What is Gravity?

1. What is Gravity? "Colliding Black Holes"Credit:National Center for Supercomputing Applications (NCSA) Fred Raab, LIGO Hanford Observatory

2. Gravity in history • Gravity is the phenomenon with which mankind has the most experience. • Aristotle believed that gravity pulled down “heavier” objects more than lighter ones because heavier objects were more earthlike • Aristotelian thought dominated history until the Renaissance… Raab: Relativity

3. Renaissance in gravity • Galileo (1564-1642) made key discoveries in study of gravity and motion • Discovery of “relativity” of motion: concept that motion can only be determined relative to a frame of reference • Discovery that the motion of a falling object was described by a simple mathematical rule that did not depend on what was falling • Isaac Newton (1643-1727), the master of motion, codified Galilean relativity into his laws of mechanics • space and motion were defined relative to a reference frame • time was an absolute quantity  all clocks ticked at the same rate • law of universal gravitation  “action at a distance” Raab: Relativity

4. Fast Forward 200 Years • About 100 years ago, Albert Einstein (1879-1955) began to explore anew the concepts of space and time • Only two forces were known at that time: electromagnetism & gravity • Theory of electromagnetism had correctly described motors, generators, radio and light transmission • Newton’s mechanics had correctly described motion of neutral matter, including an accurate description of how gravity acts, but not how it arises • But the two theories were incompatible in the description of rapidly moving electrical charges • Einstein eventually discovered that the incompatibility was caused by the presence of “absolute” time • Two travelers in relative motion, not only cannot define who is moving, if they move fast enough they also disagree on the time between events Raab: Relativity

5. e.g., explanation of muon flux at surface of earth Special Relativity • Einstein’s Special Theory of Relativity (1904) described motion of matter in the absence of gravity • Everything is relative, except the speed of light, which is absolute, in agreement with electromagnetism • Speed of light is a property of space • There is a special symmetry between space and time, mass and energy • All observers in relative motion observe the same laws of physics but they disagree on the definitions of time and space used to describe those laws • Theory was controversial at the time • Nobel committee did not mention Relativity in Einstein’s Nobel Prize citation, which was for the explanation of the photoelectric effect • Predictions: • Moving rulers contract • Moving clocks tick slower • Matter is just a “frozen” form of energy (e.g., nuclear power industry) Raab: Relativity

6. The Dawn of General Relativity • Einstein struggled another decade to discover how to incorporate gravity into relativity • A big clue was known by Newton and has often been observed by us since the dawn of the space age • “Microgravity” as we view it on video of astronauts aboard the Space Shuttle or the Space Station was predicted by Newton’s law of universal gravitation, which showed that all objects fall exactly the same under gravitation, and confirmed by Newton’s pendulum experiments • Einstein eventually realized that these objects all followed the same path through space and time and that gravitation was the consequence of geometrical properties of space and time Raab: Relativity

7. The Essence of General Relativity • Space and time are things, not concepts • Space and time affect the motion of objects and objects affect the character of space and time • Things move along the shortest path through the four dimensions of space and time • The presence of matter (or energy) warps space and time, changing their geometrical properties • The paths of all objects naturally follow the curvature of space and time • In flat spacetime, General Relativity reduces to Special Relativity Raab: Relativity

8. Presence of mass gives space the appearance of lumpy glass as evidenced by the bending of light First observed during the solar eclipse of 1919 by Sir Arthur Eddington, when the Sun was silhouetted against the Hyades star cluster Einstein Cross Photo credit: NASA and ESA Mass Warps Space, Affecting Paths of Objects and Light A massive object shifts apparent position of a star Raab: Relativity

9. Was Einstein Right? • Remarkably so, but it took time to appreciate how right he was! • Checking relativity needed good rulers and clocks, and measurements over large expanses of space and time • Golden age of experimental relativity began in late 1950’s • Radar had become an excellent ruler • Atomic timekeeping came of age • Space flight allowed controlled measurements over large distances • Dozens of new effects predicted by relativity have been confirmed to accuracies between 0.1% to 0.0001% • No test of general relativity has ever found a discrepancy! Raab: Relativity

10. Gravitational waves are ripples in space when it is stirred up by rapid motions of large concentrations of matter or energy Rendering of space stirred by two orbiting black holes: The Frontier of Relativity: Gravitational Waves Raab: Relativity

