David Hilbert was a German mathematician and one of the most influential and universal mathematicians of the 19th and early 20th centuries. Hilbert discovered and developed a broad range of fundamental ideas in many areas, including invariant theory, calculus of variations, commutative algebra, algebraic number theory, the foundations of geometry, spectral theory of operators and its application to integral equations, mathematical physics, and foundations of mathematics (particularly proof theory).
Hilbert adopted and warmly defended Georg Cantor's set theory and transfinite numbers. A famous example of his leadership in mathematics is his 1900 presentation of a collection of problems that set the course for much of the mathematical research of the 20th century.
Hilbert and his students contributed significantly to establishing rigor and developed important tools used in modern mathematical physics. Hilbert is known as one of the founders of proof theory and mathematical logic, as well as for being among the first to distinguish between mathematics and metamathematics.
Life
Early life and education
Hilbert, the first of two children of Otto and Maria Therese (Erdtmann) Hilbert, was born in the Province of Prussia, Kingdom of Prussia, either in Königsberg (according to Hilbert's own statement) or in Wehlau (known since 1946 as Znamensk) near Königsberg where his father worked at the time of his birth.In late 1872, Hilbert entered the Friedrichskolleg Gymnasium (Collegium fridericianum, the same school that Immanuel Kant had attended 140 years before); but, after an unhappy period, he transferred to (late 1879) and graduated from (early 1880) the more science-oriented Wilhelm Gymnasium. Upon graduation, in autumn 1880, Hilbert enrolled at the University of Königsberg, the "Albertina". In early 1882, Hermann Minkowski (two years younger than Hilbert and also a native of Königsberg but had gone to Berlin for three semesters), returned to Königsberg and entered the university. Hilbert developed a lifelong friendship with the shy, gifted Minkowski.
Career
In 1884, Adolf Hurwitz arrived from Göttingen as an Extraordinarius (i.e., an associate professor). An intense and fruitful scientific exchange among the three began, and Minkowski and Hilbert especially would exercise a reciprocal influence over each other at various times in their scientific careers. Hilbert obtained his doctorate in 1885, with a dissertation, written under Ferdinand von Lindemann, titled Über invariante Eigenschaften spezieller binärer Formen, insbesondere der Kugelfunktionen ("On the invariant properties of special binary forms, in particular the spherical harmonic functions").Hilbert remained at the University of Königsberg as a Privatdozent (senior lecturer) from 1886 to 1895. In 1895, as a result of intervention on his behalf by Felix Klein, he obtained the position of Professor of Mathematics at the University of Göttingen. During the Klein and Hilbert years, Göttingen became the preeminent institution in the mathematical world. He remained there for the rest of his life.
The Mathematical Institute in Göttingen. Its new building, constructed with funds from the Rockefeller Foundation, was opened by Hilbert and Courant in 1930.
Göttingen school
Portrait of David Hilbert in the 1900s, artist Anna Gorban, the cover image of the theme issue ‘Hilbert's sixth problem, Phil. Trans. R. Soc. A 2018, 376 (2118).
Among Hilbert's students were Hermann Weyl, chess champion Emanuel Lasker, Ernst Zermelo, and Carl Gustav Hempel. John von Neumann
was his assistant. At the University of Göttingen, Hilbert was
surrounded by a social circle of some of the most important
mathematicians of the 20th century, such as Emmy Noether and Alonzo Church.
Among his 69 Ph.D. students in Göttingen were many who later became famous mathematicians, including (with date of thesis): Otto Blumenthal (1898), Felix Bernstein (1901), Hermann Weyl (1908), Richard Courant (1910), Erich Hecke (1910), Hugo Steinhaus (1911), and Wilhelm Ackermann (1925). Between 1902 and 1939 Hilbert was editor of the Mathematische Annalen, the leading mathematical journal of the time.
"Good, he did not have enough imagination to become a mathematician".
— Hilbert's response upon hearing that one of his students had dropped out to study poetry.
Later years
Around 1925, Hilbert developed pernicious anemia, a then-untreatable vitamin deficiency whose primary symptom is exhaustion; his assistant Eugene Wigner
described him as subject to "enormous fatigue" and how he "seemed quite
old", and that even after eventually being diagnosed and treated, he
"was hardly a scientist after 1925, and certainly not a Hilbert."
