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If there鈥檚 one thing that eats up time, it鈥檚 working out what time is. It pops up in physical laws all over the place 鈥� but never quite as we expect it. In quantum theory, a 鈥渕aster clock鈥� ticks away somewhere in the universe, measuring out all processes. But in Einstein鈥檚 relativity, time is distorted by motion and gravity, so clocks don鈥檛 necessarily agree on how it is passing 鈥� meaning any master clock must, somewhat implausibly, be outside the universe.

Even odder, neither theory seems to place any restriction on time going backwards. The familiar one-way flow of time is expressed in only one area of physics: thermodynamics. If time flowed both ways, sometimes your coffee would warm up while sitting forgotten on your desk. Dropped eggs might spontaneously reassemble and leap from the floor into your hand. The dead might return to life and live backwards to birth, Benjamin Button style.

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The culprit is entropy, essentially the thermodynamic measure of a system鈥檚 disorder. When the universe was born, matter was randomly distributed throughout its tiny dimensions and it was the same temperature everywhere. Then gravity kicked in, pulling together matter and heated it up to form galaxies, stars, planets and other ordered imperfections. Thermodynamics has been trying to re-establish disorder, increasing entropy every which way it can.

At a local level, entropy increase seems to be associated with information loss. Broken eggs do not reassemble because information about the former ordering is lost to us in the smash. You don鈥檛 have all the information needed to put Humpty Dumpty together again 鈥� and that amounts to a barrier to travelling back in time.

Or does it? 鈥淲hen the story is told like this it appears compelling, but the moment you start looking into more detail, it becomes more convoluted,鈥� says Sandu Popescu of the University of Bristol, UK. In classical physics, you could in principle reverse a thermodynamic process if you preserved the information by measuring the trajectories and velocities of all the components of a breaking egg 鈥� suggesting that we could reverse time.

So why can鈥檛 we? One possibility, Popescu thinks, is an information gap intrinsic to the way the quantum world works. Here we are back with the phenomenon of entanglement (see 鈥�Cosmic uncertainty: Could quantum weirdness be even weirder?鈥�). When a cup of coffee cools, Popescu thinks, continual interactions between molecules of air and coffee increase their quantum entanglement. Although you can know what states an entangled particle pair contains, you can鈥檛 definitively know which one has which state 鈥� .

鈥淣o theory of nature appears to place any restriction on time going backwards鈥�

It鈥檚 still just an idea, Popescu admits. 鈥淨uantum mechanics is consistent with our macroscopic phenomenon being driven by quantum rules, but we cannot prove it,鈥� he says. And there is a huge sting in the tail: if he鈥檚 right, in some sense, time may be capable of flowing backwards after all.

Evolution in reverse

That鈥檚 because in a classical physics calculation, in theory all you need is a system鈥檚 initial state and the laws of mechanics to work out what will happen for the rest of time. But in quantum mechanics, where a system鈥檚 evolution is probabilistic, you can specify conditions for the initial state and final states of the system, and both of these conditions will influence the evolution. Apply this idea to the universe as a whole and 鈥渋nformation could be coming from plus infinity and propagating back through time鈥�, says Popescu.

There鈥檚 no evidence of any of this so far, Popescu cheerfully admits. 鈥淣o one yet has investigated it seriously,鈥� he says. 鈥淚t is speculative.鈥� But if in the future physics shows that time really can travel backwards, well 鈥� in some sense we must already know.

This article appeared in print under the headline 鈥淲hat if鈥� time went both ways?鈥