I would see two solutions here. (IANAP). 1) Energy can be negative. Not sure what that would mean though. 2) You can do the same trick as he did and do E =

I would see two solutions here. (IANAP). 1) Energy can be negative. Not sure what that would mean though. 2) You can do the same trick as he did and do E = abs(m)*c^2. I would be interested to see what the ramifications of each are.