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# Topics in String Theory | Lecture 1

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with some philosophy let's start with some general philosophical principle it's a principle that philosophers call reductionism reductionism is the principal of a philosophy that big things are made out of little things and little things are made out of Littler think so that's 1 element of reduction How many people would agree with reductions in here that now our molecules amid of Adams Adams and made out of electrons and nuclei nuclei made out of protons and neutrons protons and neutrons are made out of quarks in blue ones so for I call at the a building blocks theory that houses made out of bricks Wrexham mail molecules and nobody in their right mind mistakes a house for Brett right we all know the bricks from the houses the smaller the houses who also simpler than the houses so that's another element of reductionism as you go deeper and deeper into the layers a reduction 3 things get simpler or at least we hope they do that sets a pious hope of reductionism I think from what we've learned both from strength theory from the quantum mechanics of gravity and so forth that modern theories really do spelled the end of reduction but before In reductionism it's interesting ask how well it does as a philosophy and there's a year as a theory of elementary particles is reductionism is right particularly in that aspect which has to do with simplicity as you go deeper and deeper you might have expected that by now Elementary Particle Physics would be pretty simple gun several several layers and it is simple yes that is not what it is very very complicated and to measure how complicated is you can simply ask how many different kinds of particles are there and a particular particles which are on unexplained well something like about 75 different particles all of which in the standard model of particle physics standard model of particle physics maybe only about 20 EUR maybe a little bit less than that independent particles that don't have hiked relationships between each other so it may be 10 to 15 or 20 years distinct difference particles depending on you've count how many parameters parameters again unexplained constant I have to go into the Eureka makes sense well 20 20 distinct parameters but the fury is definitely incomplete we know that's incomplete doesn't have gravity that doesn't have thought matters doesn't have the particles the necessary for inflation are the fury of a cosmic inflation makes sense end it has a terrible fine-tuning problem we've talked about that fine-tuning problem that in order to overcome this fine-tuning problem people introduce things like supersymmetry might guess is that supersymmetry will really be discovered at the LHC but if it does it ends about a hundred new parameters well I take that that it's a hundred In way in is an unknown number but that it is now 0 because it's necessary for understanding the breaking of supersymmetry y supersymmetry is not a real another 100 parameters a doubling of the number of all of the particles classes are whole bunch of stuff that's necessary Ferrer inflation and the other by the kind you finish a standard model particle physics together with all the bells and whistles that have to be aided understand everything is a theory with about 200 parameters huge number particles are and totally unexplained connection is so as not simple it's it's a very very complicated and complicated I think you have to say Well we haven't gotten anywhere near the bottom yet always after admit that maybe reductionism isn't working butterfat the idea that reductionism is working that's not what I meant when I said modern theories so spell the end of reduction what I meant was more theoretical fact our more theoretical fact based on both quantum field theory in string theory soccer you a little bit about what is not in particular about the idea that endlessly or not necessarily endlessly but that that is we can always held the bricks from the house Is it really always true that we can't tell which things is more fundamental than which they and give some a couple of examples places where in quantum field theory that idea has broken down broken down badly the 1st example was an example of a very simple kind are not going to explain isn't the tell you about it studied quantum field theory of one-dimensional systems models models in which there was kind and only 1 dimensional space very easy to draw a dimensional space and of time and that's all areas and particles what was discovered is is a simple number fact almost every theory in the area of this type can be described in terms of particles which are fairly onto a arms are fair Meaghan's are particles that satisfy the exclusion principle there the particles which have this odd behavior that they can't go into the city can't put toward them in same state electrons affirming and quirks of firm eons of photons are after me bows on the opposite of a Fermi answer bows on and all particles are eager for me of bars are in this mathematical theory you start some particles which are fairly firmly on the field the object which describes the firm eons which creates an entirely different ones we called side

