Gravitational waves discovered—topscientists respond12 February 2016, by Keith Riles, Alan Duffy, Amanda Weltman, Daniel Kennefick, David Parkinson,Maria Womack, Stephen Smartt, Tamara Davis, And T

Massive bodies can send ripples through space time inthe form of gravitational waves. Credit: NASA

One hundred years ago, Albert Einstein publishedhis general theory of relativity, which describedhow gravity warps and distorts space-time.

While this theory triggered a revolution in ourunderstanding of the universe, it made oneprediction that even Einstein doubted could beconfirmed: the existence of gravitational waves.

Today, a century later, we have that confirmation, with the detection of gravitational waves by theAdvanced Laser Interferometer Gravitational-WaveObservatory (aLIGO) detectors.

Here we collect reactions and analysis from someof the leading astronomers and astrophysicistsfrom around the world.

Keith Riles, University of Michigan

Einstein was skeptical that gravitational waveswould ever be detected because the predictedwaves were so weak. Einstein was right to wonder– the signal detected on September 14, 2015 bythe aLIGO interferometers caused each arm ofeach L-shaped detector to change by only 2

billionths of a billionth of a meter, about 400 timessmaller than the radius of a proton.

It may seem inconceivable to measure such tinychanges, especially with a giant apparatus likeaLIGO. But the secret lies in the lasers (the real "L"in LIGO) that are projected down each arm.

Fittingly, Einstein himself indirectly helped makethose lasers happen, first by explaining thephotoelectric effect in terms of photons (for whichhe earned the Nobel Prize), and second, bycreating (along with Bose) the theoreticalfoundation of lasers, which create coherent beamsof photons, all with the same frequency anddirection.

In the aLIGO arms there are nearly a trillion trillionphotonsper second impinging on the mirrors, allsensing the precisepositions of the interferometermirrors. It is this collective,coherent sensing thatmakes it possible to determine that one mirror hasmoved in one direction, while a mirror in theotherarm has moved in a different direction. Thisdistinctive,differential motion is what characterizesa gravitationalwave, a momentary differential warpof space itself.

By normally operating aLIGO in a mode of nearlyperfect cancellation of the light returning from thetwo arms (destructive interference), scientists cantherefore detect the passage of a gravitationalwave by looking for a momentary brightening of theoutput beam.

The particular pattern of brightening observed onSeptember 14agrees remarkably well with whatEinstein's General Theory of Relativity predicts fortwo massive black holes in the final moments of adeath spiral. Fittingly, Einstein's theory of photonshas helped to verify Einstein's theory of gravity, acentury after its creation.

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Amanda Weltman, University of Cape Town

The results are in and they are breathtaking.Almost exactly 100 years ago Einstein published"Die Feldgleichungen der Gravitation" in which helaid out a new theory of gravity, his General Theoryof Relativity. Einstein not only improved on hispredecessor, Newton, by explaining theunexpected orbit of the planet Mercury, but he wentbeyond and laid out a set of predictions that haveshaken the very foundations of our understandingof the universe and our place in it. Thesepredictions include the bending of light leading to lensed objects in the sky, the existence of blackholes from which no light can escape as well as theentire framework for our modern understanding ofcosmology.

NASA’s Hubble Space Telescope captured gravitationallensing of light, as predicted by Einstein. Credit: NASA,ESA, K. Sharon (Tel Aviv University) and E. Ofek(Caltech), CC BY

Einstein's predictions have so far all proven true,and today, the final prediction has been directlydetected, that of gravitational waves, the tiniestripples through space; the energy radiated away bytwo massive heavenly bodies spiralling into eachother. This is the discovery of the century, and it isperhaps poetic that one of the places it is beingannounced is Pisa, the very place where, accordingto legend, 500 years ago, Galileo dropped twomassive objects to test how matter reacts to

gravity.

