Supposed attributes of quantum systems which, if they could be discovered in sophisticated experiments, would undercut the principles of quantum indeterminacy and superposition. Describing reality in terms of indeterminacy and probability bothered Albert Einstein. Surely, he said, a particle’s property (such as position or spin) exists before it is measured, and a theory more complete than quantum mechanics would include the existence of those properties before they were measured. Those properties before measurement must be contained in some variables hidden from the standard quantum mechanical representation. The search for those “hidden variables” pertaining to the existence of things occupied a lot of Einstein’s time in the latter part of his life, and has been a topic of concern with physicists ever since. In the 1960s John Bell proposed a number of experiments designed to test the validity of things like entanglement and indeterminacy. So far all such tests have supported the validity of quantum indeterminacy and have discouraged the idea of any hidden variables. But for some skeptics, loopholes remain, and they argue that the reality of entanglement has not yet been adequately demonstrated.