Page 109 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch08 Elementary Physical Chemistry

94 Elementary Physical Chemistry

(b) The importance of observation or measurement. In classical
mechanics, one can separate the observation of the system from
the measuring device. In quantum mechanics, the operation of
observation is built into theory and the observed results are the
properties of the system as perturbed by the measurement.
(c) The laws of quantum mechanics have meaning only if measurements
are done on the system.
(d) One of the consequences of the above is that if a system can
exist in several different states, the state of the system will be
a superposition, that is a mixture of all these states (sometimes
referred to as “ghost” states). But if a measurement is made, the
wave-function “collapses” to a particular state and only that state
is observed. The measurement causes the collapse.

Note: As an example of the contrast of observations in classical and
quantum mechanics, consider the measurement of the temperature
of a person with a thermometer. What really takes place is heat
flowing from the person to the thermometer. So the reading on the
thermometer represents the temperature of the person after the heat
loss. But the amount of heat is so small, that for all practical purposes
it is negligible. The same considerations apply to all measurements of
large bodies in classical mechanics. However, if an observation is made
on a microscopic system of say, the size of an electron, the process of
measurement will have profound effect on the results, and may not be
ignored. In quantum mechanics, the operation of observation is built
into theory and the results are properties of the system as perturbed
by the measurement.

8.8. Schr¨odinger’s Cat
Schr¨dinger had difficulties accepting this interpretation and he published
o
a “thought experiment”, which goes under the name of “Schr¨odinger’s
Cat”. In this thought-experiment, a cat is confined in a lower compartment
separated from an upper compartment by a glass ceiling (Fig. 8.4). The
upper compartment contains a poisonous gas. In the upper compartment,