BY THE time you have finished reading this, you will be a couple of minutes older. Hopefully you won鈥檛 regret those minutes, because you can鈥檛 get them back. Time, as we all know, only moves in one direction for us. The question of why, however, doesn鈥檛 come with a simple answer.
Searching for time鈥檚 arrow in the underlying, microscopic laws of physics certainly draws a blank. They give us no reason to think atoms, molecules and so on can鈥檛 move backwards as well as forwards in time, much as they (and we) move freely in three dimensions of space. The laws don鈥檛 differentiate between past, present and future, or between cause and effect.
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鈥淭hat distinction only becomes relevant in the macroscopic world, where our incomplete information about the precise physical configuration of a system leads us to perceive an arrow of time, and to put causes first and effects after,鈥� says physicist .
At issue here is the concept of entropy. This is a measure of the amount of disorder in a system, defined as the number of different microscopic configurations it can have without changing its macroscopic, or overall, appearance. A box of hot gas has high entropy, for example, with a vast number of equivalent configurations with different positions and velocities for each atom or molecule. A human, though, has low entropy 鈥� try to reconfigure us too much and things rapidly start to fall apart.
The crucial point is that there are more ways for any system of particles to have high entropy than low entropy, so things naturally tend towards high-entropy states. Where entropy is high, we lose any chance of knowing a system鈥檚 precise physical configuration, because there are so many equivalent options. If we could gain all that information, time would still exist, in the sense that the system could still evolve. But time鈥檚 arrow would disappear, because any 鈥渇orwards鈥� evolution would run equally plausibly 鈥渂ackwards鈥� from the configuration in which we see it. 鈥淭he increase of entropy is responsible for all of our impressions that the past is different from the future, including the impression that time flows,鈥� says Carroll 鈥� and our ignorance lies at its heart.
What is true in classical physics is doubly so in the quantum world, says , at the Institute for Quantum Optics and Quantum Information in Vienna, Austria. Here, uncertainties that, in the classical world, we put down to a lack of knowledge seem to represent uncertainties intrinsic to reality, although that interpretation is itself up for debate (see 鈥�Why is quantum physics so strange?鈥�). 鈥淗ere, the state is truly undetermined prior to a measurement, so the lack of knowledge 鈥� and rise of entropy 鈥� is even more fundamental,鈥� says Huber.
For some, arguments equating time鈥檚 passage with rising entropy are pass茅, anyway. , Canada, for one, believes that the problem of time鈥檚 arrow indicates a basic gap in our understanding. Others think the gradual revelation of classical certainties through quantum measurements is the true source of time鈥檚 arrow.
Even if entropy is the answer, that just shifts the question. For entropy always to rise, the universe must have started with an astoundingly low amount. Why so? That鈥檚 a question we have hardly begun to tackle.
