foreign media [Music] [Applause] [Music] thank you thank you thank you we have up to now studied a series of events ideas and discoveries which have given us a coherent view of how the world works it started with Copernicus and Kepler and Galileo culminating in Newton's theory of mechanics then there was the study of electricity which in turn culminated with Maxwell's beautiful Theory but that turned out to be an inner contradiction between these two great Sciences mechanics is based on the principle of inertia which meant that there could be no absolute state of rest and that in turn means that there can be no absolute speed but Maxwell's Theory had an absolute speed the speed of light Well Albert Einstein came along and reconciled those ideas for us but in turn we had to give up our intuitive understanding of the meaning of space and time in the meantime a third story was evolving it was the study of matter in a sense thermodynamics was part of that story because it dealt with the interactions between matter and energy and in the end it gave us a profound new view of the meaning of the flow of time but there is one issue that we haven't confronted yet and that is the age-old question of the ultimate constituents of matter by time that question had been answered everything else we had learned had been transformed profoundly and that issue is the question before the house today in the spring of 1913 the age-old question what is matter made of was fresh on the mind of Niels Hendrick David Bohr [Music] and the young Danes answer fresh to the point of being revolutionary was a model of the hydrogen atom [Music] his work was pivotal a turning point in theoretical physics but by all that was holy in those days Newton's mechanics and Maxwell's theory of electromagnetism it couldn't possibly be right nonetheless his ideas were brilliantly conceived in the first place Borah adopted a model of the atom that resembled the solar system in miniature a heavy positive nucleus took the place of the Sun and like a planet an electron was set into orbit around it but while a planet is attracted to the Sun by the force of gravity the electron is attracted to the nucleus by the force of electricity [Music] so in spite of the difference between the two both had forces of the same basic one over r squared form and both would have the same kinds of orbits [Music] as Johannes Kepler had observed and Isaac Newton had explained centuries earlier those orbits would be ellipses or depending on their eccentricity possibly even circles [Music] to simplify matters Niels Bohr assumed atoms would have circular orbits in circular orbit the potential energy is minus D over r and the kinetic energy is positive but half is big so the total energy is minus one-half D over R whether for a planet or for an atom since this seemed to be the very model of a Newtonian atom what was the matter with it in other words how had this upstart violated the physical laws of his day and if in fact he really had why didn't all right thinking physicists dismiss his ideas on the spot perhaps because at this point young Bohr was still on Solid Ground which made his work worth the Second Glance and that's when the problems became obvious being radical boar had gone further far enough to propose that an electron could exist only in certain orbits which was contrary to Newton's Laws then contrary to Maxwell Moore said the electron would radiate or absorb energy only when it jumped between these definite orbits those proposals were more than merely contrary in fact in the world of physics they were totally against the law what could possibly have led to such a turn of events a generation earlier when it came to the subject of the atom Professor James Clark Maxwell quite literally wrote the book as an editor of Encyclopedia britannica's incomparable ninth Edition his entry on the atom was not only the most advanced science of his day it summarized a body of philosophical speculation that had been around since the Golden Age four centuries before Christ a Greek philosopher called Democritus Saul Adams's self-propelled particles moving through a void but Aristotle didn't see things the way Democritus did and thus the subject was closed thank you until late in the Renaissance when Galileo came along to revive it was followed by Boyle Descartes Newton and many scientists on the same path including Maxwell in Maxwell's own words in modern times the existence of atoms has once more become conspicuous among scientific inquiries thank you John Dalton had made one of the more conspicuous inquiries in 1807 this English chemist studied the chemical combinations of the common elements and proceeded to propose the law of simple and multiple proportions [Music] when two substances combine in a chemical reaction they'd combined by mass in ratios of small whole numbers the implication was clear in order to combine into such definite ratios substances must have ultimate parts or atoms Dalton's idea was not without difficulties however and one of the most important was solved by an Italian chemist named Amadeo Avogadro first Avogadro realized that even the simplest Gases such as pure oxygen or pure nitrogen would be composed not of individual atoms but of combinations of atoms called molecules and whatever the smallest unit of any gas Avogadro said speaking of atoms or molecules a given volume of gas would have the same number of them that quantity eventually became known as Avogadro's number and finding it became a vital step in the development of atomic theory in Maxwell's words the diameter in the mass of a molecule are of course very small but by no means infinitely so about two millions of molecules of hydrogen in a row would occupy a millimeter Maxwell's knowledge came from the theory of gases and his analysis of their properties their viscosity their ability to conduct heat or in this case how long one kind of gas atom takes to move a specific distance through the atoms of another gas obviously that would depend on how fast atoms move on how far apart they are and above all on the size of each individual atom or molecule [Music] to that knowledge the size of an atom Maxwell could add new insights that had been gained from the use of a new instrument