"Annenberg Media" "¶" "CAPTIONING IS MADE POSSIBLE BY THE ANNENBERG/CPB PROJECT" "THEPERIODS OFTHEORBITINGPLANETS." "THEEBBANDFLOW OFTHETIDES." "THEACCELERATION OFA FALLINGBODY." "ALLTHESEPHENOMENA ARECONSEQUENCES" "OFTHEFORCEOFGRAVITY." "THEY'VEINSPIRED THELABOROF SCIENTISTS" "FROMKEPLERTO EINSTEIN." "WE'RE ALMOST AT OUR STORY'S END." "TODAY I HAVE A FEW LOOSE ENDS TO TIE UP." "THE FIRST IS KEPLER'S THIRD LAW." "WE'LL REACH THE END" "OF THE KEPLER PROBLEM." "THEN THE TIDES." "GALILEO HAD A THEORY OF THE TIDES," "WHICH HE CONSIDERED THE ACHIEVEMENT OF HIS CAREER" "BECAUSE IT PROVED THAT COPERNICUS WAS RIGHT." "BUT GALILEO WAS COMPLETELY WRONG." "TODAY I'LL EXPLAIN HOW THE TIDES REALLY WORK." "FINALLY, I'LL RETURN TO THE FIRST THING WE DID," "THE LAW OF FALLING BODIES." "GRAVITY APPLIES A DIFFERENT FORCE TO EACH BODY" "SO THEY ALL FALL AT THE SAME RATE." "ALBERT EINSTEIN THOUGHT THERE SHOULD BE" "A PROFOUND REASON FOR THAT." "IN SEARCHING FOR THAT REASON, HE DISCOVERED NOTHING LESS" "THAN A NEW THEORY OF THE UNIVERSE." "THOSE ARE TODAY'S LOOSE ENDS." "I THINK WE BETTER GET STARTED." "GALILEO GALILEI WAS A MATHEMATICIAN" "WHO WROTE A POPULAR BOOK THAT GOT HIM INTO TROUBLE." "250 YEARS LATER, CHARLES LUTWIDGE DODGSON," "ANOTHER MATHEMATICIAN," "WROTE ANOTHER POPULAR BOOK." "DODGSON'S MATHEMATICS WAS AS RESPECTABLE" "AS THE VICTORIAN ERA IN WHICH IT WAS PUBLISHED" "AND ALMOST AS FORGOTTEN." "YET, LIKE GALILEO, HE MAY BE REMEMBERED FOREVER," "BUT ONLY UNDER HIS PEN NAME, LEWIS CARROLL," "AND ONLY AS LONG AS CHILDREN APPRECIATE" "THE WISDOM OF PURE NONSENSE." ""THETIMEHASCOME, THE WALRUS SAID," "TOTALKOF MANYTHINGS" "OFSHOESANDSHIPS ANDSEALINGWAX," "OFCABBAGESANDKINGS," "ANDWHYTHESEA  ISBOILINGHOT," "AND WHETHER PIGS HAVE WINGS."" "LEWIS CARROLL WROTE THROUGHTHELOOKING-GLASS" "FOR A CHILD NAMED ALICE." "HE COULD HAVE HAD GALILEO'S SITUATION IN MIND." "SINCE GALILEO USED LOGIC WITH RELIGIOUS ISSUES," "HE MIGHT AS WELL HAVE DEBATED THE MAD HATTER." "FAITH AND SCIENCE HAVE NEVER BEEN IDENTICAL." "WHEN CHURCH SCHOLARS PEERED INTO GALILEO'S TELESCOPE," "THEY LOOKED THROUGH THE GLASS DARKLY." "IN GALILEO'S PERSISTENT DEFENSE OF COPERNICUS," "THEY SAW WHAT LOOKED LIKE HERESY." "THIS SERIOUS CHARGE HAD NO RELATION" "TO THE FANTASY WALRUS OR WHETHER PIGS HAVE