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Stu Shapiro on 100 years of relativity: How has Einstein’s theory shaped modern physics, astronomy? November 2015 marked the 100th anniversary of Albert Einstein’s theory of general relativity, which outlines principles of space, time and gravity. Einstein’s November 1915 presentation to the Prussian Academy sparked a century of discovery in physics and astronomy. Stuart Shapiro, professor of physics and astronomy at the U. of I. and an expert in general relativity, talked with the U. of I. News Bureau’s Liz Ahlberg about the theory and what we’ve learned in 100 years. by Liz Ahlberg Physical Sciences Editor Illinois News Bureau For us nonphysicists, what is the theory of general relativity? e theory of general relativity is Einstein’s theory of relativistic gravitation. It describes gravity as arising from the warping of space and time, or spacetime, caused by the presence of mass and energy. Mass curves spacetime, much like a stationary bowling ball curves a trampoline, and curved spacetime accelerates matter, much like a marble accelerates when placed on the warped trampoline. General relativity accounts not only for “weak-field” systems, such as the Earth and sun, which induce motions much slower than the speed of light, but also for “strong-field” systems, like black holes, where gravity is so strong that it prevents the escape of everything from its interior, including light. How has general relativity shaped the last 100 years of physics and astronomy? General relativity is one of the oldest and most successful “field theories,” and it has shaped the formulation of many other field theories. e notion that geometry, mass and energy are all intimately connected lies at the heart of theoretical physics. General relativity is the cornerstone of modern cosmology, which describes the physical history of the universe; the physics of neutron stars and black holes; the generation of gravitational waves; and countless other cosmic phenomena in which gravitation—weak-field and strong-field—is believed to play a dominant role. Much of physics and most all of astronomy fall into this category. Why was the theory such a game-changer? With general relativity, gravity is no longer described as an external force as in Newton’s theory, but as the consequence of spacetime’s warping or curvature. General relativity thus ties gravitation with geometry, a radically new concept at the time it was proposed. As a consequence of general relativity, light appears to bend as it travels past a gravitating object like the sun, the color of light reddens as it climbs out of a gravitational field, time slows down for travelers immersed in a gravitational field, and planets no longer orbit in precise, fixed, elliptical trajectories about stars, but instead move in near-elliptical orbits whose axes precess—rotate—in time. General relativity predicts phenomena that do not arise in Newton’s theory. One such phenomenon is a black hole. Another is a gravitational wave—a ripple of gravity that travels at the speed of light and causes the distance between suspended masses to oscillate when the wave passes by. Gravitational waves are generated when black holes and other compact, relativistic stars collide. Has what we’ve found in the past 100 years confirmed Einstein’s theory or found it lacking? All of the tests of general relativity to date have confirmed Einstein’s theory. However, most of these tests dealt with “weak” gravitational fields and measured the small corrections to Newton’s theory that general relativity predicts. e most important test was the slow orbital decay, or inspiral, of two neutron stars—compact stars with masses near that of the sun but confined to only a few miles across—locked in a binary orbit in our galaxy. e measured rate of the inspiral agrees closely with the prediction of general relativity, resulting in a Nobel Prize in 1994 for the two physicists who discovered the binary star and made the measurements. However, general relativity has not really been tested for “strong-field” systems, such as black holes, where it plays its greatest role. Such tests are forthcoming, as in the case of aLIGO, the Advanced Laser Interferometer Gravitational-Wave Observatory. aLIGO will look for the predicted gravitational waves from colliding black holes and neutron stars, where gravity becomes very strong. It was put into operation about one month ago at two sites in the U.S. e whole scientific community awaits the outcome with bated breath. What questions have yet to be answered surrounding general relativity? Do you foresee it continuing to shape science as we know it? A few outstanding questions are: Does general relativity correctly predict the exotic properties of black holes and of gravitational waves? Does a cosmological constant—a parameter that is sometimes referred to as “dark energy”—explain the observed acceleration of the universe, or is the acceleration due to some exotic new field of energy, or must the theory of general relativity be modified or replaced to account for this acceleration? Moreover, general relativity is only a “classical theory” in that it does not describe phenomena relating to small-scale, quantum phenomena. We are still awaiting a complete theory that unites gravity with quantum mechanics. Perhaps string theory is the seed of such a theory, but it remains largely untested. ese issues and more will be with us for many years. I suspect that general relativity in its present form—or something very close to it—will be with us, as well. n In April 2015, Stu Shapiro and his undergraduate research team released a widely acclaimed 3D movies of colliding black holes immersed in magnetized accretion disks. These visualizations were republished on the website of the journal Nature and on many other sites. To make the videos, the students used data from the computer simulations that Shapiro and his postdoctoral research team performed to solve Einstein’s equations of general relativity for this scenario. Colliding black holes are believed to occur in merging and active galaxies throughout the universe. Movies and descriptions of simulations for this and many other cosmic scenarios involving general relativity can be found at http:// tinyurl.com/shapiromovies. the Physics Illinois Bulletin Fall 2015 7 6 University of Illinois at Urbana-Champaign
