Today, I'll talk about perhaps the most famous experiment in the history of physics.

It's called the Michelson-Morley experiment.

It was designed to detect the motion of the earth 
through the Luminiferous ether.
the ether was an invention of 19th-century physicists
to explain how light could be transmitted
through empty space between the sun and the earth.

physics has its folklore and its legends, just like any other culture.

For example, the story about Isaac and the apple.

It goes like this--

Michelson and Morley did their experiment and proved there was no ether.

So Albert Einstein was forced to invent the theory of relativity in order to explain that result.

The experiment was done at what was then called the case institute of technology
in Cleveland, Ohio, In1887.

For that experiment to have been done in Cleveland in 1887 was something like the first real theory of electricity
coming out of Philadelphia more than a century before.

In the 1880s, The United States was still the scientific wilderness --and espeially Cleveland, Ohio.

It's just possible that the Michelson-Morley experiment was the most important thing to happen in Cleveland until satchel Paige pitched for the Indians.

The most remarkable thing about this whole tale may be the fact that Albert Michelson, for the remaining 50 years of his life,
considered his great experiment to have been a failure.

Today, we'll see why that happened. 

1887, the time of Victoria,
queen of England and empress of India.

London is the world capital of commerce.

Across the channel, in Germany,
a child named Albert Einstein celebrates his 8th birthday.

It's been 23 years since James Clerk Maxwell stated his equations for electromagnetic radiation.

Beyond the Atlantic, the Americans have enjoyed more than two decades of peace and prosperity since their terrible civil war ended.

Waves of migrants from Europe are swelling the population of the United States.

The western frontier is being settled-- and settling down.
Today, 1887 seems like a time of tranquility, not only in commerce and politics, but also in science.

Many physicists believed that all the great discoveries had been made-- physics had reached a state of perfection that was positively ethereal.

In fact, the compact of the ether was one of its central tenets.

And in 1887, in Cleveland, Ohio, two physicists-- Albert Abraham Michelson and

Edward W. Morley, were preparing an experiment designed to prove that the ether really existed,
at least in the minds of philosophers, for thousands of years.

Today, we can readily accept the idea of space as a vacuum,
vast stretches of the cosmos virtually devoid of matter.

But to the ancients, the concept of a perfect void was impossible to grasp.

Space had to be filled with something, they thought.

The solution that Aristotle devised was the ether.

That was the substance that filled what would otherwise be empty space.

By the 19th century,
the ether had come to serve more than a merely philosophical need.

The ether was the medium through which the sun's light waves would propagate to nourish and illuminate the earth.

Waves can propagate along the surface of water or through a crystal or through the air of a concert hall.

But any wave is a disturbance that passes through a medium from one part to another,
on down the lane.

In other words, whenever there's a wave, something must be waving.

But when light waves from the sun reach the earth through apparently empty space, what's waving ?

In the 19th century, the answer was as clear as space itself.

It was the ether.

To 19th-century physicists, the earth was not only real, it had physical properties that could be deduced by observing its behavior.

For example, the speed of any wave depends on the stiffness of the medium through which it passes.

And, of course, the speed of light is enormous.

So the ether must be very stiff indeed.

In fact, physicists said, it was nearly impossible to compress it.

On the other hand, the earth and other planets moved easily through the incompressible ether,
obeying Newton's laws as if they weren't passing through any medium at all.

If the ether behaved like a viscous fluid,
the orbiting planets would gradually lose energy and spiral inward towards the sun.

Since that didn't seem to be happening, physicists were able to reach another definite conclusion about the ether.

The ether, they said, was a perfectly mobile fluid with no viscosity at all.

it was a transparent, perfectly Nonviscous incompressible fluid that filled all of space.

Knowing so much about it, the only job that remained was to demonstrate its existence directly in a clear and irrefutable experiment.

Somehow that job had eluded the grasp of physicists throughout the century.

But in 1887, Albert Michelson, with the help of his friend Edward Morley, 
had devised an experiment that couldn't fail.

