The puzzle of time
Now is not an objective phenomenon, it is not even a material phenomenon.
We can refer to moments in time. The physicist speaks of
Objectively, now has no meaning. Suppose I look at my watch, I see it is noon--we'll cal it time t1. I write down a description of all of the relevant details. Five minutes later I look at my watch, time t2, again I write down a description. Every fact that I have written down about time t1 is still true about time t1, even though those facts may not all apply to time t2, they still apply to time t1. So, if those facts are still true about t1, then why is now no longer at time t1?
Let's examine this puzzle a little more carefully. What causes any particular time to be now? In general, a phenomenon can have multiple causes. When I flip a light switch, the light (usually) comes on. I may begin to believe that my flipping the switch causes the light to come on. But is this the only cause? One day I may flip the switch and find that the light does not come on. Since like causes should have like results, I infer that the light coming on must have more causes than the position of the light switch alone. I may not know what these other causes are, although in the case of the light I can list many of them (there must be a continuous wire from the power source to the switch, and from the switch to the lamp, the power source must be operational, no blown circuit breakers, etc.). The point is that if I catalog a list of causes, and then notice that identical causes do not produce identical effects, then I know that my catalog of causes is incomplete.
So consider a maximal catalog of
causes in the objective universe, a world book containing all
objective information about every point in space-time, from the origin of the
universe until its end. This world book will catalog my looking at my
watch at time t1 and saying
That is, from the objective standpoint of the world book, the situation at time t1 exists and does not vary depending on my perspective. However, as a sentient being--a time traveler (in the usual sense of moving forward through time in the usual way)--I realize that my subjective perspective on the universe has changed. Nothing objective has changed, only this subjective perspective. Clearly then, the perspective is not objective, since there is no objective statement about a point in time whose truth depends on that perspective.
Consider the act of reading a book. I might, at some time, look at the
page numbers. I might say
The mistake that the concept of now arises from material causes, is the same kind of mistake a reader might make if he decided that his subjective perspective on a book were caused by the book itself.
Subjectively, we know that, given the universe, we have a subjective perspective on time, we call now. I'll write that as Now(U), where U refers to the entire collection of facts about the Universe, throughout its breadth of space and time. What point in time does this select? It is just one point in time--I am only aware of one point in time being now. The problem is that on one occasion the function evaluates to t1, on another occasion, to t2. Note, that it only begs the question to say that it evaluates to t1 at time t1 and to t2 at time t2. To say that is to insist that the function must be written Now(U, t1) in the first case and as Now(U, t2) in the second case--in other words that the quality of now-ness is independent of U. More generally, however, we can say that we must write Now(U, x1) and Now(U, x2) where x1 and x2 are independent of U. In either case, we have shown that the passage of time is independent of the objective aspects of the universe.
To put this in terms of the book analogy, suppose consider the page we are on as Page(Book). This doesn't work because this can, on one occasion evaluate to page p1 and on another occasion evaluate to page p2. To say that it evaluates to page p1 when we are reading page p1 is to insist that the function in that case must be written Page(Book, p1), in other words to accept that the page we are on enters independently of the book itself.
The arrow of time
Note that the above discussion is not that closely related to the
question of the arrow of time. That problem is concerned only with why
physics is asymmetric with respect to time. This approaches a subjective
question about time when the discussion gets to such things as
Time proves to be asymmetric with respect to such things as entropy increase and wave function collapse. Intuitively, maybe the irreversible increase in the entropy of statistical ensembles, in fact, arises from the implications of wave function collapse in chaotic systems.
Given that the passage of time is not an objective concept, it is a little strange that physicists sometimes talk about time travel. Objectively, what this means is not that the subjective awareness of now moves in unexpected ways, but rather that causation can happen in unexpected ways--for example that a phenomenon might be caused by events in the future as well as in the past. The world book is invariant--at each point in space-time there are particular attributes which do not change, and objective notions of time travel relate only to whether the values of these attributes might be considered to have been caused by chains of events that bypass the normal sequences in time.