6. Let the line $l$ be perpendicular to the real axis of the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$, with the foot of the perpendicular being $M$, and intersecting the hyperbola at a point $P$ and one of the asymptotes at a point $Q$. Then $|M Q|^{2}-|M P|^{2}$ $\qquad$ .
$b^{2} ;$