As we bathe in the glory of this moment it isappropriate to ask, what is next for astronomy andphysics and who will bring about the nextrevolution? Today's discovery will becometomorrow's history. Advanced LIGO brings a newway of testing gravity, of explaining the universe,but it also brings about the end of an era of sorts. Itis time for the next frontier, with the SquareKilometre Array project finally afoot across Africaand Australia, the global South and indeed Africaitself is poised to provide the next pulse of gravityresearch.

Stephen Smartt, Queen's University Belfast

Not only is this remarkable discovery ofgravitational waves an extraordinary breakthroughin physics, it is a very surprising glimpse of a massive black hole binary system, meaning twoblack holes that are merging together.

Black holes are dark objects with a mass beyondwhat is possible for neutron stars, which are a typeof very compact stars – about 10 km across andweighing up to two solar masses. To imagine thiskind of density, think of the entire human populationsqueezed onto a tea spoon. Black holes are evenmore extreme than that. We've known about binaryneutron stars for years and the first detection ofgravitational waves were expected to be twoneutron stars colliding.

What we know about black hole pairs so far comesfrom the study of the stars orbiting around them.These binary systems typically have black holeswith masses five to 20 times that of the sun. ButLIGO has seen two black holes with about 30 timesthe mass of the sun in a binary system that hasfinally merged. This is remarkable for severalreasons. It is the first detection of two mergingblack holes, it is at a much greater distance thanLIGO expected to find sources, and the total massin the system is also much larger than expected.

This raises interesting questions about the starsthat could have produced this system. We knowmassive stars die in supernovae, and most of thesesupernovae (probably at least 60%) produce

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neutron stars. The more massive stars have verylarge cores that collapse and are too massive to bestable neutron stars so they collapse all the way toblack holes.

The LIGO detectors are sensitive to the minute ripples inspace-time caused by the merging of two black holes.Credit: University of Birmingham Gravitational WavesGroup, Christopher Berry

But a binary system with two black holes of around30 solar masses is puzzling. We know of massivebinary star systems in our own and nearbygalaxies, and they have initial masses well inexcess of 100 suns. But we see them losing massthrough enormous radiation pressure and they arepredicted, and often observed, to end their liveswith masses much smaller – typically about tentimes the sun.

If the LIGO object is a pair of 30 solar mass blackholes, then the stars that formed it must have beenat least as massive. Astronomers will be asking –how can massive stars end their lives so big andhow can they create black holes so massive? Aswell as the gravitational wave discovery, thisremarkable result will affect the rest of astronomy

for some time.

Alan Duffy, Swinburne University

The detection of gravitational waves is theconfirmation of Albert Einstein's final prediction andends a century-long search for something that evenhe believed would remain forever untested.

This discovery marks not the end, but rather thebeginning, of an era in which we explore theuniverse around us with a fundamentally newsense. Touch, smell, sight and sound all use ripplesin an electromagnetic field, which we call light, butnow we can make use of ripples in the backgroundfield of space-time itself to "see" our surroundings.That is why this discovery is so exciting.

The Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) measured the tinystretching of space-time by distant colliding blackholes, giving them a unique view into the mostextreme objects in general relativity.

The exact "ringing" of space-time as the ripplespass through the detector test this theory and ourunderstanding of gravity in ways no otherexperiment can.

We can even probe the way galaxies grow andcollide by trying to measure the gravitational wavesfrom the even larger collisions of supermassiveblack holes as the galaxies they are contained insmash together.

Australia in particular is a leading nation in thissearch, using distant pulsars as the ruler at the Parkes telescope.

Tara Murphy, University of Sydney

In addition to binary black holes, aLIGO will detectgravitational waves from other events such as thecollision of neutron stars, which are the denseremnants left over when a massive stars collapse.

Astronomers think that two neutron stars collidingmay trigger a gamma-ray burst, which we candetect with "regular" telescopes.

