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## R c h o

 Quote by S.Daedalus But this brings us right to the quaternionic and octonionic extensions of quantum mechanics discussed earlier! So, could there be a connection? (Probably not, but it's just the kind of '...but what if?'-thing that sometimes goes through my head at night...) In particular, since I've always liked the 'spacetime is made of qubits'-idea from Weizsäcker's ur-theory, to look for 'inner space' in entanglement between urs seems somehow appealing to me... But I realize this is just far-out speculation.
To be complete, is there any way to derive GR from the division algebras?

 Quote by friend To be complete, is there any way to derive GR from the division algebras?
I only know of the paper 'Non-Associativity as Gravity' by Dorofeev, which I don't think is particularly convincing. Personally, and somewhat off-topic, I consider the recent paper by Jacobson, 'Gravitation and Vacuum Entanglement Entropy', which identifies entanglement entropy with the Bekenstein-Hawking entropy (under certain assumptions, such as its non-divergence), and then uses Jacobson's old thermodynamic argument to get the EFEs, much more promising...
 Blog Entries: 6 Recognitions: Gold Member So, what happens with 4-qubits etc? I would expect it to be formulated in the usual terms of Spinorial Chessboard and Bott periodicity. The peculiar thing of division algebras in Spinors is, as you have remarked, that they beget SUSY. Is there some similar property peculiar to 2 and 3 qubits? On a different theme, I do not know of a relevant role for the third hoft fibration, with S15 sitting there. Of course it hints of SO(16) and then some of the string theory symmetries, but the standard model group seems to travel well just with the second fibration, S7, halving it so that the basis is not S4 but CP2 (there is a concept there, "branched covering", for which I would welcome an octonionic or quaternionic formulation). Also, thinking on GUT groups such as SO(10) and SO(14), it could be interesting to ask more of the S9 and S13 spheres.

 Quote by arivero On a different theme, I do not know of a relevant role for the third hopf fibration, with S15 sitting there.
It could be something to do with how you get 11-dimensional M-theory from 26-dimensional bosonic string theory.

 Quote by arivero So, what happens with 4-qubits etc?
You get the (one-loop) partition function for bosonic strings...

However, there's a qualitative difference between three- and four-qubit (or generally n > 3) entanglement: while for three qubits, you have four SLOCC-equivalence classes (two kinds of genuine three-partite entanglement, biseparable states, and fully separable states), for four qubits, there's already infinitely many. (Although some say it's nine, in accordance with the famed prediction from string theory, but those are kind of meta-classes, each depending on a continuous parameter.) So that's something peculiar to two and three qubits, but doesn't seem related to SUSY in any way...

 On a different theme, I do not know of a relevant role for the third hoft fibration, with S15 sitting there.
I thought maybe it's just that you need bioctonions for one full generation (a single octonion -- or split octonion -- in the Günaydin/Gürsey scheme incorporates only one flavor). There's another interesting paper I didn't mention earlier, 'Freudenthal Triple Classification of Three Qubit Entanglement' by Borsten et al., which collects the qubits into the Freudenthal triple $\mathfrak{M}(J)=\mathbb{C}\oplus\mathbb{C}\oplus J \oplus J$ over the Jordan algebra $J =\mathbb{C}\oplus\mathbb{C}\oplus\mathbb{C}$, and identifies the entanglement classes with the rank of its elements; but those elements are of the form of (complex) Zorn matrices, i.e. bioctonions. Not sure if it means anything, but it's kinda neat.

But maybe we should just look at the base spaces: the $S^8$ gives the octonions (or the octonion projective line), the fiber, $S^7$, is again fibered with base space $S^4$, the quaternion projective line, its fiber in turn gives the complex, then the real line -- which kinda reminds of the tensor algebra $T=\mathbb{R}\otimes\mathbb{C} \otimes \mathbb{H} \otimes \mathbb{O}$ Dixon and Furey use in their schemes...

On another note, I read somewhere (though I don't recall where) that the original octonionic/quaternionic quantum mechanics scheme fell out of favor for some reason (and certainly, it seems like it was pursued somewhat less than I would have thought it should have been), does anybody know why that might have been? I mean, there's of course the tensor product troubles etc., but is there a known reason schemes like those can't work?

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 Quote by S.Daedalus On another note, I read somewhere (though I don't recall where) that the original octonionic/quaternionic quantum mechanics scheme fell out of favor for some reason
I think to remember that the introduction to Adler's book discusses this point. Regretly it is not in my local library.

The whole issue of quaternions and how they become tainted by political fight between academical schools in the XIXth century is already part of the history of mathematics. See e.g. Felix Klein treatise about this period; his personal remarks in the last part of chapter IV are very explicit.

 Quote by arivero I think to remember that the introduction to Adler's book discusses this point. Regretly it is not in my local library.
Our university library has it, so I'll have a look, thanks for the pointer!

