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Retrospect

Space enigmas

A century ago, research on cosmic rays began

Fordham UniversityHess poses in a balloonFordham University

From a mere scientific curiosity, cosmic rays have now acquired an extraordinary status within physics research. They may help us to learn what the production and acceleration mechanisms of the most energetic particles identified to date in nature are, besides furthering our understanding of rare processes, such as quantum fluctuations in space-time. The world’s main outdoor laboratory, the Pierre Auger Observatory on the Argentine pampas, was designed to try to answer questions about this phenomenon. In the early twentieth century, however, what was sought was merely to measure the natural radiation in the environment. It was known that the subsoil contains chemical elements that emit radiation. There was also a belief that this force decreases whenever we draw away from the surface. The German physicist Theodor Wulf conducted an early experiment in 1910: he took an electroscope (radiation detector) to the Eiffel Tower, 300 meters above the ground. He found that the radiation at the top was weaker than on the ground, confirming what was thought at the time.

The Austrian physicist Victor Hess decided to try something more radical and made 10 flights taking an electroscope with him along with a 3-man crew. In one such flight, on August 7, 1912, he found that at a height of 5 thousand meters the radiation level was 16 times greater than at ground level –  the higher one goes, the less atmosphere there is to shield one from radiation. “The results of these observations seem to be better explained by a type of radiation with a great power of penetration entering our atmosphere,” wrote Hess. In 1936, he won the Nobel prize for his discovery.

“Hess’s chief merit was having the audacity to interpret that phenomenon correctly,”  says Carlos Escobar, a physicist from the State University of Campinas (Unicamp) and one of the leaders of the international consortium from 19 countries that manages the Auger projects. Before the acknowledgement of  Hess’s work, a discussion about the nature of cosmic rays made it into the first page of the New York Times. Robert Millikan thought that they were a form of electromagnetic radiation, like gamma rays. Arthur Compton, however, showed that they consisted of charged particles and closed the debate in 1932.

CERNCésar Lattes exposing his plate in ChacaltayaCERN

The experiences of the Frenchman Pierre Auger proved to be a watershed where the physics of cosmic radiation was concerned, according to Escobar. He spread detectors that were far apart along the Midi Peak in the Alps, in 1935, and found that they were all set off at the same time. “Auger believed that there were particles with extraordinary energy entering the atmosphere, clashing with other particles and causing a downpour of secondary particles.”  The Frenchman called this phenomenon “extensive aerial showers.”  In 1939, two Brazilians, Marcello Damy and Paulus Pompeia, along with Gleb Wataghin, a Russian, detected in the tunnel on Nove de Julho Avenue in São Paulo city these penetrating showers: particles able to penetrate the soil (read article about Damy).

The main repercussion of these studies in Brazil materialized in 1947, when César Lattes and Giuseppe Occhialini, an Italian, detected the pi mesons (or pions) in the Pyrenees, as part of a team led by the Englishman Cecil Powell. Lattes confirmed the existence of this particle on the Chacaltaya mountain in Bolivia. The Brazilian scientist made savvy use of the acquired prestige and became one of the creators of CGPF, the Brazilian Center of Physical Research, in Rio de Janeiro, and of laboratories at the University of São Paulo, at Unicamp and at Chacaltaya.

Studies on cosmic radiation advanced until the Pierre Auger Observatory came into being, in 2005. The project’s chief objective is to investigate very high-energy cosmic particles (1020 electron-volts, i.e., 1 followed by 20 zeros), which are detected far more rarely than the low energy ones. Billions of the latter reach the Earth every second. The Auger researcher discovered, in 2007, that the high energy particles come from galaxies in the vicinity of the Milky Way (read Pesquisa FAPESP issue 142). Now, among other doubts, one must determine exactly what type of particles they are.

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