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.

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