You might consider time to be such a fundamental feature of our existence that there really is nothing profound or meaningful that we can say about it. Maybe time, like space, simply is. Many have even argued time out of existence altogether.
For example, if we were to split it up into three components: the past, the present and the future, then we can examine each in turn. Those who deny the reality of time will say something like this: The past has happened and is gone; all we have left of it is the record of past events (in our memories, or in books, photographs, or on film), all of which can only ever form a part of the present moment (when we access those records). But the events themselves no longer exist.
In contrast, all there is of the future is our predictions, hopes, and beliefs about what might happen, which are again states of our minds in the present. Regardless of whether the future is predetermined or not, it has yet to exist. That leaves the present moment itself; surely “now” is real.
But although we “feel” that our now is a solid, changing moment steadily sweeping through time, converting the future into the past, it is nonetheless just an instant and as such does not have any duration. The physical definition of “now” is not a minute long or a second long, the time deniers will argue; it is the edge of the shadow between light and dark, between what is to happen and what has happened. The constantly changing present moment is no more than a dividing line between past and future and so does not have a tangible existence of its own.
Einstein famously stated that “the distinction between past, present and future is only a stubbornly persistent illusion.” Thus, if none of the three components of time exist, then there is nothing left, leaving the whole of time as illusory! Many thinkers going back two and half thousand years have argued thus. If this all sounds like a clever sleight-of-hand, don’t worry: A number of philosophers of time, and physicists for that matter, subscribe to a diametrically opposite view, known as “eternalism.” This holds that all three components—past, present and future—are equally real!
This idea, which we will examine more closely later on, emerges from an idea called the block universe, which gained prominence after Einstein put forward his first theory of relativity, in which time is regarded as the fourth dimension of spacetime. In this picture all moments in time coexist in the same way that all points in space exist. A related idea is the “growing block universe” view, in which the past and the present exist while the future does not, but instead gradually comes into being.
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