Quantum field theory, Lecture 8

This winter semester (2016-2017) I am giving a course on quantum field theory. This course is intended for theorists with familiarity with advanced quantum mechanics and statistical physics. The main objective is introduce the building blocks of quantum electrodynamics.

Here in Lecture 8 I begin the discussion of interacting quantum fields.

[GP-mhsb]

Hello, professor. It's puzzling for me around 1:16:00 min. How did you cancel the S and make it to U(ti, tj)s? Thank you for the lessons.

[emiliograndinetti]

Hello. Firstly thank you very much for sharing all those interesting lecrures on youtube. Secondly, i wanted to ask, all those probability calculations of quantum mechanics, how are they done in quantum field theory? Or are they just not done? In other words, is quantum field theory mainly for cross sections and decay rates, or are there also other "practical uses"?

[tillerbrady]

Around 29:00, you mention gapless vs gapped particles in the context of SUSY. What is meant by a gapless particle or gapped particles?

[simonb.]

Hello,
i'm sometimes surprised to see little to no question in your courses, oh well maybe it's just me :D Anyway around @10:00 min.
Why is the first and second line on the right upper board equivalent ? I can see the second line making sense, having to time translate the creation operators and the first line making sense.
Also why is Exp(-iH) a_dagger Exp(iH) linear in a, if H is quadratic ?
Thank you!

[karabomothupi]

Thank you for the amazing lecture prof. Besides the klein gordon equation which other free field theories do we have and can you recommend any sources?

[mrgadget]

Sir, you said the KG field is unphysical, because it doesn't interact with anything. This makes sense, but could it somehow be, that this kind of field is there (in the nature) but we can only "see" it's effect to the ground-state energy of the vacuum, i.e. "dark energy"? (And sorry, if the question didn't make any sense :) Thank you!

[douniakabakibo]

Hello. Thank you for this very nice lecture! I was wondering how the exponential factor disappears in front of the first term in the expansion in the eigenbasis of H @1:21:29...

[mckboulos]

Hello Professor,
At 1:11:40 you mention another way to think about QFT. As QFT being only about calculating those Green's functions.
Where can I find more information about it?
Thank you for the amazing lectures,
Paul

[javierrendon]

Hello, why H_{int} is infinity when using perturbation theory for quantum field theory? don't we expand in small parameters in some QFTs as QED? Thank you!

[rtheben]

What is the full ground state? I cannot easily find a definition on google, thanks