foreign media [Music] thank you thank you thank you I'd like to tell you today about what is perhaps the most famous experiment in all the history of physics it's called the Michelson Morley experiment and 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 folklore and its Legends just like any other culture for example I'm sure you remember the famous story of Isaac Newton in the Apple well there's a legend about the Michelson Morley experiment that's just as deeply rooted as the story about Isaac and the Apple it goes like this Michelson and Morley did their experiment and proved that there was no ether and so Albert Einstein was forced to invent the theory of relativity in order to explain that result the experiment was done it was then called the case Institute of Technology in Cleveland Ohio in 1887. 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 Cleveland and the United States was still the scientific wilderness and especially Cleveland Ohio [Music] in fact it's just possible that the Michelson Molly experiment was the most important thing that did happen in Cleveland until Satchel Page pitched for the Indians but 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 if we can understand why that happened 1887 the time of Victoria Queen of England and Empress of India London is the world capital of Commerce [Music] and celebrates his eighth birthday it's been 23 years since James Clark 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 Drew to a close waves of migrants from Europe are swelling the population of the United States the Western frontier is being settled and settling down looking back 1887 seems like a time of Tranquility not only in Commerce and politics but also in science [Music] many physicists believe that all the great discoveries have been made physics had reached a state of perfection that was positively ethereal in fact the concept of The Ether was one of its Central tenets [Music] and in 1887 in Cleveland Ohio two physicists Albert Abraham Michelson and Edward W Morley were preparing an experiment designed to prove once and for all that The Ether really existed certainly The Ether really had 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 that are virtually devoid of matter but to the Ancients the concept of a perfect void was impossible to grasp Ace had to be filled with something they thought and the solution that Aristotle devised was The Ether that was the substance that filled what would otherwise be empty space and by the 19th century The Ether had come to serve more than a merely philosophical need The Ether was the medium through which light waves from the Sun would propagate to nourish and illuminate the Earth [Applause] waves can propagate along the surface of water or through the body of 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 Line 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 Ether 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 at all on the other hand the Earth and other planets moved to The incompressible Ether with perfect ease 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 and since that certainly 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 non-viscous incompressible fluid that filled all of space [Music] 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 Michaelson was unquestionably qualified four years at Annapolis eight at the case School of applied science in Cleveland and three more years at Clark University in Worcester Massachusetts but it would take more than teaching Excellence to detect 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 tiniest and most precise measurements his earlier measurements of the speed of light were better than any others 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 foreign and partially reflecting mirror sends two beams traveling on perpendicular paths each is reflected back to the point where they had been split into there they combine into a single beam again Michaelson's interferometer can be seen as a race between two light beams if the race ends in a tie the result is a bright spot at the center of the interference pattern [Music] they believed it shouldn't be a tie that it shouldn't even be a horse race that's 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 and so the beam should Trace out quite different paths with respect to the ether [Music] the beam moving sideways to the Earth's Motion makes a triangle while the one moving along the 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 [Music] after all the speed of light is three times ten to the eighth meters per second while the speed of the earth through the ether is only three times ten to the fourth meters per second and to make matters very much worse since the two beams had to go forward and back before being compared the difference in the time they took would depend on the square of that ratio it would be a difference of only one part in a hundred million ironically that tiny difference in travel time would be the exact proof that 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 case if he'd had real scientific proof that the Earth moves and although it's doubtful that he had Galileo in mind that tiny difference in the travel time of the two beams of light was just what Michelson set out to measure in Berlin in 1880. a German instrument making firm built the first interferometer and in 1881 when Michelson went to Potsdam he took it with him it was sensitive all right so sensitive that horse-drawn wagons even pedestrians on the street outside his lab 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 measurements he wasn't concerned he thought correctly that his instrument 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 and 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 sensitive interferometer the new interferometer was 10 times more sensitive than his 1880 instrument had been thinking of ways to make the interferometer steady and not susceptible to passing carts and people