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.

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