Why do you think physicists need to study them?

1. A team of scientists announced that they had heard and recorded the sound of two black holes colliding a billion light-years away, a fleeting chirp that fulfilled the last prediction of Einstein’s general theory of relativity. That faint rising tone, physicists say, is the first direct evidence of gravitational waves, the ripples in the fabric of space-time that Einstein predicted a century ago. It completes his vision of a universe in which space and time are interwoven and dynamic, able to stretch, shrink and jiggle. And it is a ringing confirmation of the nature of black holes, the bottomless gravitational pits from which not even light can escape, which were the most foreboding (and unwelcome) part of his theory.

2. More generally, it means that a century of innovation, testing, questioning and plain hard work after Einstein imagined it on paper, scientists have finally tapped into the deepest register of physical reality, where the weirdest and wildest implications of Einstein’s universe become manifest. Conveyed by these gravitational waves, power 50 times greater than the output of all the stars in the universe combined vibrated a pair of L-shaped antennas in Washington State and Louisiana known as LIGO on Sept. 14. Members of the LIGO group, a worldwide team of scientists, along with scientists from a European team known as the Virgo Collaboration, published a report in Physical Review Letters on Thursday with more than 1,000 authors (You can find the report HERE: https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914%3ADetection_of_GW150914.pdf).

3. The discovery is a great triumph for three physicists — Kip Thorne of the California Institute of Technology, Rainer Weiss of the Massachusetts Institute of Technology and Ronald Drever, formerly of Caltech and now retired in Scotland — who bet their careers on the dream of measuring the most ineffable of Einstein’s notions. “Until now, we scientists have only seen warped space-time when it’s calm,” Dr. Thorne said in an email. “It’s as though we had only seen the ocean’s surface on a calm day but had never seen it roiled in a storm, with crashing waves.” The black holes that LIGO observed created a storm “in which the flow of time speeded, then slowed, then speeded,” he said. “A storm with space bending this way, then that.”

4. When Einstein announced his theory 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. A. ____________ 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.

5. 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 again. 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.

6. 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.

7. 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. B. _________ 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.

8. Another group of astronomers who go by the name Bicep made headlines in 2014 when they claimed to have detected gravitational waves from the beginning of the Big Bang, using a telescope at the South Pole. They later acknowledged that their observations had probably been contaminated by interstellar stardust.

9. Dr. Thorne of Caltech and Dr. Weiss of M.I.T. first met in 1975. Dr. Thorne was already a renowned black-hole theorist, but he was looking for new experimental territory to conquer. They stayed up all night talking about how to test general relativity and debating how best to search for gravitational waves. Dr. Thorne then recruited Dr. Drever, a gifted experimentalist from the University of Glasgow, to start a gravitational wave program at Caltech. Dr. Drever wanted to use light — laser beams bouncing between precisely positioned mirrors — to detect the squeeze and stretch of a passing wave. Dr. Weiss tried to mount a similar effort at M.I.T., also using the laser approach, but at the time, black holes were not in fashion there.

10. C. __________ The researchers calculated that a typical gravitational wave from out in space would change the distance between a pair of mirrors by an almost imperceptible amount: one part in a billion trillion. Dr. Weiss recalled that when he explained the experiment to his potential funders at the National Science Foundation, “everybody thought we were out of our minds.” In 1984, to the annoyance of Dr. Drever and the relief of Dr. Weiss, the National Science Foundation ordered the two teams to merge. Progress was slow until the three physicists were replaced in 1987 by a single director as part of the price of going forward.

11. The first version of the experiment, known as Initial LIGO, started in 2000 and ran for 10 years, mostly to show that it could work on the scale needed. There are two detectors: one in Hanford, Wash., the other in Livingston, La. Hunters once shot up the outside of one of the antenna arms in Louisiana, and a truck crashed into one of the arms in Hanford. In neither case was the experiment damaged.

12. D. _________ LIGO’s antennas are L-shaped, with perpendicular arms 2.5 miles long. Inside each arm, cocooned in layers of steel and concrete, runs the world’s largest bottle of nothing, a vacuum chamber a couple of feet wide containing 2.5 million gallons of empty space. At the end of each arm are mirrors hanging by glass threads, isolated from the bumps and shrieks of the environment.

13. The lasers in the present incarnation, known as Advanced LIGO, can detect changes in the length of one of those arms as small as one ten-thousandth the diameter of a proton — a subatomic particle too small to be seen by even the most powerful microscopes — as a gravitational wave sweeps through.

14. Even with such extreme sensitivity, only the most massive and violent events out there would be loud enough to make the detectors ring. LIGO was designed to catch collisions of neutron stars, which can produce the violent flashes known as gamma ray bursts. As they got closer together, these neutron stars would swing around faster and faster, hundreds of times a second, vibrating space-time geometry with a rising tone that would be audible in LIGO’s vacuum-tube “sweet spot.” Black holes, the even-more-extreme remains of dead stars, could be expected to do the same, but nobody knew if they existed in pairs or how often they might collide. E. _________ Dr. Thorne and others long thought these would be the first waves to be heard by LIGO. But even he did not expect it would happen so quickly.

15. On Sept. 14, the system had barely finished being calibrated and was in what is called an engineering run at 4 a.m. when a loud signal came through at the Livingston site. “Data was streaming, and then ‘bam,’ ” recalled David Reitze, a Caltech professor who is the director of the LIGO Laboratory, the group that built and runs the detectors.

16. Seven milliseconds later, the signal hit the Hanford site. LIGO scientists later determined that the likelihood of such signals landing simultaneously by pure chance was vanishingly small. Nobody was awake in the United States, but computers tagged the event, and European colleagues noticed. Dr. Turner added, “The loudest things in the gravity-wave sky are the most exotic things in the universe: black holes, neutron stars and the early universe.” The future for the dark side looks bright.

a _________ a type of star that gives off a rapidly repeating series of radio waves;

b _________ difficulties or conditions that make success unlikely;

c _________ bent or curved;

d _________ of, relating to, or being a particle making up an atom or a process occurring within atoms;

e _________ an occasion when an object that is moving crashes into something;

f _________ so slight or small that it is very difficult to notice;

g ________ made of a hard substance used in building made by mixing cement, sand, small stones, and water;

h ________ a type of radiation with a very short wavelength that can pass through solid objects;

i ________ this kind of sound is deep and sometimes difficult to hear;

j ________ A short, sharp, high-pitched sound.