 Neutron interferometry has been used to investigate the Aharonov – Bohm effect , to examine the effects of gravity acting on an elementary particle , and to demonstrate a strange behavior of fermions that is at the basis of the Pauli exclusion principle : Unlike macroscopic objects , when fermions are rotated by 360 ° about any axis , they do not return to their original state , but develop a minus sign in their wave function . In other words , a fermion needs to be rotated 720 ° before returning to its original state . 
