Elementary Particles and Their Interactions - Professor Joseph Silk FRS

so well good afternoon everybody thank you for braving the weather here in fact my lecture today is appropriate in a sense because I'm going to take you to the extreme heat that we find in certain parts of the universe in fact the beginning of the universe has been called the closest approximation to hell that we know about but that I will not go into in any detail but I want to tell you what the universe is made of basically and how we study the properties of the matter in the universe so to be in my question lecture let's just try to define what matter is so if I look it up in the Oxford English Dictionary it's physical substance in general as distinct from mind and spirit in physics that which occupies space and possesses rest mass and especially as distinct from energy okay so that's what matter is we've wondered about matter for a long time Democritus perhaps was the first the most famous person to have thought about matter being atomic by convention sweet is sweet bitter is bitter hot is hot cold is cold color is color but in truth there are only atoms and the void that from the fifth century BC and so jumping fast forward to one of the great expositors of physics richard fineman he said all things are made of atoms little particles that move around a perpetual motion attracting each other when they are a little distance apart which is why we're here basically but repelling upon being squeezed into one another also another reason for our being here otherwise we would be blown apart okay so and those the properties of atoms and particles I'm going to try to share a little light on them today so what are we talking about well there are protons there the positively charged particle that some part of the atomic nucleus and the electrons the negatively charged particles much lighter that balance out the charge in the universe so at an atom is a mixture of protons and electrons but also of some neutrons which are basically like heavy but uncharged protons so that's a constituent of all elements then there are interesting particles which have a very tiny mass called neutrinos and I'm going to tell you about those things and I will come then to some interesting applications to do with nuclear reactors and you'll see that we've made progress and all necessary in the best directions but there you are so first of all protons well one you know you like to think that diamonds are forever okay a diamond is made of of protons among other things and neutrons etc and electrons but in fact protons are unstable diamonds are unstable and why do we think this well one of the great goals of physics is trying to unify everything a very very high energy I mean today you know protons are heavy particles electrons and light particles opposite charges and all that but very very different and they even obey different forces one more the strong nuclear interaction one the electromagnetic force and the weak also respond to weak nuclear reactions so it's it's they're very different strengths but when you go to really high energy physics likes to believe and this evidence for this that things converge and we have a unified theory and what this means is that protons and electrons somehow are part of the same stuff in the universe but as the universe goes down they separate and this means that when they're part of the same stuff that means protons electrons can convert into each other you know with them neutrons playing a role proton plus electro equals Neutron whatever but at high energy that all low energy that all stops but nevertheless it must occur at a very low rate today if this unification is correct which means that protons cannot be forever if the universe began in some unified sense as we we think for many reasons that that's how things should have been at the beginning okay so let me and this is a prediction actually of unification it predicts that protons should decay now how can we test this well there's a very simple test okay my body your body contains many protons okay if they were decaying they will produce a gamma ray and this gamma ray will give you can safe there are enough of them okay so we'd be dropping dead of cancer everybody on the earth in years or weeks or whatever if the protons were really unstable now because we have an awful lot of atoms in our bodies it could be very few we don't exactly not know how many atoms you need to give mutations to cause cancer but one can guess these things we know roughly what a fatal dose is of radioactivity we call that it's measure units of Redd's a thousand Reds will kill you in a day basically and and in translating that into more useful units this lethal dose it's like a thousand Watts over one second per person which is roughly ten it doesn't see much right you know ten ten light bulbs Johnny for a second or something you know that seems tiny but if this energy is in the form of poisonous gamma rays you are doomed okay and so you can set some limit because you know basically is roughly a thousand gamma rays that's what it in energy equivalent so we have a huge number of gamma rays per gram but it grounds an awful lot of atoms and so traveling this lethal dose it turns out that if you had the tiny amount ten to the minus fourteen of a gamma ray per year that each atom was exposed to you'll be dropping dead okay within days weeks certainly within a year okay okay so that basically tells us what the lifetime the minimum possible lifetime of the proton is 10 to the 14th years which is 10,000 times the age of the universe which is roughly 10 billion years okay so that seems an incredibly stable limit you you think you might not need to worry then you know things going to be last much longer fine you know we're all here we're not too many cancer rates attributed to getting protons we assume can you prove this can you do better well it turns out that if I a human body you know 100 kilograms suppose I now go to do a controlled experiment instead of a hundred kilograms of detector looking for these rare gamma rays I now have 50,000 tons okay enormous number of atoms and I can monitor this and I can look for the very