I know that we can only read voltage across a capacitor if it's two pins are connected to the voltmeter or can't talk about any potential difference between totally different systems if they do not have a common ground. But as I think about it more, I started getting confused and can not understand the physical reason why this is the case. Let me introduce a case;
simulate this circuit Then I measured the voltage to be sure that its charged. After that;
simulate this circuit Than I connected the negative probe to any universal ground(earth, a big copper plate etc.) and from common knowledge I know that I could not measure any voltages but I could not understand the physical reason of why. If voltmeter is measuring the current across its probes than why the charge carriers do not flow from the capacitor plates to ground? If voltmeter measures the electric field like an electroscope than why a plate with charge carriers and a ground do not create an electric field? Thanks for reading and sorry for trivial question.