Relativity in Your Hand y v,^ \ When Einstein developed the general theory of relativity, he was jy trying to improve our understanding of how the universe works. At mini ~^tt rlu mi u l\v ■ r.....in - i ,i ir. ■ .is 111. -k rli.in Mirhv mií h -i in . pi.k r k .11 JllJ^^'^^^^r'fa^, '^^^ gravity calculations. However, as often happens in physics, GR has , ' $ Y^^iW\ applications that would not have been foreseen by Einstein or his How many of us have used a smartphone to get directions? Or to tag our location on social media? Or to find a recommendation for a nearby restaurant? These activities depend on GPS. GPS uses radio signals from a network of satellites orbiting Earth at an altitude of 20,000 km to pinpoint the location of a GPS receiver. The accuracy of GPS positioning depends on precision in time measurements of billionths of a second. To achieve such timing precision, however, relativity must be taken into account. Special relativity shows that if we place a clock on a satellite and compare its recorded time to an identical clock in our rest frame on Earth, the satellite's clock will appear to be running behind. For the GPS satellites, this difference amounts to about 7 microseconds per day. Now enter GR. Because the satellites are farther from Earths center, space-time is less curved and their clocks tick faster than those on the ground by about 45 microseconds per day. The combined effect is that the satellites' clocks tick faster by about 38 microseconds each day. The effects from relativity are nearly 1,000 times greater than the required timing precision for GPS. Without correcting for this, position errors would accumulate so quickly that the system would be useless for navigation in a matter of minutes. National Aeronautics and Space Administration Image Credits Background image: Galaxy Cluster Abell 2218's : "Gravitational Lens" {credit: NASA, ESA, Richard Ellis : (Caltech), and Jean-Paul Kneib {Observatoire Midi-Pyrenees, it- France)) \ Front cover: Portrait of Albert Einstein {©The University of f New Hampshire, used with permission) : Inside: Mercury's limb imaged by MESSENGER {credit: • NASA/Johns Hopkins University Applied Physics : Laboratory/Carnegie Institution of Washington); equipment used to measure the stars near the eclipse in 1919 {credit: :■' Science Museum/Science &C Society Picture Library); artist's : impression of Cygnus XT {credit: NASA/CXC/M.Weiss); | Twin Quasar SBS 0957+561 {credit: ESA/Hubble & NASA); j artist's impression of Gravity Probe B {credit: GP-B Image : Archive, Stanford University). : Back page: Artist's impression of GPS satellites {credit: : NASA/Goddard Space Flight Center). | This brochure is produced by NASA's Physics of the Cosmos (PCOS) Program Office. The PCOS Program concentrates on activities, technologies, and projects to enhance our understanding of how the universe works. The program's purpose is to explore some of the most fundamental questions regarding the physical forces and laws of the universe: the validity of Einstein's general theory of relativity and the nature of space-time; the behavior of matter and energy in extreme environments; the cosmological parameters governing inflation and the evolution of the universe; and the nature of dark matter and dark energy. Visit PCOS on the web: http://pcos.gsfc.nasa.gov Further Reading • Bartusiak, Marcia. Einstein's Unfinished Symphony. Joseph Henry Press, 2000. • Hawking, Stephen. A Brief History of Time. Bantam Books, 1988. • Schutz, Bernard. Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity. Cambridge University Press, 2004. • Thorne, Kip S. Black Holes and Time Warps: Einstein's Outrageous Legacy. W. W. Norton & Company, 1995. • Einstein Online also has a list of suggested reading: http://www.einstein-online.info/fiurther_reading NASA's Physics of the Cosmos Program A Century of General Relativity Gravity 2.0 Einstein's general theory of relativity (GR), published in 1916, stands as one of the greatest triumphs of theoretical physics. It completely revised the scientific understanding of gravity first established by the work of Isaac Newton in the late 1600s. With an earlier theory known as special relativity, published in 1905, Einstein detailed the physical relationship between space and time. Over the following decade, he worked to incorporate gravity into this picture. He presented his final result to the Prussian Academy of Science in 1915. In special relativity, Einstein showed that space and time were interwoven as a single structure he dubbed space-time. He realized in GR that gravitational fields can be understood as the motion of particles—stars, planets, and even light—on the stretched and curved surface of space-time. Testing this picture means making precise measurements of GR's subtle consequences, such as the deflection of starlight as it passes near the Sun. Scientists continue to look for cracks in the theory, testing GR's predictions using laboratory experiments and astronomical observations. For the past century, Einstein's theory of gravity has passed every hurdle. Today, GR is present in our everyday lives, through the ubiquitous use of GPS technology (see back page for more), and astronomers are embarking on a new quest to look for GR's predicted waves rippling through space-time—gravitational wave astronomy. NP-2Ü14-1Ü-194-GSFC 100 Years and Counting ... Tests of general relativity {GR) require strong gravity, massive objects and/or high-precision measurements—conditions we don't readily find on Earth. Einstein proposed three tests of GR when he first published the theory in 1915: the precession of Mercury's orbit, the bending of starlight near the Sun, and the gravitational redshift of light. These represented just the beginning of a long list of tests which could be performed to bolster the case for GR. The timeline below shows a sampling of tests which have confirmed GR's predictions over the past century, with a preference for astrophysical confirmations. The dates reflect the publication of the results, not necessarily the date of observation.