the spectroscope spectroscopes are used to analyze light by separating its various colors or frequencies when one looks at a gas of atoms of a single element the light spectrum consists of lines at certain frequencies a different Spectrum for every element and that fact was a clue into the inner nature of the atom Maxwell's words when the Spectrum consists of a number of bright lines the motion of the system must be compounded of a corresponding number of types of harmonic vibration in other words atoms would vibrate like a simple violin string and each atom would have its own frequencies if a molecule were a simple vibrating mechanical system then the frequencies of the bright lines would be related in a relatively simple way [Music] Maxwell himself didn't expect a simple relationship to be found but in 1885 a Swiss High School teacher Johann Baumer wrote a formula that nearly fit the wavelengths of all the known lines in the spectrum of hydrogen soon the Swedish Master of the spectroscope Johannes ridberg generalized balmer's formula in rydberg's version a series of lines was generated by each set of integers m and n and the quantity R called the rydberg constant was measured with great precision from that point onward it was possible to predict new lines but to understand the nature of the atom in a deeper sense would require someone to make a discovery along different lines and that someone was Professor J.J Thompson a disciple of Maxwell in the theory of electromagnetism and in his own right a man determined to look into the internal nature of the atom and to discover how it worked others would later turn the cathode ray tube into TV and computer screens but more significant in the long run Thompson's experiments showed that the Rays emanating from a heated cathode in an evacuated glass tube were electrically charged particles most important those Rays could be deflected by electric and magnetic fields and no matter what matter was in the tube they were always the same charged particles Thompson saw that all atoms contain the same working parts or in his words corpuscles which would become known as electrons with those parts in mind he developed what was called The Plum Pudding model and more than an intellectual snack it was a substantial explanation for line Spectra of the elements in Professor Thompson's Theory each atom consists of a large sphere of positive electric charge with just enough negative electrons inside to balance the turtle charge and to make it neutral this seemed a sensible idea but it would soon fail a crucial test and other new methods we are able to break up atoms in a great variety of ways and baron Ernest Rutherford Nobel Laureate and peer of the realm come from New Zealand to study Physics at Cambridge and he'd had the opportunity to work there under Professor Thompson to a definite law like a radioactive element and by the time he spoke these words it not only long since succeeded his mentor as director of the Cavendish Laboratory Rutherford had become Headmaster of the atom itself and has given us a much clearer understanding even before Rutherford began his experiments it had been discovered that atoms could split themselves naturally and emit Alpha Ray radioactivity and the progress so when he and his associates passed Alpha Rays through thin metal foils their experiments went precisely as everyone would have predicted nuclear chemistry the Rays seemed to slice through the metal as if it were a pudding with very little resistance and went straight along their course with hardly any deviation but One Fine Day in scientific history they noticed the unexpected [Music] the most incredible event that has happened in my life stated Rutherford after the fact it was almost as incredible as if you fired a 15-inch Shell at a piece of tissue paper and it came back and hit you the incredible event was that rather than winging their way right through the foil some of the alpha Rays bounced back Rutherford realized that contrary to Thompson's model all the positive charge of the atom had to be concentrated within a tiny nucleus [Music] that was because with all the positive charge concentrated in a very small region the electric force nearby would be large enough to repel or at least deflect an energetic alpha particle if one got close enough in addition if the nucleus had most of the mass of the atom it could repel the occasional alpha particle without recoiling very much his ideas LED Rutherford in 1911 to propose a planetary model of the atom according to Rutherford the solar system and the invisible atom were almost mirror images of each other both obeyed nearly identical laws In Rutherford's model each atom had a heavy nucleus that was positively charged his nucleus was surrounded by light negatively charged electrons that orbited like planets around the Sun but while the planets were bound together by the force of gravity the atoms were held together by the force of electricity like a Greek goddess in the heavens this was an idea of incredible beauty and by the same token it was too good to be true for one reason when electrons are accelerated they radiate energy in the form of electromagnetic waves but an electron in orbit would be constantly accelerating always radiating energy going round and round the electron would fall into smaller and smaller orbits radiating not spectral lines but a continuous rainbow of colors and finally necessarily it would collapse into the nucleus foreign Rutherford had obeyed the laws of Newton and Maxwell and those very laws were the downfall of his model of a planetary atom what was needed was an even Bolder step and the path would be lighted by a discovery about light that had been made just a few years earlier in 1900 a German physicist shed light on the Spectrum emitted by a heated solid body his name was Max Planck [Music] his theory held that matter emitted only discrete amounts of radiation with energy e proportional to the frequency of light f one amazing thing about Dr Planck's theory was the quantity h which became a new fundamental constant of nature called Planck's constant it was amazing because in Maxwell's Theory the energy of light depends on its intensity not its frequency but perhaps the most