Universal Kernel

commit algebra

Which operators commute with every move at a point of the Fano plane?

At a unit epe_p of the octonions, the algebra generated by the six left multiplications by the other units, each tensored with a flip of a two-state counter, and by the counter’s sign. It is the complex Clifford algebra Cl7\mathrm{Cl}_7, a sum of two matrix algebras, and its centre is spanned by the identity and one sign, the chirality D=−iLepD=-iL_{e_p} read with the parity of the number of moves.

1234567a1 = (2, 3)a2 = (4, 5)a3 = (6, 7)the counterZ = +1Z = −1I ⊗ Z, in the place of 1six Leₐ ⊗ X at the other points(−1)N = ±DCl7 on seven generators, centre spanned by I and Dage = D ⊗ Z
Plate 6.3The commit algebra at the point 1: the six other points are the six generators Lea⊗XL_{e_a}\otimes X, and the three lines through 1 pair them into the three modes whose occupation NN gives (−1)N=±D(-1)^N=\pm D.
Proposition(Each letter reverses DD)

In C⊗O\C\otimes\Oct fix an imaginary unit epe_p and put D=−iLepD=-iL_{e_p}. For each of the six imaginary units eae_a orthogonal to epe_p, Lea2=−IL_{e_a}^2=-I and LeaL_{e_a} anticommutes with LepL_{e_p}, hence with DD; the product of the six is ±Lep\pm L_{e_p}. These statements do not depend on the signs of the units.

So along a word of moves of the tetrahedron T0T_0, each paired with the left multiplication of a unit orthogonal to epe_p and acting on C2⊗(C⊗O)\C^2\otimes(\C\otimes\Oct), the first factor keeps the class of the spinor system VV while DD changes sign at each move: the product of the class and DD is reversed by every move, and after nn moves the preserved operator is (−1)nD(-1)^nD.

Proof

By alternativity LxLy+LyLx=Lxy+yxL_xL_y+L_yL_x=L_{xy+yx}, which is −2⟨x,y⟩I-2\langle x,y\rangle I for imaginary x,yx,y. The seven LeaL_{e_a} therefore generate a Clifford algebra on R8\R^8, whose volume element is central of square 1 and so acts as ±I\pm I; the product of six is ±Lep−1=∓Lep\pm L_{e_p}^{-1}=\mp L_{e_p}. Changing the sign of a unit changes only signs.

Definition(The commit algebra)

Fix a point pp of the Fano plane, with unit epe_p, and the table AA. For the three lines {p,q,r}\{p,q,r\} through pp, ordered so that eqer=epe_qe_r=e_p, put aj=(Leq+iLer)/2a_j=(L_{e_q}+iL_{e_r})/2 and N=∑jaj†ajN=\sum_ja_j^\dagger a_j, and on a second factor C2\C^2 write XX and ZZ for the Pauli matrices. The commit algebra at epe_p is

Ap=alg{Lea⊗X (a≠p), I⊗Z}⊂End((C⊗O)⊗C2),\mathcal A_p=\mathrm{alg}\bigl\{L_{e_a}\otimes X\ (a\neq p),\ I\otimes Z\bigr\}\subset\mathrm{End}\bigl((\C\otimes\Oct)\otimes\C^2\bigr),

and Dage=D⊗ZD_{\mathrm{age}}=D\otimes Z.

Theorem(The commit algebra is Cl7\mathrm{Cl}_7 with two sectors)

For each of the seven points: (1) the six LeaL_{e_a}, a≠pa\neq p, generate M8(C)M_8(\C), with commutant C\C; (2) the seven generators of Ap\mathcal A_p anticommute in pairs, six square to −I-I and one to +I+I, and their product is ±Lep⊗Z\pm L_{e_p}\otimes Z, so Ap\mathcal A_p is the complex Clifford algebra Cl7\mathrm{Cl}_7, of dimension 128; (3) the commutant of Ap\mathcal A_p is its centre, span{I,Dage}\mathrm{span}\{I,D_{\mathrm{age}}\}, and Ap≅M8(C)⊕M8(C)\mathcal A_p\cong M_8(\C)\oplus M_8(\C), with central projections P±=12(I±Dage)P_\pm=\tfrac12(I\pm D_{\mathrm{age}}) of rank 8; (4) (−1)N=±D(-1)^N=\pm D, so Dage=±(−1)N⊗ZD_{\mathrm{age}}=\pm(-1)^N\otimes Z; (5) complex conjugation of C⊗O\C\otimes\Oct fixes every generator and exchanges P+P_+ with P−P_-.

