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 Micro-World is Different
•    Quantization and Particle-Wave Duality
•    Indeterminism and the Uncertainty Principle
•    Probabilities and Amplitudes
•    Identical Particle Phenomena
•    Bosons and Spin One
•    Fermions and Spin One-Half
•    Time Evolution and the Hamiltonian Operator
•    The Two-State 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.

Y & P Measured by a Barrier With Aperture

Consider a parallel beam of particle-waves incident on a barrier with a small aperture that is followed by a detection screen, as shown in Figure 3-1. 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 3-1 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!”