Warped Passages - Lisa Randall [50]
Conversely, a photon that was moving towards a gravitational source would increase its frequency. In 1965, the Canadian-born physicist Robert Pound and one of his students, Glen Rebka, measured this effect by studying gamma rays emitted from radioactive iron that was placed at the top of the “tower” of Harvard’s Jefferson Lab, the building where I now work. (Though it’s part of the building, an elevated attic area in Jefferson Lab and the floors beneath it are known as “the tower.”). The gravitational fields at the top and bottom of the tower were slightly different, since the top is slightly further from the center of the Earth. A high tower would be best for this measurement, since it would maximize the difference in height between where the gamma rays were emitted (the top of the tower) and where they were detected (the basement). But even though the tower consists of just three floors, an attic, and some windows that peer out above the attic—it’s all of 74 feet high—Pound and Rebka managed to measure the difference in frequency between the emitted and absorbed photons with incredible precision, five parts in a million billion. They thereby established that the general relativity predictions for the gravitational redshift were correct to at least 1% accuracy.
A second experimentally observable consequence of the equivalence principle is the bending of light. Gravity can attract energy as well as mass. After all, the famous relation E = mc2 means that energy and mass are closely connected. If mass experiences gravity, then so should energy. The Sun’s gravity influences mass, and likewise affects the trajectory of light. Einstein’s theory predicted exactly the amount light should bend under the Sun’s influence. These predictions were first confirmed during the solar eclipse of 1919.
The English scientist Arthur Eddington organized expeditions to the island of Principe off the coast of West Africa and to Sobral in Brazil, where the eclipse could best be seen. Their purpose was to photograph the stars in the neighborhood of the eclipsed Sun and check whether stars that appeared near the Sun moved relative to their usual positions. If the stars did appear to be shifted, that would mean that their light was traveling along a bent trajectory. (The scientists needed to make their measurements during an eclipse so that the sunlight wouldn’t overwhelm the much dimmer light of the stars.) Sure enough, the stars appeared in just the right “wrong” places. The measurement of the correct bending angle provided strong evidence supporting Einstein’s theory of general relativity.
Incredibly, the bending of light is now so well established and understood that it is one of the tools that was used to probe the distribution of mass in the universe and look for dark matter in the form of small, burnt-out stars that no longer emit light. Like black cats on a moonless night, such objects are very hard to see. The only way to observe them is through their gravitational effects.
Gravitational lensing is one way that astronomers can learn about dark objects; dark objects, like everything else, interact via gravity. Although the burnt-out stars do not themselves emit light, there can be bright objects behind them (from our perspective) whose light we can see. Without