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here%20quantum%20gravity%20matters.%0AIgor%20Pikovski%0AIt%20would%20take%20another%20conceptual%20leap%20to%20go%20from%20a%20gravitational%20wave%20detector%20to%20a%20detector%20for%20individual%20gravitons.%20In%20the%20recent%20paper,%20which%20appeared%20in%20Nature%20Communications%20in%20August,%20Pikovski%20and%20his%20co-authors%20outlined%20how%20the%20graviton%20detector%20would%20work.%0AFirst,%20take%20a%2015-kilogram%20bar%20of%20beryllium%20(or%20some%20similar%20material)%20and%20cool%20it%20almost%20all%20the%20way%20to%20absolute%20zero,%20the%20minimum%20possible%20temperature.%20Sapped%20of%20all%20heat,%20the%20bar%20will%20sit%20in%20its%20minimum-energy%20%E2%80%9Cground%E2%80%9D%20state.%20All%20the%20atoms%20of%20the%20bar%20will%20act%20together%20as%20one%20quantum%20system,%20akin%20to%20one%20hulking%20atom.%0AThen,%20wait%20until%20a%20gravitational%20wave%20from%20deep%20space%20passes%20by.%20The%20odds%20that%20any%20particular%20graviton%20will%20interact%20with%20the%20beryllium%20bar%20are%20low,%20but%20the%20wave%20will%20contain%20so%20many%20gravitons%20that%20the%20overall%20odds%20of%20at%20least%20one%20interaction%20are%20high.%20The%20group%20calculated%20that%20approximately%20one%20in%20three%20gravitational%20waves%20of%20the%20right%20sort%20(neutron%20star%20collisions%20work%20best%20since%20their%20mergers%20last%20longer%20than%20black%20hole%20mergers)%20would%20make%20the%20bar%20ring%20with%20one%20quantum%20unit%20of%20energy.%20If%20your%20bar%20reverberates%20in%20concert%20with%20a%20gravitational%20wave%20confirmed%20by%20LIGO,%20you%20will%20have%20witnessed%20a%20quantized%20event%20caused%20by%20gravity.%0A%0AMark%20Belan/%20Quanta%20Magazine%0A%E2%80%9CIt%20would%20be%20our%20first%20window%20into%20where%20quantum%20gravity%20matters,%E2%80%9D%20Pikovski%20said.%0AAmong%20a%20handful%20of%20engineering%20hurdles%20involved%20in%20opening%20that%20window,%20the%20highest%20would%20be%20putting%20a%20heavy%20object%20into%20its%20ground%20state%20and%20sensing%20it%20jumping%20to%20its%20next-lowest-energy%20state.%20One%20of%20the%20groups%20pushing%20the%20state%20of%20the%20art%20on%20this%20front%20is%20at%20ETH%20Zurich,%20where%20Fadel%20and%20his%20collaborators%20cool%20tiny%20sapphire%20crystals%20until%20they%20display%20quantum%20properties.%20In%202023,%20the%20team%20succeeded%20in%20putting%20a%20crystal%20into%20two%20states%20simultaneously%20%E2%80%94%20another%20hallmark%20of%20a%20quantum%20system.%20Its%20mass%20was%2016%20millionths%20of%20a%20gram%20%E2%80%94%20heavy%20for%20a%20quantum%20object,%20but%20still%20half%20a%20billion%20times%20lighter%20than%20Pikovski%E2%80%99s%20bar.%20Nevertheless,%20Fadel%20considers%20the%20proposal%20to%20be%20achievable.%20%E2%80%9CIt%20wouldn%E2%80%99t%20be%20too%20crazy,%E2%80%9D%20he%20said.%0APikovski%E2%80%99s%20experiment%20%E2%80%94%20like%20Dyson%E2%80%99s%20%E2%80%94%20emulates%20the%20very%20experiment%20that%20prompted%20Einstein%20to%20propose%20in%201905%20that%20light%20is%20quantized,%20a%20watershed%20moment%20in%20the%20history%20of%20quantum%20mechanics.