i/Werner Heisenberg (1901-
1976). Heisenberg helped to
develop the foundations of quan-
tum mechanics, including the
Heisenberg uncertainty principle.
He was the scientific leader of
the Nazi atomic-bomb program
up until its cancellation in 1942,
when the military decided that it
was too ambitious a project to
undertake in wartime, and too
unlikely to produce results.
13.3.4 The uncertainty principle
Eliminating randomness through measurement?
A common reaction to quantum physics, among both early-
twentieth-century physicists and modern students, is that we should
be able to get rid of randomness through accurate measurement. If
I say, for example, that it is meaningless to discuss the path of a
photon or an electron, one might suggest that we simply measure
the particle’s position and velocity many times in a row. This series
of snapshots would amount to a description of its path.
A practical objection to this plan is that the process of measure-
ment will have an effect on the thing we are trying to measure. This
may not be of much concern, for example, when a traffic cop mea-
sure’s your car’s motion with a radar gun, because the energy and
momentum of the radar pulses are insufficient to change the car’s
motion significantly. But on the subatomic scale it is a very real
problem. Making a videotape through a microscope of an electron
orbiting a nucleus is not just difficult, it is theoretically impossi-
ble. The video camera makes pictures of things using light that has
bounced off them and come into the camera. If even a single photon
of visible light was to bounce off of the electron we were trying to
study, the electron’s recoil would be enough to change its behavior
significantly.The Heisenberg uncertainty principle
This insight, that measurement changes the thing being mea-
sured, is the kind of idea that clove-cigarette-smoking intellectuals
outside of the physical sciences like to claim they knew all along. If
only, they say, the physicists had made more of a habit of reading
literary journals, they could have saved a lot of work. The anthro-
pologist Margaret Mead has recently been accused of inadvertently
encouraging her teenaged Samoan informants to exaggerate the free-
dom of youthful sexual experimentation in their society. If this is
considered a damning critique of her work, it is because she could
have done better: other anthropologists claim to have been able to
eliminate the observer-as-participant problem and collect untainted
data.
The German physicist Werner Heisenberg, however, showed that
in quantum physics,anymeasuring technique runs into a brick wall
when we try to improve its accuracy beyond a certain point. Heisen-
berg showed that the limitation is a question ofwhat there is to be
known, even in principle, about the system itself, not of the ability
or inability of a specific measuring device to ferret out information
that is knowable but not previously hidden.
Suppose, for example, that we have constructed an electron in a
box (quantum dot) setup in our laboratory, and we are able to adjust
the lengthLof the box as desired. All the standing wave patterns900 Chapter 13 Quantum Physics