In classical mechanics, it is generally possible to measure an object's state without significantly disturbing it. This is because classical mechanics operates in a macroscopic regime where the interactions between objects are typically weak and can be neglected for most practical purposes.
For example, if you want to measure the position of an object in classical mechanics, you can use non-intrusive methods such as optical techniques, where you observe the object's position by detecting the light it reflects or emits. This measurement typically does not perturb the object's motion significantly, as the influence of the photons used for observation is negligible compared to the macroscopic forces acting on the object.
However, in quantum mechanics, the situation is different due to the fundamental principles of quantum theory, such as the uncertainty principle. According to the uncertainty principle, there is an inherent limit to how precisely certain pairs of physical properties, such as position and momentum, can be simultaneously known.
In quantum mechanics, the act of measuring a property of a quantum system generally perturbs the system itself. This is known as the measurement disturbance or the observer effect. The interaction between the measuring device and the quantum system causes the system's state to change, and the measured property can become uncertain or altered.
To minimize disturbance in quantum measurements, various techniques have been developed, such as weak measurements and quantum non-demolition measurements. These methods aim to extract information about the system's state with minimal disturbance by using delicate measurement schemes and clever experimental setups. However, even with these techniques, complete disturbance-free measurements are not possible in general.
It's worth noting that the field of quantum measurement and quantum information is still an active area of research, and scientists continue to explore new methods and technologies to perform measurements with minimal disturbance.