Until recently, gravitational waves could have been a figment of Einstein’s imagination. Before they were detected, these ripples in spacetime existed only in the physicist’s general theory of relativity, as far as scientists knew.
Now, researchers have not one but two ways to detect the waves. And they’re on the hunt for more. The study of gravitational waves is booming, says astrophysicist Karan Jani of Vanderbilt University in Nashville. “This is just remarkable. No field I can think of in fundamental physics has seen progress this fast.”
Just as light comes in a spectrum, or a variety of wavelengths, so do gravitational waves. Different wavelengths point to different types of cosmic origins and require different flavors of detectors.
Gravitational waves with wavelengths of a few thousand kilometers — like those detected by LIGO in the United States and its partners Virgo in Italy and KAGRA in Japan — come mostly from merging pairs of black holes 10 or so times the mass of the sun, or from collisions of dense cosmic nuggets called neutron stars (SN: 2/11/16). These detectors could also spot waves from certain types of supernovas — exploding stars — and from rapidly rotating neutron stars called pulsars (SN: 5/6/19).
In contrast, immense ripples that span light-years are thought to be created by orbiting pairs of whopper black holes with masses billions of times that of the sun. In June, scientists reported the first strong evidence for these types of waves by turning the entire galaxy into a detector, watching how the waves tweaked the timing of regular blinks from pulsars scattered throughout the Milky Way (SN: 6/28/23).
With the equivalent of both small ripples and major tsunamis in hand, physicists now hope to plunge into a vast, cosmic ocean of gravitational waves of all sorts of sizes. These ripples could reveal new details about the secret lives of exotic objects such as black holes and unknown facets of the cosmos.
“There’s still a lot of gaps in our coverage of the gravitational wave spectrum,” says physicist Jason Hogan of Stanford University. But it makes sense to cover all the bases, he says. “Who knows what else we might find?”
This quest to capture the full complement of the universe’s gravitational waves could take observatories out into deep space or the moon, to the atomic realm and elsewhere.
Here’s a sampling of some of the frontiers scientists are eyeing in search of new types of waves.
Go to deep space
The Laser Interferometer Space Antenna, or LISA, sounds implausible at first. A trio of spacecraft, arranged in a triangle with 2.5-million-kilometer sides, would beam lasers to one another while cartwheeling in an orbit around the sun. But the European Space Agency mission, planned for the mid-2030s, is no mere fantasy (SN: 6/20/17). It is many scientists’ best hope for breaking into new realms of gravitational waves.
“LISA is a mind-blowing experiment,” says theoretical physicist Diego Blas Temiño of Universitat Autònoma de Barcelona and Institut de Física d’Altes Energies.
As a gravitational wave passes by, LISA would detect the stretching and squeezing of the sides of the triangle, based on how the laser beams interfere with each other at the triangle’s corners. A proof-of-concept experiment with a single spacecraft, LISA Pathfinder, flew in 2015 and demonstrated the feasibility of the technique (SN: 6/7/16).
Generally, to catch longer wavelengths of gravitational waves, you need a bigger detector. LISA would let scientists see wavelengths millions of kilometers long. That means LISA could detect orbiting black holes that would be enormous, but moderately so — millions of times the mass of the sun instead of billions.
Go to the moon
With NASA’s Artemis program aiming at a return to the moon, scientists are looking to Earth’s neighbor for inspiration (SN: 11/16/22). A proposed experiment called the Laser Interferometer Lunar Antenna, or LILA, would put a gravitational wave detector on the moon.
Without the jostling of human activity and other earthly jitters, gravitational waves should be easier to pick out on the moon. “It’s almost like a spiritual quietness,” Jani says. “If you want to listen to the sounds of the universe, these is no place better in the solar system than our moon.”
Like LISA, LILA would have three stations beaming lasers in a triangle, though the sides of this one would be about 10 kilometers long. It could catch wavelengths tens or hundreds of thousands of kilometers long. That would fill in a gap between the wavelengths measured by the space-based LISA and the Earth-based LIGO.
