Sir, - Dr Sean O'Donnell (October 26th) accuses me of being rash in claiming that time moves forward in my Science Today column of October 19th. Dr O'Donnell states that there is an absence of any real present in the laws of physics. However, in my opinion physics is not as clear on the matter of time as Dr O'Donnell contends.
It is true that what are considered the most basic laws of physics are indifferent to the direction of time and provide no basis for an arrow of time. Albert Einstein expressed this memorably: "For us convinced physicists the distinction between past, present and future is an illusion, although a persistent one."
But the second law of thermodynamics, which I described in my column, is not indifferent to the direction of time. It has been assumed that thermodynamics is secondary to the primary theories of physics, and can be derived from them. This is paradoxical. How can a law that demonstrates an arrow of time be derived from time-symmetric laws?
A Belgian research group under the leadership of the 1977 Nobel Laureate Ilya Prigoginc recently claimed to resolve this paradox. They point out that, for the most part, the world is composed of unstable systems, e.g. the atmosphere. They claim to have shown that unstable systems are explained at the most fundamental level by thermodynamics, i.e. thermodynamics is the most primary theory in physics and, therefore, physics demonstrates an arrow of time.
As I explained in my article, thermodynamics demonstrates an arrow of time that points from past, through present, to future. I also pointed out that time, an everyday familiar concept, can be very difficult to grasp at a fundamental level of understanding. Dr O'Donnell's letter illustrates some of the subtleties involved. I will resist the temptation to debate these subtleties further, as the waters on this subject get very deep very quickly. - Yours, etc.,
William J. Reville,
University College,
Cork.