Transcript: Clifford Will on Was Einstein Right? | Feb 18, 2006

A man in his sixties stands on a stage. He has receding gray hair and a moustache. He wears a brown jacket, patterned tie and black trousers and shirt.
Behind him a portrait of Einstein appears.

Clifford Will says EINSTEIN'S
GENERAL THEORY OF RELATIVITY IS
ONE OF THE MOST REMARKABLE
CREATIONS OF THE HUMAN MIND.
THIS THEORY HAS CHANGED FOREVER
THE WAY WE THINK ABOUT SPACE,
ABOUT TIME, ABOUT THE UNIVERSE
AS A WHOLE.
SOME SCHOLARS AND THINKERS HAVE
EVEN WRITTEN THAT GENERAL
RELATIVITY HAS CHANGED THE
COURSE OF ART, PHIOLOSOPHY,
POETRY, PSYCHOLOGY, AND EVEN
THE COURSE OF HUMAN HISTORY.

A slide projected on the wall reads "Relativity and Art, Philosophy. It shows 3 images; one is labelled "Escher" and depicts black and white staircases. The second is labelled "Dali" and depicts melting clocks. The third shows a man in his fifties and is labelled "Reicherbach, relativity theory and a priori knowledge, 1920. Philosophy of space and time."

A caption appears below the images. It reads "and history?"

A caption appears below Clifford. It shows a light bulb icon and the words "Clifford Will, Professor of Physics, Washington University."

Clifford Will says MY FAVOURITE
EXAMPLE OF HOW RELATIVITY MAY
HAVE CHANGED THE COURSE OF
HISTORY COMES FROM THE
CONSERVATIVE, BRITISH HISTORIAN
PAUL JOHNSON.
IN 1983 JOHNSON WROTE A BOOK IN
POPULAR HISTORY OF THE 20TH
CENTURY CALLED "MODERN TIMES."

The caption changes to "Was Einstein right? Can Einstein’s theories survive today’s scientific scrutiny?"

The caption changes to "Perimeter Institute, October 5, 2005."

Clifford Will says JOHNSON'S
BOOK BEGINS WITH THE FOLLOWING
SENTENCE.
THE MODERN WORLD BEGAN ON AND
HE GIVES A DATE.
NOW IF YOU WERE WRITING A
HISTORY OF THE 20TH CENTURY
WHAT DATE WOULD YOU CHOOSE TO
BEGIN YOUR HISTORY OF THE 20TH
CENTURY?
He raises his hand and asks ANY THOUGHTS?

The audience talks in the background.

Clifford Will says BEGINNING OF
THE WAR, AUGUST 1914, VERY
IMPORTANT, SYMBOLIC DATE.
BUT THAT'S NOT THE DATE JOHNSON
CHOSE.

He puts his hands behind his back and says ANY OTHER DATES?
WHAT ABOUT JANUARY 1, 1900?

Some people laugh.

Clifford Will says OR IF YOU'RE
OBSESSIVE, COMPUSLIVE, JANUARY
1, 1901.

The Audience laughs.

Clifford Will says BUT NO THAT'S
NOT THE DATE EITHER.
JOHNSON'S BOOK BEGINS WITH A
SENTENCE, THE MODERN WORLD
BEGAN ON MAY 29TH, 1919.

He raises his brows and asks WHAT HAPPENED ON MAY 29TH 1919?
ON THAT DAY THERE WAS A TOTAL
ECLIPSE OF THE SUN THAT PASSED
OVER NORTHERN, SOUTH AMERICA,
AND ACROSS THE ATLANTIC AND
OVER WESTERN AFRICA.
AND IN THE COURSE OF THAT
ECLIPSE, TEAMS OF BRITISH
ASTRONOMERS MEASURED THE
BENDING OF STARLIGHT IN
AGREEMENT WITH THE PREDICTION
OF GENERAL, EINSTEIN'S THEORY
OF GENERAL RELATIVITY.
TO JOHNSON, THIS HISTORIAN, THE
SUCCESS OF RELATIVITY IN
SCIENCE WAS ACCOMPANIED BY A
RISE OF RELATIVISM IN MORALITY
AND POLITICS AND ETHICS.
AND TO HIM THIS MORAL
RELATIVISM WAS RESPONSIBLE FOR
ALL THE GREAT GENOCIDES AND
CATASTROPHES OF THE 20TH
CENTURY.
IN OTHER WORDS JOHNSON BLAMES
IT ALL ON EINSTEIN.

Some members of the audience laugh.

Clifford Will says NOW THAT'S A
TOPIC FOR A WHOLE OTHER LECTURE
INVOLVING EXPERTS THAT ARE MUCH
MORE EXPERT IN THIS THEN I.
BUT IT SIMPLY ILLUSTRATES THAT
TO SOME PEOPLE RELATIVITY'S
INFLUENCE EXTENDS WELL, BEYOND
THE REALM OF SCIENCE.
NOW EINSTEIN INVENTED HIS
SPECIAL THEORY OF RELATIVITY IN
1905.
DURING THIS MIRACLE YEAR WHOSE
CENTENARY WE AND OTHER
PHYSICISTS AROUND THE WORLD ARE
CELEBRATING THIS YEAR.
BUT EVEN THOUGH HE DEVELOPED,
HAD THESE GREAT SUCCESSES
DURING THAT YEAR, HE HIMSELF
REMAINED AN OBSCURE, GERMAN
PHYSICIST.
SURE ENOUGH HE ROSE RATHER
STEADILY IN THE RANKS OF GERMAN
ACADEMIC SCIENCE REACHING A
PROFESSORSHIP BY 1911.
BUT HE WAS PRETTY MUCH UNKNOWN
OUTSIDE THOSE CIRCLES.
AND WHEN HE DEVELOPED THE
GENERAL, THE GENERAL THEORY OF
RELATIVITY TEN YEARS LATER IN
1915, HE STILL REMAINED A
RATHER, OBSCURE SCIENTIST AND
EVEN NEWS ABOUT THE GENERAL
RELATIVITY ONLY SLOWLY LEAKED
OUT OF WARTIME GERMANY.
REMEMBER THIS WAS STILL AT THE
MIDDLE OF THE FIRST WORLD WAR.

He takes a sip of water and continues BUT IT WAS REALLY THESE 1919
MEASUREMENTS OF THE DEFLECTION
OF STARLIGHT THAT MADE EINSTEIN
AN INTERNATIONAL CELEBRITY.
WHEN THE RESULTS WERE ANNOUNCED
NEWSPAPERS AROUND THE WORLD
PROCLAIMED HIM THE SUCCESSOR TO
NEWTON, ESTABLISHING A NEW
WORLD ORDER WHERE NOTHING WAS
WHERE IT SEEMED AND SO ON.

The cover of a magazine in black and white shows a picture of Einstein around the age of forty, resting his chin on his hand and frowning.

Clifford says HE APPEARED ON THE COVER OF THE
BERLINER ULLUFRIRTE ZEITUNG,
SORT OF THE NEWSWEEK OF THE
DAY.
SO ALREADY IN 1919, HE WAS A
COVER BOY.

Clifford Will says AND FOR THE
REST OF HIS LIFE, HE, HE, HE
WAS FAMOUS.
HE WAS JUST THE, THE SYMBOL OF
SCIENCE.
AND YOU KNOW MANY OF US HAVE
SEEN THE FAMOUS PICTURES OF
EINSTEIN IN MANY PLACES AND
THROUGHOUT THE YEARS,

He shows a slide that reads "The public Einstein" as a title.
It shows the most famous pictures of Einstein; one riding a bike, one with Chaplin, one sticking out his tongue, etcetera.

He describes WITH LEO
SZILARD WRITING A FAMOUS LETTER
TO ROOSEVELT CONCERNING THE
ATOMIC BOMB, PLAYING THE VIOLIN
IN A BERLIN SYNAGOGUE.
WELL, WHATEVER HE'S DOING
THERE.
TICKERTAPE PARADE, DOWN SOME
STREET, WITH CHARLIE CHAPLIN IN
HOLLYWOOD.
WITH THE HOPI INDIANS IN
ARIZONA, HERE WITH DAVID BEN-
GURION PRESIDENT OF ISRAEL.
HERE IN SANTA BARBARA, SHOWING
THAT LEARNING RELATIVITY IS AS
EASY AS FALLING OFF A, WELL
NEVER MIND.

The Audience laughs.

Clifford Will says AND EVEN,
EVEN SONGS WERE WRITTEN ABOUT
EINSTEIN.
IN FACT THE MOST, WELL KNOWN
AND BELOVED SONG IN THE WORLD
ESPECIALLY IF YOU'RE A MOVIE
LOVER IS ABOUT EINSTEIN.
WELL, THE PRELUDE IS ABOUT
EINSTEIN.
THE SONG THAT EVERYBODY LOVES
AND KNOWS DOESN'T MENTION IT.
BUT THE PRELUDE IS ABOUT
EINSTEIN.
AND HERE IT IS.

He sings as he reads the lyrics from a blue slate. The title reads "Einstein and song" and the lyrics go

THIS DAY AND AGE WE’RE LIVING IN
GIVE CAISE FOR APPREHENSION.
WITH SPEED AND NEW INVENTION
AND THINGS LIKE FOURTH DIMENSION.

YET WE GET A TRIFLE WEARY
WITH MISTER EINSTEIN’S THEORY
SO WE MUST GET DOWN TO EARTH AT TIMES
RELAX RELIEVE THE TENSION.

AND NO MATTER WHAT THE PROGRESS
OR WHAT MAY YET BE PROVED
THE SIMPLE FACTS OF LIFE ARE SUCH
THEY CANNOT BE REMOVED.

Clifford Will says NAME THAT
TUNE.

The Audience laughs.

Clifford Will says NOBODY?
HERMAN HATFIELD, 1931.

A sound of talking in the background is heard.

Clifford Will says WHAT?

A picture of the movie "Casablanca" appears next to the lyrics.

He says "AS TIME GOES BY."

Clifford Will sings YOU MUST
REMEMBER THIS
A KISS IS STILL
AND LONG AFTER EINSTEIN'S DEATH
HE STILL REMAINED A CELEBRITY,
THE SYMBOL OF SCIENCE, THE
SYMBOL OF GENIUS.
HERE IN NEWSWEEK ON A COVER
STORY ABOUT GENIUS HAS
EINSTEIN.

He turns and looks at a slide with two covers showing Einstein on Time and Newsweek magazines. The Newsweek cover reads "The puzzle of genius." The cover of Time reads "Person of the century."

He says TIME MAGAZINE FAMOUSLY PROPOSED
EINSTEIN AS THE MAN OF THE
CENTURY.
BUT IRONICALLY EVEN THOUGH IT
WAS THE GENERAL THEORY OF
RELATIVITY THAT BROUGHT
EINSTEIN SUCH FAME, BY THE
MIDDLE 1920’S EINSTEIN TURNED
THE BULK OF HIS SCIENTIFIC
ATTENTION AWAY FROM THIS
THEORY, AND EMBARKED ON WHAT
WE NOW BELIEVE WAS ULTIMATELY
AN UNFULFILLED QUEST FOR A
UNIFIED THEORY OF THE
INTERACTIONS.
AND AROUND THE SAME TIME,
AROUND THE MIDDLE 1920S, THE
ENTIRE SUBJECT OF GENERAL
RELATIVITY WENT INTO A DECLINE.
SO THAT BY THE, BY THE END OF
THE 1950S GENERAL RELATIVITY
WAS CONSIDERED TO BE IN THE
BACKWATERS OF PHYSICS AND
ASTRONOMY, NOT A FIT SUBJECT
FOR A SERIOUS SCIENTIST TO
PURSUE.
BUT HOW COULD THIS HAVE
HAPPENED?
IF THIS WAS SUCH A REMARKABLE
CREATION...

Clifford Will says SUCH A
WONDERFUL THEORY HOW COULD IT
HAVE FALLEN INTO THE
BACKWATERS?
HOW COULD IT HAVE GONE INTO
SUCH A DECLINE?
I THINK THERE WERE TWO FACTORS
THAT LED TO THIS DECLINE.

Clifford Will says ONE FACTOR
WAS A PREVAILING SENTIMENT IN
THOSE DAYS THAT GENERAL
RELATIVITY WAS
INCOMPREHENSIBLE.
IT WAS TOO DIFFICULT FOR
ORDINARY MORTALS TO UNDERSTAND.
THERE ARE MANY FAMOUS STORIES
THAT, THAT EXPRESS THIS IDEA.
MY FAVOURITE HAS TO DO WITH SIR
ARTHUR STANLEY EDDINGTON, THE
BRITISH ASTRONOMER WHO HEADED
UP THE TEAMS THAT WENT TO
BRAZIL AND NORTH AFRICA TO
MEASURE THE BENDING OF
STARLIGHT IN MAY, 1919.
SIX MONTHS LATER IN NOVEMBER
1919, EDDINGTON MADE A REPORT
TO THE ROYAL SOCIETY OF LONDON
ON THE MEASUREMENTS, THE
RESULTS OF THE ANALYSIS OF THE
DATA.
AND AFTER HIS TALK AND HE'S
CLOSING UP HIS LAPTOP AND
POWERING DOWN HIS POWERPOINT,
AND OH, WAIT

The Audience laughs at his joke.

Clifford Will says AFTER HIS
TALK A COLLEAGUE CAME UP TO HIM
AND SAID, OH, WHY WITH STARS IN
HIS EYES AND SAID PROFESSOR
EDDINGTON YOU MUST BE ONE OF
THREE PEOPLE IN THE WORLD WHO
UNDERSTAND THE GENERAL THEORY
OF RELATIVITY.
TO WHICH EDDINGTON SAID OH,
TUT-TUT, HE WAS BRITISH AND
THEY SAID TUT-TUT IN THOSE DAYS
OH, NO, NO, NO.
AND THE COLLEAGUE SAID, OH,
EDDINGTON DON'T BE SO MODEST.
EDDINGTON SAID WHAT MODEST?
I CAN'T THINK WHO THE THIRD
PERSON WOULD BE.

