Related MO questions: [What is the general opinion on the Generalized Continuum Hypothesis?](https://mathoverflow.net/questions/14338/what-is-the-general-opinion-on-the-generalized-continuum-hypothesis) ; [Completion of ZFC](https://mathoverflow.net/questions/46907/completion-of-zfc) ; [Complete resolutions of GCH](https://mathoverflow.net/questions/191139/complete-resolutions-of-gch;) [How far wrong could the Continuum Hypothesis be?](https://mathoverflow.net/questions/218407/how-far-wrong-could-the-continuum-hypothesis-be;) [When was the continuum hypothesis born?](https://mathoverflow.net/questions/135912/when-was-the-continuum-hypothesis-born)


Background
----------


The [Continuum Hypothesis](https://en.wikipedia.org/wiki/Continuum_hypothesis) (CH) posed by Cantor in 1890 asserts that $ \aleph\_1=2^{\aleph\_0}$. In other words, it asserts that every subset of the set of real numbers that contains the natural numbers has either the cardinality of the natural numbers or the cardinality of the real numbers. It was the first problem on the 1900 Hilbert's list of problems. The generalized continuum hypothesis asserts that there are no intermediate cardinals between every infinite set X and its power set.


Cohen proved that the CH is independent from the axioms of set theory. (Earlier Goedel showed that a positive answer is consistent with the axioms).


Several mathematicians proposed definite answers or approaches towards such answers regarding what the answer for the CH (and GCH) should be.


The question
------------



> 
> My question asks for a description and explanation of the various approaches to the continuum hypothesis in a language which could be understood by non-professionals.
> 
> 
> 


More background
---------------


I am aware of the existence of 2-3 approaches.


One is by Woodin described in two 2001 Notices of the AMS papers ([part 1](http://www.ams.org/notices/200106/fea-woodin.pdf), [part 2](http://www.ams.org/notices/200107/fea-woodin.pdf)).


Another by Shelah (perhaps in [this paper entitled "The Generalized Continuum Hypothesis revisited "](https://arxiv.org/abs/math/9809200)). See also the paper entitled "[You can enter Cantor paradise](https://arxiv.org/PS_cache/math/pdf/0102/0102056v1.pdf)" (Offered in Haim's answer.);


There is a [very nice presentation by Matt Foreman](http://www.math.helsinki.fi/logic/LC2003/presentations/foreman.pdf) discussing Woodin's approach and some other avenues. Another description of Woodin's answer [is by Lucca Belloti](http://www2.units.it/episteme/L&PS_Vol3No1/bellotti/bellotti-html/bellotti_L&PS_Vol3No1.html) (also [suggested by Haim](https://mathoverflow.net/questions/23829/solutions-to-the-continuum-hypothesis/23831#23831)).


The proposed answer $ 2^{\aleph\_0}=\aleph\_2$ goes back according to François to [Goedel](https://books.google.com/books?id=gDzbuUwma5MC&lpg=PP1&dq=collected%2520works%2520godel&pg=PA405#v=onepage&q=collected%2520works%2520godel&f=false). It is (perhaps) mentioned in Foreman's presentation. (I heard also from Menachem Magidor that this answer might have some advantages.)


François G. Dorais mentioned an important paper by Todorcevic's entitled "[Comparing the Continuum with the First Two Uncountable Cardinals](http://www.math.toronto.edu/%7Estevo/continuum.ps)".


There is also a very rich theory (PCF theory) of cardinal arithmetic which deals with what can be proved in ZFC.


### Remark:


I included some information and links from the comments and answer in the body of question. What I would hope most from an answer is some friendly elementary descriptions of the proposed solutions.




---


There are by now a couple of long detailed excellent answers (that I still have to digest) by **Joel David Hamkins** and by **Andres Caicedo** and several other useful answers. (Unfortunately, I can accept only one answer.)


**Update** (February 2011): A new detailed answer was contributed by **Justin Moore**.


**Update** (Oct 2013) A user 'none' gave a link to [an article by Peter Koellner about the current status of CH](https://web.archive.org/web/20120403232634/http://logic.harvard.edu/EFI_CH.pdf):


**Update** (Jan 2014) A related popular article in "Quanta:" [To settle infinity dispute a new law of logic](https://www.simonsfoundation.org/quanta/20131126-to-settle-infinity-question-a-new-law-of-logic/)


**(belated) update**(Jan 2014) Joel David Hamkins links in a comment from 2012 a very interesting paper [Is the dream solution to the continuum hypothesis attainable](https://arxiv.org/abs/1203.4026) written by him about the possibility of a "dream solution to CH." A link to the paper and a short post [can be found here](http://jdh.hamkins.org/dream-solution-of-ch/).