%20%E2%80%9CIf%20carried%20through,%20it%20would%20bring%20the%20state%20of%20the%20art%20in%20the%20case%20for%20gravitons%20to%20the%20same%20level%20that%20it%20was%20for%20photons%20in%201905,%E2%80%9D%20Wilczek%20said.%0A%0AIgor%20Pikovski,%20a%20physicist%20at%20the%20Stevens%20Institute%20of%20Technology,%20has%20proposed%20a%20way%20to%20detect%20a%20quantized%20response%20to%20a%20gravitational%20wave.%0AIan%20Reitz%0ATextbooks%20often%20credit%20Einstein%E2%80%99s%20paper%20with%20establishing%20the%20photon%E2%80%99s%20existence.%20But%20the%20real%20story%20is%20far%20more%20interesting.%20At%20the%20time,%20many%20physicists%20rejected%20Einstein%E2%80%99s%20theory.%20Some%20wouldn%E2%80%99t%20come%20around%20for%20two%20decades.%20In%20their%20view,%20the%20experiment%20fell%20far%20short%20of%20conclusive%20proof.%20It%20was,%20rather,%20an%20opening%20argument%20in%20a%20decades-long%20war%20fought%20to%20determine%20the%20true%20nature%20of%20light.%0AThe%20Real%20Story%20of%20the%20Photon%0APhysicists%20saw%20the%20first%20cracks%20opening%20up%20in%20their%20classical%20understanding%20of%20reality%20in%20the%20closing%20years%20of%20the%2019th%20century.%20J.J.%20Thomson%20discovered%20that%20electric%20currents%20come%20in%20discrete%20chunks%20of%20charge%20called%20electrons.%20Meanwhile,%20physicists%20were%20puzzling%20over%20a%20string%20of%20experiments%20by%20Heinrich%20Hertz%20and%20others%20that%20used%20light%20to%20make%20a%20current%20flow%20%E2%80%94%20a%20phenomenon%20that%20came%20to%20be%20called%20the%20photoelectric%20effect.%0AThe%20puzzle%20was%20that%20when%20they%20directed%20dim%20beams%20of%20light%20at%20a%20metal%20plate,%20sometimes%20an%20electric%20current%20flowed%20across%20the%20plate%20and%20sometimes%20it%20didn%E2%80%99t.%20In%20the%20pre-quantum%20world%20this%20was%20hard%20to%20explain.%20It%20was%20believed%20that%20any%20wave%20should%20create%20at%20least%20a%20small%20current,%20and%20brighter%20waves%20should%20create%20larger%20currents.%20Instead,%20physicists%20found%20that%20there%20was%20a%20special%20color%20of%20light%20%E2%80%94%20a%20frequency%20%E2%80%94%20that%20got%20a%20current%20to%20flow.%20Only%20waves%20of%20that%20frequency%20or%20higher%20could%20start%20a%20current.%20Brightness%20had%20little%20to%20do%20with%20it.%0A%0AEinstein%20proposed%20a%20solution%20in%201905:%20A%20wave%20of%20light%20is%20made%20of%20many%20discrete%20units%20called%20%E2%80%9Cquanta,%E2%80%9D%20each%20with%20energy%20related%20to%20the%20wave%E2%80%99s%20frequency.%20The%20higher%20the%20frequency%20of%20the%20wave,%20the%20more%20energetic%20its%20quanta.%20And%20the%20brighter%20the%20wave,%20the%20more%20quanta%20there%20are.%20If%20you%20try%20to%20start%20an%20electric%20current%20in%20a%20metal%20plate%20with%20low-frequency%20red%20light,%20you%E2%80%99ll%20be%20no%20more%20successful%20than%20if%20you%20tried%20to%20knock%20over%20a%20refrigerator%20with%20ping-pong%20balls;%20no%20number%20would%20suffice.%20But%20using%20higher-frequency%20blue%20light%20is%20like%20switching%20to%20boulders.%20Each%20of%20those%20units%20has%20enough%20oomph%20to%20excite%20an%20electron,%20even%20in%20dim%20light%20with%20very%20few%20of%20them.%0AEinstein%E2%80%99s%20theory%20was%20met%20with%20skepticism.