  The theoretical calculations of the dynamic E2-M1 polarizability at the magic
wavelength of the Sr optical clock are inconsistent with experimental results.
We investigate role of negative-energy states in the E2 and M1
polarizabilities. Our result for E2-M1 polarizability difference
$-$7.74(3.92)$\times$10$^{-5}$ a.u. is dominated by the contribution from
negative-energy states to M1 polarizability and has the same sign as and
consistent with all the experimental values. In addition, we apply the present
calculations to various other optical clocks, further confirming the importance
of negative-energy states to the M1 polarizability.
