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CAPTIONING IS MADE POSSIBLE BY
THE ANNENBERG/CPB PROJECT
 OSCILLATIONS CAN TRAVEL
 THROUGH A MEDIUM
 SUCH AS AIR OR WATER
 WITHOUT CARRYING MATTER
 ALONG WITH THEM.
THESE MECHANICAL DISTURBANCES
 ARE CALLED WAVES.
IN ANY ERA,
THERE ARE CERTAIN EXPERIMENTS
WHICH ARE JUST BARELY POSSIBLE.
THESE ARE EXPERIMENTS
AT THE STATE OF THE ART.
THEY'RE A TEST
OF THE SKILL AND INGENUITY
OF THE FINEST
EXPERIMENTAL PHYSICISTS.
AN EXAMPLE TODAY MIGHT BE
THE ATTEMPT
TO DETECT GRAVITY WAVES
COMING FROM DISTANT STARS.
IN THE LATE 1600s,
A STATE-OF-THE-ART EXPERIMENT
WAS TO MEASURE
THE SPEED OF SOUND.
SOUND NORMALLY COMES TO US
THROUGH THE AIR.
IF THERE WERE NO AIR HERE,
YOU WOULDN'T HEAR ME SPEAKING.
OF COURSE, YOU WOULD HAVE
MORE SERIOUS PROBLEMS.
SOUND TRAVELS VERY FAST,
BUT ITS SPEED
IS NOT INFINITE.
WE CAN HEAR ECHOES,
SOUND REFLECTING
FROM DISTANT WALLS.
WE SEE LIGHTNING
BEFORE WE HEAR THUNDER
BECAUSE LIGHT TRAVELS
MUCH FASTER THAN SOUND.
300 YEARS AGO,
ONE OF THOSE WHO TRIED
TO MEASURE THE SPEED OF SOUND
WAS ISAAC NEWTON HIMSELF.
HIS MEASUREMENT WAS NOT
THE BEST IN HIS ERA.
EXPERIMENTAL PHYSICS
WAS NOT NEWTON'S STRONG POINT.
BUT IT'S INTERESTING TO SEE
HOW HE TRIED
TO MAKE THE MEASUREMENT.
AT TRINITY COLLEGE,
THERE WAS A CORRIDOR
WITH AN ECHO.
HE TRIED TO MEASURE
THE TIME A SOUND TOOK
TO GET DOWN THE CORRIDOR
AND RETURN.
AS A TIMING DEVICE,
HE USED A SIMPLE PENDULUM.
THE TIME A PENDULUM TAKES
TO MAKE ONE FULL SWING
DEPENDS ON ITS LENGTH.
AS A SHARP NOISE WENT OFF,
THE PENDULUM WAS RELEASED.
IF THE PENDULUM RETURNED
BEFORE THE SOUND,
THE PENDULUM
WAS MADE LONGER.
IF THE SOUND
RETURNED FIRST,
THE PENDULUM
WAS MADE SHORTER.
THAT WAY,
HE COULD GET CLOSER AND CLOSER
TO THE EXACT AMOUNT OF TIME
IT TOOK THE SOUND
TO GO DOWN THE CORRIDOR
AND COME BACK,
AND THEREBY MEASURE
THE SPEED OF SOUND.
SOUND IS A WAVE,
A DISTURBANCE THAT TRAVELS
AT A DEFINITE SPEED,
LIKE THE WAVES
IN THIS MACHINE.
A GOOD POINT TO START
OUR DISCUSSION OF WAVES
IS WITH TWO HARMONIC OSCILLATORS
LIKE THESE,
WHICH ARE COUPLED TOGETHER.
WATER,
LIGHT,
SOUND,
EVEN PRESSURE--
THEY ALL TRAVEL IN WAVES.
IN FACT,
THROUGHOUT THE PHYSICAL WORLD,
WAVES ARE ONE OF THE MOST COMMON
NATURAL PHENOMENA.
AND NATURE ISN'T THE ONLY ONE
WHO CAN MAKE WAVES.
PEOPLE CAN MAKE WAVES
OF THEIR OWN.
THEY'VE BEEN DOING IT
FOR A VERY LONG TIME.
NEW YORK CITY, 1926.
