In fields that alphabetize authors, which author to contact For papers in disciplines where the authors are alphabetized, how does one decide which author of a paper to contact for more information? Suppose for the sake of argument that one doesn't know any of the authors even by reputation, so it's impossible to decide that way.
Purely alphabetical author-list conventions and no designated contact author tends to show up in theoretical fields that usually have fairly short author lists.

As such, if you're going to contact the authors and don't know which to contact, why not put them all in the "To" line of your email?