麻豆传媒

IN 1516, Mark Anthony Zimara hit upon the ultimate idea in renewable energy. Instead of merely using windmills to generate energy, he suggested employing them to power bellows to blow air鈥� back at the windmills! The self-blowing windmill, he thought, would run forever.

Zimara鈥檚 machine failed, of course. The laws of thermodynamics put the kibosh on . No matter how cleverly designed, they will eventually grind to a halt without ever producing the desired free work.

This is due to an all-pervasive macroscopic temporal asymmetry: , a measure inversely related to the energy available for work, increases with time. That is why ice melts and gases expand, and never the reverse. Arguably it is also the reason why you can know yesterday鈥檚 stock market results but not tomorrow鈥檚.

Where does this arrow of time come from? After all, macroscopic objects are composed of microscopic particles, and the laws governing these are allegedly time symmetric. The particles can execute anti-thermodynamic behaviour just as easily as thermodynamic behaviour, so how do those comprising air, windmills and the rest 鈥渒now鈥� to execute a trajectory of increasing entropy? It seems downright conspiratorial.

鈥榮 goal in From Eternity to Here is to explain this puzzle. While it covers much of the same ground as Brian Greene鈥檚 The Fabric of the Cosmos (curved space-time, black holes) and is aimed at the same audience, Carroll鈥檚 book is distinctive in two ways: it devotes more attention to the mystery of the direction of time and it offers a speculative solution. The middle of the book is perhaps the best and most comprehensive discussion of time鈥檚 arrow that is widely accessible. Carroll explains time鈥檚 fascinating subtleties in a lucid and entertaining manner.

The standard explanation for why entropy increases is that high entropy microscopic configurations are far more likely than low entropy ones. There are vastly more ways for gas molecules to be spread out in a room than to be condensed in the corner, for example, just as there are many more poker hands with only one pair than two pairs.

The trouble is this explanation works in both directions of time: entropy is highly likely to increase towards the future and the past 鈥� the latter contrary to thermodynamics. The mystery, then, is not why entropy increases with time, but why it was lower in the past. The accepted solution is to simply say that the universe just happened to begin in a state of very low entropy. It seems innocuous enough: the low entropy posit accords with cosmological observations and is a simple add-on to the laws of physics.

鈥淭he mystery is not why entropy increases with time, but why it was lower in the early universe鈥�

Carroll, however, is unsatisfied. He finds the low entropy posit 鈥渦nnatural鈥� 鈥� after all, low entropy states are highly unlikely 鈥� and heads off in search of an explanation. Daring to speculate in the absence of well-confirmed theory, Carroll jumps from clue to clue, from black hole physics to string theory to the holographic principle, until he arrives at his destination: an eternal 鈥渕other space-time鈥� from which a multiverse of baby universes are continually bubbling up and pinching off. The mother space-time is a high entropy vacuum that gives birth to universes like our own, some of which we can expect to begin with low entropy. Problem solved, says Carroll, because that is natural.

Carroll seems slightly embarrassed by the many leaps of faith he asks of his reader in proposing this solution, and the prose of Part IV sometimes reads like the pitch of an honest used-car salesman: 鈥淭his car is a dream! True, the tyres are bald, brakes unsound and transmission sticky, but you鈥檒l love it!鈥�

Carroll and other peddlers of multiverses make us an offer: we will explain the unexplained if you add vast unconfirmable matters of fact into your ontology. In this case that includes a host of disconnected baby universes, an eternal mother universe entirely unlike ours, and half a dozen unknown mechanisms to get all this working. Assuming this explains the low entropy past 鈥� and with so much unknown it is hard to be sure another conspiracy isn鈥檛 lurking within 鈥� is this a good deal?

In most cases I don鈥檛 think so. Why is Manchester United perennially a good soccer team? Surely most solutions of the laws of physics don鈥檛 have them winning so much. How unnatural (and unfair) those initial conditions are! Nonetheless, a frothy sea of baby universes tempts no one. We shrug and say, that鈥檚 just the way it is. Sometimes it is best not to scratch explanatory itches.