The first law of thermodynamics is basically conservation of energy. The internal energy in a closed system cannot change. In this case, if 'the system does not

The first law of thermodynamics is basically conservation of energy. The internal energy in a closed system cannot change. In this case, if 'the system does not radiate its rotational energy', the particles will keep on moving indefinitely. Since you ask, QM can sort-of 'break' this law though, very temporarily, via the uncertainty principle. Many particle decays are mediated by 'virtual particles', which are heavier than the input particles. For example, a neutron can decay into an electron+neutrino+proton (Beta decay), this is mediated by a W boson which is ~80 times heavier than the neutron. So in a way energy is 'borrowed' to create the W, which then promptly decays into the lighter electron+neutrino, so in the end energy is conserved.