Unlike space, where an object can travel in the opposite directions (and in 3 dimensions), time appears to have only one dimension and only one direction – the past lies behind, fixed and immutable, while the future lies ahead and is not necessarily fixed. Yet most laws of physics allow any process to proceed both forward and in reverse. There are only a few physical phenomena, that violate the reversibility of time. This time directionality is known as the arrow of time. Acknowledged examples of the arrow of time are: