How Einstein Abolished the Aether - with John Spence

I want to talk about how Einstein abolished the ether and I'll tell you what that means I'll also talk about how it came to be understood that when we look up at the stars at night we're looking back in time the light takes years to get here I've often wondered how that affected astrologers who cast horoscopes you know I mean right now the stars is somewhere else in 1598 the king of Spain offered a prize for solving the longitude problem that's how far around the equator you've gone Spain was losing too many ships from the Spanish Navy from lost Mariners and the military value was obvious of knowing where you are Galileo who's shown here showing his newly invented telescope to the Doge of Venice had just invented the telescope and with it the first thing he did was to discover a moon of Iowa of Jupiter called IO and it occurred to him that you could see IO disappearing behind Jupiter every forty two and a half hours so the eclipses occurred every forty two 1/2 hours you could watch them as you sail around the world and use them as a kind of global clock that could be seen from anywhere on earth and give you the time back home for him that was Italy your local time you could get from the height of the Sun at noon it was overhead and as we all know from long distance air travel time differences between home and where you I give you your longitude if it's 12 hour time difference you would be in New Zealand Galileo didn't get the prize when they tried it out on a ship the ship rocked too much and they couldn't get a good sighting but it worked perfectly on land and was used ever since for hundreds of years later for example to check Harrison's chronometer at that time Cassini was head of the Paris observatory and he wanted to check this idea so he asked his friend Bart Olin and his graduate student Ola Roma at the University of Copenhagen University of Denmark to travel to the island of hven which is in the Straits between Denmark and Sweden to measure the longitude there of Tycho Brahe his old laboratory which was still there and is there still as a museum bra Hey I'm sorry bro I was dead by then Olli Roma is young graduates postgraduate student went along with his telescope and his pendulum clock and over months of observation cited the disappearance of the moon behind Jupiter every forty two and a half hours but he found something very interesting which led to one of the greatest papers in the history of physics so here's his paper it was translated to English and published in phil trans in 1677 this is the Sun here's the earth going around it now we zoom along at a fair clip here it's 67,000 miles an hour that's the speed of the earth around the Sun here's Jupiter and up here is IO orbiting like this and if you're looking say the earth is here you see IO disappear behind Jupiter every forty two and a half hours but what Roman noticed was that three months later in different season when the earth was down here somewhere going on zooming along at 67,000 miles an hour the eclipses were a little late and of course at that time everyone thought the speed of light was instantaneous and it occurred to this postgraduate student that this delay was due to the time it took light to catch up with the earth so while it was travelling toward the earth the light that you would see by looking up here as Iowa appeared the earth had moved forward and the light had to travel further the delay was about 10 minutes now he very cleverly used this knowledge to make a prediction of a future eclipse now predicting the heavens at that time was you know like magic people really took notice if you could do that his prediction came true so of course they were would burn him as a witch or something but instead of that they had a pointed him to the parents Academy he gave a presentation on this and it caused a sensation and this is his paper which resulted from it he could calculate the time it took light to travel from the earth to the Sun and he gave that numbers about eleven minutes a Newton in his book witches came soon after gives eight minutes which is remarkably close to the current it's a correct value light takes about eight minutes to get us to us from the Sun eight and a half minutes so his Roma in his observatory that he built later on here's his telescope look at his pendulum clock here's the bob swinging back and forwards he was very unlucky he went on to do many great things he became the mayor of Copenhagen he was a judge he invented the Fahrenheit temperature scale but all of his work was destroyed in a fire in 728 his devoted supporter Peter Horeb rau managed to rescue one book and by remarkable situation he had been working by window at the Copenhagen University and he'd left one of his most important notebooks there it sat there until 1920 when somebody found this thing and they're able to reconstruct a lot about the work he'd done so this raised the question of what supports this blight propagation through the vacuum you know waves on a river are supported by water what is it in if there's nothing out there in outer space how can a wave get run through it and this led to the idea which the church of course forcefully supported that there was an absolute frame of rest in the universe they used to say it was the earth and Galileo as you all know famously denied that and narrowly escaped the rack as a result during his Inquisition but the scientists came to believe there was this stuff that filled the universe sort of fixed to the most remote stars bolted on to the distant stars which was an absolute frame of god-given frame of rest if the universe is full of this sort of invisible rubber it has to have some pretty ridiculous qualities we know that for all waves their speed is given by the ratio of their stiffness to their density so if you give this stuff the stiffness of steel it has to have a density much lighter than a hydrogen 50,000 times lighter than hydrogen so that a wave would run through it at the speed of light 186,000 miles per second 186,000 miles per second it also had to not impede the motion of the planets now the planets as being all around the place that's the Greek word for wanderer and it had to permeate all forms of matter and also be completely invisible so this was absurd but before you dismiss this as an ancient foolishness we need to remember that James Clerk Maxwell got the correct equations of electrodynamics the equations that describe all electrical phenomena by using this model of the ether as a vortex sponge they called it invisible rubber now to get a speed from a time measurement you need the distance that the light's gone so they had to know the distance from the earth to the Sun and the earth to the moon this is a long long story I don't have time to go over it let me just say that the ancient Greeks had it all sorted out this is a page remarkable it's a 10th century copy of