Because orbiting objects like black holes speed up as they get closer to merging, over time they emit gravitational waves with shorter and shorter wavelengths. That means LILA could watch black holes close in on one another during the weeks before they merge, giving scientists a heads-up that a collision is about to go down. Then, once the wavelengths get short enough, earthly observatories like LIGO would pick up the signal, catching the moment of impact.
A different moon-based option would use lunar laser ranging — a technique by which scientists measure the distance from Earth to the moon with lasers, thanks to reflectors placed on the moon’s surface during previous moon landings.
The method could detect waves jostling the Earth and the moon, with wavelengths in between those seen by pulsar timing methods and LISA, Blas Temiño and a colleague reported in Physical Review D in 2022. But that technique would require improved reflectors on the moon — another reason to go back.
LISA, LIGO and other laser observatories measure the stretching and squeezing of gravitational waves by monitoring how laser beams interfere after traversing their detectors’ long arms. But a proposed technique goes a different route.
Rather than looking for slight changes in the lengths of detector arms as gravitational waves pass, this new technique keeps an eye on the distance between two clouds of atoms. The quantum properties of atoms mean that they act like waves that can interfere with themselves. If a gravitational wave passes through, it changes the distance between the atom clouds. Scientists can tease out that change in distance based on that quantum interference.
The technique could reveal gravitational waves with wavelengths between those detectable by LIGO and LISA, Hogan says. He’s part of an effort to build a prototype detector, called MAGIS-100, at Fermilab in Batavia, Ill.
Atom interferometers have never been used to measure gravitational waves, though they can sense Earth’s gravity and test fundamental physics rules (SN: 2/28/22; SN: 10/28/20). The idea is “totally futuristic,” Blas Temiño says.
Go back in time
Another effort aims to pinpoint gravitational waves from the earliest moments of the universe. Such waves would have been produced during inflation, the moments after the Big Bang when the universe ballooned in size. These waves would have longer wavelengths than ever seen before — as long as 1021 kilometers, or 1 sextillion kilometers.
But the hunt got off to a false start in 2014, when scientists with the BICEP2 experiment proclaimed the detection of gravitational waves imprinted in swirling patterns on the oldest light in the universe, the cosmic microwave background, or CMB. The claim was later overturned (SN: 1/30/15).
An effort called CMB-Stage 4 will continue the search, with plans for multiple new telescopes that would scour the universe’s oldest light for signs of the waves — this time, hopefully, without any missteps.
Go for the unknown
For most types of gravitational waves that scientists have set their sights on, they know a bit about what to expect. Known objects — like black holes or neutron stars — can create those waves.
But for gravitational waves with the shortest wavelengths, perhaps just centimeters long, “the story is different,” says theoretical physicist Valerie Domcke of CERN near Geneva. “We have no known source … that would actually give us [these] gravitational waves of a large enough amplitude that we could realistically detect them.”
Still, physicists want to check if the tiny waves are out there. These ripples could be produced by violent events early in the universe’s history such as phase transitions, in which the cosmos converts from one state to another, akin to water condensing from steam into liquid. Another possibility is tiny, primordial black holes, too small to be formed by standard means, which might have been born in the early universe. Physics in these regimes is so poorly understood, “even looking for [gravitational waves] and not finding them would tell us something,” Domcke says.
These gravitational waves are so mysterious that their detection techniques are also up in the air. But the wavelengths are small enough that they could be seen with high-precision, laboratory-scale experiments, rather than enormous detectors.
Scientists might even be able to repurpose data from experiments designed with other goals in mind. When gravitational waves encounter electromagnetic fields, the ripples can behave in ways similar to hypothetical subatomic particles called axions (SN: 3/17/22). So experiments searching for those particles might also reveal mini gravitational waves.
The gamut of gravitational waves
Gravitational waves come in a spectrum of shorter and longer wavelengths. Each wavelength range is generated by different sources. Pulsars and exploding stars, or supernovas, generate some short wavelength ripples. Other waves are produced by pairs of neutron stars, or by pairs of stellar mass black holes, with masses less than 100 times that of the sun. Still longer wavelengths are generated by pairs of supermassive black holes.