The Audience laughs again.

Clifford Will says BUT THIS IDEA
THAT THE THEORY WAS
INCOMPREHENSIBLE, WAS TOO
DIFFICULT FOR ORDINARY PEOPLE
TO UNDERSTAND WAS AN, WAS AN
IMPEDIEMENT TO PROGRESS IN THE
SUBJECT.
IT WAS NOT A SUBJECT THAT WOULD
ULTIMATELY ENTER THE NORMAL
PHYSICS CURRICULUM THAT YOUNG
PEOPLE WOULD STUDY AND IN
COMPARISON WITH THE OTHER
BRANCHES OF PHYSICS THAT WERE
GROWING IN THOSE DAYS LIKE
NUCLEAR AND ATOMIC AND PARTICLE
PHYSICS.
SO THE IDEA THAT IT WAS SO
DIFFICULT TO UNDERSTAND WAS AN
IMPEDIMENT TO ITS FROM MOVING
FORWARD.
A SECOND FACTOR THAT LED TO
THIS DECLINE I BELIEVE, WAS THE
PREVAILING VIEW AGAIN IN THOSE
DAYS THAT APART FROM A FEW
EFFECTS THAT EINSTEIN HIMSELF
CALCULATED, THE BENDING OF
STARLIGHT, THE, ADVANCE OF
MERCURY, WHICH I'LL DISCUSS
MOMENTARILY AND THE, THE
GRAVITATIONAL FREQUENCY SHIFT
OF LIGHT, WHICH I'LL ALSO
DISCUSS, A PART FROM THESE VERY
TINY, EFFECTS THAT THE THEORY
PREDICTED THERE WAS A
PREVAILING IDEA THAT THE THEORY
DIDN'T REALLY HAVE MUCH TO DO
WITH PHYSICS OR ASTRONOMY.
IT DIDN'T HAVE MANY
OBSERVATIONAL CONTACTS.
EVEN EINSTEIN HIMSELF WAS
RATHER BLASE ABOUT THE
EXPERIMENTAL IMPLICATIONS OF
THE THEORY.
IN NOVEMBER 1919, WHEN HE
RECEIVED A TELEGRAM FROM
ENGLAND REPORTING THAT THE
BRITISH ASTRONOMERS HAD
MEASURED THE BENDING OF LIGHT
IN AGREEMENT WITH HIS THEORY,
HE CASUALLY SHOWED THE TELEGRAM
TO HIS STUDENT, ILSE ROSENTHAL
SCHNEIDER.
SHE WAS ASTONISHED AT HIS
SERENE REACTION TO THIS
WONDERFUL NEWS.
AND SHE SAID, WHY PROFESSOR
EINSTEIN WHAT WOULD YOU HAVE
SAID IF THE MEASUREMENTS HAD
DISAGREED WITH YOUR THEORY?
AND HE SAID WELL, THEN I WOULD
HAVE FELT SORRY FOR THE DEAR
LORD, FOR THE THEORY IS
CORRECT.

The Audience laughs.

Clifford Will says LATER IN 1930
HE WROTE, I DO NOT CONSIDER THE
MAIN SIGNIFICANCE OF THE
GENERAL THEORY OF RELATIVITY TO
BE THE PREDICTION OF SOME TINY,
OBSERVABLE EFFECT, BUT RATHER
THE SIMPLICITY OF ITS
FOUNDATION AND THE CONSISTENCY
OF ITS STRUCTURE.
SO EVEN HIS OWN ATTITUDE SEEMED
TO BE RATHER BLASE ABOUT
EXPERIMENT.
BUT OF COURSE THIS IS NOT HOW
SCIENCE WORKS.
A SUBJECT THAT IS ALL THEORY
WITH NO EXPERIMENTAL CONTACT TO
VERIFY THE THEORY OR TO ALLOW
ONE TO MODIFY THE THEORY IN THE
PROPER DIRECTION IS IN DANGER
OF BECOMING A STERILE SUBJECT.
CONVERSELY, A SUBJECT THAT IS
ALL EXPERIMENTAL DATA WITH NO
THEORY TO TRY AND TAKE THAT
DATA AND PUT IT TOGETHER IN
SOME KIND OF COHERENT PICTURE
OF NATURE IS ALSO A STERILE
SUBJECT.
SUBJECTS THAT THRIVE IN
SCIENCE, NOT JUST PHYSICS, BUT
ALL OF SCIENCE ARE THOSE IN
WHICH THERE'S AN INTERPLAY
BETWEEN EXPERIMENT AND THEORY,
AND I THINK THIS IS WHAT
HAPPENED IN GENERAL RELATIVITY.
THE ABSENCE OF ANY KIND OF
SERIOUS, EXPERIMENTAL CONTACT
MADE GENERAL RELATIVITY INTO A
STERILE SUBJECT AND BY THE LATE
1950S IT WAS BELIEVED TO BE IN
THE BACKWATERS OF PHYSICS AND
ASTRONOMY, BUT DURING THE 1960S
ALL THAT CHANGED.
THE 1960S AND FOR THOSE OF US
OLD ENOUGH, AND I WON'T MAKE
REMARKS ABOUT AGE IN THIS, IN
THIS ARGUMENT, BUT THOSE OF US
SO OLD TO REMEMBER THE 60S WAS
A TIME OF GREAT FERMENT,
HIPPIES, VIETNAM.

He changes the slide and reads from it.

He says BUT THERE WAS ALSO A TIME OF A
REVOLUTION IN ASTRONOMY,
BECAUSE DURING THE 60S A SERIES
OF REMARKABLE DISCOVERIES WAS
MADE THAT REALLY CHANGED THE
STORY FOR GENERAL RELATIVITY.
FIRST IN 1961 CAME THE
DISCOVERY OF QUASARS.

The slide shows a picture of a very luminous point in space. A caption next to it reads "1961, discovery of Quasars."

He says QUASARS ARE EXTREMELY POWERFUL,
BUT EXTREMELY POINT LEGGED
SOUCES OF RADIOWAVES, OPTICAL
RADIATION, ALL FORMS OF
ELECTROMAGNETIC RADIATION THAT
WE SEE ALMOST TO THE EDGE OF
THE VISIBLE UNIVERSE.
THEY ARE AMONG THE MOST DISTANT
OBJECTS WE KNOW, YET THEY ARE
ON ASTRONOMICAL TERMS EXTREMELY
BRIGHT.
WE NOW BELIEVE THAT THE CENTRAL
ENGINE OF QUASARS, OF A TYPICAL
QUASAR, IS A SUPER, MASSIVE
BLACK HOLE, WEIGHING BETWEEN
MAYBE A MILLION TO A BILLION
TIMES THE MASS OF THE SUN.
CLEARLY BLACK HOLES ARE
SOMETHING THAT NEED GENERAL
RELATIVITY IN ORDER TO
UNDERSTAND THEM.

A second image appears on the slide. It shows a green oval with a red line across it. A caption next to it reads "1964 discovery of 3rd cosmic background."

He says NEXT IN 1964 CAME THE DISCOVERY
OF THE SO-CALLED THREE DEGREE
BACKGROUND RADIATION.
THIS IS THE RADIATION FROM AN
EARLIER PHASE OF THE UNIVERSE,
ROUGHLY 380 THOUSAND YEARS AFTER THE
BIG BANG, THAT WE NOW SEE TODAY
COMING AT US FROM ALL
DIRECTIONS EMITTED FROM THE
HOT, EARLY UNIVERSE.
WELL CLEARLY GENERAL RELATIVITY
HAS A ROLE TO PLAY IN
UNDERSTANDING THE ORIGIN,
EVOLUTION AND STRUCTURE OF THE
UNIVERSE.

An image shows a red mass in space. A caption next to it reads "1967 Discovery of Pulsars."

He continues NEXT IN 1967 THE DISCOVERY OF
PULSARS.
THESE ARE STRANGE STARS THAT
EMIT PULSES OF RADIOWAVES TO
THE EARTH.
EVERYWHERE BETWEEN, EVERYTHING
BETWEEN A THOUSAND PULSES PER
SECOND DOWN TO MAYBE A PULSE
EVERY FEW SECONDS, UM, WE NOW
UNDERSTAND THESE OBJECTS TO BE
NEUTRON STARS.

Clifford Will says STARS WITH
ROUGHLY THE ONE, ONE, ONE AND A
HALF, TIMES THE MASS OF THE
SUN.
BUT ABOUT A SIZE AS KITCHENER,
WATERLOO IN DIAMETER.
THESE ARE EXTREMELY CONDENSED,
EXTREMELY SMALL OBJECTS,

Clifford Will says YET WITH THE
SAME MASS AS THE SUN STAYING ON
THEIR AXIS AND EMITTING A RADIO
BEAM MUCH LIKE A, A LIGHTHOUSE,

He raises his arms and turns them in a circle over his head.

He continues ALONG THE MAGNETIC FIELD THAT'S
MISALIGNED WITH THE SPIN AXIS.
AND EVERY TIME THAT MAGNETIC
FIELD BEAMS ITS RADIATION
TOWARD US WE SEE A BEAM OF
RADIOWAVES.
NEUTRON STARS ARE SO CONDENSED
THAT GENERAL RELATIVITY IS
NEEDED TO UNDERSTAND THEIR
STRUCTURE.
FINALLY IN 1971 CAME THE
DISCOVERY OF THE FIRST BLACK
HOLE CANDIDATE.

An image shows a light blue oval on a black background.
A caption next to it reads "1971 Discovery of X ray binary black hole candidate."

He says THIS IS AN OBJECT THAT EMITS X
RAYS.
IT WAS DISCOVERED BY ONE OF THE
FIRST X RAY SATELLITES LAUNCHED
ABOVE THE EARTH'S ATMOSPHERE, A
SOURCE OF X RAYS IN ORBIT
AROUND A COMPANION STAR.
THAT'S THE SOURCE OF THE X
RAYS.
THERE'S THE STAR AND WE NOW
BELIEVE WITH VERY HIGH
CONFIDENCE THAT THE X RAY
SOURCE IS ACTUALLY A BLACK HOLE
THAT'S SUCKING MATTER OFF OF
THE ATMOSPHERE OF THE STAR.
THE MATTER HEATS UP JUST BEFORE
FALLING DOWN THE HOLE AND EMITS
X RAYS
AGAIN, BLACK HOLES YOU NEED
GENERAL RELATIVITY TO
UNDERSTAND BLACK HOLES.
SO THESE KINDS OF DISCOVERIES
GAVE ASTRONOMERS THE SENSE THAT
THEY WOULD NEED TO UNDERSTAND
GENERAL RELATIVITY IN ORDER TO
EXPLAIN THESE KINDS OF EXOTIC
PHENOMENON.
SO THIS IS ONE FACTOR THAT LED
TO THIS RENNAISSANCE FOR
GENERAL RELATIVITY BEGINNING IN
THE 60S.
A SECOND FACTOR THAT LED TO
THIS RENNAISSANCE WAS THE
DEVELOPMENT OF A NEW WAY OF
THINKING ABOUT THE THEORY.
THE OLD WAY OF THINKING SORT OF
FROM EINSTEIN'S DAY WAS VERY
MUCH FOCUSED ON THE OBSTRUSE
MATHEMATICS AND MATHEMATICAL
NICETIES THAT TENDED TO OBSCURE
WHAT WAS REALLY GOING ON.
BUT THIS NEW WAY OF THINKING
PIONEERED BY PEOPLE LIKE HERMAN
BONDI, JOHN WHEELER, KIP THORN
AND OTHERS FOCUSED ON THE
OBSERVABLE ASPECTS OF GENERAL
RELATIVITY.
THEY FOCUSED ON DOING
CALCULATIONS IN A WAY THAT
REALLY TOLD YOU WHAT, WHAT YOU
COULD MEASURE WHEN YOU WERE IN
CURVED SPACETIME.
THIS HAD A NUMBER OF IMPORTANT
EFFECTS, IT HELPED TO
STREAMLINE ONE'S UNDERSTANDING
OF THE THEORY.
IT WAS NO LONGER SO
INCOMPREHENSIBLE AS IT WAS
BELIEVED TO BE YEARS BEFORE.
AND ALSO NOT THIS IS NOT AN, AN
UNIMPORTANT FACTOR THAT ALSO
LED TO NEW GENERATIONS OF
TEXTBOOKS THAT PRESENTED
GENERAL RELATIVITY IN A
PEDAGOGICALLY SOUND WAY, IN A
SIMPLE STREAMLINED WAY THAT
ALLOWED ONE TO TEACH GENERAL
RELATIVITY IN NORMAL COURSES.
THESE DAYS MOST GRADUATE
PROGRAMS IN PHYSICS TEACH A
COURSE IN GENERAL RELATIVITY
AND MANY OF THE, THE LARGER
PHYSICS DEPARTMENTS NOW TEACH
UNDERGRADUATE COURSES IN
GENERAL RELATIVITY.
I WANT YOU TO KNOW THE IMPACT
OF THAT FACTOR BECAUSE WITHOUT
YOUNG PEOPLE LEARNING A FIELD
AND ENTERING THAT FIELD TO
PRODUCE THE RESEARCH ADVANCES
OF THE FUTURE A SUBJECT IS IN
DANGER OF BECOMING STERILE.
SO HAVING A NEW WAY OF TEACHING
THIS SUBJECT WAS EXTREMELY
IMPORTANT IN BRINGING YOUNG
PEOPLE INTO THE FIELD.
THIS WAS A DEVELOPMENT ALSO
THAT HAD ITS ORIGINS IN THE
1960S.
THE THIRD FACTOR THAT I THINK
LED TO THIS RENNAISSANCE WAS
THE DEVELOPMENT OF HIGH
PRECISION TECHNOLOGY.
THE TECHNOLOGY WITH PRECISION
MEASUREMENTS THAT ALSO BEGAN
AROUND THE 1960S AND THAT WE
ALL BENEFIT FROM TODAY.
THIS IS THE TECHNOLOGY OF
LASERS, RADAR, ATOMIC CLOCKS,
TOGETHER WITH ALL THE MACHINERY
ASSOCIATED WITH THE
INTERPLANETARY SPACE PROGRAM
THAT LED TO NEW WAYS TO MEASURE
SOME OF THOSE TINY EFFECTS THAT
GENERAL RELATIVITY PREDICTED.
AFTER ALL IF YOU WANT TO
UNDERSTAND QUASARS, BLACK
HOLES, NEUTRON STARS IN THE
UNIVERSE, YOU WOULD LIKE TO
KNOW WHETHER OR NOT GENERAL
RELATIVITY IS THE CORRECT
THEORY TO BE USING OR WHETHER
YOU SHOULD BE USING SOME OTHER
THEORY.
SO THE NEW TECHNOLOGIES OF THE
1960S LED TO NEW TOOLS, NEW
WAYS TO TEST EINSTEIN'S THEORY
TO SEE IF IT REALLY IS THE
RIGHT THEORY TO BE USED.
THE RESULT OF THAT WAS A
WORLDWIDE PROGRAM OVER THE
SUBSEQUENT YEARS TO PUT
EINSTEIN'S THEORY TO THE TEST
AND THAT'S WHAT I WANT TO FOCUS
ON IN THIS LECTURE.
FIRST I'D LIKE TO DESCRIBE THE
BENDING OF LIGHT.