the great Greek astronomer Aristarchus work showing just how well they understood principles like parallax and geometry and of course they had all Euclid's theorems that we learned as kids in school and their methods were terrific for measuring these distances the problem was they couldn't measure small angles accurately so their actual numbers for the distance from the earth to the moon were hopelessly out but their methods were good the one number they got right was the size of the earth the ancient Greeks do that you probably know they measured the length of shadow of a stick in two places 2,000 miles apart at the same time and one of them the Sun was overhead so there was no shadow at the other one a couple of thousand miles away had a sure if you know the height of the stick and the length of the shadow you can get the size of the earth but accurate values came only with the transit of Venus observations 1761 and see these happen twice every century roughly about eight years apart and I've shown here the kit which the Royal Society made up this was the first truly international scientific collaboration and the Royal Society made up a kit of a tent with a clock in it shown here and a telescope this is the one that Captain Cook on his voyage to Australia took with him and set up at Point Venus the name it still holds today of his observations so a transit of Venus is shown here this is a picture taken by a friend of mine with an ordinary camera when there was such a transit in 2012 this is Venus so this is what's happening as Venus is closer to the Sun than the earth and it passes across the disk of the Sun across a line from the earth to the Sun twice every century and if you make now of course they had no cameras they would project as Jeremiah Horrocks did even earlier in a darkroom the Sun would come through one window through a small pinhole make a shadow image of the Sun on the other wall they draw it with a pencil and trace it takes about six hours for the Venus to move across the Sun and from those dimensions and angles you can work out if you know the size of the earth the distance from the earth to the Sun and the earth to the moon these all kinds of misadventures occurred to these people who was war was disease or was famine some of the observations Catherine the Great even sent Russian scientists to Mexico but the answer they got was 93 million Mars which is within 1% of the correct answer so back in 1671 1761 from then on we knew the size of the solar system pretty accurately our next hero is greatly underrated I think because his work was so important for Einstein this is Bradley um 1762 around the time of Newton he basically lay on his back on a couch along here in his house he built it telescope onto the wall of the chimney of his house poking through the roof he lay along here looking up at the star gamma Draconis and he measured the angle between the axis of the telescope and this plumb bob says the local vertical so this is a string in a tube on the end of it is a weight and it's sitting in a bowl of water to stop the vibrations he put a tube around the string to keep the dust out and he writes that his biggest problem was the spiders which used to get up in here and mess things up so he lay here for several years all day making notes and measuring this angle and he found that this style moved in a circle throughout of course they took out the motion of the rotating earth it was understood against the background of fixed stars he was moving in a circle he didn't know why now the answer is most easily understood it came to him one day when he was sailing on the Thames and you wrote a letter about this if you're walking along with an umbrella and it's raining we can think of the rain as the starlight raining down the faster you walk the more you have to tilt the umbrella forward yeah and it's the same situation with the Starlight you have to tilt the telescope you have to look ahead of the star another way to think of it it perhaps simpler is that because the Earth's whizzing along across the rain of starlight at 70,000 miles an hour photons coming into the telescope tube will hit the side of the tube they will never get to your eye because of the sideways motion of the telescope to event that you have to tilt it forward and his simple calculations showed that this tilt angle was just equal to the ratio of the speed of the earth to the speed of light there's a fantastic experiment extremely important and crucial for Einstein's theory of relativity we move on now to Fornell he was an absolutely brilliant mathematician and physicist he actually worked in the roads construction depots a civil engineer during that just enough to the French Revolution the thing he was most proud of because he put her on his tomb it's the only thing on his tomb were the lighthouse lenses that he invented because they saved so many lives these sick Fornell lenses that he but equally important for our story is that he came up with a theory of the ether drag now I have to say that if we go back to this queries of the ether if you are on the earth here with a flashlight and you shine it that way now here's the earth going along seventy thousand miles per hour around the Sun if the ethers bolted onto the most distant stars the earth would be rushing through it and you'd expect a headwind it's like waves on a river where there's a current it's like waves on a river where there's a current so there'd be this because we're moving through the ether which is fixed to the most distant stars you'd expect light going to goes more slowly in this direction and the tailwind to speed it up going the other way and it's fair to say that scientists looked for this effect for a couple of hundred years and was it the absence of that effect the absence of any headwind or tail wind was something which really only Einstein was able to explain fully so we have this measurement and we now have for now his idea was that the ether is like some fog let's say as it gets close to the earth it slows down and it's dragged around by the earth that the fog at the Earth's surface is moving with the earth like the molecules whizzing over the wing of an aircraft when you look out the window the the Airstream actually at the wing is moving with the airplane and as you can move further away it speeds up but his wave theory was extremely important his boss was a fascinating man arauco I think who had the most interesting life of any of the people in my story and araga suspected arauco spoke good English Fornell didn't speak any language other than French Moraga came to think that there was an Englishman James young whom I might have scooped them by showing that life was a wave earlier than they had and so he writes home to his family when he's in London visiting young in 1816 now this is the time of soon after Jane Austen and and the Napoleonic time and the French Revolution would mean a fresh memory