Hilbert lived to see the Nazis purge many of the prominent faculty members at University of Göttingen in 1933. Those forced out included Hermann Weyl (who had taken Hilbert's chair when he retired in 1930), Emmy Noether and Edmund Landau. One who had to leave Germany, Paul Bernays, had collaborated with Hilbert in mathematical logic, and co-authored with him the important book Grundlagen der Mathematik (which eventually appeared in two volumes, in 1934 and 1939). This was a sequel to the Hilbert-Ackermann book Principles of Mathematical Logic from 1928. Hermann Weyl's successor was Helmut Hasse.
About a year later, Hilbert attended a banquet and was seated next to the new Minister of Education, Bernhard Rust. Rust asked whether "the Mathematical Institute really suffered so much because of the departure of the Jews". Hilbert replied,
"Suffered? It doesn't exist any longer, does it!"
Death
Hilbert's tomb:
Wir müssen wissen
Wir werden wissen
Wir müssen wissen
Wir werden wissen
By the time Hilbert died in 1943, the Nazis had nearly completely
restaffed the university, as many of the former faculty had either been
Jewish or married to Jews. Hilbert's funeral was attended by fewer than a
dozen people, only two of whom were fellow academics, among them Arnold Sommerfeld, a theoretical physicist and also a native of Königsberg. News of his death only became known to the wider world six months after he died.
The epitaph on his tombstone in Göttingen consists of the famous
lines he spoke at the conclusion of his retirement address to the
Society of German Scientists and Physicians on 8 September 1930. The
words were given in response to the Latin maxim: "Ignoramus et ignorabimus" or "We do not know, we shall not know":
- Wir müssen wissen.
- Wir werden wissen.
In English:
- We must know.
- We will know.
The day before Hilbert pronounced these phrases at the 1930 annual meeting of the Society of German Scientists and Physicians, Kurt Gödel—in
a round table discussion during the Conference on Epistemology held
jointly with the Society meetings—tentatively announced the first
expression of his incompleteness theorem. Gödel's incompleteness theorems show that even elementary axiomatic systems such as Peano arithmetic are either self-contradicting or contain logical propositions that are impossible to prove or disprove.
Personal life
Käthe Hilbert with Constantin Carathéodory, before 1932
In 1892, Hilbert married Käthe Jerosch (1864–1945), "the daughter of a
Königsberg merchant, an outspoken young lady with an independence of
mind that matched his own". While at Königsberg they had their one child, Franz Hilbert (1893–1969).
Hilbert's son Franz suffered throughout his life from an
undiagnosed mental illness. His inferior intellect was a terrible
disappointment to his father and this misfortune was a matter of
distress to the mathematicians and students at Göttingen.
Hilbert considered the mathematician Hermann Minkowski to be his "best and truest friend".
Hilbert was baptized and raised a Calvinist in the Prussian Evangelical Church. He later on left the Church and became an agnostic. He also argued that mathematical truth was independent of the existence of God or other a priori assumptions.
Hilbert solves Gordan's Problem
Hilbert's first work on invariant functions led him to the demonstration in 1888 of his famous finiteness theorem. Twenty years earlier, Paul Gordan had demonstrated the theorem
of the finiteness of generators for binary forms using a complex
computational approach. Attempts to generalize his method to functions
with more than two variables failed because of the enormous difficulty
of the calculations involved. In order to solve what had become known in
some circles as Gordan's Problem, Hilbert realized that it was necessary to take a completely different path. As a result, he demonstrated Hilbert's basis theorem, showing the existence of a finite set of generators, for the invariants of quantics in any number of variables, but in an abstract form. That is, while demonstrating the existence of such a set, it was not a constructive proof — it did not display "an object" — but rather, it was an existence proof and relied on use of the law of excluded middle in an infinite extension.
Hilbert sent his results to the Mathematische Annalen. Gordan, the house expert on the theory of invariants for the Mathematische Annalen,
could not appreciate the revolutionary nature of Hilbert's theorem and
rejected the article, criticizing the exposition because it was
insufficiently comprehensive. His comment was:
- Das ist nicht Mathematik. Das ist Theologie.