Proof

(1) The six LeaL_{e_a} anticommute and square to −I-I, so they generate a quotient of Cl6≅M8(C)\mathrm{Cl}_6\cong M_8(\C), which is simple; on C8\C^8 the image is all of M8(C)M_8(\C). (2) (Lea⊗X)(Leb⊗X)=LeaLeb⊗I(L_{e_a}\otimes X)(L_{e_b}\otimes X)=L_{e_a}L_{e_b}\otimes I anticommutes for a≠ba\neq b, each Lea⊗XL_{e_a}\otimes X anticommutes with I⊗ZI\otimes Z, and the six LeaL_{e_a} multiply to ±Lep\pm L_{e_p}. (3) On an odd number of generators the centre of the complex Clifford algebra is spanned by II and the volume element, here ±iDage\pm iD_{\mathrm{age}}; since tr⁡Dage=0\operatorname{tr}D_{\mathrm{age}}=0, both simple summands act. (4) 1−2aj†aj=iLeqLer1-2a_j^\dagger a_j=iL_{e_q}L_{e_r}, so (−1)N=i3∏jLeqjLerj=∓iLep(-1)^N=i^3\prod_jL_{e_{q_j}}L_{e_{r_j}}=\mp iL_{e_p}. (5) The generators are real matrices, and Dage=−iLep⊗ZD_{\mathrm{age}}=-iL_{e_p}\otimes Z is imaginary. All five were also checked for all seven points, the dimensions by ranks over a prime field and the commutants with explicit commuting elements.

Remark(The counter in the clock’s place)

On O\Oct the seven LexL_{e_x} anticommute and their product is ±I\pm I, since the six LeaL_{e_a} multiply to ±Lep\pm L_{e_p} and Lep2=−IL_{e_p}^2=-I. So O\Oct is a single module of Cl7\mathrm{Cl}_7, whose volume element is a scalar, and there is one sector. The commit algebra rebuilds Cl7\mathrm{Cl}_7 from six of the seven and I⊗ZI\otimes Z, which stands in the place of LepL_{e_p}; its volume element is then ±Lep⊗Z\pm L_{e_p}\otimes Z, which is not a scalar, and its two eigenspaces are the sectors.

Replacing C2\C^2 by ℓ2(N)\ell^2(\mathbb N), with the generators Lea⊗SL_{e_a}\otimes S for the shift SS and every function of the count, leaves the commutant spanned by II and D⊗(−1)nD\otimes(-1)^n: an element commuting with every function of the count is ⨁nAn\bigoplus_nA_n, commuting with every Lea⊗SL_{e_a}\otimes S gives An+1=LeaAnLea−1A_{n+1}=L_{e_a}A_nL_{e_a}^{-1}, so AnA_n commutes with the even products and An=αI+(−1)nβDA_n=\alpha I+(-1)^n\beta D. The parity carries all of the counter’s contribution.

Proposition(What the centre leaves out)

(1) Each LeaL_{e_a}, a≠pa\neq p, changes NN by ±1\pm1. A function of NN, tensored with II or with ZZ, commutes with Ap\mathcal A_p only if it lies in span{I,(−1)N⊗Z}\mathrm{span}\{I,(-1)^N\otimes Z\}, and the products LeaLebL_{e_a}L_{e_b} generate M4(C)M_4(\C) on each eigenspace of DD. (2) With u=e0+⋯+e6u=e_0+\dots+e_6, LeaLuLea−1=2Lea−LuL_{e_a}L_uL_{e_a}^{-1}=2L_{e_a}-L_u for every imaginary unit, so no LeaL_{e_a} sends J0=Lu/7J_0=L_u/\sqrt7 to ±J0\pm J_0; J0⊗IJ_0\otimes I does not commute with DageD_{\mathrm{age}}, so it is not in Ap\mathcal A_p. (3) LuLep+LepLu=−2IL_uL_{e_p}+L_{e_p}L_u=-2I for every point, so the +i+i-eigenspaces WW of J0J_0 and QQ of LepL_{e_p} are isoclinic: PWPQPW=12(1±17)PWP_WP_QP_W=\tfrac12\bigl(1\pm\tfrac1{\sqrt7}\bigr)P_W, the sign depending on conventions.