WINGS," "BUT IT DID HAVE SOMETHING TO DO WITH THE TIDES." "TO GALILEO, THE TIDES WERE MORE" "THAN THE EBB AND FLOW OF OCEANS." "THEY WERE HIS EVIDENCE THAT THE EARTH MOVES." "TIDES EXIST" "BECAUSE OF THE EARTH'S MOTION AROUND THE SUN," "HE ARGUED." "GALILEO CONSIDERED HIS THEORY OF THE TIDES" "TO BE HIS CAREER'S HIGH POINT." "IF HE HAD BEEN RIGHT," "THERE WOULD BE ONE HIGH TIDE, DAILY AT NOON." "THE SAD TRUTH IS GALILEO'S TIDE THEORY WAS ALL WET." "THE TIDES DO RISE." "THEY RECEDE AS WELL." "BUT AS ANYONE CAN SEE WITH A BIT OF OBSERVATION," "THEY DO SO TWICE EVERY DAY." "TO UNDERSTAND THE TIDES," "IT HELPS TO TAKE THE LONG VIEW." "IN THE EARTH-MOON SYSTEM," "BOTH ROTATE AROUND A COMMON CENTER OF MASS." "THAT POINT IS ACTUALLY INSIDE THE EARTH," "ABOUT 3/4 OF THE DISTANCE" "FROM THE EARTH'S CENTER TO ITS SURFACE." "THE WOBBLING MOTION OF THE EARTH" "IS A KEPLERIAN ORBIT OF THE CENTER OF THE EARTH" "AROUND THE CENTER OF MASS OF THE EARTH-MOON SYSTEM." "ONLY THE CENTER OF THE EARTH" "IS IN EXACTLY THE RIGHT ORBIT." "ON THE SIDE CLOSEST TO THE MOON," "THE MOON'S GRAVITY IS TOO STRONG." "THE WATER IS PULLED TOWARD THE MOON AND BULGES." "ON THE OPPOSITE SIDE, THE MOON'S GRAVITY" "IS TOO WEAK TO HOLD THE WATER IN PLACE" "THERE TOO, THE WATER BULGES OUTWARD," "TRYING TO ESCAPE." "AS THE EARTH WOBBLES" "AROUND THE EARTH-MOON CENTER OF MASS," "IT ALSO ROTATES ON ITS AXIS." "AS THE EARTH ROTATES, IT PASSES BENEATH THE BULGES." "AT THOSE LOCATIONS WHERE THE ROTATING EARTH" "PASSES UNDER THE RISING WATER," "HIGH TIDES OCCUR," "AND AT THE POINTS BETWEEN, LOW TIDES OCCUR." "TWO HIGH TIDES AND TWO LOW TIDES EVERY DAY." "THE SUN ALSO PLAYS A ROLE IN THE DRAMA OF THE TIDES." "SINCE THE SUN'S GRAVITY TUGS AT THE EARTH," "IT ALSO CONTRIBUTES." "THOUGH LARGER, THE SUN'S EFFECT IS ABOUT HALF OF THE MOON'S" "BECAUSE IT'S FARTHER AWAY." "AT FULL MOON AND AT NEW MOON," "EARTH, THE SUN, AND THE MOON FALL INTO A STRAIGHT LINE." "THE TIDAL EFFECTS OF SUN AND MOON" "REINFORCE EACH OTHER" "AND CREATE THE HIGHEST HIGH TIDES" "AND THE LOWEST LOW TIDES." "HIGH AND LOW, THE TIDES RISE AND FALL" "BECAUSE THE EARTH MOVES." "IN WRITING HIS THEORY, GALILEO WAS NOT WRONG" "IN RAISING HIS PEN IN DEFENSE OF COPERNICUS." "IT WAS A LONELY BATTLE." "AT LEAST IN PRINCIPLE, HE WAS NOT