A BIG WELCOME
FOR GERTRUDE EDERLE,
THE FIRST WOMAN
TO SWIM THE ENGLISH CHANNEL.
THERE'S MORE GOING ON HERE
THAN MEETS THE EYE.
THIS IS A HUMAN SHOCK WAVE,
WITH A CIRCULAR WAVE FRONT
LIKE THE MOLECULAR MOTIONS
OF A SOUND WAVE IN A GAS.
THE WAVE MOVES
FROM PERSON TO PERSON,
LEAVING A PARTIAL VACUUM
IN ITS WAKE.
THIS NONRECURRENT WAVE FRONT
WAS HARMLESS.
THIS ONE WAS NOT.
ENIWETOK, 1952.
THE FIRST HYDROGEN BOMB TEST.
THE WORLD AT LARGE
FEELS THE HEAT
OF THE FIRST NUCLEAR SHOCK WAVE
IN THE COLD WAR.
BUT THE PHENOMENON
OF SPREADING WAVES
GOES BACK CONSIDERABLY FARTHER
THAN 1952,
MUCH FARTHER BACK THAN 1926.
IT GOES BACK
TO THE VERY BEGINNING OF TIME
AND THE UNIVERSE ITSELF,
IN FACT.
THE BIG BANG.
AND FROM THE VERY BEGINNING
OF IT ALL,
A DISTURBANCE IN ONE PLACE,
NATURAL OR OTHERWISE,
INEVITABLY CAUSES A REACTION
IN ANOTHER.
AND SO IT GOES.
DISTURBED WEATHER
IN A POLAR REGION CAUSES TROUBLE
ON A TROPICAL ISLE
HALFWAY AROUND THE GLOBE.
[THUNDER]
THE CHAIN OF EARTHLY EVENTS
AND PHENOMENON,
WHICH ARE HOOKED TOGETHER
BY INFINITELY COMPLEX LINKS,
IS OFTEN BETTER SEEN
AT A DISTANCE.
BUT IT CAN BE BETTER UNDERSTOOD
CLOSE UP.
THAT'S BECAUSE NO MATTER
HOW COMPLEX THE SYSTEM APPEARS,
AN UNDERLYING PRINCIPLE
BEGINS TO EXPLAIN
ITS PHYSICAL ASPECTS.
WHEN SIMPLE MECHANICAL SYSTEMS
ARE JOINED,
A DISTURBANCE IN ONE
WILL PASS ON TO THE NEXT.
WHEN ANY STABLE
MECHANICAL SYSTEM IS DISTURBED,
NATURE'S RESPONSE
IS SIMPLE HARMONIC MOTION.
FOR A SINGLE OSCILLATOR,
THAT'S AS FAR AS IT GOES.
BUT WHEN OSCILLATORS
ARE LINKED
FROM ONE TO THE NEXT
AND TO THE NEXT,
A DISTURBANCE IN ONE
PASSES ON TO THE NEXT
AND CONTINUES ON
DOWN THE LINE.
THIS IS THE ESSENCE
OF A MECHANICAL WAVE.
SOMETIMES, INDIVIDUAL
MECHANICAL OSCILLATORS
ARE EASY TO DISCERN.
IN OTHER CASES,
THE WAVE ITSELF IS EASIER TO SEE
THAN THE INDIVIDUAL OSCILLATORS.
PERHAPS MORE SURPRISING,
WAVES PROPAGATE NOT ONLY
ALONG THE SURFACE OF WATER,
BUT EVEN THROUGH THE INTERIOR
OF A CRYSTALLINE SOLID.
A DISTURBANCE'S SPEED
DEPENDS ON THE MEDIUM
THROUGH WHICH IT PASSES
AND ON THE CONNECTION
BETWEEN ONE BIT OF MATTER
AND THE NEXT.
IF THE LINKAGE IS WEAK,
THE DISTURBANCE PASSES SLOWLY.
IF IT'S STRONG,
THE DISTURBANCE TRAVELS RAPIDLY.
BUT NO MATTER
THE SPEED OR THE MEDIUM--
WATER, AIR,
OR EVEN A SOLID--
ALL WAVES THAT PROPAGATE
THROUGH ANY MEDIUM
ARE CALLED
MECHANICAL WAVES.
