1.8 The Arrow of Time
Unlike physical space, physical time is inherently directional: it flows in one direction, from the future to the past; this is a necessary truth. In thermodynamics the arrow of time for the world is expressed through what is called the ‘entropy’, which is a description of the degree of order of a system; a highly ordered system is said to have smaller entropy, and vice versa. The world’s entropy is always increasing (i.e., its order is decreasing): this is a free ‘one-way ticket’, and one has to pay dearly for the return. For example, the process of mixing hot water into cold water to get warm water is never reversed, though in principle we may think of some complicated machine that can do the reverse. The arrow of an irreversible physical process is the way it normally goes, the way it normally unfolds through time.
The problem with the arrow of time is that the variable time is symmetric in most equations of physical laws. This means that if the variable ‘t’ is replaced by its negative ‘-t’ in those laws, the result is still a law; the basic equations are unchanged. Some scientists theorize that the cosmological arrow of time will one day reverse direction when the force of gravity will halt the further expansion of the Universe and start a collapse to its initial state, just like a movie played backwards (Price 2002: 19-56).