ALONE." "IN 1618, TWO YEARS AFTER THE CHURCH WARNED GALILEO" "TO STOP TEACHING SUCH DANGEROUS IDEAS," "JOHANNES KEPLER PUBLISHED HARMONYOFTHEWORLDS." "MORE THAN AN ARGUMENT FOR THE TRUTH OF COPERNICUS," "THIS BOOK WAS KEPLER'S PROOF" "THAT HE HIMSELF HAD NOT LIVED A LIE." "KEPLER NOT ONLY ATTEMPTED TO UNWRAP" "THE SECRETS OF THE UNIVERSE," "HE TRIED TO EXPLAIN THEM ALL IN ONE BOOK" "WITH ONE ALL-ENCOMPASSING SYNTHESIS" "OF GEOMETRY, MUSIC, AND ASTROMONY." "NO WORK SINCE PLATO'S" "COULD MATCH THE AMBITIOUS NATURE" "OF KEPLER'S HARMONYOFTHEWORLDS." "WHEN NEWTON DUG INTO KEPLER'S IDEAS," "HE FOUND MATHEMATICAL TREASURES OF INVALUABLE WORTH." "IT MIGHT HAVE BEEN TEDIOUS" "TO SIFT THROUGH MUSIC THAT COULDN'T BE HEARD" "AND FANTASIES AND PLANETARY MODELS" "THAT MADE LITTLE SENSE." "BUT HOW REWARDING TO UNEARTH KEPLER'S THREE LAWS." "ACCORDING TO NEWTON, KEPLER'S FIRST LAW" "SAID THE PLANETS' ORBITS ARE ELLIPSES." "KEPLER'S SECOND STATED THAT THE RADIUS VECTOR OF A PLANET" "SWEEPS OUT EQUAL AREAS IN EQUAL TIMES." "AND FINALLY, KEPLER'S THIRD LAW," "THE SQUARE OF ANY PLANET'S PERIOD" "IS PROPORTIONAL TO THE CUBE OF ITS SEMIMAJOR AXIS." "BUT WHY?" "INTEGRATING KEPLER'S SECOND LAW THROUGH ONE COMPLETE PERIOD" "SHOWS THAT A PLANET'S PERIOD" "ISPROPORTIONALTO  THEAREAOF ITSORBIT," "BUTALSOGOESINVERSELY" "ASTHEANGULARMOMENTUM DIVIDEDBYTHEMASS." "THISRELATIONIS TRUE OFMANYKINDSOFMOTION." "ITWOULDBE TRUE" "IFTHESOLARSYSTEM ROTATEDLIKEASOLIDBODY" "WITHALLPLANETS HAVINGTHESAMEPERIOD." "ITWOULDALSOBETRUE" "IFTHESOLARSYSTEMS WHIRLEDLIKEAVORTEX," "WITHPLANETSHAVING THESAMEANGULARMOMENTUM" "DIVIDEDBYMASS." "EACHPLANET'SPERIOD WOULDBEPROPORTIONAL" "TO THE SQUARE OF ITS ORBIT." "IT'SALSOTRUE FOR THE PLANETS' REAL MOTION." "THIS MOTION, PRODUCED AND DIRECTED" "BY ISAAC NEWTON'S LAW OF UNIVERSAL GRAVITATION," "FEATURES AN ELLIPTICAL ORBIT," "WHOSE SIZE DEPENDS ON L/M." "SO IN THE END, 4 PI-SQUARED," "THE GRAVITATIONAL CONSTANT, AND THE MASS OF THE SUN" "CONNECT T-SQUARED TO a-CUBED FOR EVERY PLANET." "KEPLER'S THIRD LAW" "RELATES THE PERIODS OF THE PLANETS" "TO THEIR ORBIT'S SIZE." "THE CONNECTION IS SIMPLE, ELEGANT," "AND ALTOGETHER PROFOUND." "THE HISTORY OF SCIENCE" "CAN BE SEEN AS A SERIES OF SUCH INSIGHTFUL CONNECTIONS" "THE CONNECTION BETWEEN A COPERNICAN IDEA" "AND