in 1816 I went to England with my friend Gale Lewis sector discussed Fornells work was young he told us that Fornells experience experiments was already published in his own work in 1867 this did not appear to us correct and rendered the discussion long and minut as you can imagine but look at this this is fascinating mrs. young was present but did not appear to take any interest in this conversation but as we know that fear however puerile of passing for learning ladies of being designated a bluestocking this is 1800 blue stockings is that old a blue stocking made the English ladies very reserved in the presence of strangers suddenly mrs. young rose up and left the room and returned with the enormous quarter of Young's natural philosophy she placed it on the table as her husband she placed it on the table opened it without saying a word and pointed with a finger to a figure where the curved lines of diffracted bands was shown and on which the discussion turned and was theoretically established so arago is saying and I think this is true from Madame de pins diary writing at the same time that the French women really had much more authority and influence at the French Court then did the England were more liberated than their English women at the time it was later found understood that frog 4lz the drag theory did give the right answer fortuitously but for the wrong reason a raga went on to do many amazing things he wrote fabulous memoirs of all the French academicians he went into politics and a ball slavery in all the colonies and also flogging in the Navy next we have hippolyte fizeau he was a professor we move now to the Paris observatory it's still there part of it as a museum which still has his apparatus there and these people made the first measurements of the speed of light on earth terrestrial measurements and fizeau did it with this tooth tuile so he had a fabulous technician working for him Fremont and he was able to use an ivory comb on a spinning wheel so there were gaps between the teeth this would spin round at didn't have to go very fast 12 revolutions per second but there was 720 teeth so like going up here would go through a gap in between the teeth it travelled off over five miles across the chimneys and rooftops of Paris to a mirror he'd put on his father's house and was reflected back but by the time I got back the next tooth had come round and blocked it so he because he could adjust the speed of his tooth wheel so he had adjusted the speed until he didn't see any light and then he knew that the time it took light to go ten miles around the round-trip was equal to the time a tooth took to move around one place now he got the speed of this thing with a piano because this is zero 440 is a so you can and it makes a whizzing sound so you if you cut you piano tuned you can get the speed his answer was that speed of light is 3.4 by 10 to the 8th meters per second and that's within 5% of the modern correct value working with him was léon foucault now photon of course is much more famous for his pendulum and the two of them work together on many experiments in electricity before one day they came across the works of Charles Wheatstone who had measured the speed of electricity at King's College his apparatus is still there I went and saw it last summer in the basement he'd measured the speed of electricity was a spinning mirror and Foucault who was not a mathematician at all he was rather looked down on in the French Academy because he wasn't a theoretician but he was a better experimentalist I think than fizz and got actually a much more accurate answer and his idea was to have a mirror here spinning he had a friend who'd built the pipe organ that just built the pipe organ from dr. dom and so this friend built him an air pump and he made it pneumatic up here is the thing is it's like a siren or um it's a spinning disc here that's blown around by this air here turbine and here's the mirror which spins about the vertical axis a lot across its diameter so light from a source here and they had a thing called a heliostat they used sunlight coming into the window and then of a mirror which rotated by clockwork to keep the spot focus spot of light in the same place throughout the day as the Sun moved then that sunlight went up to this mirror out to here now only 20 meters not across the rooftops anymore and by the time it came back this vastly rapidly rotating mirror had rotated slightly so the image of the original source was displaced a bit to the side because when the by the time the light came back the mirror had rotated slightly and if he measured the distance between the original source image and the source and it's image this displacement X here would increase with the speed of the mirror and if you make a graph of the speed of the mirror against the displace when you get the speed of light his answer was within about half a percent of the modern value fabulous work this is fuko's pendulum I put this up here so you can just remember this it's very useful these things are in many buildings around the world now we have one in my university in the physics department so he put a huge pendulum in the path in the Parthenon I think it was pantheon in Paris it would swing across along this line here but what happened was just throughout the day the plane in which it was swinging would appear to rotate so if it swung along that line at noon a few hours later to be swinging across here later across here to understand it imagine it's at the North Pole and you give it a push to start moving and as a Newton has taught us it keeps on moving in the same direction but the Earth rotates underneath it so if you're standing here it's the plane of Swing appears to rotate this was deeply shocking to the people of Paris when he set it up and you know the village folk would come in and say that it's magic and again he should be burned at the stake but that was the first direct evidence of the Earth's rotation you could just see it with your own eyes all of this happened just prior to the siege of Paris the Paris Commune in 1870 a fascinating time when an extreme left-wing government got into power and it's a it's just amazing to see how their policies map on to today's Elizabeth Warren for example what they wanted was free childcare childcare they wanted no gender discrimination they wanted separation of church and state all the issues which are hot-button issues now were urgent issues for them then Bismarck came with troops from Germany bombarded the city shelled it there was more damage to the buildings of Paris at that time than in any other war before or since and here's the menu basically Bismarck starved the population into submission here's the menu with a restaurant at that time you see there start starvation diet they're living on dog cutlets regular cat donkey fricassee of rats and mice and this little note here says that only the brave American ambassador