About that third Hopf map: the decomposition is such that of an entangled triplet, two qubits live in the $S^7$ fiber, while the third lives in the $S^8$ base, together with the additional degrees of freedom coming from the entanglement (if there's no entanglement, the whole construction collapses -- as one would expect -- to $S^2 \times S^2 \times S^2$, i.e. three single-qubit state spaces). So this third qubit is somehow 'augmented' by the entanglement to an octonionic being -- lepton/quark as per Günaydin/Gürsey -- living in 9+1 dimensions (because of the connection between $\mathbb{O}P^1$ and $SL(2,\mathbb{O})\cong SO(9,1)$). That's probably a bit much free association I realize, but I think there's a story to be told here, even though I don't yet fully see it.
 I've skimmed Adler's book, and he points out that octonion quantum mechanics really only exists in the case of the exceptional Jordan algebra $J_3(\mathbb{O})$, describing a single quantum system over the Moufang plane $\mathbb{O}P^2$ as constructed by Günaydin, Prion, and Ruegg. (The book also backs up -- and might be the source of -- Tony Smith's statement son the quaternion electroweak paper referenced above by Finkelstein et al.) Also, I've stumbled across an approach that I think of as 'RCHO in disguise', propagated by Greg Trayling, which has been briefly mentioned before on this forum here and here. There's only two papers on this, 'A Geometric Approach to the Standard Model' and 'A Geometric Basis for the Standard Model Gauge Group', the latter of which is the more extensive one. Basically, the bid is to get the standard model from the Clifford algebra $C\ell_7$, introducing four additional spatial dimensions. Nevertheless, I think this is related, in particular, to Dixon's approach: firstly, $C\ell_7 = \mathbb{R}[8]\oplus\mathbb{R}[8] = \mathbb{C}\otimes\mathbb{R}[8] \cong \mathbb{C}\otimes\mathbb{O}_L$ (where the 'L' denotes left-action), whose spinor space is just $\mathbb{C}\otimes\mathbb{O}$ (cf. Dixon's 'Division Algebras; Spinors; Idempotens; The Algebraic Structure of Reality', which contains a nice presentation of his model (and its link to the Hopf fibrations!)). However, use is also made of $C\ell_3 \cong \mathbb{C}\otimes\mathbb{H}_L$, introducing all our favourite players after all. There's however some awkwardness in the treatment of the right-handed neutrino, which has to be artificially suppressed in order to get the right structure.
 Blog Entries: 6 Recognitions: Gold Member I am happy to know that Adler had some valuable info; I really was doing a partly blind shot, as I had read it in 2006 last time. Jordan algebra seems to have some role, yep. It does appear also when extending the idea of the relationship between SUSY and division algebras, I think that some paper of German Sierra is really about it. And of course Jordan algebras have a deep history of its own, in the context of quantum mechanics, foundations, etc. In a private comunication from someone (perhaps M Porter? Other?), I have been told about seeing octonions as a set of 8 roots, 7 of them imaginary, the other the trivial 1, and then arguing that this 7+1 decomposition could be used to explain why 12 out of 96 states of the particle spectrum (ie 1 out of 8) have peculiar mass properties. Perhaps the 1 is to be related to the 12 neutrino states, perhapt to the 12 top quark states. Final rumiation, I have already mentioned it in this tread, and a lot elsewhere, but perhaps not enough in this one: Michael Atiyah, Jurgen Berndt http://arxiv.org/abs/math/0206135 should be the tool to explain why colour is SU(3) and not SO(5), and the contexts for it is either RHCO or Hoft (-like) fibrations with S4 (and CP2, resp) base spaces.
 Yes, that was me... Baez and Huerta have a connection between the imaginary split octonions and the group G2. Someone tell Gordon Kane!
 I don't think he will listen! He is now claiming that SUSY particles to be found around 50TeV...

 Quote by arivero I am happy to know that Adler had some valuable info; I really was doing a partly blind shot, as I had read it in 2006 last time.
I had the book on my radar, your recommendation bumped it to the top of the list, and it's certainly very interesting, though I'm not sure I buy into all of it. He proposes some Harari-Shupe like preon model, which I've decided I'm not a great fan of, and I'm also not sure about the idea of a quaternion quantum mechanics underlying complex QM. Though it's interesting that the S-matrix is complex, asymptotically at least -- perhaps one could think of this as a mechanism for dimensional reduction, i.e. macroscopic experimenters only 'see' the 3+1 dimensional world associated with the complex numbers, instead of the quaternionic 5+1 (in my own vague ruminations, I have supposed that this role is played by the fact that quantum correlations only get weaker by admixture of states -- i.e. genuine tri- or bipartite entanglement generally doesn't survive to the macroscopic level, effectively reducing octonions and 9+1 dimensional space time to complex numbers and 3+1 dimensions...).