He changes the slide. This one reads "Was Einstein right?" as a title. Below the title it continues "Introduction; Putting General relativity to the test; light’s departure from the straight and narrow. Mercury’s perihelion from trouble to triumph; does space time do the twist? The search for gravity waves; Effect of gravity on time; Einstein and daily life. Epilogue."

Clifford Will says THIS FAMOUS
EXPERIMENT, THAT MADE EINSTEIN
FAMOUS.
THEN I'LL TALK ABOUT A SECOND
OF THESE FAMOUS TESTS THAT
EINSTEIN HIMSELF DEVISED HAVING
TO DO WITH THE ORBIT OF THE
PLANET MERCURY.

Clifford Will says AND I'LL
DESCRIBE HOW THAT HAS, HAS HAD
ITS UPS AND DOWNS BUT IS NOW A
TRIUMPH FOR GENERAL RELATIVITY.
THEN I'LL TALK ABOUT WHETHER OR
NOT SPACETIME DOES THE TWIST.

Clifford Will says I PROMISE YOU
WILL NOT SEE ME DANCE.
YOU'VE HEARD ME SING, BUT YOU
WON'T HAVE TO SEE ME DANCE BUT
I'LL EXPLAIN WHAT THAT MEANS.
THEN I'LL DESCRIBE THE SEARCH
FOR GRAVITATIONAL WAVES, A VERY
IMPORTANT PREDICTION OF GENERAL
RELATIVITY.

Clifford Will says AND I'LL
DESCRIBE WHY WE KNOW THEY
EXIST, BUT WHY WE STILL HAVEN'T
YET DETECTED THEM.

Clifford Will says FINALLY I'LL
TALK ABOUT THE EFFECT OF
GRAVITY ON TIME AND TALK ABOUT
WHY EINSTEIN IS IMPORTANT IN
YOUR, DAILY LIFE.

Clifford Will says AND THAT'S
THE PLACE WHERE YOU'RE ALLOWED
TO LAUGH.

Some laugh.

Clifford Will says BUT LET ME
START WITH THE BENDING OF
LIGHT.
ACCORDING TO GENERAL RELATIVITY
SPACE IS NOT FLAT LIKE THIS
STAGE.
BUT RATHER IT'S WARPED, IT'S
CURVED, IT'S BENT BY THE
PRESENCE OF MASS.

A slide behind him shows a red star on the left and a telescope on the right. A curved line draws itself and travels over the sun. A caption on the slide reads "Light’s departure from the straight and narrow."

He says MASS WARPS SPACE, SPACE AND
TIME AND IT'S THAT WARPAGE OF
SPACETIME THAT IS RESPONSIBLE
FOR GRAVITY.
NOW ONE IMMEDIATE CONSEQUENCE
OF THAT IS THAT LIGHT WILL NO
LONGER TRAVEL ON WHAT YOU THINK
OF AS A STRAIGHT LINE IF IT
PASSES BY A BODY LIKE THE SUN,
BUT WILL IN FACT BE DEFLECTED
IN ITS PATH.
SO THAT LIGHT FROM A STAR OVER
HERE, PASSING BY THE SUN AND
ENTERING A TELESCOPE OVER HERE
WILL APPEAR FROM THE POINT OF
VIEW OF THE TELESCOPE TO CAUSE
THE STAR TO SHIFT IN ITS
POSITION SLIGHTLY.
SO IT WILL APPEAR THE STARS
MOVED FROM THERE TO THERE
BECAUSE OF THAT BENDING OF
LIGHT.
SO THIS SUGGESTS THEN A VERY,
SIMPLE EXPERIMENT THAT YOU
COULD DO TO TEST WHETHER OR NOT
BENT LIGHT, LIGHT IS BENT BY A
BODY SUCH AS THE SUN.

Other stars appear scattered around the image.

He explains YOU SIMPLY GO OUT AT NIGHT AND
TAKE A PHOTOGRAPH OF SOME FIELD
OF STARS IN THE SKY AND THEN
SIX MONTHS LATER, AFTER THE
EARTH'S ORBIT AROUND THE SUN
HAS PUT THE SUN IN THAT SAME
POSITION OF THE SKY, YOU THEN
TAKE A PHOTOGRAPH OF THE SAME
GROUP OF STARS.

A second yellow circle appears representing the sun.

He continues AND THE IMPORTANT THING ABOUT
THE BENDING OF LIGHT IS THAT
THE FURTHER THE LIGHT IS IN ITS
PASSAGE FROM THE SUN, LIKE FOR
A STAR WAY OUT HERE THE LESS
THE EFFECT.
SO HERE, THE STAR IS HARDLY
EFFECTED AT ALL.
STARS CLOSER TO THE SUN HAVE
THEIR POSITIONS DEFLECTED BY A
LARGER AMOUNT.
AND A STAR VERY CLOSE TO THE
SUN WHOSE RAY JUST GRAZES THE
SURFACE OF THE SUN IS DEFLECTED
BY AN AMOUNT OF 1.75 ARC
SECONDS.

A line appears from the sun pointing upwards in a 45 degree angle. It’s labelled "1.75."

He says I REMIND YOU THAT AN ARC SECOND
IS 1, 36 HUNDREDTH OF A DEGREE.

Clifford Will says AN ARC SECOND
IS THE ANGLE SUBTENDED BY MY
FINGER AT TWO AND A HALF,
MILES.
THIS WAS A VERY SMALL ANGLE,
BUT IT'S NOT, ITS, ASTRONOMERS
CAN ROUTINELY MEASURE SUCH
ANGLES.

Clifford Will says SO WHAT YOU
DO IS SIMPLY TAKE A PHOTOGRAPH
OF THE STARS WHEN THE SUN IS
THERE AND THEN MEASURE HOW MUCH
HEAT TO THE STARS HAS BEEN
SHIFTED AND SEE IF THE AMOUNT
AGREES WITH GENERAL RELATIVITY.
JUST ONE PROBLEM WITH THIS
EXPERIMENT.
WHAT'S THE PROBLEM?

Some people talk in the background.

Clifford Will says THERE ARE NO
STARS IN THE SKY WHEN THE SUN
IS THERE BECAUSE THE SUN IS
THERE, RIGHT?
THIS IS WHY OF COURSE YOU HAVE
TO WAIT FOR A TOTAL ECLIPSE OF
THE SUN, WHEN THE MOON GOES IN
FRONT OF THE SUN BLOCKING ITS
LIGHT AND YOU CAN START TO SEE
STARS NEAR THE SUN.

A white slate appears. It reads "the 1919 Eclipse expedition Principe."
A white circle on a black patch of colour appears.

He continues AND THIS IS ACTUALLY A
REPRODUCTION FROM ONE OF THE
PLATES FROM A 1919 EXPEDITION
OF EDDINGTON.
SO THERE'S THE MOON IN FRONT OF
THE SUN.
THERE'S THE SOLAR PHOTOSPHERE,
AND THERE ARE TICK MARKS NOT
THEY'RE NOT PROVIDED BY GOD,
BUT ASTRONOMERS ADDED THEM
LATER.

The Audience laughs.

Clifford Will says AND YOU CAN'T
TELL THESE STARS FROM DIRT ON
THE VIEWGRAPH, BUT ANYWAY THERE
ARE ASSURE I YOU, STARS BETWEEN
THESE TICK MARKS THAT YOU CAN
PHOTOGRAPH.
AND YOU COMPARE THE POSITIONS
OF THOSE STARS WITH THE
PHOTOGRAPH OF THE SAME STARS
TAKEN AT NIGHT AND SEE HOW MUCH
EACH ONE HAS SHIFTED.
AND THIS WAS THE BASIS FOR
EDDINGTON'S REPORT OF A
SUCCESSFUL CONFIRMATION OF
EINSTEIN'S THEORY.
BUT THERE'S ONE PROBLEM WITH
THESE EXPERIMENTS AND IT'S A
PROBLEM THAT I DISCOVERED A FEW
YEARS AGO AND I NAMED IT WILL'S
LAW, AND I'M JUST AWAITING MY
CALL FROM STOCKHOLM ANY DAY
NOW.

The Audience laughs.

Clifford Will says I, I MISSED
OUT ON THIS YEAR, SO I'LL HAVE
TO WAIT TILL NEXT YEAR FOR MY
PRIZE.
WILL'S LAW STATES THAT TOTAL
SOLAR ECLIPSES MUST NEVER OCCUR
OVER FINE ASTRONOMICAL
OBSERVATORIES, BUT INSTEAD
SOLAR ECLIPSES MUST ONLY OCCUR
OVER ROLLING OCEANS, STEAMY
JUNGLES OR SWIRLING DESERT
SANDS.

The Audience laughs again.

Clifford Will says AND THIS WAS
TRUE OF EDDINGTON'S EXPEDITION.
FOR EXAMPLE, IN AFRICA THERE
WAS A HUGE RAINSTORM RIGHT UP
UNTIL THE LAST MOMENT BEFORE
TOTALITY OF THE ECLIPSE AND THE
CLOUDS ONLY BEGAN TO MOVE CLEAR
SLOWLY WHEN THE ECIPSE WHEN IN
ITS MAXIMUM PHASE, AND SO HE
COULD ONLY SEE A SMALL FRACTION
OF THE STARS THAT HE COULD HOPE
TO SEE.
SIMILARLY IN 1973, WHEN A TEAM
FROM PRINCETON UNIVERSITY AND
THE UNIVERSITY OF TEXAS WENT TO
MORATANIA, IN WEST AFRICA
DURING A TOTAL ECLIPSE TO DO
THE SAME KIND OF EXPERIMENT,
THEY OF COURSE IN 1973 HAD THE
BENEFIT OF A COMPUTER
CONTROLLED TELESCOPE AND THE
FANCIEST KODAK PHOTOGRAPHIC
EMULSIONS FOR TAKING THEIR
PHOTOGRAPHS, BUT THEY COULDN'T
CONTROL THE WEATHER ANY BETTER
THEN EDDINGTON DID.
THERE WAS A HUGE DUSTSTORM
DURING THE, THE ECLIPSE AND SO
THEY COULD ONLY SEE ABOUT 15 PERCENT
OF THE STARS THEY HOPED TO SEE
THROUGH THE DUST.
UNFORTUNATELY MY LAW WAS BROKEN
SEVERAL YEARS AGO, ABOUT FIVE
YEARS AGO WHEN THERE WAS A
TOTAL SOLAR ECLIPSE OVER
HAWAII, OVER MONOKAIA, WHICH
HAPPENS TO HAVE SOME OF THE
WORLD'S FINEST ASTRONOMICAL
TELESCOPES IN IT.
SO I'M CALLING THAT SPONTANEOUS
SYMMETRY BREAKING, SO MAYBE
THERE'S STILL HOPE FOR MY, FOR
MY WILL'S LAW.

The Audience laughs.

Clifford Will says BUT THESE
KINDS OF ECLIPSE EXPERIMENTS
WERE BESET BY THESE SORTS OF
ENVIRONMENTAL PROBLEMS AND SO
THEY WERE ALWAYS LIMITED IN
ACCURACY.
BUT IN THE MEANTIME AN ENTIRELY
NEW METHOD CAME ALONG FOR
MEASURING THE BENDING OF LIGHT,
AND THAT EXPLOITED TWO OF THE
THINGS I'VE ALREADY MENTIONED
THAT CAME ALONG AROUND THE
1960S.

A slide shows large satellite dishes pointing up to the sky.

He says FIRST THE DEVELOPMENT OF RADIO
ASTRONOMY.
A VERY, HIGH PRECISION WAY TO
MEASURE ANGLES OF RADIO
SOURCES, AND IN FACT THESE DAYS
RADIO TELESCOPES MORE
PARTICULARLY RAYS OF RADIO
TELESCOPES CALLED RADIO
INTERFEROMETERS CAN MEASURE
ANGLES BETWEEN RADIO SOURCES AT
THE LEVEL OF HUNDREDS, A
HUNDRED MICRO, ARC SECONDS.
THAT'S 100, MILLIONTHS OF AN
ARC SECOND SO THEY HAVE VERY
FANTASTIC PRECISON IN, IN
ANGULAR MEASUREMENTS.
ON TOP OF THAT ARE THESE
QUASARS THAT I'VE ALREADY
MENTIONED THAT WERE DISCOVERED
IN THE 1960S, THESE VERY POINT
LIKE INTENSE SOURCES OF
RADIOWAVES.
SO THE IDEA WOULD BE THEN TO
MEASURE THE BENDING OF
RADIOWAVES FROM THESE QUASARS
USING RADIO INTERFEROMETRY.
AND THIS TECHNIQUE WAS
DEVELOPED IN THE LATE 1960S AND
MANY MEASUREMENTS WERE MADE.
AND HERE'S AN EXAMPLE OF HOW
SUCH A MEASUREMENT MIGHT WORK.

