A paradox is a statement that, despite sound reasoning from acceptable premises, leads to a conclusion that is contradictory. For instance, “I always tell a lie” is a paradox because if it is true, it must be false.
Zeno of Elea (c. 450 BCE) is credited with creating several famous paradoxes, and perhaps the best known is the paradox of the Tortoise and Achilles which is embedded in the study of infinite series, and even more specifically, convergent series.
The set up is pretty simple: Achilles is racing a tortoise. Achilles gives the tortoise a 100 yard head start, and while Achilles runs at 20 mph, the tortoise “runs” at 2 mph. So, when does Achilles pass the tortoise?
Zeno used the following argument to prove that Achilles would never catch or pass the tortoise. Figure 1, below, provides two snapshots once the race is underway.
The first snapshot is the instant Achilles reaches the point where the tortoise started. By the time Achilles gets there, the tortoise has raced forward and is now 10 yards ahead of Achilles (remember, the tortoise moves a tenth as fast as Achilles, so in the time it takes Achilles to travel 100 yards, the tortoise covers a tenth as much ground, or 10 yards).
For the sake of Zeno’s argument, let’s pretend you have a really fast Polaroid and every time Achilles reaches the point where the tortoise was, you take another photo. Using the picture above, the tortoise’s position is point A in the first photo. Then you take your next photo when Achilles reaches point A, which takes him about one more second. In that second, the tortoise has moved ahead to point B. You take your third photo (not shown) when Achilles reaches point B and the tortoise has moved ahead to point C. And so on…
Assuming you and your camera can work infinitely fast, there is no end to this series of photographs. Thus, the argument goes: because you can never get to the end of your infinite series of photos, Achilles can never catch the tortoise. The tortoise will always cover a tenth of the distance of Achilles.
Well, as everyone knows, Achilles does in fact surpass the tortoise. Therein lies the paradox.
