Weyl pondered on the reasons why the structure group of the physical automorphisms still contained the “Euclidean rotation group” (respectively the Lorentz group) in such a prominent role:
The Euclidean group of rotations has survived even such radical changes of our concepts of the physical world as general relativity and quantum theory. What then are the peculiar merits of this group to which it owes its elevation to the basic group pattern of the universe? For what ‘sufficient reasons’ did the Creator choose this group and no other?”
He reminded that Helmholtz had characterized ∆o ≅ SO (3, ℜ) by the “fact that it gives to a rotating solid what we may call its just degrees of freedom” of a rotating solid body; but this method “breaks down for the Lorentz group that in the four-dimensional world takes the place of the orthogonal group in 3-space”. In the early 1920s he himself had given another characterization living up to the new demands of the theories of relativity in his mathematical analysis of the problem of space.
He mentioned the idea that the Lorentz group might play its prominent role for the physical automorphisms because it expresses deep lying matter structures; but he strongly qualified the idea immediately after having stated it:
Since we have the dualism of invariance with respect to two groups and Ω certainly refers to the manifold of space points, it is a tempting idea to ascribe ∆o to matter and see in it a characteristic of the localizable elementary particles of matter. I leave it undecided whether this idea, the very meaning of which is somewhat vague, has any real merits.
. . . But instead of analysing the structure of the orthogonal group of transformations ∆o, it may be wiser to look for a characterization of the group ∆o as an abstract group. Here we know that the homogeneous n-dimensional orthogonal groups form one of 3 great classes of simple Lie groups. This is at least a partial solution of the problem.
He left it open why it ought to be “wiser” to look for abstract structure properties in order to answer a natural philosophical question. Could it be that he wanted to indicate an open-mindedness toward the more structuralist perspective on automorphism groups, preferred by the young algebraists around him at Princetion in the 1930/40s? Today the classification of simple Lie groups distinguishes 4 series, Ak,Bk,Ck,Dk. Weyl apparently counted the two orthogonal series Bk and Dk as one. The special orthogonal groups in even complex space dimension form the series of simple Lie groups of type Dk, with complex form (SO 2k,C) and real compact form (SO 2k,ℜ). The special orthogonal group in odd space dimension form the series type Bk, with complex form SO(2k + 1, C) and compact real form SO(2k + 1, ℜ).
But even if one accepted such a general structuralist view as a starting point there remained a question for the specification of the space dimension of the group inside the series.
But the number of the dimensions of the world is 4 and not an indeterminate n. It is a fact that the structure of ∆o is quite different for the various dimensionalities n. Hence the group may serve as a clue by which to discover some cogent reason for the di- mensionality 4 of the world. What must be brought to light, is the distinctive character of one definite group, the four-dimensional Lorentz group, either as a group of linear transformations, or as an abstract group.
The remark that the “structure of ∆o is quite different for the various dimensionalities n” with regard to even or odd complex space dimensions (type Dk, resp. Bk) strongly qualifies the import of the general structuralist characterization. But already in the 1920s Weyl had used the fact that for the (real) space dimension n “4 the universal covering of the unity component of the Lorentz group SO (1, 3)o is the realification of SL (2, C). The latter belongs to the first of the Ak series (with complex form SL (k + 1,C). Because of the isomorphism of the initial terms of the series, A1 ≅ B1, this does not imply an exception of Weyl’s general statement. We even may tend to interpret Weyl’s otherwise cryptic remark that the structuralist perspective gives a “at least a partial solution of the problem” by the observation that the Lorentz group in dimension n “4 is, in a rather specific way, the realification of the complex form of one of the three most elementary non-commutative simple Lie groups of type A1 ≅ B1. Its compact real form is SO (3, ℜ), respectively the latter’s universal cover SU (2, C).
Weyl stated clearly that the answer cannot be expected by structural considerations alone. The problem is only “partly one of pure mathematics”, the other part is “empirical”. But the question itself appeared of utmost importance to him
We can not claim to have understood Nature unless we can establish the uniqueness of the four-dimensional Lorentz group in this sense. It is a fact that many of the known laws of nature can at once be generalized to n dimensions. We must dig deep enough until we hit a layer where this is no longer the case.
In 1918 he had given an argument why, in the framework of his new scale gauge geometry, the “world” had to be of dimension 4. His argument had used the construction of the Lagrange density of general relativistic Maxwell theory Lf = fμν fμν √(|detg|), with fμν the components of curvature of his newly introduced scale/length connection, physically interpreted by him as the electromagnetic field. Lf is scale invariant only in spacetime dimension n = 4. The shift from scale gauge to phase gauge undermined the importance of this argument. Although it remained correct mathematically, it lost its convincing power once the scale gauge transformations were relegated from physics to the mathematical automorphism group of the theory only.
Our question has this in common with most questions of philosophical nature: it depends on the vague distinction between essential and non-essential. Several competing solutions are thinkable; but it may also happen that, once a good solution of the problem is found, it will be of such cogency as to command general recognition.