MECHANICAL WAVES,
OR IMPULSES,
PASS THROUGH A CRYSTAL
FROM ATOM TO ATOM
BECAUSE EACH ATOM IS BOUND
TO AN EQUILIBRIUM POSITION
BY ELECTRICAL FORCES.
WHEN DISTURBED,
THEY ACT MECHANICALLY,
EXACTLY LIKE MASSES
CONNECTED BY SPRINGS.
WHEN AN IMPULSE
MOVES THROUGH A SYSTEM,
NO SINGLE OSCILLATOR
GOES VERY FAR,
BUT THE DISTURBANCE
MOVES RIGHT ALONG.
[ORCHESTRA PLAYS]
SOMETIMES,
MECHANICAL OSCILLATIONS
CAN BE MORE MOVING
THAN DISTURBING...
CAN BE ART
AS WELL AS SCIENCE,
AND IT CAN BE ENJOYED
AS BOTH.
MUSICAL INSTRUMENTS
SEND CONTINUOUS WAVES
THROUGH THE AIR
THAT CAN BE VISUALIZED
AS WELL AS HEARD.
MASSES CONNECTED BY SPRINGS
MAY NOT LOOK
LIKE BEAUTIFUL MUSIC,
BUT THE PRINCIPLE
IS THE SAME.
IT CAN BE DESCRIBED
USING A FEW TERMS.
EACH WAVE HAS AN AMPLITUDE,
THE SIZE OF THE DISTURBANCE,
WHICH IS PRESERVED
AS THE WAVE MOVES ALONG.
IT ALSO HAS A DEFINITE TIME
FOR EACH COMPLETE CYCLE,
CALLED THE PERIOD.
THE INVERSE OF THE PERIOD
IS CALLED THE FREQUENCY.
THE TONE OF MUSIC
DEPENDS ON ITS FREQUENCY.
THE HIGHER THE FREQUENCY,
THE HIGHER THE TONE
OR PITCH OF THE SOUND.
AND THE LOUDNESS OF MUSIC
DEPENDS ON THE AMPLITUDE.
BUT NO MATTER HOW LOUD,
SOUNDS ARE ALWAYS
ON THE MOVE.
AND REGARDLESS OF PITCH
OR LOUDNESS,
ALL SOUND TRAVELS
THROUGH THE AIR
AT EXACTLY THE SAME SPEED.
IF IT DIDN'T,
EVERY LISTENER WOULD HEAR
A DIFFERENT PERFORMANCE.
WHETHER IT'S SHORT AND SWEET
OR LOUD AND LONG,
EVERY WAVE
HAS A DEFINITE DISTANCE
FROM ONE COMPRESSION
TO THE NEXT
CALLED THE WAVELENGTH.
THE WAVELENGTH
= THE PERIOD
TIMES THE SPEED
OF THE WAVE.
OR, IN OTHER WORDS,
FREQUENCY TIMES WAVELENGTH
= SPEED.
LOWER FREQUENCY GENERATES
A LONGER WAVELENGTH,
BUT THE SPEED
REMAINS THE SAME.
FOR SOUND WAVES IN AIR,
THE SPEED IS
ALWAYS THE SAME,
REGARDLESS OF THE FREQUENCY,
THE WAVELENGTH,
OR EVEN THE AMPLITUDE.
BUT IN THIS WORLD,
ALL WAVES DON'T GET
AN EVEN BREAK.
SOUND AND WATER WAVES TRAVEL
AT DIFFERENT SPEEDS.
AND UNLIKE SOUND WAVES,
WATER WAVES CAN TRAVEL
AT SPEEDS DIFFERENT
FROM EACH OTHER.
FAR AWAY FROM A CONTINENT,
WAY OUT THERE IN THE SEA,
LONG WAVES TRAVEL
FASTER THAN SHORT ONES.
BUT CLOSER TO THE SHORE,
WAVES TRAVEL TOGETHER.
AND NO MATTER
THEIR LENGTH,
ALL WAVES
DECREASE IN SPEED
AS THEY APPROACH
THE SHORE.
IN THE MECHANICS OF WAVES,
THAT'S THE LONG
AND THE SHORT OF IT.
