LIGO/Virgo Listens to Neutron Stars

On August 8, 2017, LIGO/Virgo detected the merging of two neutron stars 130 million light years away. Just 1.7 seconds later, the Fermi Gamma Ray Space Telescope received an optical signal—a short gamma ray burst (GRB). Thus began a new era of “multimessenger astronomy.” The GRBs are very energetic explosions observed in galaxies. The neutron star merger offers the first evidence that heavy metals such as gold, platinum, and uranium were created by the collision of neutron stars in a “kilonova.” The resulting gravitational waves offer a new way of measuring the Hubble constant, which determines the rate of expansion of the universe. An important result from the neutron star merger is an extremely accurate determination of the speed of gravitational waves; they move at the speed of light. This has significant ramifications for gravitational theory. It falsifies many proposed modified gravity models.

The Biggest Ears in the Sky: LIGO

At a press conference on February 11, 2016, David Reitz, LIGO Executive Director, announced, “We did it!” They detected gravitational waves for the first time. Both LIGO sites, in Washington state and Louisiana, registered the incoming gravitational waves from two black holes colliding and merging far away. Over the following months, more mergers were detected. Gravitational waves are caused by the acceleration of a massive object, which stretches and compresses spacetime in a wave-like motion that is incredibly small and difficult to detect. Numerical relativity research over decades has produced over a quarter of a million template solutions of Einstein’s equations. The best template fit to the wave form data identifies the masses and spins of the two merging black holes. Much of this chapter describes the technology of the LIGO apparatus. On October 3, 2017, Barish, Thorne, and Weiss, the founders of LIGO, received the Nobel Prize for Physics.

Origins of Gravitational Waves and Detectors

Civita criticized Einstein’s papers on gravitational waves: their energy momentum is frame dependent and therefore does not fit the covariance of Einstein’s gravity theory. Infeld and Rosen did not believe gravitational waves existed, and Einstein changed his mind on their existence repeatedly. Others did believe in them, such as Fock and Feynman. Weber constructed his “Weber bar” to detect gravitational waves, but when he claimed success, he was criticized. He then proposed using a Michelson-Morley type of interferometer with lasers to detect gravitational waves, as did Weiss. Merging black holes and neutron stars were proposed as detectable sources of gravitational waves. Taylor and Hulse, using the large Arecibo radio telescope, indirectly detected gravitational waves from inspiraling neutron stars. Primordial gravitational waves, still emanating from the Big Bang, were claimed to have been detected by BICEP2, but the waves were eventually shown to be a result of foreground dust.

Early Observations of Black Holes

Early observations of black holes, before the LIGO/Virgo detection of gravitational waves, were made by observing electromagnetic processes involving atomic spectral lines. X-ray binary systems were observed consisting of a progenitor star such as a neutron star and a dark companion. X-rays emitted from the gas accreting the dark companion tells us whether it is a black hole. Evidence indicated supermassive black holes at the centers of galaxies. From observations of orbits of stars near the supermassive black holes, one could determine their masses, which proved they were black holes. Observations of quasars, among the brightest objects in the universe, showed they contain black holes. It is important to establish the existence of an event horizon with the black hole, as predicted by general relativity. The current evidence for the event horizon is circumstantial, based on controversial theoretical models about the accretion disks surrounding the collapsed dark objects.

Thermodynamics, Quantum Physics, and Black Holes

A major question confronting physicists studying black holes was whether thermodynamics applied to them—that is, whether the black holes radiated heat and lost energy. Bekenstein considered heat and thermodynamics important for the interior of black holes. Based on the second law of thermodynamics, Hawking proposed that black holes evaporate over a very long time through what we now call Hawking radiation. This concept contradicts the notion that nothing can escape a black hole event horizon. Quantum physics enters into Hawking’s calculations, and he discovered the conundrum that the radiation would violate quantum mechanics, leading to what is called the information loss problem. These ideas are still controversial, and many physicists have attempted to resolve them, including Russian theorists Zel’dovich and Starobinsky. Alternative quantum physics interpretations of black holes have been proposed that address the thermodynamics problems, including so-called gravastars.

