"Annenberg Media" "¶" "I'LL START BY SHOWING YOU A LITTLE DEMONSTRATION" "THAT YOU'VE ALREADY SEEN BEFORE." "I HAVE A LOOP OF WIRE CONNECTED TO A GALVANOMETER," "WHICH WILL TELL US WHETHER AN ELECTRIC CURRENT" "IS RUNNING AROUND IN THIS LOOP." "THE LASER BEAM, THE SPOT ON THE SCREEN," "WILL MOVE WHENEVER AN ELECTRIC CURRENT RUNS" "IN THIS LOOP." "TO MAKE A CURRENT RUN THROUGH THIS WIRE," "I TAKE THIS ORDINARY BAR MAGNET" "AND MOVE IT INTO THE LOOP LIKE THAT" "OR OUT OF THE LOOP" "OR MOVE THE LOOP INTO THE MAGNET" "AND BACK LIKE THAT." "THAT MAKES A CURRENT FLOW." "THAT WAS ONE OF THE GREAT DISCOVERIES" "OF THE 19th CENTURY." "BY NOW, WE'VE MASTERED ALL THE FORMIDABLE MACHINERY" "OF ELECTRICITY AND MAGNETISM." "WE KNOW HOW TO DESCRIBE THEM," "SO LET'S DESCRIBE HOW THAT EXPERIMENT WORKS." "A CURRENT FLOWS THROUGH THE WIRE." "ELECTRIC CHARGES INSIDE THE WIRE" "ARE SET INTO MOTION," "WHICH MEANS SOMETHING APPLIED A FORCE TO THEM." "THE FORCE ON THE ELECTRIC CHARGE" "CAN EITHER BE EQUAL TO q TIMES AN ELECTRIC FIELD," "IF THERE'S ONE PRESENT," "OR IT COULD BE EQUAL TO THE CHARGE q," "TIMES THE VELOCITY OF THE CHARGE," "CROSSED INTO THE MAGNETIC FIELD." "EITHER WOULD MAKE THE CHARGE MOVE." "THAT'S OUR EQUATION." "LET'S SEE HOW WE USE IT" "TO DESCRIBE THE EXPERIMENT WE JUST SAW." "THERE ARE TWO POSSIBILITIES." "ONE, THE LOOP IS HELD STATIONARY," "AND I MOVE THE MAGNET INTO THE LOOP." "THAT CAUSES THE CURRENT TO FLOW." "IN THAT CASE, THE CHARGES IN THE LOOP" "WERE NOT THEMSELVES IN MOTION," "SO IT CAN'T HAVE BEEN THE BAR MAGNET'S MAGNETIC FIELD" "THAT MADE THEM MOVE." "BUT SINCE THEY DID MOVE," "THERE THEREFORE WAS AN ELECTRIC FIELD." "WE CONCLUDE THAT A CHANGING MAGNETIC FIELD," "MOVING THE BAR MAGNET," "CREATED AN ELECTRIC FIELD." "THAT WAS FARADAY'S GREAT DISCOVERY" "OF ELECTROMAGNETIC INDUCTION IN THE 19th CENTURY," "AND THAT'S ONE EXPLANATION OF THAT EXPERIMENT." "NOW, THERE'S ANOTHER COMPLETELY DIFFERENT," "INDEPENDENT EXPLANATION," "AND THAT GOES THIS WAY." "SUPPOSE INSTEAD I HOLD THE BAR MAGNET STATIONARY" "AND I MOVE THE LOOP." "OF COURSE, EXACTLY THE SAME THING HAPPENS," "BUT WE HAVE NO MOVING MAGNET," "NO CHANGING MAGNETIC FIELD." "INSTEAD, THE CHARGES IN THE LOOP ARE MOVING" "BECAUSE I'M MOVING THE LOOP." "THEY HAVE THE VELOCITY v." "THIS VELOCITY CROSSED INTO THE BAR MAGNET'S MAGNETIC FIELD" "GIVES US A FORCE WHICH CAUSES THE CURRENT TO FLOW." "THAT ALSO DESCRIBES THE EXPERIMENT WE JUST SAW." "SO THOSE TWO DIFFERENT