1. The Art of Estimation Physics
also known as
Ph 2239
II. Forces, etc.
Patrick Diamond, George M. Fuller, Tom Murphy
Department of Physics, University of California, San Diego

2. Handy Facts: Solar System
radius of earth’s orbit around sun

3. Handy Facts: the Universe

4. Electromagnetism
(see handout on comparison of S.I. & c.g.s. units/dimensions)
Thompson Cross Section: (photon energies >> electron binding)
(energy-independent photon-electron scattering cross section)
Klein-Nishina
(relativistic electron targets; very high energy photons)
Rayleigh Scattering:
(when photon energies
lower than atomic electron binding)

5. fine structure constantSI
cgs

6. Eddington Luminosity
Photon scattering-induced momentum transfer rate to electrons/protons
must be less than gravitational force on proton
Electrons tied to protons via Coulomb force
proton
At radius r where interior mass is M( r ) and photon energy luminosity
(e.g., in ergs s-1) is L( r ) the forces are equal when
Sir Arthur Eddington
electron
Flux of photon
momentum
Gravitational force on proton with mass mp

7. The Weak Interaction changes neutrons to protons and vice versa
strength of the Weak Interaction:
typically some 20 orders of magnitude weaker than
electricity (e.g., Thompson cross section)

8. Gravitation
Newton’s constant

9. The Strong Interaction
Low Momentum Transfer
(i.e., where quark/gluon degrees of freedom can be ignored)
The nucleon-nucleon potential is strong, short ranged (~ 1 fm)
– if they touch, they interact!
--- “sizes” = geometric cross sectional area
--- but for charged ions must convolve with Coulomb barrier penetration probability

10. Examples
neutron capture

11. Examples
lab proton energy 460 keV
lab proton energy 100 keV

12. The Strong Interaction
High Momentum Transfer
(i.e., where degrees of freedom are quarks/gluons)
the interaction between quarks and gluons is weak,
Coulomb-like
asymptotic freedom implies zero interaction
at very high energy/short distance – a “gas” of free quarks & gluons
Quark-gluon plasma in the very early universe, the deep interior of neutron stars,
or the central (low rapidity) regime in relativistic heavy ion collisions

13. Mean Free Path/Time
diffusion constant

14. Random Walk
time to random walk a distance R