The arrow of time is not the same thing as time itself, but a feature of the universe and its contents and the way it has evolved. The perception of an arrow of time that we have in our everyday life therefore appears to be nothing more than an illusion of consciousness in this model of the universe, an emergent quality that we happen to experience due to our particular kind of existence at this particular point in the evolution of the universe. Perhaps even more interesting and puzzling is the fact that, although events and processes at the macroscopic level — the behaviour of bulk materials that we experience in everyday life — are quite clearly time-asymmetric i.
If a physical process is physically possible, then generally speaking so is the same process run backwards, so that, if you were to hypothetically watch a movie of a physical process, you would not be able to tell if it is being played forwards or backwards, as both would be equally plausible. In theory, therefore, most of the laws of physics do not necessarily specify an arrow of time.
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There is, however, an important exception: the Second Law of Thermodynamics. Most of the observed temporal asymmetry at the macroscopic level — the reason we see time as having a forward direction — ultimately comes down to thermodynamics , the science of heat and its relation with mechanical energy or work, and more specifically to the Second Law of Thermodynamics. This law states that, as one goes forward in time, the net entropy degree of disorder of any isolated or closed system will always increase or at least stay the same.
The concept of entropy and the decay of ordered systems was explored and clarified by the German physicist Ludwig Boltzmann in the s, 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.
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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, as discussed below, it is not quite as simple as that, and a better way of thinking of it may be as a tendency towards randomness. It should be noted that, in thermodynamic systems that are NOT closed, it is quite possible that entropy can decrease with time e.
Any localized or temporary instances of order within the universe are therefore in the nature of epiphenomena within the overall picture of a universe progressing inexorably towards disorder.
Physical Origins Of Time Asymmetry
It is also perhaps counter-intuitive, but nevertheless true, that overall entropy actually increases even as large-scale structure forms in the universe e. Likewise, the relatively smooth configuration of the very early universe see the section on Time and the Big Bang is actually an indication of very low overall entropy i.
Most of the processes that appear to us to be irreversible in time are those that start out, for whatever reason, in some very special, highly-ordered state. For example, a new deck of cards are in number order, but as soon as we shuffle them they become disordered; an egg is a much more ordered state than a broken or scrambled egg; etc. 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. So, 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.
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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. Thermodynamics, then, appears to be one of the only physical processes that is NOT time-symmetric, and so fundamental and ubiquitous is it in our universe that it may be single-handedly responsible for our perception of time as having a direction. Indeed, several of the other arrows of time noted below arguably ultimately come back to the asymmetry of thermodynamics.
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It became apparent towards the beginning of the 20 th Century, thanks to the work of Edwin Hubble and others, that space is indeed expanding, and the galaxies are moving ever further apart. Logically, therefore, at a much earlier time, the universe was much smaller, and ultimately concentrated in a single point or singularity , which we call the Big Bang.
Thus, the universe does seem to have some intrinsic outward directionality. In our everyday lives, however, we are not physically conscious of this movement, and it is difficult to see how we can perceive the expansion of the universe as an arrow of time.
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This accords with the Second Law of Thermodynamics in that the overall direction is from the current semi-ordered state, marked by outcroppings of order and structure, towards a completely disordered state of thermal equilibrium. What remains a major unknown in modern physics, though, is exactly why the universe had a very low entropy at its origin, the Big Bang. It is also possible — although less likely according to the predictions of current physics — that the present expansion phase of the universe could eventually slow, stop, and then reverse itself under gravity.
As the universe contracts and collapses, entropy will in theory start to reduce and, presumably, the arrow of time will reverse itself and time will effectively begin to run backwards. In this scenario, then, the arrow of time that we experience is merely a function of our current place in the evolution of the universe and, at some other time, it could conceivably change its direction. 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. It is possible that the radiative arrow may also be linked to the thermodynamic arrow, because radiation suggests increased entropy while convergence suggests increased order.
In this paper, we address the problem of the arrow of time from a cosmological point of view, rejecting the traditional entropic approach that defines the future direction of time as the direction of the entropy increase: from our perspective, the arrow of time has a global origin and it is an intrinsic, geometrical feature of spacetime.
Physical Origins of Time Asymmetry
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