The number of closed ideals in $L(L_p)$

We show that there are $2^{2^{aleph_0}}$ different closed ideals in the Banach algebra $L(L_p(0,1))$, $1<p ot= 2<infty$. This solves a problem in A. Pietschs 1978 book Operator Ideals. The proof is quite different from other methods of producing closed ideals in the space of bounded operators on a Banach space; in particular, the ideals are not contained in the strictly singular operators and yet do not contain projections onto subspaces that are non Hilbertian. We give a criterion for a space with an unconditional basis to have $2^{2^{aleph_0}}$ closed ideals in terms of the existence of a single operator on the space with some special asymptotic properties. We then show that for $1<q<2$ the space ${frak X}_q$ of Rosenthal, which is isomorphic to a complemented subspace of $L_q(0,1)$, admits such an operator.