was the only person foreign country to sit out and remain in Paris throughout the siege we now come to Faraday of course who worked in this building here's a picture of the rhubarb tree and you can see this downstairs Martin took me around before and I saw exactly this here he is working over in the corner here his work is very important it's nizzle I'm leading up to Einstein's synthesis of all of this it was crucially important because he founded field theory so we imagined that the universe is filled with this invisible rubber which supports light waves running through it at 186,000 miles per second you know light takes about an hour to get here from Saturn Faraday was the first to see field lines which he thought of his lines of tension in this ISA he took a magnet a horseshoe magnet like this I remember doing this as a kid you put a card across the top sprinkle iron filings on it and they line up on these lines of tension they're like rubber bands now you need rotor you need sideways forces of course also to keep them apart here's the North Pole here's the South Pole looking down on this thing he could pump put this in a jar glass Bell jar and pump out the air and the lines were still there so they were there in vacuum what are they what are these lines of tension in this non-existent ether he wrote an extraordinary perceptive sentence in one of his letters to Maxwell I consider radiation to be the high species of vibration in the lines of force which are known to connect particles and also masses of matter together so he's there leading into field theory for gravity from electrostatics so he thought that there were lines of tension in this ISA which were responsible for radiation that's an incredible statement for such an early time that picture came out of lodge's book in 19th century he discovered another effect which I saw before the talk Martin showed me the magneto optical effect this was just basically that when light goes through certain kinds of stuff if you put a magnet near it it returned it rotates the plane of polarization of the light now why was that important because at that time there was no connection whatsoever between electricity and light it was Maxwell who explained all that there was also no connection until Maxwell between electric statics that's when you get electric shock in a hotel with rubber shoes on and magnets they thought they were completely separate things so all of that was unified by Maxwell so this was very important because he wrote a letter to Maxwell describing this and it's the first experimental connection between light and electricity now I guess you could say that sparks are that I mean they knew during a lightning storm that as barks gave off light but other than that there wasn't much to connect them that led max to a very famous term in his equation called the displacement current and the symmetry that that gave his equations was a crucial clue for Einstein in his theory of relativity so we now come to Maxwell here he is with his dog and his wife she was tremendously religious lady he grew up in a country school in Scotland here they are with their country Scots accents firm religious convictions social awkwardness and James's dry iconic wit he was about 40 years younger than Faraday but they did write to each other and to some extent Maxwell's work is based on Faraday's experiments he unified electricity magnetism and optics using a mechanical model of diseases that's the extraordinary thing he applied Newton's laws to this invisible rubber and got his four great equations now in fact he got twenty equations which he put in his book and then promptly died soon after and it was an extraordinary fellow called Heaviside an autodidact who lived with his mother in southern england and did nothing but study Maxwell's equations throughout his whole life and write papers about it he was eventually recognized and he reduced Maxwell's equations to the four that we teach our students today in Electrical Engineering and physics departments he designed the old Cavendish lab which is still there he died of stomach cancer in 1879 now he never predicted radio people say that he did but he was focused on light and it was on it was eight years after he died that Hertz discovered radio and it was realized that his equations could be extended to radio waves that we use for our mobile phones radio waves are just light at a longer wavelength and lower frequency and there's a nice story about them in this room when Maxwell had been giving a lecture on his kinetic theory of gases that's the theory about molecules bumping into each other in a crowded space and the apparently was only one exit door which is still the case and there was a bit of a crash in a bottleneck as people were leaving and Faraday called out to Maxwell ho Maxwell can't you get out if any man can find his way through a crowd it should be you they sort of come alive when you see what they said well they say that an equation is worth a thousand pictures so I've given Maxwell's equations here for it's worth but more important is this the mysterious thing here really we think of Maxwell as something of a magician because his mechanical model of the East is shown here this is his diagram he had to have these loop currents circulating to get magnetic coupling and to get the coupling directions correct like gearwheels he had to put in these idler wheels what on earth are they they're vortices in this invisible rubber vortex Foam stuff which just happens the right properties so that it will transmit a light wave he had as I'm sorry he found that a change by introducing these idler wheels he could make his equation symmetrical so that a changing magnetic field caused a electric field and vice versa and that produced a thing called his displacement current which was his really original contribution to all of this from that he could work out the speed of light because as I said before we knew that the speed of a river wave or a sound wave on a guitar string is the stiffness over the density and he says in the happiest day of his life was he got this number he was at his house up near Edinburgh it's still there the ruins of it at least and he got this number when he worked out the speed for lights in the invisible rubber and he thought he recognized it from somewhere but he couldn't go home because of family obligations for a month he says it's the most frustrating month in his life because back in London he had Roma's value and fizzers value for the speed of light he couldn't find it locally in the library in Edinburgh so eventually he was able to get the coach up to London and go to his flat in in here and London and look up his notes and found for Zoe's value was the same as his theoretically predicted value for the speed of light if the vacuum is filled with invisible rubber with these properties and he realized then that light was an electromagnetic wave and