the mirrors rested on an enormous base of sandstone that floated in a pool of Mercury 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 easily to detect the tiny expected difference in travel time of the two beams 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 orientation with respect to the motion Through The Ether [Music] and then rotating it if the two beams arrived in a tie for example constructive interference would make a bright spot at the center of the pattern [Music] but if one beam arrives slightly ahead of the other destructive interference could make the center dark in other words the effective motion Through The Ether would be seen in the shift of the interference fringes as the instrument was rotated [Music] to the complete surprise of Michaelson 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 [Music] no matter how they varied the experiment summer or winter night or day the result was the same light apparently travels at the same speed in all directions in complete disagreement with the idea of Earth moving through the ether for the next 50 years numerous physicists repeated the Michelson Morley experiment with ever increasing precision and sophistication the vain hope of finding some mistake in that perplexing conclusion but even in 1887 the facts were clear the experiment had failed to detect The Ether and the consequences were shattering the negative result was almost as disturbing from 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 the day of Copernicus the location of the Sun no experiment had been better conceived no more carefully executed and in science when such an experiment fails it not only creates a theoretical dilemma it can provoke fantastic responses in the scientific community [Music] in 1892 the Irish physicist Gia Fitzgerald suggested that the size of one arm of the Michelson and Morley interferometer might have somehow contracted how much it contracted he said depended on its velocity [Music] to most physicists Fitzgerald's contraction sounded absurd but not to Europe's Most distinguished physicist H.A lorentz 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 transformations in 1899 the great French mathematician examined the results obtained by Michelson and Morley offered a general explanation calling it The principle of relativity the idea behind it was that absolute motion will never be detected in the laboratory there must arise an entirely new kind of Dynamics he said 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 Galileo had said that an object in motion would tend to remain in that same state of uniform motion that was the law of inertia it was correct because in its own frame of reference every state of uniform motion was in reality the state of rest in other words Galileo understood not only the law of inertia but also the reason 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 [Music] 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 either way success or failure the results of Michaelson'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 Hendrick Lawrence of course we can imagine lorentz if we choose as a pitcher for the Brooklyn Dodgers [Music] suppose he had been capable of launching a fastball at six tenths the speed of light while rolling past Albert at the same speed [Applause] [Music] surely fast as Henry'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 watched 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 Hendrick Lawrence 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 and 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 lived on measuring the speed of light with ever increasing precision measuring the diameter of a star for the first time ever and even admitting finally that his interferometer experiment had provided the verification of Albert Einstein's theory of relativity to these was the measurement of the velocity of light the second the measurement of the diameter of a star and the third was a test of the Einstein theory of relativity [Music] Michaelson 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 admitted he confessed in 1931 these experiments are not sufficient to justify the hypothesis 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 spots in a career of 50 years he tended to focus on his so-called failure of 1887. [Music] since the result of the original experiment was negative he concluded the problem is still demanding a solution but in reality Michaelson's experiment if it was a failure was the most brilliant failure in all the history of science and the solution he sought but refused to accept Albert Einstein's theory of relativity would change forever the very meaning of space and time [Music] we often teach physics in a historical way the purpose of course is not to teach history but to teach physics and so very often we redesign the history we tell you not what really happened but what should have happened well I suppose that it serves some pedagogical purpose to do that so let me tell you what should have happened at this point there should have been a patent clerk in Switzerland named Albert Einstein who should have said look the Michelson Morley experiment compels us to do two things one is to restore the principle of relativity and 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 fundamental postulates follow all the astonishing results of the theory of relativity now let me tell you what the facts were one is that Michelson of course never for the rest of his life believed the results of his own experiment but the other one is that Einstein not only didn't base his theory on the Michelson Morley experiment but he later claimed that when he wrote his famous paper in 1905 he had never even heard of the Michelson Morley experiment now we'll see how that was possible when we go on with this next time together thank you foreign foreign media [Music] for information about this and other Annenberg media programs call 1-800 learner and visit us at www.learner.org [Music]