BECAUSE ON LAND OR AT SEA--
IN FACT, ANYWHERE AT ALL--
ALL MECHANICAL WAVES FOLLOW
THE SAME BASIC PRINCIPLES.
WHEREVER ANYTHING MAKES WAVES,
HARMONIC OSCILLATORS RESPOND
AND THEN SPRING BACK.
THE OSCILLATORS ARE LINKED
IN SUCH A WAY
THAT EVERY CYCLE OF ONE
EXCITES THE OSCILLATOR
BESIDE IT.
BUT WHAT DETERMINES
HOW FAST THE WAVE MOVES?
 FOR MASSES
 CONNECTED BY SPRINGS,
 THE SPEED OF THE WAVE
 DEPENDS ON THE STIFFNESS
 OF THE SPRING,
THE MASS OF EACH OSCILLATOR,
AND THE EQUILIBRIUM DISTANCE
 BETWEEN THEM.
 IN WATER,
 GRAVITY IS THE FORCE
 THAT MAKES THE WATER
 SPRING BACK.
 THAT'S WHY GRAVITY
 DETERMINES THE SPEED
 AT WHICH
 A WATER WAVE TRAVELS.
 SO DOES THE LENGTH
 OF THE WAVE ITSELF.
IF THE WATER IS MUCH DEEPER
 THAN THE WAVELENGTH,
 THE WAVE SPEED
 IS APPROXIMATELY EQUAL TO
 THE SQUARE ROOT OF g,
THE ACCELERATION OF GRAVITY,
 TIMES THE WAVELENGTH,
 DIVIDED BY 2 PI.
IN OTHER WORDS,
LONG OCEAN WAVES MOVE RAPIDLY,
WHILE SHORT RIPPLES
MOVE MORE SLOWLY.
THE RESULT IS
THAT IN DEEP WATER,
LONG WAVES PASS RIGHT UNDER
THE SURFACE RIPPLES.
AND WHETHER THEY'RE
LONG WAVES OR RIPPLES,
WATER WAVES DIFFER
FROM SIMPLE MECHANICAL ONES
IN SOME OTHER WAYS.
FOR EXAMPLE, MASSES
AND SPRINGS CAN OSCILLATE
ALONG THE DIRECTION
THAT CONNECTS THEM.
THESE ARE CALLED
LONGITUDINAL WAVES.
AND THEY CAN ALSO BE MADE
TO WAVE SIDEWAYS.
THESE ARE CALLED
TRANSVERSE WAVES.
BUT WATER WAVES ARE NEITHER
LONGITUDINAL NOR TRANSVERSE.
INSTEAD, EACH BIT OF WATER
ON THE SURFACE
MOVES AROUND
IN A LITTLE CIRCLE,
EACH CIRCLE SLIGHTLY OFFSET
FROM THE NEXT,
ALTOGETHER, GIVING
THE FAMILIAR UNDULATION
OF THE WATERY SURFACE.
AS A WATER WAVE
APPROACHES THE SHORE,
IT COMES INTO EVER-INCREASING
CONTACT WITH THE SEABED,
WHICH SLOWS IT DOWN.
THE CLOSER THE SURFACE IS
TO THE BOTTOM,
THE SLOWER THE WAVE.
IN SHALLOW WATER,
THE WAVE SPEED
IS APPROXIMATELY EQUAL TO
TO THE SQUARE ROOT
OF THE ACCELERATION OF GRAVITY
TIMES THE DEPTH OF THE WATER.
WHEN THIS HAPPENS,
THE THICK PART OF THE WAVE
MOVES FASTER
THAN THE THIN PART,
SPOILING ITS SINE-WAVE FORM,
AND ULTIMATELY
CAUSING THE WAVE TO BREAK.
SOUND WAVES CAN BE HEARD
BUT NOT SEEN.
WAVES OF SOUND
ARE GENERATED
BY SOMETHING
VIBRATING THE AIR.
A VIBRATING OBJECT
SETS THE AIR NEXT TO IT
INTO MOTION,
COMPRESSING AND EXPANDING
ITS DENSITY
WITH EACH VIBRATION.
A GENERATED SOUND WAVE
CARRIES THE SAME FREQUENCY
AS ITS SOURCE.