As far as academic credentials go,
Michelson was unquestionably qualified--
four years at Annapolis, eight at Cleveland's case school of applied science,
and three more years at Clark University in Worcester, Massachusetts.

But it would take more than teaching excellence to defect the absolute motion of the earth through space,
which, in effect, would detect the ether itself.

It would take an extraordinary experiment with an unheard of degree of accuracy.

Fortunately, Michelson had an established talent for designing instruments that could make the most precise measurements.

His earlier measurements of the speed of light were better than any before him.

When he was in Berlin,
Michelson designed a new instrument that was exquisitely precise.

It was called an interferometer.

Shown here using a laser,
a Michelson interferometer begins by splitting a beam of light into two beams.

A partially transparent and partially reflecting mirror sends two beams traveling on perpendicular paths.

Each is reflected back to the point where they had been split in two.
There, they combine into a single beam again.

Michelson's interferometer can be seen as a race between two lights beams.

If the race ends in a tie,
the result is a bright spot at the center of the interference pattern.

But in Michelson”Ēs time, they believed it shouldn”Ēt even be a horse race. 

That”Ēs because the two beams of light were racing on a moving track.

Everyone suspected the earth
And therefore the interferometer
Were moving through the ether.

The beam moving sideways to the earth”Ēs motion has less distance to go one way,
But more the other, on their round trips,
the beam along the direction of motion has a little farther to go.

So the sideways beam should always win.

Michelson knew that the expected difference in the arrival of the two beams would be very small.

After all, the speed of light is 3X10 to the 8th meters per second, 
while earth”Ēs speed through the ether is only 3X10 to the 4th meters per second.
Since the two beams went forward and back before being compared, 
their time difference would depend on the square of that ratio.

It would be a difference would of only 1 part in 100 million.

Ironically, that tiny difference in travel time would be the exact proof Galileo desperately needed three centuries earlier

The absolute, concrete proof he needed to show that Copernicus had been right, 
And that the earth really moves around the sun.

Perhaps Galileo could have won his cast.

     If he'd had
     real scientific proof
   
     That the earth moves.

     Althogh it's doubtful  
     that he had GALILEO in mind,

     that tiny difference 
     in the travel time 
    
     of the two beams of light 
     
     was what michelson wanted 
     to measure in berlin in 1880.

     A german 
     instrument-making firm

     built the first 
     interferometer.

     In 1881,when Michelson 
     went to Potsdam,

     he took it with him.

    It was sensitive all rihgt.

    So sensitive 
    that horse-drawn wagons,

    even pedestrians on 
    the street outside his rab,

    affected his measurements.

    Each time he tried,
    
    it took almost a day
    to get a single measurement.

    So when Michelson saw 
    no "Ether effect" 

    in his mesurements, 
    he hasn't concerned

    he thought,correctly,

    that his instruement
    wasn't dependable enough.

    Time passed,
    and Michelson did no more 

    with the interferometer.

    he returned to 
    the United States

    and took a position with
    case institute in cleveland.

    During that period,
    he and Edward W. morley,

    a physicist and chemist
    at Western reserve university,

    became colleagues
    and close friends.

    Other physicists,meanwhile,

    urged him to try again 
    to detect the ether.

    So in 1887,Michelson
    asked his friend morley 

    to work with him
 
    on a new and highly sesitive
    interferometer.

    The new interferometer
    was 10 times more sensitive

    than his 1880 instrument.

    Michelson had been 
    thinking of ways

    to make

    the interferometer steady

    and not suscepitable 
    to passing carts and peaple.

    the ,irrors rested on 
    an enormous base of sandsotne

    that floated in 
    a pool of merucury.

    When Michelson and morley
    tested the new interferometer,

    they were thrilled 
    and delighted.

    the instrument had all 
    the stability and sensitivity

    they had hoped for 
    and should be able,easly,

    to detect the tiny expected
    difference in travel time

    of the two beans of light

    due to motion
    through the ether.

    the actual experiment  
    was to be done

    by observing
    the interference pattern

    with the interferometer 
    in one orienation
  
    with respect to the motion
    through the ether...

    and then roating it.

