General Relativity in the Universe of Astronomy and Physics

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General Relativity in the Universe of Astronomy
and Physics

• Bernard Schutz
• Max-Planck-Institut für Gravitationsphysik
• (Albert-Einstein-Institut -- AEI)
• Golm bei Potsdam
• and
• Department of Physics and Astronomy
• University of Cardiff, Wales
• http//www.aei-potsdam.mpg.de

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At the beginning of the new century...

• As we enter the 21st Century, it is interesting
  to remember what some physicists thought 100
  years ago. Typical was the American Albert
  Michelson, one of the finest experimenters of his
  day. His experiments on light, begun in Potsdam,
  prepared the way for Einstein. But in 1898 he
  did not see this. Instead he wrote
• it seems probable that most of the grand
  underlying principles have been firmly
  established and that further advances will be
  in the application of these principles.
• Within 7 years, Albert Einstein and Max Planck
  had made this complacency look silly. The 20th
  Century began with relativity and atomic theory
  and closed with a revolution in our understanding
  of the Universe. Today there is no complacency.
  Physicists have a job to do.
• We have to explain where the Universe came from
  and why the laws of physics are the way they are.

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The 20th century a century for relativity

• In 1905, Albert Einstein created a revolution in
  physics with special relativity. The revolution
  was mainly in the way we think about time.
• The main idea moving clocks run slowly.

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After Einstein, time was relative. The rate at
which a clock ticks goes to zero as the speed
with which it moves approaches the speed of light
c.

• This effect can now be measured accurately by
  watching how slowly an elementary particle decays
  when it is moving at nearly the speed of light in
  a modern accelerator, such as those at CERN and
  DESY.

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General Relativity

• In 1915 Einstein introduced his new theory of
  gravity, general relativity. This modernized the
  250-year-old theory of Isaac Newton.
• Within weeks, the Potsdam astronomer Karl
  Schwarzschild found the first solution of
  Einsteins equations. It describes a black hole.

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What is relative about general relativity? Why
use the same word, relativity? The answer is
time gravity also makes time relative.
Einstein was the first to understand that(1)
gravity deforms time, and (2) the deformation of
time is gravity.

• Gravity slows time. The stronger gravity gets,
  the slower time goes.
• Near a black hole, time slows dramatically.

normal
slow
Rate of time near a black hole
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Even near the Earth, gravity slows time. The
effect is large enough that the GPS navigation
system has to correct for it.

• The GPS system has many satellites (like this one
  being built) that carry precise clocks that must
  stay synchronized with the master clocks on the
  ground far below. But the master clocks are slow!

To give accurate positions, the satellites
clocks are built to run too slow. Otherwise they
would have to correct their clocks for the
slowing of by gravity almost once a minute!
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General Relativity a tool for astronomy

• General relativity is central to modern
  astronomy. Astronomers use it as an every-day
  tool to understand the Universe they see in their
  telescopes.

• We will look at
• Neutron stars
• Black holes

• Gravitational lenses
• Gravitational waves
• The Big Bang

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Neutron stars as pulsars

• Neutron stars are formed by the collapse of the
  core of a star when it explodes as a supernova.

They are the last stop before a black hole 1.4
times the mass of our Sun inside a sphere only 10
km in radius. Pulsars are neutron stars that
emit a lighthouse beam that sweeps past the Earth
as they spin. Some have been observed to spin at
up to 600 times per second.
(Duncan Lorimer 1999)
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Black holes and neutron stars orbiting larger
stars can be powerful X-ray sources

• Gas from a big star (white) falls onto a neutron
  star or black hole and is heated as it gets
  compressed (red disk).

This hot gas gives off X-rays. There are
hundreds of such systems in the Milky Way, many
with black holes.
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There is a giant black hole (weighing 1 million
Suns or more) at the center of almost every
galaxy...
including our own.
Quasars are powered by black holes made from a
billion Suns!
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The end of time inside a black hole
Even for a large black hole, say 1 million solar
masses, the particle has only minutes to live.
After that, time simply ends. Scientists
believe that a quantum theory of gravity will
change this story, and tell us what really
happens to time inside a black hole.

• What happens to an object that falls into a black
  hole?
• If it is not torn apart by strong gravity, then
  it will still not last long. According to
  general relativity, it soon reaches the end of
  time.

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Gravitational lenses

• Einstein showed that gravity bends the path of
  light refraction, just as in a glass lens.
• The Universe has given us many examples. In this
  one, called the Einstein Cross, the central
  galaxy makes 4 images of a distant quasar.

Recent observations of lensing in our own
Galaxy have revealed a large population of
black holes made from stars, many more than
previously believed.
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Lensing by a cluster of galaxies
All the blue galaxies are much further away than
the cluster of yellow galaxies, which acts as a
lens. One blue galaxy has at least 5 different
images in this photograph.
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Gravitational waves

• Relativity requires that nothing moves faster
  than light. This means that changes in
  gravity also move outwards, as ripples of
  gravity. They move at the speed of light.

