Why Does E=mc2_ - Brian Cox [91]
You may well have noticed that nothing we have said has singled out one type of object over another. In particular, light itself should also move through spacetime along a geodesic. In each spacetime patch that it passes over, the light travels along one of the 45-degree lines we introduced in Chapter 4 but, upon sewing all the patches together, we will find a trajectory that bends through space. The bending simply reflects the way in which the spacetime is warped by the presence of mass and energy. Just as for the case of the earth in orbit around the sun, its path through space is a shadow of its four-dimensional geodesic. The power of the equivalence principle and the implied bending of light can be illustrated nicely by another thought experiment.
Imagine that you are standing on the earth and you fire a laser beam horizontally. What happens to it? The principle of equivalence tells us what happens. The light falls toward the ground at exactly the same rate as would an object that is released from rest at the precise moment that the laser is fired. If Galileo had access to a laser and he fired it horizontally off the Leaning Tower of Pisa at the same time as dropping a cannonball, then Einstein predicts that the laser beam would hit the ground at the same time as the cannonball. The problem with this experiment in reality is that the earth’s surface curves away very quickly and the laser would never actually hit the ground because it would run out of earth. If we imagine instead that we are standing on a flat earth, then that problem goes away and we would expect the laser beam to hit the ground at exactly the same time as the cannonball, only a very great deal farther away. In fact, if the cannonball took a second to hit the ground, then the laser would hit the ground one light-second from the tower, which is just over 186,000 miles away.
The description of gravity as geometry is certainly immensely satisfying and it leads to quite startling conclusions but, as we have emphasized throughout this book, it is ultimately useless unless it leads to predictions that can be tested against experiment. Fortunately for Einstein, he had to wait only four years for his exotic predictions to be confirmed.
The first great test of Einstein’s new theory came in 1919 when Arthur Eddington, Frank Dyson, and Charles Davidson wrote a paper titled “A Determination of the Deflection of Light by the Sun’s Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919.” The paper was published in the Philosophical Transactions of the Royal Society of London and contains the immortal words “both of these point to the full deflection of 1.”75 of Einstein’s generalized relativity theory.” Overnight, Einstein became a global superstar. His esoteric theory of curved spacetime had been vindicated following the not inconsiderable efforts of Eddington, Dyson, and Davidson: To see the eclipse, they had to make expeditions to Sobral in Brazil and Principe, off the western coast of Africa. The eclipse allowed them to look at stars lurking very close to the sun that would otherwise be obscured by its light. This is the starlight best suited to testing Einstein’s theory, because it should be deflected the most since the spacetime curvature is greater the closer you get to the sun. In essence, Eddington, Dyson, and Davidson were looking to see whether the stars shifted their position in the sky as the sun passed by. Quite literally, the sun bends spacetime and acts like a lens, distorting