PHENOMENA" "LOOP STATIONARY AND MAGNET MOVING" "AND MAGNET STATIONARY AND LOOP MOVING" "ARE ACTUALLY TWO COMPLETELY DISTINCT," "INDEPENDENT PHENOMENA" "THAT HAVE COMPLETELY DIFFERENT EXPLANATIONS." "WHEN ALBERT EINSTEIN SAW THAT, HE SAID," ""LOOK, GUYS, YOU'VE JUST GOT TO BE KIDDING." "ANY YO-YO CAN SEE THAT THOSE TWO ARE THE SAME THING."" "SO IT WAS THIS SIMPLE LITTLE EXPERIMENT" "THAT WAS THE STARTING POINT OF THE THEORY OF RELATIVITY," "NOT THE MICHELSON-MORLEY EXPERIMENT," "NOT SOME EXOTIC EXPERIMENT" "TO DETECT THE EARTH'S MOTION THROUGH THE ETHER," "BUT THIS SIMPLE LITTLE PHENOMENON" "THAT OF COURSE EVERYBODY KNEW ABOUT," "BUT WHICH DISTURBED NOBODY ELSE EXCEPT ALBERT EINSTEIN." "WHAT DISTURBED EINSTEIN" "WAS NOT THAT WE HAD DIFFICULTY EXPLAINING THESE PHENOMENA." "THIS EQUATION EXPLAINS THEM PERFECTLY IN EVERY CASE." "WHAT DISTURBED HIM WAS THE LACK" "IN THE INNER PERFECTION OF THE THEORY." "WHAT HE DID IN RESPONSE WAS TO PRODUCE A THEORY," "THE SPECIAL THEORY OF RELATIVITY," "WHICH HAD JUST THAT KIND OF INNER PERFECTION," "AND IT'S THAT THEORY" "THAT WE'RE IN THE MIDST OF STUDYING RIGHT NOW." "HISTORIANS RECALL THE TURN OF THIS CENTURY" "AS A REVOLUTION." "NO MATTER THE FIELD," "FROM AVIATION TO ANTHROPOLOGY TO ART," "DEPARTURE FROM THE NORM WAS RADICAL ON ALL FRONTS." "AND THE YEAR 1905 STANDS AS A CRUCIAL LANDMARK." "THAT'S WHEN ALBERT EINSTEIN ASKED THE WORLD" "TO GET ON THE RIGHT TRACK," "TO THINK FOR A MOMENT ABOUT WHAT A MOMENT REALLY IS." "TIME IS RELATIVE, HE SAID," "AND A MATTER OF SIMULTANEOUS EVENTS." "AND IN EINSTEIN'S RELATIVITY," "WHICH IS REALITY NO MATTER HOW IT'S LOOKED AT," "SIMULTANEITY IS A MATTER OF OPINION." "NOTICE, FOR EXAMPLE," "WHAT HAPPENS WHEN A LIGHT IS SWITCHED ON" "AS HENRY'S TRAIN MOVES BY." "THE LIGHT REACHES THE FRONT AND REAR OF THE TRAIN" "AT THE SAME TIME." "AT LEAST THAT'S HOW HENRY SEES IT." "BUT CONSIDER AN OUTSIDER'S POINT OF VIEW." "TO ALBERT, AT REST ON THE GROUND," "THE REAR WALL COMES UP TO MEET THE LIGHT," "AND THE FRONT WALL MOVES AWAY FROM IT." "THESE TWO EVENTS DON'T LOOK SIMULTANEOUS AT ALL," "AND THAT'S THE ESSENCE OF THE TIME PROBLEM." "AND IN PHYSICS," "THE EXPLANATION OF THE PROBLEM STARTS WITH TIME DILATION." "GIVEN THE CONSTANCY OF THE SPEED OF LIGHT," "THERE'S NO DOUBT THAT A SIMPLE LIGHT CLOCK, IN MOTION," "RUNS SLOW BY A FACTOR OF GAMMA." "BUT IF A MORE CONVENTIONAL CLOCK WENT ALONG FOR THE RIDE," "WOULD IT SLOW DOWN BY THE SAME AMOUNT?" "THE ANSWER