- (This is not Mathematics. This is Theology.)
Klein,
on the other hand, recognized the importance of the work, and
guaranteed that it would be published without any alterations.
Encouraged by Klein, Hilbert extended his method in a second article,
providing estimations on the maximum degree of the minimum set of
generators, and he sent it once more to the Annalen. After having read the manuscript, Klein wrote to him, saying:
- Without doubt this is the most important work on general algebra that the Annalen has ever published.
Later, after the usefulness of Hilbert's method was universally recognized, Gordan himself would say:
- I have convinced myself that even theology has its merits.
For all his successes, the nature of his proof stirred up more trouble than Hilbert could have imagined at the time. Although Kronecker
had conceded, Hilbert would later respond to others' similar criticisms
that "many different constructions are subsumed under one fundamental
idea" — in other words (to quote Reid): "Through a proof of existence,
Hilbert had been able to obtain a construction"; "the proof" (i.e. the
symbols on the page) was "the object". Not all were convinced. While Kronecker would die soon afterwards, his constructivist philosophy would continue with the young Brouwer and his developing intuitionist "school", much to Hilbert's torment in his later years. Indeed, Hilbert would lose his "gifted pupil" Weyl
to intuitionism — "Hilbert was disturbed by his former student's
fascination with the ideas of Brouwer, which aroused in Hilbert the
memory of Kronecker".[33]
Brouwer the intuitionist in particular opposed the use of the Law of
Excluded Middle over infinite sets (as Hilbert had used it). Hilbert
would respond:
- Taking the Principle of the Excluded Middle from the mathematician ... is the same as ... prohibiting the boxer the use of his fists.
Axiomatization of geometry
The text Grundlagen der Geometrie (tr.: Foundations of Geometry) published by Hilbert in 1899 proposes a formal set, called Hilbert's axioms, substituting for the traditional axioms of Euclid. They avoid weaknesses identified in those of Euclid,
whose works at the time were still used textbook-fashion. It is
difficult to specify the axioms used by Hilbert without referring to the
publication history of the Grundlagen since Hilbert changed and
modified them several times. The original monograph was quickly followed
by a French translation, in which Hilbert added V.2, the Completeness
Axiom. An English translation, authorized by Hilbert, was made by E.J.
Townsend and copyrighted in 1902.
This translation incorporated the changes made in the French
translation and so is considered to be a translation of the 2nd edition.
Hilbert continued to make changes in the text and several editions
appeared in German. The 7th edition was the last to appear in Hilbert's
lifetime. New editions followed the 7th, but the main text was
essentially not revised.
Hilbert's approach signaled the shift to the modern axiomatic method. In this, Hilbert was anticipated by Moritz Pasch's work from 1882. Axioms are not taken as self-evident truths. Geometry may treat things,
about which we have powerful intuitions, but it is not necessary to
assign any explicit meaning to the undefined concepts. The elements,
such as point, line, plane, and others, could be substituted, as Hilbert is reported to have said to Schoenflies and Kötter, by tables, chairs, glasses of beer and other such objects. It is their defined relationships that are discussed.
Hilbert first enumerates the undefined concepts: point, line,
plane, lying on (a relation between points and lines, points and planes,
and lines and planes), betweenness, congruence of pairs of points (line segments), and congruence of angles. The axioms unify both the plane geometry and solid geometry of Euclid in a single system.
The 23 problems
Hilbert put forth a most influential list of 23 unsolved problems at the International Congress of Mathematicians in Paris
in 1900. This is generally reckoned as the most successful and deeply
considered compilation of open problems ever to be produced by an
individual mathematician.
After re-working the foundations of classical geometry, Hilbert
could have extrapolated to the rest of mathematics. His approach
differed, however, from the later 'foundationalist' Russell-Whitehead or
'encyclopedist' Nicolas Bourbaki, and from his contemporary Giuseppe Peano.
The mathematical community as a whole could enlist in problems, which
he had identified as crucial aspects of the areas of mathematics he took
to be key.
