It really just depends on how you define love. If you define love as physical attraction or the chemical processes that cause such, then yes, love at first sight is entirely valid. If you regard love as a strong emotional bond between people, then it's possible but unlikely. If you consider love as understanding, empathy, caring and a deep friendship between people, then not a snow-balls chance in hell. Sadly, the first tends to fade with time, unless you actively work at keeping the feelings fresh. The second and third not so much but the third is really the only one that can keep things going indefinitely. In the end, yeah, it could happen under the first or second definition but you'd better hope you develop the third or it ain't gonna last.