Near the end of the medieval era we saw the invention of the mechanical clock. Before then people relied on sun clocks, which of course gave rise to differences all over the world in terms of duration of an hour, the synchronization of time, and, moreover, obviously did not work after sunset. The synchronization of time over large distances has quite some history. There were at least two reasons why civilization wanted to have reliable time synchronization. One was the discovery of America and the other one the development of a reliable train schedule which required the synchronization of clocks in different cities. There were many occasions of colliding trains simply because of differences in time at different locations along the train rail network. Especially in France, several attempts were made to synchronize clocks; one used pneumatic systems driven by steam pressure. However, even at short distances, such as within the boundaries of Paris, this did not work at all. The introduction of a telegraph allowed synchronization over large distances and Britain and the USA were leading those efforts. In 1885 there was an international conference that agreed on a standard time (Greenwich Time) and the division of the world into 24 time zones. Humans, it seemed, captured time and put it in a convenient box.
However, one can say that since the birth of the theory of relativity our thinking about what time exactly is has changed dramatically. Einstein had to accept that the time could impossibly be the same for each observer traveling at different speeds in order to explain why the speed of light was the same for each observer. Since then insights have grown but the discussion about time continues. This can be illustrated by a recent article from Sean M. Caroll. The article poses some fundamental questions about what is generally called the “time’s arrow”. What is the time’s arrow? This is simple: it is the arrow that points from the past to the future. However, according to Stephen Hawking there are at least three arrows of time:
– The thermodynamic time’s arrow: this is basically the second law of thermodynamic that states that in an isolated system the entropy always increases.
– The psychological time’s arrow: we remember the past but not the future
– The cosmological time’s arrow: the direction in which the universe is expanding rather than contracting
Hawking identifies the psychological arrow with the thermodynamic arrow. Then he argues that a universe that contracts rather than expands will not be able to support life as we know it because there are essentially no gradients (for example temperature gradient or particles concentration gradient) left; this is basically the Heat Death, the final end situation the universe is suppose to arrive at.
The big question that Caroll, and with him many others, addresses is why is the time asymmetric? This question is relevant to pose as the laws of physics are definitely time symmetric. One way to get more insight in this asymmetry is to focus on the thermodynamic arrow of time. Why does the entropy increases all the time in the universe? Well implicit in this statement one assumes that the entropy at the birth of the universe was very low.
Aha! You will say, that assumption seems to be a quite unnatural thing since low entropy situations are not so likely, as we learned from Boltzmann’s micro- and macro states considerations. Then the thought emerges that maybe the big bang was not the real beginning of the universe but that it was merely an intermediate stage. Before the big bang there was a prior universe that possessed high entropy. In that prior universe the time will run backward and in this way the asymmetry of time could be taken away (and people would remember the future rather then the past). Interesting idea isn’t it? See for more mind provoking thoughts the article by Caroll and references cited there and the ones provided below.
Thus, do we understand time? I guess not yet but we are making progress!
© 2008 John Schmitz
 Saint Augustine wrote already around 400, “I know what time is, if no one asks me, but if I try to explain it to one who asks me, I no longer know.” (from Confessiones)
 Caroll Sean M., The Cosmic Origins of Time’s Arrow, Scientific American, June 2008
 Hawking Stephen, A Brief History of Time,
 A system that does not exchange energy or material with its surroundings
 The argument runs as follows. It will take a very long time before the universe starts to contract. At that point in time all stars will have burned up their fuel and the particles and energy will be evenly distributed in space.