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« Last post by Casey Enos on August 08, 2012, 08:18:00 pm »
Another essay I wrote while rehearsing for last years exams; and actually used on the test. Every year Zeno comes up, with a slightly different twist on the question: for instance, can Zeno teach us anything positive about motion; does it dissolve Zeno's paradoxes to hold that space can be divided infinitely, and a few others.
"Can we we learn anything positive about the nature of motion from Zeno's paradoxes?"
In treating Zeno's paradoxes, possibly the single most satisfying answer was that of Doignes the Cynic, who refuted Zeno by walking around the room. In declaring movement to be impossible, Zeno is clearly running afoul of empirical evidence, and it is therefore unlikely that his paradoxes have much to say on the nature of movement. However, such hasty answers miss the opportunity to examine exactly where the fault in his reasoning lies, and therefore to learn a great deal about the tools and models with which we examine movement.
Poor treatment of dynamic processes is symptomatic o f classical Greek thought, with its focus on understanding static states. To Zeno, a finite section, be it of time, motion of space, can only be divided into finite sized chunks, although he held, oddly that the division can be performed an infinite number of times, leading directly to a paradox. Using a system of mathematics without the concept of zero and with an undeveloped idea of infinity, Zeno could not do other than generate a paradox in his treatment of both physical extension and motion. The infinitely small points of modern calculus were as unavailable to him as to any of his contemporaries.
Modern treatment of Zeno, as most famously presented by Russell, hinges on the idea of an infinite number of points which can be fitted, in a gap-free manner, on a line finite extension so that the addition or subtraction of points cannot cause an alteration in the size of the line. Importantly the modern calculus based solution is primarily advocated over Zeno's presentation or classical attempts at solutions, largely because of the success at calculus in building scientific models describing motion. No other set of solutions has provided such support to theories describing empirical reality, surely an indication of which direction practical handling of the nature of motion may lie.
Zeno's most discussed paradoxes are "The Dichotomy" and "The Achilles". In the Dichotomy, a runner must complete a course from point A to B. In order to reach B, she first must traverse 1/2 the distance, then 3/4, then 7/8...Each diminishing fraction must take a finite amount of time to complete; presumably as there is no lower limit to the fractions remaining yet each requires a finite time to traverse, the result is an ever growing time spiraling to infinity to traverse a finite space. The same treatment can actually be applied to the runner's first step, so that the course can not only be completed, it will not even be started. The Achilles, involving the efforts of a swift runner to overtake a tortoise who enjoys a head start, is essentially the same paradox with a more developed story line. Each time the runner arrives at the animal's previous position, it will have crawled ahead, resulting in an ever-diminishing gap that cannot be closed.
A standard solution is point out that the continuously decreasing fractions will sum to 1;g with the along with the diminishing amount of time required to traverse each. Such a treatment is not entirely satisfactory, although it does point in the right direction. Zeno is not dealing with a completed division which can be summed to one, but with an ongoing and never completed one that never reaches the sum of one.
Aristotle famously refuted to even consider the division into points, addressing the fact that lines may be potentially, or mathematically, divisible, yet real distances in nature do not consist of points or fractions. In this assertion he was clearly correct an established an extremely salient point about the limitations of human ability to conduct accurate analysis of nature; however in taking such a route he also closes off any access to the practical application of mathematics to analyze movement. Building the modern edifice of physics on Aristotle's foundations would be simply impossible...
Another potential solution, similar to the calculus based one, to regard the division of the line and the time required to traverse it as already complete; this time into infinitely small units, or points. As before, the runner is faced with an infinity of tasks to complete, however for each task she has a corresponding infinitely small fragment of time in which to complete it. Both series sum to one, in the same way a finite line can be divided into an infinity of points and a decimal number between zero and be assigned to each point. The key is to regard the division as already completed, not as one that occurs continuously as the runner moves and thus never arrives at its sum.
The completion of "super-tasks" of course remains controversial and has spawned a great deal of philosophical literature, most famously "Thompson's Lamp". This lamp is equipped with a toggle switch which is flicked from "on" to "off" an infinite number of times in a minute, and the question, probably unanswerable, is posed of which position it comes to rest at after the minute. However, for our purposes a better analysis is not of the end position of the switch, but the motion of the switch as it traverses an infinite number of points back forth in its movement. Under such an analysis, more relevant to consideration of Zeno, a super-task is performed every time a light switch is turned on, indeed every time this pen draws a line on the paper (*note: the actual exam paper was, of course, hand written). While not exactly resolving the problem of super-tasks, it does shed light on the irrelevance of the question to that of motion.
The third paradox of motion reported in the doxology is that of "The Arrow", which when shot from the bow will occupy a space equal to itself at any "instant". Since time consists only of instants, motion is impossible as the arrow never moves from the space it is occupying.
Again, Aristotle makes a correct answer, that the division into instants is artificial, yet again it is an answer that does not allow anything further to be learned. A more enlightening answer is that at every instant the arrow possesses "speed", that is the quality of movement, a characteristic which is carried over from instant to instant. Commentators have pointed out that in modern terms, by assigning a value of 0 to the arrow's speed at any instant, Zeno is effectively dividing by zero and thus rendering the analysis of the arrow's movement meaningless.
Another solution, similar to that of the runner, is to look at the arrow's flight as a whole, divided into an infinity of points, each one of which is traversed at each instantly, of which there are also an infinity. Since the points in both time and space are completely gap-less, the result would be a smooth flight.
The final paradox of motion, and perhaps the easiest to resolve, is that of "the stadium", or "moving bands". As reconstructed by Aristotle, three bands of equally sized objects are presented, two moving parallel to one another in opposite directions, and both passing an unmoving one. The paradox is generated by the fact that the two moving bands pass each other more quickly than the third:
(*here there was a hand drawn diagram difficult to reproduce in type, but similar to
T1: AAA
BBB
CCC
T2:
AAA
BBB
CCC
with A stationary, B moving to the right and C to the left)
In the diagram, the leading edge of the two moving bands have passed three units of one another, and only two of the stationary one.
This paradox is usually regarded as simply a serious mishandling on Zeno's part of the notion of relative speed. More charitable reconstructions involve the motion of point-particles or quantum style "jumping", unfortunately there is very little in the doxology to support such reconstructions and Zeno lacked the technical equipment to postulate points of zero size. Therefore the moving bands, at least as Zeno presented them, is probably the least edifying if the four paradoxes of motion.
As a whole, we can learn a great deal more about motion really consists of from Aristotle and his successors, such as Aquinas, who denied the actual divisibility of time and space, than from Zeno. In maintaining the use of calculus to dispel the paradoxes, we must be careful to remember that we are operating with an approximate tool. Mathematics is internally consistent only because of the definitions of the units and operators involved, just as Zeno's text works only because of the definitions in the terms used. The most important lesson may be simply that nature contains motion and it does not contain points, fractions or calculus.