Different wavelengths can be spotted using different types of detectors, including ground-based detectors such as LIGO, space-based detectors such as LISA, and measurements of blips from dead stars called pulsars. Especially long wavelengths may be detected by studying the light released shortly after the Big Bang, the cosmic microwave background. Other detector types (not pictured) could fill in the gaps.
A new view
Catching gravitational waves is like paddling against the tide: tough going, but worth it for the scenic views. “Gravitational waves are really, really hard to detect,” Hogan says. It took decades of work before LIGO spotted its first swells, and the same is true of the pulsar timing technique. But astronomers immediately began reaping the rewards. “It’s a whole new view of the universe,” Hogan says.
Already, gravitational waves have helped confirm Einstein’s general theory of relativity, discover a new class of black holes of moderately sized masses and unmask the fireworks that happen when two ultradense objects called neutron stars collide (SN: 2/11/16; SN: 9/2/20; SN: 10/16/17).
And it’s still early days for gravitational wave detection. Scientists can only guess at what future detectors will expose. “There’s way more to discover,” Hogan says. “It’s bound to be interesting.”
Hadi hivi karibuni, mawimbi ya uvutano yangeweza kuwa ni jambo la kuwaziwa tu na Einstein. Kabla ya kugunduliwa, mawimbi hayo katika nafasi-wakati yalitokea tu katika nadharia ya jumla ya fizikia ya upendeleo, kadiri wanasayansi walivyojua. Sasa, watafiti wana njia mbili za kugundua mawimbi hayo. Na wao ni juu ya uwindaji kwa ajili ya zaidi. "Kama ilivyoelezwa katika makala hii, ""Kuna uwezekano mkubwa kwamba kuna uwezekano wa kuibuka kwa mawimbi ya uvutano,"" anasema Karan Jani, mtaalamu wa fizikia ya nyota wa Chuo Kikuu cha Vanderbilt huko Nashville, New York." <unk>Hii ni ya ajabu tu. Hakuna uwanja wowote ninaoweza kufikiria katika fizikia ya msingi umeona maendeleo ya haraka kama hii. Kama vile mwanga unakuja katika wigo, au urefu wa mawimbi mbalimbali, ndivyo mawimbi ya uvutano yanavyofanya. Vipimo tofauti-tofauti vya mawimbi huonyesha aina tofauti-tofauti za chanzo cha ulimwengu na huhitaji vipimo tofauti-tofauti. Mawimbi ya mvuto yenye urefu wa mawimbi ya maelfu ya kilomita, kama yale yaliyotambuliwa na LIGO nchini Marekani na washirika wake Virgo nchini Italia na KAGRA nchini Japan, hutoka kwa kuunganishwa kwa jozi za mashimo meusi yenye uzito wa mara 10 au zaidi ya jua, au kutoka kwa mgongano wa nuggets za anga zenye uzito mkubwa zinazoitwa nyota za neutroni. Vipimo hivyo vinaweza pia kugundua mawimbi kutoka kwa aina fulani za supernova - nyota zinazopuka - na kutoka kwa nyota za neutroni zinazozunguka haraka zinazoitwa pulsars (SN: 5 6 19). Kwa kulinganisha, mawimbi makubwa ambayo huenea kwa miaka ya nuru yanafikiriwa kuundwa na jozi za mashimo meusi makubwa yenye uzito mara mabilioni ya ule wa jua. Katika mwezi Juni, wanasayansi waliripoti ushahidi wa kwanza wa nguvu kwa aina hizi za mawimbi kwa kugeuza galaxy nzima katika detector, kuangalia jinsi mawimbi tweaked muda wa blink mara kwa mara kutoka