The problem set was launched as a talk "The Problems of
Mathematics" presented during the course of the Second International
Congress of Mathematicians held in Paris. The introduction of the speech
that Hilbert gave said:
- Who among us would not be happy to lift the veil behind which is hidden the future; to gaze at the coming developments of our science and at the secrets of its development in the centuries to come? What will be the ends toward which the spirit of future generations of mathematicians will tend? What methods, what new facts will the new century reveal in the vast and rich field of mathematical thought?
He presented fewer than half the problems at the Congress, which were
published in the acts of the Congress. In a subsequent publication, he
extended the panorama, and arrived at the formulation of the
now-canonical 23 Problems of Hilbert. See also Hilbert's twenty-fourth problem.
The full text is important, since the exegesis of the questions still
can be a matter of inevitable debate, whenever it is asked how many have
been solved.
Some of these were solved within a short time. Others have been
discussed throughout the 20th century, with a few now taken to be
unsuitably open-ended to come to closure. Some even continue to this day
to remain a challenge for mathematicians.
Formalism
In an account that had become standard by the mid-century, Hilbert's
problem set was also a kind of manifesto, that opened the way for the
development of the formalist
school, one of three major schools of mathematics of the 20th century.
According to the formalist, mathematics is manipulation of symbols
according to agreed upon formal rules. It is therefore an autonomous
activity of thought. There is, however, room to doubt whether Hilbert's
own views were simplistically formalist in this sense.
Hilbert's program
In 1920 he proposed explicitly a research project (in metamathematics, as it was then termed) that became known as Hilbert's program. He wanted mathematics to be formulated on a solid and complete logical foundation. He believed that in principle this could be done, by showing that:
- all of mathematics follows from a correctly chosen finite system of axioms; and
- that some such axiom system is provably consistent through some means such as the epsilon calculus.
He seems to have had both technical and philosophical reasons for
formulating this proposal. It affirmed his dislike of what had become
known as the ignorabimus, still an active issue in his time in German thought, and traced back in that formulation to Emil du Bois-Reymond.
This program is still recognizable in the most popular philosophy of mathematics, where it is usually called formalism. For example, the Bourbaki group
adopted a watered-down and selective version of it as adequate to the
requirements of their twin projects of (a) writing encyclopedic
foundational works, and (b) supporting the axiomatic method
as a research tool. This approach has been successful and influential
in relation with Hilbert's work in algebra and functional analysis, but
has failed to engage in the same way with his interests in physics and
logic.
Hilbert wrote in 1919:
- We are not speaking here of arbitrariness in any sense. Mathematics is not like a game whose tasks are determined by arbitrarily stipulated rules. Rather, it is a conceptual system possessing internal necessity that can only be so and by no means otherwise.
Hilbert published his views on the foundations of mathematics in the 2-volume work Grundlagen der Mathematik.
Gödel's work
Hilbert and the mathematicians who worked with him in his enterprise
were committed to the project. His attempt to support axiomatized
mathematics with definitive principles, which could banish theoretical
uncertainties, ended in failure.
Gödel
demonstrated that any non-contradictory formal system, which was
comprehensive enough to include at least arithmetic, cannot demonstrate
its completeness by way of its own axioms. In 1931 his incompleteness theorem
showed that Hilbert's grand plan was impossible as stated. The second
point cannot in any reasonable way be combined with the first point, as
long as the axiom system is genuinely finitary.
Nevertheless, the subsequent achievements of proof theory at the very least clarified
consistency as it relates to theories of central concern to
mathematicians. Hilbert's work had started logic on this course of
clarification; the need to understand Gödel's work then led to the
development of recursion theory and then mathematical logic as an autonomous discipline in the 1930s. The basis for later theoretical computer science, in the work of Alonzo Church and Alan Turing, also grew directly out of this 'debate'.
Functional analysis
Around 1909, Hilbert dedicated himself to the study of differential and integral equations;
his work had direct consequences for important parts of modern
functional analysis. In order to carry out these studies, Hilbert
introduced the concept of an infinite dimensional Euclidean space, later called Hilbert space.
His work in this part of analysis provided the basis for important
contributions to the mathematics of physics in the next two decades,
though from an unanticipated direction.
