L19.3 Differential and total cross section

MIT 8.06 Quantum Physics III, Spring 2018
Instructor: Barton Zwiebach
View the complete course: ocw.mit.edu/8-...
UA-cam Playlist: • MIT 8.06 Quantum Physi...
L19.3 Differential and total cross section
License: Creative Commons BY-NC-SA
More information at ocw.mit.edu/terms
More courses at ocw.mit.edu

[@gibbs-13]

If there are 100 people explaining differential cross section, they do in 100 different ways from each other.
This is one of the best explanations to me!

[@jacobvandijk6525]

This only means you didn't understand the other explanations. Nothing else.
You (and these likers of yours) have a very subjective mind. Wake up!
Teaching is about giving EVERYONE the right example to understand.

[@TheCrunchyGum]

3:39 Finally the differential cross section is explained clearly... I have no idea why textbooks and instructors are obtuse about something so simple.

[@not_amanullah]

Thanks ❤️🤍

[@jacobvandijk6525]

@ 16:37 A STRANGE EQUATION physically. As if a change in solid angle (determined by the experimenter) would change something about the cross-section of the material in target?

[@JimmyGray]

Great lecture. Thank you.

[@not_amanullah]

This is helpful ❤️🤍

[@abhinovenagarajan.s7237]

Given that the wave function psi is a 'leading term' solution, how do we know that the remaining terms may not contain functions that depend on (theta,phi) and hence, affect our differential cross section as well?

[@scienceandstudy6431]

please explain why u took incident wave imaginary part

[@onderozenc4470]

It is plane wave representation. Though the outgoing wave is spherical in terms of both phase and amplitude, its argument becomes suppressed in the intensity expression (complex conjugate).

[@jacobvandijk6525]

@@onderozenc4470 Now try to answer his question. Only the first sentence has anything to do with it.

[@onderozenc4470]

In turkish universities, even the emeretus professors😄don't know about these representations 😝

[@Shaziakhan75557]

A cmplt knowledge of the topic could be found here

[@cikif]

2:02 I'm finding as you -should- *shoot* in particles
3:58 an -iv- *idea*
5:39 -floods- *flux*
19:03 approximate solution -for a wave- *far away*

[@MANOJKUMAR-ev7pl]

At 9.12 min why he used imaginary

[@parasharchatterjee3223]

The probability current density has the formula of the type he wrote. There's another one where its (psi*)del psi - psi del (psi*), which is generally what is derived in textbooks.

[@onderozenc4470]

Wave function multiplied by its imaginary part gives the probability density.

[@zakirreshi6737]

Playlist link ua-cam.com/play/PLUl4u3cNGP60Zcz8LnCDFI8RPqRhJbb4L.html

[@scienceandstudy6431]

please explain why u took incident wave imaginary part

[@Tom-iv5pw]

It is the definition of 'Probability Current'. Since the sum (or integral) over all possible states will always conserve to 1 - before and after the scattering process - what we assume is that the probability redistributes itself, like how the intermediate state between two different charge distributions would require an electric current to redistribute the conserved overall charge.
A quick way to obtain this quantity is to take the time-dependent Schrdöedinger equation (Let's call this Eq.1) and its complex conjugate (Eq.2). If we multiply Eq.1 by the complex conjugate of the wave function and Eq. 2 by the wave function and subtract these expressions, we will get an expression which we can be manipulated into something very similar to a continuity equation (divergence of a vector + partial time derivative of a scalar = 0).
This vector (the one that is acted upon with a divergence in the paragraph above) is the 'Incident Flux' the professor defined in this video.