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In Australia, we have been using the MurchisonWidefield Array and the Australian SquareKilometre Array Pathfinder) to follow-up aLIGOcandidates.

aLIGO is an incredibly sensitive instrument but ithas very poor ability to determine where thegravitational waves are coming from. Our radiotelescopes can scan large areas of sky extremelyquickly, so can play a critical part in identifying theevent.

This project has been like no other one I haveworked on. When aLIGO identifies a candidate, itsends out a private alert to an international networkof astronomers. We respond as quickly as possiblewith our telescopes, scanning the region the eventis thought to have occurred in, to see if we candetect any electromagnetic radiation.

Everything is kept top secret – even the otherpeople using our telescopes are not allowed toknow where we are pointing them.

To make sure their complex processing pipelinewas working correctly, someone in the aLIGO teaminserted fake events into the process. Nobody onthe team, or those of us doing follow-up, had anyidea whether what we were responding to was realor one of these fake events.

We are truly in an era of big science. Thisincredible result has been the work of not onlyhundreds of aLIGO researchers and engineers, buthundreds more astronomers collaborating aroundthe globe. We are eagerly awaiting the next aLIGOobserving run, to see what else we can find.

Tamara Davis, University of Queensland

Rarely has a discovery been so eagerly anticipated.

When I was a university undergraduate, almost 20years ago, I remember a physics lecturer telling usabout the experiments trying to detect gravitationalwaves. It felt like the discovery was imminent, and itwas one of the most exciting discoveries that couldbe made in physics.

Mass and energy warping the fabric of space is one

of the pieces of general relativity that most capturesthe imagination. However, while it has enormousexplanatory power, the reality of that curvature ishard to grasp or confirm.

For the last few months I've had to sit quietly andwatch as colleagues followed up the potentialgravitational wave signal. This is the one and onlytime in my scientific career that I wasn't allowed totalk about a scientific discovery in progress.

But that's because it is such a big discovery that wehad to be absolutely sure about it beforeannouncing it, lest we risk "crying wolf".

Every last check had to be done, and of course, wedidn't know whether it was a real signal, or a signalinjected by the experimenters to keep us on ourtoes, test the analysis and follow-up.

I work with a project called the Dark Energy Survey,and with our massive, wide-field, half-billion pixelcamera on a four metre telescope in Chile, mycolleagues took images trying to find the source ofthe gravitational waves.

An example signal from an inspired gravitational wavesource. Credit: A. Stuver/LIGO, CC BY-ND

The wide-field is important, because thegravitational wave detectors aren't very good atpinpointing the exact location of the source.

Unfortunately if it was a black hole merger, wewouldn't expect to see any visible light.

Now that we're in the era of detecting gravitationalwaves, though, we'll be able to try again with the

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next one.

Maria Womack, University of South Florida

This is a momentous change for astronomy.Gravitational-wave astronomy can now truly begin,opening a new window to the universe. Normaltelescopes collect light at different wavelengths,such as Xray, ultraviolet, visible, infrared and radio,collectively referred to as electromagnetic radiation(EM). Gravitational waves are emitted fromaccelerating mass analogous to the wayelectromagnetic waves are emitted fromaccelerating charge; both are emitted fromaccelerating matter.

The most massive objects with the highestaccelerations will be the first events detected. Forexample, Advanced LIGO, funded by the U.S.National Science Foundation, can detect binaryblack holes in tight, fast orbits. GWs carry awayenergy from the orbiting pair, which in turn causesthe black holes to shrink their orbit and accelerateeven more, until they merge in a violent event,which is now detectable on Earth as a whistling"chirp."

The gravitational-wave sky is completely uncharted,and new maps will be drawn that will change howwe think of the universe. GWs might be detectedcoming from cosmic strings, hypothetical defects inthe curvature of space-time. They will also be usedto study what makes some massive stars explodeinto supernovae, and how fast the universe isexpanding. Moreover, GW and traditionaltelescopic observing techniques can be combinedto explore important questions, such as whether thegraviton, the presumed particle that transmitsgravity, actually have mass? If massless, they willarrive at the same time as photons from a strongevent. If gravitons have even a small mass, theywill arrive second.

