5.
已知正方体\(AC\)\({\,\!}_{1}\)的棱长为\(1\),点\(P\)是面\(AA\)\({\,\!}_{1}\)\(D\)\({\,\!}_{1}\)\(D\)的中心,点\(Q\)是面\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\) \(C\)\({\,\!}_{1}\)\(D\)\({\,\!}_{1}\)的对角线\(B\)\({\,\!}_{1}\)\(D\)\({\,\!}_{1}\)上一点,且\(PQ\)\(/\!/\)平面\(AA\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(B\),则线段\(PQ\)的长为( )