After over 100 years of research in materials characterization, possibly starting from Rontgen's discovery of X-ray, we are at a new dawn in measuring materials properties. Quantum mechanics tells us that  "a Hermitian operator is an observable", and if we consider a probe measurement that perturbs a system, correlation functions are typical measurables. On the contrary, very few maeterials' properties have actually been accessible with current-generation measurement techniques.