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Lecture 23: Fundamental Forces and Interactions
April 24, 2025
Reading Assignment
- Read: Supplementary Reading Ch 10
Objectives
- (Continuing objective) Describe quantum and elementary particle theories in your own words, and discuss applications of the material.
- State the four “fundamental” interactions and describe their properties; in particular, what kinds of particles they act on, their range, and their role in atomic and subatomic (as well as macroscopic) processes.
- State and use the colorless rule for physically observable particles. Explain in words and sketches how gluons can change quark color.
- Describe the role of virtual particles in Quantum Field Theory's description of particle interactions. Use the Energy-Time uncertainty principle to relate the mass of a virtual messenger particle to the range of the corresponding force.
- Distinguish between quarks and leptons, describe which interactions they feel, and list which properties (color, flavor, family membership) are affected by which interactions.
- Draw and use Feynman diagrams that include quark-gluon vertices, quark W-boson vertices, lepton-W boson vertices, or electron-photon vertices.
Homework
- Friday's Assigned Problems: A106; Supp CH 10: 1a, 2, 3, 5, 6, 7, 8, 9, 10
- Monday's Hand-In Problems from Lecture 23:
Supp CH 10: 1b, 4, 11, 12
Note: this is only the second half of the hand-in set.
Lecture Materials
- Click here for the Lecture overheads. Answers: CT1 - 6; CT2 - 3; CT3 - 2
Videos of example problems
To see the problem statement, click on the link below. To play the video example, click on the underlined words "Video Demonstration" near the top of the page with the problem statement.- Video example: range for Forces
- Example of Feynman diagrams (a vertex).
- Another example of working with Feynman diagrams, this time with gluons.
Pre-Class Entertainment
- Hills of Connemara - Gaelic Storm
- Back On the Chain Gang - The Pretenders
- Uncle John's Band - The Grateful Dead
- Feel Like a Natural Woman - Carole King
- Rip this Join - The Rolling Stones
Assigned Problems Guide
- A106: medium quick. Practice conserving at the vertices.
- Supp 10-1a: medium quick. A look at the world around you with an eye towards fundamental forces.
- Supp 10-2: quick. Compare to what we did in lecture with the $W^-$.
- Supp 10-3: medium. Exactly what we did in lecture with the $W^-$ range, only now with the $Z^0$, which has a slightly different mass.
- Supp 10-5: medium. You can only have three lines meet at a vertex. Hint: the virtual particle in this case isn't a messenger! That's allowed.
- Supp 10-6: medium. Practice with switching particles from before to after by taking their antiparticle.
- Supp 10-7: medium quick. Step 1: the colorless rule implies that you can have $qqq$, but not $qq$ or $q$. You can have $q\bar q$, but not $qq\bar q$, etc. So now see what charge results from the allowed $qqq$ or $q\bar q$.
- Supp 10-8: quick. Make the diagram, with time flowing up.
- Supp 10-9: Yakka Foob Mog. Grug Pubbwup Zink Wattoom Gazork. Chumble Spuzz.(explanation). If you're shaky on camouflage, talk with your PS instructor.
- Supp 10-10: medium. Practice with using the Gumby rule and drawing vertices.