 In a private comunication from someone (perhaps M Porter? Other?), I have been told about seeing octonions as a set of 8 roots, 7 of them imaginary, the other the trivial 1, and then arguing that this 7+1 decomposition could be used to explain why 12 out of 96 states of the particle spectrum (ie 1 out of 8) have peculiar mass properties. Perhaps the 1 is to be related to the 12 neutrino states, perhapt to the 12 top quark states.
Interesting, but how is mass related to the octonion roots?

 Final rumiation, I have already mentioned it in this tread, and a lot elsewhere, but perhaps not enough in this one: Michael Atiyah, Jurgen Berndt http://arxiv.org/abs/math/0206135 should be the tool to explain why colour is SU(3) and not SO(5), and the contexts for it is either RHCO or Hoft (-like) fibrations with S4 (and CP2, resp) base spaces.
I think I don't understand this stuff well enough to comment much... Perhaps there's some relation to the non-compact Hopf maps defined using the split-algebras (see here)?...

 Quote by mitchell porter Yes, that was me... Baez and Huerta have a connection between the imaginary split octonions and the group G2. Someone tell Gordon Kane!
Oh, that one slipped past me! I'm usually on the lookout for Baez' stuff, so thanks for the pointer...

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 Quote by S.Daedalus Interesting, but how is mass related to the octonion roots?
No idea. Naively, when one has a natural scale and some masses with are near zero respect to such scale, then one is supposed to search for a symmetry that protects such zero masses. What is intriguing is that in the standard model we have two different 7+1 (or 84+12) scenarions: respect to the Dirac mass scale of neutrino, all the others are almost zero. And respect to the mass scale of the top quark, the same. So the most elegant solution could be, instead needing to decide for one or another, to have a duality. Thus I was inclined to look into the M2-brane M-5 brane duality, because its source is a tensor with 84 components. Does such duality (which is simply the Pascal Triangle equality between (9 2+1) and (9 5+1)) has some parallel in octonions?

(yep, "scenarions" is a typo, but a funny one)
 Recognitions: Gold Member Is there any way to extract a real number from the quaternions and octonions like there is for complex numbers? In complex numbers we can multiply by the complex conjugate to get a real number. Is there an analogous procedure for quaternions and octonions?

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 Quote by friend Is there any way to extract a real number from the quaternions and octonions like there is for complex numbers? In complex numbers we can multiply by the complex conjugate to get a real number. Is there an analogous procedure for quaternions and octonions?
Sure; they are normed division algebras.
 Blog Entries: 6 Recognitions: Gold Member I noticed this old one from Nahm. No surprises, but interesting ancient reference http://inspirehep.net/record/129766?ln=es

 Quote by S.Daedalus Now this is quite a surprising way for the division algebras to turn up in entanglement! In particular, this appears to allow us to consider a two-qubit state as a single quaternionic qubit, and similarly, a three-qubit state as a single octonionic qubit ... an analogous construction works for the three-qubit case). (The connection between Hopf fibrations and qubits over division algebras was also noticed in the paper 'Extremal Black Holes as Qudits', by M. Rios who I think posts here occasionally.)
Yes, extremal black hole paper also deals with the split-composition algebras, which show up in toroidal M-theory compactifications. The basic picture is that M-theory acquires the symmetries of geometries (projective, symplectic, metasymplectic, etc.) over the split-octonions upon compactification down to d=6, 5, 4 and 3 dimensions. These symmetries give the U-duality groups for the corresponding supergravity theories (which includes the U-duality group E7(7) for N=8 supergravity in the d=4 case).

The black hole/qudit paper covers the d=6,5 cases where the U-duality groups SO(5,5) and E6(6) provide SLOCC gates/transformations for split-octonion qubit and qutrits. Two states are defined as SLOCC equivalent if there is a non-vanishing probability to convert one into the other (and back) via LOCC (local quantum operations assisted by classical communication). Geometrically, SO(5,5) and E6(6) are determinant preserving collineation transformations acting on the (2x2 and 3x3 Hermitian matrix) black hole charge spaces in d=6,5. The transformations are generally not isometries (i.e., not always unitary), but do preserve rank, hence preserve the entropy and fraction of supersymmetry of each black hole.

 But this brings us right to the quaternionic and octonionic extensions of quantum mechanics discussed earlier! So, could there be a connection? (Probably not, but it's just the kind of '...but what if?'-thing that sometimes goes through my head at night...) In particular, since I've always liked the 'spacetime is made of qubits'-idea from Weizsäcker's ur-theory, to look for 'inner space' in entanglement between urs seems somehow appealing to me... But I realize this is just far-out speculation.
Yes, there is a connection. Toroidally compactified M-theory and N=8 supergravity make use of (split) quaternionic and octonionic extensions of quantum mechanics. Moreover, if M-theory in d=11 does have hidden Cayley plane fibers arXiv:0807.4899, then M-theory becomes a d=27 theory inherently equipped with a 16-dimensional (over ℝ) octonionic qutrit state space.