There are three generations of electrons, neutrinos, and quarks. The second and third generations of electrons and quarks are unstable and decay into lighter particles.

Why are there exactly three generations? Is it possible that there are more generations of increasingly massive and unstable particles that we aren't able to discover yet (for instance, at higher energy levels that we are capable of), or is there some known reason why it has to be exactly (and can't be more than) three?

edit: in addition to the link above, I also found this question helpful: What Do We Get From Having Higher Generations of Particles?

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    $\begingroup$ possible duplicate of Why do we think there are only three generations of fundamental particles? $\endgroup$ – user10851 Jun 17 '13 at 1:36
  • $\begingroup$ Why do the duplicates never show up before I submit? $\endgroup$ – Michael Jun 17 '13 at 1:46
  • $\begingroup$ I would delete but it won't let me for 2 days... :-( $\endgroup$ – Michael Jun 17 '13 at 1:46
  • $\begingroup$ The stackexchange internal search feature is pretty bad; I don't know why. I always search for things via google with the site:physics.stackexchange.com restriction. In any event, no harm done - hopefully the answers there work for you (though the theory side of those answers is not as fleshed out as the experimental side...). $\endgroup$ – user10851 Jun 17 '13 at 1:59
  • $\begingroup$ We don't generally delete duplicates...they provide another chance to show up in a search so that the next person with this question is that much more likely to find one of these questions before they ask. $\endgroup$ – dmckee --- ex-moderator kitten Jun 17 '13 at 2:01