June 21, 1997

This Week's Finds in Mathematical Physics (Week 105)

John Baez

There are some spooky facts in mathematics that you'd never guess in a million years... only when someone carefully works them out do they become clear. One of them is called "Bott periodicity".

A 0-dimensional manifold is pretty dull: just a bunch of points. 1-dimensional manifolds are not much more varied: the only possibilities are the circle and the line, and things you get by taking a union of a bunch of circles and lines. 2-dimensional manifolds are more interesting, but still pretty tame: you've got your n-holed tori, your projective plane, your Klein bottle, variations on these with extra handles, and some more related things if you allow your manifold to go on forever, like the plane, or the plane with a bunch of handles added (possibly infinitely many!), and so on.... You can classify all these things. 3-dimensional manifolds are a lot more complicated: nobody knows how to classify them. 4-dimensional manifolds are a lot more complicated: you can prove that it's impossible to classify them - that's called Markov's Theorem.

Now, you probably wouldn't have guessed that a lot of things start getting simpler when you get up around dimension 5. Not everything, just some things. You still can't classify manifolds in these high dimensions, but if you make a bunch of simplifying assumptions you sort of can, in ways that don't work in lower dimensions. Weird, huh? But that's another story. Bott periodicity is different. It says that when you get up to 8 dimensions, a bunch of things are a whole lot like in 0 dimensions! And when you get up to dimension 9, a bunch of things are a lot like they were in dimension 1. And so on - a bunch of stuff keeps repeating with period 8 as you climb the ladder of dimensions.

(Actually, I have this kooky theory that perhaps part of the reason topology reaches a certain peak of complexity in dimension 4 is that the number 4 is halfway between 0 and 8, topology being simplest in dimension 0. Maybe this is even why physics likes to be in 4 dimensions! But this is a whole other crazy digression and I will restrain myself here.)

Bott periodicity takes many guises, and I already described one in "week104". Let's start with the real numbers, and then throw in n square roots of -1, say e₁,...,e_n. Let's make them "anticommute", so

e_i e_j = - e_j e_i

when i is different from j. What we get is called the "Clifford algebra" C_n. For example, when n = 1 we get the complex numbers, which we call C. When n = 2 we get the quaternions, which we call H, for Hamilton. When n = 3 we get... the octonions?? No, not the octonions, since we always demand that multiplication be associative! We get the algebra consisting of pairs of quaternions! We call that H + H. When n = 4 we get the algebra consisting of 2x2 matrices of quaternions! We call that H(2). And it goes on, like this:

C₀ = R

C₁ = C

C₂ = H

C₃ = H + H

C₄ = H(2)

C₅ = C(4)

C₆ = R(8)

C₇ = R(8) + R(8)

C₈ = R(16)

Note that by the time we get to n = 8 we just have 16x16 matrices of real numbers. And that's how it keeps going: C_n+8 is just 16x16 matrices of guys in C_n! That's Bott periodicity in its simplest form.

Actually right now I'm in Vienna, at the Schroedinger Institute, and one of the other people visiting is Andrzej Trautman, who gave a talk the other day on "Complex Structures in Physics", where he mentioned a nice way to remember the above table. Imagine the day is only 8 hours long, and draw a clock with 8 hours. Then label it like this:

                                    0
                                 
                                    R

                 7                                    1
                                                   
                   R+R                             C





             6   R                                       H   2
 




                    C                             H+H
                 
                  5                                    3

                                    H

                                    4

Now let's completely change the subject and talk about rotations in infinite-dimensional space! What's a rotation in infinite-dimensional space like? Well, let's start from the bottom and work our way up. You can't really rotate in 0-dimensional space. In 1-dimensional space you can't really rotate, you can only reflect things... but we will count reflections together with rotations, and say that the operations of multiplying by 1 or -1 count as "rotations" in 1-dimensional space. In 2-dimensional space we describe rotations by 2x2 matrices like

cos t     -sin t
sin t      cos t

cos t      sin t
sin t     -cos t

Note that we can think of a rotation in 2 dimensions

cos t     -sin t
sin t      cos t

cos t     -sin t    0
sin t      cos t    0
  0          0      1

(Now if you know your math, or you read "week82", you'll realize that I didn't really change the subject, since the Clifford algebra C_n is really just a handy way to study rotations in n dimensions. But never mind.)

