Leggett-Garg violation with a massive mechanical oscillator
In plain words
Macrorealism is the everyday assumption that a large object always has a definite state that can be checked without disturbing it. A Leggett-Garg test checks this by measuring one object at several times; a convincing test with a heavy mechanical object is missing.
Precise statement
For a mechanical mode of effective mass $m\ge 1e-6\,\mathrm{g}$, measure two-time correlators $C_{ij}$ of a two-valued observable at $t_1<t_2<t_3$ and test $K=C_{12}+C_{23}-C_{13}\le 1$, using ideal negative-result or control-experiment protocols that close the clumsiness loophole (disturbance by the measurement itself). An answer is $K>1$ at stated significance with invasiveness bounded by independent controls.
What would settle it
A loophole-controlled Leggett-Garg violation on a microgram-scale mechanical mode.
Status in the literature
Cat states of a 16 microgram bulk acoustic resonator were made in 2023 (Bild et al., Science); no loophole-controlled Leggett-Garg test at such masses was found in the literature checked.