Octonions

Octonions are the fourth, the last, and the greatest of the so-called normed division algebras based on the Real numbers. When I first learned about the Octonions way back when, I didn’t like them because they weren’t associative like the Quaternions, the Complex numbers, and the Reals. But now I’m fine with that, and they may be important for new theories of physics!

Octonions have the general form:

a0e0 + a1e1 + a2e2 + a3e3 + a4e4 + a5e5 + a6e6 + a7e7

Where the coefficients ai are Real and the bases ei have (something like) the following relations:

e0 = 1 (and -1 = e12 = e22 = …)

e1 = I = e2e3 = e7e6 = e4e5

e2 = J = e5e7 = e3e1 = e4e6

e3 = IJ = e1e2 = e6e5 = e4e7

e4 = K = e5e1 = e6e2 = e7e3

e5 = IK = e7e2 = e1e4 = e3e6

e6 = JK =e5e3 = e1e7 = e2e4

e7 = IJK = e6e1 = e3e4 = e2e5

In addition, if any of the products like e2e3 = e1 are reversed you get the negative, so e3e2 = -e1.

Non-associativity is demonstrated by going through the list of triples:

(e1e2)e3 = e32 = -1

e1(e2e3) = e12 = -1

(e1e2)e4 = e3e4 = e7

e1(e2e4) = e1e6 = -e7

(e1e2)e5 = e3e5 = -e6

e1(e2e5) = e1e7 = e6

so it is hit or miss I guess. Also note that e7 = (IJ)K = -I(JK). And for all (eiej)ek and ei(ejek), if they are not equal, is one equal to the negative of the other? And do I have to multiply them all out to find out?

Further Reading:

https://en.wikipedia.org/wiki/Octonion

http://math.ucr.edu/home/baez/octonions/

https://www.quantamagazine.org/the-octonion-math-that-could-underpin-physics-20180720/

http://mathworld.wolfram.com/Octonion.html

http://homepages.wmich.edu/~drichter/octonions.htm

https://ncatlab.org/nlab/show/normed+division+algebra

[*11.33]

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One Response to “Octonions”

  1. Degen’s Eight-square Identity | Equivalent eXchange Says:

    […] Euler’s Four-square Identity has a similar simpler symmetry. There is also a connection with Octonions if you are interested in digging for it. If you are anticipating that there is such a formula for […]

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