- Erlangen program
An influential research program and manifesto was published in
1872 byFelix Klein , under the title "Vergleichende Betrachtungen über neuere geometrische Forschungen". This Erlangen Program ("Erlanger Programm") — Klein was then atErlangen — proposed a new solution to the problem how to classify and characterize geometries on the basis ofprojective geometry andgroup theory .At that time, a family of new non-Euclidean geometries already emerged, without adequate clarifications of their mutual hierarchy and relationships. What Klein suggested was fundamentally innovative in three ways:
:*
projective geometry was emphasized as the unifying frame for all other geometries considered by him. In particular, the affine, metric, and Euclidean geometries are just special and gradually more restrictive cases of theprojective geometry .:* Klein proposed that
group theory , a branch of mathematics that uses algebraic methods to abstract the idea ofsymmetry , was the most useful way of organizing geometrical knowledge; at the time it had already been introduced into thetheory of equations in the form ofGalois theory .:* Klein made much more explicit the idea that each geometrical language had its own, appropriate concepts, so that for example
projective geometry rightly talked aboutconic section s, but not aboutcircle s orangle s because those notions were not invariant underprojective transformation s (something familiar ingeometrical perspective ). The way the multiple languages of geometry then came back together could be explained by the waysubgroup s of asymmetry group related to each other.The problems of nineteenth century geometry
Was there one 'geometry' or many? Since
Euclid , geometry had meant the geometry ofEuclidean space of two dimensions (plane geometry ) or of three dimensions (solid geometry ). In the first half of the nineteenth century there had been several developments complicating the picture. Mathematical applications required geometry of four or more dimensions; the close scrutiny of the foundations of the traditional Euclidean geometry had revealed the independence of theParallel Axiom from the others, andnon-Euclidean geometry had been born. Klein proposed an idea that all these new geometries are just special cases of theprojective geometry , as already developed byPoncelet ,Möbius ,Cayley and others. Klein also strongly suggested to mathematical "physicists" that even a moderate cultivation of the projective purview might bring substantial benefits to them.With every geometry, Klein associated an underlying group of symmetries. The hierarchy of geometries is thus mathematically represented as a hierarchy of these groups, and hierarchy of their invariants. For example, lengths, angles and areas are preserved with respect to the Euclidean group of symmetries, while only the
incidence structure and thecross-ratio are preserved under the most general projective transformations. A concept ofparallelism , which is preserved inaffine geometry , is not meaningful inprojective geometry . Then, by abstracting the underlying groups of symmetries from the geometries, the relationships between them can be re-established at the group level. Since the group of affine geometry is asubgroup of the group of projective geometry, any notion invariant in projective geometry is "a priori" meaningful in affine geometry; but not the other way round. If you add required symmetries, you have a more powerful theory but fewer concepts and theorems (which will be deeper and more general).Homogeneous spaces
In other words, the "traditional spaces" are
homogeneous space s; but not for a uniquely determined group. Changing the group changes the appropriate geometric language.In today's language, the groups concerned in classical geometry are all very well-known as
Lie group s: theclassical groups . The specific relationships are quite simply described, using technical language.Examples
For example the group of
projective geometry in "n" dimensions is the symmetry group of "n"-dimensionalprojective space (the matrix group of size "n" + 1, quotiented by scalar matrices). Theaffine group will be the subgroup respecting (mapping to itself, not fixing pointwise) the chosenhyperplane at infinity . This subgroup has a known structure (semidirect product of the matrix group of size "n" with the subgroup of translations). This description then tells us which properties are 'affine'. In Euclidean plane geometry terms, being a parallelogram is affine since affine transformations always take one parallelogram to another one. Being a circle is not affine since an affine shear will take a circle into an ellipse.To explain accurately the relationship between affine and Euclidean geometry, we now need to pin down the group of Euclidean geometry within the affine group. The
Euclidean group is in fact (using the previous description of the affine group) the semi-direct product of the orthogonal (rotation and reflection) group with the translations.Influence on later work
The long-term effects of the Erlangen program can be seen all over pure mathematics (see tacit use at