he had its speed and that was one of the greatest discoveries in all of physics so what I'm getting out here is just that these equations he came up with were kind of metaphor you see if you ask what does the field consists of we don't know nobody knows physicists find equations which work they don't know why they work and they don't know the stuff that they actually represent we don't know what is an electric field in vacuum because there's nothing there it's a vacuum by filling it with this metaphorical invisible rubber will you get the right answer it's a deeply mysterious process his Maxwell he died soon after the telephone had been invented and he says here we've all been conversing on the phone his friend garnet actually recognized the voice of a man who called by chance but the phonograph will preserve for posterity the voices of our best singers and speakers later his biographer Campbell writes about him he had a strong sense of humor a keen relish for witty repartee the art were designed being a particular twinkle of the eye when working he whistled a soft accompaniment to his inner thoughts he could pursue his studies under distractions such as loud conversation then he would take his dog into his confidence and say to the dog Toby Toby Toby it must be so and then he'd join in the conversation Maxwell played the guitar he loved Burns's poetry he and he wrote poetry lots of it and of course he invented color photography his first color photograph is in the Cavendish Museum in Cambridge and here they are around that time laying the first transatlantic cable I put this in because this is really the birth of the web so they tried to string a wire between America and England the first one broke pretty soon afterwards but eventually they got it they could pick it up with a grappling hook the bottom of the Atlantic is pretty soft sand pick parts from this trench and they could keep on going now the way they did was two ships met in mid-atlantic one each with this huge spool of hable on it these sailors it's pulling out running out the cable over the side of the ship down to the bottom of the Atlantic Ocean here's Kelvin explaining to the sailors around here why it doesn't all work with the speed of light but they ships met in the middle tied the ends of the rope this is the cable to get electrical cable together covered covered with gutta-percha she just discovered and strung it out back to Newfoundland on one side and Island on the other the driving force from the investors was share prices you could get them earlier if this thing worked instead of waiting twelve days for someone to sail across and military and diplomatic information so that's how they got their investors and it was a constant struggle for money of course the first message from Queen Victoria to President Buchanan took 16 hours for a hundred words so it's five seconds per bit your mobile phone works at gigahertz rates and now the last experimentalist I want to talk about before Einstein was Albert Michelson he was of a Polish family he was the first American to win the Nobel Prize they settled in San Francisco he tried to get a job with the President Ulysses Grant and and grant eventually took him on and sent him to the Naval Academy from where he went on a scholarship to Helmholtz the greatest German physicist at the time in Berlin he simply decided to devote his entire life to pinning down and locating the ether which everybody believed must be there given by God I would say all physicists born before 1900 believed in the ISA in its existence so the great puzzle for him is why is there no headwind or tailwind due to the Earth's motion through the ether what he found as a result of his life's work was that the speed of light is the same in every direction and of course for him that was a great failure he'd expected to find that light went faster when it was going in this opposite direct dependence pede how the earth speed was with respect to the station of the distant stars so here's his interferometer it's a clever idea it's entirely his original own original invention a Michelson interferometer he sent a beam of life let's say the earth is moving this way to the right then this ether wind the ether being stationary would be to the left on the surface of the earth he sent a beam of light across the ether and back from a mirror and then into it and back on this mirror and if you think of people swimming across rivers with a current you'll understand that the time it takes a light to go across it and back is different from the time to go into it and back so where the two beams meet he let them interfere and by that you could tell how long they had the times for the two trips who he expected there to be a change you know certainly six months later when the earth was going in the opposite direction it goes in a circle and when he rotated his infer Amida it could be swung around about its vertical axis but he got no result he published it I mean he got no effect there's no change in the time for light across or with these throughout the seasons of the year as expected so he wrote this paper and you see this was all sort of consistent with Fornells idea that these sticks gets more sticky to the earth as it gets closer to the earth and is dragged around with it he wrote a paper in which the conclusion was there is no there was no ether wind the ether is not stationary with respect to the distant stars now that paper he thought was an indication of his failure as a scientist he went back to America planning to give up science altogether and he believed that nobody had read his paper at all he got no responses whatsoever well it happened that he was seated at a dinner with Lord the Lord's Kelvin and Rayleigh in 1884 and they in fact had read it they were fascinated by it and Rayleigh in particular took a vascular interest in this young American and wrote letters to him for the rest of his life Michelson went and stayed with the Rayleigh's in their grand house in southern South of England and the two of them urged him to try again because it was so important it's been described as the greatest negative result in the history of so he who took a job at Case Western and built a much better interferometer with his friend and colleague Morley this whole great slab here is floating on mercury liquid mercury and he took measurements when this was rotated through 90 degrees and he took it six months later and he got the same result nothing so that well perhaps just very quickly I'll interject now before we start on how Einstein made sense of all of this I'll just say something about the discovery of radio because I became in writing this book I became a huge admirer of Heinrich Hertz I think he was really the most gifted experimentalist of all of these people because the experiments he did was so difficult he did them in karlsville he discovered radio he certainly didn't set out to do that now it's