GALILEO'S TIDAL THEORY." "THE CONNECTION AMONG THE RISING MOON, THE RISING SUN," "AND THE RISING TIDES ON EARTH." "THE CONNECTION OF ALL THREE," "LINKED AND LOCKED IN ETERNAL ORBIT." "THE CONNECTION BETWEEN THE ORBITS OF ALL THE PLANETS" "AND KEPLER'S THREE LAWS OF PLANETARY MOTION." "THE CONNECTION BETWEEN KEPLER'S LAWS," "WHICH EXPLAIN HOW THE PLANETS BEHAVE," "AND NEWTON'S LAWS, WHICH EXPLAIN WHY." "AND FINALLY, FROM GALILEO'S LAW OF FALLING BODIES," "TO KEPLER'S THIRD LAW," "TO NEWTON'S MECHANICS AND HIS UNIVERSAL LAW OF GRAVITATION," "THE CONNECTION EXPLAINED THE MECHANICAL UNIVERSE." "A FEW RARE THINGS IN THE UNIVERSE" "SEEMED TO GO BEYOND EVEN NEWTON'S EXPLANATION," "REQUIRING A CHANGE IN EVEN THE MEANING OF SPACE AND TIME." "TO ALBERT EINSTEIN, THE ENIGMA OF GRAVITY, TIME, AND SPACE" "WAS OF NO SMALL CONSEQUENCE." "IT WAS A PUZZLE THAT PROMISED ENORMOUS CHALLENGE," "AND IF SOLVED, THE ANSWER TO PERHAPS" "THE DEEPEST MYSTERIES IN THE COSMOS." "LIKE NEWTON BEFORE HIM, EINSTEIN BEGAN" "WITH THE LAW OF FALLING BODIES." "GALILEO DIDN'T QUITE UNDERSTAND GRAVITY" "NO ONE REALLY WOULD UNTIL NEWTON" "BUT HE DISCOVERED THAT ALL BODIES FALL" "WITH THE SAME CONSTANT ACCELERATION." "WHY SHOULD THE FORCE OF GRAVITY LEAD TO SUCH A STRANGE RESULT?" "WHEN CALCULATING THE ACCELERATION" "OF A FALLING APPLE," "THE MASS CANCELS OUT," "LEAVING THE SAME ACCELERATION FOR ANY FALLING BODY." "BUT WHY SHOULD THAT BE THE CASE?" "IN F = ma" "THE MASS DETERMINES HOW MUCH FORCE IS NEEDED" "TO PRODUCE A GIVEN ACCELERATION." "THE MASS DETERMINES" "HOW STRONGLY A BODY RESISTS CHANGING ITS INERTIA." "THIS PROPERTY OF A BODY HAS NO RELATION TO GRAVITY." "IT'S CALLED THE INERTIAL MASS." "IN THE LAW OF GRAVITY," "THE MASS DETERMINES HOW STRONGLY A BODY" "ATTRACTS EVERY OTHER BODY IN THE UNIVERSE." "THIS PROPERTY HAS NOTHING TO DO WITH INERTIA." "IT'S CALLED THE GRAVITATIONAL MASS." "ISAAC NEWTON UNDERSTOOD THAT THE LAW OF FALLING BODIES" "MEANT THESE TWO DIFFERENT MASSES" "HAD TO BE EXACTLY THE SAME" "FOR EVERY BODY IN THE UNIVERSE." "BUT WHY IS GRAVITATIONAL MASS" "ALWAYS EQUAL TO INERTIAL MASS?" "COULD THAT BE SOME COSMIC COINCIDENCE?" "SCIENTISTS AS A RULE" "DON'T PLACE MUCH STOCK IN PHENOMENA" "THAT CAN BE EXPLAINED BY COINCIDENCE." "ALBERT EINSTEIN PLACED NONE." "HE