IS TIMELY," "BECAUSE IF ONLY THE LIGHT CLOCK SLOWS DOWN," "IT MIGHT BE POSSIBLE TO COMPARE THEIR TIMES." "THEN IF THE TIMES WERE THE SAME, THEY WOULD BE AT REST." "BUT IF THE LIGHT CLOCK WERE SLOWER," "THAT WOULD BE BECAUSE THEY'RE IN MOTION." "BUT IN MOTION WITH RESPECT TO WHAT?" "THE BASIC PREMISE OF RELATIVITY" "IS THAT THERE CAN BE NO ABSOLUTE MOTION" "AND NO ABSOLUTE REST." "AND IF THOSE CONDITIONS DON'T EXIST," "NO EXPERIMENT CAN POSSIBLY DETECT THEM." "SO IF THE THEORY OF RELATIVITY IS CORRECT," "EVERY CLOCK IN THE UNIVERSE" "MUST BEHAVE EXACTLY AS A LIGHT CLOCK DOES," "AND THAT'S NOT JUST A DISCOVERY ABOUT CLOCKS." "THAT'S A FACT ABOUT THE NATURE OF TIME ITSELF." "TIME DILATION PLACES THE RELATIONSHIP" "BETWEEN TIME AND SPACE" "IN AN ENTIRELY NEW LIGHT," "WHICH IS EXACTLY WHAT WAS HAPPENING" "IN ART AND LITERATURE" "IN THE EARLY YEARS OF THIS CENTURY." "IN ART AS WELL AS SCIENCE," "THE IDEA OF TIME AND SPACE" "WAS NOW FAR MORE THAN A PASSING FANCY." "JUST AS EINSTEIN WAS TRYING TO PLACE EVERYTHING" "IN A NEW PERSPECTIVE," "VIRGINIA WOOLF URGED WRITERS TO MOVE BEYOND" ""THE FORMAL RAILWAY LINE OF A SENTENCE."" "NO WRITER INNOVATED MORE WITH TIME AND SPACE" "THAN JAMES JOYCE." ""LIKE EINSTEIN'S WORLD," WRITES CRITIC EDMUND WILSON," ""JOYCE'S WORLD IS ALWAYS CHANGING" ""AS IT IS PERCEIVED BY DIFFERENT OBSERVERS" "AT DIFFERENT TIMES."" "AND SO IT WENT AROUND THE CLOCK" "WITH SCIENTISTS AND ARTISTS," "MUSICIANS AND MATHEMATICIANS," "POETS AND PHILOSOPHERS." "AND EINSTEIN'S THEORY OF RELATIVITY" "HAD GONE AT LEAST AS FAR AS ANY CREATIVE IMAGINATION." "ON THE TRACK OF RELATIVITY," "VELOCITY NOT ONLY CHANGES THE NATURE OF THINGS," "IT CHANGES THE WAY THINGS ARE PERCEIVED." "THAT'S BECAUSE EINSTEIN'S RELATIVITY" "IS A THEORY OF MOTION" "AS VIEWED BY DIFFERENT OBSERVERS." "THE THEORY APPLIES EVERYWHERE, EVEN BROOKLYN," "AND INCLUDES ALL OBSERVERS," "EVEN A GOOD SPORT NAMED HENRY." "THEY SAY HENRY HAD ONE HECK OF A FAST BALL." "WITH IT AND A DESIRE TO PITCH FOR THE BROOKLYN DODGERS," "HE WAS ON HIS WAY TO MAKING THE TEAM." "SOME SAID HENRY COULD FLING A BASEBALL" "AT NEARLY THE SPEED OF LIGHT," "AND FROM HIS POINT OF VIEW, IT DOES LOOK PRETTY FAST." "BUT FROM WHERE HIS PAL ALBERT STANDS," "IT LOOKS EVEN FASTER." "COULD THE SPEED OF THE BALL," "ADDED TO THE SPEED OF THE TRAIN," "MAKE HENRY'S PITCH LOOK EVEN FASTER" "THAN THE SPEED OF LIGHT?" "OF COURSE NOT," "BECAUSE EVEN ON A TRAIN IN SPRING TRAINING," "NOTHING TRAVELS