THE FORCE
THAT DRIVES A SOUND WAVE
COMES FROM PRESSURE CHANGE
AS AIR DENSITY
INCREASES OR DECREASES.
 JUST AS THE SPEED
 OF WATER WAVES
 DEPENDS ON GRAVITY,
 THE SPEED OF SOUND
 DEPENDS ON
 THE PRESSURE
 AND DENSITY OF AIR.
 IN AIR, THE SPEED OF SOUND
 IS APPROXIMATELY EQUAL TO
 THE SQUARE ROOT
 OF THE PRESSURE
 DIVIDED BY THE DENSITY
 OF THE AIR.
AROUND THE WORLD,
IN THE MOST GENTLE SETTING,
OR THE MOST VIOLENT,
COUNTLESS BITS OF MATTER
VIBRATE IN UNISON.
AND ACCORDING TO THE PRINCIPLES
OF HARMONIC MOTION,
THEY CREATE THE PHENOMENON
OF THE MECHANICAL WAVE.
NOW, WE UNDERSTAND WHY
ISAAC NEWTON
WAS PLAYING WITH A PENDULUM
IN A CORRIDOR
AT TRINITY COLLEGE.
HE HAD A THEORY
OF THE SPEED OF SOUND
AND DECIDED IT SHOULD EQUAL
THE SQUARE ROOT
OF THE ATMOSPHERIC PRESSURE
DIVIDED BY AIR DENSITY.
WHEN HE PUT IN THOSE VALUES
TO FIND OUT HOW BIG
IT SHOULD BE,
THE RESULT WAS
979 ENGLISH FEET PER SECOND.
NEWTON WAS TRYING
TO MEASURE THE SPEED
IN ORDER TO SEE
IF HIS THEORY WAS RIGHT.
NOW, NEWTON'S MEASUREMENT
WAS NOT THE VERY BEST.
THE BEST OF THAT TIME
WAS MADE BY WILLIAM DERHAM
WHO GOT A RESULT
OF 1,142 FEET PER SECOND.
AT THIS POINT,
NEWTON HAD TO DO
WHAT EVERY SCIENTIST
MUST DO.
HE HAD TO MAKE
A VALUE JUDGMENT.
HIS THEORY
PREDICTED THIS NUMBER.
THE EXPERIMENT
MEASURED THIS RESULT.
THE QUESTION IS,
"IS THAT GOOD OR BAD?
"IS THE AGREEMENT
OR DISAGREEMENT
BETWEEN THESE NUMBERS
SATISFACTORY OR UNSATISFACTORY?"
WE KNOW IN RETROSPECT,
THAT WHAT NEWTON HAD DONE
WAS AN ABSOLUTELY STUNNING
INTELLECTUAL ACCOMPLISHMENT.
BEFORE NEWTON,
NOBODY HAD THE FOGGIEST NOTION
OF WHAT THE SPEED OF SOUND
SHOULD BE.
NEWTON CAME ALONG,
HAD THE RIGHT IDEA
OF WHAT SOUND WAS,
AND CALCULATED A SPEED
THAT WAS CORRECT TO WITHIN 20%.
SO HE SHOULD HAVE BEEN
SATISFIED.
NEWTON CONSIDERED
THIS SITUATION,
AND HE FOUND IT TOTALLY,
COMPLETELY, UNACCEPTABLE
BECAUSE THE ENTIRE THRUST
OF THE SCIENTIFIC REVOLUTION
FROM THE TIME OF COPERNICUS ON,
WAS TO CAST MAGIC
AND THE OCCULT OUT OF SCIENCE.
NEWTON HAD INTRODUCED
HIS THEORY OF GRAVITATION
IN WHICH INVISIBLE FORCES
ACTED BETWEEN BODIES
AT VAST DISTANCES
WITH NOTHING IN BETWEEN.
THAT SMELLED TO NEWTON
AND HIS CONTEMPORARIES
LIKE MAGIC.
NEWTON'S DEFENSE
OF HIS THEORY WAS
THAT IT WAS RIGHT
BECAUSE IT WORKED.
IT GAVE PRECISELY CORRECT
NUMERICAL PREDICTIONS.
THAT IS THE TRUE TEST
OF THE CORRECTNESS
OF A SCIENTIFIC IDEA.