A slate shows 2 quasars marked by red stars on a white slate.

He says TWO PARTICULAR QUASARS, 3C273
AND 3C279 ARE IN THE SKY.
THOSE ARE JUST CATALOGUE NAMES
FROM A CERTAIN CATALOGUE OF
QUASARS AND EVERY FALL THE SUN
PASSES BY THEM.

A circle representing the sun appears. It reads "September 28" next to it.

He continues SO THAT ON SEPTEMBER 28TH THE
SUN IS HERE, AND IF YOU MEASURE
THE ANGLE BETWEEN THE TWO
QUASARS AND THAT'S WHAT RADIO
INTERFEROMETERS ARE BEST AT,
MEASURING THE RELATIVE ANGLE
BETWEEN TWO RADIO SOURCES,
BECAUSE IF YOU MEASURE THE
ANGLE HERE THEN YOU'RE GOING TO
GET THE NOMINAL UNDEFLECTED
ANGLE BETWEEN THE TWO, BECAUSE
THE SUN IS WAY OVER HERE AND
NONE OF THE LIGHT PASSES CLOSE
TO THE WARPED SPACETIME NEAR
THE SUN.
BUT AS DAYS PASS, THE SUN GETS
CLOSER AND CLOSER.

The sun then moves closer to the quasars and its label changes to "October 3."

He says ON OCTOBER THREE, IT'S HERE.
ON OCTOBER EIGHTH IT SITS HERE.

The sun moves closer to the lower quasar and its label changes to "October 8."

He continues SO NOW THE LIGHT FROM THIS
QUASAR PASSES CLOSE TO THE SUN,
AND SO THE IMAGE OF THIS QUASAR
IS GOING TO BE SHIFTED.
HERE, IS THE FIRST QUIZ OF THIS
TALK.
WHICH WAY DOES THE IMAGE OF
3C279 SHIFT?
DOES IT GO THIS WAY

He lowers his right arm and continues
OR DOES IT
GO THIS WAY?

He raises his left arm.

He then says AND WE'LL HAVE TO VOTE.
HOW MANY, HOW MANY VOTE THIS
WAY?

He keeps his left arm raised.

He then lowers his right hand and asks HOW MANY VOTE THIS WAY?

He says I'M, I'M IMPRESSED ON TWO
THINGS.

Clifford Will says "A" THAT YOU
GOT THE RIGHT ANSWER.
THAT'S THIS WAY.

He points down with his right arm.

He continues AND "B" THAT YOU RATE, YOU
VOTED ABOUT DOUBLE THE RATE OF
A TYPICAL AMERICAN ELECTION.

Some members of the audience laugh and applaud.

Clifford Will says OKAY?
SO HERE'S, HERE'S QUESTION
NUMBER TWO.
SO WHAT HAPPENS TO THE ANGLE
BETWEEN THE TWO QUASARS?

He opens and closes his arms as he asks WILL IT INCREASE OR DECREASE?
SORRY.
EVERYONE AGREE WITH THAT?
INCREASE, OKAY.
OCTOBER NINTH, ACTUALLY YOU GET
NO DATA, ACTUALLY GOES RIGHT IN
FRONT OF 3C279 IN THIS CASE.

The sun covers the quasar.

He continues OCTOBER TENTH WHICH WAY DOES
THE IMAGE OF 3C279 GO?

The sun appears below the quasar and is labelled "October 10."

He lowers his right arm and asks HOW MANY VOTE THIS WAY?
WE'RE GETTING GOOD.
HOW MANY VOTE THIS WAY?

He extends his left arm up.

He continues GOOD.
ACTUALLY THE VOTING THERE WAS
LESS.
NOW I'M A LITTLE TROUBLED BY
THAT.

The Audience laughs.

Clifford Will says AND THEN ON
OCTOBER 15TH THE ANGLE RETURNS
TO THE NOMINAL UNDEFLECTED
ANGLE.

The sun then appears below and to the left of the quasar.

He continues SO WHAT YOU DO IS PLOT THE
ANGLE BETWEEN THE TWO QUASARS
AS A FUNCTION OF TIME.

A graph on the right of the screen shows two curved lines on a vertical axis.

He explains IT INCREASES YOU KNOW DATA FOR
ONE DAY THEN IT'S LESS THEN
NORMAL, THEN BACK TO NORMAL.
AND COMPARE THE SHAPE OF THAT
CURVE WITH THE DETAILED
PREDICTION OF GENERAL
RELATIVITY, THEN YOU CAN ASK
WHETHER THE RESULTS AGREE.
IN A RECENT MEASUREMENT
ACTUALLY IN 2004 THAT INVOLVED
MEASURING ANGLES AT 541 QUASARS
DISTRIBUTED OVER THE ENTIRE SKY
LED TO AN AGREEMENT WITH THE
BRITISH THAT GENERAL RELATIVITY
TO TWO PARTS IN 10 THOUSAND.
SO THIS IS A VERY, HIGH
PRECISION MEASUREMENT IN
AGREEMENT WITH EINSTEIN'S
THEORY.
BUT THAT'S NOT THE END OF THE
STORY FOR THE DEFLECTION OF
LIGHT.
IN 1979, ASTRONOMERS
DISCOVERED A VERY, STRANGE
QUASAR.
IN FACT THEY DISCOVERED TWO
QUASARS.

A slide shows nine images of the quasars. Luminous red and orange shapes on a dark background formed by what appear as two nucleus and matter around them.

He continues THESE QUASARS WERE VERY STRANGE
BECAUSE THEY WERE EXTREMELY
CLOSE TOGETHER ON THE SKY, MUCH
CLOSER THEN YOU WOULD EVER HAVE
EXPECTED ON THE BASIS OF RANDOM
CHANCE.
AND FURTHERMORE, THEY LOOK
ALMOST LIKE TWINS OF EACH
OTHER.
THEY LOOK ALMOST IDENTICAL.
VERY QUICKLY THE ASTRONOMERS
CONCLUDED THAT WHAT THEY WERE
SEEING WAS NOT TWO QUASARS, BUT
A SINGLE QUASAR.
AND THAT SOMEWHERE BETWEEN THAT
QUASAR AND US THERE'S A BIG
CLUSTER OF GALAXIES.
AND AS THE LIGHT PASSES THROUGH
THE WARPED SPACE SURROUNDING
THAT CLUSTER OF GALAXIES IT'S
TAKING TWO DIFFERENT PATHS TO
GET TO US.
SO THE ONE QUASAR IS BEING
SPLIT INTO TWO SEPARATE IMAGES.
I MEAN SOME OF YOU MAY HAVE
EXPERIENCED THIS.
I'VE NEVER DONE THIS MYSELF.
BUT WHEN YOU'RE INSIDE A BAR
AND YOU'RE LOOKING THROUGH THE
MODEL OF GLASS THAT YOU SEE IN
THE GLASS AT THE BAR AND YOU
SEE MULTIPLE IMAGES OF PEOPLE
OUTSIDE TRYING TO LOOK IN AND
SEE WHO'S THERE.
I WOULDN'T KNOW ANY, ANY OF
THIS MYSELF.
BUT MAINLY YOU SEE MULITPLE
IMAGES FROM GLASS.
THIS IS THE SAME KIND OF AN
EFFECT.
BUT HERE IT'S CURVED, SPACETIME
THAT'S ACTING AS THE, AS THE
LENS NOT GLASS.
AND SINCE THAT TIME THIS FIELD
OF GRAVITATIONAL LENSES HAS
EXPLODED AND BECOME A MAJOR
ASTRONOMICAL TOUR, AH, TOOL.
AND HERE ARE SOME JUST
BEAUTIFUL EXAMPLES OF
ASTRONOMICAL LENSES.
HERE'S ONE CASE WHERE A
QUASAR'S BEEN IMAGED INTO FIVE
SEPARATE IMAGES BY AN
INTERVENING GALAXY.

The slide reappears. It reads "Gravitational lenses, Einstein’s gift to astronomy."

He says IN SOME CASES IF YOU HAVE A
QUASAR AND THE GALAXY ARE
CLUSTERS TO COME PERFECTLY
LINED UP BETWEEN US AND THE
QUASAR THEN LIGHT CAN TAKE
INFINITELY MANY WAYS TO GET
AROUND THIS.

He holds one hand out and mimics light shining through at different angles.

He says IT CAN GO ANY WAY IT WISHES
AROUND THE, THE GALAXY AND SO
THIS ONE QUASAR CAN BE IMAGED
INTO A PERFECT RING.
IT'S CALLED, NOW CALLED THE
EINSTEIN RING IN HONOUR OF
EINSTEIN AND THERE'S AN EXAMPLE
OF AN ALMOST PERFECT EINSTEIN
RING.

He points to the image at the center. It shows a luminous ring.

He continues THERE'S ANOTHER ONE.
AND HERE ARE PIECES WHERE A
QUASAR, IS THESE POINT-LIKE
QUASARS, ARE DISTORTED INTO
LITTLE ARC LIKE SHAPES BY THE
INTERVENING CURVED SPACETIME
SURROUNDING A CLUSTER OR A
GALAXY.
HERE'S ANOTHER EXAMPLE OF A
GRAVITATIONAL LENS THAT IS ONE
OF MY FAVOURITES.
HERE'S AN IMAGE OF A HUGE
CLUSTER OF GALAXIES TAKEN BY
THE HUBBLE SPACE TELESCOPE.

The slide changes and shows a dark space background with several golden lights shining on it.

He continues IF YOU TAKE THIS REGION AND
BLOW IT UP YOU SEE TWO, FAINT
RED DOTS HERE.

A square of the total image appears enlarged and shows two small red circles.

He explains THAT IS A SINGLE GALAXY.
THIS HAS BEEN IMAGED INTO TWO
GALAXIES BY THE LENSING ACTION
OF THIS CLUSTER, AND THIS
GALAXY IS TOO FAR AWAY FOR
HUBBLE TO DETECT.
YET IT DETECTED IT BECAUSE THE
LENS PRODUCED BY THIS CLUSTER
OF GALAXIES HAS MADE THE GALAXY
APPEAR CLOSER AND SO IT'S
BRIGHT ENOUGH THAT HUBBLE CAN
SEE IT.
SO IT'S LIKE ADDING AN
ADDITIONAL LENS TO ALL OF THE
OPTICS OF THE HUBBLE SPACE
TELESCOPE SO IT CAN SEE FURTHER
THEN IT MIGHT HAVE OTHERWISE.
GRAVITATIONAL LENSES TODAY ARE
BEING USED IN THE SEARCH FOR
DARK MATTER, FOR TRYING TO
UNDERSTAND THE ACCELERATION OF
THE UNIVERSE OR LOOKING FOR
MINI, BLACK HOLES IN AND AROUND
OUR GALAXY.
ITS BEING USED AS NOW AN EVERY
DAY WORKING TOOL OF ASTRONOMERS
AND IT'S REALLY ANOTHER
IMPORTANT LEGACY OF EINSTEIN.
NOW LET ME TURN TO ANOTHER
IMPORTANT TEST OF GENERAL
RELATIVITY AND THIS IS ONE THAT
EINSTEIN HIMSELF WORKED ON IN
HIS 1915 PAPERS ON GENERAL
RELATIVITY.
IN THE LATE 17TH AND EARLY 18TH
CENTURY,

Clifford Will says YOU MIGHT
HAVE SAID THAT NEWTON'S THEORY
OF GRAVITY WAS ONE OF THE MOST
REMARKABLE CREATIONS OF THE
HUMAN MIND.
AFTER ALL, THIS UNIVERSAL
THEORY OF GRAVITY WAS CAPABLE
WITH ONE SIMPLE PROPOSAL TO
EXPLAIN THE FALL OF APPLES ON
EARTH

Clifford Will says AND THE
MOTION OF THE MOON AROUND THE
EARTH, AND THE MOTION OF
PLANETS AROUND THE SUN AND THE
MOTION OF SATELLITES AROUND
THEIR PLANETS AND SO ON.
IT CAN EVEN EXPLAIN THE TIDES,
ACCOUNT IN DETAIL FOR THE TIDES
OF THE EARTH.
ALL THE DETAILS OF CELESTIAL
MECHANICS COULD, COULD BE
EXPLAINED BY THIS REMARKABLE
NEWTONIAN THEORY OF GRAVITY.
BUT IN THE MIDDLE OF THE 19TH
CENTURY A PROBLEM AROSE.

A white slate appears.
It reads "Mercury’s perihelion, from trouble to triumph. 1687, Newtonian triumph; 1859 Leverrier’s conundrum; 1900, a turn of the century crisis."