FASTER THAN THE SPEED OF LIGHT." "SO WHAT'S REALLY HAPPENING HERE?" "BASEBALL STATISTICIANS WORK IT OUT" "USING A SPACE-TIME DIAGRAM," "WHERE FROM HENRY'S POINT OF VIEW," "THE BALL SPEEDS ONE WAY AND ALBERT SPEEDS THE OTHER WAY." "AND SINCE DISTANCE IS ONE AXIS" "AND TIME IS THE OTHER," "SPEED, SAY v PRIME," "CAN BE SEEN IN THE SLOPE OF EACH PATH," "JUST AS THE SPEED OF LIGHT" "IS GIVEN BY THE SLOPE OF ITS PATH." "ON THIS GRAPH," "LIGHT GOES 1 UNIT OF DISTANCE FOR EACH UNIT OF TIME," "SO c = 1." "MEANWHILE, HENRY'S PITCH HAS A SPEED u SUB x PRIME OF .6." "WHILE ALBERT, GOING THE OTHER WAY," "HAS A SPEED OF -.6." "THAT'S FROM HENRY'S POINT OF VIEW." "ON THE OTHER HAND, AS ALBERT SEES THINGS," "THE SPEED OF LIGHT REMAINS THE SAME," "BUT ALL OTHER SPEEDS ARE TRANSFORMED" "BECAUSE ALL SLOPES ARE MEASURED ON TILTED AXES." "OF COURSE, IT'S TRUE THAT LIGHT STILL GOES 1 UNIT OF DISTANCE FOR EACH UNIT OF TIME," "SO c IS STILL EQUAL TO 1." "BUT NOW, HENRY'S MOVING FORWARD" "AT A SPEED v = .6," "AND THE BASEBALL'S SPEED, u SUB x MEASURES .88." "IN THE STRANGE ARITHMETIC OF RELATIVITY," "NO MATTER HOW YOU LOOK AT IT," "WHEN ONE SPEED IS ADDED TO ANOTHER," "THE RESULT IS ALWAYS LESS" "THAN THE SPEED OF LIGHT." "SO, WHILE ALBERT WILL SURELY AGREE" "THAT HENRY THROWS ONE HECK OF A FAST BALL..." "HE CAN ALWAYS THROW A LITTLE LIGHT ON THE MATTER" "AND BE SURE IT WILL GET THERE FIRST." "EVENALONGTHEDIRECTION OFRELATIVEMOTION," "COMPONENTSOFVELOCITY NEVERADDTO ASPEED" "GREATERTHAN THESPEEDOF LIGHT," "ANDTHEVELOCITY TRANSFORMATION" "DOESAFFECTCOMPONENTS PERPENDICULAR TO THE MOTION," "BECAUSE ALTHOUGH DISTANCES REMAINTHESAME," "TIMECHANGES FROM ONE FRAME TO THE OTHER." "EVEN WHEN THE THEORY'S AS BEAUTIFUL AS EINSTEIN'S," "IT NEEDS TESTING," "AND THERE IS A TEST OF RELATIVITY" "IN A PHENOMENON CALLED THE DECAY OF COSMIC RAY MU MESONS." "A QUARTER OF A CENTURY AGO," "DR. DAVID FRISCH OF M.I.T." "AND DR. JAMES SMITH OF THE UNIVERSITY OF ILLINOIS" "FILMED A CLASSIC EXPERIMENT" "PROVING THE REALITY OF THE RELATIVISTIC TIME DILATION." "THEY DID IT BY MEASURING THE FLUX THROUGH THE ATMOSPHERE" "OF A RADIOACTIVE SUBATOMIC PARTICLE" "CALLED THE MU MESON." "CREATED AT THE TOP OF THE EARTH'S ATMOSPHERE" "BY COSMIC RAYS FROM OUTER SPACE," "MU MESONS ZOOM STRAIGHT DOWNWARD AT NEARLY THE SPEED OF LIGHT." "BUT LABORATORY EXPERIMENTS," "SUCH AS THE ONE DONE HERE BY SMITH," "SHOW THAT MU MESONS DISINTEGRATE NATURALLY" "AFTER AN AVERAGE LIFE OF ONLY 2 MILLIONTHS