• Feynman Rules
• Feynman Diagrams
• Feynman Parametrization
• Feynman Gauge
• Feynman Cut-off
• Feynman Propagator
• Feynman Path integral
• Feynman Parton Model
• ….
Feynman invented the language of
modern particle physics – Feynman
diagrams and covariant Feynman rules
Sin-Itiro Tomonaga
Julian Schwinger
Richard P. Feynman
"for their fundamental work in quantum electrodynamics, with deepploughing consequences for the physics of elementary particles“
What I am going to tell you about is what we
teach our physics students in the third or
fourth year of graduate school... It is my task
to convince you not to turn away because
you don't understand it. You see my physics
students don't understand it... That is
because I don't understand it. Nobody does.
QED, The Strange Theory of Light and
Matter, (London, 1990) 9.
To those who do not know
mathematics it is difficult to get
across a real feeling as to the
beauty, the deepest beauty, of
nature ... If you want to learn about
nature, to appreciate nature, it is
necessary to understand the
language that she speaks in.
The Character of Physical Law
Emitted photon
Emitted electron (absorbed positron)

Absorbed photon

Flying photon
g 
i 2
p
v
“I took the observation that positrons could
electron
simplyAbsorbed
be represented
as (emitted
electronspositron)
going
from the future to the past in a back section
of their world lines”

R.Feynman, Nobel Lecture
v
Flying Electron
Flying Positron
p̂  m
p̂  m
i
p  m  i
2
2
i
p 2  m 2  i
Interaction between electron, positrons and photons
Absorption of a photon
by an electron
e

Emission of a photon
by an electron
e

e e
Annihilation
e 
All physical processes in Quantum Electrodynamics can be
constructed from these elements like LEGO structures
Amplitudes of probability = M
Probability = |M|2
Compton Scattering
Mueller Scattering
p

v  
( pˆ  m)   v   
p 2  m2
p
Annihilation
p

g


v   v (i 2 )v   v
p
Electrons (e-) and positrons
(e+) collide at accelerators at
very high energy and produce
a pair of muons (μ+ μ-)
LEP Accelerator
v=0. 999 999 999 987 c
N  Flux 
Number of events
Cross-section
Cross-section
Angular Deps
d
1
|k| 1
2

|
M
|

2
2
d  2 E 16 E 4 spin
E=Energy
Ω = Angle
Matrix element

g
2


2
| M | | v   v (i 2 )v   v |
p
d

m2

1  E2
2
d  4E
2
(1 

4 2
m2

1  E2
2
3E

)

(1

)
cos

E2
E2

m2
1 

m2
2
m

1
2 E2


2
Total x-section
Area view of the Large
Hadron Collider (CERN)
27 km ring
If I could explain it to the average person, I wouldn't
have been worth the Nobel Prize.
R.Feynman
"The electron does anything it likes," he said. "It just
goes in any direction at any speed, forward or
backward in time, however it likes, and then you
add up the amplitudes and it gives you the wavefunction." I said to him, "You're crazy." But he wasn't.
Freeman J.Dyson, 1983
Feynman was fond of saying that all of
quantum mechanics can be gleaned from
carefully thinking through the implications of
this single experiment.
The most baffling part of this experiment comes when only one photon at a time
is fired at the barrier with both slits open. The pattern of interference remains the
same as can be seen if many photons are emitted one at a time and recorded on
the same sheet of photographic film. The clear implication is that something with
a wavelike nature passes simultaneously through both slits and interferes with
itself — even though there is only one photon present. (The experiment works
with electrons, atoms, and even some molecules too.)
“I don't know why this is - it remains a mystery,
but it was something I `learned from
experience. There is always another way to
say the same thing that doesn't look at all like
the way you said it before. I don't know what
the reason for this is. I think it is somehow a
representation of the simplicity of nature”.
.
R.Feynman
The most beautiful thing we can experience is the mysterious. It is
the source of all true art and all science. He to whom this emotion
is a stranger, who can no longer pause to wonder and stand rapt
in awe, is as good as dead: his eyes are closed.
A. Einstein
I learned very early the difference between knowing the
name of something and knowing something.
Some people say, "How can you live without knowing?" I do not
know what they mean. I always live without knowing. That is
easy. How you get to know is what I want to know.
The Feynman Problem-Solving Algorithm:
(1) write down the problem;
(2) think very hard;
(3) write down the answer.
If it turns out there is a simple
ultimate
law
which
explains
everything, so be it — that would be
very nice to discover. If it turns out it's
like an onion with millions of layers...
then that's the way it is. R.Feynman
No problem is too small or too trivial if we
can really do something about it.
The worthwhile problems are the ones you
can really solve or help solve, the ones you
can really contribute something to.
The wonderful thing about
science is that it's alive.
Physics isn't the most
important thing. Love is.
He is by all odds the most brilliant young physicist here, and
everyone knows this.
J.Oppenheimer on Feynman's status among the physicists at Los Alamos.