pulsars kutawanyika katika Milky Way. Kwa kuwa sasa wanasayansi wana uwezo wa kuchunguza mawimbi madogo na mawimbi makubwa ya tsunami, wanatazamia kugundua kwamba kuna mawimbi mengi ya uvutano yenye ukubwa mbalimbali. Mageuzi haya yanaweza kufunua maelezo mapya kuhusu maisha ya siri ya vitu vya kigeni kama vile mashimo meusi na pande zisizojulikana za ulimwengu. "Bado kuna mapungufu mengi katika chanjo yetu ya wimbi la mvuto", anasema mwanafizikia Jason Hogan wa Chuo Kikuu cha Stanford. Lakini ni jambo la busara kufunika mambo yote, asema. "Ni nani anayejua nini kingine tunaweza kupata? ""Hii utafutaji wa kukamata kamili ya mawimbi ya mvuto wa ulimwengu inaweza kuchukua observatories nje katika nafasi ya kina au mwezi, kwa ulimwengu wa atomu na mahali pengine. """ Hapa ni sampuli ya baadhi ya mpaka wanasayansi ni eyeing katika kutafuta aina mpya ya mawimbi. Laser Interferometer Space Antenna (LISA) ni kifaa cha kupima mawimbi ya angani. Trio ya vyombo vya angani, vilivyowekwa katika pembetatu yenye pande milioni 2.5 za kilomita, vingeweza kupeleka miale ya laser kwa kila mmoja huku vikizunguka jua. Lakini ujumbe wa Shirika la Anga la Ulaya, uliopangwa kwa katikati ya miaka ya 2030, si ndoto tu. Ni tumaini bora zaidi la wanasayansi wengi la kuvunja katika nyanja mpya za mawimbi ya uvutano. "LISA ni jaribio la akili-blowing", anasema theoretical physicist Diego Blas Temiño wa Universitat Autònoma de Barcelona na Institut de Física d'Altes Energies. Wakati mawimbi ya mvuto yanapopita, LISA itagundua kunyoosha na kukandamiza kwa pande za pembetatu, kulingana na jinsi miale ya laser inavyoingiliana kwenye pembe za pembetatu. Majaribio ya uthibitisho wa dhana na chombo kimoja cha angani, LISA Pathfinder, iliruka mnamo 2015 na kuonyesha uwezekano wa mbinu hiyo (SN: 6 7 16). Kwa kawaida, ili kukamata mawimbi marefu zaidi ya mawimbi ya uvutano, unahitaji kifaa kikubwa zaidi cha kugundua. LISA itawawezesha wanasayansi kuona mawimbi yenye urefu wa mamilioni ya kilometa. LISA inaweza kugundua mashimo meusi makubwa, lakini ya wastani, mamilioni ya mara ya wingi wa jua. NASA ina mpango wa Artemis wa kurudisha watu kwenye mwezi, na wanasayansi wanaangalia jirani ya Dunia kwa msukumo. Majaribio ya Laser Interferometer Lunar Antenna (LILA) yanaonyesha jinsi ya kugundua mawimbi ya mvuto kwenye mwezi. Bila shughuli za kibinadamu na mambo mengine ya dunia, mawimbi ya uvutano yangekuwa rahisi kuona kwenye Mwezi. "Ni karibu kama utulivu wa kiroho", Jani anasema. "Kama unataka kusikiliza sauti za ulimwengu, hakuna mahali bora katika mfumo wa jua kuliko mwezi wetu.""Like LISA, LISA itakuwa na vituo vitatu vya laser katika pembetatu, ingawa pande za hii itakuwa karibu kilomita 10 kwa urefu." Inaweza kukamata urefu wa mawimbi ya maelfu au mamia ya kilometa. Hii ni kwa sababu ya kutokuwa na uwezo wa kupima mawimbi ya LIGO na LISA. Kwa sababu vitu vinavyozunguka, kama vile mashimo meusi, huongeza mwendo wanapokaribia kuungana, kwa muda wao hutokeza mawimbi ya uvutano yenye urefu wa mawimbi mfupi zaidi na zaidi. Lila inaweza kuona mashimo meusi yakikaribia kwa karibu katika wiki kabla ya kuunganishwa, na kuwapa wanasayansi