Dimension expanders

We show that there exists $k \in \bbn$ and $0 < \e \in\bbr$ such that for every field $F$ of characteristic zero and for every $n \in \bbn$, there exists explicitly given linear transformations $T_1,..., T_k: F^n \to F^n$ satisfying the following: For every subspace $W$ of $F^n$ of dimension less or equal $\frac n2$, $\dim(W+\suml^k_{i=1} T_iW) \ge (1+\e) \dim W$. This answers a question of Avi Wigderson [W]. The case of fields of positive characteristic (and in particular finite fields) is left open.