Joseph S. Merola, associate dean for research at Virginia Polytechnic Institute, responds.
There are several different ways of approaching this question, but I won't beat around the bush. The simple answer is that wave/particle duality, as it is called, is present in the macroscopic world--but we can't see it.
Scientists have developed a number of indirect methods for observing wave/particle duality. One of the earliest experiments showed that a regular array of atoms could diffract an electron beam. Because diffraction is a property of a wave, this test indicated that particles--electrons in this case--could also behave as waves.
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The physicist Louis deBroglie proved that any particle in motion has a wave-like nature. He developed the following relationship: the wavelength of a particle's wave aspect is equal to Planck's constant divided by the momentum of the particle.
Now, most of the objects that we encounter have incredibly large masses compared with atomic and sub-atomic particles, which can give them a relatively huge momentum. For example, if you calculate the wavelength of a one-pound basketball traveling at one foot per second (I hope the purists will forgive my use of units, but they work out quite well in this case), you will find that it has a wavelength of around 10-34 meters. This wavelength is incredibly small--too small, in fact, to measure using modern instrumentation.
An electron, though, is less massive than our basketball by a factor of about 1030. So, an electron traveling at the same speed as our basketball has a wavelength of some 10-4 meters, which is quite measurable. Thus, we can determine the wavelength of very small particles, but not of large macroscopic items such as basketballs.