Now O(∞) is a very big group, but it elegantly summarizes a lot of information about rotations in all dimensions, so it's not surprising that topologists have studied it. One of the thing topologists do when studying a space is to work out its "homotopy groups". If you hand them a space X, and choose a point x in this space, they will work out all the topologically distinct ways you can stick an n-dimensional sphere in this space, where we require that the north pole of the sphere be at x. This is what they are paid to do. We call the set of all such ways the homotopy group π_n(X). For a more precise description, try "week102" - but this will do for now.

So, what are the homotopy groups of O(∞)? Well, they start out looking like this:

n       πn(O(∞))

0         Z/2
1         Z/2
2          0
3          Z
4          0
5          0
6          0
7          Z

But what do they mean?

Well, luckily Jim Dolan has thought about this a lot. Discussing it repeatedly in the little cafe we tend to hang out at, we came up with the following story. Most of it is known to various people already, but it came as sort of a revelation to us.

The zeroth entry in the table is easy to understand. π₀ keeps track of how many connected components your space has. The rotation group O(∞) has two connected components: the guys that are rotations, and the guys that are rotations followed by a reflection. So π₀ of O(∞) is Z/2, the group with two elements. Actually this is also true for O(n) whenever n is higher enough, namely 1 or more. So the zeroth entry is all about "reflecting".

The first entry is a bit subtler but very important in physics. It means that there is a loop in O(∞) that you can't pull tight, but if you go around that loop twice, you trace out a loop that you can pull tight. In fact this is true for O(n) whenever n is 3 or more. This is how there can be spin-1/2 particles when space is 3-dimensional or higher. There are lots of nice tricks for seeing that this is true, which I hope the reader already knows and loves. In short, the first entry is all about "rotating 360 degrees and not getting back to where you started".

The second entry is zero.

The third entry is even subtler but also very important in modern physics. It means that the ways to stick a 3-sphere into O(∞) are classified by the integers, Z. Actually this is true for O(n) whenever n is 5 or more. It's even true for all sorts of other groups, like all "compact simple groups". But can I summarize this entry in a snappy phrase like the previous nonzero entries? Not really. Actually a lot of applications of topology to quantum field theory rely on this π₃ business. For example, it's the key to stuff like "instantons" in Yang-Mills theory, which are in turn crucial for understanding how the πon gets its mass. It's also the basis of stuff like "Chern-Simons theory" and "BF theory". Alas, all this takes a while to explain, but let's just say the third entry is about "topological field theory in 4 dimensions".

The fourth entry is zero.

The fifth entry is zero.

The sixth entry is zero.

The seventh entry is probably the most mysterious of all. From one point of view it is the subtlest, but from another point of view it is perfectly trivial. If we think of it as being about π₇ it's very subtle: it says that the ways to stick a 7-sphere into O(∞) are classified by the integers. (Actually this is true for O(n) whenever n is 7 or more.) But if we keep Bott periodicity in mind, there is another way to think of it: we can think of it as being about π_-1, since 7 = -1 mod 8.

But wait a minute! Since when can we talk about π_n when n is negative?! What's a -1-dimensional sphere, for example?

Well, the idea here is to use a trick. There is a space very related to O(∞), called kO. As with O(∞), the homotopy groups of this space repeat modulo 8. Moreover we have:

π_n(O(∞)) = π_n+1(kO)

Combining these facts, we see that the very subtle π₇ of O(∞) is nothing but the very unsubtle π₀ of kO, which just keeps track of how many connected components kO has.

But what is kO?

Hmm. The answer is very important and interesting, but it would take a while to explain, and I want to postpone doing it for a while, so I can get to the punchline. Let me just say that when we work it all out, we wind up seeing that the seventh entry in the table is all about dimension.

