Whither Time’s Arrow?

In our everyday lives we have the sense that time flows inexorably from the past into the future; that time has a definite direction; and that the arrow of time points towards a future of greater entropy and disorder. But in the microscopic world of atoms and molecules the direction of time is indeterminate and ambiguous. The true nature of time is somewhat mysterious. We creatures of time sense that time flows, that the present is real, the past remembered, the future anticipated. Rain falls; rivers flow downhill; mountains erode; we are born, grow old, and die; order decays into chaos. In our everyday experience, time has a definite direction, and the arrow of time points towards the future. But despite the blatant time-asymmetry of our everyday lives, the accepted, fundamental theories of physics are time symmetric1. Neither Newtonian mechanics, special or general relativity, quantum mechanics, nor quantum field theory picks a preferred direction in time, anymore than these theories picks out a preferred direction in space. Physics provides no objective reason to believe that our present is in any way special, or more real than any other instant of time. Why then does the past appear different from the future? The one fundamental theory that does pick out a preferred direction to time is the second law of thermodynamics. The second law asserts that the entropy of any closed system, the Universe included, tends to increases towards a state of maximum en-