PHY.9:2 - Problem

A property can affect a physical system without appearing in the chosen readout. Mass occurs in the force and inertia of a falling body but cancels from ideal gravitational acceleration. A field can change a quantum state’s phase while the selected measurement remains insensitive to it.

A responsive instrument can also confuse the sought property with its own interaction. A capillary responds to pressure through a liquid column, but surface forces contribute to the height. Repeating the same reading more precisely does not separate those contributions.

The difficulty is to construct a physical comparison that exposes the wanted difference under the available conditions.