interesting when he did they call it invisible light you see it was described perfectly by Maxwell's equations it's just a different frequency Maxwell never had the curiosity to ask would his equations describe waves other than light at lower frequency you know all the way down up to x-rays and down to across the electromagnetic spectrum Hertz was working also for Helmholtz and he started out with a slightly different name now what he did is what he did it was noticed sometimes at enlightening storms you'd see sparks between the ions by the fireplace yeah so the reason radio wasn't discovered we have detectors for light your eye for example there were no detectors for radio that's why it wasn't discovered but Hertz had the idea of using a spark gap as a detector so here's what he did he made a spark gap with a micrometer where you could screw down the gap as small as you like and it was connected to a coil to give it a resonant frequency in a completely darkened room he had a machine in one corner making sparks with a little coil and that would create as we know radio waves he walked around in the dark looking through an optical microscope at this micrometer gap he screw it down until it started to make a spark and the gap length was then the strength of the electric field at that point the lines that Faraday had seen running between his magnets and he'd write that in his notebook this went on for months and months or years in fact he was lucky because the wavelength just fitted into his room had he you know worked at a lot more less turns on the coil or more turns on the coil than ways might endure feel it because they fitted into his room a few meters I used to be a radio ham so we built all these things in the 60s and 70s so he would he could create standing waves in like a guitar string note across the room in this radio frequency field and he was able to plot it out and he noticed maxima and minima just like a standing wave on a guitar string where there's you know guitarists tuned the instruments by running their finger up to a harmonic position where there's no displacement of the spring and he was able to map out the standing waves which he published now so he had them wavelength from the number of turns on the coil Kelvin had published the frequency of that circuit so the frequency was known the wavelength was done he could combine them and get the speed and once again the happiest day in his life was when he combined these numbers and got a dumber equal to the speed of light he then knew that radio waves travel at the speed of light and there's a wonderful letter in 1887 his wife Elizabeth drew the field lines for this work that's the first plotting of them elect electromagnetic field lines from a dipole ever which she turned out to be extremely important but his wife wrote to his parents that Heinrich had again succeeded in the most beautiful experiments which make him very happy and me as well when he tells me about it with such a radiant face I want to say something about the spacecraft here because I think it is just fantastic you know this thing what happens Cassini was the name of named after the head of the named after Romans boss it was put up in 1997 and expected to last for years in lasted 20 years here it is this picture of Jupiter so Saturn was actually taken by this spaceship which has then been Photoshop owner of the spacecraft has then been photoshopped onto this large picture of the planet these are the rings and in fact Maxwell's first piece of serious physics was to calculate the motions of the rings of Saturn and explain why they all didn't just crash into the surface through gravity or get flung out into space why rings were stable which he did the tour de force so what I want to emphasize here is just the amazing thing that the transmitter in this thing is 50 watts can you believe it that's like the lamp in your refrigerator it's a very weak bedside lamp and yet that 50 watts pointed towards Earth can send ones and zeroes like Morse codes dots and dashes back to an object a bit like a television antenna on the roof of the house at the Goldstone facility in the outer suburbs of Los Angeles which are 700 million miles away and the radio waves take an hour to get there so we have to believe that these field lines that Faraday had seen between his magnet poles can stretch out can reach out across the complete vacuum of outer space and cause electrons in a wire very slippery electrons to slosh up and down on the antenna at Goldstone near Los Angeles and make a signal now it works for us it's got several things going for it first this is directional so it's a it's a dish but it's not a very good dish very good very much very directional the beam that it sends out has a width of 5 million miles when it gets to earth much wider than the earth because you can't have a very big digit too heavy the one at Goldstone is 70 meters I think it's a bit better secondly they're transmitting and receiving on the same frequency of course that helps a lot but most important they cool the receiver Goldstone down to about zero temperature and what that does is to quieten all the hiss and static that you normally hear when you tune a radio between stations okay so the net effect of all this in 1900 was that things were in a complete mess there lots of papers with the million crazy theories some of the craziest were that moving clocks slowed down because if they're going very fast and that moving objects get shorter in the direction of their length if they move very fast but no one had a coherent unified exploration for all of this information here's a summary of what we've said so far Maxwell's equations suggested an absolute reference frame Maxwell's equations gave a speed of light it didn't depend on the speed of the source if you have a car coming towards you at night shining its headlights at you you would expect that if the car sped up the light would speed up it doesn't no matter how fast the car is going the light coming at you always comes at you at the same speed there's no headwind as we were saying unlike waves on a river light speed did not seem to add to the ether current speed mikaelsons result was that there was no stationary ether fixed to the remote stars that didn't help Bradley's result was the opposite here's the point if if the ether was fixed to the O I mean one explanation for mikaelsons result which he didn't really want to put in his paper was that the ether is bolted onto the earth I mean the church would have loved that and that as the earth rotated it dragged this ether stuff throughout the entire universe around with it and if that were the case that could not be true because if the ether was dragged around with the earth you wouldn't need to tilt your experience your telescope in Bradley's experiment because the light waves will be bolted on to the ether so the the Bradley's result was contradictory to mikaelsons there