He continues THE PROBLEM WAS POINTED OUT BY
JEAN JOSEPH LEVERRIER, WHO WAS
THEN THE DIRECTOR OF THE
OBSERVATOIRE DU PARIS.
LEVERRIER HAD ALREADY BECOME
FAMOUS SOME TEN TO FIFTEEN
YEARS LATER BY NOTICING THAT IF
YOU LOOK AT THE ORBIT OF
URANUS, IT UNDERGOES SOME
SLIGHT WOBBLES, SOME DEVIATIONS
THAT COULD NOT BE ACCOUNTED FOR
BY NEWTONIAN GRAVITY.
LEVERRIER AND INDEPENDENTLY
ADAMS IN ENGLAND, REALIZED THAT
THESE DEVIATIONS WOULD BE,
COULD BE PRODUCED BY AN
ADDITIONAL PLANET FURTHER OUT
WHOSE GRAVITATIONAL TUG ON
URANUS COULD PRODUCE SUCH
WOBBLES.
THEY EVEN COULD PREDICT ROUGHLY
WHERE THAT PLANET SHOULD BE
AND, AND ITS AND ITS
CHARACTERISTICS AND LATER THAT
PLANET WAS INDEED DISCOVERED BY
GERMAN ASTRONOMERS AND THAT'S
THE PLANET WE NOW CALL NEPTUNE.
SO LEVERRIER WAS ALREADY
FAMOUS, BUT HE TURNED HIS
ATTENTION TO ANOTHER ISSUE IN
THE SOLAR SYSTEM AND THAT WAS
MERCURY.

A drawing on the right hand of the screen shows the sun and a planet orbiting around it with several parallel orbits simultaneously turning and a caption next to it that reads "575 per century."

Clifford says ACCORDING TO NEWTONIAN GRAVITY
THE TYPICAL ORBIT OF ONE BODY
AROUND ANOTHER IS AN ELIPSE,
WITH THE SUN SITTING AT A FOCUS
OF THE ELIPSE AND THE SHORT
AXIS OF THE ELIPSE IS CALLED
THE PERIHELION WHERE THE PLANET
GETS CLOSEST TO THE SUN.
ACCORDING TO NEWTONIAN THEORY
THAT ELIPSE SHOULD ALWAYS
REMAIN FIXED IN SPACE.
ITS AXIS SHOULD ALWAYS BE THE
SAME, NEVER CHANGING.
BUT THAT'S NOT WHAT ASTRONOMERS
OBSERVED.
ASTRONOMERS OBSERVED THAT THE
ELIPSE WAS ROTATING VERY SLOWLY
AT A RATE OF 575 ARC SECONDS
PER CENTURY.
AGAIN, IT'S A SMALL EFFECT, BUT
MEASUREABLE AND THEY REALLY
KNEW WHAT IT WAS BY
OBSERVATION.
SO IS THIS A CATASTROPHE FOR
NEWTONIAN GRAVITY?
WELL, TO LEVERRIER OBVIOUSLY
NOT.
YET SINCE HE WAS ALREADY AN
EXPERT ON PERTEVATIONS OF
PLANETARY ORBITS BY OTHER
PLANETS HE SAID, WELL CLEARLY
THE SOLUTION OF THIS PROBLEM IS
THAT MERCURY IS BEING PERTURBED
BY THE OTHER PLANETS IN THE
SOLAR SYSTEM AND IF I WORK OUT
THOSE PERVATIONS I SHOULD BE
ABLE TO ACCOUNT FOR THIS SHIFT
IN MERCURY'S PERIHELION.
SO LEVERRIER SAT DOWN TO
CALCULATE THE CONTRIBUTIONS OF
ALL THE PLANETS TO THIS, TO
MERCURY'S ORBIT.

Two columns appear. One reads "Cause" and the second reads "Rate"
The list goes "Venus 278; etcetera."

He reads from the slide THE FIRST BIGGEST,
CONTRIBUTION, THE, THE FIRST
CONTRIBUTION COMES FROM VENUS
THE NEXT PLANET OUT AND HE GOT
278 ARC SECONDS.
IS THAT IS THE CONTRIBUTION TO
THE VENUS' PERTURBING GRAVITY
FIELD.
EARTH THE NEXT PLANET OUT
CONTRIBUTES ABOUT 90 ARC
SECONDS.
THE NEXT BIG CONTRIBUTION COMES
FROM JUPITER.
EVEN THOUGH IT'S MUCH FURTHER
AWAY IT'S A VERY, MASSIVE
PLANET SO IT PRODUCES A HEFTY
AMOUNT.
AND THEN MARS AND ALL THE OTHER
PLANETS THAT LEVERRIER KNEW IN,
IN THOSE DAYS, THERE WAS ABOUT
TEN AND IF YOU'RE ADDING THESE
THINGS UP IN YOUR HEAD YOU
DISCOVER THE PROBLEM THAT
LEVERRIER FOUND.
THE TOTAL IS 532 ARC SECONDS
PER CENTURY, SHORT BY 43 ARC
SECONDS.
DID NOT AGREE WITH THE
OBSERVATION.
NO ONE COULD ACCOUNT FOR THIS
DISCREPANCY.
PEOPLE PROPOSED IN THE SPIRIT
OF LEVERRIER ANOTHER PLANET,
ANOTHER PLANET BETWEEN MERCURY
AND THE SUN.
THEY EVEN GAVE IT A NAME.
THEY CALLED IT VULCAN AFTER
THEIR ROMAN GOD OF FIRE BECAUSE
IT WOULD HAVE TO BE A VERY, HOT
PLANET.
BUT ASTRONOMERS SEARCHED FOR
VULCAN FOR DECADES AND FOUND NO
EVIDENCE WHATSOEVER OF AN
ADDITIONAL PLANET BETWEEN
MERCURY AND THE SUN.
AND FOR THE REST OF THE 19TH
CENTURY NOBODY COULD COME UP
WITH AN EXPLANATION FOR THIS
DISCREPANCY IN THE PERIHELION
ADVANCE OF MERCURY.
AND IT WAS ONE OF THE GREAT
CRISES OF 19TH CENTURY PHYSICS,
SIMILAR IN SOME WAYS TO THE
CRISIS THAT ULTIMATELY LED TO
QUANTUM MECHANICS AND SPECIAL
RELATIVITY.
IT WAS AN UNSOLVED PROBLEM.
WELL, WHEN EINSTEIN COMPLETED
THE THEORY OF GENERAL
RELATIVITY IN NOVEMBER OF 1915,
HE REALIZED THAT HIS EQUATIONS
FOR GRAVITY, ALTHOUGH THEY HAD
NEWTON'S THEORY AS A FIRST
APPROXIMATION HAD ADDITIONAL
CORRECTION TERMS THAT HE WOULD
HAVE TO TAKE INTO ACCOUNT.
AND SO HE SAID WELL, LET'S TAKE
INTO ACCOUNT THESE ADDITIONAL
RELATIVISTIC TERMS AND SEE WHAT
THEY PRODUCE.
AND LO AND BEHOLD HE CAME UP
WITH 43 ARC SECONDS PER
CENTURY, EXACTLY WHAT WAS
NEEDED TO ACCOUNT FOR THE
DISCREPANCY.

The columns appear once more. At the end of the list more information appears.

It reads "Discrepancy, 43. Modern value, 42.98, GR prediction 42.98."

He continues HE LATER WROTE TO A FRIEND THAT
HE HAD PALPITATIONS OF THE
HEART WHEN HE FINISHED THIS
CALCULATION, HE WAS SO EXCITED.
HE COULD NOT SLEEP FOR WEEKS
BECAUSE OF THIS, THIS WONDERFUL
RESULT.
IF YOU CAN IMAGINE HAVING YOU
KNOW SOLVED WITH A SIMPLE, ONE,
FELL, SWOOP, THIS 50-YEAR OLD
PROBLEM ONE, I CAN ONLY IMAGINE
WHAT IT MUST HAVE FELT LIKE.
HE WAS BESIDE HIMSELF.

Clifford Will says WELL THE
STORY HERE ALSO DOESN'T END
WITH THAT TRIUMPH OF 1915.
WE KNOW NOW WITH MUCH HIGHER
PRECISION THE, THE
DISCREPANCY THAT HAS TO BE
EXPLAINED.
FIRST OF ALL WE KNOW MUCH,
BETTER THE ORBIT OF MERCURY,
MAINLY BY BOUNCING RADAR
SIGNALS OFF THE SURFACE OF
MERCURY.
SO WE KNOW ITS ORBIT TO VERY,
HIGH PRECISION.
FURTHERMORE WE KNOW FROM ALL
THE VARIOUS PLANETARY
SPACECRAFT THAT HAVE GONE BY
THE OTHER PLANETS, WE KNOW
THERE, MASSES MUCH BETTER THEN
THEY DID IN 1915.
WE KNOW THEIR, ORBITS MUCH
BETTER, SO THAT WE CAN
CALCULATE THESE CORRECTION
TERMS WITH EXTREMELY HIGH
PRECISION.
SO THE MODERN VALUE OF THE
DISCREPANCY THAT HAS TO BE
EXPLAINED AFTER YOU'VE
SUBTRACTED ALL THE PLANETARY
PERTEVATIONS IS 42.98, PLUS A
MINUS .04 ARC SECONDS PER
CENTURY.
SO THIS IS A PART IN A
THOUSANDTH ACCURACY IN THE
DISCREPANCY THAT HAS TO BE
EXPLAINED.
THAT'S WHAT'S OBSERVED, AND
WHAT GENERAL RELATIVITY
PREDICTS WHEN YOU PUT IN THE
NUMBERS AND KEEP ALL THE
SIGNIFICANT FIGURES EXACTLY
AGREES WITH THE OBSERVED VALUE.
SO TO A PART IN A THOUSANDTH
AGAIN IT'S A TRIUMPH FOR
GENERAL RELATIVITY.
LET ME TURN NOW TO WHETHER
SPACETIME DOES THE TWIST.
FOR THE PAST 16 MONTHS THERE'S
BEEN AN EXPERIMENT ORBITING THE
EARTH LAUNCHED BY NASA AND
STANFORD UNIVERSITY CALLED
GRAVITY PROBE B.
THE EXPERIMENT JUST ENDED
ACTUALLY A FEW WEEKS AGO.
THE IDEA OF THIS EXPERIMENT
WAS, WAS TO PUT INTO ORBIT
AROUND THE EARTH, IT'S ACTUALLY
IN A POLAR ORBIT AN ARRAY OF
GYROSCOPES.

A slide shows planet Earth with an orbit around it. It reads "Gravity Probe B" as a title.

He explains THERE ARE FOUR GYROSCOPES,
SPINNING BALLS.
AND THE IDEA IS TO MEASURE
WHETHER OR NOT THOSE, SPIN,
THOSE GYROSCOPES ALWAYS POINT
IN THE SAME DIRECTION.
THEY'RE INITIALLY ALIGNED
POINTING ALL TOWARD A GIVEN
DISTANT STAR.

A green star appears on the left hand side of the screen. It’s labelled "guide star."

He says IT'S THE STARION PEGASUS
ACTUALLY.
AND THE IDEA IS TO ASK THE
QUESTION DO THESE GYROSCOPES
POINT ALWAYS IN THAT DIRECTION
OR DO THEY SOMEHOW SHIFT A
LITTLE BIT IN THEIR DIRECTION?
ORDINARILY YOU THINK OF A
GYROSCOPE AS ALWAYS POINTING IN
THE SAME DIRECTION.
THAT'S HOW YOU USE GYROSCOPES
FOR INITIAL GUIDANCE.
THEY ALWAYS SEEM TO POINT IN
THE SAME DIRECTION.
BUT ACCORDING TO GENERAL
RELATIVITY GYROSCOPES IN A
WARPED SPACETIME CAN SHIFT A
LITTLE BIT.
THERE ARE TWO SEPARATE EFFECTS
THAT CAN CAUSE A LITTLE SHIFT
IN THE DIRECTION OF A SPINNING
GYRO.
ONE IS CALLED THE GEODETIC
EFFECT AND THAT'S SIMPLY THE
EFFECT OF PROPOGATING A
GYROSCOPE AROUND A CURVED
SPACETIME THAT SURROUNDS THE
EARTH.
YOU CAN ACTUALLY DO A SIMPLE
EXAMPLE OF THIS TAKING A GLOBE
IN YOUR, OWN LIVING ROOM, JUST
A GLOBE OF THE EARTH.
TAKE A STICK AND MOVE IT SO
IT'S ALWAYS PARALLEL TO ITSELF
ALONG THE GLOBE.
SO YOU'RE STARTING FROM THE
EQUATOR AND GOING UP TO THE
NORTH POLE.
THEN COME BACK DOWN FROM THE
NORTH POLE TO THE EQUATOR ALONG
A LINE OF LONGITUDE 90 DEGREES
DISPLACED FROM THE FIRST.
AND THEN GO BACK ALONG THE
EQUATOR TO WHERE YOU STARTED
FROM AND YOU'LL SEE THAT THE
GYRO, THAT YOUR STICK IS NOT
POINTING IN THE SAME DIRECTION
THAT IT WAS.
EVEN THOUGH YOU'VE KEPT IT
PARALLEL TO ITSELF ALL THE WAY
AROUND.
THAT CHANGE IN DIRECTION OF
SOMETHING THAT ORDINARILY IS
ALWAYS IN THE SAME DIRECTION IS
THE HALLMARK OF A CURVED SPACE
AND IS AN ANALOGOUS THING IN
THE CURVED SPACETIME
SURROUNDING THE EARTH.
THAT PRODUCES AN EFFECT OF
6600, milli ARC SECONDS PER
YEAR, 66, 6.6 ARC SECONDS PER
YEAR, A VERY TINY EFFECT BUT
THAT'S MEASUREABLE.
BUT THE SECOND EFFECT, A MUCH
SMALLER EFFECT IS WHAT THIS
MISSION REALLY WANTS TO
MEASURE.
I'VE TOLD YOU THAT A BODY LIKE
THE EARTH OR THE SUN WARPS
SPACETIME.
BUT IF THE BODY IS ALSO
ROTATING THEN IT HAS A TENDENCY
TO DRAG SPACETIME AROUND WITH
IT.
MUCH AS IF YOU TOOK A STICK IN
A, IN A VAT OF MOLASSES AND
TURNED THE STICK, THE MOLASSES
WILL START TO SWIRL AROUND THE
STICK CAUSED BY VISCOUSITY IN
THAT CASE.
BUT HERE IT'S ACTUALLY
SPACETIME THAT'S BEING SLIGHTLY
DRAGGED AROUND WITH THE EARTH
AS THE EARTH ROTATES.

