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 mostphysicists who view it as a dimension rather than linear path. According to thetheory of Relativity, the reality of the universe can be described byfour-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 lifetherefore is nothing more than an illusion of consciousness. The events andprocesses at the macroscopic level are quite clearly time-asymmetric (i.e.natural processes do have a natural temporal order, and there is an obviousforward direction of time), but the physical processes and laws at themicroscopic level, are entirely or mostly time-symmetric. If a process isphysically possible, then generally speaking, so is the same process runningbackwards, so that if we were to hypothetically watch a movie of a physicalprocess, one would not be able to tell if it is being played forwards orbackwards, as both would be equally plausible. Watching a single smoke particlebuffeted by air, it would not be clear if a video was playing forwards or inreverse; as it drifts left or right qualitatively it looks no different. It isonly when we study that gas at a macroscopic scale that the effects of entropybecome noticeable.
In theory, therefore, most of the laws of physics do notnecessarily specify an arrow of time. Waves always radiate away from a sourceWaves, like light, radio waves, sound waves, water waves, etc, are alwaysradiative and expand outwards from their sources. While theoretical equationsdo allow for the opposite (covergent) waves, this is apparently never seen innature. This asymmetry is regarded by some as a reason for the asymmetry oftime.
The apparent asymmetry of time is really just an asymmetryof chance – things evolve from order to disorder not because the reverse isimpossible, but because it is highly unlikely. There is nothing in the laws ofphysics that prevents the act of shuffling a deck of cards from producing aperfectly ordered set of cards – there is always a chance of that, it is just avanishingly small chance. To give another example, there are many more possibledisordered arrangements of a jigsaw than the one ordered arrangement that makesa complete picture. Entropy is therefore the only quantity in the physicalsciences (apart from certain rare interactions in particle physics) thatrequires a particular direction for time. As one goes “forward” intime, the second law of thermodynamics says, the entropy of an isolated systemcan increase, but not decrease. Hence, from one perspective, entropymeasurement is a way of distinguishing the past from the future. However, inthermodynamic systems that are not closed, entropy can decrease with time: manysystems, including living systems, reduce local entropy at the expense of anenvironmental increase, resulting in a net increase in entropy. Examples ofsuch systems and phenomena include the formation of typical crystals, theworkings of a refrigerator etc. The law is therefore more a statistical principlethan a fundamental law. But the upshot is that, provided the initial conditionof a system is one of relatively high order, then the tendency will almostalways be towards disorder. The concept of entropy and the decay of orderedsystems was explored and clarified by the German physicist Ludwig Boltzmann inthe 1870s, building on earlier ideas of Rudolf Clausius, but it remains adifficult and often misunderstood idea. Entropy can be thought of, in mostcases, as meaning that things (matter, energy, etc) have a tendency todisperse. Thus, a hot object always dissipates heat to the atmosphere and coolsdown, and not vice versa; coffee and milk mix together, but do not thenseparate; a house left unattended will eventually crumble away, but a pile ofbricks never spontaneously forms itself into a house; etc. However, it is notquite as simple as that, and a better way of thinking of it may be as atendency towards randomness. Entropy increases as available energydecreases. In other words, the purelynatural tendency of things is to move toward chaos not order; and availableenergy necessary for work is lost (mostly as heat) in this process. Eventually, the universe will run down, andall life and motion will cease because all the energy that exists will be, moreor less, evenly distributed so that no work can be performed and no life canexist. This is the natural tendency ofall things. Batteries run down, machinesbreak, buildings crumble, roads decay, living things die, etc. Left to the natural state, all things wouldeventually cease to function. The universe is not infinitely old because it hasnot “run down.” If the universe were infinitely old, it would havereached 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. Itis not infinite in size as it would require an infinite amount of time tobecome infinite in size. Since theuniverse 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 ofthe arrow of time on what is sometimes called the weak anthropic principle, theidea that the laws of physics are as they are solely because those are the lawsthat allow the development of sentient, questioning beings like ourselves. Itis 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 waywith quite different laws.
Thus, Hawking argues, a strong thermodynamic arrow of timeis a necessary condition for intelligent life as we know it to develop. Forexample, beings like us need to consume food (a relatively ordered form ofenergy) and convert it into heat (a relatively disordered form of energy), forwhich a thermodynamic arrow like the one we see around us is necessary. If theuniverse were any other way, we would not be here to observe it.
Qudsia Gani is Assistant Professor, Department of Physics,Cluster University Srinagar