In his theory of relativity Albert Einstein reasoned that, because energy and matter are one, gravity affects not only matter, but also energy. Einstein proposed that the light energy of stars bends when subjected to a strong gravitational field (such as the field of a planet). In 1919, teams from the Royal Astronomical Society set up in Brazil and West Africa to observe and measure the position of stars during a solar eclipse. They found that starlight did indeed bend as it passed through the sun's gravitational field, just as Einstein had predicted. By testing Einstein's hypothesis, astonomers proved that the theory of relativity was valid. They could then use the theory as a basis for further scientific investigation.