He turns his hand in a circle.

He continues AND THAT DRAGGING OF SPACETIME
WILL ALSO CAUSE GYROSCOPES TO
POSSESS A SMALL AMOUNT, A MUCH
SMALLER AMOUNT, ONLY 42 MILLI-
ARC SECONDS PER YEAR.
WELL THIS GRAVITY PROBE B WAS
LAUNCHED TO MEASURE THESE TWO
EFFECTS.
ITS GOAL WAS TO MAKE A
MEASUREMENT AT THE LEVEL OF
FOUR-TENTHS OF A MILLI-ARC
SECOND PER YEAR.

A square down below reads "goal 0.4 Mas/yr; Launch April 20, 2004; Mission ends September 2005."

He explains SO IT HOPES TO MEASURE THIS
EFFECT TO ABOUT ONE PERCENT
PRECISION.
IT WAS LAUNCHED IN APRIL 2004
AND JUST ENDED.
THE EXPERIMENT EXPLOITS SUPER
CONDUCTIVITY TO HAVE VERY
SENSITIVE MEASURING DEVICES ON
BOARD, AND IT ONLY HAD SO MUCH
LIQUID HELIUM TO MAKE
EVERYTHING RUN AT SUPER,
CONDUCTOR TEMPERATURES, AND THE
LIQUID HELIUM RAN OUT AS
SCHEDULED JUST A FEW WEEKS AGO.
HERE'S WHAT THE GYROSCOPES LOOK
LIKE.

He points to a slide above him. It shows a hand holding a small sphere about the size of a walnut. The slide reads "gravity probe B, The experiment, gyroscopes."
Next to the first image, a second one shows the inner circuits.

He continues THERE'S SPHERES OF FUSED
QUARTZ.
THAT'S A HAND AND A THUMB AND A
FINGER, SO WE SEE HOW BIG THEY
ARE, THEY'RE JUST LITTLE
THINGS, COATED WITH A THIN
LAYER OF NIOBIUM WHICH BECOMES
A SUPER CONDUCTOR AT LOW
TEMPERATURES.
IN ORDER TO REDUCE UNWANTED
DRIFTS IN THE DIRECTION OF THE
GYROSCOPE PRODUCED BY GRAVITY
AND OTHER EXTRANEOUS
DISTURBANCES IT TURNS OUT THESE
GYROSCOPES HAVE TO BE SPHERICAL
AND HOMOGENEOUS IN THEIR
DENSITY TO ABOUT A PART IN
10 MILLION.
MANUFACTURING SUCH OBJECTS WAS
A MAJOR CHALLENGE FOR THIS,
THIS PROJECT AND HERE'S AN
EXAMPLE OF HOW WELL THEY DID.
THIS IS A, A DIAGRAM USING
LASERS SHOWING THE DIFFERENCE
BETWEEN THE HIGHEST MOUNTAIN ON
SUCH A GYROSCOPE AND THE
DEEPEST VALLEY.

He points to an image of two gyroscopes. They show deep blue patterns on a black background resembling islands on a sphere.

He continues THAT DIFFERENCE IS SO SMALL
THAT IF YOU TOOK ONE OF THOSE
GYROSCOPES AND BLEW IT UP TO
THE SIZE OF THE EARTH THEN THE
DIFFERENCE BETWEEN THE HIGHEST
MOUNTAIN ON THAT EARTH, SIZED

Clifford Will says GYROSCOPE AND
THE DEEPEST VALLEY WOULD BE MY
HEIGHT.
THAT'S HOW SPHERICAL THEY ARE.
BUT THAT REDUCES UNWANTED
EFFECTS THAT THEY REALLY AREN'T
INTERESTED IN,

Clifford Will says AND LEAVES
ONLY THE RELATIVISTIC DRIFTS AS
THE THINGS THEY WANT TO
MEASURE.
AND HERE'S SOME OTHER PRETTY
PICTURES OF SOME OF THE
APPARATUS.
I'M A THEORIST, SO I LIKE TO
SHOW PICTURES OF APPARATUS SO
YOU THINK THAT I ACTUALLY
UNDERSTAND WHAT THESE PEOPLE
DO.

The Audience laughs.

Clifford Will says BUT IT WILL
BE ABOUT A YEAR TO A YEAR AND A
HALF BEFORE ALL THE DATA FROM
THIS MISSION IS ANALYZED.
SO WE'LL HAVE TO WAIT FOR SOME
FUTURE TALK TO FIND OUT IF
EINSTEIN WAS RIGHT ABOUT
SPACETIME DOING THE TWIST.
NEXT LET ME TURN TO
GRAVITATIONAL WAVES, A VERY,
IMPORTANT PREDICTION OF
EINSTEIN'S GENERAL THEORY OF
RELATIVITY.
THE ONLY WAY TO TALK ABOUT
GRAVITATIONAL WAVES IS TO TRY
TO COMPARE AND CONTRAST THEM
WITH ELECTROMAGNETIC WAVES,
WITH WHICH MANY OF US ARE MORE
FAMILIAR.

He shows another slide that reads "the search for gravitational waves" as a title. Below it, another title reads "Electro magnetic waves, light."
And another title reads "Gravitational waves."

He explains SO ELECTROMAGNETIC WAVES YOU
KNOW ARE PRODUCED SAY AND
INTENDED WHEN YOU HAVE MOVING
ELECTRIC CHARGES IN SOME
VARYING CURRENT.
THOSE MOVING CHARGES THEN EMIT
AN ELECTROMAGNETIC WAVE, WHICH
IS AN UNDULATING, WAVE OF
ELECTRIC AND MAGNETIC FIELDS
THAT PROPOGATE THROUGH SPACE
WITH THE SPEED OF LIGHT.
WHEN THOSE WAVES REACH ANOTHER
ELECTROCONDUCTOR THEY CAN CAUSE
CHARGES TO MOVE.
THOSE MOVING CHARGES PRODUCE A
CURRENT.
THE CURRENT GOES DOWN AND
PRODUCES SOMETHING, SAY IN YOUR
TV SET.
THESE WERE DISCOVERED IN 1887
BY HERTZ, HAD MANY PRACTICAL AS
WELL AS IMPRACTICAL USES.

The Audience laughs.

Clifford Will says THERE ARE
SOME SIMILARITIES AND SOME
IMPORTANT DIFFERENCES BETWEEN
ELECTROMAGNETIC WAVES AND
GRAVITATIONAL WAVES.
ELECTROMAGNETIC WAVES ARE
PRODUCED BY MOTION.
SO ARE GRAVITATIONAL WAVES.
THAT'S A SIMILARITY.
BUT HERE IT'S MOVING MASSES.
SO HERE, MOVING, A DOUBLE STAR
SYSTEM WILL EMIT GRAVITATIONAL
WAVES.
A BLACK HOLE EATING A STAR WILL
PRODUCE GRAVITATIONAL WAVES.
THE COLLAPSE OF THE CORE OF THE
SUPER NOVA, YOU THINK OF A
SUPER NOVA AS AN EXPLOSION AND
MUCH OF THE, WHAT WE SEE IS
PART OF THE EXPLODING OUTER
SHELL OF THE STAR THAT'S
UNDERGONE THIS CATASTROPHE.
BUT THE INNER PART OF THE STAR
TYPICALLY IMPLODES TO PRODUCE
EITHER A NEUTRON STAR OR A
BLACK HOLE.
UNDER THE RIGHT CIRCUMSTANCES,
THAT IMPLODING CORE CAN PRODUCE
GRAVITATIONAL WAVES.
BUT UNLIKE ELECTROMAGNETIC
WAVES, WHICH ARE VARYING
ELECTRIC AND, AND MAGNETIC
FIELDS, GRAVITATIONAL WAVES ARE
RIPPLES IN SPACETIME ITSELF.
MUCH LIKE THE RIPPLES ON A
POND, ARE RIPPLES IN A TWO
DIMENSIONAL SURFACE OF A POND,
GRAVITATIONAL WAVES ARE RIPPLES
IN A FOUR DIMENSIONAL
FABRIC OF SPACETIME.
ACCORDING TO GENERAL RELATIVITY
THEY PROPOGATE WITH EXACTLY THE
SAME SPEED AS LIGHT.
THAT'S A FIRM PREDICTION THAT
COMES OUT OF THE MATHEMATICS.
AND WHEN THESE GRAVITATIONAL
WAVES IMPINGE ON SOME OBJECTS
WHERE, WHEREAS ELECTROMAGNETIC
WAVES WILL MAKE CHARGES MOVE,
GRAVITATIONAL WAVES WILL MAKE
MASSES MOVE.
BUT THEY MAKE THE MASSES MOVE
IN A PARTICULAR MANNER.
SO IF I IMAGINE A PLANE WITH
FOUR MASSES ARRAYED LIKE THIS
ON THE PLANE, TWO HERE AND TWO
HERE,

He moves his hands to either sides forming an imaginary cross.

He continues THEN GRAVITATIONALLY
COMING IN PERPENDICULAR TO THE
PLANE AS IN THIS DIAGRAM.
THEN WHEN THE GRAVITATIONAL
WAVE COMES BY AT WARPED
SPACETIME IN SUCH A WAY THAT
FIRST THESE TWO MASSES WILL
MOVE TOGETHER

He moves his hands horizontally together and vertically apart.

He says AS THESE TWO
MASSES A PART, AND THEN ON THE
NEXT HALF CYCLE THESE WILL MOVE
APART WHILE THESE MOVE BACK
TOGETHER.

He repeats the movement in opposite direction.

He continues SO IT'S A PATTERN SOMETHING
LIKE THIS.

He then shows a pattern of four dots on the slide moving one way and the other.

He says WHEN I WAS A KID WE USED TO DO
THIS WITH PAPER.
YOU KNOW YOU'D FOLD OVER THE
PAPER AND PUT DIFFERENT
COLOURS, RED, GREEN, BLUE AND
VARIOUS NUMBERS AND FORTUNES.
AND YOU'D GO LIKE THIS AND, AND
MAKE THE PAPER GO AND YOU'D
OPEN UP YOUR FORTUNE.

He moves his hands together and up, then together again and down.

He says SO THE PATTERN IS JUST LIKE
THAT, FIRST, APART THEN
TOGETHER, THEN APART AND
TOGETHER.
SO IF YOU CAN THINK BACK TO
THOSE OLD DAYS OF THOSE, THAT
LITTLE PAPER GAME YOU KNOW A
LOT ABOUT GRAVITATIONAL WAVES
ALREADY.
SO THAT'S THE PATTERN THAT IT
PRODUCES.

The Audience laughs.

Clifford Will says BUT NOW
HERE'S ONE BIG, BIG DIFFERENCE
BETWEEN GRAVITATIONAL WAVES AND
ELECTROMAGNETIC WAVES.
AND THE STRONGEST SOURCE WE
COULD IMAGINE,

A jagged yellow shape appears on the slate. It reads "Collapse or merger in our galaxy causes change in a 1 m antenna of 1 1 thousandth diameter of an atomic nucleus."

He says THE COLLAPSE OF
A CORE IN SUPER NOVA OR THE
MERGER OF TWO BLACK HOLES FOR
EXAMPLE IN OUR GALAXY, VERY
NEARBY US WOULD CAUSE A CHANGE,
IF YOU HAD TWO MASSES SEPARATED
BY ONE METRE WOULD CAUSE THEM
TO MOVE BACK AND FORTH BY A
DISTANCE OF ONE-THOUSANDTH OF
THE DIAMETER OF AN ATOMIC
NUCLEUS.
SO THIS IS A REALLY, SMALL
EFFECT, AND PRACTICAL
CONSEQUENCES YOU CAN JUST ABOUT
FORGET ABOUT IT.
SO FAR THERE'S BEEN NO DIRECT
DETECTION OF GRAVITATIONAL
WAVES.
YET WE KNOW WITH ALMOST
COMPLETE CERTAINTY THAT
GRAVITATIONAL WAVES EXIST.
AND I KNOW THEY EXIST BECAUSE
OF A REMARKABLE DISCOVERY THAT
WAS MADE AT THIS MOVIE SET.
NOW, HERE'S THE SECOND QUIZ.
HUH, WHO CAN NAME THE MOVIE SET
OR WHO CAN NAME THE MOVIES THAT
WERE OVER THE SET?

An aerial image shows a large white circular surface with three posts around it and a sphere hanging in the centre.

The audience talks in the background.

Clifford Will says SORRY.

"GOLDEN EYE."

Clifford Will says "GOLDEN EYE."
AND?

Someone says "CONTACT."

Clifford Will says "CONTACT,"
RIGHT.
WELL, WHEN IT'S NOT BEING A
MOVIE SET THIS PLACE DOUBLES AS
THE ARECIBO RADIO TELESCOPE IN
PUERTO RICO.

The Audience laughs.

Clifford Will says ONE OF THE
FINEST RADIO TELESCOPES IN THE
WORLD.
AND IN 1974 JOE TAYLOR AND
RUSSELL HULSE DISCOVERED A
REMARKABLE PULSAR.
I'VE ALREADY MENTIONED PULSARS.
THESE ARE THESE ROTATING
NEUTRON STARS, OBJECTS OF ABOUT
ONE AND A HALF, TIMES THE MASS
OF THE SUN, WITH RADIAI OF
ABOUT OH, TEN TO TWENTY
KILOMETRES, VERY CONDENSED,
COMPACT OBJECTS SPINNING ON
THEIR AXIS.

A white slide appears. It reads "The binary pulsar; gravitational waves exist!"
Below it; another title reads "Discovery, 1974; Pulse, 59 ms; orbit period 8 hours."

