Question: In an audio circuit, such as a guitar's electronics (the only thing I ever dabble with!), I have always thought (probably simplistically) of every point along a continuous earth path being essentially all at the same voltage (a 'zero volt' reference voltage).

However, I was wondering - if you extend an earth wire from some point along that path, is it possible for that in itself to introduce noise to the circuit by changing the '0V' of the earth reference? (Possibly I am wondering "What is the difference between a long earth wire and an antenna" - but I don't know enough about antennas to know if that's what I'm asking :)

Answer: 
Yes, long ground connections can pick up noise, which means the ground in one place is a different voltage from the ground at another place. For single-ended signals, this ground offset voltage usually appears as part of the signal.

There are two common strategies for avoiding this situation:

- Use differential signals. The actual signal is encoded as the difference between two signals that are driven oppositely from each other. Ground offset then looks like a common mode signal, which can be largely ignored by the receiver.

- Pay attention to how things are grounded. Make sure all the grounds are tied back to one place without other connections to elsewhere, like the local power outlet ground. This ground net is then connected to real ground in one place only. Also avoid having deliberate current flow thru any ground connection. There should be separate returns for power current. Never use ground as a power return.
The answer is YES if your application depends upon the the difference of voltage between the two ends of the wire to be close to zero. In other words if a functioning system is sensitive to signal levels at both ends of the wire at once then there can be reason to be aware that noise pickup can happen.

On the other hand if you connect two systems together using a cable such as a twisted pair (or a coax cable for high frequency) and make sure that the current traveling in one wire is equal and opposite to the current in the other wire and the target system is only sensitive to the signalling between the target ends of the two wires then any common mode pickup in the pair of wires will go unnoticed by the target equipment.