very rare decays of gamma rays it just takes one gamma ray to give me a light flash if I have a certainly dark run when it interact when it's produced and ionizes etc the scintillations whatever in this liquid and if the water is purified it's a great great system okay so this is what we're doing one experiment that's been looking for a few years and you can improve the precision of the stability the proton by a huge amount okay so here's the experiment it's in an abandoned zinc mine in Japan the biggest one currently running and it was designed to look for proton decay events using 50 thousand tons of highly purified water and they look for the rare gamma-ray induced flashes of light all these little dots around here are photo tubes thousands of them that they did have one catastrophe this experiment when some years ago one of the photo tubes popped and it caused like a chain reaction of popping photo tubes not experiment you know it had to be rebuilt from scratch but anyway now it's working fine this all gets filled with purified water and you and you look for signals like flashes that have color codes like this and they've seen nothing okay so they've not seen a single proton decay event and from the fact that they're monitoring all these many many atoms they can say we can do 20 factors of 10 20 powers of 10 better than studying human bodies decaying okay so now they have a minimum lifetime for the proton against decay into you know get gamma ray plus plus whatever us because your particles meals etc of 10 to 20 times better which is incredibly the universe is 10 to the 10th years okay so we're talking about protons are decayed for many many times each of the universe so we don't need to worry at all about probably a but we do worry because if the theory of unification is correct we should eventually be seeing this decay and there isn't that much room so if we were to have a bigger experiment than this one and the increase this limit by a factor of 10 a theory says you'd better be seeing the protons decay otherwise your theory is wrong and so colleagues in Japan are preparing a million ton purified water detector to go in there the same mine which is being built over the next few years and they will either discover the decay or revolutionize their understanding of physics actually ok right so now that's the proton so you know we like to think that protons are fundamental building blocks of nature but they're not actually a proton I like the electron which is basically a point particle the proton is a composite particle it's made of smaller particles called quarks and there are three of them in the proton and they're about a thousandth of the size of a proton okay and electrons as far as we can tell you know have no extension at all that point like but and quarks probably like that too so electrons and quarks are the fundamental particles out of which the atoms and the nuclei of atoms are made and and we and our basic understanding of atomic physics is that you have a charged nucleus with protons and neutrons and it's surrounded by a cloud of electrons and the whole thing has you know no charge things balance out and it's and because you know electrons tend to like to the clouds tend like to attach to other atoms this this is how chemistry works basically and this sort of mixture of clouds of electrons around positive charged nuclei controls all of chemistry - okay so over the last few decades we've developed a better understanding of the fundamental bits of nature the fundamental particles and here I've tried to show you the conclusions that came from these three gentlemen each of whom got Nobel Prizes for their contributions and they basically developed what we now call the standard model of physics in which you have light particles so electrons and also neutrinos which I'll come to in a moment they're they're part is almost zero mass the lightest ones we know your particles but we have measured their mass now and then partners of the electron called muons which are first discovered in cosmic rays long before we started colliding detectors together we used cosmic ray detection to prove the short-lived particles exist we found them in the cosmic rays at high altitude by the time they'd have gotten to earth they out of decayed they were unstable but we found them by looking for the cosmic ray which was a natural Collider when it ran in the atmosphere it made it made new on since that's how we discovered them since made them into in colliders of course and there so there are two types of muons and then there are particles which connect everything the equivalents of other photons for these particles called bosons and and then we come to the to the quarks and these are different types of quarks all of them make up over hill or all of them make up protons and neutrons and all in this model had a prediction there was one missing element from it and this was called the Higgs boson which is the way we can understand how particles acquire their masses and this was predicted and it took many many years and just three or four years ago they discovered it finally and and so that was a major triumph of of the standard model okay okay so now let me come to something truly bizarre because we've never seen a quark oh this is hypothesis I mean hypothesis led to a prediction and we then measure they expose on but we've never directly seen one of these particles and the reason is there such short-lived particles so the nucleus of an atom is a stable configuration contains protons which themselves made of quarks the fundamental particles but we've never been we can see the protons all right okay but we've never been able to separate the quarks but and the reason is the quarks are just they hold together the proton and the neutrons that go with it into the nucleus of an atom and they're stable and the forces between the quarks don't even though they may have different charges they don't repel each other but we believe that outside of the nucleus the stuff which keeps the quarks together simply doesn't exist and we we think that quarks simply don't exist in in free space okay that's what the theory says and and that's what these guys