Let's imagine that you are watching a planet in its orbit. Let's further imagine that you can speed up time, to watch the planet in a sort of "fast forward". In theory, you wouldn't be able to know, from watching the movement of the planet, whether the time is running forward or backward. This is because, regardless of the direction in which time is moving, physics follows the same laws. This property is known as time symmetry. Time symmetry describes how the laws of physics work the same way whether the time is moving forward or backward.
In reality, however, time symmetry cannot be broken in order to turn back time. For example, a broken cup cannot suddenly reassemble itself. Until now, scientists explained this by pointing to the large numbers of particles involved. The cup has two many particles for them to be able to interact in exact reverse order.
In physics, this inability to turn back time emerges from the study of n-body problems. These involve the problem of predicting the individual motions of a number (n) of celestial objects which are interacting with each other gravitationally.
An example of this can be seen in our solar system. If we wanted to map out the movement of Earth's Moon with respect to the Sun, we would need to take into account the relative positions of the Sun, the Moon, and the Earth, as well as the gravitational forces they cause on each other. Because we are taking into account three objects, this would be called a 3-body problem.
It turns out that if there are more than two bodies, the problem cannot be solved precisely - the movements of more than two bodies cannot be predicted with 100 percent accuracy.
Additionally, when scientists run n-body simulations backwards, they do not get back to their starting point. Until now, they did not know if this inability was resulting from the chaotic nature of these systems, or if the simulations themselves were unreliable.
Now, however, three astronomers may have shown that it is, in fact, impossible to turn back time, and that it only takes three particles to break time symmetry. A team led by astronomer Tjarda Boekholt, from the University of Aveiro, has shown that time symmetry can be broken by three gravitationally interacting bodies.
The astronomers were able to calculate how the orbits of three black holes would influence each other, using two simulations.
In each simulation, the researchers shifted the initial positions of the black holes to see how it would affect their motion over time.
In the first simulation, the black holes start from a resting position and move towards and past each other in complex orbits, before one black hole finally leaves the company of the two others. The second simulation begins where the first one ends, and attempts to turn back the time to the starting position.
The researchers found that 5% of the time, the simulation could not be reversed. All it took for this to happen was a disturbance to the system the size of a Planck length. This is the smallest possible unit of length, measuring 1.6 x 10^-35 m.
Boekholt explained 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 their movements. The disturbances the size of the Planck length have an exponential effect and break the time symmetry."
This is an important breakthrough, since it shows that simulations are not at fault for the inability to solve the n-body problem or to turn back time. 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".
In other words, according to study co-author Portegies Zwart, "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.