With a further factor C2\C^2 for the spinor, each of the two sectors has dimension 16. In the gauge in which (−1)N=−D(-1)^N=-D, the sector Dage=−1D_{\mathrm{age}}=-1 holds Z=+1Z=+1, D=−1D=-1, N∈{0,2}N\in\{0,2\}, which is 1⊕3\mathbf1\oplus\mathbf3 for SU(3)ep\mathrm{SU}(3)_{e_p}, together with Z=−1Z=-1, D=+1D=+1, N∈{1,3}N\in\{1,3\}, which is 3ˉ⊕1\bar{\mathbf3}\oplus\mathbf1; the sector Dage=+1D_{\mathrm{age}}=+1 holds the other two combinations. Neither DD nor NN is constant on a sector.

Proof

(1) Leq=aj+aj†L_{e_q}=a_j+a_j^\dagger and Ler=−i(aj−aj†)L_{e_r}=-i(a_j-a_j^\dagger) flip the occupation of the mode jj; commutation of f(N)⊗If(N)\otimes I with every Lea⊗XL_{e_a}\otimes X forces ff constant, and for f(N)⊗Zf(N)\otimes Z it forces f(N±1)=−f(N)f(N\pm1)=-f(N). The products LeaLebL_{e_a}L_{e_b} span so(6)≅su(4)\mathfrak{so}(6)\cong\mathfrak{su}(4), irreducible on each eigenspace of DD. (2) LeaLexLea−1=−LexL_{e_a}L_{e_x}L_{e_a}^{-1}=-L_{e_x} for x≠ax\neq a. (3) If two complex structures satisfy JK+KJ=−2cJK+KJ=-2c, then PJPKPJ=12(1+c)PJP_JP_KP_J=\tfrac12(1+c)P_J; here c=±1/7c=\pm1/\sqrt7. All three items were also checked by computation.

Proposition(The invariant pairs of the quartets) computed

At epe_p, with the quartet 4\mathbf4 the −1-1-eigenspace of DD, put B=13(PN=2−PN=1)B=\tfrac13(P_{N=2}-P_{N=1}) and L=PN=0−PN=3L=P_{N=0}-P_{N=3}. (1) The fifteen products LeaLebL_{e_a}L_{e_b}, a,b≠pa,b\neq p, span su(4)\mathfrak{su}(4) and commute with DD; those annihilating 1 span su(3)ep\mathfrak{su}(3)_{e_p}. (2) On 4⊗4ˉ\mathbf4\otimes\bar{\mathbf4} the su(3)ep\mathfrak{su}(3)_{e_p}-invariants have dimension 2 and the su(4)\mathfrak{su}(4)-invariants dimension 1, spanned by ∑iei⊗eˉi\sum_ie_i\otimes\bar e_i. (3) Every su(3)ep\mathfrak{su}(3)_{e_p}-invariant vector of 4⊗4ˉ\mathbf4\otimes\bar{\mathbf4} is annihilated by B⊗I+I⊗BB\otimes I+I\otimes B, L⊗I+I⊗LL\otimes I+I\otimes L and D⊗I+I⊗DD\otimes I+I\otimes D.

So an element of (V⊗V)⊗(4⊗4ˉ)(V\otimes V)\otimes(\mathbf4\otimes\bar{\mathbf4}) invariant under SL⁡(2,Z[ω])\SL(2,\Z[\omega]) and su(4)\mathfrak{su}(4) is unique up to scale, ε⊗∑iei⊗eˉi\varepsilon\otimes\sum_ie_i\otimes\bar e_i.

Theorem(Pairs fix the exchange sign relative to the interior) computed

A bilinear pairing realized by operators as [ψ(f),ψ(g)]±=⟨f,g⟩[\psi(f),\psi(g)]_\pm=\langle f,g\rangle is symmetric for the anticommutator and antisymmetric for the commutator, so a symmetric pairing admits only the canonical anticommutation relations. On the stabilizer of II in SL⁡(2,Z[ω])\SL(2,\Z[\omega]), the dicyclic group of order twelve, VV is irreducible with Frobenius–Schur indicator −1-1, so its invariant bilinear forms are the multiples of the antisymmetric ε\varepsilon: the pairing of the spinor alone selects no statistics.

Identify (C⊗O)⊗2(\C\otimes\Oct)^{\otimes2} with the complex 8×88\times8 matrices and put Dp=−iLepD_p=-iL_{e_p}. A tensor τ\tau with (Dp⊗I+I⊗Dp)τ=0(D_p\otimes I+I\otimes D_p)\tau=0 for all seven pp is a multiple of τ0=1⊗1+∑kek⊗ek\tau_0=1\otimes1+\sum_ke_k\otimes e_k, the tensor of the octonion norm, which is symmetric; at one point the su(4)\mathfrak{su}(4)-invariant ones are spanned by τ0\tau_0 and the antisymmetric tensor τD\tau_D of DpD_p, and ∑iei⊗eˉi=12(τ0−τD)\sum_ie_i\otimes\bar e_i=\tfrac12(\tau_0-\tau_D).