%20Physicists%20felt%20fiercely%20protective%20of%20James%20Clerk%20Maxwell%E2%80%99s%20then-40-year-old%20theory%20of%20light%20as%20an%20electromagnetic%20wave.%20They%20had%20seen%20light%20refracting,%20diffracting,%20and%20doing%20all%20the%20things%20waves%20do.%20How%20could%20it%20be%20made%20up%20of%20particles%3F%0AEven%20after%20Einstein%20won%20the%201921%20Nobel%20Prize%20in%20Physics%20for%20his%20theory%20of%20the%20photoelectric%20effect,%20debate%20continued%20among%20physicists.%20The%20effect%20suggested%20that%20something%20is%20quantized;%20otherwise%20there%20wouldn%E2%80%99t%20be%20a%20minimum%20threshold%20required%20to%20get%20electrons%20flowing.%20But%20some%20physicists,%20including%20Niels%20Bohr,%20who%20is%20considered%20one%20of%20the%20founders%20of%20quantum%20theory,%20continued%20to%20explore%20the%20possibility%20that%20only%20the%20matter%20was%20quantized,%20not%20the%20light.%20Today,%20this%20type%20of%20theory%20is%20called%20%E2%80%9Csemiclassical%E2%80%9D%20because%20it%20describes%20a%20classical%20field%20interacting%20with%20quantized%20matter.%0ATo%20see%20how%20a%20semiclassical%20theory%20can%20explain%20the%20photoelectric%20effect,%20imagine%20a%20kid%20on%20a%20swing.%20They%E2%80%99re%20kind%20of%20like%20an%20electron%20in%20a%20metal.%20They%20have%20a%20ground%20state%20(not%20swinging)%20and%20an%20excited%20state%20(swinging).%20A%20classical%20wave%20is%20like%20giving%20the%20kid%20a%20series%20of%20pushes.%20If%20the%20pushes%20come%20at%20some%20random%20frequency,%20nothing%20happens.%20The%20kid%20might%20bounce%20around%20a%20little,%20but%20they%20will%20basically%20stay%20in%20their%20ground%20state.%20It%E2%80%99s%20only%20when%20you%20push%20with%20just%20the%20right%20frequency%20%E2%80%94%20the%20swing%E2%80%99s%20%E2%80%9Cresonant%E2%80%9D%20frequency%20%E2%80%94%20that%20the%20kid%20accumulates%20energy%20and%20starts%20swinging.%20(Electrons%20in%20a%20metal%20are%20a%20little%20different;%20they%20resonate%20with%20a%20whole%20continuous%20%E2%80%9Cband%E2%80%9D%20of%20frequencies%20instead%20of%20just%20the%20one.%20But%20the%20upshot%20is%20the%20same:%20Any%20wave%20below%20that%20frequency%20band%20does%20nothing,%20whereas%20any%20wave%20in%20that%20frequency%20band%20excites%20electrons%20and%20makes%20a%20current%20flow.)%0A%0AEinstein%20was%20eventually%20vindicated,%20but%20not%20on%20the%20strength%20of%20the%20photoelectric%20effect%20alone.%20Later%20experiments%20that%20collided%20electrons%20and%20photons%20like%20projectiles%20found%20that%20momentum,%20too,%20came%20in%20chunks.%20This%20research%20eventually%20ruled%20out%20the%20main%20alternative%20%E2%80%94%20a%20semiclassical%20theory%20of%20light%20and%20matter%20from%20Bohr%20and%20his%20collaborators.%20In%201925,%20seeing%20the%20data,%20Bohr%20agreed%20to%20%E2%80%9Cgive%20our%20revolutionary%20efforts%20as%20honorable%20a%20funeral%20as%20possible%E2%80%9D%20and%20welcomed%20light%20into%20the%20quantum%20fold.%20Light%20quanta%20became%20known%20as%20photons.%0AFew%20doubted%20the%20photon%20after%201925,%20but%20physicists%20are%20nothing%20if%20not%20thorough.%20Just%20because%20no%20one%20could%20think%20of%20a%20viable%20semiclassical%20theory%20didn%E2%80%99t%20mean%20one%20didn%E2%80%99t%20exist.