I often hear that a superconductor can hold a current indefinitely, I have a thought experiment which relates to this claim. Consider a closed square loop of superconducting wire, this wire carries some current. Will the electrons in the circuit transfer momentum and energy to the metal lattice when the electrons accelerate around the corners of the square circuit? If yes, then the current will actually slowly dissipate, if no then you could accelerate the electrons in the wire, giving a recoil force, and the electrons would travel all around the wire without transferring momentum where they can be accelerated again. This would be a momentum conservation violating thruster...So is current or momentum conserved?