29/01/2020

Is light made of waves or particles? Find out in Folha.

Reproduction of Marcelo Viana's column in Folha de S.Paulo.

At the beginning of the 17th century, questioning old ideas of Aristotle, science began to ask what light is made of: small particles (corpuscular theory) or vibrating waves (wave theory)? Despite Newton's support, three centuries later the corpuscular theory was in trouble: it did not explain phenomena such as refraction, and it was also discredited by advances in electromagnetism.

The empirical discovery of electricity (static electricity) and magnetism (magnets) dates back to antiquity, but their scientific study also began in the 17th century. It became clear that the two are directly related: moving magnets generate electric current—this is how hydroelectric plants transform the movement of water into electricity—and electric current generates magnetism.

This culminated in the unification achieved in 1865 by the British mathematician James Clerk Maxwell (1831–1879): his mathematical theory explains electricity and magnetism as facets of a single entity, called the electromagnetic field, which moves through space at the speed of light in the form of waves (depending on their wavelength, these are radio waves, microwaves, visible light, X-rays, gamma rays, etc.). It was through Maxwell's equations that the divine proclaimed, "Let there be light!"

A decade later, the young physicist Max Planck (1858–1947) began his postgraduate studies in Munich, but his advisor advised him not to study physics because "virtually everything has already been discovered, we just need to fill in a few gaps." Planck persisted nonetheless.

One of these "holes" concerned the electromagnetic radiation emitted by so-called black bodies. Desperate to obtain a theory compatible with observations, in 1900 Planck adopted a strange assumption: there would be minimum quantities ("quanta") of electromagnetic energy, which cannot be subdivided. A bit like money, whose smallest quantity is a penny.

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