%20The%20final%20proof%20that%20photons%20are%20real%20came%20in%20the%20late%201970s,%20when%20quantum%20optics%20researchers%20showed%20that%20light%20arrived%20at%20a%20detector%20in%20a%20pattern%20no%20semiclassical%20theory%20could%20mimic.%20The%20experiments%20were%20akin%20to%20firing%20a%20photon%20gun%20once%20per%20second%20and%20confirming%20that%20the%20detector%20clicked%20once%20per%20second%20in%20response.%20The%20photon%20wars%20ended%20with%20a%20whimper.%0A%E2%80%9CThere%20were%20just%20mountains%20of%20evidence%20that%20this%20photon%20concept%20was%20useful%20and%20vital,%E2%80%9D%20Wilczek%20said.%0AThe%20Graviton%20Wars%20Begin%0AIn%20August%20of%202023,%20Daniel%20Carney%20and%20his%20collaborators%20fired%20the%20first%20shot%20in%20a%20new%20war.%0AIt%20started%20when%20Carney%E2%80%99s%20colleague%20Nicholas%20Rodd%20had%20an%20insight%20similar%20to%20Pikovski%E2%80%99s,%20about%20a%20possible%20way%20to%20detect%20a%20graviton.%20%E2%80%9CWe%20got%20super%20pumped,%E2%80%9D%20said%20Carney,%20a%20physicist%20at%20Lawrence%20Berkeley%20National%20Laboratory.%0ABut%20when%20he%20and%20his%20collaborators%20dug%20into%20the%20literature,%20they%20uncovered%20the%20messy%20history%20of%20the%20photon,%20and%20the%20lengths%20to%20which%20quantum%20optics%20researchers%20had%20gone%20in%20the%201970s%20to%20close%20the%20final%20loopholes.%20They%20translated%20those%20more%20stringent%20tests%20into%20the%20gravitational%20context%20and%20found%20that%20Dyson%20had%20been%20right.%20Really%20proving%20quantumness%20by%20detecting%20lone%20gravitons%20one%20after%20another%20%E2%80%94%20as%20opposed%20to%20plucking%20one%20out%20of%20a%20tsunami,%20in%20the%20style%20of%20Pikovski%E2%80%99s%20proposal%20%E2%80%94%20would%20indeed%20take%20planetary-scale%20machinery.%0A%0ADaniel%20Carney,%20a%20physicist%20at%20Lawrence%20Berkeley%20National%20Laboratory,%20argues%20that%20a%20proposed%20experiment%20would%20not%20offer%20conclusive%20proof%20of%20quantum%20gravity.%0ALawrence%20Berkeley%20National%20Laboratory%0A%E2%80%9CIt%20was%20crazy%20to%20have%20to%20revise%20your%20hypothesis%20by%20100%25%20really%20fast,%E2%80%9D%20Carney%20said.%0ANow%20graviton%20chasers%20find%20themselves%20in%20a%20peculiar%20position.%20On%20the%20main%20facts,%20everyone%20is%20in%20agreement.%20One,%20detecting%20a%20quantum%20event%20sparked%20by%20a%20gravitational%20wave%20is%20%E2%80%94%20surprisingly%20%E2%80%94%20possible.%20And%20two,%20doing%20so%20would%20not%20explicitly%20prove%20that%20the%20gravitational%20wave%20is%20quantized.%20%E2%80%9CCould%20you%20make%20a%20classical%20gravitational%20wave%20that%20would%20produce%20the%20same%20signal%3F%20The%20answer%20is%20yes,%E2%80%9D%20said%20Carney,%20who%20along%20with%20two%20co-authors%20analyzed%20this%20type%20of%20experiment%20in%20Physical%20Review%20D%20in%20February.%0AHow%20much%20physicists%20feel%20they%20would%20learn%20from%20the%20experiment%20varies.%20To%20some,%20it%20would%20strongly%20suggest%20that%20gravity%20is%20a%20quantum%20force%20because%20the%20alternative%20%E2%80%94%20a%20semiclassical%20theory%20of%20gravity%20and%20matter%20%E2%80%94%20is%20disfavored%20on%20other%20grounds.