Computing in a backwards world

Thermodynamic entropy can be a tricky area even for physicists. Suppose you fell through a wormhole into a part of the universe where everything went backwards. Broken eggs would put themselves together again, if you had a beer you would feel drunk beforehand and thirsty afterwards.

But Stephen Hawking theorised you wouldn't be able to tell this had happened, because you'd start seeing and remembering things backwards too - your past would become your future. Now, Dr Owen Maroney, a postdoctoral research fellow in Foundations of Physics at The University of Sydney, says this is not correct: we can remember in a reversed universe and it's all to do with the heat coming from your computer.

Already working on the problem of heat generated by computers, Maroney went to a lecture at Imperial College, London, on computers, the direction of time, and their relation to our memories and perceptions, when he started wondering what would happen if you calculated the heat given off by a computer in the reversed universe. "People had assumed that it wouldn't change things," he said, "but I realised that's all it was - an assumption."

If you break an egg, entropy goes up. You can't put the egg back together, because physics says entropy can't decrease. The theory is that in the reversed universe entropy has to go down, so broken eggs can get put back together again.

“Computers get hot. That increases entropy as well. So Hawking had said that this means computers can't work in the reversed universe. They'd have to start running backwards. The brain is a kind of computer, so you'd go into reverse too and still end up seeing the egg breaking apart."

But until now, no-one had ever done the calculation. Maroney worked it out and found that it works in the opposite way to that expected. “In the reversed universe, computers will absorb heat instead. They'll get cold. That makes entropy go down, so they can work after all. And that means so can you,” explains Maroney.

So Humpty Dumpty might have a chance after all. But there may be a problem: all this has only been worked out for classical computers. With a quantum computer things may be different, and Maroney is still working on the details.

“It is interesting, given my research could have an impact on quantum computing, that it’s mainly quantum physicists who argue with my findings,” he says with a bemused look.

Maroney's paper Does a Computer Have an Arrow of Time? is to be published in the February 2010 issue of Foundations of Physics. Maroney works in the areas of physics, philosophy and time at the University of Sydney.