basically got the Nobel Prize for among others but we we convinced that they're made of quarks now I will show you you know one of these even though we can't see the quarks directly okay we can see what the quacks decay so over here you're looking at this is taken from the Large Hadron Collider one of their experiments with many many detectors giving you these these light flashes from interacting particles and and you see the traces of party was decaying quacks decaying basically so we can see them decay so that's that's why we're sure there exist we see them we see them die basically okay now you may wonder what all those got to do with the universe okay which is where I'm coming from now so in this diagram of the history of the universe from beginning to now we have stars and galaxies and then there was a time as you go back in time this is the arrow of time which we call the Dark Ages and the reason all this is so highly compressed is that the total time from here to here is the age of the Big Bang which is 13.7 billion the but you're now looking at most of that time and this is just a few hundred million years over here okay and back here it's a few hundred thousand years and this greenish thing is what we see when we look at the primordial radiation the cosmic microwave background the fossil radiation from the Big Bang and so we can see back this far between those tiny fluctuations the radiation where in structure form we call this the dark ages where we there are no stars we're beginning finally to understand how to look at that but it's very very indirect but before then things were really dense and hot and the protons were so hot and crammed together that they were just quark so we call this the era of quark soup if you like and this is now one nanosecond after the Big Bang which is why it seems so highly compressed over here but you know that's how long it took and then even before then there was this period of very very rapid expansion well that we call inflation from the beginning so that is sort of the history of how you know development of galaxies and stars and finally maybe because of dark energy things of moving apart bit faster today so that's the whole story of the universe okay so here's another mystery about all of this the quark soup was there that you know the universe is this unique really hot place we can study particle physics that's that's the lesson we're learning you can't reproduce these extreme conditions in accelerators you can get close to make it seeing quacks decay but you can't have this incredible dense version that was the early it's a wonderful laboratory for studying things so here's one prediction that one might have expected but we know obviously is another thing that is wrong just as protons don't decay on a short timescale where is the antimatter I mean if you have a truly symmetric beginning everything is unified there should be as much matter as antimatter okay therefore you know out there this should be stars and galaxies made of antimatter maybe there are planets made of antimatter maybe one day if we ran into a Martian and we shook hands from the earth with the Martian and anti Martian right there'll be a dramatic explosion right because matter and antimatter annihilate they give you pure energy and this simply is not happening we're not seeing a universe glowing with gamma rays so I think if we met a Martian we probably could shake hands it wouldn't be a problem but you know we're pretty sure there were no anti lot there's not lots of antimatter of them and there must be very little actually there must be a fraction of a fraction fraction percent just because we're not seeing the universe glowing gamma rays okay so where'd all this come from well we measure in the microwave background all these photons quanta of radiation okay lots of them and we believe that early in the universe and when there was a lot more matter but probably antimatter too very very early that that matter no matter did annihilate and produce photons which is why we have so many photons today so the photons we measure which are most Dept quote particles far more photons quanta of light than there are actual protons in here today by a huge factor that tells us that once the universe was very very close to being symmetric but here's the rub it can't be completely symmetrical as we wouldn't be here if it was exactly balance between man Ryan Tamara there'd be nothing left over at the end just just gamma rays cool down to radiation photons so there must have been a very very tiny asymmetry between the matter and antimatter built into the beginning of the universe at the very beginning of the Big Bang and it's a tiny amount it's about a few parts in a billion okay well ten billion that's the number morphing 1-point abilities are doing the first nanosecond so we know what is this telling us where did this will come from well I'll come to that in a moment so the moral is the universe is wonderful testbed for these ideas about particle physics about the nature of what matter is and the elementary particles and and if you would try to do the same thing with a particle collider you know that the longer the collider the more you accelerate you've more space from Mack bouncing you know building up magnetic field and pushing things along that's how colliders work basically with accelerators you'd have to build something so at we have an accelerator a Thomas circular tunnel with a diameter of some twenty seven thirty qalaat kilometers and that gets us up to huge energies certainly but he wanted to test what matter is really made of to test the beginning of universe stuff you'd have to build an accelerating halfway to the moon okay which is hugely different and totally inconceivable in terms of building and ever in terms of cost so the wonderful thing then is the early universe is a natural laboratory for studying how matter is made so that that's that's wonderful for cosmology and it's also attracted enormous attention from the people trying to understand particles the particle physics community they're becoming cosmologists too and so then we believe that in the very beginning of the universe there was this unification