I am pleased to announce the publication of my new book Einstein’s Theories of Relativity, now available in paperback at Amazon. This books combines, updates, and extends my prior ebooks on both special and general relativity. I have added a new comprehensive exploration of gravitational waves, covering both the theoretical foundation and the observational developments.
The timing is fortuitous as LIGO recently announced the first-ever detection of gravity waves produced by colliding neutron stars.
Gravity waves are like tiny ripples spreading outward from jet skis on the opposite side of a lake. But gravity waves don’t move water; they alternately stretch and compress space.
My February 2016 newsletter reported on the first-ever direct detection of gravity waves by LIGO, the Laser Interferometer Gravitational Wave Observatory, comprised of two identical NSF-funded facilities, one in Hanford, Washington, and the other in Livingston, Louisiana.
LIGO’s interferometers are the most precise instruments ever built. With their latest enhancements, they can detect the distance from the star Alpha Centauri to Earth changing by 1/10th of the width of a human hair, or the diameter of Earth changing by the width of one proton.
The waves LIGO first detected were produced by the collapse of a black hole binary system. Astrophysicists speculate that two black holes, attracted by their mutual gravity, may have orbited one another for millions or even billions of years, while gradually spiraling inward toward a cataclysmic event. At one moment, when they were 6,000 miles apart, one full orbit (their “year”) took only 2 seconds. Fifty minutes later, they were 600 miles apart, orbiting 60 times per second. And 0.3 seconds later, while orbiting 500 times per second, their event horizons collided, when their central singularities were 60 miles apart. While shedding some weight, they merged into a single black hole.
Since then, during 8 months of total observing time, LIGO has detected gravity waves from three more black hole collisions. By comparison, the world’s most powerful atom smasher, the LHC, can produce 200 million events per second. But the world’s physicists are far more interested in LIGO’s handful of events. The four black hole merger events are:
Above, the distance from Earth to the colliding black holes is measured in light-years (each about 6 trillion miles), and all masses are measured in solar masses, the mass of our Sun. Thus in the latest event, black holes weighing 31× and 25× our Sun’s mass collided, merging into one black hole weighing 53 solar masses, and converting 3 solar masses into energy (recall E = m c2) that was carried away by gravity waves. This amount of energy, released in less than 1 second, exceeds the total luminosity of all the 30 billion, trillion stars in our observable universe.
LIGO’s fifth event is a new beast. Neutron stars are the remains of heavy stars that exhausted their nuclear fuel and collapsed under their own self-gravity. Neutron star masses are between 1.4 and a bit more than 2 solar masses. The LIGO event, dubbed GW170817 for its date of observation, is believed due to a binary neutron star merger at a distance of 130 million light-years in the constellation Hydra. The initial masses are both estimated to be about 1.4 solar masses. The energy released in this merger was about 100 times less than was released in LIGO’s four black hole mergers.
GW170817’s gravity waves were detected by both LIGO observatories and by VIRGO, a similar observatory near Pisa, Italy, which recently joined the hunt.
In addition, the Fermi Gamma-Ray Space Telescope (FGST) detected a short gamma ray burst (GRB) 1.7 seconds after GW171017. The tight time correlation is convincing evidence that neutron star mergers are an important progenitor of GRB’s. Within 14 seconds, FGST alerted the astronomical community. The LIGO-VIRGO announcement came 40 minutes later. (Gamma-ray physicists have optimized their automated alert systems over many years; gravitational wave physicists are at the start of this learning curve.)
These announcements caused an astronomical feeding frenzy — nearly 100 preprints and dozens of preliminary papers were submitted within 24 hours of this gravity wave’s arrival. In total, these publications were coauthored by almost 4000 astronomers (about one-third of those alive and breathing), representing 900 institutions. All that activity alone must have created its own gravity wave.
This event fits models of binary neutron star mergers that produce kilonovae, a category of supernova that are prodigious producers of heavy elements. It is believed this kilonova produced 16,000 Earth masses of heavy elements (elements heavier than iron), including 10 Earth masses of gold and platinum. Too bad it’s so far away. Over 70 optical observatories studied the GRB. Perhaps strangely, no neutrinos were detected from this event.
The end result of this neutron star merger is not yet established — it might be the most massive neutron star, or the least massive black hole, ever seen.
A neutron star is essentially one solid nucleus containing a billion, trillion, trillion, trillion, trillion neutrons. This is the densest form of matter that physicists think we understand. Black holes can pack billions of times more mass into trillions of trillions of trillions of times less volume. We are certain that we don’t understand what happens in the center of a black hole.
December 13, 2017
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