He says BUT THEY DISCOVERED ONE OF
THESE THINGS IN A BINARY SYSTEM
WITH A COMPANION STAR.
THE COMPANION STAR WE NOW
BELIEVE IS ALSO A NEUTRON STAR.
BUT IT'S NOT A PULSAR.
IT'S A DEAD PULSAR.
IT NO LONGER EMITS PULSE RADIO,
EMISSION BASED ON THE STILL
ALIVE PULSAR.
IT'S AN ORBITAL COMPANION WITH
AN ORBITAL PERIOD OF ABOUT
EIGHT HOURS AND IT PULSES WITH
A PERIOD OF ABOUT 59
MILLISECONDS.
BUT HERE'S WHERE THE ADVANCE OF
TECHNOLOGY COMES INTO PLAY.
WITH USE OF ATOMIC CLOCKS AND
THINGS LIKE THE GLOBAL
POSITIONING SYSTEM, TAYLOR AND
HIS COLLEAGUES HAVE BEEN ABLE
TO MEASURE THE RELEVANT
PERAMETERS OF THIS SYSTEM WITH
UNBELIEVABLE PRECISION.
SO I SAID THE PULSE PERIOD WAS
59 MILLISECONDS.
BUT THERE'S ITS ACTUAL MEASURED
VALUE,

He reads the value from the slide .05902999 BLAH-BLAH-
BLAH-BLAH, AND THE ERROR THERE
IS ON THE LAST DIGIT.
AND THERE'S THE ORBITAL PERIOD
IN SECONDS, 27,000906.-BLAH-
BLAH-BLAH, AGAIN ERROR ON THE
LAST DIGIT.
IT JUST ILLUSTRATES AGAIN HOW
ADVANCING TECHNOLOGY,
ESPECIALLY THE TECHNOLOGY OF
HIGH PRECISION ALLOWS ONE TO
MAKE VERY PRECISE MEASUREMENTS
OF THESE KINDS OF ASTRONOMICAL
QUANTITIES.
BUT WHAT DOES THIS HAVE TO DO
WITH GRAVITATIONAL WAVES?
WELL, I'VE ALREADY MENTIONED
THAT A BINARY STAR SYSTEM EMITS
GRAVITATIONAL WAVES.
THAT'S A PREDICTION OF
EINSTEIN'S THEORY OF GRAVITY.

A blue caption on the screen reads "Binary systems emit gravitational waves."

He continues THOSE WAVES CARRY ENERGY.
THAT'S ANOTHER PREDICTION.
THAT WILL TAKE ENERGY AWAY FROM
THE SYSTEM.

Another blue caption reads "Gravitational waves carry energy."

He says IN RESPONSE TO THAT LOSS OF
ENERGY BEING RADIATED AWAY, THE
BODIES WILL SPIRAL TOWARD EACH
OTHER.

Another caption reads "System loses energy, spirals inward."

He continues THEY'LL GET CLOSER AND CLOSER
TOGETHER.
THEY'LL START GOING FASTER AND
FASTER AROUND EACH OTHER.
AND SO THE ORBITAL PERIOD, THE
TIME IT TAKES TO GO AROUND ONCE
WILL DECREASE.

Another blue caption reads "Orbit period decreases at a predicted rate of 0.0000758 seconds per year."

He says AND IT TURNS OUT THAT WITH ALL
THE VARIOUS NUMBERS THAT HAVE
BEEN MEASURED YOU CAN MAKE A
FIRM PREDICTION FROM GENERAL
RELATIVITY AS TO THE RATE OF
DECREASE OF THE ORBITAL PERIOD
AND IT TURNS .000758 SECONDS
PER YEAR, OR 75.8 MICROSECONDS
PER YEAR.

A red caption reads "Observed rate; 0.0000759 minus plus seconds per year."
Next to the prediction, a blue square reads "1993 Nobel prize to Joe Taylor and Russell Hulse."

He says SO THAT'S THE PREDICTION OF
GENERAL RELATIVITY AND THE
OBSERVED VALUE IS SMACK ON,
WITHIN THE MEASUREMENT ERROR,
TWO, THE ERROR HERE IS ON THE
LAST DIGIT.
SO AGAIN THIS IS NOT ONLY A
CONFIRMATION OF THE IDEA THAT
GRAVITATIONAL WAVES EXIST,
BECAUSE THAT'S THE ONLY
PHENOMENON THAT COULD ACCOUNT
FOR SUCH A DECREASE OF THE
ORBIT PERIOD.
BUT ALSO A QUANTITATIVE
CONFIRMATION OF THE FORMULAS
FROM GENERAL RELATIVITY THAT
MADE THIS KIND OF PREDICTION IN
SUCH CLOSE AGREEMENT, NOW
BETTER THEN A FEW PERCENT,
BETTER THEN TWO TENTHS OF A
PERCENT WITH EINSTEIN'S THEORY.
WELL WITH THIS KIND OF
CONFIDENCE THAT GENERAL
RELATIVITY...

Clifford Will says THAT
GRAVITATIONAL WAVES EXIST
ASTRONOMERS ARE NOW, HAVE NOW
EMBARKED ON A MISSION TO DETECT
THEM DIRECTLY.
THIS IS AN INDIRECT DETECTION.
WE DON'T SEE THE ACTUAL WAVES.
WE SEE THE EFFECT OF THE LOSS
OF THOSE WAVES ON THE ORBIT.

Another slide appears. It reads "Interferometers around the world."
It shows a purple world map in the center of the image. Different photographs appear around the map. They are labelled "Ligo, Hanford 482 km; GEO Hannover 600m; Ligo Livingston 4 km; Virgo Cascina 3 km; tama Tokyo 300 m."

Clifford Will says WE'D REALLY
LIKE TO SEE THOSE WAVES
THEMSELVES DIRECTLY.
AND SO CURRENTLY IN OPERATION
AROUND THE WORLD IS AN, IS AN
ARRAY OF LASER INTERFEROMETER
GRAVITATIONAL WAVE DETECTORS.
THE IDEA OF THESE DETECTORS IS
TO AGAIN GOING BACK TO THE
PICTURE I SHOWED EARLIER OF
MASSES, IS TO TAKE WHEN TWO
MASSES, THE GRAVITATIONAL WAVE
COMES BY TWO MASSES MOVE BACK
AND FORTH TOWARD EACH OTHER.
IN THIS CASE THE TWO MASSES
CONSIST OF A MIRROR AT ONE END
OF A LONG EVACUATED TUBE AND
ANOTHER MIRROR HERE.
AND IN THIS SIMILAR MIRROR AT
THE END OF ANOTHER, LONG,
EVACUATED TUBE OVER HERE AND
ONE MEASURES THE RELATIVE
CHANGE IN DISTANCE BETWEEN THE
MIRRORS AND THE CENTRAL STATION
BY SENDING LASER BEAMS BACK AND
FORTH BOUNCING OFF THESE
MIRRORS.
USING LASER BEAMS AS A WAY TO
KIND OF MEASURE THE DISTANCE
BETWEEN THESE MIRRORS.
SO THIS TECHNIQUE OF LASER
INTERFEROMETRY NOW HAS BEEN
ESTABLISHED AND PUT INTO THESE
DETECTORS.
IN THE U.S. THERE ARE TWO
OBSERVATORIES.
ONE IN HANFORD, WASHINGTON,
JUST THE, THE HANFORD NUCLEAR
RESERVATION OF THE DEPARTMENT
OF ENERGY.
THESE EVACUATED TUBES THAT I
DESCRIBED ARE FOUR KILOMETRES
LONG.
SO THESE ARE EXTREMELY LONG
DEVICES.
AND THE OTHER LIGO, THE OTHER
OBSERVATORY WITHIN THE LIGO
PROJECT IS IN LIVINGSTON,
LOUISIANA, NOT FAR FROM BATON
ROUGE AND IT'S ALSO A FOUR
KILOMETRE INSTRUMENT.
AND I'M HAPPY TO SAY THAT THE
LIVINGSTON OBSERVATORY DID NOT
SUFFER MAJOR DAMAGE FROM
HURRICANE KATRINA.
THERE'S A THREE KILOMETRE
INSTRUMENT NEAR THE TOWN OF
CASCINA, JUST OUTSIDE PISA,
ITALY.
THAT'S A FRENCH-ITALIAN
PROJECT.
GEO 600, A 600 METER INSTRUMENT
IS A BRITISH, GERMAN
COLLABORATION, JUST OUTSIDE OF
HANNOVER.
AND THERE'S A 300 METER
INTERFEROMETER IN TOKYO RUN BY
THE JAPANESE.
ALL FIVE OBSERVATORIES ARE
OPERATIONAL.
THE LIGO OBSERVATORY IS ABOUT
TO ENTER AN 18-MONTH SCIENCE
RUN LATER THIS, FALL.
AND AFTER SOME UPGRADES AND
IMPROVEMENTS IN SENSITIVITY,
MANY OF US ARE CONFIDENT THAT
WITHIN THE NEXT FIVE TO TEN
YEARS THEY WILL BE DETECTING
GRAVITATIONAL WAVES FROM THINGS
LIKE COLLIDING BLACK HOLES,
SUPER NOVA, IMPLOSIONS AND THE
LIKE WITH REGULARITY.
AND AT THAT TIME A WHOLE NEW
FIELD OF ASTRONOMY WILL OPEN UP
AS WELL AS POSSIBLE NEW WAYS TO
TEST EINSTEIN'S THEORY OF
GRAVITY.

A black slate appears. It reads "Lisa; a space interferometer for 2013." The image on it shows a ring of light with a yellow triangle on it on the left hand side of the screen and two luminous rings around a bright nucleus on the left hand side of the screen.

He says FINALLY THERE'S ALSO WELL
UNDERWAY TO PUT SUCH AN
INTERFEROMETER SYSTEM INTO
SPACE.
THIS IS CALLED LISA FOR LASER
INTERFEROMETER SPACE ANTENNA,
AND HERE THIS WILL CONSIST OF
THREE SPACECRAFT ORBITING THE
SUN, EACH INDEPENDENTLY OF EACH
OTHER, NOT CONNECTED BY ANY
STRAPS BECAUSE EACH SPACECRAFT
IS 5 million KILOMETRES FROM
THE OTHER SPACECRAFT.
SO THEY'LL ORBIT THE SUN IN A
TRIANGULAR WAY, ARRAY THAT IS
SET IN JUST SUCH A WAY THAT AS
THEY GO AROUND THE SUN THEY
RETAIN THEIR TRIANGULAR SHAPE.
ALL THAT HAPPENS IS THE
TRIANGLE ROTATES AROUND AS THEY
ORBIT THE SUN.
BUT TO FIRST ORBIT THEY STILL
RETAIN AN EQUALATERAL
TRIANGULAR CONFIGURATION.
AND BY SENDING LASER BEAMS BACK
AND FORTH BETWEEN THESE
SPACECRAFT THROUGH THE VACUUM
OF SPACE THEY WILL AGAIN
MONITOR ANY RELATIVE MOTIONS OF
THE SPACECRAFT RELATIVE TO EACH
OTHER IF A PASSING
GRAVITATIONAL WAVE SHOULD COME
THROUGH THE SOLAR SYSTEM.
THAT MISSION IS SCHEDULED
TENTATIVELY FOR LAUNCH IN THE
2013 TIMEFRAME.
SO WHEN THESE SYSTEMS START
DETECTING GRAVITATIONAL WAVES
THERE WILL BE REALLY A NEW
WINDOW, THERE WILL BE A NEW
WINDOW ON THE UNIVERSE.
NOW EARLIER I MENTIONED THAT
GENERAL RELATIVITY PREDICTS
THAT SPACE IS WARPED.
BUT IT ALSO PREDICTS THAT
SPACE, THAT TIME IS WARPED.
IT REALLY PREDICTS THAT
SPACETIME IS CURVED.
SO TIME ITSELF IS ALSO WARPED
OR CURVED.

A white slide reads "Gravity’s effect on time" as a title.
Below the title it reads "NASA Smithsonian gravity probe A; 1976." It shows the Earth on the lower left hand side of the screen and a cone at the top right hand side with a label that reads "10 thousand km."
Below a caption reads "General relativity, clocks at altitude tick faster than clocks on the ground; Special relativity; moving clocks tick slower than Stationary clocks."

He says IN THE SENSE THAT A CLOCK HIGH
ON A GRAVITATIONAL FIELD, SAY
AT A HIGH ALTITUDE TICKS MORE
QUICKLY THEN A CLOCK ON THE
GROUND BECAUSE OF THE DIFFERENT
AMOUNT OF SPACE CURVATURE, TIME
CURVATURE IN THESE TWO
LOCATIONS.
SO A CLOCK AT A HIGH ALTITUDE
TICKS FASTER THEN CLOCKS ON THE
GROUND.
OF COURSE WE SHOULD KEEP IN
MIND THAT MOVING CLOCKS TICK
MORE SLOWLY THEN AN ARRAY OF
CLOCKS IN A LABORATORY.
THAT'S DUE TO SPECIAL
RELATIVITY.
SO THERE ARE REALLY TWO EFFECTS
THAT, THAT IMPINGE UPON THE
BEHAVIOUR OF CLOCKS, THE
CURVATURE OF TIME AND MOTION BY
A SPECIAL RELATIVITY.
EINSTEIN ACTUALLY PREDICTED
THIS EFFECT IN 1907 AND THEN
LATER INCORPORATED IT INTO
GENERAL RELATIVITY, BUT IT
WASN'T FIRST MEASURED UNTIL
1960 WHEN TECHNOLOGY REALLY
REACHED THE STAGE WHERE IT
COULD BE MEASURED WITH ANY SORT
OF PREDICTION.
THE BEST MEASUREMENT SO FAR WAS
DONE IN 1976, BY LAUNCHING INTO
A FLIGHT AT ABOUT 10 THOUSAND
KILOMETRES ALTITUDE, A HYDROGEN
LASER CLOCK, WHICH IN THE 1970S
WAS THE MOST STABLE KIND OF
ATOMIC CLOCK THEN KNOWN.
WE HAVE MUCH BETTER ONES TODAY
BUT THAT WAS THE BEST KNOWN
THEN.
AND THE RATE OF THAT CLOCK WAS
COMPARED WITH AN IDENTICAL
HYDROGEN LASER CLOCK ON THE
GROUND.
AND THE RESULTS OF THAT
MEASUREMENT, CAN BE MEASURED
BOTH THE COMBINATION OF THE
GENERAL RELATIVISTIC AND THE
SPECIAL RELATIVISTIC SHIFTS OF
THE CLOCKS,

Clifford Will says AGREED WITH
GENERAL RELATIVITY TO A FEW
PARTS IN A 10,000TH SO ONCE
AGAIN A VERY, HIGH PRECISION
CONFIRMATION OF EINSTEIN'S
PREDICTIONS.

