# 7.4: Does All This Really Work?

• Eric Lehman, F. Thomson Leighton, & Alberty R. Meyer
• Google and Massachusetts Institute of Technology via MIT OpenCourseWare

So this is where mainstream mathematics stands today: there is a handful of ZFC axioms from which virtually everything else in mathematics can be logically derived. This sounds like a rosy situation, but there are several dark clouds, suggesting that the essence of truth in mathematics is not completely resolved.

• The ZFC axioms weren’t etched in stone by God. Instead, they were mostly made up by Zermelo, who may have been a brilliant logician, but was also a fallible human being—probably some days he forgot his house keys. So maybe Zermelo, just like Frege, didn’t get his axioms right and will be shot down by some successor to Russell who will use his axioms to prove a proposition $$P$$ and its negation $$\overline{P}$$. Then math as we understand it would be broken—this may sound crazy, but it has happened before.

In fact, while there is broad agreement that the ZFC axioms are capable of proving all of standard mathematics, the axioms have some further consequences that sound paradoxical. For example, the Banach-Tarski Theorem says that, as a consequence of the axiom of choice, a solid ball can be divided into six pieces and then the pieces can be rigidly rearranged to give two solid balls of the same size as the original!

• Some basic questions about the nature of sets remain unresolved. For example, Cantor raised the question whether there is a set whose size is strictly between the smallest infinite set, $$\mathbb{N}$$ (see Problem 7.9), and the strictly larger set, pow$$(\mathbb{N})$$? Cantor guessed not:

Cantor’s Contiuum Hypothesis: There is no set, $$A$$, such that

$\nonumber \mathbb{N} \text{ strict } A \text{ strict pow}(\mathbb{N}).$

The Continuum Hypothesis remains an open problem a century later. Its difficulty arises from one of the deepest results in modern Set Theory— discovered in part by Gödel in the 1930’s and Paul Cohen in the 1960’s— namely, the ZFC axioms are not sufficient to settle the Continuum Hypothesis: there are two collections of sets, each obeying the laws of ZFC, and in one collection the Continuum Hypothesis is true, and in the other it is false. Until a mathematician with a deep understanding of sets can extend ZFC with persuasive new axioms, the Continuum Hypothesis will remain undecided.

• But even if we use more or different axioms about sets, there are some unavoidable problems. In the 1930’s, Godel ¨ proved that, assuming that an axiom system like ZFC is consistent—meaning you can’t prove both $$P$$ and $$\overline{P}$$ for any proposition, $$P$$—then the very proposition that the system is consistent (which is not too hard to express as a logical formula) cannot be proved in the system. In other words, no consistent system is strong enough to verify itself.

## Large Infinities in Computer Science

If the romance of different-size infinities and continuum hypotheses doesn’t appeal to you, not knowing about them is not going to limit you as a computer scientist. These abstract issues about infinite sets rarely come up in mainstream mathematics, and they don’t come up at all in computer science, where the focus is generally on “countable,” and often just finite, sets. In practice, only logicians and set theorists have to worry about collections that are “too big” to be sets. That’s part of the reason that the 19th century mathematical community made jokes about “Cantor’s paradise” of obscure infinities. But the challenge of reasoning correctly about this far-out stuff led directly to the profound discoveries about the logical limits of computation described in Section 7.2, and that really is something every computer scientist should understand.

This page titled 7.4: Does All This Really Work? is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Eric Lehman, F. Thomson Leighton, & Alberty R. Meyer (MIT OpenCourseWare) .