,PROTON DECAY IS THE HYPOTHESIS THAT THE PROTON THE HEART OF EVERY ATOMIC NUCLEUS1 THE PARTICLE YOU'VE NEVER SEEN BREAK IS NOT ETERNAL4 ,GRAND ,UNIFIED ,THEORIES1 THE FRAMEWORKS THAT SEEK TO MERGE THE STRONG1 WEAK AND ELECTROMAGNETIC FORCES1 GENERALLY REQUIRE IT TO EVENTUALLY FALL APART INTO LIGHTER PARTICLES4 ,NO PROTON HAS EVER BEEN OBSERVED DECAYING3 EXPERIMENTS HAVE WATCHED TENS OF THOUSANDS OF TONS OF WATER FOR FOUR DECADES AND PUSHED THE LIFETIME LIMIT BEYOND @9#B4D #AJ;T YEARS A TRILLION TRILLION TIMES THE AGE OF THE UNIVERSE4 ,YET THE QUESTION IS ONE OF THE MOST CONSEQUENTIAL IN PHYSICS3 IF PROTONS DECAY1 THEN EVERY ATOM THAT EXISTS EVERY PLANET1 EVERY STAR1 EVERY FUTURE GALAXY HAS AN EXPIRY DATE4 ,WHY ANYONE THINKS THE PROTON MIGHT DIE ,THE ,STANDARD ,MODEL KEEPS MATTER STABLE BY ACCIDENT BARYON NUMBER CONSERVATION ISN'T A LAW1 IT'S AN EMERGENT HABIT4 ,IN ,GRAND ,UNIFIED ,THEORIES "<,G,U,TS"> THE ATTEMPT TO UNIFY THE THREE NON-GRAVITATIONAL FORCES INTO ONE QUARKS AND LEPTONS SIT IN THE SAME FAMILY1 AND THE THEORY'S NEW ULTRA-HEAVY GAUGE BOSONS CAN CONVERT ONE INTO THE OTHER4 ,IN THE SIMPLEST ,G,U,TS THE PROTON'S PREFERRED EXIT IS ;P EZ;P3 IT TURNS INTO A POSITRON AND A NEUTRAL PION1 LEAVING A TELLTALE FLASH OF ,CHERENKOV LIGHT IN A DETECTOR4 ,THE FACT THAT UNIFICATION PREDICTS THE DECAY IS WHAT MAKES IT PHYSICS RATHER THAN PHILOSOPHY3 IF THE FORCES ARE TRULY ONE1 THE PROTON'S PERMANENCE WAS ALWAYS AN ILLUSION4 ,FORTY YEARS OF WATCHING WATER ,THE SEARCH STRATEGY IS MONUMENTAL PATIENCE3 FILL A MOUNTAIN WITH ULTRAPURE WATER1 LINE IT WITH PHOTOMULTIPLIER TUBES1 AND WAIT FOR ONE OF @9#AJ PROTONS TO DIE4 "9 ,,IMB AND ,KAMIOKANDE "<#AIHJ;S"> THE FIRST GENERATION1 WATCHING THOUSANDS OF TONS OF WATER2 NO DECAYS SEEN "9 ,SUPER-,KAMIOKANDE "<#AIIFPRESENT"> #EJ1JJJ TONS A KILOMETRE UNDERGROUND IN ,KAMIOKA1 ,JAPAN2 HAS WATCHED CONTINUOUSLY FOR DECADES1 PUSHING THE FAVORED-MODE LIFETIME LIMIT PAST #B4D #AJ;T YEARS STILL ZERO DECAYS "9 ,HYPER-,KAMIOKANDE " FIVE TIMES ,SUPER-,K'S MASS1 @9#BEH1JJJ TONS2 WILL PROBE LIFETIMES PAST #AJ;U YEARS "9 ,,DUNE "<,FERMILAB_/,,SURF"> LIQUID-ARGON DETECTOR WITH A DIFFERENT DECAY-MODE SENSITIVITY2 COMPLEMENTARY SEARCH ,THE NULL RESULT ISN'T FAILURE IT'S ELIMINATION4 ,THE SIMPLEST ,,GUT ""> WAS ALREADY RULED OUT BY THE EARLY LIMITS2 EACH DECADE OF SILENCE KILLS OR CONSTRAINS ANOTHER THEORY4 ,THE LIFETIME BOUND HAS GROWN SO EXTREME THAT FINDING THE DECAY AT ALL WOULD BE A DISCOVERY ON THE SCALE OF THE ,HIGGS BOSON4 ,WHAT IT MEANS FOR THE FUTURE OF EVERYTHING ,IF THE PROTON DECAYS1 THE UNIVERSE HAS A SHELF LIFE4 ,ON TIMESCALES OF @9#AJ;T#AJ;U YEARS FAR BEYOND ANY STELLAR ERA EVERY ATOM IN EVERY OBJECT WILL EVENTUALLY DISINTEGRATE3 PLANETS CRUMBLE PARTICLE BY PARTICLE1 STARS EXHAUST NOT JUST FUEL BUT SUBSTANCE1 AND THE COSMOS DESCENDS INTO THE 8BLACK DWARF0 ERA A THIN SOUP OF ELECTRONS1 POSITRONS1 PHOTONS1 NEUTRINOS AND BLACK HOLES1 WITH NO CHEMISTRY AT ALL4 ,IF PROTONS DON'T DECAY1 MATTER IS EFFECTIVELY IMMORTAL1 AND THE UNIVERSE'S FAR FUTURE PLAYS OUT VERY DIFFERENTLY4 ,THE ANSWER TO WHETHER THE COSMOS CONTAINS ATOMS FOREVER IS DECIDED BY PHYSICS AT SCALES OF @9#AJ_< METRES DEEPER THAN THE ZEPTOVERSE1 DEEPER THAN ANYTHING ANY MACHINE WILL EVER DIRECTLY IMAGE4 ,THE HONEST ACCOUNTING "9 ,PROTON DECAY HAS NEVER BEEN OBSERVED FORTY-PLUS YEARS OF NULL RESULTS ,THE LIFETIME LIMIT IS @9#AJT"6 YEARS BUT LIFETIME"9 IS A STATISTICAL HALF-LIFE1 NOT A FUSE3 INDIVIDUAL PROTONS MAY DECAY AT ANY TIME2 WE JUST HAVEN'T CAUGHT ONE "9 ,MOST PHYSICISTS' HONEST POSITION3 IF ,,GUT UNIFICATION IS REAL1 THE PROTON IS MORTAL BUT WHETHER THE DECAY IS ACCESSIBLE TO ANY POSSIBLE EXPERIMENT IS GENUINELY UNKNOWN "9 ,THE SEARCH BUILT NEUTRINO PHYSICS AS A SIDE-EFFECT ,KAMIOKANDE AND ,SUPER-;,K1 BUILT TO WATCH PROTONS DIE1 ENDED UP CATCHING SUPERNOVA NEUTRINOS AND DISCOVERING NEUTRINO OSCILLATION INSTEAD4 ,THE HUNT FOR THE DECAY OF MATTER PRODUCED THE DISCOVERY OF THE GHOST PARTICLE ,SEE ALSO "9 ZEPTOVERSE THE SMALLEST SCALES DECIDE MATTER'S FATE "9 NEUTRINO THE PARTICLE THESE DETECTORS ACTUALLY FOUND "9 WHY DOES MATTER EXIST THE TWIN OPEN PROBLEM OF BARYON PHYSICS "9 SMALLEST THINGS EVER MEASURED ,REFERENCES - ,SUPER-,KAMIOKANDE COLLABORATION PROTON-DECAY LIFETIME LIMITS " - ,GEORGI @& ,GLASHOW ,,SU"<#E"> GRAND UNIFICATION "<#AIGD"> - ,HYPER-,KAMIOKANDE DESIGN REPORT @9#BEH1JJJ-TON NEXT-GENERATION DETECTOR - ,ADAMS @& ,LAUGHLIN ,THE ,FIVE ,AGES OF THE ,UNIVERSE "<#AIII">1 FAR-FUTURE SCENARIOS