OF A SECOND." "THAT MEANS THAT EVEN GOING AT NEAR THE SPEED OF LIGHT," "THEY SHOULDN'T MAKE IT VERY FAR THROUGH THE ATMOSPHERE" "BEFORE DECAYING." "OR, TO PUT IT DIFFERENTLY," "WHEN FRISCH AND SMITH OBSERVE 568 MU MESONS" "IN THEIR DETECTOR AT THE TOP OF MT." "WASHINGTON," "THEY CALCULATE THAT ONLY 27 PER HOUR" "OUGHT TO MAKE IT TO THE BASE OF THE MOUNTAIN." "BUT WHEN THEY CART THEIR EQUIPMENT" "DOWN TO SEA LEVEL," "THEY FIND ALMOST AS MANY MU MESONS" "AS THEY HAD AT THE TOP-- 412 PER HOUR." "OF COURSE, FRISCH AND SMITH REALLY AREN'T A BIT SURPRISED." "THE REASON FOR THE DISCREPANCY" "IS SPELLED OUT BY THE THEORY OF RELATIVITY." "TRAVELING AT NEARLY THE SPEED OF LIGHT," "THE MU MESON'S INTERNAL CLOCK RUNS SLOW," "SO THAT AS VIEWED FROM EARTH," "THEY TAKE MUCH LONGER ON THE AVERAGE" "THAN 2 MILLIONTHS OF A SECOND" "TO DISINTEGRATE." "OR, TO PUT IT DIFFERENTLY," "AS VIEWED FROM THE FRAME OF REFERENCE OF THE MU MESON," "THE EARTH'S ATMOSPHERE," "RUSHING UPWARD AT NEARLY THE SPEED OF LIGHT," "IS LORENTZ CONTRACTED TO SUCH A DEGREE" "THAT THE PARTICLE CAN EASILY MAKE IT TO THE GROUND" "IN ONLY 2 MILLIONTHS OF A SECOND." "A SET OF TWINS," "BORN IN VIRTUALLY THE SAME TIME AND SPACE," "ARE ON THE SURFACE VERY MUCH ALIKE." "AND YET, ON THE INSIDE," "MAY TURN OUT AS DIFFERENT AS NIGHT AND DAY." "BEING SO DIFFERENT AND SO ALIKE AT THE SAME TIME" "MAY BE THE ULTIMATE PARADOX OF IDENTICAL TWINS." "BUT IN PHYSICS," "THE TWINS' PARADOX CONJURES UP ANOTHER IMAGE." "AND, LIKE EINSTEIN'S THEORY," "IT BEGINS WITH AN EXTRAORDINARY FACT ABOUT CLOCKS." "NO MATTER HOW LONG THEY'VE BEEN AROUND," "NO MATTER WHETHER THEY TICKTOCK, CHIME, OR SAY CUCKOO," "ALL CLOCKS OBEY THE RULES OF RELATIVITY." "NATURALLY, THESE RULES APPLY TO THE HUMAN CLOCK." "WITH A TIME-KEEPING DEVICE CALLED METABOLISM," "IT STARTS RUNNING EVEN BEFORE THE MOMENT OF BIRTH," "AND IN A RELATIVELY PREDICTABLE FASHION," "IT KEEPS RUNNING TO THE END." "BUT WHAT WOULD HAPPEN IF THE HUMAN CLOCK" "WERE MOVING AT NEARLY THE SPEED OF LIGHT?" "TO FIND OUT," "CONSIDER THESE IDENTICAL TWINS, ALBERT AND HENRY." "FROM THE VERY OUTSET," "THEIR BIOLOGICAL CLOCKS START OUT" "IN PERFECT SYNCHRONIZATION." "SOON ENOUGH, HOWEVER," "THE BEST-LAID PLANS OF MOTHER AND FATHER ALIKE" "GET OUT OF SYNC." "INSTEAD OF AN ORTHODONTIST, ALBERT BECOMES A SCIENTIST." "HENRY CHOOSES WHAT'S LEFT OF THE WIDE-OPEN SPACES," "OPEN SPACE ITSELF." "ESPECIALLY FOR IDENTICAL TWINS," "THE FIRST PARTING IS SUCH SWEET