Gravitational Waves: A New Era of Astronomy Begins
OLLI Lecture,     April 17, 2018
by Sam Wormley

On September 14th, 2015, a ripple in the fabric of spacetime,
created by the violent collision of two distant black holes
over a billion years ago, washed across the Earth. As it did,
two laser-based Michelson-like interferometers, 50 years in
the making-one in Louisiana and the other in Washington
State-momentarily twitched, confirming a century-old
prediction by Albert Einstein and marking the opening of a new
era in astronomy.

On August 17th, 2017, the Advanced Laser Interferometer
Gravitational-Wave Observatory (LIGO) registered tiny ripples
in spacetime, produced by a pair of frantically orbiting
neutron stars right before they collided. A Gamma Ray Burst
was observed at the same time from the same region of the sky.
The excitement is fully justified--observing both
gravitational waves and electromagnetic radiation from the
catastrophic coalescence of two hyper-dense neutron stars
provides astronomers with a wealth of new, detailed

We will explore this multi-messenger astronomy, the study of
the universe using fundamentally different types of emission.

Sam Wormley earned degrees in mathematics and electrical
engineering and taught as an adjunct professor of astronomy at
Marshalltown Community College for 17 years. Wormley regularly
teaches OLLI classes on science and technology.


The Michelson-Morley experiment (1887)

Michelson-Morley experiment was devised on the premise that if
Ether exists in space, then Earth moving through that Ether
medium would feel 'Ether winds', just like how a bike rider
moving through air would feel air winds.

And it was premised that light beams passed in different
directions i.e. one beam passed perpendicular to the direction
of the Ether wind and another beam passed along the direction
of the Ether wind would take different times to travel the
same distance.


   o  Length of orthogonal arms are identical and fixed.

   o  The speed of light along orthogonal arms was expected 
      to be different.
To the astonishment of the scientific minds, the experiment
yielded no interference between the returning beams. It
implied that both the returning beams have arrived at the half
silvered mirror at the same time contrary to their expectation
that the 'perpendicular beam' would take longer for the return
trip than the 'parallel beam'.

On June 30, 1905, Abert Einstein in his paper, "ON THE 

  Examples of this sort, together with the unsuccessful
  attempts to discover any motion of the earth relatively to
  the "light medium," suggest that the phenomena of
  electrodynamics as well as of mechanics possess no
  properties corresponding to the idea of absolute rest. They
  suggest rather that, as has already been shown to the first
  order of small quantities, the same laws of electrodynamics
  and optics will be valid for all frames of reference for
  which the equations of mechanics hold good. We will raise
  this conjecture (the purport of which will hereafter be
  called the "Principle of Relativity") to the status of a
  postulate, and also introduce another postulate, which is
  only apparently irreconcilable with the former, namely, that
  light is always propagated in empty space with a definite
  velocity c which is independent of the state of motion of
  the emitting body. These two postulates suffice for the
  attainment of a simple and consistent theory of the
  electrodynamics of moving bodies based on Maxwell's theory
  for stationary bodies. The introduction of a "luminiferous
  ether" will prove to be superfluous inasmuch as the view
  here to be developed will not require an "absolutely
  stationary space" provided with special properties, nor
  assign a velocity-vector to a point of the empty space in
  which electromagnetic processes take place.
When Einstein announced his General Relativity Paper in 1915,
he rewrote the rules for space and time that had prevailed for
more than 200 years, since the time of Newton, stipulating a
static and fixed framework for the universe. Instead, Einstein
said, matter and energy distort the geometry of the universe
in the way a heavy sleeper causes a mattress to sag, producing
the effect we call gravity.

A disturbance in the cosmos could cause space-time to stretch,
collapse and even jiggle, like a mattress shaking when that
sleeper rolls over, producing ripples of gravity:
gravitational waves.

Einstein was not quite sure about these waves. In 1916, he
told Karl Schwarzschild, the discoverer of black holes, that
gravitational waves did not exist, then said they did. In
1936, he and his assistant Nathan Rosen set out to publish a
paper debunking the idea before doing the same flip-flop

According to the equations physicists have settled on,
gravitational waves would compress space in one direction and
stretch it in another as they traveled outward.


In 1969, Joseph Weber, a physicist at the University of
Maryland, claimed to have detected gravitational waves using a
six-foot-long aluminum cylinder as an antenna. Waves of the
right frequency would make the cylinder ring like a tuning
fork, he said.

Others could not duplicate his result, but few doubted that
gravitational waves were real. Dr. Weber's experiment inspired
a generation of scientists to look harder for Einsteinian
marks on the universe.


