The concept of time itself has evolved over the centuries. To Newton, time was like an arrow; once fired, it never changed course and traveled unerringly and uniformly to its target. Einstein then introduced the concept of warped space, so time was more like a river that gently speeded up or slowed down as it meandered through the universe. But Einstein worried about the possibility that perhaps the river of time can bend back on itself. Perhaps there could be whirlpools or forks in the river of time.
