Listening to the chirps of gravitational waves

Three solar masses were converted into gravitational waves

February 13, 2016 05:41 pm | Updated November 17, 2021 05:10 am IST

A century after Albert Einstein hypothesised the existence of gravitational waves, after decades of hard work to detect them directly and nearly four months of rigorous scrutiny of data, an international team of scientists, which also included researchers from India, finally detected the most elusive waves stretching and compressing the fabric of space-time.

The gravitational waves >detected on September 14, 2015 were produced 1.3 billion years ago when two black holes, one 36 times the mass of the Sun and the other 29 solar masses collided and fused together to produce one huge black hole 62 times the mass of the Sun; it happened 1.3 billion light-years away from Earth.

Gravitational waves

In his theory of general relativity, Albert Einstein described space-time as a fabric permeating the universe. He proposed that space-time was not a rigid and static stage but is instead flexible, so much so that it can be distorted and warped as large masses move through it.

For instance, a bowling ball kept on a trampoline causes a depression in the fabric at the place it is kept. The heavier the ball, the greater the depression will be. Gravitational waves are produced when objects accelerate. For instance, if two bowling balls were set to roll around each other on the trampoline, the fabric of the trampoline would undulate and oscillate, like ripples on a pond. The bowling balls tend to lose energy as a result of causing these oscillations, and they would ultimately come closer and closer to each other and finally collide.

This is precisely what happened when the two black holes orbiting around each other collided and merged into a single massive black hole. The ripples in the fabric of space-time (like oscillations seen in the trampoline when the balls moved) caused by the orbiting black holes travelled outward as gravitational waves.

Also read: >Gravitational waves explained

According to the theory of general relativity, the pair of black holes orbiting around each other lost energy through the emission of gravitational waves. As the holes lost energy, they were gravitationally drawn closer to each other. As they camecloser, the spinning became faster and they emitted even more gravitational waves and lost even more energy. By the end of 20 millisecond of data, they were whirling so fast that each orbit lasted just a few milliseconds; the black holes had whipped up to nearly the speed of light before colliding and merging into one bigger black hole.

“The total power output of gravitational waves during the brief collision was 50 times greater than all of the power put out by all the of the stars in the universe put together,” Kip Thorne of Caltech, one of LIGO’s founders, told New Scientist .

Though the black holes were 36 times and 29 times the mass of the Sun, the fused black hole weighed only 62 solar masses and not 65 solar masses. The three solar masses that went missing werewhat created the gravitational waves.

Primary sources While all objects emit gravitational waves when they orbit each other, including the Earth orbiting the Sun, only certain cosmic events — like the merger of neutron-neutron stars and two black holes moving around each other — can disturb the fabric of space-time with sufficient force to produce gravitational ripples that propagate through the Universe. According to Nature , the “Earth should be awash with such waves — but by the time they reach us, the disturbances that they produce are minute and cannot be detected.”

Uniqueness This is the first time that scientists have been able to observe a black-hole merger. The collision was such a violent event that it radiated more energy in the form of gravitational waves than all the stars in the observable universe emitted as light in the same amount of time, says Nature . The gravitational waves stretched space, making the entire Earth expand and contract by 1/100,000 of a nanometre, about the width of an atomic nucleus, notes Science .

Chance observation The gravitational waves were detected at 11:50 a.m. Central European Time on September 14, 2015. It was a chance discovery made during the experiment's first observing run. The upgraded version, Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), was to officially begin collecting data only from September 18, 2015.

LIGO’s twin detectors located about 3,000 km apart at Hanford, Washington and in Livingston, Louisiana split a single laser into twin beams, and shoot each beam down a pair of perpendicular L-shaped listening devices 4 km long. The beams bounce back and forth off a series of mirrors in the long tunnels and ultimately return to the location where they were originally split. Since the beams are tuned to be in opposite phase they cancel each other when they meet; hence no signal reaches the detector. But very small changes in the relative distance travelled by the beams take place (down to smaller than one-ten-thousandth the diameter of a proton) — the beam gets stretched in one arm and compressed in the other — when they encounter gravitational waves. The two beams do not cancel each other completely and the signal reaches the detector.

The significance Everything we know about the universe comes from observations made through telescopes and particles of light. But unlike light, gravitational waves can pass through the universe unobstructed, so they carry information that we cannot obtain otherwise.

According to Nature , the ability to detect gravitational waves opens up the “new field of gravitational-wave astronomy”, in which scientists will now “listen to the waves” to know more about the objects that can produce them, including black holes, neutron stars and supernovae and directly study violent events like mergers of very massive and dense stars. Till LIGO came into the picture, black holes were never directly detected.

Another significant possibility is the ability to explore the very early stages of the universe. “These stages are impossible to study with light because the very early universe was so hot and dense that it was opaque to light,” Dejan Stojkovic, associate professor of physics at University at Buffalo said in a release. “But gravitational waves could freely propagate through the hot plasma of the very early universe, so scientists can use them to gain insight into what occurred during that otherwise invisible time.”

Hearing the waves The frequency of these waves that LIGO is designed to catch are actually in the audible range for humans. It is possible to hear gravitational wave signals if the frequency of the waves is translated into sound waves. Physicists call it a “chirp” — a rise in pitch and volume as the black holes circle each other faster and faster. In this case, the two observatories had seen a wave resembling the chirp of a bird. The oscillation began at 35 cycles per second (hertz) and rapidly increased to 250 hertz. It then became chaotic and rapidly died down; the whole thing was over within one-fourth of a second, journal Science notes. The signal LIGO received of the black hole merger was played on speakers to audiences of eager scientists. The ‘loudness’ of the recorded signal also provides a rough measure of when the merger occurred.

Spotting the source of gravitational waves with very high degree of confidence and accuracy will become easier when more observatories are located in different parts of the world. The observatory in Italy was not working when LIGO detected the signals. An interferometer is under development in Japan and India may have one too if the proposal to build one is accepted.

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