To summarize:

π₀(O(∞)) = Z/2 is about REFLECTING

π₁(O(∞)) = Z/2 is about ROTATING 360 DEGREES

π₃(O(∞)) = Z is about TOPOLOGICAL FIELD THEORY IN 4 DIMENSIONS

π₇(O(∞)) = Z is about DIMENSION

But wait! What do those numbers 0, 1, 3, and 7 remind you of?

Well, after I stared at them for a few weeks, they started to remind me of the numbers 1, 2, 4, and 8. And that immediately reminded me of the reals, the complexes, the quaternions, and the octonions!

And indeed, there is an obvious relationship. Let n be 1, 2, 4, or 8, and correspondingly let A stand for either the reals R, the complex numbers C, the quaternions H, or the octonions O. These guys are precisely all the "normed division algebras", meaning that the obvious sort of absolute value satisfies

|xy| = |x||y|.

Thus if we take any guy x in A with |x| = 1, the operation of multiplying by x is length-preserving, so it's a reflection or rotation in A. This gives us a function from the unit sphere in A to O(n), or in other words from the (n-1)-sphere to O(n). We thus get nice elements of

π₀(O(1))

π₁(O(2))

π₃(O(4))

π₇(O(8))

which turn out to be precisely why these particular homotopy groups of O(∞) are nontrivial.

So now we have the following fancier chart:

π₀(O(∞)) is about REFLECTING and the REAL NUMBERS

π₁(O(∞)) is about ROTATING 360 DEGREES and the COMPLEX NUMBERS

π₃(O(∞)) is about TOPOLOGICAL FIELD THEORY IN 4 DIMENSIONS and the QUATERNIONS

π₇(O(∞)) is about DIMENSION and the OCTONIONS

Now this is pretty weird. It's not so surprising that reflections and the real numbers are related: after all, the only "rotations" in the real line are the reflections. That's sort of what 1 and -1 are all about. It's also not so surprising that rotations by 360 degrees are related to the complex numbers. That's sort of what the unit circle is all about. While far more subtle, it's also not so surprising that topological field theory in 4 dimensions is related to the quaternions. The shocking part is that something so basic-sounding as "dimension" should be related to something so erudite-sounding as the "octonions"!

But this is what Bott periodicity does, somehow: it wraps things around so the most complicated thing is also the least complicated.

That's more or less the end of what I have to say, except for some references and some remarks of a more technical nature.

Bott periodicity for O(∞) was first proved by Raoul Bott in 1959. Bott is a wonderful explainer of mathematics and one of the main driving forces behind applications of topology to physics, and a lot of his papers have now been collected in book form:

1) The Collected Papers of Raoul Bott, ed. R. D. MacPherson. Vol. 1: Topology and Lie Groups (the 1950s). Vol. 2: Differential Operators (the 1960s). Vol. 3: Foliations (the 1970s). Vol. 4: Mathematics Related to Physics (the 1980s). Birkhauser, Boston, 1994, 2355 pages total.

A good paper on the relation between O(∞) and Clifford algebras is:

2) M. F. Atiyah, R. Bott, and A. Shaπro, Clifford modules, Topology (3) 1964, 3-38.

For more stuff on division algebras and Bott periodicity try Dave Rusin's web page, especially his answer to "Q5. What's the question with the answer n = 1, 2, 4, or 8?"

3) Dave Rusin, Binary products, algebras, and division rings, http://www .math.niu.edu/~rusin/known-math/95/division.alg

Let me briefly explain this kO business. The space kO is related to a simpler space called BO(∞) by means of the equation

kO = BO(∞) x Z,

so let me first describe BO(∞). For any topological group G you can cook up a space BG whose loop space is homotopy equivalent to G. In other words, the space of (base-point-preserving) maps from S¹ to BG is homotopy equivalent to G. It follows that

π_n(G) = π_n+1(BG).

This space BG is called the classifying space of G because it has a principal G-bundle over it, and given any decent topological space X (say a CW complex) you can get all principal G-bundles over X (up to isomorphism) by taking a map f: X → BG and pulling back this principal G-bundle over BG. Moreover, homotopic maps to BG give isomorphic G-bundles over X this way.