was good agreement with the fernell theory fruit for so had done experiments running light through running water water running anti parallel to the light and measuring the speed of light as he sped up the water and he got a change and it fit with pronounced theory so people tended think Fidel's old theory of 1820 was probably right and then the biggest problem of all was that Newton's laws couldn't accept this idea about the speed of light being constant look if you are on a Beltway at the airport and you're walking along the Beltway at two miles an hour and the belts going at three miles an hour with respect to the ground you're obviously walking at five miles an hour with respect to the ground the some of the belt and your own speed was fixed the belt that's Newton's law and it didn't fit with this results about the speed of light being constant the car coming at you with its headlights on so Kelvin gave a great talk in 1900 it was called clouds over 19th century physics where he said the greatest unsolved problem see he basically said physics has solved all problems there only two outstanding small problems to worry about now one is was called the ultraviolet catastrophe that gave birth to quantum mechanics and the other was the ether and in nature and that gave verse to relativity two of the great revolutions in physics so there were three possible solutions you could continue as Michelson has devoted his life to finding the ether you could assume that Maxwell had made a mistake and fixed Maxwell's equation or you could assume Newton made in a mistake and fixed Newton's equations Newton of course had immense Authority and it really did require tremendous boldness and confidence on the part of a 26 year old patent attorney in Zurich Albert Einstein to conclude that it was Newton who was wrong Einstein as a patent attorney he didn't he wasn't working in university had three portraits in his office one of Newton one of Faraday and one of Maxwell they were his heroes so what to say about Einstein I could summarize it by saying it's slowly sunk into people that all motion is relative now of course Einstein became a great celebrity in the 20s particularly because of the work in general relativity so clever students from Cambridge and Oxford when they went to the railway station here in London would ask the stationmaster things like does Oxford stop at this train so it was pop it was in the air and written about in newspapers and so forth his paper of 1905 clarified everything at a stroke he abolished these are entirely he assumed Maxwell's equations were correct and he made changes to Newton's laws and those changes led to the equation e equals MC squared now equals MC squared his most famous equation tells you the amount of energy E in a nuclear explosion when an amount of mass M is disappears completely and it's all a result of modifying Newton's laws so that all motion is relative in the speed of light is constant to put it in simple terms so this equation of course is of profound importance it tells us how the stars are powered after all the star is just a continuous succession of hydrogen bombs and it tells us nuclear energy and it well many other things of course his theory agreed with both Bradley and Mac and finals results to fernell had got the right answer for the wrong reason he came to America in 1921 where he was struck by the joyous positive attitude to life of the people who he found to be friendly self-confident optimistic and without envy he played the violin he was once asked if he hadn't been a physicist what would he have done with his life he said oh I'd have been a musician after playing with the Julliard quartet someone went up to one of the musicians in the quartet and said what was he like what's he like as a violinist and then the musician said eyestone he's alright but you got no sense of time in 1933 the Nazis stole his sailboat that was kind of the last straw at that point he moved to Princeton and stayed there till he died in 1955 but before that he'd asked Churchill for help moving Jewish children out of Germany which Churchill did in response to Einstein's letter and my boss at Oxford Sir Peter Hirsch was one of those children he said he was a deeply religious non-believer and very appointed words for our times now without ethical culture there is no salvation for Humanity so I want to put in here plug for our own research because it seems so appropriate but just by pure chance one other thing my main job in America is as director of science for a consortium we've got a big 50 million dollar grant from the NSF for ten years among seven universities Stanford Cornell ASU which is by far the biggest of them and others Reisz UCSF and we got the grant to use the x-ray laser which was just invented in 2009 to try to make movies of molecular machines at work these are the little molecular things going in your body all the time that heal a wound for example and this is one professor Mara Schmidt's team led in a big collaboration in 2016 these are molecules proteins and they're the proteins in the material at the back of your eye and all this it's the same protein here it's called a sis trans isomerization reaction this across the top is the top view these are frames of a movie stepping across and this is a side view and what we do is to flash some light as you would if light was coming into your eye on this protein and then a little later take an x-ray snapshot of it and each delay between when we have flashed a light on it and when we take its snapshot gives us one frame of a movie yeah and then we repeat that many times it's like a stroboscope and we build up a movie by changing the delay and this is the result and what happens is that this molecule along the dashed line here changes at this thick line from a left-handed conformation left-handed shape like you're left-handed like your left hand to a right-handed shape because it's absorbed a photon of light and when it changes from left to right it sends a signal to your brain which says you've seen a flash of life that's how it works so this is the photo detector in your eye and this is a move you had happening but what is remarkable is the time scale these x-ray lasers there's now five of them around the world in South Korea there's one in Hamburg and there's a British consortium to use that and there's an important discussion now if I can put in a plug to build one of these in England which I've been a bit involved with so you can get these movies of molecular machines in operation but the time resolution is the extraordinary thing the time between this frame and this frame is about a millionth of a millionth of a second so these are in femtoseconds down the bottom here this is okay so a millions of millionths of a second because our x-ray pulse is so brief we can get we can resolve the motion of this wriggling protein and it just struck me this is exactly relevant to something