It is an early golden morning in mid-July. A warm summer rain had fallen the night before, leaving tattle-tale puddles on the street, where little children play, tossing pebbles into puddles while watching propagating ripples. Gravitational waves propagating throughout Spacetime are similar to those pebble-induced ripples propagating in a puddle of water where children play. According to Albert Einstein’s General Theory of Relativity (1915), gravity is a phenomenon that causes Spacetime to curve in the presence of mass–and the more mass that is contained within a particular volume of Space, the greater the curvature of Spacetime will be at the boundary of this particular volume. As objects with mass travel around in Spacetime, the curvature changes in order to reflect the altering location of those objects–and in certain circumstances, accelerating objects will generate changes in this curvature that propagate outwards at the speed of light in a wave-like way–these propagating phenomena are called gravitational waves. In February 2016, for the first time, scientists announced that they have observed these ripples in the fabric of Spacetime, confirming a major prediction of Einstein’s Theory of General Relativity.
Gravitational waves can arrive at Earth from a catastrophic event in the distant Universe, and this very first observation of their real existence in nature opens up an unprecedented new view into the hidden mysteries of the Cosmos. This is because such propagating ripples in Spacetime carry with them important information about their violent and dramatic origins that cannot be obtained by scientists in other ways. The reason for this is that gravitational waves can cut through regions of space that electromagnetic waves cannot penetrate. Astronomers can now study the Universe using gravity as a tool, as well as light. Therefore, gravitational waves can provide a precious gift to astronomers on our planet, providing them with extremely important information about exotic objects in the very distant Universe, such as black holes. Such systems cannot be seen using more traditional means–such as optical telescopes or radio telescopes. Gravitational wave astronomy provides a valuable new understanding into how our mysterious, wonderful, weird, and indisputably bizarre Universe operates. This is especially true for cosmologists, because gravitational waves provide a potential way of observing the primordial Universe. This is not possible with conventional methods of astronomy, since during its early, dark years, the Universe was opaque to electromagnetic radiation. Exact measurements of gravitational waves provide astronomers with a new, one-of-a-kind tool to test Einstein’s Theory of General Relativity. By attaining an understanding of gravitational waves, astronomers can then gain an understanding of what happened at the initial singularity–which is usually thought to have given birth to the Universe almost 14 billion years ago.
According to the inflationary Big Bang model, our Universe was born about 13.8 billion years ago when all of Space emerged from an unimaginably small Patch, that was much smaller than a proton–the initial singularity–and then, in the tiniest fraction of a second, expanded exponentially to attain macroscopic size. That unimaginably tiny Patch, that was much too small for a human being to see, was almost–but not exactly–nothing. That little Patch was so extremely hot and dense that everything we know is thought to have emerged from it. In the madly expanding fireball of the inflationary Big Bang, the newborn Cosmos danced and dazzled with extremely energetic radiation, a searing-hot, seething, turbulent cauldron of sparkling little particles of light (photons). The entire neonatal Universe glared with a blinding brilliance of light. What we are now able to observe, almost 14 billion years later, is the greatly expanded and expanding, dimming aftermath of that primordial blast of newborn, screaming brilliance. And today, Earthlings watch helplessly from their very small and obscure, rocky and watery blue planet, as the primordial fires of the Universal formation fade and cool, as our Cosmos races in its expansion to ultimately darken into the ashes of Eternity–like the haunting smile of the Cheshire Cat in the dream of a sleeping child.
According to the physicists who detected the Spacetime ripples, they were produced by the cataclysmic merger of a duo of black holes, that resulted in the formation of a single, massive, spinning black hole. The head-on collision of two black holes had previously been predicted–but had never been observed before.
The ripples in Spacetime were spotted on September 14, 2015 at 5:51 a.m. Eastern Daylight Time by both of the two twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington. The LIGO observatories were funded by the National Science Foundation (NSF), and are operated, built, and conceived of by scientists at the California Institute of Technology (Caltech) in Pasadena, California, and the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. The discovery is to be published in the journal Physical Review Letters.
