In recent years the precise control involved in quantum measurements has allowed the testing of Leggett-Garg Inequality violations in microscopic systems, and a renewed interest in understanding precisely what such a violation implies. Doubts have been raised whether such violations are a problem for macrorealism, as Leggett and Garg thought, or are just a problem for non-invasive measurements, and whether the Leggett Garg Inequalities themselves could be replaced with better tests.
I will attempt to bring some clarity to this issue by suggesting a root theorem that incorporates both the Leggett-Garg Inequality and some of the suggestions for replacing it, and argue that it is the possibility of an explanation in terms of non-invasive measurements that is at stake in these tests. However, I will also show that whenever a non-invasive explanation is possible, then a macrorealist non-invasive model is also possible. This is analogous to the situation with Bell's inequality, where it is local causality, rather than hidden variables, that is at stake, but that whenever a locally causal explanation is possible, a locally causal hidden variable model is also possible.
Finally I will show how macrorealism is no more committed to non-invasive measureability than hidden variables are restricted to be locally causal, and will discuss how some classes of macrorealist theories may still be experimentally ruled out by different kinds of experimental tests.
Philosophy of Physics Seminar Convenors: Dr Adam Caulton and Dr Christopher Timpson