Time has stretched from the big bang to the present, and is further stretching off into the future. As we experience it, time flows in a single direction. The past lies behind us and is fixed and immutable, and accessible by memory or written documentation. The future, on the other hand, lies ahead and is not necessarily fixed and although we can perhaps predict it to some extent, we have no firm evidence or proof of it and this is what we think of as the arrow of time. The arrow of time seems to express the fact that in the world about us, the past is distinctly different from the future e.g the milk spills but doesn’t un-spill; gases mix and do not un-mix; eggs splatter but do not un-splatter; waves break but do not un-break; we always grow older and never younger. It appears inconceivable to us that that this progression could go in any other direction. Most of the events we experience appear irreversible: for example, it is easy for us to break an egg, and hard, if not impossible, to unbreak an already broken egg.
But this experience of time doesn’t match with most physicists who view it as a dimension rather than linear path. According to the theory of Relativity, the reality of the universe can be described by four-dimensional space-time, so that time does not actually “flow” but it just “exists”. The perception of an arrow of time that we have in our everyday life therefore is nothing more than an illusion of consciousness. The events and processes at the macroscopic level are quite clearly time-asymmetric (i.e. natural processes do have a natural temporal order, and there is an obvious forward direction of time), but the physical processes and laws at the microscopic level, are entirely or mostly time-symmetric. If a process is physically possible, then generally speaking, so is the same process running backwards, so that if we were to hypothetically watch a movie of a physical process, one would not be able to tell if it is being played forwards or backwards, as both would be equally plausible. Watching a single smoke particle buffeted by air, it would not be clear if a video was playing forwards or in reverse; as it drifts left or right qualitatively it looks no different. It is only when we study that gas at a macroscopic scale that the effects of entropy become noticeable.
In theory, therefore, most of the laws of physics do not necessarily specify an arrow of time. Waves always radiate away from a source Waves, like light, radio waves, sound waves, water waves, etc, are always radiative and expand outwards from their sources. While theoretical equations do allow for the opposite (covergent) waves, this is apparently never seen in nature. This asymmetry is regarded by some as a reason for the asymmetry of time.
The apparent asymmetry of time is really just an asymmetry of chance – things evolve from order to disorder not because the reverse is impossible, but because it is highly unlikely. There is nothing in the laws of physics that prevents the act of shuffling a deck of cards from producing a perfectly ordered set of cards – there is always a chance of that, it is just a vanishingly small chance. To give another example, there are many more possible disordered arrangements of a jigsaw than the one ordered arrangement that makes a complete picture. Entropy is therefore the only quantity in the physical sciences (apart from certain rare interactions in particle physics) that requires a particular direction for time. As one goes “forward” in time, the second law of thermodynamics says, the entropy of an isolated system can increase, but not decrease. Hence, from one perspective, entropy measurement is a way of distinguishing the past from the future. However, in thermodynamic systems that are not closed, entropy can decrease with time: many systems, including living systems, reduce local entropy at the expense of an environmental increase, resulting in a net increase in entropy. Examples of such systems and phenomena include the formation of typical crystals, the workings of a refrigerator etc. The law is therefore more a statistical principle than a fundamental law. But the upshot is that, provided the initial condition of a system is one of relatively high order, then the tendency will almost always be towards disorder. The concept of entropy and the decay of ordered systems was explored and clarified by the German physicist Ludwig Boltzmann in the 1870s, building on earlier ideas of Rudolf Clausius, but it remains a difficult and often misunderstood idea. Entropy can be thought of, in most cases, as meaning that things (matter, energy, etc) have a tendency to disperse. Thus, a hot object always dissipates heat to the atmosphere and cools down, and not vice versa; coffee and milk mix together, but do not then separate; a house left unattended will eventually crumble away, but a pile of bricks never spontaneously forms itself into a house; etc. However, it is not quite as simple as that, and a better way of thinking of it may be as a tendency towards randomness. Entropy increases as available energy decreases. In other words, the purely natural tendency of things is to move toward chaos not order; and available energy necessary for work is lost (mostly as heat) in this process. Eventually, the universe will run down, and all life and motion will cease because all the energy that exists will be, more or less, evenly distributed so that no work can be performed and no life can exist. This is the natural tendency of all things. Batteries run down, machines break, buildings crumble, roads decay, living things die, etc. Left to the natural state, all things would eventually cease to function. The universe is not infinitely old because it has not “run down.” If the universe were infinitely old, it would have reached a state where all usable energy was gone. But, we are not in this state; therefore, the universe is not infinitely old and must have had a beginning. It is not infinite in size as it would require an infinite amount of time to become infinite in size. Since the universe had a beginning, it has not had an infinite amount of time to expand; therefore, it is finite in size.
Thinkers like Stephen Hawking , have pinned the direction of the arrow of time on what is sometimes called the weak anthropic principle, the idea that the laws of physics are as they are solely because those are the laws that allow the development of sentient, questioning beings like ourselves. It is not that the universe is in some way “designed” to allow human beings, merely that we only find ourselves in such a universe because it is as it is, even though the universe could easily have developed in a quite different way with quite different laws.
Thus, Hawking argues, a strong thermodynamic arrow of time is a necessary condition for intelligent life as we know it to develop. For example, beings like us need to consume food (a relatively ordered form of energy) and convert it into heat (a relatively disordered form of energy), for which a thermodynamic arrow like the one we see around us is necessary. If the universe were any other way, we would not be here to observe it.
Qudsia Gani is Assistant Professor, Department of Physics, Cluster University Srinagar