Richard Feynman on Quantum Electrodynamics
X-ray crystallography in a basic sense studies the result of photons interacting with electrons.
How do we describe how photons are reflected?
Richard Feynman won the Nobel Prize in Physics in 1965 for fundamental work in quantum electrodynamics.
Vega Science has put together a series lectures that Richard give at the University of Auckland in 1978.
Below is a time line with a couple of notes from his 77 minute lecture entitled:
Part 1: Photons – Corpuscles of Light
00:00 Introduction
04:40 Light
09:35 Theory of interaction with light
14:42 Great analogy between checkers and nature
20:20 Explains the theory
23:15 ‘I enjoyed your lecture, but didn’t understand it’
26:15 ‘Nobody understands it’
33:13 Possible meaning
34:45 Describes theory
40:48 Reflection
47:50 Science is based on probabilities
57:10 Answer to the reflection Problem
68:10 Questions
The lecture makes me wonder about the relationship between protein crystal size and the probability of photons being diffracted. I am starting to get uneasy about how much is explained away due to crystal packing.
Sili
December 23rd, 2009 at 10:27 AM #
I’m pretty sure I don’t understand your question.
Sean
December 23rd, 2009 at 12:15 PM #
Hi Sili,
I don’t have one explicitly, but the lecture made me think.
For example, Feynman discusses two issues that play a role in how photons are reflected:
1) the material
2) the depth of interaction
I wonder if a larger crystal of the same protein (and packing) will always reflect more photons. If photons are reflected based on probabilities in a peak and valley type of manner then perhaps reducing the crystal size will improve diffraction by moving from a valley to a peak.
Artem
December 23rd, 2009 at 2:42 PM #
Cheers,
In an idealized situation larger crystals should have stronger scattering since each incoming photon has more chances to interact with the electrons (there are more electrons to interact with/longer interference cross-section).
In a somewhat more realistic approximation there’s a ’soft’ practical limit on the crystal size imposed by absorption , lattice effects, and mosaic spread. If we take an even more realistic view then there’s yet another limitat imposed by practicalities of crystal cryoprotection (very large protein crystals are difficult to freeze properly).
A.