habari kwamba mgongano unakaribia. Wakati wa kupima mawimbi, vituo vya uchunguzi vya LIGO vinaweza kugundua wakati wa kugonga. Njia nyingine ya kupima umbali wa mwezi ni kutumia laser range, mbinu ambayo wanasayansi hutumia kupima umbali kutoka dunia hadi mwezi kwa kutumia lasers, kwa sababu ya reflectors zilizowekwa kwenye uso wa mwezi wakati wa kutua kwa mwezi. "Kama vile, kuna uwezekano wa kugundua mawimbi ya umeme kutoka kwa pulsar, ambayo inaweza kuonekana katika ""LISA"" na ""Blas Temenio"" katika jarida la Physical Review D." Lakini mbinu hiyo ingehitaji vipimo vilivyoboreshwa kwenye mwezi - sababu nyingine ya kurudi. LISA, LIGO na vituo vingine vya uchunguzi vya laser hupima kunyoosha na kukandamiza kwa mawimbi ya mvuto kwa kuangalia jinsi miale ya laser inavyoingiliana baada ya kuvuka mikono yao mirefu ya detector. Lakini mbinu inayopendekezwa inafuata njia tofauti. Badala ya kuangalia mabadiliko madogo katika urefu wa mikono ya detector wakati mawimbi ya mvuto hupita, mbinu hii mpya inaweka jicho juu ya umbali kati ya mawingu mawili ya atomu. Sifa za quantum za atomu humaanisha kwamba zinafanya kazi kama mawimbi ambayo yanaweza kuingiliana yenyewe. Mawimbi ya uvutano yanapopita, yanabadilisha umbali kati ya mawingu ya atomu. Wanasayansi wanaweza kugundua badiliko hilo la umbali kwa kutegemea kuingiliana huko kwa quantum. Hogan anasema mbinu hiyo inaweza kufunua mawimbi ya mvuto yenye urefu wa mawimbi kati ya yale yanayoweza kugunduliwa na LIGO na LISA. "Mwanasayansi huyo wa sayansi ya anga, ambaye anafanya kazi katika maabara ya Fermilab huko Batavia, Illinois, Marekani, amekuwa akifanya utafiti wa majaribio ya ""magnetic interferometer"" ambayo inaweza kupima mawimbi ya uvutano wa dunia na sheria za msingi za fizikia." Wazo hilo ni "la wakati ujao kabisa", Blas Temiño anasema. Jitihada nyingine inakusudia kugundua mawimbi ya uvutano kutoka nyakati za mapema zaidi za ulimwengu. Mawimbi hayo yangetokea wakati wa mlipuko wa mawimbi, wakati ambao ulimwengu uliongezeka ukubwa baada ya mlipuko mkubwa. Mlipuko huo una urefu wa mawimbi ya urefu wa kilomita 1021 au sekta milioni moja. "Hata hivyo, utafiti huo ulifanyika mwaka 2014 wakati wanasayansi waliotumia jaribio la ""Bip2"" walipotangaza kugundua mawimbi ya mvuto yaliyoandikwa katika mwelekeo wa mwangaza wa zamani zaidi katika ulimwengu, mwangaza wa nyuma wa cosmic microwave." Madai hayo baadaye yalikataliwa (SN: 1/30/15). Jitihada inayoitwa CMB-Stage 4 itaendelea na utafutaji, na mipango ya telescopes nyingi mpya ambazo zitasaka mwanga wa zamani zaidi wa ulimwengu kwa ishara za mawimbi - wakati huu, kwa matumaini, bila hatua yoyote mbaya. Kwa sababu ya aina nyingi za mawimbi ya mvuto ambayo wanasayansi wameweka macho yao juu yake, wanajua kidogo kuhusu nini cha kutarajia. Vitu vinavyojulikana - kama vile mashimo meusi au nyota za neutroni - vinaweza kutokeza mawimbi hayo. Lakini kwa mawimbi ya uvutano yenye urefu mfupi zaidi wa mawimbi, labda sentimita chache tu, "hadithi ni tofauti", asema mwanafizikia Valerie Domcke wa CERN karibu na Geneva. "Hatuna chanzo kinachojulikana... ambacho kwa kweli kingeweza kutupa [mawe haya] ya uvutano wa