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Zenos Paradoxes – The motionless runner & Achilles and the Tortoise

zenos paradoxes

Zeno of Elea was a Greek philosopher and mathematician, whom Aristotle called the inventor of dialect. He is especially known for his paradoxes that contributed to the development of logic.

zenos paradoxes

Image credit: Colm Kelleher on TED-Ed

The first paradox we’ll explain goes like this:

A runner wants to run a certain distance, for example 500 meters. But if he wants to reach that distance he must first reach the first 250 meters, and to reach that he must first run 125 meters. Since space is infinitely divisible, we can repeat that forever. Thus, the runner has to reach an infinite number of midpoints in a finite time which is impossible. This leads us to thinking that anyone who wants to move from one point to another must meet these requirements, and so motion is impossible, and what we perceive as motion is an illusion.

One assumption in this paradox is that space is infinitely divisible. Another is that it is impossible to reach infinite number of points in a finite time. But why time couldn’t be infinitely divided to?

$ Distance = speed * time$

$ Time = distance / speed$

If you divide an infinitesimal distance by a finite speed, you get an infinitesimal time.

This means that time can be infinitely divided; that’s why infinitesimal distance can be travelled in an infinitesimal time, and the times add up to a finite time in the same way that the infinite number of distances adds up to a finite distance. This is where this paradox fails.


Achilles and the tortoise

archilles tortoise

Image credit: Affordable Housing Institute

Let’s say that, for some reason, Achilles and the tortoise are racing. The tortoise, if of course a lot slower that the Achilles and for that reason she has a 100 meters head start.

Achilles sprints 100 meters and catches up to a tortoise. By that time tortoise also moved, but only 10 meters.
Now Achilles sprints another 10 meters to catch up to a tortoise. But now, tortoise moved another meter. And this keeps happening. The paradox is that the Achilles can never catch the tortoise which is nonsense because he obviously has to.

archilles catches tortoise

Image credit: IB Maths Resources

So how can an infinite process end?

If we have the distance of 2 meters and we’re constantly dividing it by two we get:

$ S = 1 + (\frac{1}{2}) + (\frac{1}{4}) + (\frac{1}{8}) + … $

If we divide whole sum by 2:

$\frac{S}{2} = (\frac{1}{2}) + (\frac{1}{4}) + (\frac{1}{8}) + (\frac{1}{16}) + … $
Now let’s subtract those two sums:

$\frac{S}{2} = 1$
Which means that $ S = 2$.

This is a well behaved sum. At infinity it is equal to a real number exactly. But when we’re talking about infinity, we know that it doesn’t have the last step. So how can something without a last step be completed? That is the paradox. But even though this is an infinite process, it can be completed. This is the same problem as drawing a length of an irrational number. How can you draw accurately segment whose length has infinitely many decimals? But for example, if you have a right triangle with length of both of its legs 1, its hypotenuse will be $\sqrt{2}$, which is an irrational number.