SORROW." "NONETHELESS, TRAVELING AT NEARLY THE SPEED OF LIGHT," "HENRY HAS NO TROUBLE LEAVING ALBERT BEHIND." "THIS IS HENRY'S FIRST LONG HAUL FOR HIS NEW BOSS," "THE INTERSTELLAR BAKING COMPANY." "DESTINATION" "ZOG, 10 LIGHT-YEARS AWAY." "ASSIGNMENT--TAKE A LOAD OF ORDINARY WHITE BREAD" "AND EXCHANGE IT FOR ZOGGIE CAKES," "THOSE DELICIOUS LITTLE BUNS THAT ALL THE EARTHLINGS CRAVE." "BUT THAT'S NOT WHAT'S SO EXTRAORDINARY." "IF HENRY COULD COMPARE HIS CLOCK TO ALBERT'S CLOCK," "HE WOULD FIND ALBERT'S CLOCK RUNNING SLOW." "AND IF ALBERT COULD COMPARE HIS CLOCK TO HENRY'S," "HE WOULD FIND HENRY'S CLOCK RUNNING SLOW," "AND THAT SPELLS PARADOX." "HOW CAN EACH ONE BE SLOWER THAN THE OTHER?" "FORTUNATELY, IN THE THEORY OF RELATIVITY," "THERE'S NO PROBLEM AT ALL." "THERE WON'T REALLY BE A PROBLEM" "AS LONG AS THE TWO OF THEM ARE FAR APART," "BUT WHAT WILL HAPPEN WHEN THEY'RE NOT FAR APART," "SAY, WHEN THEY GET BACK TOGETHER FOR A FAMILY REUNION?" "AGAIN, IN THE THEORY OF RELATIVITY," "THAT'S NO PROBLEM." "BROTHER HENRY, AT NEARLY THE SPEED OF LIGHT," "WILL TAKE ABOUT 10 YEARS TO REACH ZOG, 10 LIGHT-YEARS AWAY," "AND THEN ANOTHER DECADE TO HAUL HIS BUNS BACK TO EARTH." "TO ALBERT, THAT'S AROUND 20 YEARS." "BUT TO HENRY," "THE TWIN WHO'S ACTUALLY EN ROUTE," "THE SITUATION SEEMS DIFFERENT." "SINCE IT'S LORENTZ-CONTRACTED," "THE DISTANCE TO ZOG SEEMS LIKE A MILK RUN." "A FEW HANDS OF SOLITAIRE, 40 WINKS," "AND AS FAR AS HENRY'S CONCERNED," "IT'S HELLO TO THE BRIGHT LIGHTS OF ZOG CITY." "THEN, EQUALLY QUICK," "BECAUSE THE DISTANCE IS AGAIN LORENTZ-CONTRACTED," "IT'S BACK TO EARTH JUST AS FAST." "AND SO, WHEN THEY DO GET TO COMPARE THOSE BIOLOGICAL CLOCKS," "WHILE THE RESULT IS AMAZING," "THERE'S NO PARADOX AT ALL." "HENRY, THE TWIN WHO ZOOMED AWAY" "AND THEN CAME BACK," "HAS ONLY AGED A COUPLE OF MONTHS," "BUT ALBERT," "THE TWIN WHO NEVER SWITCHED INERTIAL FRAMES," "HAS AGED 20 YEARS." "AND SO, EVEN AMONG RELATIVES, TIME IS RELATIVE." "TRY TO IMAGINE, IF YOU CAN, A COMPLETELY EMPTY UNIVERSE," "A UNIVERSE WITH NOTHING WHATSOEVER IN IT." "IN SUCH A UNIVERSE," "IT WOULD MAKE NO SENSE AT ALL" "TO SPEAK OF TIME OR SPACE." "THE VERY CONCEPTS WOULD BE MEANINGLESS." "IF THERE WERE ONE SINGLE OBJECT IN THE UNIVERSE," "IT WOULD STILL BE TRUE." "TIME AND SPACE WOULD HAVE NO MEANING AT ALL." "IF THERE WERE TWO OBJECTS," "THEN YOU COULD IMAGINE SAYING" "THEY'RE GETTING CLOSER TOGETHER OR FURTHER APART." "THEN YOU'RE SAYING SOMETHING" "ABOUT