%20Such%20theories%20violate%20the%20conservation%20of%20energy,%20for%20instance.%20If%20the%20beryllium%20bar%20gains%20one%20quantum%20of%20energy,%20then%20energy%20conservation%20requires%20that%20the%20gravitational%20wave%20must%20have%20lost%20one%20quantum%20of%20energy%20%E2%80%94%20and%20therefore%20it%20must%20be%20quantized,%20too.%20(Einstein%20advanced%20this%20sort%20of%20argument%20for%20the%20photon%20in%201911.)%20Semiclassical%20theories%20save%20gravity%E2%80%99s%20classicality%20by%20sacrificing%20this%20revered%20principle.%0AWe%E2%80%99re%20so%20biased%20to%20think%20that%20everything%20is%20quantum%20that%20you%20should%20really%20be%20doing%20a%20lawyerly%20thing.%0ADaniel%20Carney%0A%E2%80%9CUnless%20you%20use%20very%20artificial%20interpretations,%E2%80%9D%20Wilczek%20said,%20%E2%80%9Cit%20does%20tell%20you%20that%20you%20really%20should%20apply%20quantum%20mechanics%20to%20the%20gravitational%20wave.%E2%80%9D%0A%E2%80%9CIf%20I%20want%20to%20see%20signatures%20of%20quantumness,%20it%E2%80%99s%20not%20my%20first%20goal%20to%20rule%20out%20these%20pathological%20things,%E2%80%9D%20Pikovski%20said.%0ATo%20physicists%20such%20as%20Carney,%20however,%20a%20mere%20strong%20suggestion%20that%20gravity%20is%20quantized%20isn%E2%80%99t%20all%20that%20informative.%20We%20already%20have%20an%20abundance%20of%20strong%20suggestions%20that%20all%20of%20reality%20is%20quantized,%20he%20says.%20What%E2%80%99s%20needed%20is%20proof%20%E2%80%94%20such%20as%20experiments%20that%20would%20close%20the%20remaining%20loopholes,%20no%20matter%20how%20bizarre%20they%20might%20seem.%0A%E2%80%9CWe%E2%80%99re%20so%20biased%20to%20think%20that%20everything%20is%20quantum%20that%20you%20should%20really%20be%20doing%20a%20lawyerly%20thing,%E2%80%9D%20he%20said.%0AA%20Starting%20Point%0AWhile%20Pikovski%E2%80%99s%20proposal%20is%20not%20a%20loophole-closing%20experiment,%20many%20physicists%20would%20still%20like%20to%20see%20it%20happen.%20It%20would%20mark%20the%20dawn%20of%20an%20era%20of%20experimental%20quantum%20gravity,%20which%20until%20recently%20seemed%20quite%20far%20off.%0A%E2%80%9CThis%20is%20an%20exciting%20paper,%E2%80%9D%20said%20Alex%20Sushkov,%20an%20experimental%20physicist%20at%20Boston%20University.%20%E2%80%9CThese%20are%20hard%20experiments,%20and%20we%20need%20bright,%20smart%20people%20to%20move%20in%20this%20direction.%E2%80%9D%0A%E2%80%9CWe%20can%20take%20it%20as%20a%20starting%20point,%E2%80%9D%20said%20Myungshik%20Kim,%20a%20physicist%20at%20Imperial%20College%20London.%0ARELATED:%0A%0A%09%09Physicists%20Find%20a%20Way%20to%20See%20the%20%E2%80%98Grin%E2%80%99%20of%20Quantum%20Gravity%20%E2%80%A8How%20the%20Bits%20of%20Quantum%20Gravity%20Can%20Buzz%20%E2%80%A8Why%20Gravity%20Is%20Not%20Like%20the%20Other%20Forces%20%0AIt%20might%20motivate%20subsequent%20experiments%20that%20would%20take%20physicists%20deeper%20into%20the%20quantum%20gravity%20era,%20just%20as%20scattering%20experiments%20once%20took%20them%20deeper%20into%20the%20era%20of%20the%20photon.%20Physicists%20now%20know%20that%20quantum%20mechanics%20is%20much%20more%20than%20quantization.