A Series About Time: Reality or an Illusion? Part III: Philosophical Point of View
In philosophy, time was questioned throughout the centuries; what time is and if it is real or not. There are different theories about time and the universe. St. Augustine of Hippo asks himself, `What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not.´ He begins to define time by what it is not rather than what it is. Augustine ends up calling time a `distention´ of the mind by which we simultaneously grasp the past in memory, the present by attention, and the future by expectation.
The theory that past and future don´t exist and are a construct of our mind is called presentism. In presentism, only present objects exist. This implies that necessarily only present objects and present experiences are real, and we conscious beings recognize this in the special vividness of our present experience compared to our memories of past experiences and our expectations of future experiences. Following this theory, Hitler is not reality anymore, though our memories and current ideas about him are still real.
They are one and the same concept, and the present simply is the real considered in relation to two particular species of unreality, namely the past and the future.
Prior
The idea that the past is not real is challenging to grasp if we have pictures of Hitler and find dinosaurs’ fossils. Luckily this was not the last word of philosophy about time. The theory that postulates that past and present are real, but the future is not is called the growing block theory. It is also called the growing-past theory. Past and present are real, but the future is not because the future is indeterminate or merely potential. Following this theory, Hitler is real. The past is real, and it is growing bigger all the time.
It will not surprise you that there is also a theory that considers past, present, and future as real. The theory of eternalism states that all existence in time is equally real. There are no objective ontological differences between present, past, and future because the differences are subjective. You can compare it to the ontological difference between here and there; there is no objective difference. Here and there are subjective. The differences among past, present, and future are subjective, depending upon whose experience is being implicitly referred to. For example, Hitler’s rise to power in Germany is not simply in the past; it is in the past for you, but in the future for Aristotle.
We return to fields where physics and philosophy intertwine. The multiverse theory implies that anything that is a physical possibility does exist in some universe or another. Our universe is one among many universes, there are universes with time, and there are universes without time. Our universe arose at random, by a process in which any possible universe inevitably will arise.
An even more disturbing metaphysical claim made by some philosophers of physics about relativity theory is that the future already exists but just has not yet been experienced by us. This theory is called the block universe theory. Our universe may be looked at as a four-dimensional block of space-time, containing all the things that ever happen. In the block universe, there is no `now` or present. All moments that exist are just relative to each other within the three spacial dimensions and one time dimension. Your sense of the present is just reflecting where in the block universe you are at that instance. The `past` is just a slice of the universe at an earlier location while the `future` is at a later location. If you see this as a static model, one of the problems is that our future is already there, and that would mean there is no self-determination, so what would be the point of anything? The answer to this is the `evolving block universe` model which sees the block of the universal space-time growing rather than staying the same.
The block universe is not necessarily a deterministic one. …Strictly speaking, to say that the occurrence of a relatively later event is determined vis à vis a set of relatively earlier events, is only to say that there is a functional connection or physical law linking the properties of the later event to those of the earlier events. …Now in the block universe we may have partial or even total indeterminacy — there may be no functional connection between earlier and later events.
McCall
I will end this series about time with the fun part, time travel. Time travel is not possible yet. We need to figure out how to travel at some reasonable percentage of the speed of light. And we have to figure out how to use wormholes, which are like short cuts through space-time. But not being able to travel in time does not mean we can already think about it. A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of a temporal paradox. This is also called the grandfather paradox. The name comes from the paradox’s description: a person travels to the past and kills their own grandfather before the conception of their father or mother, which prevents the time-traveler’s existence. A solution to this problem is that a time traveler would not be able to change the past from the way it is; they would only act in a way that is already consistent with what necessarily happened. The Novikov self-consistency principle states that history is an unchangeable constant but that any change made by a hypothetical future time traveler would already have occurred in his or her past, resulting in the reality that the traveler moves from. More practical, if you could travel back in time, and you tried to kill Hitler, you would just not be able to do it (maybe you were involved in one of the at least 42 failed assassination plots). Another solution to the problem is to use Tegmark´s theory that time is not as a line, but time is as a branching tree. At each next instant, the universe splits into new universes, all of which have a common past. The time-traveler travels between branch points, parallel realities, or universes. You could travel back in time, and you managed to kill Hitler. But the moment you killed him, you created a new branch in time. A branch of time we are not experiencing now.
Interesting links:
A Series About Time: Reality or an Illusion? Part I: History and Culture
A Series About Time: Reality or an Illusion? Part II: Physicists Point of View
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