Daniel Kennefick, University of Arkansas

Almost 100 years ago, in February 1916, Einsteinfirst mentioned gravitational waves in writing.Ironically it was to say that he thought they did notexist! Within a few months he changed his mindand by 1918 had published the basis of our modern

theory of gravitational waves, adequate to describethem as they pass by the Earth. However hiscalculation does not apply to strongly gravitatingsystems like a binary black hole.

It was not until 1936 that Einstein returned to theproblem, eventually publishing one of the earliest exact solutions describing gravitational waves. Buthis original sceptical attitude was carried forward bysome of his former assistants into the postwarrebirth of General Relativity. In the 1950s, doubtswere expressed as to whether gravitational wavescould carry energy and whether binary star systemscould even generate them.

One way to settle these disputes was to carry outpainstaking calculations showing how the emissionof gravitational waves affected the motion of thebinary system. This proved a daunting challenge.Not only were the calculations long and tedious, buttheorists found they needed a much moresophisticated understanding of the structure ofspace-time itself. Major breakthroughs included thedetailed picture of the asymptotic structure of space-time, and the introduction of the concept of matched asymptotic expansions. Prior tobreakthroughs such as these, many calculationsgot contradictory results. Some theorists even gotanswers that the binary system should gain, notlose, energy as a result of emitting gravitationalwaves!

While the work of the 1960s convinced theoriststhat binary star systems did emit gravitationalwaves, debate persisted as to whether Einstein's1918 formula, known as the quadrupole formula,correctly predicted the amount of energy theywould radiate. This controversy lasted into the early1980s and coincided with the discovery of thebinary pulsar which was a real-life system whose orbit was decaying in line with the predictions ofEinstein's formula.

In the 1990s, with the beginnings of LIGO, theorists'focus shifted to providing even more detailedcorrections to formulas such as these. Researchersuse descriptions of the expected signal astemplates which facilitate the extraction of thesignal from LIGO's noisy data. Since nogravitational wave signals had ever been seen

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before, theorists found themselves unusuallyrelevant to the detection project – only they couldprovide such data analysis templates.

David Parkinson, University of Queensland

Gravitational waves can be used to provide a directprobe of the very early universe. The further awaywe look, the further back in time we can see. Butthere is a limit to how far back we can see, as theuniverse was initially an opaque plasma, andremained so even as late as 300,000 years afterthe Big Bang.

This surface, from which the cosmic microwavebackground is emitted, represents the furthest backany measurement of electromagnetic radiation candirectly investigate.

But this plasma is no impediment for gravitationalwaves, which will not be absorbed by anyintervening matter, but come to us directly.Gravitational waves are predicted to be generatedby a number of different mechanisms in the earlyuniverse.

For example, the theory of cosmic inflation, whichsuggests a period of accelerated expansionmoments after the Big Bang, goes on to predict notjust the creation of all structure that we see in theuniverse, but also a spectrum of primordialgravitational waves.

It is these primordial gravitational waves that the BICEP2 experiment believed it had detected inMarch 2014.

BICEP2 measured the polarisation pattern of thecosmic microwave background, and reported astrong detection of the imprint of primordialgravitational waves. These results turned out in factto be contamination by galactic dust, and notprimordial gravitational waves.

But there is every reason to believe that futureexperiments may be able detect these primordial gravitational waves, either directly or indirectly, andso provide a new and complementary way tounderstand the physics of the Big Bang.

This article was originally published on TheConversation. Read the original article.

Source: The Conversation

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APA citation: Gravitational waves discovered—top scientists respond (2016, February 12) retrieved 16November 2021 from https://phys.org/news/2016-02-gravitational-discoveredtop-scientistsrespond.html

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