory. Participants explore definitions and characteristics of these diagrams, as well as their implications in various physical phenomena.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants propose that tree diagrams are diagrams without loops, while loop diagrams contain loops, defined topologically by their homotopy groups.
  • One participant suggests that vacuum diagrams are diagrams without vertices, while another clarifies that they are actually diagrams without external legs.
  • A participant questions how vacuum diagrams can be detected without external legs, referencing the Casimir Effect and the Lamb Shift as examples of closed loop effects that involve external legs.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and characteristics of vacuum diagrams, particularly regarding their detection and the role of external legs. The discussion remains unresolved with multiple competing interpretations.

Contextual Notes

Definitions of diagrams may depend on specific theoretical frameworks, and the implications of these diagrams in physical phenomena are not fully explored. The discussion includes assumptions about the nature of diagrams that may not be universally accepted.

Higgsy
Messages
18
Reaction score
0
Could someone please tell me the difference between tree diagrams and loop diagrams? If I'm thinking correctly tree diagrams are before contracting? Also how do vacuum diagrams fit into the picture?

Thanks!
 
Physics news on Phys.org
Higgsy said:
Could someone please tell me the difference between tree diagrams and loop diagrams?

A tree diagram is a diagram which does not contain loops. A diagram contains (at least) a loop if it is possible to go along edges and return to the same point without backtracking along the same edges. If you are topologically inclined, a diagram ##T## is a tree diagram if it has a trivial first homotopy group ##\pi_1(T)##. An ##n## loop diagram ##D_n## has the first homotopy group ##\pi_1(D_n) = \mathbb Z_2^n##.
 
And vacuum diagrams are just diagrams without vertices?
 
Higgsy said:
And vacuum diagrams are just diagrams without vertices?
No, vacuum diagrams are diagrams without external legs.
 
  • Like
Likes   Reactions: Higgsy
Oh right of course. Sorry it's getting late... thanks!
 
Isn't the only way it can detected by by the external legs? How could it's presence could be in any detected without external legs? For example, it's the Casimir Effect a closed loop effect, with external legs, in the same way the Lamb Shift is a closed loop effect with external legs?
 

Similar threads

Graduate How to know the number of Feynman diagrams for a given order?
  • · Replies 0 ·
Replies
0
Views
1K
Graduate Propagator from a space-time point to itself
  • · Replies 6 ·
Replies
6
Views
2K
Graduate Scalar-scalar scattering Feynman diagram
  • · Replies 7 ·
Replies
7
Views
3K
Undergrad Some questions in QFT (EM vertex, Ward identity, etc.)
  • · Replies 5 ·
Replies
5
Views
3K
Graduate Srednicki's QFT: Feynman Rules and Feynman Diagrams
  • · Replies 4 ·
Replies
4
Views
3K
Undergrad Scattering, 4 point correlator, number of distinct Feynman diagrams
  • · Replies 15 ·
Replies
15
Views
3K
Undergrad Common interaction vacuum for QED + QCD?
  • · Replies 4 ·
Replies
4
Views
2K
Graduate Trace of a product of Dirac Matrices in a Fermion loop
  • · Replies 5 ·
Replies
5
Views
3K
Graduate Renormalizability conditions for a real scalar field in d dimensions
  • · Replies 2 ·
Replies
2
Views
2K
Graduate QM is Feynman path ensemble - is QFT Feynman field ensemble?
  • · Replies 89 ·
3
Replies
89
Views
6K