When Should We Question Infinity?

A version of this article appears in TWISTED LOGIC: Puzzles, Paradoxes, and Big Questions. By Leighton Vaughan Williams, Chapman & Hall/CRC Press. 2024.

Introduction

Born in the 5th century BC in Elea (a Greek colony in southern Italy), Zeno of Elea is one of the most intriguing figures in the field of philosophy. Zeno’s paradoxes are a set of problems generally involving distance or motion. While there are many paradoxes attributed to Zeno, the most famous ones revolve around motion and are extensively discussed by Aristotle in his work, ‘Physics’. These paradoxes include the Dichotomy paradox (that motion can never start), the Achilles and the Tortoise paradox (that a faster runner can never overtake a slower one), and the Arrow paradox (that an arrow in flight is always at rest). Through these paradoxes, Zeno sought to show that our common-sense understanding of motion and change was flawed and that reality was far more complex and counterintuitive.

The Achilles and the Tortoise paradox, as one example, uses a simple footrace to question our understanding of space, time, and motion. While it’s clear in real life that a faster runner can surpass a slower one given enough time, Zeno uses the race to craft an argument where Achilles, no matter how fast he runs, can never pass a tortoise that has a head start. This thought experiment forms a remarkable philosophical argument that challenges our perceptions of reality and creates a fascinating paradox that continues to engage scholars to this day.

These paradoxes might seem simple, but they invite us into deep philosophical waters, questioning our perception of reality and illustrating the complexity of concepts we take for granted like motion, time, and distance. In this way, Zeno’s contributions continue to have profound relevance in philosophical and scientific debates, encouraging us to critically explore the world around us.

The Paradox of the Tortoise and Achilles

In one version of this paradox, a tortoise is given a 100-metre head start in a race against the Greek hero Achilles. Despite Achilles moving faster than the tortoise, the paradox argues that Achilles can never overtake the tortoise. As Aristotle recounts it, ‘In a race, the quickest runner can never overtake the slowest, since the pursuer must first reach the point whence the pursued started, so that the slower must always hold a lead’.

The Underlying Infinite Process

This paradox lies in the infinite process Zeno presents. When Achilles reaches the tortoise’s original position, the tortoise has already moved a bit further. By the time Achilles reaches this new position, the tortoise has again advanced. This sequence of Achilles reaching the tortoise’s previous position and the tortoise moving further seems to continue indefinitely, suggesting an infinite process without a final, finite step. Zeno argues that this eternal chasing renders Achilles incapable of ever catching the tortoise.

A Mathematical Solution to the Paradox

The resolution to Zeno’s paradox lies in the mathematical understanding of infinite series. Using a stylised scenario where Achilles is just twice as fast as the tortoise (it’s a very quick tortoise!), we define the total distance Achilles runs (S) as an infinite series: S = 1 (the head start of the tortoise) + 1/2 (the distance the tortoise travels while Achilles covers the head start) + 1/4 + 1/8 + 1/16 + 1/32 …

By mathematical properties of geometric series, this infinite series sums to a finite value. In other words, despite there being infinitely many terms, their sum is finite: S = 2. Hence, Achilles catches the tortoise after running 200 metres, demonstrating how an infinite process can indeed have a finite conclusion.

Philosophical Implications: Is an Infinite Process Truly Resolved?

Zeno’s paradoxes, while they might be resolved mathematically, open a Pandora’s box of philosophical questions, particularly concerning the nature of infinity and the real-world interpretation of mathematical abstractions. How can a seemingly infinite process with no apparent final step culminate in a finite outcome?

The Thomson’s Lamp thought experiment, proposed by philosopher James F. Thomson, provides an insightful analogy. Imagine you have a lamp that you can switch on and off at decreasing intervals: on after one minute, off after half a minute, on after a quarter minute, and so forth, with each interval being half the duration of the previous one. Mathematically, the total time taken for this infinite sequence of events is two minutes. However, a critical philosophical question emerges at the end of the two minutes: is the lamp in the on or off state?

This question is surprisingly complex. On the one hand, you might argue that the lamp must be in some state, either on or off. However, there is no finite time at which the final switch event takes place, given the infinite sequence of switching. Hence, the state of the lamp appears indeterminate, raising questions about the applicability of infinite processes in the physical world. More prosaically, of course, you may just have blown the bulb!

This conundrum mirrors the situation in Zeno’s paradox of Achilles and the Tortoise. Just as the state of Thomson’s Lamp after the two-minute mark seems ambiguous, so does the concept of Achilles catching the tortoise after an infinite number of stages. While mathematics gives us a definitive point at which Achilles overtakes the tortoise, the philosophical interpretation of reaching this point through an infinite process is not as clear-cut.

The Thomson’s Lamp thought experiment highlights that while we can use mathematical tools to deal with infinities, interpreting these results in our finite and discrete physical world can be philosophically challenging. It reminds us that philosophy and mathematics, while often harmonious, can sometimes offer different perspectives on complex concepts like infinity, sparking ongoing debates that fuel both fields.

Zeno’s Paradoxes, the Quantum World, and Relativity

Zeno’s paradoxes, which have puzzled thinkers for millennia, find surprise echoes in the realms of quantum mechanics and the theory of relativity, two foundational components of modern physics. Thse paradoxes, originally aimed at challenging the coherence of motion and time, intersect with quantum and relativistic concepts in thought-provoking ways.

In quantum mechanics, the principle of superpoition allows particles to exist in multiple states a once until observed. This phenomenon reflects the essence of Zeno’s Arrow Paradox, where an arrow in flight is paradoxically motionless at any instant. This comparison highlights how quantum theory disrupts traditional views on motion, suggesting that at a microscopic level, movement doesn’t conform to our standard or philosophical expectations.

Meanwhile, the theory of relativity introduces the conceot of time dilation, where times appears to ‘slow down’ for an object moving at speeds close to the speed of light. This idea provides a moden perspective on Zeno’s Dichotomy Paradox, which argues that motion is impossible due to the infinite divisibility of time and space. Through relativity, we see that motion and time are relative, not absolute, concepts – illustrating a deep connection to Zeno’s philosophical challenges, even after two millennia.

Conclusion: Philosophical Debate and Contemporary Relevance

Contemporary philosophers continue to grapple with Zeno’s paradoxes, not only as historical curiosities but also as fundamental challenges to our understanding of reality. These paradoxes force us to reconsider how we conceptualise time, space, and motion. They remind us that our intuitive grasp of the world is often at odds with its underlying complexities. In today’s world, where scientific and technological advancements continually push the boundaries of what we understand, Zeno’s paradoxes remain as relevant as ever, reminding us of the enduring power and limits of human reason and the ongoing journey to comprehend the universe in which we live.