THE FLOW OF TIME AND DISTANCES." "IF THERE WERE MANY OBJECTS IN YOUR UNIVERSE," "YOU MIGHT IMAGINE USING SOME OF THEM" "TO CONSTRUCT A DEVICE" "THAT COULD MEASURE THE FLOW OF TIME" "OR MEASURE THE DISTANCE BETWEEN OBJECTS." "BUT IT WOULD STILL MAKE NO SENSE AT ALL IN THAT UNIVERSE" "TO SAY ONE OBJECT IS AT REST" "AND ONLY EVERYTHING ELSE IS IN MOTION." "IN SUCH A UNIVERSE," "THE SIMPLEST LAW OF MOTION IT WOULD BE POSSIBLE TO IMAGINE" "IS TO SAY THAT EACH BODY CONTINUES" "IN WHATEVER STATE OF MOTION IT HAS" "UNTIL SOMETHING INTERFERES WITH IT." "THAT, OF COURSE, IS THE LAW OF INERTIA." "BUT THAT HAS TO KEEP ON MAKING SENSE" "IN A UNIVERSE IN WHICH LIGHT CAN PROPAGATE THROUGH THE VOID." "NOW, IF LIGHT HAS SOME DEFINITE SPEED" "AS OBSERVED FROM A PARTICULAR FRAME OF REFERENCE," "THEN THERE IS SUCH A THING AS A STATE OF REST" "IN THAT FRAME OF REFERENCE," "AND THE WHOLE ARGUMENT" "FOR THE LAW OF INERTIA COLLAPSES." "NOW, I DON'T KNOW" "IF THERE ARE ANY OTHER LOGICALLY CONSISTENT UNIVERSES," "BUT IN OUR UNIVERSE, THAT PROBLEM HAS BEEN SOLVED" "BY MAKING THE SPEED OF LIGHT THE SAME TO ALL OBSERVERS" "REGARDLESS OF THEIR STATE OF MOTION." "KNOWING THAT, AND ONLY THAT," "IT'S POSSIBLE TO DEDUCE" "THAT IF THERE ARE A PAIR OF TWINS" "AND ONE OF THEM STAYS BEHIND ON EARTH" "AND THE OTHER ONE TRAVELS AT HIGH SPEED TO A DISTANT STAR," "TURNS AROUND AND COMES BACK," "THE TRAVELING TWIN, WHEN THEY GET BACK TOGETHER," "WILL BE YOUNGER THAN THE ONE WHO STAYED BEHIND." "I DON'T NEED TO KNOW ANYTHING ABOUT HOW CLOCKS WORK," "ABOUT QUARTZ CRYSTAL OSCILLATORS," "ABOUT HUMAN METABOLISM." "THOSE THINGS ARE COMPLETELY IRRELEVANT." "ALL I NEED TO KNOW IS THE PRINCIPLE OF RELATIVITY" "AND THE CONSTANCY OF THE SPEED OF LIGHT." "FROM THAT I CAN DEDUCE" "THAT THE TRAVELING TWIN WILL COME BACK YOUNGER" "THAN THE TWIN THAT WAS LEFT BEHIND." "THAT IS AN ABSOLUTELY BREATHTAKING FEAT" "OF PURE HUMAN LOGIC." "THAT IS THE THEORY OF RELATIVITY," "AND WE'LL GET ON WITH IT NEXT TIME." "CAPTIONING IS MADE POSSIBLE BY THE ANNENBERG/CPB PROJECT" "CAPTIONING PERFORMED BY THE NATIONAL CAPTIONING INSTITUTE, INC." "CAPTIONS COPYRIGHT 1986 CALIFORNIA INSTITUTE OF TECHNOLOGY," "THE CORPORATION FOR COMMUNITY COLLEGE TELEVISION, THE ANNENBERG/CPB PROJECT" "PUBLIC PERFORMANCE OF CAPTIONS PROHIBITED WITHOUT PERMISSION OF NATIONAL CAPTIONING INSTITUTE" "Annenberg Media" "¶" "For information about this and other Annenberg Media programs call 1-800-LEARNER and visit us at"