Clifford Will says WELL I
PROMISED THAT I'D TELL YOU WHY
GENERAL RELATIVITY IS IMPORTANT
IN YOUR DAILY LIFE AND THIS HAS
TO DO WITH THE GLOBAL
POSITIONING SYSTEM.
SO LET ME SAY A FEW WORDS ABOUT
WHAT THIS SYSTEM IS.
IT'S A NAVIGATION SYSTEM THAT
CONSISTS OF 24 SATELLITES THAT
ORBIT THE EARTH.
EACH SATELLITE CARRIES AN
ATOMIC CLOCK ON BOARD,
TYPICALLY A CESIUM OR RUBIDIUM
CLOCK STANDARD.

Another slate appears. It reads "Relativity and daily life" as a title. Below that title; a subtitle reads "The Global positioning system GPS."
An image of intercrossing lines forming an intertwined structure over and around the Earth appears.

He explains EACH CLOCK TRANSMITS TO EARTH A
CODED SIGNAL AND THE CODE IN
THE SIGNAL TELLS YOU WHEN THE
SIGNAL WAS EMITTED BY THAT
CLOCK AS DETERMINED BY THE ON
BOARD ATOMIC CLOCK AND WHERE
THE SATELLITE WAS IN ITS ORBIT
AT THE TIME THAT SIGNAL WAS
EMITTED.
SO IF YOU ARE SITTING HERE ON
EARTH AND YOU HAVE AN ATOMIC
CLOCK BESIDE YOU, YOU CAN
MEASURE A TIME WHEN THAT SIGNAL
ARRIVES.
SO IF YOU KNOW WHEN IT WAS
EMITTED FROM THE CODED PART OF
THE SIGNAL AND YOU KNOW WHEN IT
ARRIVES AT, WHAT CAN YOU
DETERMINE?
DISTANCE, BECAUSE YOU KNOW THE
SPEED OF LIGHT.
SO KNOWING THE TIME EMITTED AND
THE TIME RECEIVED YOU KNOW THE
DISTANCE, YOU CAN DETERMINE THE
DISTANCE BETWEEN YOU AND THAT
SATELLITE.
SO FOR EXAMPLE IF YOU RECEIVED
THE SIGNAL FROM THIS SATELLITE
AND YOU DETERMINE ITS DISTANCE
YOU THEN KNOW THAT YOU MUST SIT
SOMEWHERE ON THIS ARC, GIVEN BY
THAT DISTANCE FROM THAT
SATELLITE, RIGHT?
THEN IF YOU CAN GET A SIMILAR
SIGNAL FROM A SECOND SATELLITE
YOU CAN IN THE SAME WAY
DETERMINE IF YOU'RE AT A
CERTAIN DISTANCE FROM THAT
SATELLITE SO YOU MUST SIT ON
THAT ARC.
THEREFORE YOU MUST SIT AT THE
INTERSECTION OF THOSE TWO ARCS
SO YOU LOCATE YOURSELF THERE.
IF YOU CAN DO THIS WITH A THIRD
SATELLITE, THEN YOU CAN WHERE
THE THREE ARCS INTERSECT
DETERMINE, BOTH YOUR, LOCATION
IN LATITUDE, LONGITUDE AND
ALTITUDE AND GPS IS DESIGNED TO
GIVE YOU PRECISION MEASUREMENTS
OF LATITUDE, LONGITUDE AND
ALTITUDE.
NOW LET ME SEE BY A SHOW OF
HANDS HOW MANY PEOPLE TYPICALLY
CARRY AN ATOMIC CLOCK AROUND
WITH THEM?

He raises his own hand and some people laugh.

Clifford Will says NOT THAT,
MANY.
SO WHAT DO YOU DO?
WELL, IF YOU CAN SEE A FOURTH
SATELLITE THEN YOU CAN USE THAT
FOURTH SATELLITE AS A WAY TO
DETERMINE INDIRECTLY BY SORT OF
GOING BACK THROUGH THE
MATHEMATICS WHAT YOUR TIME
WOULD HAVE BEEN.
THIS REALLY IS WHEN YOU'RE
USING GPS SOLVING YOUR LOCATION
IN SPACETIME.
YOU'RE REALLY DOING A FOUR-
DIMENSIONAL PROBLEM.
YOU HAVE FOUR UNKNOWNS, YOUR
LOCAL LOCATION, LATITUDE,
LONGITUDE AND ALTITUDE AND
LOCAL TIME.
BUT YOU'VE GIVEN FOUR PIECES OF
DATA INFORMATION FROM FOUR
SATELLITES.
THERE'S A LITTLE CHIP IN YOUR
GPS RECEIVER THAT DOES ALL THE
MATH FOR YOU,

A caption below the image reads "Relativistic effects, 39 thousand NS per day!"

He continues BUT IT WILL
DETERMINE LOCAL POSITION,
WITHIN A 15 METRE ACCURACY FOR
THE TYPICAL, COMMERCIAL
RECEIVERS AND YOU CAN ALSO
DETERMINE LOCAL TIME TO 15
NANOSECOND PRECISION.
NANOSECOND IS A BILLIONTH OF A
SECOND.
AND DID YOU GUESS THE WAY IT
WAS DESIGNED WITH THIS NUMBER
OF SATELLITES SO THAT ANY, AT
ANY MOMENT OF TIME YOU COULD
TYPICALLY SEE FOUR SATELLITES
IN THE SKY.
SO WHAT DOES THIS HAVE TO DO
WITH RELATIVITY?
WELL, I JUST EXPLAINED TO YOU
THAT CLOCKS ON SATELLITES UP
HIGH TICK MORE QUICKLY THEN
CLOCKS ON THE GROUND BECAUSE OF
THE CURVATURE OF TIME.
ON A, CONVERSELY HOWEVER THESE
OPENING CLOCKS MOVE MUCH MORE
QUICKLY THEN CLOCKS ON THE
GROUND, SO THERE MUST BE SOME
OFFSET DUE TO SPECIAL
RELATIVITY.
THOSE CLOCKS MUST BE MOVING A
BIT SLOWER.
BUT YOU'RE SAYING, WELL,
GENERAL RELATIVITY IS ALL ABOUT
VERY, TINY EFFECTS.
SURELY THESE EFFECTS ARE, ARE
VERY TINY OR NEGLIBLE.
BUT IN THIS CASE THEY ARE HUGE.
THEY'RE ENORMOUS.
THE DIFFERENCE IN RATE BETWEEN
GPS CLOCKS AND CLOCKS ON THE
SURFACE OF THE EARTH IS 39,000
NANOSECONDS PER DAY.
COMPARE THAT WITH THE 50
NANOSECOND REQUIREMENT FOR
ACCURACY FOR THE SYSTEM TO
WORK.
MOST OF THIS EFFECT IS THE
GENERAL RELATIVISTIC EFFECT,
THE CURVATURE OF TIME.
THEN THERE'S A SMALL OFFSET,
NEGATIVE EFFECT DUE TO THE
VELOCITY OF THESE SATELLITES OF
ABOUT 7 THOUSAND ARC SECONDS.
THIS IS ENORMOUS COMPARED TO
THE ACCURACY REQUIRED.
AND THE SYSTEM REQUIRES THAT
ALL CLOCKS, EARTH AND SATELLITE
CLOCKS BE KEPT IN SYNCH AND BE
KEPT AT THE SAME RATE AND ALL
THAT BE UNDERSTOOD TO THIS KIND
OF ACCURACY.
SO RELATIVITY MUST BE TAKEN
INTO ACCOUNT FOR THE GPS SYSTEM
TO FUNCTION.
QUITE DIFFERENTLY IF YOU WERE
TO TURN GPS ON NOW AND NOT TAKE
THESE RELATIVISTIC EFFECTS INTO
ACCOUNT, GPS WOULD FAIL TO
FUNCTION WITHIN A MINUTE AND A
HALF, BECAUSE THE ERRORS DUE TO
THIS WOULD BE BIGGER THEN THE
REQUIREMENT FOR THE SYSTEM TO
WORK.
SO, HERE'S A PRACTICAL, EVERY
DAY APPLICATION OF GENERAL
RELATIVITY WHEN YOU'RE LANDING
AT SOME AIRPORT IN A, IN A FOG
AND YOU KNOW THAT YOUR PLANE IS
EQUIPPED WITH A GPS TRACKING
SYSTEM YOU CAN SORT OF RECALL
THAT EINSTEIN IS LOOKING OVER
YOUR SHOULDERS HELPING YOU TO A
SAFE LANDING.
BACK IN THE 1980S THE AIR
FORCE, WHICH IN THOSE DAYS WAS
RUNNING GPS WAS ONLY, THERE
WERE ONLY SIX SATELLITES UP AT,
AT THAT TIME, THE AIR FORCE
BECAME WORRIED ABOUT WHETHER OR
NOT THEY WERE PUTTING THE
RELATIVISTIC CORRECTIONS IN
CORRECT.
SO THEY ASKED ME TO FORM A
SMALL STUDY COMMITTEE TO
EXAMINE THE QUESTION AND ALSO
TO CHECK IN, TO LOOK INTO SOME
OTHER OPERATIONAL ISSUES
RELATED TO GPS.

Clifford Will says AND SO WE DID
AND WE DID A STUDY AND
DETERMINED IN FACT VERY QUICKLY
THAT THE AIR FORCE WAS INDEED
PUTTING IN THE RELATIVISTIC
TERMS ABSOLUTELY CORRECTLY.
THEY HAD ALL THE RIGHT
FORMULAS.
THEY HAD TALKED TO THE RIGHT
PEOPLE, EVERYTHING WAS
COMPLETELY KOSHER AND IT WAS
ALL, FINE.

Clifford Will says AT THE END OF
OUR STUDY I HAD TO MAKE A FINAL
REPORT TO A BODY CALLED THE AIR
FORCE STUDIES BOARD OF THE
NATIONAL RESEARCH COUNCIL,
WHICH RUNS THESE KINDS OF
STUDIES FOR THE MILITARY AND SO
I HAD TO MAKE, GIVE A LECTURE
TO THIS GENERAL, WHO WAS THEN
IN CHARGE OF THE AIR FORCE
SYSTEMS COMMAND.

A black and white picture of a group of men dressed in suits and uniforms appears.

He continues I HAD TO EXPLAIN TO HIM A BIT
ABOUT GENERAL RELATIVITY AND
WHY IT WAS IMPORTANT IN GPS.
SO I PREPARED THE SIMPLEST
LECTURE I COULD POSSIBLY PUT
TOGETHER, WITH THE MOST
COLOURFUL FLASH AND VIEWGRAPHS
AND I JUMPED UP AND DOWN AND
WAVED MY ARMS.
AND HIS EYES CLOSED WITHIN 30
SECONDS.
BUT LUCKILY HIS, HIS LIEUTENANT
BACK, THERE, SHE TOOK FURIOUS
NOTES DURING MY TALK SO I
PRESUME LATER SHE BRIEFED HIM
ON WHAT I SAID.

The Audience laughs.

Clifford Will says BUT FOR A
GENERAL RELATIVIST TO BE
BRIEFING AN AIR FORCE GENERAL
ON GENERAL RELATIVITY BECAUSE
IT'S IMPORTANT FOR NATIONAL
SECURITY, I, I WAS IN SEVENTH
HEAVEN, BECAUSE RELATIVISTS ARE
ALWAYS LOOKING FOR SOMETHING
USEFUL FOR WHAT THEY DO AND SO
THIS SORT OF MADE MY DAY, IT
MADE MY LIFE IN SOME WAYS.
AND JUST, JUST AS A, JUST AS A
REALITY CHECK, THERE, THERE I
AM WITH MUCH LONGER HAIR THEN I
HAVE NOW.

A red circle marks Clifford in the picture.

The Audience laughs.

Clifford Will says AND, AND SO
TO SPEAK MUCH YOUNGER THEN I AM
NOW.
SO IN THIS TALK I'VE TRIED TO
GIVE YOU SOME IDEA FOR THE
IMPERICAL FOUNDATION OF GENERAL
RELATIVITY.
SOME OF THE EXPERIMENTS THAT
HAVE BEEN DONE TO TRY TO VERIFY
WHETHER THE THEORY IS THE
CORRECT THEORY TO BE USED IN
OUR ATTEMPTS TO UNDERSTAND THE
UNIVERSE.
BUT TO ME THE MOST REMARKABLE
PART ABOUT THIS STORY IS THE
FOLLOWING.
AS YOU REMEMBER EINSTEIN
DEVELOPED GENERAL RELATIVITY
NOT DRIVEN BY A DESIRE TO
EXPLAIN SOME EXPERIMENTAL FACT,
BUT RATHER DRIVEN BY CRITERIA
OF ELEGANCE AND BEAUTY AND
SIMPLICITY.
TO ME WHAT IS SO AMAZING THAT
THE THEORY THAT HE CAME UP WITH
IN 1915 TURNED OUT TO BE SO
RIGHT STILL.
THANKS VERY MUCH.

The Audience gives him a round of applause.

Watch: Clifford Will on Was Einstein Right?