**(belated) update** (Sept 2015) Here is a link to an interesting article: [Can the Continuum Hypothesis be Solved?](https://www.ias.edu/about/publications/ias-letter/articles/2011-fall/continuum-hypothesis-kennedy) By Juliette Kennedy


**Update** A videotaped lecture [The Continuum Hypothesis and the search for Mathematical Infinity](https://www.youtube.com/watch?v=nVF4N1Ix5WI) by Woodin from January 2015, with reference also to his changed opinion. (added May 2017)


**Update (Dec '15):** A very nice answer was added (but unfortunately deleted by owner, (2019) now replaced by a new answer) by Grigor. Let me quote its beginning (hopefully it will come back to life):


"One probably should add that the continuum hypothesis depends a lot on how you ask it.


1. $2^{\omega}=\omega\_1$
2. Every set of reals is either countable or has the same size as the continuum.


To me, 1 is a completely meaningless question, how do you even experiment it?


If I am not mistaken, Cantor actually asked 2..."


**Update** A 2011 videotaped lecture by Menachem Magidor: [Can the Continuum Problem be Solved?](https://youtu.be/3b_wbObaNgA) (I will try to add slides for more recent versions.)


**Update** (July 2019) Here are [slides of 2019 Woodin's lecture](https://www.uni-muenster.de/imperia/md/content/MathematicsMuenster/woodin-opening-colloquium.pdf) explaining his current view on the problem. (See the answer of Mohammad Golshani.)


**Update** (Sept 19, 2019) Here are [videos of the three 2016 Bernay's lectures by Hugh Woodin](https://gess.ethz.ch/en/news-and-events/paul-bernays-lectures/bernays-2016.html) on the continuum hypothesis and also the videos of the [three 2012 Bernay's lectures on the continuum hypothesis and related topics by Solomon Feferman](https://gess.ethz.ch/en/news-and-events/paul-bernays-lectures/bernays-2012.html).


**Update** (Sept '20) Here are [videos of the three 2020 Bernays' lectures](https://video.ethz.ch/speakers/bernays/2020.html) by Saharon Shelah on the continuum hypothesis.


**Update** (May '21) In a [new answer](https://mathoverflow.net/a/391766/1532), Ralf Schindler gave a link to his 2021 videotaped lecture in Wuhan, describing a result with David Asperó that shows a relation between two well-known axioms. It turns out that Martin's Maximum$^{++}$ implies Woodin's ℙ$\_{max}$ axiom. Both these axioms were known to imply the $\aleph\_2$ answer to CH. A link to the paper: <https://doi.org/10.4007/annals.2021.193.3.3>



Solutions to the Continuum Hypothesis
Since you have already linked to some of the contemporary
primary sources, where of course the full accounts of those
views can be found, let me interpret your question as a
request for summary accounts of the various views on CH.
I'll just describe in a few sentences each of what I find
to be the main issues surrounding CH, beginning with some
historical views. Please forgive the necessary simplifications.


**Cantor.** Cantor introduced the Continuum Hypothesis
when he discovered the transfinite numbers and proved that
the reals are uncountable. It was quite natural to inquire
whether the continuum was the same as the first uncountable
cardinal. He became obsessed with this question, working on
it from various angles and sometimes switching opinion as
to the likely outcome. Giving birth to the field of
descriptive set theory, he settled the CH question for
closed sets of reals, by proving (the Cantor-Bendixon
theorem) that every closed set is the union of a countable
set and a perfect set. Sets with this perfect set property
cannot be counterexamples to CH, and Cantor hoped to extend
this method to additional larger classes of sets.


**Hilbert.** Hilbert thought the CH question so
important that he listed it as the first on his famous list
of problems at the opening of the 20th century.


**Goedel.** Goedel proved that CH holds in the
constructible universe $L$, and so is relatively consistent
with ZFC. Goedel viewed $L$ as a device for establishing
consistency, rather than as a description of our (Platonic)
mathematical world, and so he did not take this result to
settle CH. He hoped that the emerging large cardinal
concepts, such as measurable cardinals, would settle the CH
question, and as you mentioned, favored a solution of the form $2^\omega=\aleph\_2$.


**Cohen.** Cohen introduced the method of forcing and
used it to prove that $\neg$CH is relatively consistent
with ZFC. Every model of ZFC has a forcing extension with
$\neg$CH. Thus, the CH question is independent of ZFC,
neither provable nor refutable. Solovay observed that CH
also is forceable over any model of ZFC.


**Large cardinals.** Goedel's expectation that large
cardinals might settle CH was decisively refuted by the
Levy-Solovay theorem, which showed that one can force
either CH or $\neg$CH while preserving all known large
cardinals. Thus, there can be no direct implication from
large cardinals to either CH or $\neg$CH. At the same
time, Solovay extended Cantor's original strategy by
proving that if there are large cardinals, then increasing
levels of the projective hierarchy have the perfect set
property, and therefore do not admit counterexamples to CH.
All of the strongest large cardinal axioms considered today
imply that there are no projective counterexamples to CH. This can be seen as a complete affirmation of Cantor's original strategy.


**Basic Platonic position.** This is the realist view
that there is Platonic universe of sets that our axioms are
attempting to describe, in which every set-theoretic
question such as CH has a truth value. In my experience,
this is the most common or orthodox view in the
set-theoretic community. Several of the later more subtle
views rest solidly upon the idea that there is a fact of
the matter to be determined.