BELIEVED THERE MUST BE SOME MORE PROFOUND LAW OF NATURE" "THAT WOULD MAKE THE LAW OF FALLING BODIES" "NOT A MYSTERY," "BUT SOMETHING SO SIMPLE IT WOULD BE OBVIOUS." "HIS PRINCIPLE BECAME THE BASIS FOR HIS THEORY OF RELATIVITY." "IT'S CALLED THE PRINCIPLE OF EQUIVALENCE." "EQUIVALENCE, THAT IS," "BETWEEN CONSTANT GRAVITY AND CONSTANT ACCELERATION." "IMAGINE AN EXPERIMENT" "IN WHICH DIFFERENT OBJECTS WITH DIFFERENT MASSES" "ALL FALL WITH EXACTLY THE SAME ACCELERATION, CALLED g." "THAT MIGHT HAPPEN" "BECAUSE THE LABORATORY IS ON THE SURFACE OF THE EARTH." "THE SAME THING WOULD HAPPEN" "IF THE LABORATORY WERE FAR FROM EARTH," "SAY, IN INTERGALACTIC SPACE." "IT WOULD HAPPEN IF THE LABORATORY" "WERE ACCELERATING UPWARD WITH ACCELERATION g." "ALL THE OBJECTS INSIDE, OBEYING THE LAW OF INERTIA," "WOULD NOT ACCELERATE," "BUT THEY WOULD ALL SEEM TO FALL" "WITH THE SAME CONSTANT ACCELERATION." "ALBERT EINSTEIN UNDERSTOOD THAT THE LAW OF FALLING BODIES" "AND THE MYSTERIOUS EQUALITY" "OF GRAVITATIONAL MASS AND INERTIAL MASS" "WOULD BE EXPLAINED IF THE FOLLOWING WERE TRUE." "NO EXPERIMENT OF ANY KIND" "DONE ENTIRELY INSIDE THAT LABORATORY" "COULD DETERMINE WHETHER THE OBJECTS FALL" "BECAUSE OF THE PULL OF GRAVITY" "OR BECAUSE THE LABORATORY IS ACCELERATING UPWARDS." "THE IDEA SEEMS TOO SIMPLE," "BUT EINSTEIN DIDN'T TAKE IT LIGHTLY." "HE ASKED HOW THE EXPERIMENT WOULD COME OUT" "IF IT WERE PERFORMED WITH A BEAM OF LIGHT." "WITH THE LABORATORY IN SPACE," "THE ANSWER WAS EASY TO SEE." "IF THE ROCKET ACCELERATES WHILE THE BEAM GOES ACROSS," "IT SEEMS TO BEND DOWNWARDS A BIT." "BUT WHAT WOULD HAPPEN ON EARTH?" "THE ANSWER COMES FROM THE PRINCIPLE ITSELF." "SINCE NO EXPERIMENT," "NOT EVEN THIS ONE," "CAN TELL WHETHER THE LABORATORY" "IS ON EARTH OR IN SPACE," "THE RESULT MUST BE THE SAME." "EINSTEIN CONCLUDED" "THAT LIGHT DOESN'T TRAVEL IN STRAIGHT LINES." "IT BENDS EVER SO SLIGHTLY" "BECAUSE OF THE GRAVITATIONAL FORCE OF THE EARTH." "EINSTEIN PREDICTED LIGHT WOULD BEND PASSING THE SUN." "WHEN HE SAID SO IN THIS LETTER TO GEORGE ELLERY HALE," "IT LOOKED EXTREMELY GOOD ON PAPER," "BUT NOT GOOD ENOUGH" "FOR ASTRONOMERS TO RISK THEIR PRECIOUS TELESCOPES," "MUCH LESS THEIR PRECIOUS EYESIGHT," "LOOKING DIRECTLY AT THE SUN." "SO GEORGE ELLERY HALE," "ONE OF THE GREAT