在长方体\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,\(AA_{1}=AD=2\),点\(E\)在棱\(CD\)上,且\(CE= \dfrac {1}{3}CD\).
\((1)\)求证:\(AD_{1}⊥\)平面\(A_{1}B_{1}D\);
\((2)\)在棱\(AA_{1}\)上是否存在点\(P\),使\(DP/\!/\)平面\(B_{1}AE\)?若存在,求出线段\(AP\)的长;若不存在,请说明理由;
\((3)\)若二面角\(A-B_{1}E-A_{1}\)的余弦值为\( \dfrac { \sqrt {30}}{6}\),求棱\(AB\)的长.