The term "Arrow of Time" was first coined by Arthur Eddington in 1927. It is used to describe the "one way direction" in which we perceive the passage of time. Time is a rather curious dimension in that we only see it increasing in one direction. The laws of physics at the microscopic level stay the same under something called the "T-Symmetry". T-Symmetry states that the laws of physics stay invariant under the time reversal transformation. A notable exception to this is the second law of thermodynamics. Entropy of the universe increases with passage of time. Or in other words we perceive time in the direction of increase of entropy. Since the beginning of universe started at big bang the entropy of universe has been increasing. The early universe had a low level of entropy as all the mass was concentrated at one point. With the expansion of universe this mass gets distributed across the space. The following figure shows a graph plot of Entropy v/s Time.
Thus we see that the arrow of time points to a state of increasing entropy (or expanding space). The direction of the arrow in a way gives a default form of Time Travel. We are all traveling in the future at all times. Even though the universe had a single big bang singularity in the past, we have several black hole singularities in our observable universe. We know now that information loss can occur when a black hole evaporates. As such the entropy at the edge of a black hole may actually increase. What does it mean for the direction of Time ? We will explore some of these ideas in the next post on possibilities for Time Travel.
For now we update our previous picture of the pear shape of time with a direction as follows.
Not only does Time have a space but also it has a direction. Possibilities of Time Travel should take into account our physical understanding of Time. The shape and arrow of Time limit the forms of Time Travel we may imagine.