Particles or Waves? What are they? If matter exhibits properties of waves and particles, then which one are they? Matter can behave as a particle or a wave, but they can never be both of these at the same time. Something very strange is that it changes based on what kind of measurement technique you are using. In the double slit experiment the electrons produced an interference pattern when they were fired through the barrier, but in a modified experiment with detectors placed at the slits in order to determine which slit the electrons were traveling through, they began to behave as particles. The complementary principle says that electrons can have properties of both, but can never coexist, and that if you "ask" the electron a wave-type question you will receive a wave-type answer; ask it a particle-type question, and you will receive a particle-like answer. Waves and particles are two sides of the same coin. They both heavily contradict each other, but go very well together, to the point where you need both in order to have a full description in physics. DeBroglie Waves All matter has a wavelength. A matter wave is called a DeBroglie wave. The wavelength of a DeBroglie wave is directly proportional to Planck's constant, 6.626 x 10^(-34) Js, divided by the momentum of the object. With the objects we deal with in every day life, we never notice anything showing wave-type behavior, because in order to see these behaviors the wavelength has to be comparable to the size of the system. If you take a grain of sand that weighs about 5 x 10^(-10) kg and have it moving at around .5 metres per second, the DeBroglie wavelength of the grain of sand will be about a trillionth of the size of an atom's nucleus. For things we deal with the DeBroglie wavelength is too small for it to matter. When dealing with an object like an electron, the DeBroglie wavelength is comparable to the size of the electron, so the wave-like behaviors become apparent.