,A GRAVITATIONAL WAVE IS A RIPPLE IN SPACETIME ITSELF A STRETCHING AND SQUEEZING OF THE FABRIC OF SPACE THAT RADIATES OUTWARD AT THE SPEED OF LIGHT WHEN A MASSIVE OBJECT ACCELERATES4 ,PREDICTED BY ,ALBERT ,EINSTEIN IN #AIAF AS A CONSEQUENCE OF GENERAL RELATIVITY1 THEY WENT UNDETECTED FOR A CENTURY NOT BECAUSE THEY WERE RARE1 BUT BECAUSE THEY ARE UNIMAGINABLY WEAK3 BY THE TIME A WAVE FROM A CATACLYSM ON THE FAR SIDE OF THE UNIVERSE REACHES ,EARTH1 IT CHANGES THE LENGTH OF A #D-KILOMETRE DETECTOR ARM BY ABOUT A THOUSANDTH OF THE WIDTH OF A PROTON4 ,ON #AD ,SEPTEMBER #BJAE1 THE ,LASER ,INTERFEROMETER ,GRAVITATIONAL-,WAVE ,OBSERVATORY HEARD ONE ANYWAY A CHIRP FROM TWO COLLIDING BLACK HOLES AND OPENED AN ENTIRELY NEW SENSE FOR ASTRONOMY4 ,THE PREDICTION ,EINSTEIN DIDN'T QUITE BELIEVE ,GENERAL RELATIVITY SAYS MASS CURVES SPACETIME1 AND WHEN MASS ACCELERATES1 THE CURVATURE PROPAGATES OUTWARD AS A WAVE ANALOGOUS TO HOW ACCELERATING CHARGES RADIATE LIGHT4 ,EINSTEIN DERIVED THE RESULT IN #AIAF1 THEN NEARLY TALKED HIMSELF OUT OF IT1 AND THE PHYSICS COMMUNITY SPENT DECADES ARGUING WHETHER THE WAVES WERE REAL OR A COORDINATE ARTIFACT4 ,THE FIRST PROOF CAME INDIRECTLY3 IN #AIGD1 ,RUSSELL ,HULSE AND ,JOSEPH ,TAYLOR FOUND A BINARY PULSAR TWO NEUTRON STARS ORBITING EACH OTHER WHOSE ORBIT WAS DECAYING EXACTLY AS FAST AS GENERAL RELATIVITY PREDICTED IT WOULD LOSE ENERGY TO GRAVITATIONAL RADIATION4 ,THEY NEVER SAW THE WAVES2 THEY WATCHED THEM CARRY THE ENERGY AWAY4 ,IT WON THEM THE #AIIC ,NOBEL ,PRIZE4 ,THE CHIRP THAT BROKE THE SILENCE ,,GW#AEJIAD ,DIRECT DETECTION TOOK A MACHINE OF ALMOST ABSURD SENSITIVITY4 ,,LIGO'S TWO DETECTORS IN ,HANFORD1 ,WASHINGTON AND ,LIVINGSTON1 ,LOUISIANA ARE #D-KILOMETRE ;,L-SHAPED LASER INTERFEROMETERS3 MIRRORS HUNG ON VIBRATION ISOLATORS1 LASER BEAMS SPLIT AND RECOMBINED SO THAT A PASSING WAVE'S STRAIN "<@9#AJ_<1 A FRACTIONAL LENGTH CHANGE OF ONE PART IN A SEXTILLION"> SHOWS UP AS AN INTERFERENCE SHIFT4 ,ON #AD ,SEPTEMBER #BJAE DAYS INTO A NEWLY UPGRADED OBSERVING RUN BOTH DETECTORS CAUGHT THE SAME SIGNAL #G MILLISECONDS APART3 TWO BLACK HOLES1 ABOUT #CF AND #BI SOLAR MASSES1 SPIRALING INTO EACH OTHER #A4C BILLION LIGHT-YEARS AWAY1 MERGING INTO A SINGLE @9#FB-SOLAR-MASS BLACK HOLE AND RADIATING THE REMAINING @9#C SOLAR MASSES AS PURE GRAVITATIONAL ENERGY BRIEFLY OUTSHINING EVERY STAR IN THE OBSERVABLE UNIVERSE IN GRAVITATIONAL POWER ALONE4 ..