What is Gravity?

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What is Gravity?
"Colliding Black Holes"CreditNational Center
for Supercomputing Applications (NCSA)

• Fred Raab,
• LIGO Hanford Observatory

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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

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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

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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
• Newtons 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

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Special Relativity

• Einsteins 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
  Einsteins 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)

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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 Newtons law of
  universal gravitation, which showed that all
  objects fall exactly the same under gravitation,
  and confirmed by Newtons 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

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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

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Mass Warps Space, Affecting Paths of Objects and
Light

• 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

A massive object shifts apparent position of a
star

• Einstein Cross
• Photo credit NASA and ESA

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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 1950s
• 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!

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The Frontier of Relativity Gravitational Waves

• 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

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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

Hulse, Taylor receive Nobel Prize
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Basic Signature of Gravitational Waves for All
Detectors
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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.
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Is General Relativity the final word on gravity?
No!
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What was going on in the subatomic world

• Spurred on by Einsteins 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

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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

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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

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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

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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

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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

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Recommended Reading

• Was Einstein Right, by Clifford Will. A bit
  ancient (more than 10 years old) accounting of
  progress in verifying Einsteins theory.
• Black Holes and Time Warps Einsteins
  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.
• Einsteins Unfinished Symphony, by Marcia
  Bartusiak. A highly readable account of the
  search for gravitational waves.