# Significance of Irreducible Representations

I am reading texts on QFT and they place a lot of emphasis on finding irreducible representations of the Lorentz/Poincare Group. I also recall some level of discussion from non-relativistic quantum mechanics, and I've also read up a bit on them in texts on representation theory. I am wondering however why they are particularly significant to physicists. If anyone could clarify this for me it would be greatly appreciated.

• Is the question about the theory of representation, or about the irreducible representation of group theory ? – FraSchelle Nov 29 '17 at 9:45
• An irreducible representation indicates that the quantum system is elementary; please see the following question: physics.stackexchange.com/questions/73593/… – David Bar Moshe Dec 3 '17 at 15:21