Based on the signals observed, LIGO scientists calculate that the doomed black holes that caused the event were about 29 and 36 times solar-mass, and the collision and merger occurred about 1.3 billion years ago. About three times the mass of our Sun was converted into gravitational waves in a mere fraction of a second–with a peak power output that amounted to about 50 times that of the entire visible Universe. By looking at the time of the arrival of the signals–the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford–scientists can now say that the event occurred in the Southern Hemisphere.
According to General Relativity, a doomed black hole duo loses energy by emitting gravitational waves. This causes the black holes to slowly approach one another over a span of billions of years, and then move much more rapidly towards their final embrace in the last lingering moments before their great farewell performance. Before their final blaze of glory, in only a fraction of a second, the doomed duo crash together at almost half the speed of light, giving birth to a single more massive black hole–converting a percentage of the combined black holes’ mass to energy, according to Einstein’s famous formula E=mc squared. This energy is tossed out as a final strong burst of gravitational waves–and these are the gravitational waves that LIGO detected.
The LIGO discovery represents the first observation of gravitational waves themselves, accomplished by measuring the tiny disturbances the waves make to Space and Time as they zip through the Earth. Prior to this very first direct detection of gravitational waves, there had already been indirect evidence for their existence. For example, measurements of the Hulse-Taylor binary system indicated that gravitational waves are more than merely a hypothetical concept. Potential sources of detectable gravitational waves include binary stellar systems composed of a duo of white dwarfs, which are the relic cores of dead sunlike stars, and neutron stars that are the relics of more massive stars that perished in the ferocious blast of a supernova explosion. In addition, crashing black holes also provide evidence of the real existence in nature of gravitational waves–and in February 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced their historic direct detection of gravitational waves emanating from a pair of merging black holes.
As a gravitational wave passes a distant observer, that observer will see Spacetime distorted by the effects of that traveling ripple. Distances that exist between free objects increase, and then decrease, rhythmically as the wandering wave propagates–and it does so at a frequency corresponding to that of the wave. The magnitude of this effect decreases inversely with distance from the source of the wave. Doomed duos of neutron stars, that are in the process of spiraling inward towards one another, are an especially powerful source of gravitational waves when they finally blast into each other and merge–as a result of the very large acceleration of their masses, as they orbit ever closer, and closer, and closer in the final, fatal movement of their catastrophic dance. Alas, because of the immense distances to these sources, the effects when measured by scientists on Earth, are predicted to be extremely small–having strains of less than 1 part in 10 to the twentieth power. Scientists have demonstrated the real existence of these ripples with increasingly more sensitive detectors.
The existence of these propagating Spacetime ripples was first demonstrated in the 1970s and 1980s by Dr. Joseph Taylor, Jr. and his team. In 1974, Dr. Taylor and Dr. Russell Hulse spotted a binary stellar system composed of a pulsar in orbit around a neutron star. A pulsar is a young neutron star that is spinning very rapidly, emitting regular beams, that have frequently been compared to the bright beams of a lighthouse on Earth.
Dr. Taylor and Dr. Joel M. Weisberg, in 1982, discovered that the orbit of the wildly spinning pulsar was in the process of shrinking over time because of the release of energy in the form of gravitational waves. Dr. Hulse and Dr. Taylor were awarded the 1993 Nobel Prize in Physics for their discovery of this pulsar and showing that it would make a possible target for gravitational wave measurement.
The historic LIGO discovery represents the first observation of gravitational waves themselves, made by measuring the minute disturbances the ripples make in Spacetime as they pass through our planet.
“Our observation of gravitational waves accomplishes an ambitious goal set out over five decades ago to directly detect this elusive phenomenon and better understand the Universe, and, fittingly, fulfills Einstein’s legacy on the 100th anniversary of his General Theory of Relativity,” Dr. David H. Reitze commented in a February 11, 2016 National Science Foundation (NSF) Press Release. Dr. Reitze is executive director of the LIGO Laboratory.
The discovery was made using the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments, when compared to the first generation LIGO detectors. These improvements made for a significant increase in the volume of the Universe that astronomers could probe–and this resulted in the important discovery of gravitational waves during its first observation run. NSF is the lead financial supporter of Advanced LIGO. Funding organizations in Germany (Max Planck Society), Australia (Australian Research Council), and the U.K. (Science and Technology Facilities Council, STFC).
