The smartest man in the world answered this question for me 40 years ago but I would think longer words have been introduced since then.The word is "antidisestablishmentarianism".
I'm moving this to the Puzzles Etc Forum, where I think you will find another thread on this topic.
you know what is the longest word in English?
That´s a piece of cake.
it´s "SMILES".
you know why?
Because between the two "S´s" there is "ONE MILE".
Good isn´t it?