The Arrow of Time
by Roel Snieder, WM Keck Distinguished Professor of Basic Exploration Science
Department of Geophysics, Colorado School of Mines

ABSTRACT
It is great irony of science that the most fundamental concepts are often most dificult to understand. The concept "time" is an important example of this. The laws that describe the basic forces in nature are symmetric for time reversal. This means that they do not change when one changes the direction of time by replacing the time t by -t. However, this clearly contradicts our experience; we perceive a direction of time. This direction is called the "arrow of time." Different arrows of time can be distinguished: the thermodynamic arrow of time, the biological arrow of time, the radiative arrow of time, the mechanical arrow of time and the cosmological arrow of time. The relation with natural laws that are not invariant for time reversal is discussed and some pitfalls are shown in "deriving" equations with a direction of time from the fundamental laws of physics. The symmetry of time reversal has important applications in geophysics, examples are shown of this. The final question remains: "what explains the arrow of time that seems to pervade our daily experience?"