period and then there's a period now where the strong and the weak was very different quarks and protons are reading from electrons the forces that hold them together totally different and so there was a some sort of transition and we call this as fresh in phase from this symmetrical stage to now the highly asymmetrical stage and when the force is separated and it's a bit like the one we're most familiar with maybe is especially today is ice the melting of ice right so you know we're lucky that we have lots of fish and the reason and if the oceans did freeze completely there'd be no fish okay and the reason they don't freeze that lake for example I'm only the surface freezes is that this transition of the melting of ice releases energy okay that keeps stops too much ice from forming so this is the energy in a tradition of phase from ice to water and the same thing something similar in terms of physics happen as the universe cooled down and this energy which was released at the very very beginning caused a huge increase in expansion we call that inflation so that's the origin of why we think neither of us began very high and suddenly inflated as the if you like the ice melted the the symmetry vanished and became a universe much more akin as with the size of the one we have today okay so let's get let's get back to these fundamental forces so this is a cartoon that shows you how things work so this is the energy scale and so this is where we are today where electric forces magnetic forces are very different from nuclear forces and these are the forces that control radioactivity with the Large Hadron Collider we can sort of probe this range roughly over here only by going to the beginning of the universe can we start getting to these really high energies there's a big mystery at the beginning because we have not got the ultimate unification theory which must include gravity as well as the nuclear forces so we have a big question mark at the beginning many many physicists reign desperately to look iron Stein's spent most of his life looking for this - he failed and many are still looking for the unification of all the fundamental forces the four of them okay but so far with we're pretty sure that we have found the strong force the weak force thing reticles how these will get at high enough energy these unified together so sorry so this is the weak nuclear force the strong nuclear force a brilliant force and finally gravity okay weak weak nuclear force so that's the force that controls radioactivity okay that controls nuclear decay of unstable isotopes which I'm going to come to in a second okay anyway so this is the the big mystery at the beginning but apart from that we we have a good theoretical understanding of how this happened then we're trying very much to prove that this really did happen by testing our theories of cosmology of inflation for example where these forces would have the separation forces would have had something implication for cosmology so inflation then when this transition in phase happens when the forces separate does give you all this energy and there as the universe expands dramatically by an enormous amount when you lose energy and instead of being maybe very tiny very regular just like blowing up a balloon it becomes incredibly smooth and very very big okay so we this naive theory at first it seemed naive now we have more confident in it could explain you know the size of the universe and the relative smoothness that it's the same everywhere you look and these are the two people who pioneer this particular theory Alan Guth and ralindi in the 1981 okay so let's get back to matter vs. antimatter okay so that's you know one implication of particle physics why this is a big wonderful story okay what is this asymmetry that we have in nature well believe it at all left-handedness predominance in nature amino acids all the ones that make life are left-handed so left by left-handed I mean there if you imagine moving a corkscrew forward it can either turn right or left okay and so we we call turning left left-handed and right right handed and and so there's a unique sense with propagation of being left or right and it's like being you know if you look at yourself in a mirror left and right switch but if you are you know that you can't imagine a different symmetry in the mirror so amino acids are all left-handed and I once wrote a book called the left hand of creation and the story there was that you know when we look at amino acids they essentially all the ones that that are involved in life our left hand there are very rare examples of right-handed ones and they simply you know left hand and right hand just don't fit together as well so once you imagine start developing DNA with left-handed amino acids then it sort of runs away and everything becomes more asymmetrical okay so that that's that's the life story in a nutshell things are left-handed when we find me amino acids in space immediate writes there are a mixture of left and right so something has has affected their distribution when life evolved on the earth we don't exactly know what that is anyway so moving back to the universe this is now the story of one of the greatest physicists of the 20th century and researcher of who most notable achievement probably was that he was the father of the Soviet hydrogen bomb so he was able to he designed this basically with as far as we know little help from from the from the Americans because it came very soon after the u.s. development and but you know he went on to regret the use of nuclear bombs very much and testing of them and got the Nobel Peace Prize in 1975 anyway as a side effect of his nuclear work he was a cosmologies as well he in particle physics and he was fascinated by this problem of why is there so much more matter than antimatter in the universe and so he said that well you know we normally you might want to think that baryons you know that is the number of protons over antiprotons they're both baryons should be equal okay but in you know and you and whenever you make them by having nuclear interactions you should conserve the excess of one over the other well he said no that can't be