ASTRONOMERS OF THE 20th CENTURY," "MADE A GENTLE SUGGESTION." "WHY NOT OBSERVE THE SUN DURING AN ECLIPSE?" "IN 1919, THAT'S PRECISELY WHAT SIR ARTHUR EDDINGTON" "AND HIS PARTY OF SOLAR EXPLORERS DID." "THE TRIUMPHANT SUCCESS OF THAT EXPEDITION" "WAS THE FIRST GREAT PROOF OF RELATIVITY," "AND TURNED THE QUIET, SHY ALBERT EINSTEIN" "INTO AN INSTANT HERO." "EVERYONE KNEW ABOUT HIM," "EVEN THOUGH FEW COULD UNDERSTAND RELATIVITY." "NO ONE REALLY KNOWS" "WHY THE WORLD TOOK EINSTEIN TO ITS HEART." "EXHAUSTED BY WORLD WAR I, PEOPLE NEEDED A HERO." "HIS DISCOVERY WAS BENIGN," "AND EINSTEIN LOOKED LIKE EVERYONE'S FAVORITE UNCLE." "HE ALSO BECAME THE SYMBOL FOR ALL SCIENTISTS." "IT DIDN'T MAKE ANY DIFFERENCE" "THAT FEW COULD UNDERSTAND EINSTEIN'S GREAT WORK." "THE PRINCIPLE OF EQUIVALENCE RAISES AN INTERESTING QUESTION." "IF LIGHT TRAVELS CURVED PATHS, WHAT IS A STRAIGHT LINE?" "EINSTEIN SAID THAT IT MAKES NO SENSE" "TO TALK OF STRAIGHT LINES." "SPACE ITSELF IS CURVED." "NOT ONLY SPACE, BUT SPACE-TIME." "THAT MEANS THAT SPACE AND TIME CHANGE" "AS THEY MOVE THROUGH GRAVITATIONAL FIELDS." "BUT HOW DOES THAT RELATE TO NEWTONIAN PHYSICS?" "NEWTON SAID A BODY MOVING IN A STRAIGHT LINE" "WILL KEEP MOVING UNTIL ACTED UPON" "BY AN OUTSIDE FORCE." "IF THERE ARE NO STRAIGHT LINES," "WHAT IS THE MEANING OF THE LAW OF INERTIA?" "ON A FLAT MAP OR CHART," "THE SHORTEST PATH BETWEEN TWO POINTS" "IS THE FAMILIAR STRAIGHT LINE," "BUT NOT ON THE SURFACE OF A GLOBE." "HERE, THE SHORTEST PATH BETWEEN TWO POINTS" "IS NOT A STRAIGHT LINE, BUT A GREAT CIRCLE." "TO GENERALIZE, THE SHORTEST PATH" "BETWEEN ANY TWO POINTS ON ANY SURFACE" "IS CALLED A GEODESIC." "EINSTEIN SAID THAT STARLIGHT MAY NOT BE BENT" "BY THE SUN'S GRAVITATIONAL FORCE." "INSTEAD, IT TRAVELS INERTIALLY" "ALONG THE SHORTEST PATH BETWEEN ANY TWO POINTS" "IN THE LOCAL CURVED SPACE-TIME." "IT SEEMS TO BEND TO ANY OBSERVER." "FOR THE SAME REASON, EARTH'S ORBIT DOESN'T HAVE TO BE" "A COMPROMISE BETWEEN INERTIA," "WHICH MAKES THE EARTH WANT TO GO STRAIGHT," "AND THE SUN'S GRAVITY," "WHICH KEEPS THE EARTH CIRCLING THE SUN." "LIKE LIGHT, THE EARTH MOVES INERTIALLY" "ALONG A GEODESIC AND THE LOCAL SPACE-TIME" "CREATED BY THE SUN'S PRESENCE." "CURVED SPACE-TIME, OR EVEN JUST CURVED SPACE," "CAN CREATE THE APPEARANCE OF A