# Kurt Gödel

**Kurt
Gödel** (April 28, 1906 - January 14, 1978) was a mathematician whose
biography lists quite a few nations, although he is usually associated with Austria.
He was born in Austria-Hungary (which broke up after World War I), became Czechoslovak
citizen at age 12, and Austrian citizen at age 23. When Austrian-born Hitler annexed
Austria, Gödel automatically became German at age 32. After WW-II, at age 42,
he also obtained US citizenship in addition to his Austrian one.

He was a deep logician whose most famous work was the Incompleteness Theorem stating that any self-consistent axiomatic system powerful enough to describe integer arithmetic will allow for propositions about integers that can neither be proven nor disproven from the axioms. He also produced celebrated work on the Continuum hypothesis, showing that it cannot be disproven from the accepted set theory axioms, assuming that those axioms are consistent. Also of note is Gödel's ontological proof, a formalization of St. Anselm's ontological argument for God's existence.

Arguably, Kurt Gödel is the greatest logician of the 20th century and one of the three greatest logicians of all time, with the other two of this historical triumvirate being Aristotle and Frege. He published his most important result in 1931 at age 25 when he worked at Vienna University, Austria.

## Short biography

### Childhood

Kurt Gödel
was born April 28, 1906, in Brünn (today Brno), Austria-Hungary (now Czech Republic)
as the son of the manager of a textile factory. In his family little Kurt was
known as *Der Herr Warum* (Mr. Why). He attended German-language primary
and secondary school in Brno and completed them with honors in 1923. Although
Kurt had first excelled in learning languages he later became more fond of history
and mathematics. His interest in mathematics increased when in 1920 his older
brother Rudolf (born 1902) left for Vienna to go to Medical School at the University
of Vienna (UV). Already during his teens Kurt studied Gabelsberger shorthand,
Goethe's theory of colors and criticisms of Isaac Newton, and the writings of
Kant.

### Studying in Vienna

At the
age of 18 Kurt joined his brother Rudolf in Vienna and entered the UV. By that
time he had already mastered university-level mathematics. Although initially
intending to study theoretical physics he also attended courses on mathematics
and philosophy. During this time he adopted ideas of mathematical realism. He
read Kant's *Metaphysische Anfangsgründe der Naturwissenschaft*, and participated
in the Vienna Circle with Moritz Schlick, Hans Hahn, and Rudolf Carnap. Kurt then
studied number theory, but when he took part in a seminar run by Moritz Schlick
which studied Bertrand Russell's book *Introduction to mathematical philosophy*
he became interested in mathematical logic.

While at UV Kurt met his future wife Adele Nimbursky (née Porkert). He started to publish papers on logic and attended a lecture by David Hilbert in Bologna on completeness and consistency of mathematical systems. In 1929 Gödel became an Austrian citizen and later that year he completed his doctoral dissertation under Hans Hahn's supervision. In this dissertation he established the completeness of the first-order predicate calculus (also known as Gödel's completeness theorem).

### Working in Vienna

In 1930 a doctorate in Philosophy was granted to Gödel.
He added a combinatorial version to his completeness result, which was published
by the Vienna Academy of Sciences. In 1931 he published his famous Incompleteness
Theorems in *Über formal unentscheidbare Sätze der Principia Mathematica und
verwandter Systeme*. In this article he proved that for any axiomatic system
that is powerful enough to describe the natural numbers it holds that:

- It
cannot be both consistent and complete. (It is this theorem that is generally
known as
*the*Incompleteness Theorem.) - If the system is consistent, then the consistency of the axioms cannot be proved within the system.

In hindsight, the basic idea of the incompleteness theorem is rather simple. Gödel essentially constructed a formula that claims that it is unprovable in a given formal system. If it were provable it would be wrong, so one could prove wrong statements in this system. Otherwise there would be at least one true but unprovable statement.

To make this precise, however, Gödel needed to solve several technical issues, such as encoding proofs and the very concept of provability within integer numbers. Such formal details are the main reason why his 1931 paper is rather long and not so easy to read.

Gödel
earned his Habilitation at the UV in 1932 and in 1933 he became a *Privatdozent*
(unpaid lecturer) there. When in 1933 Hitler came to power in Germany this had
little effect on Gödel's life in Vienna since he had little interest in politics.
However after Schlick, whose seminar had aroused Gödel's interest in logic, was
murdered by a National Socialist student, Gödel was much affected and had his
first nervous breakdown.