    If the two beans arrived
    in a tie,for example,

    constructive interference

    would make a bright spot 
   
    at the center of the pattern.

    but if one beam arrived
    slightly ahead of the other,

    destructive interference
    could make the center dark.

    the effect of motion
    through the ether

    

    would be seen in the shift
    of the interference frings 

    as the instrument was rotated.

    To the compleate surprise
    of Michelson and Morley

    and just about everyone else,

    there was no shift at all
    in the fringes,

    which means there was no delay.

    Nothing.
    No difference at all.

    No matter how they varied 
    the experiment,

    summer or winter,
    night or day,

    the resurt was the same.

    Light apparently travels
    at the same speed
  
    in all directions,
    
    in compleate disagreement
    with the idea of earth

    moving trough the ether

    for the next 50 years,
    numerous physicists repeated
  
    the Michelson-Morley experiment

    with ever-increasing
    precision and sophistication

    in the vain hope 
    of finding some mistake.

    But even in1887,
    the facts were clear.

    The experiment had failed 
    to detect the ether 

    and the consequences
    were shattering.

    The negative result

    was almost as disturbing
    for Michelson

@@as it was 
    for physics as a whole.

    He truly believed
    the ether existed,

    and that he and Morley
    had simply failed to detect it.

    Sustained by a legendary
    sense of humor,

    Michelson carried on 
    through the years,

    and he continued
    to succeed 
 
    as a virtuoso
    of experimental physics.

    nonetheless,he remained
    haunted by the results- -
 
    or the lack of them--

    in his experiment 
    with Edward Morley.

    Putting the idea of the ether 
    in historical perspective,
 
    it had been important
    beyond measure.

    In 1887,its existence 
    was as important

    as in Copernicus day 
    the suns location.

    No experiment 
    had been better conceived

    nor more carefully executed.

    In science,when such 
    an experiment fails,

    it not only creates
    a theoretical dilemma,

    it can provoke
    fantastic responses

    in the scientific community.

    In 1892,the irish phycist
    G.F.fitzgerald

    suggested that the size
    of one arm

    of the Michelson and Morley
    interferometer

    might have somehow contracted.

    how mush it contracted

    he said

    depended on its velocity 

    to most physicists,

    fitzgerald's contraction 
    sounded absurd.

    but not to europe's 
    most distinguished physicist,
  
    h.a. Lopentz

    he worked out 
    a quantitative model

    to explain the phenomenon 
 
    and in the process, developed
    mathematical expressions

    that would change 
    the face of physics.

    they came to be called
    "the Lorentz transformation."

    In 1899, the great French 
    mathematician Henri Poincare

    examined the results obtained
    by Michelson and Morley.

    He offered 
    a general explanation,

    calling it the principle 
    of relativity.

    It was the idea 
    that absolute motion 
   
    will never be detected
    in the labolatry.

    "there most arise

    an entirely new kind
    of dynamics," he said.

    Poincare was right.
   
    an entirely new kind 
    of dynamics would arise,

    but the principle 
    of relativity,

    which would be its cornerstone,
    was far from new.

    It went back at least as far 
    as Galileo Galilei.

    Gallileo had said 
    that an object in motion

    would tend to remain

    in that state 
    of uniform motion.

    That was the law of intertia.

    It was correct because, 
    
    in its own frame
    of reference,
 
    every state of uniform motion
    was a state of rest.

    Galileo understood
    the law of intertia

    and why it was true.

    There is no difference at all

    between being at rest 
    
    and being 
    in uniform motion
   
    but on the other hand 
  
    Albert Michelson,
    in his experiment,

    had set out to detect

    the absolute motion 
    of the earth.

    Had he succeeded,
    he would have proved 


    that there is a difference 
    between motion and rest 

    a difference that can be 
    detected in an experiment.

    The truth is that, euther way,
    success or failire,

    the results of 
    Michelson's experiment 
  
    would have faced physics 
    with a grave dilemma.