right you must violate this somehow them you must develop an excess of one over the other and this is what's eventually going to lead to the symmetry in nature the fact that we don't have any anti people around us and their stars around us or whatever and it was a challenge because in the grand unification idea of everything things really are symmetrical so this had to break down at some point so first there were to be some built-in violation of the net number of particles over anti particles then you have then these particles or anti particles when they decay into electrons or or the positive electrons called positrons they can't actually balance each other there must be so many symmetry to otherwise you'll also end up with exactly the same thing and then finally the universe had better be expanding so when things do finally decay you're left over with them with whatever is left and that that would explain what you have today so that was what what he was arguing for okay and that is a basis of our current understanding of this okay so now let's talk a little bit about I've said a lot about protons now let me move briefly on to electrons so they were discovered electricity was discovered by the Greeks if you rub amber then that as you know it gives you static and that's electricity basically and the electron was discovered by Joseph Thompson measured empirically at the end of the 19th century then it was realized that instigated but by Maxwell James Clerk Maxwell Scott that when you move electrons when you accelerate them they make waves electromagnet waves and these are the basis of all we know about radio waves okay so moving electrons accelerate electrons make waves and it was first applied ideas like this by you know while it wired connections transmitting you know voice into current impulses so using wave-like properties of electrons and and by mile curtain that bar this is by Alexander Graham Bell and also by Marconi who first did transmission without wires of radio waves and showed that and so this is now the basis of our whole modern society with radio television it whatever okay okay so this came about the early 20th century so particles electrons are particles but they're also waves and so the idea that waves are particles and vice versa was first brought up by French aristocrat physicist called Lewis debroglie and this of course has been used in devices like the electron microscope we can use the wave-like part of electrons to focus electron beams and get incredibly smaller they're tiny wavelengths Curie high resolution and so the classical notion of an electron looks something like this that invented by also Niels Bohr one of the founders of the quantum theory that here we have the nucleus with our neutrons and protons electrons going orbits around it okay and so that's the clarity of electrons in fact the quantum theory showed that this this model is much more naive and there's something called intrinsic uncertainty invented by Werner Heisenberg famous Heisenberg uncertainty principle which is that you can never exactly say where an electron is you can only define it with a certain probability its position and this is the probability of observing electrons they and here's the central nucleus and the electron will have some position in this orbit but you can never say exactly it's at a certain point okay okay so let me stop there with electrons and move on to neutrons the third major component both matter and maybe one of the most crucial components of all because the neutron weighs very slightly less than the proton and it's this difference between the proton and the neutron when you combine them together you release this difference in mass by Einstein's famous equation energy equals mass times C squared you produce energy and so if you so you start off with hydrogen which is pure protons and you build up helium which is a mixture of protons and neutrons and by doing that you can release the mass difference and that is the source of all energy so electrons and protons and neutrons make atoms and we call the result the periodic table of the elements and again this was also a discovery from the late 19th century by a Russian and so this these all the the color coding in indicates groups of elements I mean these are the rare the noble elements supposedly like argon and so forth and and you have metals on one side and nonmetals on the other but ignore all the colors but basically you can see that all of these elements the most from up to element number 94 wherever there is a natural elements all the rest were made in accelerators and are unstable and we go up now to element 103 are there abouts okay so these are the these are the various combinations of electrons in the orbits and protons and neutrons to balance the charges in the nuclei and and the numbers mean you have up to this is the number of charges that you have this so hydrogen would have a charge of 1 and helium of 2 to proton to the nucleus and we're going up to 103 okay charges combined with various numbers of neutrons to give you the mass they have via masses okay so how do you release this energy difference between electrons and protons again the genius behind this was Hans bethe a German American scientist and he showed that when you bring protons together and eventually build them up into helium you do release this mass difference by nuclear reactions and these go on inside the Sun and so this is the source of energy of the Sun it has to be incredibly hot to bring two protons together many millions of degrees in fact but that's sufficient to do that and it's caused by the gravity of the Sun the center's very hot and this is naturally a force that can produce the power that shines at us and we're very grateful for that so the bottom line then is that all stars in fact are powered by a fusion of of protons and and neutrons nuclear together and in fact all the elements up to iron are made by this type of fusion involving just bringing together things of thermal energy and so it's sort of interesting that you can divide up all the elements and this is the number of the elements of different masses okay going up starting from hydrogen and so there are