### Visiting the USA

In this year he took his first trip to the USA, during which he met Albert Einstein who would become a good friend. He delivered an address to the annual meeting of the American Mathematical Society. During this year he also developed the ideas of computability and recursive functions to the point where he delivered a lecture on general recursive functions and the concept of truth. This work was developed in number theory, using the construction of the Gödel numbers.

In 1934 Gödel
gave a series of lectures at the Institute for Advanced Study (IAS) in Princeton
entitled *On undecidable propositions of formal mathematical systems*.
Stephen Kleene who had just completed his Ph.D. at Princeton, took notes of these
lectures which have been subsequently published.

Gödel would visit the IAS again in the autumn of 1935. The travelling and the hard work had exhausted him and the next year he had to recover from a depression. He returned to teaching in 1937 and during this time he worked on the proof of consistency of the Continuum hypothesis; he would go on to show that this hypothesis cannot be disproved from the common system of axioms of set theory. He married Adele on September 20, 1938. In the autumn of 1938 he visited again the IAS. After this he visited the USA once more in the spring of 1939 at the University of Notre Dame.

### Working in Princeton

After the Anschluss in 1938 Austria had become a part
of Nazi Germany. Since Germany had abolished the title of *Privatdozent*
Gödel would now have to fear conscription into the Nazi army. In January 1940
he and his wife left Europe via the trans-Siberian railway and traveled via Russia
and Japan to the USA. After they arrived in San Francisco on March 4, 1940, Kurt
and Adele settled in Princeton, where he resumed his membership in the IAS. At
the Institute, Gödel's interests turned to philosophy and physics. He studied
the works of Gottfried Leibniz in detail and, to a lesser extent, those of Kant
and Edmund Husserl.

In the late 1940s he demonstrated the existence of paradoxical solutions to Albert Einstein's field equations in general relativity. These "rotating universes" would allow time travel and caused Einstein to have doubts about his own theory.

He also continued to work on logic and in 1940 he published his
work *Consistency of the axiom of choice and of the generalized continuum-hypothesis
with the axioms of set theory* which is a classic of modern mathematics. In
that work he introduced the constructible universe, a model of set theory in which
the only sets which exist are those that can be constructed from simpler sets.
Gödel showed that both the axiom of choice and the generalized continuum hypothesis
are true in the constructible universe, and therefore must be consistent.

He became a permanent member of the IAS in 1946 and in 1948 he was naturalized as an U.S. citizen. He became a full professor at the institute in 1953 and an emeritus professor in 1976.

Gödel was awarded (with another nominee) the first Einstein Award, in 1951, and was also awarded the National Medal of Science, in 1974.

In the early seventies, Gödel, who was deeply religious, circulated among his friends an elaboration on Gottfried Leibniz' ontological proof of God's existence. This is now known as Gödel's ontological proof.

### Death and afterwards

Gödel was a shy and withdrawn person. Towards the end of his life he was extremely concerned about his health; eventually he became convinced that he was being poisoned. To avoid this fate he refused to eat and thus starved himself to death. He died January 14, 1978, in Princeton, New Jersey, USA.

The Kurt Gödel Society (founded in 1987) was named in his honor. It is an international organization for the promotion of research in the areas of logic, philosophy, and the history of mathematics.

## Important publications

*Über formal unentscheidbare Sätze der Principia Mathematica und verwandter Systeme*,*Monatshefte für Mathematik und Physik,*vol. 38 (1931). (Available in English at http://home.ddc.net/ygg/etext/godel/ )*The Consistency of the Axiom of Choice and of the Generalized Continuum Hypothesis with the Axioms of Set Theory.*Princeton University Press, Princeton, NJ. (1940)

## Links and references

### Further reading

- John W. Dawson,
*Logical Dilemmas: The Life and Work of Kurt Godel*, published by A K Peters. (ISBN 1568810253) - Werner Depauli-Schimanovich and John L. Casti,
*Gödel: A Life of Logic*, published by Perseus publishing. (ISBN 0738205184) - Douglas Hofstadter,
*Gödel, Escher, Bach*(ISBN 0465026567) - Ernst Nagel and James R. Newman,
*Gödel's Proof*, published by New York University Press. (ISBN 0-8147-5816-9)