    As it turned out, the dilemma
    was almost beyond imagining--

    even to the imagination 
    of Hendric Lorentz.

    Of course, we can image
    Lorentz, if we choose

    as a pitcher 
    for the Brooklyn Dodgers.

    Suppose he had been capable 
    of launching a fastball

    at 6/10 the speed of light 

    while Rolling past Albert

    at the same speed.

    Surely, fast as 
    Henly's pitch had been,
   
    it would seem  
    much faster to Albert.

    but--and here's what 

    the Michelson-Morley experiment
    said--

    if both Albert and Henry
    wathed the same light beam,

    each would think
    it was traveling

    at the speed of light 
    relative to himself,

    even though they were moving 
    relative to each other.

    Such mind-boggling consequences 
    were hard to accept,

    even for a mind such as
    that of Hendrik Lorentz.

    He worked out 
    a mathematical theory

    that accounted for 
    the experiment,

    but it was based on
   
    the mysterious,
    unverified properties

    of a newly discovered particle
    called the electron. 
   
    It took a younger man,
    a fresh point of view,

    to see the speed of light 
    in an entirely new light.

    His name was Albert Einstein.

    In physics, his was
    the way of the future.

    But no one had done more

    to illuminate that future
    than Albert Michelson himself.

    For nearly 50 years
    after his first interferometer.

    he live on,
    measureing the speed of light
  
    with ever increasing precision,
  
    measuring a star's diameter
    for the first time ever,
  
    and even admitting, finally,
 
    that his interferometer 
    experiment

    had verified Albert Einstein's 
    theory of relativity.

    ...The measurement of 
    the velocity of light.

    the secound, a measurement
    of the diameter of a star,

    and the third was a test

    of the Einstein theory
    of relativity.

    Michelson won the respect 
    of the scientific community.

    the friendship
    of Albert Einstein,

    and became 
    the first american

    to win the Nobel prize.

    But to the end of his days,
    
    he never fully accepted

    the implications
    of his famous experiment.

    "It must be admited,"
    he confessed in 1931,

    "These experiments
    are not suffiicient

    to justfy the hypoyhesis
    of an ether."

    "But then," he wondered,

    "how can the negative results
    be explained?"

    on the surface,

    like unintentional shadows
    on a canvas,

    there are dark areas

    in the life of 
    Albert Abraham Michelson.

    For despite
    all the bright spot

    in a career of 50 years,

    he tended to focus on
    his so-called failure of 1887.

    "since the results of 
    the original experiment

    was negative,"he concluded,

    "The problem is still 
    demanding a solution."

    But in reality,
    Michelson's experiment,

    if it was a failure,
   
    was most brilliant failure
    in all the history of science.

    And the solution he thought
    but refused to accept--

    Albert Einstein's 
    theory of relativity--

    would change forever the very 
    meaning of space and time.

    We often teach physics
    in a historical way.

    the purpose, of course,
    is to teach physics.

    Very often,
    we redesign the history.

    We tell you 
    not what really happened,

    but what 
    should have happened,

    well, I suppose it serves

    some pedagosical purpose
    to do that,

    so let me tell you
    what should have happened.

    A Patent Clerk in Switzerland
    named Albert Einstein

    should have said, "look,
    the Michelson- Morley experiment

    "compels us to do things.

    "one is to restore
    the principle of relativity.

    "The other one is to believe
    that the speed of light

    "will be the same
    to all observers,

    "regardless of 
    their state of motion.

    "And from those 
    two fundermental postulates

    "follow all 
    the astonishing results

    of the theory of relativity."

    Now let me tell you
    what the facts wwere.

    One is that Michelson never,
    for the rest of his life,

    believed the results
    of his own experiment.

    The other one 
    is that Einstein

    not only didn't
    base his theory

    on the Michelson-Morley
    experiment,
  
    but when he wrote
    his famous paper in 1905,
  
    he claimed he never heard of 
    the Michelson-Morley experiment.

    We'll see why next time.

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    Captions copyright 1986
    California institute
    of technology,


    
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