# The Continuum and Borel Sets

A continuum is a series or range of things that gradually change. It doesn’t have dividing points, and its extremes are quite different from one another. This term is commonly used in science to describe the properties of fluids, such as liquids, gases, and plasmas.

#### The Continuum Hypothesis

In the late nineteenth century, Georg Cantor invented a theory that is now known as the continuum hypothesis. He attempted to prove that it holds for all sets of reals, but he struggled. He also met with serious opposition from other mathematicians, especially those who were reluctant to accept infinite objects into their work.

The Continuum Hypothesis has become one of the most important open problems in set theory, and it remains unsolved to this day. It was considered so important by Hilbert that he placed it first on his list of “open questions” to be tackled by the 20th century.

Several seminal figures have tried to solve the continuum hypothesis, including Saharon Shelah and Godel.

Shelah’s approach to the problem centered on a new question: How many “small” subsets of a set should there be in order to cover every small subset by only a few of them? Shelah was able to use this approach to prove a variety of impressive results in cardinal arithmetic, and he also reversed a fifty-year trend of independence results.

He did this by making his universe of constructible sets as small as possible, and then using the resulting space to prove that it is consistent with the continuum hypothesis. This result did not prove that the continuum hypothesis is true, but it did prove that it is consistent with current mathematical methods.

After this achievement, other mathematicians began to wonder if the continuum hypothesis might be solvable in general, and whether we could find a model of the mathematical world that would lead to its failure. This could mean that we cannot solve the continuum hypothesis with current methods, which is what both Godel and Hilbert thought was true.

As of the mid-nineties, mathematicians were able to prove that the continuum hypothesis holds for a special class of sets called Borel sets. These sets are not the usual sets that mathematicians work with, but they are concrete.

These Borel sets are so large that they represent a vastly larger part of the mathematical universe than the usual sets that mathematicians work in. Nonetheless, it is a remarkable result that makes it more likely that the continuum hypothesis will be solved in the future.

Moreover, by focusing on sets that are definable, rather than arbitrary sets of reals, mathematicians have been able to develop better methods for proving that the continuum hypothesis holds. In fact, this is the basis of most modern approaches to the problem.

In this way, the continuum hypothesis was able to take hold of a much wider field of study than it did in the past. It is now a central component of mathematical set theory, a field that is expanding and building on itself, rather than being stuck in its prehistory. This is a very exciting development in the history of mathematics!