Time has a precise direction. We know the past is past and the future is yet to come. But for many laws of physics, this division is arbitrary. Several laws that have to do with motion, for example, are time-symmetric. If you were to look at footage of an egg rolling across a table, you could easily mistake it for the same video when played in reverse. But if you saw an egg fall off a table and shatter, the reverse scenario of it forming together and jumping up on the table would look wrong.
In statistical mechanics, scientists have generally considered the interaction between many particles to be necessary for time symmetry to break. Now in the Monthly Notices of the Royal Astronomical Society, researchers show that just three particles can do that.
The approach used two simulations with three black holes that influenced each other. The first began with the three black holes at rest, then followed their complex orbits until one of the black holes escaped. The second tried to turn back time on the first simulation, which it could not do in 5 percent of the cases.
The team found that even when using a simulation with 100 decimal places of accuracy, one out of 20 simulations could not be recreated backward. In this small fraction of the reconstructed simulations, huge differences developed over time, even when the original positions were only sightly off. Some of the differences between the original and recreated scenarios were so tiny that they were smaller than the smallest distance physicists believe we can measure (the so-called Planck length). And yet this minuscule difference grew to be many light years by the end of it!
The Planck length is about 1.6 x 10-35 meters – so tiny that a human hair is about 5,000,000,000,000,000,000,000,000,000,000 Planck lengths in thickness. And yet even deviations smaller than that had huge consequences in the simulation.
"The movement of the three black holes can be so enormously chaotic that something as small as the Planck length will influence the movements. The disturbances the size of the Planck length have an exponential effect and break the time symmetry," lead author Tjarda Boekholt, from the University of Coimbra, said in a statement.
"So not being able to turn back time is no longer just a statistical argument. It is already hidden in the basic laws of nature. Not a single system of three moving objects, big or small, planets or black holes, can escape the direction of time," the paper’s co-author Simon Portegies Zwart from Leiden University added.