So I know a voltage can be negative with respect to another relatively higher voltage, but can a voltage ever be lower than or negative with respect to earth ground (0 volts).

 

So I know a voltage can be negative with respect to another relatively higher voltage, but can a voltage ever be lower than or negative with respect to earth ground (0 volts).

Yes it can. A simple example would be where you connect the +ve pole of an AA cell to ground. Its -ve connection will then be at a potential of -1.5V relative to ground.

 

Yes it can be negative in respect to ground.

 

An AC signal is both positive and negative with respect to ground.

 

And a Split Rail supply (common in Amplifiers and also PC supplies) has both Positive AND Negative rails with respect to ground.

 

Here's another way of looking at it: "Negative" and "positive" simply indicate whether current would flow to the reference or from the reference as a result of the potential.