Later on, Stefan Banach amplified the concept, defining Banach spaces. Hilbert spaces are an important class of objects in the area of functional analysis, particularly of the spectral theory of self-adjoint linear operators, that grew up around it during the 20th century.
Physics
Until 1912, Hilbert was almost exclusively a "pure" mathematician.
When planning a visit from Bonn, where he was immersed in studying
physics, his fellow mathematician and friend Hermann Minkowski
joked he had to spend 10 days in quarantine before being able to visit
Hilbert. In fact, Minkowski seems responsible for most of Hilbert's
physics investigations prior to 1912, including their joint seminar in
the subject in 1905.
In 1912, three years after his friend's death, Hilbert turned his
focus to the subject almost exclusively. He arranged to have a
"physics tutor" for himself. He started studying kinetic gas theory and moved on to elementary radiation
theory and the molecular theory of matter. Even after the war started
in 1914, he continued seminars and classes where the works of Albert Einstein and others were followed closely.
By 1907 Einstein had framed the fundamentals of the theory of
gravity, but then struggled for nearly 8 years with a confounding
problem of putting the theory into final form. By early summer 1915, Hilbert's interest in physics had focused on general relativity, and he invited Einstein to Göttingen to deliver a week of lectures on the subject. Einstein received an enthusiastic reception at Göttingen.
Over the summer Einstein learned that Hilbert was also working on the
field equations and redoubled his own efforts. During November 1915
Einstein published several papers culminating in "The Field Equations of
Gravitation".
Nearly simultaneously David Hilbert published "The Foundations of
Physics", an axiomatic derivation of the field equations (see Einstein–Hilbert action).
Hilbert fully credited Einstein as the originator of the theory, and no
public priority dispute concerning the field equations ever arose
between the two men during their lives.
Additionally, Hilbert's work anticipated and assisted several advances in the mathematical formulation of quantum mechanics. His work was a key aspect of Hermann Weyl and John von Neumann's work on the mathematical equivalence of Werner Heisenberg's matrix mechanics and Erwin Schrödinger's wave equation and his namesake Hilbert space
plays an important part in quantum theory. In 1926 von Neumann showed
that if atomic states were understood as vectors in Hilbert space, then
they would correspond with both Schrödinger's wave function theory and
Heisenberg's matrices.
Throughout this immersion in physics, Hilbert worked on putting
rigor into the mathematics of physics. While highly dependent on higher
mathematics, physicists tended to be "sloppy" with it. To a "pure"
mathematician like Hilbert, this was both "ugly" and difficult to
understand. As he began to understand physics and how physicists were
using mathematics, he developed a coherent mathematical theory for what
he found, most importantly in the area of integral equations. When his colleague Richard Courant wrote the now classic Methoden der mathematischen Physik
(Methods of Mathematical Physics) including some of Hilbert's ideas, he
added Hilbert's name as author even though Hilbert had not directly
contributed to the writing. Hilbert said "Physics is too hard for
physicists", implying that the necessary mathematics was generally
beyond them; the Courant-Hilbert book made it easier for them.
Number theory
Hilbert unified the field of algebraic number theory with his 1897 treatise Zahlbericht (literally "report on numbers"). He also resolved a significant number-theory problem formulated by Waring in 1770. As with the finiteness theorem,
he used an existence proof that shows there must be solutions for the
problem rather than providing a mechanism to produce the answers. He then had little more to publish on the subject; but the emergence of Hilbert modular forms in the dissertation of a student means his name is further attached to a major area.
He made a series of conjectures on class field theory. The concepts were highly influential, and his own contribution lives on in the names of the Hilbert class field and of the Hilbert symbol of local class field theory. Results were mostly proved by 1930, after work by Teiji Takagi.
Hilbert did not work in the central areas of analytic number theory, but his name has become known for the Hilbert–Pólya conjecture, for reasons that are anecdotal.
Works
His collected works (Gesammelte Abhandlungen) have been published several times. The original versions of his papers contained "many technical errors of varying degree";
when the collection was first published, the errors were corrected and
it was found that this could be done without major changes in the
statements of the theorems, with one exception—a claimed proof of the continuum hypothesis. The errors were nonetheless so numerous and significant that it took Olga Taussky-Todd three years to make the corrections.
