[ Pobierz całość w formacie PDF ]
.However, in order to make sure that the memory is in the right state, it isnecessary to use a certain amount of energy (to move the bead or to power the computer, for example).Thisenergy is dissipated as heat, and increases the amount of disorder in the universe.One can show that thisincrease in disorder is always greater than the increase in the order of the memory itself.Thus the heatexpelled by the computer s cooling fan means that when a computer records an item in memory, the totalamount of disorder in the universe still goes up.The direction of time in which a computer remembers the pastis the same as that in which disorder increases.Our subjective sense of the direction of time, the psychological arrow of time, is therefore determined within ourbrain by the thermodynamic arrow of time.Just like a computer, we must remember things in the order in whichentropy increases.This makes the second law of thermodynamics almost trivial.Disorder increases with timefile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/h.html (2 of 5) [2/20/2001 3:15:38 AM] A Brief History of Time - Stephen Hawking.Chapter 9because we measure time in the direction in which disorder increases You can t have a safer bet than that!But why should the thermodynamic arrow of time exist at all? Or, in other words, why should the universe be ina state of high order at one end of time, the end that we call the past? Why is it not in a state of completedisorder at all times? After all, this might seem more probable.And why is the direction of time in whichdisorder increases the same as that in which the universe expands?In the classical theory of general relativity one cannot predict how the universe would have begun because allthe known laws of science would have broken down at the big bang singularity.The universe could havestarted out in a very smooth and ordered state.This would have led to well-defined thermodynamic andcosmological arrows of time, as we observe.But it could equally well have started out in a very lumpy anddisordered state.In that case, the universe would already be in a state of complete disorder, so disorder couldnot increase with time.It would either stay constant, in which case there would be no well-definedthermodynamic arrow of time, or it would decrease, in which case the thermodynamic arrow of time would pointin the opposite direction to the cosmological arrow.Neither of these possibilities agrees with what we observe.However, as we have seen, classical general relativity predicts its own downfall.When the curvature ofspace-time becomes large, quantum gravitational effects will become important and the classical theory willcease to be a good description of the universe.One has to use a quantum theory of gravity to understand howthe universe began.In a quantum theory of gravity, as we saw in the last chapter, in order to specify the state of the universe onewould still have to say how the possible histories of the universe would behave at the boundary of space-time inthe past.One could avoid this difficulty of having to describe what we do not and cannot know only if thehistories satisfy the no boundary condition: they are finite in extent but have no boundaries, edges, orsingularities.In that case, the beginning of time would be a regular, smooth point of space-time and theuniverse would have begun its expansion in a very smooth and ordered state.It could not have beencompletely uniform, because that would violate the uncertainty principle of quantum theory.There had to besmall fluctuations in the density and velocities of particles.The no boundary condition, however, implied thatthese fluctuations were as small as they could be, consistent with the uncertainty principle.The universe would have started off with a period of exponential or inflationary expansion in which it wouldhave increased its size by a very large factor.During this expansion, the density fluctuations would haveremained small at first, but later would have started to grow.Regions in which the density was slightly higherthan average would have had their expansion slowed down by the gravitational attraction of the extra mass.Eventually, such regions would stop expanding and collapse to form galaxies, stars, and beings like us.Theuniverse would have started in a smooth and ordered state, and would become lumpy and disordered as timewent on.This would explain the existence of the thermodynamic arrow of time.But what would happen if and when the universe stopped expanding and began to contract? Would thethermodynamic arrow reverse and disorder begin to decrease with time? This would lead to all sorts ofscience-fiction-like possibilities for people who survived from the expanding to the contracting phase
[ Pobierz całość w formacie PDF ]