I'm no
nuclear physicist, but here's what I've gathered from various
confusingly technical websites:
Assume that
all matter is energy.
Everything we deal with
on a daily basis has a
positive energy.
Protons,
elections,
neutrons,
photos,
radio waves,
all of it.
Dirac's
equations (based on
Einstein's, i believe)
predicted that there should also be
negative energy. Today we know this energy as
antimatter. But how can you have
negative energy?
Less than none?
Dirac explained this by saying that all
measurements are taken
relative to
empty space, in which all of the
negative energy levels have been filled.
Imagine a graph with
positive energy levels at the top and
negative energy at the bottom. All
energy levels below zero are filled in. This is the "
Sea of Dirac" or "
Dirac Sea".
According to Dirac's theory (as I understand it), antimatter would not be matter with negative mass, but an absence of "negative mass". Antiparticles are the holes in the negative energy sea.