elements that go up to roughly I'm a bit about iron over here this is the peak of iron and then you get rarer very heavy elements this is the basically the particle number the number of charges in the nucleus and if you look at the stability of a nucleus you find that iron is the most stable of all the nuclei so when you fuse things together you build up to iron as stars heat up as they shrink in an age and you build up to iron and then it can't get any more energy out the only energy after that is from something catastrophic which must involve something like an explosion of the star and I show that example I've talked about that already in these lectures and that explosion then it radiates so many neutrons that you could then use capturing neutrons to make all these heavier elements so this is you can think of this as cooking and then this is something much more violent to bring things together so fusion give gives you energy and here it you have to provide lots of energy to make build things up okay so that's the difference iron is it's like the ultimate slag heap of the universe okay so this gets us into the notion of what we do with thermonuclear fission as well as fusion because some elements are going to be unstable and if you have an unstable element that will break up and give you gamma rays and release energy maybe in a violent way so if you have the right number of neutrons to protons they roughly balance each other the things are stable but all the elements which have large excesses one way or the other are radioactive because they break up okay and they give you gamma rays as well as these products and we call this process of breaking up beta decay and if and you can convert you the protons to neutrons and neutrons to protons and it can either if you have too many or too few so this is a way of understanding why it is that we have a stable regime of nuclei so this is the belt of the value of stability we call this okay as you go up in in in mass element if I have too many neutrons or too few neurons I there are lots of isotopes they can be made but they they're unstable okay okay and so they can and so we can study we can by you know having collisions in accelerators we study how all of this works we can synthesize many of these things and and work out how radioactive they are how rapidly they decay but basically we we can understand why it is that many elements higher than helium on words up to iron and way beyond silicon these are the stable elements there are isotopes of them that is with a certain number of neutrons and protons some of them there the stable ones and then you have others that are and say one way or the other okay so what I want to do now is talk to you a bit more about radioactivity so the unsung pioneer over this field is Mary Curie of course and she managed you know was created with two Nobel prizes actually for her work in in discovering radioactivity and using radium was not necessarily the best thing for her to do in those days and she did die very young probably cancer the before her the discovery activity was Ernest Rutherford and the neutron was discovered by James Chadwick um both in the UK and Mary Curie was polish but worked in France war for Korea okay and so the idea is that if you have again proton plus Neutron that's very very stable that's a hydrogen nucleus but as soon as you have in excess of neutrons you can get a serious deficit when you build up heavier things things can be radioactive and you get x-rays gamma rays whatever which are the more dangerous things so again going back to what we see around us the earth is not a good place to do this because there's no helium the second most of mine element the universe after hydrogen was discovered in the Sun so you wouldn't you know so the earth isn't nests are your best place to study what we called cosmic abundance distribution you want a larger sample of stars which should have everything in okay so looking at the Sun and the solar system then as you go to heavier elements you see hydrogen dominates then helium is next and then less and less elements with a an excess near ion and then things go down so again this really shows you a blown-up version of this and so here here from hydrogen helium so these are the relative amounts of these things these are all factors of terms a hydrogen is immensely more abundant than whatever you know platinum or uranium as we as we know of course but you know nevertheless you can you measure all of these things and you notice this is interesting zig zagging okay so this is between art and even nuclei and this zig zagging in abundance is proof that it's nuclear reactions that did this okay so this is a natural prediction of nuclear fission and nuclear fusion that you naturally get this it's odd-even structure of the relative abundances and then we believe that when you go to the light ones below I and all of these are made in stars first you burn hydrogen is in the Sun and then when the star heats up it gets hot enough in the middle to burn its helium and so on eventually we wind up with iron when that that's the end of the star and if it's massive enough it may then explode collapse even more and give you the environment which can they make all these heavier elements by producing lots and lots of neutrons again that explosion and we call that a supernova process for example and and these things where the star does not explode that might be what we end up as a white dwarf which is the likely fate of the Sun in five billion years time so to summarise the Big Bang makes helium we're pretty sure the stars make too little of it but almost all the other stuff is made in stars are the lower mass stars like the Sun to make the elements up to iron and then the massive stars which are unstable at the end to make all the heavier elements because they have this wonderful supply of nuclear instability and neutrons to build up all you have the elements okay so here is briefly then a picture of a star that like the Sun the future of son and in the center you see a white dwarf star this very compact ball of heavy stuff iron maybe not in this case maybe more oxygen and and carbon may be traces of iron - that's the end point and all of this stuff has been shed off as the Sentra heats up and burns away more and more effectively and all this stuff is being shed off to be the eventually infiltrate the other set of clouds from which our solar system then condenses so this explains where all the carbon and our bodies come from it's come from events like this and then as you move so this is the magic line of of protons versus neutrons and as you go more and more up in neutrons who have now to go to more exotic environments to make these things and then of those exploding stars and that's this is an example of an exploding star and this is the eventually you'll make what we call a neutron star a black hole in the center but all this stuff is now highly Neutron radiated and gives you the heavier elements so that's some a story for neutrons I'll say a little about neutrinos they were discovered because there was something missing when people studied the first collisions radioactive in directions putting particles beta decays something was carrying away energy that was not a particle was not an electron or a positive electron and the name was invented a little neutral particle little Neutron sorry and and this is the source of radioactivity and these two gentlemen Enrico Fermi Wolfgang Pauli with with the with the pioneers of neutrinos and it was invented to account for radioactivity basically when you have too many protons versus neutrons or vice versa and these things carry energy away basically because there was no mass coming out but since then we've studied them and we found they do have a mass okay and how can you study them well they are incredibly weakly interacting particles neutrinos pass through us very freely far more easily than any other particle can they are not particularly dangerous but to detect them you need a huge amount of material to have a probability of observing one neutrino and so and they were first detected in in experiments in mines deep underground to avoid the cosmic rays from the atmosphere that gives you a contamination ceylon produced in two units - so you want to avoid all of that stuff in the first experiment the pioneering one they had a hundred thousand gallons of cleaning fluid while cleaning fluid because there's a reaction between a chlorine atom which becomes a radioactive organ item when it absorbs neutrino very rare and so what you do is you study a hundred thousand gallons of cleaning fluid you bubble there every few months you then bubble some rare gas through it to pick up a new radioactive argon and they look for the radioactivity and and they manage to detect neutrinos in in this experiment and and in particular these were neutrinos that came from the center of the Sun so this was the so one now had direct proof that the nuclear reactions proton proton helium have to produce neutrinos and finally they were discovered by this experiment involving cleaning fluid since then there's many more experiments I showed you this one already this is the one in Japan where they have these 50 thousand gallons of ultra purified water you see the engineers going around testing the photo tubes before they will fill this even further again they detected the fact that neutrinos coming from the Sun but there was a major discovery they made to because it turned out that the neutrinos that the previous people had found were from the Sun there were too few of them they missed by a factor of three we know exactly how much the Sun radiates it's a wonderful precise machine the Sun and they found to a few and it was the new experiment that that looked for this difference and a combination of this experiment and a later one in in Canada also in a mine found that neutrinos actually changed from the type of the electron neutrino to once associate with muons yet the other light particles electron like and that the chance that are giving them and since there are three types altogether we get 1/3 arrives at the earth and that was proven by these two types of experiments okay in the future there are new experiments going on and we're gonna it's a wonderful future actually because what we're gonna discover what is predicted by the theory is not just that there are the energy neutrinos what what they've measured up to now is is just you know a broad rate a broad range of energy excuse me but what you predict in particular it neutrinos come with a very specific energy to test the reactions and these are what we call you know spectral lines features neutrinos you experiment to look for those and can even learn even more about the Sun so the experiments have done so far they've talked about just just measure this part they they measure neutrinos they measure the efficiency of neutrinos but the new experiments going to look down here and lower energies and get much more detail so that's another major activity in the future for the whole field okay finally let me tell you about nuclear reactors okay so here's the most important one of all the one on the left here the Sun it's incredibly inefficient this is the number of what's the Sun produces the earth captures a tiny fraction okay about 10 to minus 9 one billionth of all the energy well this is waste it goes out of space so we conjecture that some future civilization will do a better job than we have so far it using solar energy in the meantime the best we can do on the earth is this damn hydroelectric dam which gives 22 gigawatts so a tiny fraction of what the earth needs but that's our biggest man-made nuclear hydro reactor not using the nuclear power but of course what about we're trying to reproduce the effects of nuclear reactions thermonuclear fusion in the lab these will be the first steps to controlling nuclear fusion which we are in principle wonderful source of energy much more efficient than nuclear fission because that's current nuclear reactors you know which used uranium-235 you know very messy things but if you can actually make energy from light stuff from water say or hydrogen then you'd be doing much cleaner job and people are designing experiments to test for this no success yet but they're on the way to showing that in principle it can be possible