amplitude kubwa ya kutosha ambayo tunaweza kugundua kwa kweli. " Bado, wanasaikolojia wanataka kuangalia ikiwa mawimbi madogo yapo huko nje. Mageuzi haya yanaweza kutokea kutokana na matukio ya ghafla katika historia ya ulimwengu, kama vile mabadiliko ya awamu, ambapo ulimwengu hubadilika kutoka hali moja hadi nyingine, kama vile maji yanayokithiri kutoka mvuke hadi umajimaji. Uwezekano mwingine ni kwamba kuna mashimo meusi madogo, madogo sana yasiyoweza kufanyizwa kwa njia ya kawaida, ambayo huenda yalitokea katika ulimwengu wa mapema. Fizikia katika mifumo hii haijulikani vizuri, <unk>hata kutafuta [mahali pa mawimbi ya uvutano] na kutowapata ingetuambia kitu,<unk> Domcke anasema. Mawimbi hayo ya uvutano ni ya ajabu sana hivi kwamba mbinu zao za kugundua mawimbi hayo hazijulikani. Lakini urefu wa mawimbi ni mdogo vya kutosha kwamba yanaweza kuonekana kwa majaribio ya kiwango cha juu cha maabara, badala ya vyanzo vikubwa. Wanasayansi wanaweza hata kutumia tena data kutoka kwa majaribio yaliyoundwa kwa makusudi mengine. Wakati mawimbi ya mvuto yanapokutana na mashamba ya umeme, mawimbi yanaweza kutenda kwa njia sawa na chembechembe za subatomic zinazoitwa axions (SN 3 17 22). Kwa hiyo majaribio ya kutafuta chembe hizo yaweza pia kufunua mawimbi madogo ya uvutano. Mzunguko wa mawimbi ya uvutano Mawimbi ya uvutano huja katika wigo wa urefu wa mawimbi mafupi na marefu. Kila safu ya urefu wa wimbi hutokezwa na vyanzo tofauti. Pulsari na nyota zinazolipuka, au supernova, hutokeza mawimbi ya muda mfupi. Mawimbi mengine hutokezwa na nyota za neutron au mashimo meusi yenye uzito wa nyota, yenye uzito wa chini ya mara 100 ya jua. Vipindi virefu zaidi hutokezwa na jozi za mashimo meusi yenye uzito mkubwa sana. Vipimo vya mawimbi mbalimbali vinaweza kupatikana kwa kutumia aina tofauti za vifaa, ikiwa ni pamoja na vifaa vya angani kama vile LIGO, vifaa vya angani kama vile LISA, na vipimo vya vipimo vya nyota zilizokufa zinazoitwa pulsar. Mwanga mrefu zaidi unaweza kugunduliwa kwa kuchunguza mwanga uliotolewa muda mfupi baada ya Big Bang, mwangaza wa nyuma wa cosmic microwave. Aina nyingine za vifaa vya kugundua (visivyoonyeshwa) zinaweza kujaza mapungufu hayo. Kuchukua mawimbi ya uvutano ni kama kupiga makasia dhidi ya mawimbi: ni vigumu, lakini inafaa kwa ajili ya maoni ya mandhari. "Mawimbi ya uvutano ni ngumu sana kugundua", Hogan anasema. LIGO ilichukua miaka kadhaa kabla ya kuona mawimbi ya kwanza, na ndivyo ilivyo na mbinu ya kupima wakati wa pulsar. Lakini mara moja wataalamu wa nyota walianza kupata matokeo mazuri. "Ni mtazamo mpya kabisa wa ulimwengu", Hogan anasema. Kwa sasa, mawimbi ya mvuto yamesaidia kuthibitisha nadharia ya Einstein ya jumla ya upendeleo, kugundua darasa jipya la mashimo meusi yenye uzito wa wastani na kufichua moto unaotokea wakati vitu viwili vya ultradense vinavyoitwa nyota za neutroni vinapogongana (SN: 2 11 16; SN: 9 12 20; SN: 10 16 17). Na bado ni siku za mapema kwa ajili ya kugundua mawimbi ya uvutano. Wanasayansi wanaweza tu kukisia kile ambacho vifaa vya baadaye vya kugundua vitakifunua. "Kuna mengi zaidi ya kugundua", Hogan anasema. <unk>Ni lazima kuwa ya kuvutia.<unk>