11. Emission of Energy by Gravitational Waves Has Been Observed Emission of gravitational waves Neutron Binary System – Hulse & Taylor PSR 1913 + 16 -- Timing of pulsars 17 / sec · · ~ 8 hr • Neutron Binary System • separated by 106 miles • m1 = 1.4m; m2 = 1.36m; e = 0.617 • Prediction from general relativity • spiral in by 3 mm/orbit • rate of change orbital period Raab: Relativity Hulse, Taylor receive Nobel Prize

12. Basic Signature of Gravitational Waves for All Detectors Raab: Relativity

13. The Laser Interferometer Gravitational-Wave Observatory LIGO (Washington) LIGO (Louisiana) Brought to you by the National Science Foundation; operated by Caltech and MIT; the research focus for more than 500 LIGO Scientific Collaboration members worldwide. Raab: Relativity

14. Is General Relativity the final word on gravity? No! Raab: Relativity

15. What was going on in the subatomic world: • Spurred on by Einstein’s early work, quantum mechanics happened • Einstein confirmed reality of atoms • Einstein confirmed that energy could only be exchanged in units of “quanta” • Quantum mechanics was birthed (principally due to the work of others) from these basic ideas • Quantum mechanics explains the structure of atoms, the emission and absorption of light and the stability of matter • New forces (nuclear force, weak force) are discovered • Eventually all forces (electromagnetism, nuclear and weak) are shown to obey quantum laws and it becomes understood that they were all of comparable strength in the conditions that exited in the early universe Raab: Relativity

16. What is not talked about in polite company… • General Relativity gives a beautiful and accurate description of the universe on scales extending from macroscopic objects to the edge of the visible universe and back to the age when atoms first formed • GR has passed many experimental and observational tests and never failed • Quantum Mechanics gives an extremely accurate description of the universe on scales comparable to and smaller than an atom • QM has passed many experimental tests of incredible accuracy (up to 12 decimal places!) without failure • Unfortunately these two theories are incompatible Raab: Relativity

17. Incompatibilities of GR and QM • QM describes an indeterminate world ruled by chance whereas GR is determinate • QM objects can only have discrete energy levels and exchange quanta of energy; GR objects have a continuum of energy levels and can exchange energy smoothly • In the early universe and in the interiors of black holes, the subatomic and the cosmic scales become the same, so a single set of laws must apply; GR and QM must be approximations that apply as the universe evolves Raab: Relativity

18. A popular contender: string theory • Basic idea is that the objects we call particles and the “exchange particles” that mediate the forces of nature are actually made up of tiny vibrating strings • By making particles of strings (which have some spatial extent) rather than of points, one avoids the troublesome infinities that crop up • The same pieces of string can emulate different particles depending on how they vibrate • To have the mathematics reproduce the richness of particles, forces and symmetries in nature requires that the strings be able to vibrate in many dimensions Raab: Relativity

19. Extra dimensions? What extra dimensions? • The most popular variants of string theory require at least 10 spatial dimensions • But if there are 10 spatial dimensions how come we only see 3? • One possibility is that the extra dimensions are small and rolled up; think about a human hair • Another possibility is intriguing • Suppose the electrical, weak and strong forces only act in 3 spatial dimensions, but gravity can leak out into the extra dimensions • Since almost all our information about the universe has come from electromagnetic signals, why would we know about the other dimensions? • Furthermore, the leakage of gravity into the other dimensions might explain why gravity is weak compared to the other 3 phenomena Raab: Relativity

20. Should we believe in string theory? No! • A scientific theory must make falsifiable predictions, subject to experimental or observational testing! • Until string theory makes unique, falsifiable predictions that are confirmed, it is merely a mathematical model, not science. • A few predictions from string theory that might be observable: • Sparticles might produced in the next generation of high energy colliders • Cosmic strings might be detectable by gravitational wave detectors like LIGO Raab: Relativity

21. Recommended Reading • “Was Einstein Right”, by Clifford Will. A bit ancient (more than 10 years old) accounting of progress in verifying Einstein’s theory. • “Black Holes and Time Warps: Einstein’s Outrageous Legacy”, by Kip Thorne. From basics to time travel in a long, but rewarding read. • “Black Holes, Wormholes and Time Machines”, by Jim Al-Khalili. Some of the most challenging concepts of modern physics described in a humorous and plain-spoken style. • “Einstein’s Unfinished Symphony”, by Marcia Bartusiak. A highly readable account of the search for gravitational waves. Raab: Relativity