in the next 50 years probably then here's another application of nuclear fusion maybe not the ones that we should be proudest of but there was an epoch that we did many use the first nuclear bombs in Hiroshima for example and then did many years of testing okay and going up from the original 15 this is dynamite equivalent of explosion 15 kilotons of dynamite equivalent going up to many millions of tons in you know with French and US tests these two US tests and Russian tests as well and the Russians were even discussing using nuclear fusion to develop big dams and do but although all of them all of this has long since passed we've now living in a more civilized era with okay so to summarize where we are I've said that big bang makes only the lylat mostly helium but tiny traces of other elements and that's simply because the massive ones are not stable you can't fuse things together and build up all the heavy elements stars make helium as well a small amount of it but all the other elements and then supernovae give you all these neutrons because the extreme temperatures and the in the centers to make all the elements okay so that sort of is a story my final point is that here is one of the most curious things in the whole subject in Gabon in France as a uranium mine okay and in this uranium mine it's one of the world's most important uranium mines the French scientists found traces of excess uranium-235 now that isotope is critical for making bombs and it's got a certain age as its partner isotope you're any of the dominant one 100 times more um 238 and this a half flight of uranium-238 is about four billion years and from this you can actually measure the age of the earth okay by measuring the amounts of product relative to what you began with in uranium and so when they started the uranium deposits in this mine they found to their big surprise that the amount of um 235 one percent of uranium in this enriched uranium region was miss a tiny amount was missing a few percent was missing and so this worried them for a while and then they finally realized that aha once about and for dating they could figure this out about two billion years ago there was a natural explosion deep in the mine probably caused by seepage of underground water it was a uranium rich to start with and the water acts as a neutron moderator stops there flying away in enables you get more chain reactions and so that seems to be the story it's the one example we have of a natural nuclear reactor two billion years ago in the universe okay that's okay so my final comment I think the the what I've told you about so far doesn't work okay completely we've been looking for new you know what does work so the Higgs boson was a wonderful thing I mentioned that before these two gentlemen are two of the primary people that found it and he confirmed it a standard model but what's missing is it can't account for neutrinos which have a small mass that's why they oscillate and we see one-third of them they don't include gravity and they don't account for dark matter so finally the new thing that's come into this field it's called string theory and the idea now is that we no longer have point masses I said the electron was a point mass the cork was a point mass so the new theory says well maybe we need something new instead of being point mass let's make them not and this symbolizes interactions of particles point base particles with each other to come into new ones come out that's some sort of complex interaction instead let's imagine these particles are little strings ok tubes of string in some energetic sense okay instead of being point like and so this has been an intriguing theory because it's had it's got a lot of equations in this is one of the prime X positive string theory but the beauty of string theory is that it can account it's our best hope at the moment of going beyond the standard model of particle physics has not succeeded because although it could explain many things in nature it has not been tested it occurs at such high energy this is the energy of stuff at the beginning of the universe effectively all the mass that its untestable unreproducible okay so this has led to an intriguing controversy we have people like Brian Greene writing what I recommend these books writing wonderful energies to string theory of the quest for the ultimate theory and skeptics string theory is a theory that's not even wrong ok the failure ok so there you are I think in we know we've made enormous advances in in in understanding what the matter is we haven't got the ultimate theory yet so the usual you know things you say now is at this point give us some more funding we want to build bigger bigger telescopes look at the beginning of the universe or we want to build bigger and bigger particle colliders to go beyond what we can do at the LHC and in Geneva or have we in fact reached the limit of what we can do given any reasonable funding source you know to give you one final example a new telescope spoke which would have been had 100 times the field of view of a Hubble telescope done a wonderful job so the universe will cost roughly half half a an aircraft carrier nuclear aircraft carrier and it was just cancelled by by the American administration so you know because they want to prioritize other things so we're at a point now this is what just one one an example of where we seriously have to decide and want to do next in in fundamental physics do we just build bigger and bigger or maybe have just be cleverer so thank you that that's my final message [Applause]

Channel: Gresham College

Views: 57,081

Keywords: gresham, gresham talk, gresham lecture, lecture, gresham college, gresham college lecture, gresham college talk, free video, free education, education, public lecture, Event, free event, free public lecture, free lecture, astronomy, matter, elementary particles, space, SOLAR SYSTEM, joseph silk, Physics, energy, democritus, atoms, richard feynman, perpetual motion, protons, electrons, neutrons, neutrinos, nuclear reactors, Decay, Super-Kamiokande, universe, quarks, fundamental particles

Id: 7orJnDFrClI

Length: 55min 41sec (3341 seconds)

Published: Thu Mar 08 2018