Leggett-Garg Inequality

The Leggett-Garg inequality, named for Anthony James Leggett and Anupam Garg, is an inequality that any macrorealistic physical theory must obey. Macrorealism combines two assumptions: that a macroscopic object with two or more distinguishable states is always definitely in one of them, and that a system's state can in principle be determined without disturbing it or its later behavior. Experiments that violate the inequality, as quantum-mechanical systems can, show that at least one of these two classical assumptions fails to hold for the system actually being measured.