**Old-school dream solution of CH.** The hope was that
we might settle CH by finding a new set-theoretic principle
that we all agreed was obviously true for the intended
interpretation of sets (in the way that many find AC to be
obviously true, for example) and which also settled the CH
question. Then, we would extend ZFC to include this new
principle and thereby have an answer to CH. Unfortunately,
no such conclusive principles were found, although there
have been some proposals in this vein, such as [Freilings
axiom of symmetry](https://en.wikipedia.org/wiki/Freiling%27s_axiom_of_symmetry).


**Formalist view.** Rarely held by mathematicians,
although occasionally held by philosophers, this is the
anti-realist view that there is no truth of the matter of
CH, and that mathematics consists of (perhaps meaningless)
manipulations of strings of symbols in a formal system. The
formalist view can be taken to hold that the independence
result itself settles CH, since CH is neither provable nor
refutable in ZFC. One can have either CH or $\neg$CH as
axioms and form the new formal systems ZFC+CH or
ZFC+$\neg$CH. This view is often mocked in straw-man form,
suggesting that the formalist can have no preference for CH
or $\neg$CH, but philosophers defend more subtle versions,
where there can be reason to prefer one formal system to
another.


**Pragmatic view.** This is the view one finds in
practice, where mathematicians do not take a position on
CH, but feel free to use CH or $\neg$CH if it helps their
argument, keeping careful track of where it is used.
Usually, when either CH or $\neg$CH is used, then one
naturally inquires about the situation under the
alternative hypothesis, and this leads to numerous consistency or independence results.


**Cardinal invariants.** Exemplifying the pragmatic view, this is a very rich subject
studying various cardinal characteristics of the continuum,
such as the size of the smallest unbounded family of
functions $f:\omega\to\omega$, the additivity of the ideal
of measure-zero sets, or the smallest size family of
functions $f:\omega\to\omega$ that dominate all other such
functions. Since these characteristics are all uncountable
and at most the continuum, the entire theory trivializes
under CH, but under $\neg$CH is a rich, fascinating
subject.


**Canonical Inner models.** The paradigmatic canonical
inner model is Goedel's constructible universe $L$, which
satisfies CH and indeed, the Generalized Continuum
Hypothesis, as well as many other regularity properties.
Larger but still canonical inner models have been built by
Silver, Jensen, Mitchell, Steel and others that share the
GCH and these regularity properties, while also satisfying
larger large cardinal axioms than are possible in $L$. Most
set-theorists do not view these inner models as likely to
be the "real" universe, for similar reasons that they
reject $V=L$, but as the models accommodate larger and
larger large cardinals, it becomes increasingly difficult
to make this case. Even $V=L$ is compatible with the
existence of transitive set models of the very largest
large cardinals (since the assertion that such sets exist
is $\Sigma^1\_2$ and hence absolute to $L$). In this sense,
the canonical inner models are fundamentally compatible
with whatever kind of set theory we are imagining.


**Woodin.** In contrast to the Old-School Dream
Solution, Woodin has advanced a more technical argument in
favor of $\neg$CH. The main concepts include $\Omega$-logic
and the $\Omega$-conjecture, concerning the limits of
forcing-invariant assertions, particularly those
expressible in the structure $H\_{\omega\_2}$, where CH is
expressible. Woodin's is a decidedly Platonist position,
but from what I have seen, he has remained guarded in his
presentations, describing the argument as a proposal or
possible solution, despite the fact that others sometimes
characterize his position as more definitive.


**Foreman.** Foreman, who also comes from a strong
Platonist position, argues against Woodin's view. He writes
supremely well, and I recommend following the links to his
articles.


**Multiverse view.** This is the view, offered in
opposition to the Basic Platonist Position above, that we
do not have just one concept of set leading to a unique
set-theoretic universe, but rather a complex variety of set
concepts leading to many different set-theoretic worlds.
Indeed, the view is that much of set-theoretic research in
the past half-century has been about constructing these
various alternative worlds. Many of the alternative set
concepts, such as those arising by forcing or by large
cardinal embeddings are closely enough related to each
other that they can be compared from the perspective of
each other. The multiverse view of CH is that the CH
question is largely settled by the fact that we know
precisely how to build CH or $\neg$CH worlds close to any
given set-theoretic universe---the CH and $\neg$CH worlds
are in a sense dense among the set-theoretic universes. The
multiverse view is realist as opposed to formalist, since
it affirms the real nature of the set-theoretic worlds to
which the various set concepts give rise. On the Multiverse
view, the Old-School Dream Solution is impossible, since
our experience in the CH and $\neg$CH worlds will prevent
us from accepting any principle $\Phi$ that settles CH as
"obviously true". Rather, on the multiverse view we are to study all the possible set-theoretic worlds and especially how they relate to each other.


I should stop now, and I apologize for the length of this answer.


