💾 Archived View for spam.works › mirrors › textfiles › humor › timetr.hum captured on 2023-06-14 at 17:19:59.
-=-=-=-=-=-=-
Time Travel: A Discussion Howard Ullman The possibilities of time travel have captured the imaginations of many people, most notably science-fiction writers. Indeed, time travel may be the modus operandi of plot in the science fiction genre. Time travel as a plot vehicle opens up wonderful avenues of exploration. However, the treatment of time travel in most books and movies is usually ill-conceived and poorly planned. Paradoxes are usually created that disturb any serious appreciation of the plot. In this short essay we will first examine precisely what is meant by the term "time travel," and will then proceed to explore the paradoxes associated with the concept and their resolutions. Time Travel: A Definition (N.B.: In the course of this discussion, we will use the term "time travel" to describe a backwards displacement in time. Forward displacements generate no paradoxes and can be completely described by Special and General Relativity.) What do we mean when we say "time travel?" Let us imagine that the inventor Dr. Leopold Fechner (of Tonight Show fame) has invented a "time machine." Using this machine, he is able to travel back in time. Dr. Fechner is a collection of many, many molecules. As a system of n molecules, he can be described at any time t by 6n variables - 3 variables describing the position of each molecule, and 3 variables describing the momenta of each molecule. (Of course, our description is bounded in precision by the Uncertainty Principle.) Because Dr. Fechner is only about two meters long, we may approximate reality and say that we may describe all his molecules existing simultaneouly at any time t. (Over such a short distance the effects of the Lorentz contractions are negligible.) When we say that Dr. Fechner as travelled back in time, we mean that all 6n variables at time t are "duplicated" at some time t-a. Dr. Fechner's molecules have disappeared at time t and have reappeared at time t-a in precisely the same relative configuration. Problem One: The Violation of The First Law The First Law states that the mass-energy of the universe (Ue) is constant (within the limits of the Uncertainty Principle.) No observer can detect a violation of the First Law. Now, an observer at time t-a at rest relative to Dr. Fechner would observe an increase in Ue of mc^2 Joules, where m is Dr. Fechner's rest mass in kgs. This is a very large increase in Ue. Dr. Fechner's trip back through time would thus violate the First Law. Problem Two: The Violation of The Second Law The Second Law states that the entropy (S) of the universe is always increasing at a non-negative rate. At Dr. Fechner's starting point (at time t), the entropy of the universe is S1. At time t-a, the entropy is S2. Now, S1-S2 is a positive number. Dr. Fechner would observe a decrease in the universe's entropy. However, there is no process which can reduce S from the vantage point of any observer. A machine that would reduce S by deltaS would increase its own entropy by deltaS+epsilon, where epsilon>0. Dr. Fechner would have to have an energy source "outside the universe" (a meaningless phrase) to cross what would appear to him as an infinite entropy barrier. (Recently some physicists have suggested that the Second Law does not hold when the effects of gravity are strong. Other physicists, however, believe the entropy burden is merely shifed to the space-time curvature in an as yet unquantified way. Needless to say, I subscribe to the latter view.) Problem Three: The Paradox of Causality This is the most popular objection to time travel. Dr. Fechner goes back in time and shoots his grandfather. Then he is never born, so he can't go back in time and shoot his grandfather, but then he IS born and DOES go back in time... I think a lot of the confusion comes from an outdated perspective on time. Most people still have the Newtonian concept fixed firmly in their minds: that time is a "fluid," flowing by and through us which carries everything along with it. The modern physicist has a much different view. Objects move in space as they move in time. In some sense, time simply exists. The future is already "there," and the past is still "there." As well, the phrase "travel into the past" has no meaning. There is no universal past to which one can travel. Doesn't the idea of past an future events being "there" violate quantum theory? In fact, relativity has no implications regarding determinism vs. uncertainty. As the physicist Paul Davies writes: "Does not the collapse of determinism conflict with the theory of relativity? In this theory there is no universal present, and the entire past and futue of the universe are regarded as existing as an indivisible whole. The world is four-dimensional (three of space, one of time), and all events are simply there: the future does not 'happen' or 'unfold.' Any conflict is, in fact, illusory. Determinism concernes the question of whether every event is completely determined by a prior cause. It says nothing about whether the event is there. After all, the future will be what it will be regardless of whether it is determined by prior events or not. The four-dimensional perspective of relativity simply forbids us to slice up spacetime, in any absolute way, into universal instants of time. The notion of two events in different places being 'simultaneous' is relative to one's state of motion. They may be judged to occur at te same moment by on observer, but one moment after the other by another observer. We must therefore regard the universe as extended in time as well as space. But the theory tells us nothing about whether the temporal extension includes rigid links of caue and effect between the events there displayed. So in spite of the fact that past, present, and future seem to have no objective meaning, the theory of relativty does not forbid a human being fom deciding later events by his earlier acions. (Recall that the earlier-later ordering relation is an objective property of time, even though the past and the future are not.)" Thus, time travel presents paradoxes that cannot be resolved, even by the most clever author. The simple fact is that the phrase "travel back in time" is meaningless. Some authors may hide the holes in their arguments better, but all time travel descriptions have unresolvable problems.