  Mass is an important concept in classical mechanics, which regards a particle
as a corpuscular object. But according to wave-particle duality, we know a free
particle can behave like a wave. Is there a wave property that corresponds to
the mass of a particle? This is an interesting question that has not been
extensively explored before. We suggest that this problem can be approached by
treating the mass on the same footing as energy and momentum. Here we propose
that, all particles are excitation waves of the vacuum and different particles
are represented by different excitation modes. Based on such a model, we found
that mass is not an intrinsic property of the particle. Instead, mass is
basically a measure of the particle energy. The relations between energy and
mass can be directly derived based on the wave properties of the particle. This
work explains why some particles are "wave-like" while others are
"particle-like". Furthermore, this work has several interesting implications.
It directly explains why photons can interact with a gravitational field. It
also suggests a possible origin of dark matter; they are thought to be composed
of excitation waves that fail to interact with each other. From this model, one
can easily see why our universe has more dark matters than visible matters.