In 1978, the radio astronomers Joseph H. Taylor Jr. and
Russell A. Hulse, then at the University of Massachusetts
Amherst, discovered a pair of neutron stars, superdense
remnants of dead stars, orbiting each other. One of them was a
pulsar, emitting a periodic beam of electromagnetic radiation.
By timing its pulses, the astronomers determined that the
stars were losing energy and falling closer together at
precisely the rate that would be expected if they were
radiating gravitational waves.

The radiation of gravitational waves has been inferred from
the Hulse-Taylor binary (and other binary pulsars). Precise
timing of the pulses shows that the stars orbit only
approximately according to Kepler's Laws: over time they
gradually spiral towards each other, demonstrating an energy
loss in close agreement with the predicted energy radiated by
gravitational waves. For their discovery of the first binary
pulsar and measuring its orbital decay due to
gravitational-wave emission, Hulse and Taylor won the 1993
Nobel Prize in Physics.

Hulse-Taylor Pulsar PSR B1913+16 (3 min)


Gravitational Waves Explained (3 min)

  Mass and Energy (Energy-momentum) warp spacetime
  creating a gravitational field.  Whereas accelerating mass 
  and energy, such as orbiting black holes, neutron stars, or
  you and me with locked arms spinning, creates ripples
  in spacetime--gravitational waves.


Gravitational Waves LIGO interferometer capture (1 min)

September 14, 2015 - Observation of Gravitational Waves from 
a Binary Black Hole Merger



Gravitational Waves Explained Simply to the Public, LIGO video
animation documentary introduction (9 min)

On 15 June 2016, LIGO announced the detection of a second
gravitational wave event, recorded on 26 December 2015, at
3:38 UTC. Analysis of the observed signal indicated that the
event was caused by the merger of two black holes with masses
of 14.2 and 7.5 solar masses, at a distance of 1.4 billion
light years. The signal was named GW151226.

In 2017, LIGO saw four further gravitational waves: the first
(GW170104) in January; the second (GW170608) in June; the
third (GW170814) and the fourth (GW170817) both in August.
GW170814 and GW170817 were also seen by the Virgo


On August 17th, 2017, the Advanced Laser Interferometer
Gravitational-Wave Observatory (LIGO) registered tiny ripples
in spacetime, produced by a pair of frantically orbiting
neutron stars right before they collided (GW170817). A Gamma
Ray Burst was observed at the same time from the same region
of the sky.

The excitement is fully justified--observing both
gravitational waves and electromagnetic radiation from the
catastrophic coalescence of two hyper-dense neutron stars
provides astronomers with a wealth of new, detailed

Ripples of Gravity, Flashes of Light (4 min)

The gravitational wave signal indicated that it was produced
by the collision of two neutron stars with a total mass of
2.82 +0.47 -0.09 solar masses.

If low spins are assumed, consistent with those observed in
binary neutron stars that will merge within a Hubble time (the
age of the universe), the total mass is 2.74 +0.04 -0.01 solar

Scientific interest in the event has been enormous, with
dozens of preliminary papers (and almost 100 preprints)
published the day of the announcement, including eight letters
in Science, six in Nature, and 32 in a special issue of The
Astrophysical Journal Letters devoted to the subject. The
interest and effort was global: the paper describing the
multi-messenger observations is coauthored by almost 4,000
astronomers (about one-third of the worldwide astronomical
community) from more than 900 institutions, using more than 70
observatories on all seven continents and in space.

This is not the first observation that is known to be of a
neutron star merger; GRB 130603B was the first observed
kilonova. It is however, by far the best observation, making
this the strongest evidence to date to confirm the hypothesis
that mergers of binary stars are the cause of short gamma-ray

The event also provides a limit on the difference between the
speed of light and that of gravity. Assuming the first photons
were emitted between zero and ten seconds after peak
gravitational wave emission, the difference between the speeds
of gravitational and electromagnetic waves, vGW - vEM, is
constrained to between -3x10-15 and +7x10-16 times the speed
of light, which improves on the previous estimate by about 14
orders of magnitude.

In addition, it allowed investigation of the equivalence
principle (through Shapiro delay measurement) and Lorentz
invariance. The limits of possible violations of Lorentz
invariance (values of 'gravity sector coefficients'') are
reduced by the new observations, by up to ten orders of

GW170817 also excluded some alternatives to general
relativity, including variants of scalar-tensor theory,
Horava-Lifshitz gravity, Dark Matter Emulators and bimetric
gravity, the details of which are beyond the scope of this

Gravitational wave signals such as GW170817 may be used as a
standard siren to provide an independent measurement of the
Hubble constant. An initial estimate of the constant derived
from the observation is 70.0 +12.0 -8.0 (km/s)/Mpc, broadly
consistent with current best estimates.

