May 19, 2010
Serway and Jewett Chapter 28 – Quantum Mechanics
Blackbody radiation, the photo & Compton effects
proved beyond doubt that light comes in little packets of energy
and yet somehow is able to exhibit wave-like interference effects.
– The Double slit experiment and Wave-Particle duality
In this lecture you will learn that quantum effects are universal – every
degree of freedom, every coordinate associated with any physical system,
is both wave-like and particle-like – light, electrons, buckyballs, all!
The double slit experiment or its equivalent has been done with photons
(1803), electrons (1927), positrons (1980), and buckyballs (1999), proving
de Broglie’s (1922) hypothesis correct -
The experiment has been done with very weak beams of photons and
electrons, proving that whatever constitutes the wave that is interfering is
associated with single particles that are detected all at once at a point.
Finally, the experiment has been done with a delayed choice, such that a
single photon cannot know before it is launched into a beam splitter that
produces two interfering beams whether it will eventually have to interfere
with itself or choose one or the other of two possible paths.
This seems to prove that, as the electric field has a reality independent of
the charge that produced it and is not merely a device for calculating
distant forces, that the probability amplitude distribution, or wavefunction,
has an existence that is independent of what measurement will be
performed on it.
sketch of two-
slit diffraction of light.
Narrow slits at A and B act as
sources, and waves interfering
constructively are shown at C,
D, E, and F.
Royal Soc. 1803.
The 2007 delayed choice experiment of Vincent Jacques et al. brought to
life the gedanken experiment envisioned by John A. Wheeler (1984).
The delayed choice experiment is the Michelson-Morley experiment of
Louis de Broglie and the wave nature of “particles”
De Broglie’s (1923) Hypothesis: Because photons have particle as well as
wave natures, perhaps all forms of matter have
as well as
The inspiration for this amazing hypothesis was likely the Rydberg series
for the energy levels of the hydrogen atom
, where the fact that
= 1,2,3, … suggests standing waves have something to do with the states
of electrons in “orbit” around a nucleus.