12. What does one mean by wave-particle duality? (Peter J)

A. Everything in the physical world is either matter or energy.
We observe matters as concentrated mass: from tiny dust particles to planets and stars.
Matter is characterized by momentum (mass x velocity). It is always localized at a point (region in space).
Energy is often in the form of radiation: light, infra-red (heat), ultra-violet, x-rays, etc. These are all (electromagnetic) waves.
Energy is characterized by its frequency. Waves are spread out in a region, never localized at a point.
Let us refer to this as the Energy-Matter double: two separate kinds.
B. When we study the physical world at the atomic and subatomic levels we find that there is an important difference.
(a) Matter, i.e. material particles like electrons and protons also have a wave-aspect (matter-waves). This is called the De Broglie wave.
(b) Radiant energy like light also has a particle aspect. This is known as a quantum of energy (or photon).
C. Thus every microcosmic entity has both a particle and a wave aspect. This is called wave-particle duality.
D. It is important to recognize that the same microscopic entity has both the wave and the particle features simultaneously. But as soon as one makes an observation on it, one or the other (particle or wave) aspect is what appears. We say that at any instant there is a superposition of states until a measurement is made.
E. Analogy: Consider a coin. A microscopic entity (electron, proton, etc.) may be compared to a coin that is falling to the ground. It has both a head and a tail all the time. But as soon as it falls to the ground it is seen as a head or a tail. This would be like making an observation of an electron. In that experiment it will seem to be a particle or a wave.

January 5, 2015


About Varadaraja V. Raman

Physicist, philosopher, explorer of ideas, bridge-builder, devotee of Modern Science and Enlightenment, respecter of whatever is good and noble in religious traditions as well as in secular humanism,versifier and humorist, public speaker, dreamer of inter-cultural,international,inter-religious peace.
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