that firemen said in 1950 when quantum electrodynamics another branch of physics was being established he said that when an atom in the Sun shakes my eye electron shakes eight minutes later because of a direct interaction that is truly remarkable it's correct remember Romo got 11 minutes for light to get from the Sun to the earth Newton thought about I think 8 or 9 minutes in his book how he came so close I don't know but this is what the quantum electronics s tells us actually happens a bit of light comes from the Sun eight minutes later you see it it's interesting that that Newton and Maxwell had opposite kinds of luck Newton used his ideas about gravity to predict the distance from the earth to the Sun when he looked up the best known value at that time it was wildly different so he gave up theoretical physics for 20 years he was actually correct but he thought it was all wrong because the experiment didn't fit it unfortunately for him it was just a bad experiment the numerical value with that he looked up was was hopeless Maxwell had kind of the opposite luck he predicted the speed of light from his equations and the value he chose agreed almost perfectly and but there were other values around and he just happened to lock onto the right one okay I'll finish up now and let me just sort of in a more philosophical direction speculate what a what is this ether thing of what stuff does an electric field consist now the vacuum state in modern physics is thought to be alive with virtual particles and things and this vacuum energy thought to be responsible for the origin of the universe in the Big Bang so you could say that modern QED just replaces the ether with another thing with a different name and you're just replacing one unknown with another that would be pretty correct Maxwell himself understood that mathematics is a metaphor in physics he wrote the analogy between light and vibrations of an elastic ether although its importance and fruitfulness cannot be overestimated we must recollect that it's based on a resemblance in form between the laws of light and the laws of vibrations so he knew that these equations of his were metaphorical but they worked he didn't understand what was the underlying reality well what is reality is a deep question of course in quantum mechanics what is real Steven Hauk and Hawking spoke once of a model dependent reality and the idea that consciousness is nothing but the sum of all this sort of anticipatory modeling that our brain is doing all the time and it leaves you with a fundamental question is reality out there waiting to be discovered or do we somehow impose our imagination on it to create it that's what john von neumann one of the greatest mathematicians and physicists of the last century believed the speed of light if we come back to that I'm afraid to say it's no longer measured in 1986 it was defined in terms of other cons known constants so there's no point in measuring anymore we know we've defined its value very precisely and it's been fundamental to many things for GPS I think driverless cars you know if you didn't allow for relativistic Corrections they'd crash into each other so you know it's in point you can get it right you have to have the right numbers in there for the speed of light but more interesting I think is acceptance of Darwin's theory you know that the reason they had it was so difficult to accept Darwin's ideas in around 1900 was simply people couldn't imagine that the earth was as old as it is it's about 4 billion years old and life's been around for more than 3 billion they couldn't comprehend such times and only after measuring the speed of light and using it for the redshift and the expansion's universe can we scale that put a time scale on the universe and on the earth so scaling those times really aided acceptance of darwin's theory and I'll end with usually in Wigner's famous essay on the unreasonable it's the physicists love talking giving this quote in their talks the unreasonable effectiveness of mathematics in the Natural Sciences he wrote that the miracle of the appropriateness of the language of mathematics for the formulation of the laws of physics is a wonderful gift which we've neither understand nor deserve we should be grateful for it and hope that it will remain valid in future research and that it will extend for better or worse for our pleasure even though perhaps also to our bafflement to wider branches of learning so that's the end this is a picture of Cassini's that I'm sorry the Paris observatory it's still there that building I went and saw it last summer when Cassini was boss and then the time that a roma was measuring the speed of light from the moons of Jupiter and I hope you don't feel that as Mark Twain said the professor has cast great darkness on the subject and if he continues we shall soon know nothing at all down the bottom some of the topics I've left out didn't have time for which are all covered in the book but I think that the story of the measurement of the speed of light really is one of mankind's greatest intellectual adventures I've tried to show the challenge created for our most fundamental ideas about the foundations of science space and time and universe and the inspiring ingenuity of the experimentalists who are part of this great adventure and it does seem to me that the speed of light is the most important of our physical constants appearing widely throughout science and also providing a kind of unify playing a kind of unifying role in physics thank you [Applause] [Music] [Applause] you

Channel: The Royal Institution

Views: 199,508

Keywords: Ri, Royal Institution, lightspeed, john spence, speed of light, quantum, quanta, aether, discovery, history of science, how to measure the speed of light, physics, lecture, science lecture, talk

Id: 0LjQt1-D0Pc

Length: 60min 16sec (3616 seconds)

Published: Thu Jun 04 2020

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The fact that light travels at the same speed in all frames of reference is one of the greatest revelations of modern science. John's book "Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light" is available now: https://geni.us/3NJlCIB

How have physicists from the ancient Greeks to Einstein tackled the problem of how light travels and what discoveries did it lead them to along the way? John Spence tells the stories of some of the greatest experimental scientists in history as they searched in vain for the mysterious 'aether' and a frame of absolute rest in the universe.

Watch the Q&A: https://youtu.be/GXpgkdOBefo

Prof John C.H. Spence is a Fellow of the Royal Society and the Richard Snell Professor of Physics and a Regents Professor at Arizona State University. He is also the director of science for the NSF BioXFEL Science and Technology Center on the application of X-Ray Free-electron lasers to structural biology.

This talk was filmed in the Ri on 11 March 2020.