informationphilosopher | The laws of nature, except the second law of thermodynamics, are
symmetric in time. Reversing the time in the dynamical equations of
motion simply describes everything going backwards. The second law is
different. Entropy must never decrease in time.
Many natural processes are apparently irreversible. Irreversibility
is intimately connected to the direction of time. Identifying the
physical reasons for the observed irreversibility, the origin of
irreversibility, would contribute greatly to understanding the apparent
asymmetry of nature in time, despite nature's perfect symmetry in space.
In 1927, Arthur Stanley Eddington coined the term "Arrow of Time" in his book The Nature of the Physical World.
He connected "Time's Arrow" to the one-way direction of increasing
entropy required by the second law of thermodynamics. This is now known
as the "thermodynamic arrow."
In his later work, Eddington identified a "cosmological arrow,"
the direction in which the universe is expanding, as shown by Edwin
Hubble about the time Eddington first defined the thermodynamic arrow.
There are now at least five other proposed arrows of time (discussed
below). We can ask whether one arrow is a "master arrow" that all the
others are following, or perhaps time itself is just a given property of
nature that is otherwise irreducible to something more basic, as is
space.
Given the four-dimensional space-time picture of special relativity, and
given that the laws of nature are symmetric in space, we may expect the
laws to be invariant under a change in time direction. The laws do not depend on position in space or direction, they are invariant
under translations and rotations, space is assumed uniform and
isotropic. But time is not just another spatial dimension. It enters
into calculations of event separations as an imaginary term (multiplied
by the square root of minus 1). Nevertheless, all the dynamical laws of
motion are symmetric under time reversal.
So the basic problem is - how can macroscopic irreversibility result from microscopic processes that are fundamentally reversible?
