This is taken from Appendix 4 of my new book, How to Time Travel, to be published by early September 2013.

Let us examine the three major philosophical schools on the nature of time and their implications regarding time travel.

1. Presentism theory of time

The presentism theory of time holds that only the present is real. The past is over. Therefore, it is no longer real. The future has yet to occur. Therefore, the future is not real. Presentists argue that our mind remembers a past and anticipates a future, but neither is real. They are mental constructs.

Arguably, the most famous presentist is Augustine of Hippo (a.k.a. St. Augustine), who compared time to a knife edge. The present represents a knife edge cutting between the past and future. Ironically, this means Augustine of Hippo is not real, since he lived and died in the past. Therefore, should we study Augustine of Hippo, who, by his own philosophy, is not real? Of course, I am only being contentious to make a point.

Presentism has a large following, especially among Buddhists. Fyodor Shcherbatskoy (1866–1942), often referred to as the foremost Western authority on Buddhist philosophy, summed up the Buddhist view of presentism with these few words: “Everything past is unreal, everything future is unreal, everything imagined, absent, mental…is unreal…Ultimately real is only the present moment of physical efficiency.” Uncountable millions of Buddhists still ascribe to this view of time and reality.

A cogent philosophical argument can be made for presentism, but presentism is problematic from a scientific viewpoint. When we discussed the special theory of relativity, we learned that the present is a function of the position and speed of the observer. Therefore, what is the present to one observer may be the past to another.

From the standpoint of time travel, presentism renders the question “how to time travel” moot. If we embrace presentism, there is no past or future, and time travel is meaningless. Fortunately, though, other philosophies of time open the door to time travel. Let us examine the next one.

2. Growing universe theory of time

This theory of time is also referred to as “growing block universe” and “the growing block view.” However, regardless of the name, they all hold the same philosophy of time. The past is real, and the present is real. The future is not real. The logic goes something like this: The past is real because it actually happened. We experience it, and we document it. We call it history. The present is real because we experience it. We often share the present with others. The future is not real because it has not occurred.

Why do all the names for this theory of time start with the word “growing”? The concept is that the passage of time continually expands the history of the universe. Actually, this is logical. The history of the world, and the universe, continues to expand with the passage of time. The history section of any library is destined to grow with time.

In this philosophy of time, only time travel to the past makes sense, since for growing-universe philosophers, the past is real. We cannot time travel to the future, since the future has yet to occur. Therefore, it is not real.

As logical as this theory of time may appear, there is scientific evidence that the future is real and can influence the present. We discussed this evidence in the section titled “Twisting the arrow of time” in chapter 1. Now, let us examine the last significant philosophy of time.

3. Eternalism theory of time

The eternalism theory of time holds that the past, present, and future are real. The philosophy of this theory rests on Einstein’s special theory of relativity. Essentially, the special theory of relativity holds that the past, present, and future are functions of the speed and position of an observer.

While Einstein never equated time with the fourth dimension, Minkowski’s geometric interpretation of Einstein’s special theory of relativity gave birth to four-dimensional space, with time as part of the fourth dimension. In Minkowski’s interpretation, often termed “Minkowski space” or “Minkowski spacetime,” the fourth dimension includes time and is on equal footing with the normal three-dimensional space we currently encounter. However, Minkowski’s fourth dimension borders on the strange. In Minkowski spacetime, the fourth dimension, X4, is equal to ict, where i = √-1, an imaginary number, c is the speed of light in a vacuum, and t is time as measured by clocks. The mathematical expression ict is dimensionally correct, meaning that it is a spatial coordinate, not a temporal coordinate, but is essentially impossible to visualize, since it includes an imaginary number. What is an imaginary number? It is a number that when squared (multiplied by itself) gives a negative number. This is not possible to do with real numbers. If you multiply any real number, even a negative real number like minus one, by itself, you always get a positive number. Therefore, it is impossible to solve for the square root of minus one.

Although we can express it mathematically as √-1, it has no solution, and it is termed an imaginary number. Does that mean Minkowski was wrong about the fourth dimension? Actually, it does not. It does say that it is a mathematical construct, and intuitively, for most of us, impossible to visualize. However, the special theory of relativity continues to be taught using Minkowski spacetime, which the bulk of the scientific community considers a valid geometric interpretation. In either its algebraic form, as first presented by Einstein, or its geometric form, as interpreted by Minkowski, the majority of the scientific community considers the special theory of relativity the single most successful theory in science. It has withstood over a century of experimental investigation, and it is widely considered verified.

Because of its scientific underpinnings, the eternalism theory of time is widely accepted in the scientific community. If we adopt the eternalism theory of time, then time travel to the past or future becomes equally valid. In addition, no scientific theory contradicts or prohibits time travel. Said more positively, based on Einstein’s theories of relativity, which lay a theoretical foundation for time dilation (i.e., time travel to the future) and closed timelike curves (i.e., time travel to the past), most of the scientific community would support the scientific possibility of time travel.