What is the difference between first and second-order vacuum polarization?

  • Context: Undergrad 
  • Thread starter Thread starter raracon
  • Start date Start date
  • Tags Tags
    Polarisation Vacuum
Click For Summary

Discussion Overview

The discussion revolves around the differences between first and second-order vacuum polarization in quantum electrodynamics, with participants examining specific Feynman diagrams and their interpretations. The scope includes theoretical aspects and conceptual clarifications related to vacuum polarization and light-by-light scattering.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions what determines the order of vacuum polarization and provides Feynman diagrams for reference.
  • Another participant challenges the classification of the second diagram as a vacuum polarization diagram.
  • Some participants assert that the second diagram represents light-by-light scattering rather than vacuum polarization.
  • There is a reference to Wikipedia describing the second diagram as "vacuum polarization second order," which raises further confusion among participants.
  • A participant explains that vacuum polarization typically refers to photon self-energy diagrams with two external photon lines, while the second diagram involves four external lines and describes elastic scattering of photons.
  • Concerns are raised about the observable consequences of self-energy insertions, with a participant noting that such effects are negligible and relate to the concept of renormalization.

Areas of Agreement / Disagreement

Participants express disagreement regarding the classification of the second diagram, with some asserting it is light-by-light scattering while others reference its description as vacuum polarization. The discussion remains unresolved regarding the correct interpretation of the diagrams.

Contextual Notes

There are limitations in the discussion regarding the definitions of vacuum polarization and light-by-light scattering, as well as the implications of the diagrams presented. The participants do not reach a consensus on these points.

raracon
Messages
35
Reaction score
14
TL;DR
What is the difference for i.e vaccumpolarisation of the lowest order and vacuumpolarisation second order?
What determines the order of the vacuumpolarisation? I've added 2 Feynman Diagrams for refference. The first one shows the vacuumpolarisation of the lowest order the second shows the vacuumpolarisation of the 2nd order. What is the difference?

VP.png
VPHO.png
 
Physics news on Phys.org
Are you sure that the second diagram is a vacuum polarization diagram?
 
Gaussian97 said:
Are you sure that the second diagram is a vacuum polarization diagram?
Yup
 
  • Like
Likes   Reactions: gentzen and Nugatory
Well, it's not. It's light-by-light scattering.
 
  • Like
Likes   Reactions: vanhees71
Vanadium 50 said:
Well, it's not. It's light-by-light scattering.
Yeah, but on Wikipedia the second pic is described by the words: Vacuumpolarisation second order
 
I defer to your expertise as an 11th grader, then.
 
  • Haha
Likes   Reactions: berkeman
Vanadium 50 said:
I defer to your expertise as an 11th grader, then.
Nah, I am just madly confused haha. It is indeed Light by Light scattering, but why does it have that name given to it? Or how would a vacuumpolarisation second order look like?
 
raracon said:
Yeah, but on Wikipedia the second pic is described by the words: Vacuumpolarisation second order
Wikipedia has been known to be wrong. Just sayin’
 
What's called "vacuum polarization" is usually referring to the photon self-energy diagrams, i.e., Feynman diagrams with two external wiggly photon lines. So the one-loop 1st diagram is a vacuum-polarization diagram, the 2nd diagram has four external photon lines, and it describes the elastic scattering of photons. This is a process that's not included in classical electrodynamics, i.e., it is purely due to the radiative corrections of the quantized theory, QED. You can get a contribution to the vacuum polarization by connecting two of the external lines to an internal photon line. Another example is if you take the 1st one-loop diagram and draw a photon line connecting the two internal electron-positron lines. These are then two-loop contributions to the vacuum polarization of the photon.
 
  • Informative
Likes   Reactions: gentzen and hutchphd
  • #10
vanhees71 said:
he one-loop 1st diagram is a vacuum-polarization diagram
But a terrible example of one, as it has no measurable effects. Photon goes in, photon with the same kinematics comes out.
 
  • Like
Likes   Reactions: vanhees71
  • #11
Of course self-energy insertions to external legs have no observable consequences. That's the point of renormalization after all!
 

Similar threads

Undergrad Four Questions on vacuum phase transitions in the Universe...
  • · Replies 29 ·
Replies
29
Views
3K
Undergrad The theory on the effects of polarizers on particles wavefunctions
  • · Replies 6 ·
Replies
6
Views
1K
Graduate Propagator from a space-time point to itself
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad What happens when circular polarization meets a diagonal polarizer?
  • · Replies 20 ·
Replies
20
Views
3K
Undergrad Observables on the "3 polarizers experiment"
  • · Replies 2 ·
Replies
2
Views
1K
Undergrad Normal order and overlap of states
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Common interaction vacuum for QED + QCD?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Scattering Theory: Difference between use of scattering amplitude and Born approximation?
  • · Replies 1 ·
Replies
1
Views
569
High School Why do photon polarization experiments show similar outcomes in different situations?
  • · Replies 11 ·
Replies
11
Views
2K
Undergrad Problem with the polarization of entangled photons
  • · Replies 7 ·
Replies
7
Views
2K