The Johnson-Lindenstrauss lemma is optimal for linear dimensionality reduction

For any $n>1$ and $0<\varepsilon<1/2$, we show the existence of an $n^{{O(1)}$-point} subset $X$ of $\mathbb{R}^n$ such that any linear map from $(X,\ell2)$ to $\ell2^m$ with distortion at most $1+\varepsilon$ must have $m = \Omega(\min{n, \varepsilon^{-2}\log n})$. Our lower bound matches the upper bounds provided by the identity matrix and the Johnson-Lindenstrauss lemma, improving the previous lower bound of Alon by a $\log(1/\varepsilon)$ factor.