The Illusion of Time

This is a very good article. Reality is not what you think it is.

That Rovelli's approach yields the correct probabilities in quantum mechanics seems to justify his intuition that the dynamics of the universe can be described as a network of correlations, rather than as an evolution in time. "Rovelli's work makes the timeless view more believable and more in line with standard physics," says Dean Rickles, a philosopher of physics at the University of Sydney in Australia.

With quantum mechanics rewritten in time-free form, combining it with general relativity seems less daunting, and a universe in which time is fundamental seems less likely. But if time doesn't exist, why do we experience it so relentlessly? Is it all an illusion?

Yes, says Rovelli, but there is a physical explanation for it. For more than a decade, he has been working with mathematician Alain Connes at the College de France in Paris to understand how a time-free reality could give rise to the appearance of time. Their idea, called the thermal time hypothesis, suggests that time emerges as a statistical effect, in the same way that temperature emerges from averaging the behaviour of large groups of molecules (Classical and Quantum Gravity, vol 11, p 2899).

Imagine gas in a box. In principle we could keep track of the position and momentum of each molecule at every instant and have total knowledge of the microscopic state of our surroundings. In this scenario, no such thing as temperature exists; instead we have an ever-changing arrangement of molecules. Keeping track of all that information is not feasible in practice, but we can average the microscopic behaviour to derive a macroscopic description. We condense all the information about the momenta of the molecules into a single measure, an average that we call temperature.

According to Connes and Rovelli, the same applies to the universe at large. There are many more constituents to keep track of: not only do we have particles of matter to deal with, we also have space itself and therefore gravity. When we average over this vast microscopic arrangement, the macroscopic feature that emerges is not temperature, but time. "It is not reality that has a time flow, it is our very approximate knowledge of reality that has a time flow," says Rovelli. "Time is the effect of our ignorance."