# L1.2 Fermions, Bosons, and Fields: Feynman Diagram | Summary and Q&A

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June 24, 2021
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L1.2 Fermions, Bosons, and Fields: Feynman Diagram

## TL;DR

This lecture provides an introduction to Feynman diagrams, which are used for perturbative calculations of amplitude in reactions.

## Key Insights

• 🔨 Feynman diagrams are a tool used in perturbative calculations in physics.
• 🧑‍🏭 Each diagram represents a factor in the calculation and can be used to perform calculations.
• 📶 The number of vertices in a diagram determines the strength of the coupling and can affect the contribution to the calculation.
• ⌛ The direction of time is an important consideration when interpreting Feynman diagrams.
• ❓ Antiparticles can be represented in Feynman diagrams by replacing particles or relabeling vertices.
• 🪈 Calculating reactions requires multiple vertices in order to satisfy energy and momentum conservation.
• 🧑‍🤝‍🧑 The strength of the coupling is represented by the charge or coupling constant.

## Transcript

MARKUS KLUTE: Welcome to 8.701. So in this lecture, we'll give you the first introduction to Feynman diagram. This is part 1 out of a few sections on Feynman diagrams. So this is really meant to introduce the topic such that we can use the same language to talk about Feynman diagrams before we then later on are able to use them as a tool to calcula... Read More

### Q: What are Feynman diagrams used for?

Feynman diagrams are used for perturbative calculations of amplitude in reactions. They provide a visual representation of the mathematical terms in the calculation.

### Q: How are the diagrams used for calculations?

After drawing the diagrams, the pieces can be put together following a specific rule in order to perform the calculation. Each diagram represents a particular factor in the calculation.

### Q: What is the significance of the vertices in Feynman diagrams?

Vertices in Feynman diagrams indicate where interactions take place. The number of vertices in a diagram determines the strength of the coupling and can affect the contribution to the probability calculation.

### Q: What is the relationship between alpha, the coupling strength, and the number of vertices?

The probability calculation is affected by the coupling strength, which is represented by the charge or coupling constant (alpha). Diagrams with more vertices have a greater factor of alpha to the power of the number of vertices.

## Summary & Key Takeaways

• Feynman diagrams are a visual representation of perturbative calculations for reactions.

• Each diagram represents a factor in the calculation, and the diagrams can be used to perform calculations.

• The number of vertices in a diagram is related to the strength of the coupling and can determine the contribution to the perturbation series.