What discovery of Einstein’s gravitational waves mean

Posted on February 12, 2016 12:10 am

Gravitational waves are tiny ripples in the fabric of space-time first proposed by Albert Einstein a century ago.Yesterday a team of international scientists from the Advanced LIGO project confirmed they had detected gravitational waves caused by two black holes merging about 1.3 billion years ago through their publication on Physical Review Letters. LIGO executive director David Reitze confirmed the news at a media conference, announcing: “I think we’re opening a window on the universe.””We have detected gravitational waves, we did it,” he said.”It took us months of careful checking, rechecking, analysis. This is not just about the detection of gravitational waves … what’s really exciting is what comes next.”France Cordova, director of the US National Science Foundation, which funded the work, said the development would “lead to unexpected discoveries”.”Like Galileo first pointing his telescope upward, this new view of the sky will deepen our understanding of the cosmos,” she said. In 1915, Albert Einstein as part of his General Theory of Relativity predicted gravitational waves would be produced in extremely violent events such as collisions between black holes or neutron stars. However, they had never been directly observed until now.On September 14 last year, scientists working at LIGO observatories 3,000 kilometres apart in Livingston, Louisiana and Hanford Washington simultaneously detected vibrations from two rapidly spinning black holes one about 29 times the mass of the Sun and the other about 36 times the Sun’s mass as they spiraled together and merged into a single black hole.

The black holes gave out a burst of gravitational waves that travelled through the universe at the speed of light, causing everything in their path to vibrate back and forth.The technique used at LIGO and other observatories hunting for gravitational waves is a highly refined version of a method that has been around since the 1880s.Called laser interferometry, it uses a split laser beam to measure extremely small distances with incredible accuracy.Einstein’s general theory of relativity tells us that gravity is the curvature of space and time.The stronger the gravity an object has, the greater the deformation of space and time it causes.Gravitational waves are caused when objects with strong gravity accelerate. As they accelerate, ripples of space travel away from them at the speed of light.They are not like light waves traveling through space, they are actual waves in space: rhythmic stretching and squeezing of space.All objects sitting in the path of gravitational waves rhythmically move further apart and closer together as the space they exist in is stretched and squeezed.The strongest gravitational waves, the only ones we have a hope of detecting are formed when objects with enormous gravity undergo dramatic acceleration. Like when two black holes merge to form another.If scientists have indeed detected the to and fro movement caused by passing gravitational waves it will be a monumental achievement.

A view of the LIGO detector in Hanaford, Washington. LIGO research is carried out by the LIGO Scientific Collaboration - Picture courtesy of National Science Foundation
A view of the LIGO detector in Hanaford, Washington. LIGO research is carried out by the LIGO Scientific Collaboration – Picture:National Science Foundation

These ripples are so small only a fraction the size of an atom that Einstein thought they had to be beyond our technology.No doubt in my mind that gravitational wave astronomy is in the business of serious data collection.Detecting gravitational waves tells us more about the types of events that create them, and allows astronomers to point telescopes in the directions of the source to complete the picture.And the regularity with which gravitational waves are detected indicates just how common these massively energetic collisions and explosions are in our universe.As well as Earth-based observatories like the Advanced LIGO experiment, a huge space-based detector is due to go live in 2028.The 1 million kilometre-wide space antenna eLISA relies on three spacecraft orbiting the sun in a triangular formation.Although vastly different in scale, eLISA works on the same interferometer principle as Advanced LIGO, without the need for pipes and minus the background noise on Earth.However, instead of detecting waves that we can hear the kind of signal expected from LIGO could be played on a cello the space detectors will listen for waves with a frequency of about one cycle per hour created by much bigger black holes.The information we gather from these and other detectors will reveal aspects of the universe that would otherwise remain invisible.

The new discovery means scientists will be able to develop new techniques for mapping the universe.All existing knowledge has relied on light, which is affected by gas and dust.Now, gravitational waves will allow scientists to see inside distant objects.Gravitational wave astronomy would allow us to look further back in time and deeper inside the most extreme objects in the sky to the earliest instant after the Big Bang.All of our existing knowledge of the universe comes from telescopes, and all telescopes like optical, radio, X-ray etc rely on light coming from distant objects.Telescopes tell us a lot, but the light they detect has been absorbed and scattered by lots of gas and dust between the source and the telescope. At best, we are getting blurry images.Like light waves, gravitational waves are imprinted with information about their source but it is information that light could never provide.Gravitational wave astronomy will reveal the insides of distant objects because it will let us “see” their mass.The pattern of movement as black holes coalesce, the changes inside a supernova, the mechanisms of a gamma ray burst will all become visible to us.And because gravitational waves only interact with gas and dust to a tiny extent, their signal is much cleaner than those from light.Our picture of the universe will come into much sharper focus.

An aerial view of the Laser Interferometer Gravitational-wave Observatory (LIGO) detector in Livingston, Louisiana.Picture :National Science Foundation

While Einstein’s general theory of relativity predicted gravitational waves, he thought that if they did exist, they would be far too small to ever be detected.And that was certainly true with the technology used in previous detectors.Although they are produced by some of the most massive accelerations in the universe like black holes colliding, or a supernova explosion gravitational waves are incredibly tiny wiggles in space.And detecting movement on that scale is no mean feat, hence Einstein’s skepticism and the failure of previous detectors built over the last 50 years to pick up a signal their technology was simply too blunt.There were high hopes for the discovery of gravitational waves at the upgraded LIGO an acronym for Laser Interferometer Gravitational-wave Observatory facility.Dubbed Advanced LIGO, it is three times more sensitive than the original LIGO detector and was designed to detect vibrations in the range expected.A single laser beam was split in two, with each beam traveling down one arm of the interferometer.Mirrors at either end of the arms bounce the beam back and forth, and it is then recombined. Because the two arms are identical in length – 4 kilometres, the recombined laser beams perfectly cancel each other out.When a gravitational wave from a distant cataclysmic event reaches the detectors, the rhythmic stretching and squeezing of space and time make the pipes longer and shorter in turns, and the recombined beams no longer cancel out perfectly. Instead, a telltale pulsing signal is detected.Such a signal gives direct evidence for gravitational waves.

Contador Harrison