If I was at point $A$ and I wanted to walk directly to point $B$, I would have to walk half way to point $B$, but before that I would have to walk half way to halfway to halfway to point $B$ and half of that again and so on and so fourth. if I halved this distance an infinite amount of times then there would be an infinite amount of actions I would need to perform in order to cross from $A$ to $B$. Therefore, if each action took any quantity of time at all, then it would take me an infinite amount of time to cross from $A$ to $B$, even if $A$ and $B$ were only a few centimetres apart!
Because I am not infinitely old and I can move, there must be a flaw in this logic. Where is it?
Answer
You have stumbled onto one of Zeno's many paradoxes - the so-called Dichotomy paradox.
The resolution lies in the fact that sum of terms in an infinite series do not necessarily add up to produce an infinity, so the basic premise is flawed. In particular, the series
$$S = \sum_{n=1}^\infty \left(\frac{1}{2}\right)^n = \frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \frac{1}{16} \ldots$$
is a convergent infinite series. The sum is
$$ S = 1 \, .$$
Since Zeno belonged to the BC era, this paradox stood as a paradox for many years. In fact, Zeno was satisfied with the reasoning than it had an infinite number of steps, but didn't find the sum. However, things changed with the development of Analysis and Calculus in the 19th century, and with these convergence arguments, it is settled now.
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