Electromagnetic observations helped to support the theory that
the mergers of neutron stars contribute to rapid neutron
capture r-process nucleosynthesis and are significant sources
of r-process elements heavier than iron, including gold and

Rising stars of multi-messenger astronomy

  Gravitational waves have been described as ripples in the
  fabric of spacetime. They are the wake left behind when an
  object with gravitational pull-so, any object with
  mass-changes its speed. When two enormous neutron stars go
  through a particularly drastic change in speed by colliding
  and merging, the gravitational waves released are powerful
  enough for scientists to detect them on Earth, 130 million
  light-years away.

  That's what happened at 12:41 universal time last August.
  The Laser Interferometer Gravitational-Wave Observatory
  detectors in Hanford, Washington, and Livingston, Louisiana,
  recorded gravitational waves the likes of which no one had
  ever seen. Previous gravitational-wave signals had lasted a
  few seconds; this one was in range for more than 100.
  Additional data from the Virgo gravitational-wave detector
  near Pisa, Italy, helped scientists triangulate the
  gravitational waves' origin in the sky.

  Astrophysicist Wilson-Hodge, the principal investigator on
  the NASA's Fermi Gamma-ray Space Telescope's Gamma-ray Burst
  Monitor team, was in training, learning how to motivate her
  team. When she received the notification from LIGO-Virgo,
  she knew motivation wouldn't be hard to find: The GBM sent
  out an alert of its own that morning from a high-energy
  blast of light called a gamma-ray burst, recorded just two
  seconds after the gravitational-wave signal passed the
  Earth. "It was pretty clear it was something really
  exciting," she says.

Neutron-rich matter in heaven and on Earth

  Despite a length-scale difference of 18 orders of magnitude,
  the internal structure of neutron stars and the spatial
  distribution of neutrons in atomic nuclei are profoundly

  Where do neutrons go? The elusive answer to such a seemingly
  simple question provides fundamental new insights into the
  structure of both atomic nuclei and neutron stars. To place
  the question in the proper context, consider lead-208, the
  element's most abundant isotope, which contains 82 protons
  and 126 neutrons. As the heaviest known doubly magic
  nucleus, 208Pb holds a special place in the nuclear-physics
  community. Just as noble gases with filled electronic shells
  exhibit low levels of chemical reactivity, doubly magic
  nuclei with filled proton and neutron shells display great
  stability. Because 208Pb is heavy, the Coulomb repulsion
  among its protons leads to a large neutron excess. The Lead
  Radius Experiment, or PREX, at the Thomas Jefferson National
  Accelerator Facility in Virginia was built to measure the
  location of 208Pb's 44 excess neutrons. In turn, a detailed
  knowledge of the neutron distribution in 208Pb illuminates
  the structure of a neutron star.
  Anatomy of a neutron star

  With masses comparable to that of our sun but radii of only
  10-15 km, neutron stars are unique laboratories for the
  study of phenomena that lie well outside the realm of
  terrestrial laboratories.

  The stellar composition at the highest densities encountered
  in a neutron star's inner core is unknown. Depending on the
  unknown compressibility of neutron-rich matter, the stellar
  core may harbor exotic states of matter, such as deconfined
  quark matter, a novel state in which quarks are allowed to
  roam freely at enormously high densities. Yet the canonical
  picture of the stellar core is that of a uniform liquid
  consisting of neutrons, protons, and neutralizing
  leptons—electrons and muons—in chemical equilibrium. The
  stellar core accounts for practically all the mass and about
  90% of a neutron star's size.

  Above the uniform core lies the nonuniform stellar crust, a
  region about 1 km thick that develops as a consequence of
  the short-range nature of the nuclear force. Indeed, at the
  subsaturation densities of the stellar crust, it becomes
  energetically favorable for neutrons and protons to cluster
  into complex nuclei that display highly exotic shapes, often
  referred to as nuclear pasta.

  The outermost surface of the neutron star constitutes the
  very thin atmosphere that is composed of hydrogen but may
  also contain heavier elements such as helium and carbon. To
  date, most of the information on neutron star radii has been
  obtained from the thermal emission from its surface, often
  assumed to be consistent with a blackbody spectrum.
  Unfortunately, complications due to both distortions to the
  blackbody spectrum and distance measurements make the
  determination of stellar radii a challenging task. Yet the
  discovery of gravitational waves from GW170817 has opened a
  new window into the study of neutron star properties and
  will nicely complement electromagnetic observations. 

All in the family: Kin of gravitational wave source discovered

Watch Physicist Brian Greene Explain Gravitational Waves To
Stephen Colbert  (8 min)