FORCE" "EVEN IF THERE IS NONE." "IMAGINE TWO BEINGS ON A JOURNEY, OBEYING INERTIA," "ON WHAT THEY THINK ARE TWO STRAIGHT LINES." "BUT SINCE THEY'RE ON A SPHERE," "THEY'RE REALLY GREAT CIRCLES." "AFTER A WHILE, THEY START GETTING CLOSER TOGETHER," "AS IF DRAWN BY SOME MYSTERIOUS FORCE" "WHICH THEY MIGHT EVEN CALL GRAVITY." "EINSTEIN DID EXACTLY THE OPPOSITE." "HE ELIMINATED THE FORCE OF GRAVITY," "REPLACING IT WITH THE CURVATURE OF SPACE-TIME." "BUT EINSTEIN HAD CREATED A MAMMOTH DILEMMA." "HOW DOES MASS CAUSE SPACE-TIME TO CURVE?" "HE SPENT THE MOST DIFFICULT YEARS OF HIS LIFE," "FROM 1909 TO 1916, WORKING ON THE RIDDLE." "WHEN FINISHED, HE HAD PRODUCED NOTHING LESS" "THAN A NEW THEORY OF THE COSMOS." "IT WAS NOT DESIGNED" "TO EXTEND OR EXPAND NEWTON'S MECHANICAL UNIVERSE," "BUT TO REPLACE IT COMPLETELY." "COULD THAT BE POSSIBLE?" "COULD IT BE THAT NEWTON'S THEORY" "WAS WRONG AFTER ALL?" "DOES THE MOON NOT FALL 1/20 OF AN INCH EVERY SECOND?" "DO PLANETS NOT SWEEP OUT EQUAL AREAS IN EQUAL TIMES?" "WHEN A SPACECRAFT LEAVES EARTH, WHO'S DRIVING?" "[ASTRONAUT] THAT'SA GOODQUESTION." "ITHINKISAACNEWTON'S DOINGMOSTOF THEDRIVING." "NEWTON'S LAWS WORK JUST AS WELL AS ALWAYS." "IN MOST PLACES AND TIMES," "NEWTON'S UNIVERSE AND EINSTEIN'S UNIVERSE" "ARE SO NEARLY IDENTICAL" "THAT ONLY THE MOST PRECISE SCIENTIFIC INSTRUMENTS" "STRETCHED TO THE LIMITS OF THEIR ABILITIES" "CAN DETECT THE TINY DIFFERENCES." "THERE ARE TIMES AND PLACES IN THE UNIVERSE" "WHERE CONDITIONS ARE SO EXTREME" "THAT ONLY EINSTEIN'S THEORY CAN EXPLAIN THEM." "WHEN A LARGE STAR, MUCH LARGER THAN THE SUN," "EXHAUSTS ITS NUCLEAR FUEL," "IT STARTS TO COLLAPSE UNDER ITS OWN GRAVITY." "SOME OF THE MASS IS BLASTED AWAY" "IN A VIOLENT EXPLOSION." "BUT WITH NO FORCE STRONG ENOUGH TO OPPOSE THE COLLAPSE," "THE INWARD FALL CONTINUES UNTIL GRAVITY BECOMES SO INTENSE" "THAT SPACE-TIME ITSELF IS STRETCHED AND WARPED" "INTO A POINT OF INFINITE CURVATURE" "CALLED A BLACK HOLE." "LIKE AN ORDINARY PROJECTILE FALLING BACK TO EARTH," "LIGHT AND ANYTHING ELSE IS DRAWN BACK" "BY THAT AWESOME CONCENTRATION OF MASS." "IT TAKES AN IMMENSE AMOUNT OF MASS" "CROWDED INTO A TINY SPACE" "TO CREATE A BLACK HOLE." "IF BLACK HOLES EXIST," "THEY ARE REMNANTS OF COLLAPSED GIANT STARS" "OR ARE LEFT OVER FROM THE BIG