The Biggest Eyes in the Sky: The EHT

The international Event Horizon Telescope (EHT) project aims to observe the supermassive black holes at the centers of galaxies, such as Sagittarius A* at the center of the Milky Way and the more distant M87* in the galaxy M87. Using Very-Long-Baseline Interferometry, the project can observe the shadows of the supermassive black holes that block the bright light emitted by their accretion disks. The EHT ties together radio telescopes ranging across the western hemisphere of Earth to create, in effect, a planet-size telescope. The EHT will determine the size of the shadow, which can be compared to the predictions of general relativity and modified gravity theories. The EHT will also observe the physics of the accretion disks surrounding supermassive black holes. These observations can potentially determine whether a black hole event horizon exists.

Wormholes, Time Travel, and Other Exotic Theories

In 1935, Einstein and Rosen described what is now called the Einstein-Rosen bridge. Wheeler called this a wormhole, which could connect two distant parts of the universe. Thorne and Morris showed the wormhole cannot be traversable unless exotic matter with negative energy props it up. Using the Penrose mechanism of superradiance, one can produce rotational energy from a black hole, which could be used to detect dark matter particles. Higher dimensional objects such as branes in superstring theory have been considered as sources of gravitational waves. Black holes have even been proposed to be giant atoms, related to Hawking radiation and black hole entropy. Bekenstein and Mukhanov postulated that black holes radiated quantum radiation. Many such speculative ideas have been put forth that could potentially be verified by detecting gravitational waves. Yet, many physicists work with mathematical equations, unconcerned with whether their ideas can be verified or falsified by experiments.

Stars and Black Holes

Physicists began to believe in black holes when research revealed new information about the constitution of stars and their life cycles, indicating that a black hole represents the death of certain massive stars. Chandrasekhar used quantum mechanics and the notion of a degenerate electron gas to obtain the maximum mass of a white dwarf. A degenerate neutron gas produced enough pressure to stop the gravitational collapse of a massive star, producing a neutron star or pulsar. For a massive-enough star, the degenerate neutron gas fails to prevent gravitational collapse into a black hole. Supernovae explosions and implosions produce a neutron star or black hole as remnants. Oppenheimer and Volkoff used general relativity to derive the maximum mass of a star that would produce a black hole. Wheeler conceived of a “hairless black hole” in which only the mass, charge, and angular momentum determined the properties of the black hole.

Gravitation and Black Holes

Einstein’s theory of general relativity introduced the concept of black holes to physics and astronomy. But, eminent physicists such as Einstein and Eddington did not believe in the existence of black holes. Schwarzschild solved Einstein’s equations and disclosed a possible dark compact object with an event horizon. This is a membrane in spacetime surrounding the object, from which nothing can escape, not even light. The chapter describes the contributions of Chandrasekhar, Kruskal, Wheeler, Oppenheimer, Landau, Kerr, and others to help promote black holes in the physics and astronomy communities. It details ideas about the origins of black holes and the conundrums they still present, such as the singularities (infinitely dense matter) associated with them.

Prologue: LIGO

We left the city of Richland,Washington, on the desert road leading to the Hanford LIGO site to spend two days visiting the now-famous gravitational wave detector observatory. When my wife, Patricia, and I left our hotel, the sky was overcast with dark-gray clouds. There was a chilly, damp wind, and we had taken our coats to shield us from a typical March day in eastern Washington. The road to the site was straight, and we looked out over the unobstructed landscape covered with sagebrush and tumbleweeds blowing in the wind, all ringed by purple mountains in the distance. Hanford was one of the sites in the Manhattan Project, which produced the nuclear bombs that destroyed Hiroshima and Nagasaki in August 1945. We met very little traffic on our way, other than trucks leaving the infamous reactor site left by the Manhattan Project. The legacy of that project, the largest nuclear waste site in the United States, has contaminated the groundwater underneath 61 square miles of the site, and it threatens the headwaters of the Columbia River....