0
$\begingroup$

I came across this statement in the book "Quantum Field theory and the Standard Model".

"A particle can be defined as a set of states that mix only among themselves under Poincare transformations."

What does this statement mean?

$\endgroup$

marked as duplicate by stafusa, Jon Custer, Cosmas Zachos, John Rennie special-relativity Nov 7 '17 at 9:37

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.