Now a principal O(n)-bundle is basically the same thing as an n-dimensional real vector bundle - there are obvious ways to go back and forth between these concepts. A principal O(∞)-bundle is thus very much like a real vector bundle of arbitrary dimension, but where we don't care about adding on arbitrarily many 1-dimensional trivial bundles. If we take the collection of isomorphism classes of real vector bundles over X and decree two to be equivalent if they become isomorphic after adding on trivial bundles, we get something called KX, the "real K-theory of X". It's not hard to see that this is a group. Taking what I've said and working a bit, it follows that

KX = [X, BO(∞)]

where the right-hand side means "homotopy classes of maps from X to BO(∞)". If we take X to be Sⁿ⁺¹, we see

KSⁿ⁺¹ = π_n+1(BO(∞)) = π_n(O(∞))

It follows that we can get all elements of π_n of O(∞) from real vector bundles over Sⁿ⁺¹.

Of course, the above equations are true only for nonnegative n, since it doesn't make sense to talk about π_-1 of a space. However, to make Bott periodicity work out smoothly, it would be nice if we could pretend that

KS^-1 = π₀(BO(∞)) = π_-1(O(∞)) = π₇(O(∞)) = Z

Alas, the equations don't make sense, and BO(∞) is connected, so we don't have π₀(BO(∞)) = Z. However, we can cook up a slightly improved space kO, which has

π_n(kO) = π_n(BO(∞))

when n > 0, but also has

π₀(kO) = Z

as desired. It's easy - we just let

kO = BO(∞) x Z.

So, let's use this instead of BO(∞) from now on.

Taking n = 0, we can think of S¹ as RP¹, the real projective line, i.e. the space of 1-dimensional real subspaces of R². This has a "canonical line bundle" over it, that is, a 1-dimensional real vector bundle which to each point of RP¹ assigns the 1-dimensional subspace of R² that is that point. This vector bundle over S¹ gives the generator of KS¹, or in other words, π₀(O(∞)).

Taking n = 1, we can think of S² as the "Riemann sphere", or in other words CP¹, the space of 1-dimensional complex subspaces of C². This too has a "canonical line bundle" over it, which is a 1-dimensional complex vector bundle, or 2-dimensional real vector bundle. This bundle over S² gives the generator of KS², or in other words, π₁(O(∞)).

Taking n = 3, we can think of S⁴ as HP¹, the space of 1-dimensional quaternionic subspaces of H². The "canonical line bundle" over this gives the generator of KS⁴, or in other words, π₃(O(∞)).

Taking n = 7, we can think of S⁸ as OP¹, the space of 1-dimensional octonionic subspaces of O². The "canonical line bundle" over this gives the generator of KS⁸, or in other words, π₇(O(∞)).

By Bott periodicity,

π₇(O(∞)) = π₈(kO) = π₀(kO)

so the canonical line bundle over OP¹ also defines an element of π₀(kO). But

π₀(kO) = [S⁰,kO] = KS⁰

and KS⁰ simply records the difference in dimension between the two fibers of a vector bundle over S⁰, which can be any integer. This is why the octonions are related to dimension.

If for any pointed space we define

Kⁿ(X) = K(Sⁿ smash X)

we get a cohomology theory called K-theory, and it turns out that

Kⁿ⁺⁸(X) = K(X)

which is another say of stating Bott periodicity. Now if * denotes a single point, K(*) is a ring (this is quite common for cohomology theories), and it is generated by elements of degrees 1, 2, 4, and 8. The generator of degree 8 is just the canonical line bundle over OP¹ and multiplication by this generator gives a map

Kⁿ(*) → Kⁿ⁺⁸(*)

which is an isomorphism of groups - namely, Bott periodicity! In this sense the octonions are responsible for Bott periodicity.