IF PRECISE AGREEMENT
IS THE TRUE TEST
OF HIS THEORY OF GRAVITY,
IT HAD TO BE
THE TRUE TEST
OF HIS THEORY
OF THE SPEED OF SOUND
WHICH WAS WRONG BY 20%.
NEWTON SET OUT
TO MAKE HIS THEORY CORRECT.
WHAT HE DID
HAS A SPECIAL NAME.
IT'S CALLED FUDGING.
NO SCIENTIST EVER DOES IT.
AND THIS IS FUDGING
BY THE GREAT SIR ISAAC NEWTON
IN THE ULTIMATE
CLASSIC OF SCIENCE,
THE PRINCIPIA ITSELF.
I'LL TELL YOU WHAT HE DID.
LISTEN CAREFULLY.
HE SAID, "SOUND TRAVELS
979 FEET EACH SECOND,
"NOT THROUGH THE AIR,
BUT THROUGH THE SPACE
BETWEEN THE MOLECULES OF AIR."
IT ACTUALLY GOES FURTHER
IN EACH SECOND
BECAUSE THE MOLECULES THEMSELVES
TAKE UP SPACE.
HOW MUCH DO THEY TAKE UP?
HE SAID, "WE KNOW
THAT THE DENSITY OF AIR
IS 470 TIMES LESS
THAN THE DENSITY OF WATER."
IF WE IMAGINE A VOLUME
LIKE THIS CONTAINING AIR,
THAT AIR COULD BE CONDENSED
INTO 1/470 OF THAT VOLUME.
THEN THE LINEAR DISTANCE
IT TOOK UP
WOULD BE 1/9
OF THE ORIGINAL LINEAR DISTANCE.
THE EXTRA DISTANCE
THE SOUND TRAVELS IN 1 SECOND
IS 979 DIVIDED BY 9,
WHICH IS AN EXTRA 108.
IF HE ADDS THAT TO THIS,
WE GET 1,088 FEET PER SECOND.
THAT'S CLOSER,
BUT NOT GOOD ENOUGH FOR NEWTON.
HE SAYS, "WELL, AIR
IS 10% WATER VAPOR."
HE DOESN'T SAY
HOW HE KNOWS THAT.
THE WATER DOESN'T PARTICIPATE
IN THIS PROCESS.
HE DOESN'T SAY WHY.
WE SHOULD HAVE DIVIDED BY
THE SQUARE ROOT
OF 9/10
OF THE DENSITY OF AIR.
THE OTHER 1/10 IS WATER.
WE MUST ADD TO THIS
1,088 DIVIDED BY
THE SQUARE ROOT OF 0.9.
THAT'S AN ADDITIONAL 5%,
ABOUT 55 FEET.
IF HE ADDS 55 TO THIS,
HE GETS 1,143 FEET PER SECOND,
COMPARED TO
1,142 FEET PER SECOND.
AND NOW,
NEWTON WAS SATISFIED.
THAT'S HOW SIR ISAAC NEWTON
MADE SCIENCE AND REASON
TRIUMPH OVER
MAGIC AND THE OCCULT.
I'LL SEE YOU NEXT TIME.
 THE REAL REASON
 FOR THE DISCREPANCY
 BETWEEN
 NEWTON'S CALCULATION
 AND THE MEASURED
 SPEED OF SOUND
 IS THAT AIR IS HEATED
 WHEN IT'S COMPRESSED
 CAUSING IT TO SPRING BACK
 SLIGHTLY FASTER.
 THIS SUBTLE EFFECT
 WASN'T DISCOVERED
 UNTIL A CENTURY
 AFTER NEWTON'S DEATH.
CAPTIONING PERFORMED BY
THE NATIONAL CAPTIONING
INSTITUTE, INC.
CAPTIONS COPYRIGHT 1985
CALIFORNIA INSTITUTE
OF TECHNOLOGY,
THE CORPORATION FOR
COUNITY COLLEGE TELEVISION,
AND THE ANNENBERG/CPB PROJECT
Annenberg Media
♪
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about this
and other Annenberg
Media programs
call 1-800-LEARNER
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♪
CAPTIONING IS MADE POSSIBLE BY
THE ANNENBERG/CPB PROJECT
