Let's say that you're watching a planet orbit. You can't know whether it is running forward or backward since regardless of your direction of time, physics follows the same laws. This property of laws of physics is known as time symmetry.
However, that may not be the case in our "real" universe. A team of scientists led by astronomer Tjarda Boekholt from the University of Aveiro has shown that it takes three gravitationally interacting bodies to break time symmetry.
They studied chaotic three-body systems in free fall, and by doing so, they were able to calculate the orbits of three black holes that were influenced by each other. This experiment was done in two simulations.
The simulations were computer models of black holes in a gravitational dance. The researchers would shift the initial positions of the black holes to see how it would affect their motions over time.
This demonstrates a chaotic system that is a perfect example of the "n-body problem". It arises when we add more bodies to a gravitationally interacting system. We can't exactly solve these problems, which is why we can only predict a few million years into the future even in our home, the Solar System.
The thing is, when scientists run n-body simulations, they run the simulations backwards, but this doesn't get them to their starting point. Until now, we didn't know if this occurrence was resulting from the chaos of these systems, or if the simulations were unreliable.
In order to learn the reason behind this, the researchers designed a test.
In the first simulation, the black holes started from a resting position. Then, they would move towards each other and past each other in complicated orbits, in a dance of gravitational forces. Finally, one black hole would leave the company of the two others.
The second simulation starts with the end-position of two black holes and the one that got away. In this position, the simulation tries to turn back the time to the initial situation.
They found that 5% of the time, the simulation could not be reversed. All it took was a disturbance to the system the size of a Planck length, which is the smallest length possible [1.6 x 10^-35 m].
The lead researcher Boekholt explains the findings by saying, "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."
This is such a breakthrough since it shows that simulations are not at fault. It appears that we can never predict which part of the simulations will fall within that 5%, and the conclusion is that n-body systems are "fundamentally unpredictable".
Moreover, co-author Simon Portegies Zwart adds: "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."
Their findings are published in the journal The Monthly Notices of the Royal Astronomical Society.