Let τ\tau have symmetry sτ=±1s_\tau=\pm1 under the swap of its factors and put κ=ε⊗τ\kappa=\varepsilon\otimes\tau, a tensor on the sixteen modes of V⊗(C⊗O)V\otimes(\C\otimes\Oct). In the Fock space of these modes with exchange sign η\eta, −1-1 for fermions and +1+1 for bosons, the pair ∑A,BκAB cA†cB† Ω\sum_{A,B}\kappa_{AB}\,c_A^\dagger c_B^\dagger\,\Omega is nonzero if and only if η=−sτ\eta=-s_\tau. As a two-particle tensor κ\kappa has eigenvalue −sτ-s_\tau under the swap and −1-1 under a rotation by 2π2\pi of one particle, and the two agree exactly when τ\tau is symmetric. So the invariant pair ε⊗∑iei⊗eˉi\varepsilon\otimes\sum_ie_i\otimes\bar e_i has a half that makes pairs only for fermions and a half only for bosons, and a pair neutral for all seven points is ε⊗τ0\varepsilon\otimes\tau_0 up to scale and makes pairs only for fermions.

Proof

[ψ(g),ψ(f)]±=±[ψ(f),ψ(g)]±[\psi(g),\psi(f)]_\pm=\pm[\psi(f),\psi(g)]_\pm. With MM the matrix of τ\tau, neutrality reads DpM=MDpD_pM=MD_p, and the seven left multiplications generate EndR(O)\mathrm{End}_\R(\Oct), so for all seven points MM is scalar. Since cA†cB†=η cB†cA†c_A^\dagger c_B^\dagger=\eta\,c_B^\dagger c_A^\dagger, the pair depends only on the part of κ\kappa of symmetry η\eta, and κ\kappa has symmetry −sτ-s_\tau because ε\varepsilon is antisymmetric; the rotation by 2π2\pi is −I-I on VV. Checked with Jordan–Wigner fermions and truncated bosons on the sixteen modes.

Theorem(The pairs fermions allow) computed

At epe_p, let PℓP_\ell and PqP_q be the projections onto the plane ⟨1,ep⟩\langle1,e_p\rangle and onto the six units orthogonal to it. The symmetric pair tensors that are su(3)ep\mathfrak{su}(3)_{e_p}-invariant and annihilated by Dp⊗I+I⊗DpD_p\otimes I+I\otimes D_p are spanned by τ0\tau_0 and τ0∘(Pℓ−13Pq)\tau_0\circ(P_\ell-\tfrac13P_q), and Pℓ−13Pq=−σ (B−L)DpP_\ell-\tfrac13P_q=-\sigma\,(B-L)D_p, where Dp=σ(−1)ND_p=\sigma(-1)^N. For each of the fifteen generators XX of su(4)\mathfrak{su}(4), and for X=B−LX=B-L, the tensor τ0∘X\tau_0\circ X is antisymmetric, and (B−L)Dp(B-L)D_p commutes with no LeqL_{e_q}, q≠pq\neq p.

If each generation’s lepton and quark pair coefficients are equal, and the lepton and quark terms are dressed by factors that do not depend on the generation, then (ms/mb)/(mμ/mτ)=1(m_s/m_b)/(m_\mu/m_\tau)=1. If instead the second generation carries only the second structure and the third only the first, the ratio of ratios is 1/31/3 in absolute value.

Proof

The invariant neutral tensors are spanned by the parts of τ0\tau_0 and τD\tau_D on ⟨1,ep⟩\langle1,e_p\rangle and on the six units, and the symmetric ones are the parts of τ0\tau_0. With B−L=23N−1B-L=\tfrac23N-1 and NT=3−NN^{\mathsf T}=3-N, (B−L)T=−(B−L)(B-L)^{\mathsf T}=-(B-L); since DpT=−DpD_p^{\mathsf T}=-D_p commutes with NN, (B−L)Dp(B-L)D_p is symmetric, equal to −σ-\sigma on N∈{0,3}N\in\{0,3\} and σ/3\sigma/3 on the six units. With one structure ms/mμ=mb/mτm_s/m_\mu=m_b/m_\tau; with two, the second generation’s quark entry carries the extra factor −13-\tfrac13. The rest was computed at one point.

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