Why do you have to ground two power supplies together for PWM

[protobit]

So i have two power supplies on my breadboard, one for the micro controller and whatnot and the other for 2 servos. I was wondering why (for the PWM to work correctly) do i have to connect the two grounds together. Couldn't find anything google-ing and i was just wondering why you have to connect them. Maybe somebody can recommend some reading material on basic electronics because i apparently lack in the area atm.

[Soeren]

Hi,

[...] why (for the PWM to work correctly) do i have to connect the two grounds together.

Two answers, take your pick...

1) Because the electrons need a return path - you cannot communicate on a single line, as there is no reference telling the other side whether the line is high or low.
If you just connected one pole of a battery to a circuit, would you expect it to work then?

2) You don't, just get an opto coupler and communicate via light (the opto coupler needs to have two connections to each circuit though.

[Abdulla M.A.]

You had to connect the ground of the two sources in order to get a complete circuit, let us assume you did not connect
these grounds, the current will not flow in the servo, because there is no voltage difference making the current  flow in the circuit.

Abdulla