3A: Quantum
Mechanics
Part One
Feynman
Simplified 3A covers
the first half of Volume 3 and chapters 37 and 38
of Volume 1 of The Feynman Lectures on Physics. The
topics we explore include:
 Why the MicroWorld is Different
 Quantization and ParticleWave Duality
 Indeterminism and the Uncertainty
Principle
 Probabilities and Amplitudes
 Identical Particle Phenomena
 Bosons and Spin One
 Fermions and Spin OneHalf
 Time Evolution and the Hamiltonian
Operator
 The TwoState Ammonia Maser
Readers
will greatly benefit from a prior understanding of the material in
Feynman Simplified 1A, 1B and 1C. A familiarity with elementary
calculus is assumed.
Excerpt:
Y & P Measured by a Barrier
With Aperture
Consider
a parallel beam of particlewaves incident on a barrier with a small
aperture that is followed by a detection screen, as shown in Figure
31. Let D be the aperture width, L be the distance between barrier and
screen, and let y=0 on the horizontal axis of symmetry.
Figure 31 Plane Wave
Diffracting
from Barrier with Aperture
Assume
the incident particles have momenta py=0 and pz=p0. What do we know
about the particles after they pass through the aperture?
Particles exiting the aperture must have y values
between +D/2 and –D/2.
Feynman
cautions: we might assume the particle’s exiting y momentum is zero
with zero uncertainty, “but that is wrong. We once knew the momentum
was [entirely] horizontal, but we do not know it any more. Before the
particles passed through the hole, we did not know their vertical
positions. Now that we have found the vertical position … we have lost
our information about the vertical momentum!”
