A more detailed explanation would be that any measurement in quantum mechanics involves at least two parties, an Object and a Meter. The former is the system whose observable, say x ^ ^ , we want to measure. The latter is the system we couple to the Object in order to infer the value of x ^ ^ of the Object by recording some chosen observable, O ^ ^ , of this system, e.g. the position of the pointer on a scale of the Meter. This, in a nutshell, is a model of most of the measurements happening in physics, known as indirect measurements (see pp. 38–42 of ). So any measurement is a result of interaction and that acts in both ways. Therefore, the Meter acts on the Object during each measurement, usually via the quantity, F ^ ^ , conjugate to the readout observable O ^ ^ , thus perturbing the value of measured observable x ^ ^ and modifying the results of subsequent measurements. This is known as back action (quantum) of the Meter on the system under measurement.