LIGO was originally proposed as a method of detecting those elusive gravitational waves in the 1980s by Dr. Rainer Weiss, professor of physics, emeritus, from MIT; Dr. Kip Thorne, Caltech’s Richard P. Feynman Professor physics, emeritus; and Dr. Ronald Drever, professor of physics, emeritus, also from Caltech.
“The description of this observation is beautifully described in the Einstein Theory of General Relativity formulated 100 years ago and comprises the first test of the theory in strong gravitation. It would have been wonderful to watch Einstein’s face had we been able to tell him,” commented Dr. Weiss in the February 11, 2016 NSF Press Release.
“With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the Universe–objects and phenomena that are made from warped Spacetime. Colliding black holes and gravitational waves are our first beautiful examples,” Dr. Thorne noted in the same Press Release.
“The Advanced LIGO detectors are a tour de force of science and technology, made possible by a truly exceptional international team of technicians, engineers, and scientists. We are very proud that we finished this NSF -funded project on time and on budget,” Dr. David Shoemaker of MIT noted in the NSF Press Release. Dr. Shoemaker is the project leader for Advanced LIGO.
At each observatory, the 2 1/2-mile long, L-shaped LIGO interferometer uses laser light split into two beams that travel back and forth down the arms (four-foot diameter tubes kept under a near-perfect vacuum). The beams are used to measure the separation between mirrors that are exactly positioned at the tips of the arms. According to Einstein’s Theory of General Relativity, the distance between mirrors will change by a very small amount when a gravitational wave wanders by the detector. A change in the lengths of the arms smaller than one-ten-thousandth the diameter of a proton can be detected.
Independent and widely separated observatories are necessary to determine the direction of the event causing the gravitational waves, and also to verify that the signals come from space and are not from some other local phenomenon.
The Ripples Of Spacetime
For the past half-century, gravitational waves have escaped direct detection. Catastrophic mergers of binary systems–such as the merging duo of black holes that emitted the newly discovered gravitational waves–can trigger explosive, brilliant fireworks of dazzling light. This fabulous light display enabled a team of astronomers, shortly thereafter, to hunt for evidence of a visible afterglow left by the Cosmic smash-up. Although none was spotted, this work represents the first detailed search for a visible counterpart of a gravitational event. It also will serve as a model for similar event follow-up in the future.
“Our team has been anxiously waiting for the first detection of gravitational waves so that we can rapidly point the Dark Energy Camera at this location and search for the associated visible light. It’s one of the most powerful instruments in the world for this purpose,” explained Dr. Edo Berger in a February 13, 2016 Harvard-Smithsonian Center for Astrophysics (CfA) Press Release. Dr. Berger, who is of the CfA, located in Cambridge, Massachusetts, is the Principal Investigator of the follow-up team.
The team was quick to observe the sky location of the first gravitational wave source–discovered by LIGO–within only a day of its announced discovery on September 16, 2015.
The joint discovery of gravitational waves and light is a difficult task, demanding that large and wide field telescopes quickly scan the sky location of a gravitational wave source. The team used the 3 square-degree Dark Energy Camera (DECam) Imager mounted on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile.The search program represents a collaboration between astronomers from many institutions in the United States, the Dark Energy Survey (DES), and members of the LIGO Scientific Collaboration.
“Planning and executing these observations immediately became our top priority. It was hectic, but also thrilling to be able to follow up on such a significant result,” noted Dr. Marcelle Soares-Santos in the same CfA Press Release. Dr. Soares-Santos is of Fermilab, a member of DES, and lead author of the paper describing the search and results.
The astronomers had to overcome an important hurdle because the search area was very large: 700 square degrees of the sky, which is about 2,800 times the size of the full Moon. For more than three weeks, the scientists observed large swaths of this region several times–but they did not spot any strange bursts of visible light. They then went on to use this information to assign a limit on the brightness that can serve as a benchmark for future endeavors.
“This first attempt to detect visible light associated with gravitational waves was very challenging, but it paves the way to a whole new field of astrophysics,” Dr. Berger noted in the February 13, 2016 CfA Press Release.
The astronomers plan to continue on their the hunt for visible light emanating from future gravitational wave sources.
This research was submitted for publication in The Astrophysical Journal Letters.