Time

Time is used to measure a progress of events that occur from the past, through the present, into the future. Time is the fourth dimension, along with three spatial dimensions. It is an essential component of time travel.

Newton thought that time was like an arrow, and was consistent throughout the universe. Einstein correctly said that time is like a river that can speed up and slow down, and time beats faster on the moon than it does on the Earth. Michio Kaku extends this by saying that the river of time can fork into two, and this can resolve time travel paradoxes where you would have multiple instances in a multiverse.