The subject is controversial, after all light is not something material that you can pick up a handful and put on the scale. But, in a recent article, American astrophysicist Laura Whitlock, from NASA, said that a photon (a particle of light) must weigh something around 4 x 10-48 grams, that is, 0.000000000000000000000000000000000000000000000004 grams. But this is far from being the final word on the topic. “Light has no weight”, says physicist Ernesto Kemp, from Unicamp’s Institute of Physics. The NASA astrophysics article does not make it very clear how it arrived at the number presented, but it certainly followed the same ideas as the scientists who defend such weight of light. “They take Einstein’s classic equation that says that energy is equal to mass times the speed of light squared (E = mc2) and mix it with another that measures the amount of electromagnetic energy in a body”, says Ernesto Kemp. This juggling involving Einstein’s famous mathematical definition of energy may be exaggerated, but, in fact, the German physicist has everything to do with the subject. His studies on the characteristics of photons – when he defined that they have, at the same time, aspects of an electromagnetic wave and of a particle – earned him the Nobel Prize in Physics in 1921.
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