%20Quantum%20systems%20can%20take%20on%20combinations%20of%20states%20known%20as%20superpositions,%20for%20instance,%20and%20their%20parts%20can%20become%20%E2%80%9Centangled%E2%80%9D%20in%20such%20a%20way%20that%20measuring%20one%20reveals%20information%20about%20the%20other.%20Experiments%20establishing%20that%20gravity%20exhibits%20these%20phenomena%20would%20provide%20stronger%20evidence%20for%20quantum%20gravity,%20and%20researchers%20are%20already%20exploring%20what%20it%20would%20take%20to%20carry%20them%20out.%0ANone%20of%20these%20tests%20of%20gravity%E2%80%99s%20quantum%20side%20are%20completely%20ironclad,%20but%20each%20would%20contribute%20some%20hard%20data%20regarding%20the%20finest%20features%20of%20the%20universe%E2%80%99s%20weakest%20force.%20Now%20a%20frigid%20quantum%20bar%20of%20beryllium%20appears%20to%20be%20a%20prime%20candidate%20for%20an%20experiment%20that%20will%20mark%20the%20first%20step%20down%20that%20long%20and%20winding%20road.%0A%0A%0ABy%0ACharlie%20Wood%0AStaff%20Writer%0A%0AOctober%2030,%202024%0A%0Aexperimental%20physics%0A%0Afundamental%20physics%0A%0Agravity%0A%0Aphysics%0A%0Aquantum%20gravity%0A%0Aquantum%20physics%0A%0AAll%20topics%0A%0A%0AShare%20this%20article%0A%0A%0A%0A%0A%0ACOPIED!%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0ASubscribe%20now%0ARecent%20newsletters%0A%0AThe%20Quanta%20Newsletter%0AGet%20highlights%20of%20the%20most%20important%20news%20delivered%20to%20your%20email%20inbox%0AEmail%0A%0ASubscribe%0A%0ARecent%20newsletters%0A%0AAlso%20in%20Physics%0AThe%20Cosmos%20Teems%20with%20Complex%20Organic%20Molecules%0A%0AASTRONOMY%0AThe%20Cosmos%20Teems%20with%20Complex%20Organic%20Molecules%0ABy%0AELISE%20CUTTS%0A%0ANOVEMBER%2013,%202024%0A%0A%0APhysicists%20Spot%20Quantum%20Tornadoes%20Twirling%20in%20a%20%E2%80%98Supersolid%E2%80%99%0A%0ACONDENSED%20MATTER%20PHYSICS%0APhysicists%20Spot%20Quantum%20Tornadoes%20Twirling%20in%20a%20%E2%80%98Supersolid%E2%80%99%0ABy%0AZACK%20SAVITSKY%0A%0ANOVEMBER%206,%202024%0A%0A%0AHow%20Do%20Merging%20Supermassive%20Black%20Holes%20Pass%20the%20Final%20Parsec%3F%0A%0AASTROPHYSICS%0AHow%20Do%20Merging%20Supermassive%20Black%20Holes%20Pass%20the%20Final%20Parsec%3F%0ABy%0AJONATHAN%20O'CALLAGHAN%0A%0AOCTOBER%2023,%202024%0A%0A%0AComment%20on%20this%20article%0AQuanta%20Magazine%20moderates%20comments%20to%C2%A0facilitate%20an%20informed,%20substantive,%20civil%20conversation.%20Abusive,%20profane,%20self-promotional,%20misleading,%20incoherent%20or%20off-topic%20comments%20will%20be%20rejected.%20Moderators%20are%20staffed%20during%20regular%20business%20hours%20(New%20York%20time)%20and%20can%20only%20accept%20comments%20written%20in%20English.%C2%A0%0A%0AShow%20comments%0A%0A%0ANEXT%20ARTICLE%0AMeet%20the%20Eukaryote,%20the%20First%20Cell%20to%20Get%20Organized%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%09%09About%20Quanta%0A%09%09Archive%0A%09%09Contact%20Us%0A%09%09Terms%20%26%20Conditions%0A%09%09Privacy%20Policy%0AAll%20Rights%20Reserved%20%C2%A9%202024%0AAn%20editorially%20independent%20publication%20supported%20by%20the%20Simons%20Foundation.%0ASimons%20Foundation%0A%0AWe%20care%20about%20your%20data,%20and%20we'd%20like%20to%20use%20cookies%20to%20give%20you%20a%20smooth%20browsing%20experience.%20Please%20agree%20and%20read%20more%20about%20our%20privacy%20policy.%0AAGREE%0ADISMISS%0A%0A%3C/text%3E