congruence (geometry) , for example); and the idea of transformations and of synthesis using groups of symmetry is of course now standard too inphysics .When
topology is routinely described in terms of properties invariant underhomeomorphism , one can see the underlying idea in operation. The groups involved will be infinite-dimensional in almost all cases - and notLie group s - but the philosophy is the same. Of course this mostly speaks to the pedagogical influence of Klein. Books such as those byH.S.M. Coxeter routinely used the Erlangen program approach to help 'place' geometries. In pedagogic terms, the program becametransformation geometry , a mixed blessing in the sense that it builds on stronger intuitions than the style ofEuclid , but is less easily converted into alogical system .In his book "Structuralism" (1970)
Jean Piaget says, "In the eyes of contemporary structuralist mathematicians, likeBourbaki , the Erlangen Program amounts to only a partial victory for structuralism, since they want to subordinate all mathematics, not just geometry, to the idea of structure."For a geometry and its group, an element of the group is sometimes called a motion of the geometry. For example, one can learn about the
Poincaré half-plane model ofhyperbolic geometry through a development based onhyperbolic motion s. Such a development enables one to methodically prove theultraparallel theorem by successive motions.The Erlangen Program is carried into mathematical logic by
Alfred Tarski in his analysis of propositional truth.Abstract returns from the Erlangen program
Quite often, it appears there are two or more distinct geometries with
isomorphic automorphism group s. There arises the question of reading the Erlangen program from the "abstract" group, to the geometry.One example:
oriented (i.e., reflections not included)elliptic geometry (i.e., the surface of ann-sphere with opposite points identified) andoriented spherical geometry (the same non-Euclidean geometry, but with opposite points not identified) haveisomorphic automorphism group , SO(n+1) for even "n". These may appear to be distinct. It turns out, however, that the geometries are very closely related, in a way that can be made precise.To take another example, elliptic geometries with different radii of curvature have
isomorphic automorphism group s. That does not really count as a critique as all such geometries areisomorphic . GeneralRiemannian geometry falls outside the boundaries of the program.Some further notable examples have come up in physics.
Firstly, "n"-dimensional
hyperbolic geometry , "n"-dimensionalde Sitter space and ("n"−1)-dimensionalinversive geometry all have isomorphicautomorphism group s,:O(n,1)/mathbb{Z}_2,
the
orthochronous Lorentz group , for "n" ≥ 3. But these are apparently distinct geometries. Here some interesting results enter, from the physics. It has been shown that physics models in each of the three geometries are "dual" for some models.Again, "n"-dimensional
anti de Sitter space and ("n"−1)-dimensionalconformal space with "Lorentzian" signature (in contrast withconformal space with "Euclidean" signature, which is identical toinversive geometry , for 3 dimensions or greater) haveisomorphic automorphism group s, but are distinct geometries. Once again, there are models in physics with "dualities" between both spaces. SeeAdS/CFT for more details.More intriguingly, the covering group of SU(2,2) is isomorphic to the covering group of SO(4,2), which is the symmetry group of a 4D conformal Minkowski space and a 5D anti de Sitter space AND a complex four dimensional
twistor space .The Erlangen program can therefore still be considered fertile, in relation with dualities in physics.
ee also
*
homogeneous space
*Klein geometry References
*Guggenheimer, Heinrich, 1977. "Differential Geometry", Dover, NY, ISBN 0-486-63433-7. :An inexpensive book that's still in print, not too difficult, with many references to Lie, Klein and Cartan. P. 139, "A Klein geometry is the theory of geometric invariants of a transitive transformation group (Erlangen program, 1872)".
*
Felix Klein , 1872. "Vergleichende Betrachtungen über neuere geometrische Forschungen" ('A comparative review of recent researches in geometry'), Mathematische Annalen, 43 (1893) pp. 63-100 (Also: Gesammelte Abh. Vol. 1, Springer, 1921, pp. 460-497).:An English translation by Mellen Haskell appeared in "Bull. N. Y. Math. Soc" 2 (1892-1893): 215--249.:The original German text of the Erlangen Program can be viewed at the University of Michigan online collection at [http://www.hti.umich.edu/cgi/t/text/text-idx?c=umhistmath;idno=ABN7632] , and also at [http://www.xs4all.nl/~jemebius/ErlangerProgramm.htm] in HTML format.:A central information page on the Erlangen Program maintained by John Baez is at [http://math.ucr.edu/home/baez/erlangen/] .
*
Felix Klein , 2004 "Elementary Mathematics from an Advanced Standpoint: Geometry", Dover, NY, ISBN 0-486-43481-8:(translation of Elementarmathematik vom höheren Standpunkte aus, Teil II: Geometrie, pub. 1924 by Springer). Not a hard book. Has a section on the Erlangen Program.
Wikimedia Foundation. 2010.