BANG," "WHEN THE UNIVERSE BEGAN." "OF COURSE, IT'S TANTALIZING TO TRY TO IMAGINE" "WHAT IT WOULD BE LIKE TO BE INSIDE OF A BLACK HOLE." "BUT WE'LL NEVER KNOW." "THAT IS THE ULTIMATE IMPOSSIBILITY." "YOU MIGHT ARRANGE TO FALL INTO ONE" "IF YOU REALLY TRIED," "BUT EVEN IF YOU SURVIVED," "THERE'D BE NO WAY TO TELL US WHAT YOU FOUND." "HERE'S SOMETHING TO THINK ABOUT." "WHEN THE UNIVERSE WAS CREATED IN THE BIG BANG," "IT STARTED TO EXPAND RAPIDLY." "CLUMPS OF MATTER CONDENSED INTO GALAXIES," "BUT THEY'RE FLYING AWAY FROM EACH OTHER" "AT GREAT SPEED, EVEN TODAY." "THAT EXPANSION DOES SLOW DOWN WITH TIME." "THE GALAXIES DO ATTRACT EACH OTHER" "BY MEANS OF GRAVITY." "IF THE DENSITY OF MATTER IS HIGH ENOUGH," "THAT EXPANSION WILL NOT CONTINUE FOREVER." "SOMEDAY THE UNIVERSE WILL START CONTRACTING AGAIN." "IT WILL BECOME SMALLER AND SMALLER," "APPROACHING ANOTHER CATACLYSMIC EVENT" "WHICH IS THE EXACT REVERSE OF THE BIG BANG." "THAT EVENT HAS A NAME IN COSMOLOGY, TOO." "IT'S CALLED THE BIG CRUNCH." "NOW, WE DON'T KNOW" "IF THERE'S ENOUGH MATTER IN THE UNIVERSE" "TO CAUSE THAT TO HAPPEN," "BUT LET'S SUPPOSE THERE IS." "IN THAT CASE, NOTHING WILL EVER ESCAPE THAT CONTRACTION," "NOT EVEN LIGHT ITSELF." "BUT IF NOTHING, NOT EVEN LIGHT, CAN ESCAPE," "WHY, THAT'S EXACTLY THE WAY WE DESCRIBE" "THE INSIDE OF A BLACK HOLE." "IT'S POSSIBLE THAT WE ALREADY ARE INSIDE A BLACK HOLE." "MAYBE OUR UNIVERSE" "IS THE INSIDE OF ANOTHER UNIVERSE'S BLACK HOLE." "NOW, THAT'S A BIG ENOUGH IDEA" "TO KEEP YOU BUSY UNTIL NEXT TIME." "EINSTEIN'SGENERALTHEORY OFRELATIVITY" "REPLACESGRAVITYBY  THE CURVATURE OF SPACE-TIME." "THE MORE MASSIVE AN OBJECT," "THEGREATERTHECURVATURE OF SPACE-TIME AROUND AN OBJECT." "SPACE-TIMEISSO WARPED FORA BLACKHOLE" "THATNOTHINGCAN ESCAPEFROMIT ." ""THETIMEHASCOME, THE TEACHER SAID," "TOTIEUP MANYTHREADS." "HOWTYCHO'SCLAWS ANDKEPLER'SLAWS" "JUSTTORETHEGREEKS TOSHREDS." "WHYEINSTEIN'SSPACE INNEWTON'SPLACE" "ISLOOSEIN MANYHEADS."" "CAPTIONING PERFORMED BY THE NATIONAL CAPTIONING INSTITUTE, INC." "CAPTIONS COPYRIGHT 1985 CALIFORNIA INSTITUTE OF TECHNOLOGY," "THE CORPORATION FOR COMMUNITY COLLEGE TELEVISION, AND THE ANNENBERG/CPB PROJECT" "Annenberg Media" "¶" "For information about this and other Annenberg Media programs call 1-800-LEARNER and visit us at"