amazing thing of all was the fact that young Niels Bohr took Planck's formula and used it to create a model of the hydrogen atom Bohr assumed that the electron of Rutherford's model of the atom could exist in certain special orbits without radiating energy he went on to assume that radiation at the frequency given by Planck's formula would be emitted or absorbed whenever an electron jumped from one of these orbits to another boar couldn't say why such special orbits should exist but he knew that without them there'd be no hope of explaining spectral lines he also knew that in assuming those special orbits he was stepping beyond the physics of Newton and Maxwell and into the unknown but there still was a problem what would determine the sizes of those special orbits he tried various ideas but the one he chose amounted to this each orbit would have a different angular momentum and Planck's constant just happens to have the units of an angular momentum could it be that the allowed orbits have angular momenta given by Planck's constant well almost in fact the angular momenta are multiples of H over 2 pi the combination given its own symbol called h-bar so in Niels Bohr's model of the hydrogen atom the lowest orbit has angular momentum h-bar the next orbit has two h-bar the next has three h-bar and in fact an allowed orbit exists for any integer times H bar [Music] putting these ideas together the sizes of the allowed orbits are easy to find the radius of each orbit is proportional to the square of the integer n the radius of the lowest orbit with n equal to one would then give the natural size of a hydrogen atom in terms of the fundamental constants of physics its value turned out to be just about half an angstrom unit this number was small but its value in physics was enormous Beyond Compare previously the size of an atom could have been just about anything but now if Bohr's bold assumptions were correct the size of an atom could be seen as a combination of the fundamental constants of nature but for Niels Bohr that was only the beginning he was now prepared to calculate the precise frequencies of the lines in the hydrogen Spectrum each frequency would be the result of a Quantum jump that is the atom would emit or absorb light only when the electron jumped from one allowed orbit to another [Music] the elements of the calculation were all at hand ready to be put together first the sizes of the allowed orbits then the energies of the allowed orbits and finally the energy of a jump between orbits when these were put in rydberg's equation out came a formula for the rydberg constant itself [Music] [Music] its components were once again fundamental constants of physics and when they were all put together they yielded a prediction that could be compared to experiment the agreement between Niels Bohr's model and the experimental measurement was nothing short of amazing [Music] like Newton's idea of action at a distance Bohr's model of the hydrogen atom was accepted not because it was easy to grasp or comfortable to live with instead like Newton Bohr provided compelling agreement with observation that couldn't be denied nobody knew why Bohr's model of the atom should work and yet the best Minds agreed that it did work why because in Bohr's physics like Newtons there's an agreement between Theory and experiment that is quite simply too precise to ignore Niels Bohr took the old physics and with his profound insight into the atom propelled it directly into the future [Music] the nature of the atom is particularly interesting because people were trying to explain something that they couldn't see and which obeyed rules that hadn't even been formulated yet the real question is how is it possible to do something like that where do you begin what you need when you're confronting a set of confusing information is some idea that sorts it out and explains the most important features it may not explain everything but it gets the key things the key features an idea like that actually has a definite name in science it's called a model because every once in a while you get an idea that you like so much that you think it does more than just get the key features and not everything else you get an idea that you think is really true really right when that happens it's got another name that's called a theory well once you've formulated your model or your theory The Next Step really is the kind of thing that we do teach you how to do it's a little like doing a homework problem you have to work out the consequences of your model or your theory you must get the right answer that is right in the sense that it is indeed what your model predicts but then you've got a problem all over again once again you've got to do something that we can't teach you how to do you have to use judgment because inevitably the consequences of your model or your theory will disagree with some piece of information with some datum and the question is what do you do about that how important is it what is the reason for it is it because of some approximation that you had to make in order to work out the consequences of your theory or is it because the experiment was wrong that sometimes happens or is it the crucial clue that shows that your idea was wrong and should be discarded and you must start all over again if I were teaching you how to paint I could teach you how to mix colors and I could teach you the rules of perspective and I could teach you how to choose a canvas and prepare it but I could never teach you how to imagine and then create a painting like the Mona Lisa well it's exactly the same in science I can teach you the facts of Science and I can teach you the rules the mechanical rules for how to do science but I can never teach you to have the imagination the creativity and the judgment and above all the Relentless dedication to work that all great scientists always have if you want to be a great scientist those are the things that you're going to have to supply for yourself so I'd like you to pick those things up and bring them with you when you come back next time I'll see you then [Music] Annenberg media for information about this and other Annenberg media programs call 1-800 learner and visit us at www.learner.org [Music]