To the ancients time was something alive and cyclic. The endless cycles of life were not just happening over time again and again rather they were time itself. However as man became increasingly aware of his long history and increasingly separated from the cycles of the natural world cyclic time eventually would give way to linear time.
It was newton who made the switch when he published his works on mechanics and described an absolute time which moved forward at the same rate regardless of outside influences. According to Newton time did not go faster when one was having fun. Newtons absolute time was quite the step forward because it established time as something not pliable to mood or human nature. To people with no indicators of time but only the sun's position in the sky a very personal time would have been perfectly plausible, to them time could very well fly when one was having fun after all it certainly seemed to. Without the minute to minute second to second precision of the modern world there certainly was no trouble with believing that the sun moved a little faster accross the sky when we were playing stones than when we were hauling cargo at the docks.
The absolute trust that most people now put into newtons concept of absolute time is instructive of its usefullness. The usefullness of an absolute time is its saving grace. It alows us to set appointments at 2:00 and expect people to show up. If someone were to show up to the meeting an hour late claiming that they arrived on time because their time runs differently than ours we would probably dismiss such a story as completely impossible. But time does not quite seem to be so well behaved as newton seemed to think it was. Einstein showed that newtons time reflected reality only at relative speeds much slower than the speed of light. Much earlier than this however there were some interesting theories that had dire implications for the absolutivity of time. Newton proposed a model of time but not a theory of what it was. The first insight into that particular mystery came with the advent of thermodynamics. Thermodynamics was so named because it came about first from the study of heat. However thermodynamics is a branch of physics with implications as far reaching as is it is possible for a theory to have. Thermodynamics is the basic theory of energy and as such is one of the pillars of modern science. Albert Einstein once said that every physical theory he knew might eventually be proven wrong except thermodynamics. Given its extremely important place in science one might expect it to have extremely complicated rules. However the basis of thermodynamics is the very essence of simplicity with the entire realm based on only two laws. The first law of thermodynamics is: Energy cannot be created or destroyed. The second law of thermodynamics is: Entropy always increases. The concept of entropy was introduced to the scientific world simply as the ratio of heat to teperature but it has since evolved into something more general. Today when entropy is described it is most often talked about in terms of disorder. Disorder viewed from the standpoint of thermodynamics is largely a matter of homogeneity. The more homogenous a thing the higher its entropy. Take the example of a hot glass of cocoa. The cocoa will cool over time, the first law ensures that the heat of the liquid does not just dissapear but the total amount of heat will stay the same. The difference in temperature of the cup and the air represent a kind of order and order that is destroyed when the cup and air come to rest at the same temperature called tye equilibrium temperature. The second law of thermodynamics therefore ensures that the temperature of air and hot cocoa will become homogenous with each other since that state has a higher entropy and a lower order. The second law of thermodynamics is written with absolute certainty entropy always increases. The striking similarities between time and entropy are too many to be ignored. When trying to describe time phsicists are at a severe disadvantage since almost all the laws of physics seem to work equally well backwards in time as forwards. The first law of thermodynamics for instance bears no preference for past or future it doesn't care what form the energy is in so long as there is always the same amount of it. The same is true for electromagnetism, every electromagnetic wave traveling forward in time defines a perfectly possible wave traveling backwards in time instead. Even einsteins theory of relativity which replaced newtons does not have any intrinsic arrow of time built into it. The only law that seems to hold a preference between past and future is the second law of thermodynamics and for this reason many physicists have equated entropy with time.
Because of the strong association between time and increasing entropy many have looked for clues about the nature of time in the science of thermodynamics. One of the most startling and difficult things to understand about thermodynamics is the reconciliation of macroscopic thermodynamics with microscopic thermodynamics. On extremely small scales the second law softens to become something more like entropy will probably increase. Time may be no different from entropy in this respect. That is to say that time flows forward only as an average and that statistical fluctuations in the flow of time may occur.