> ,THE WAVEFORM A RISING 8CHIRP0 THAT SWEEPS UP IN FREQUENCY AS THE OBJECTS SPIRAL FASTER IS UNMISTAKABLE1 AND IT CAN BE LISTENED TO3 THE VIDEO ABOVE PLAYS THE REAL DETECTION1 THE SOUND OF SPACETIME BEING RUNG LIKE A BELL BY A COLLISION THAT HAPPENED WHEN MULTICELLULAR LIFE WAS YOUNG ON ,EARTH4 ,THE DISCOVERY WON THE #BJAG ,NOBEL ,PRIZE FOR ,,LIGO PIONEERS ,RAINER ,WEISS1 ,BARRY ,BARISH1 AND ,KIP ,THORNE AND IT LAUNCHED GRAVITATIONAL-WAVE ASTRONOMY3 A WAY TO OBSERVE THE UNIVERSE THAT DOESN'T USE LIGHT AT ALL4 ..> ,WHAT GRAVITATIONAL WAVES SEE THAT LIGHT CAN'T ,GRAVITATIONAL WAVES PASS THROUGH EVERYTHING DUST1 PLASMA1 THE EARLY OPAQUE UNIVERSE CARRYING INFORMATION NO TELESCOPE CAN REACH3 "9 ,BINARY BLACK HOLES ,,GW#AEJIAD OPENED THE CATALOGUE2 DOZENS OF MERGERS ARE NOW CATALOGUED EACH OBSERVING RUN BY ,,LIGO1 ,VIRGO "<,ITALY">1 AND ,,KAGRA "<,JAPAN"> "9 ,NEUTRON-STAR MERGERS ,,GW#AGJHAG "<#BJAG"> PRODUCED THE FIRST MULTI-MESSENGER EVENT3 GRAVITATIONAL WAVES PLUS A GAMMA-RAY BURST PLUS A KILONOVA THAT FORGED GOLD1 PLATINUM AND URANIUM DIRECT EVIDENCE HEAVY ELEMENTS ARE MADE IN THESE COLLISIONS "9 ,THE STOCHASTIC BACKGROUND PULSAR-TIMING ARRAYS "<,N,A,N,O,GRAV AND PARTNERS"> ANNOUNCED EVIDENCE IN #BJBC FOR A PERSISTENT HUM OF GRAVITATIONAL WAVES1 LIKELY FROM SUPERMASSIVE BLACK HOLE BINARIES THROUGHOUT THE UNIVERSE "9 ,THE FIRST SECOND OF THE ,BIG ,BANG LIGHT CAN'T REACH EARLIER THAN #CHJ1JJJ YEARS1 BUT GRAVITATIONAL WAVES FROM INFLATION COULD1 IN PRINCIPLE1 CARRY DIRECT ECHOES OF THE UNIVERSE'S FIRST INSTANT SEE ZEPTOVERSE FOR HOW THE SMALLEST SIGNALS BECAME THE GALAXIES ,WHAT'S NEXT THE INSTRUMENTS ,,LIGO_/,VIRGO_/,,KAGRA CONTINUE COORDINATED OBSERVING RUNS1 CATALOGUING MERGERS BY THE HUNDREDS4 ,,LISA " WILL MOVE THE TECHNIQUE TO SPACE THREE SPACECRAFT FORMING A TRIANGLE OF #B4E-MILLION-KILOMETRE ARMS1 SENSITIVE TO THE LOW-FREQUENCY RUMBLE OF SUPERMASSIVE BLACK HOLE MERGERS AND GALACTIC BINARIES4 ,AND PROPOSED INSTRUMENTS LIKE THE ,EINSTEIN ,TELESCOPE AND ,COSMIC ,EXPLORER AIM FOR TENFOLD SENSITIVITY HEARING NEARLY EVERY STELLAR-MASS BLACK HOLE MERGER IN THE OBSERVABLE UNIVERSE4 ,SEE ALSO "9 ZEPTOVERSE ,,LIGO'S STRAIN IS ITSELF A ZEPTO-SCALE SIGNAL "9 SMALLEST THINGS EVER MEASURED THE DISPLACEMENT RECORD "9 NEUTRINO THE OTHER NON-OPTICAL SENSE ASTRONOMY JUST GAINED "9 HISTORY OF PHYSICS ,REFERENCES - ,ABBOTT ET AL4 "<,,LIGO_/,VIRGO"> 8,OBSERVATION OF ,GRAVITATIONAL ,WAVES FROM A ,BINARY ,BLACK ,HOLE ,MERGER10 ,PHYS4 ,REV4 ,LETT4 #AAF1 #JFAAJB "<#BJAF"> - ,HULSE @& ,TAYLOR BINARY PULSAR ENERGY LOSS "<#AIGE">2 ,NOBEL ,PRIZE #AIIC - ,NOBEL ,PRIZE IN ,PHYSICS #BJAG ,WEISS1 ,BARISH1 ,THORNE1 FOR ,,LIGO_/,,GW#AEJIAD - ,,GW#AGJHAG MULTI-MESSENGER PAPERS "<#BJAG"> - ,N,A,N,O,GRAV COLLABORATION NANOHERTZ BACKGROUND EVIDENCE "<#BJBC">