Let the foot of perpendicular from a point $P(1,2,-1)$ to the straight line $L: \frac{x}{1}=\frac{y}{0}=\frac{z}{-1}$ be $N$. Let a line be drawn from $P$ parallel to the plane $x+y+2 z=0$ which meets $L$ at point $Q$. If $\alpha$ is the acute angle between the lines PN and PQ , then $\cos \alpha$ is equal to $\_\_\_\_$ .