  Long-ranged, or power-law, behavior of correlation functions in both space
and time is discussed for classical systems and for quantum systems at finite
temperature, and is compared with the corresponding behavior in quantum systems
at zero temperature. The origin of the long-ranged correlations is explained in
terms of soft modes. In general, correlations at zero temperature are of longer
range than their finite temperature or classical counterparts. This phenomenon
is due to additional soft modes that exist at zero temperature.
