Research direction: Schrödinger’s cat
The unusual features of quantum theory resulted
in heated debate about the correctness of the
theory, and scientists discussed the relative mer-
its of classical and quantum theory throughout
the 1920s and 1930s. One approach that they used
to question the usefulness of each concept was
the use of gedenken experiments; that is, thought
experiments that are not performed in the
laboratory. One of the most famous examples
of these thought experiments was originally
poised by Schrödinger in 1935 and is known
as Schrödinger’s cat (Schrödinger 1935). In the
original version of his experiment, a cat is placed
into a sealed chamber (Figure 9.10). In the
box is a tiny amount of radioactive material so small that perhaps one
of the atoms will decay within an hour. If the atom does decay, a Geiger
counter measures the event, which triggers the release of a hammer that
then shatters a flask containing a deadly poison. Therefore, if the atom
decays then the vial will break releasing the deadly gas and killing the
poor cat but if the decay does not occur the cat lives. The question is: does
the cat live or die? How can the cat live without allowing for a trick, such
as an unseen hole in the box? The answer is that in quantum mechanics,
after the box is sealed, then we no longer know what happened to the cat.
The atom may either decay or not decay, which means that the atom has
the properties of both states at the same time. Thus, the cat can be con-
sideredto be in a superposition of two states, alive and dead. For the real
world the cat cannot be in both states. However, in the world of quantum
mechanics such possibilities are not only allowed but must be considered
in order to understand the possible outcomes. For example, when we
consider chemical bonds, electrons are thought to be present in orbitals
involving two atoms rather than belonging to either one individually.
Historically, Schrödinger used this gedenken to test the unusual fea-
tures of quantum mechanics as the outcome of the superposition of the
macroscopically distinguishable states did not seem reasonable. Recently,
the thought experiment involving Schrödinger’s cat has been tested
experimentally. In these experiments the production of Schrödinger-cat-
like states is hindered by the phenomenon that the superposition of states
is destroyed quickly under most circumstances. The larger the object the
better it must be isolated from outside influences to remain unchanged.
In one experiment (Blatter 2000; Friedman et al. 2000) the system
was not a cat but rather a special instrument called a SQUID, or super-
conducting quantum-interference device. In superconducting devices,
electrons can circulate without any decay; that is, with no resistance.194 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY
Geiger counter linked to poisonous gasFigure 9.10
Schrödinger’s cat.
This is a thought
experiment in which
a cat is trapped
inside a sealed box.
The box contains a
poisonous gas that
is released after a
radioactive decay.
Does the cat live
or die?