Likely sources of gravitational waves binary
stars, exploding stars, spinning neutron stars,
the Big Bang.
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Gravitational waves have been measured (but only
indirectly)!

• Radio astronomers Russell Hulse and Joseph Taylor
  won the 1993 Nobel Prize for Physics for
  discovering the Binary Pulsar.

Taylor
Hulse
The loss of energy to gravitational waves makes
the orbit shrink, at exactly the rate predicted
by Einsteins theory.
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Now we want to detect gravitational waves
directly, a new kind of astronomy

• On the University farm at Ruthe, near Hanover,
  German and British physicists are building
  GEO600, one of 3 detector systems being built
  around the world that should make the first-ever
  direct detections.

Visits to GEO600 are part of Expo2000!
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The international gravitational wave network

• The major projects
• LIGO (USA)
• VIRGO (FRA-IT)
• GEO (D-GB)
• will work together to improve sensitivity
• x10 by 2005 using technology already developed
  (e.g. for GEO)
• another x10 by 2010 using completely new designs.

• These detectors will see
• Black holes as they collide in complete silence
• Neutron stars as they collide and produce
  Natures most spectacular events, a gamma ray
  burst
• Supernovae as they create neutron stars and black
  holes from ordinary stars
• Pulsars as they spin hundreds of times per second

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Moving into space LISA

• By 2010, ESA and NASA may launch a joint mission
  to study the giant black holes in galaxies.
• First proposed in the USA, LISA has been
  developed in Europe and the USA by an
  international team of scientists. It is an ESA
  Cornerstone Mission.
• These studies have produced an affordable mission
  with wide appeal to astronomers and physicists.

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Computer Simulations of Black Holes and
Gravitational Waves

• The following two computer simulations were
  performed by members of the Albert Einstein
  Institute in Golm. They both start with no black
  hole, just focused gravitational waves. In the
  first, the waves do not have enough energy to
  make enough gravity to form a black hole, so they
  disperse. In the second, they trap themselves in
  a black hole.
• The bottom part of both diagrams shows the
  slowing of time, as a measure of the strength of
  gravity.
• (Simulations performed by AEI scientists and US
  collaborators at the NCSA supercomputer center,
  Illinois, USA, 1999.)

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Gravitational waves that almost trap themselves
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Gravitational waves strong enough to trap
themselves in a black hole
(Watch the Horizon form, the surface of the black
hole.)
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Computer simulations of colliding black holes

• In the following simulation, also by the same
  group, two nearby black holes come together to
  form a single one, giving off gravitational
  waves. Future gravitational wave detectors will
  look for waves like these.
• One black hole is 50 heavier than the other, and
  both spin in different directions.
• This is one of the most detailed simulations of
  black hole collisions so far performed.
• (Simulation performed by AEI scientists and US
  collaborators at the NCSA supercomputer center,
  Illinois, USA, 1999.)

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Collision of two spinning black holes
Wave strength and hole boundaries
Slowing of time
(Look for the formation of the common horizon,
the outer boundary around both black holes.)
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Cosmology the study of the Universe
All of this is explained within the Big Bang
model, which is described by general relativity.
In the Big Bang, the Universe began at a certain
time (about 15 billion years ago) in a big
explosion of primeval matter.

• Astrophysicists have made great progress
  understanding
• the formation of galaxies
• the creation of the chemical elements
• the radiation from the Big Bang that fills the
  universe
• the ages of known stars and planets

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Our earliest picture of the Universe radiation
emitted about 100 000 years after the Big Bang
The microwave background, imaged by the COBE
satellite.
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Time and the Universe

• The Big Bang was the beginning of time, according
  to general relativity. Within this theory, it is
  meaningless to ask about before the Big Bang.
• Relativity explains much of the Universe, but it
  raises new questions that it cannot answer
• Why was there a Big Bang?
• Is there only one Universe?
• What is the nature of time?

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The future of gravitational physics

• Physicists believe that, in spite of its great
  success, general relativity is incomplete it
  cant deal with the beginning of time or with the
  end of time inside black holes.
• It is also not a quantum theory, so it is not
  consistent with our other theories of physics.

• Physicists are looking for a new theory, unifying
  all the forces of nature. This should explain
  gravity, electromagnetism, and the nuclear
  forces. It should tell us what happens inside
  black holes.
• The current best hope is String Theory / M Theory.

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How do we find the new theory?
Theoretical studies
Experiments

• Astronomical
• observations
• Gravitational waves
• Big Bang
• Dark matter

New Understanding

• The origin of matter
• Quantum gravity

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The 21st century time for quantum gravity

• When physicists eventually create a complete
  theory of quantum gravity, it may answer the
  questions general relativity cant answer, about
  the beginning and end of time.
• More deeply, remember that gravity is the warping
  of time. If we learn something new about
  gravity, then we learn something about time.
• For me, this is the most exciting prospect for
  the 21st century Quantum gravity can change our
  understanding of time itself.