Addendum: The Clifford algebra clock is even better than I described above, because it lets you work out the fancier Clifford algebras C_p,q, which are generated by p square roots of -1 and q square roots of 1, which all anticommute with each other. These Clifford algebras are good when you have p dimensions of "space" and q dimensions of "time", and I described the physically important case where q = 1 in "week93". To figure them out, you just work out p - q mod 8, look at what the clock says for that hour, and then take NxN matrices of what you see, with N chosen so that C_p,q gets the right dimension, namely 2^p+q. So say you're a string theorist and you think there are 9 space dimensions and 1 time dimension. You say: "Okay, 9 - 1 = 8, so I look and see what's at 8 o'clock. Okay, that's R, the real numbers. But my Clifford algebra C_9,1 is supposed to have dimension 2^{9 + 1} = 1024 = 32², so my Clifford algebra must consist of 32x32 matrices with real entries."

By the way, it's not so easy to see that the canonical line bundle over OP¹ is the generator of KS⁸ - or equivalently, that left multiplication by unit octonions defines a map from S⁷ into SO(8) corresponding to the generator of π₇(O(∞)). I claimed it's true above, but when someone asked me why this was true, I realized I couldn't prove it! That made me nervous. So I asked on sci.math.research if it was really true, and I got this reply:

From: Linus Kramer 
Newsgroups: sci.math.research
Subject: π_7(O) and octonions
Date: Tue, 09 Nov 1999 12:44:33 +0100

John Baez asked if π_7(O) is generated by
the (multiplication by) unit octonions.

View this as a question in KO-theory: the claim is
that H^8 generates the reduced real K-theory
\tilde KO(S^8) of the 8-sphere; the bundle
H^8 over S^8 is obtained by the standard glueing
process along the equator S^7, using the octonion
multiplication. So H^8 is the octonion Hopf bundle.
Its Thom space is the projective Cayley plane
OP^2. Using this and Hirzebruch's signature theorem,
one sees that the Pontrjagin class of H^8 is
p_8(H^8)=6x, for a generator x of the 8-dimensional
integral cohomology of S^8 [a reference for this
calulation is my paper 'The topology of smooth
projective planes', Arch. Math 63 (1994)].
We have a diagram

        cplx         ch
KO(S^8) ---> K(S^8) ---> H(S^8)

the left arrow is complexification, the second arrow
is the Chern character. In dimension 8, these maps form
an isomorphism. Now ch(cplx(H^8))=8+x (see the formula
in the last paragraph in Husemoller's "Fibre bundles",
the chapter on "Bott periodicity and integrality
theorems". The constant factor is unimportant, so the
answer is yes, π_7(O) is generated by the map
S^7---> O which sends a unit octonion A to the
map l_A:x → Ax in SO(8).

Linus Kramer

From: Todd Trimble
Subject: Hopf bundles
To: John Baez 
Date: Fri, 25 Mar 2005 16:37:11 -0500

John, 

In the book Numbers (GTM 123), there is an article by 
Hirzebruch where the Bott periodicity result is formulated 
as saying that the generators of \tilde KO(S^n)  in the cases 
n = 1, 2, 4, 8  are given by [η] - 1  where η is the Hopf 
bundle corresponding to R, C, H, O  and 1 is the trivial 
line bundle over these scalar "fields" (of real dimension 
1, 2, 4, 8), and is 0 for n = 3, 5, 6, 7 [p. 294].  Also that 
the Bott periodicity isomorphism  

        \tilde KO(S^n) ---> \tilde KO(S^{n+8}) 

is induced by  [η(O)] - 1  [p. 295].  I know you are aware 
of this already (courtesy of the response of Linus Kramers 
to your sci.math.research query), but I thought you might 
find a published reference, on the authority of no less than 
Hirzebruch, handier (should you need it) than referring to a 
sci.math.research exchange. 

Unfortunately no proof is given.  Hirzebruch says (p. 295), 

 Remark.  Our formulation of the Bott periodicity theorem 
 will be found, in essentials, in [reference to Bott's Lectures 
 on K(X), without proofs].  A detailed proof within the 
 framework of K-theory is given in the textbook [reference 
 to Karoubi's K-theory].  The reader will have a certain amount 
 of difficulty, however, in extracting the results used here from 
 Karoubi's formulation.

Todd

© 1997 John Baez
baez@math.removethis.ucr.andthis.edu