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mathematical terminology so important he wasn't portent is the basic starting point is a field theory of fairy now what happens if you put 2 fermions together we make and if you put Toyota identical ones together you Meg 0 but supposing tool not quite identical 1 so right next to the other but not right on top of the baby maker bows out pay for example protons of surveillance electrons of trivial hydrogen atoms are bows on Citicorp from and evolves asked to firmly on the you put together almost together maybe this summer attractive force between them which holds them together and these particles which it now come has clearly composites they can move up and down there also particles protect composite particles those composite particles of both guns and bowls ions are also described by fields you would think that that's not really a fundamental field you think it's a field there is some effective description of the 2 particles start together new qualify fight creates bows on secrete pairs of these fermionic but sounds very very very straightforward but was discovered in these fields theories is that you could rewrite exactly the same field theory in terms of underlying starting point which started with the bows on the Strathfield theory over he said now you start with the buzz around the Fermi eons cannot be composites of the blows up because any number of bows aren't put together they bows on but these firm Iran's can be described a totally new way that described the plot is not plumbing kind anymore is plotting the feels fine as what's called a key in the field for life a place where the feel fine jumps normally jump-start the present a jump around a submarine that small only jumps Over a distance some distance jumps from 1 value Co another its call the King it's called a kink if you wanna picture issue taken don't worry arm of rebelled but imagine belt periods in here out the blackboard so that it's not twisted that's gotten OK now take the belt and twist it rotate send around by 2 pie twisted once it comes back to its original configuration over here its original configuration over here but in between its twisted battercake that's a kink in my I often do that often do their work of my belt on discoverer 2 hours later when talking to somebody belt was on wrong at that's school kink the bizarre thing is that these on that we start we thought we started with the break which built the houses firmly on bricks which build bows on houses can now be Re represented as kinks and the HMOs on-field kinks which are extended I think and massive much heavier than the bows on which other houses which of the bricks nobody could say it's part of equal Philip parameters researchers theory field theories have parameters it turns out that for some values of the song ranges of the parameters it's much more convenient think our Beyond the fundamental building blocks and Ono range of the parameters the bows on a much more efficient at studying the theory and they are the more useful building blocks it simply becomes a matter which is more useful as more useful for think of signed firmly and or a fine bows on the fundamental objects of theory and that depends on the parameters that depends on coupling constant coupling constant but usually called G matter what's called Stop starting with very small g you discover that much more useful to think Our be firmly on the starting point easy deal with easy to study these things are calm posits that are held together by complicated forces now she this coupling constant and increasing in an increasing 0 incidentally when that GE is very small but that King fears I'm very tight theory that now they start changing the cup and that's why the firm was a small object as a small object because it's a small cave nite you start combing the coupling cars making it stronger and stronger what happens is the king spread out it gets less West Point Light but the bows on starts to behave more and more like a simple elementary object so the question of which it is fundamental and which is composted doesn't have a unique cancer by varying the constants you could make you could morph you make up 1 thing going to the upper house is going to the bricks or speak on that was a case that was known 0 since I was a very young physicist the key word is called boson a it's making from here on out of Burroughs on for 4 HBO's arms other anything like this in elementary particle physics it is believed that the R and the belief centers around electric and magnetic monopoles for me you electric and magnetic monopole is elected on appalled could be electron could be any charged particle would never charged particle surrounded by an electric field is electric field and let's take these zealots take particle Toby let's say an electron electric charge of an electron is very weak as a matter of fact it's a very weak electric charge the amount of charge I'm not because electron small that's actually not be reissued Isabel mention was quantity what's a dimensionless quantity called the fine structure constant before the fine structure constant it is a measure of how much an electron would radiate if you know if you a accelerated its suddenly or if Europe Inc the an old plowed into stopping radio how much order radiate comedy photons were radio yesterday is an electron will radiate on average about worn over a hundred photons the live saying it is electron only radiate about 1 out of a hundred times when stop radiative photon so the electron charge is very weak the electron very simple electric field around it is rather areas rather weak because electric charge is weak and it doesn't do we very much to the neighboring space around it it doesn't polarized vacuum it doesn't create electron-positron pairs to any appreciable amount but chargeable it is also believed that in nature they do exist magnetic monopoles most physicists like myself believe in the existence of magnetic monopoles they they come up in so many theories that there by now they but they've never been discovered that never been discovered in a laboratory a magnetic Amana pour is like the end of a bar magnet North South of you just unimagined end of the bar magnet with a bar magnet itself being so thin you couldn't see it then waving around the bar magnet would look like waving her a magnetic Mama Paul where the magnetic field comes out magnetic by Paul exactly the same way the electric field comes out of electric model remember the Mont polls that theoretical physics seems to for doses however a magnetic charge which is huge very large before us between 2 magnetic monopoles would be 10 thousand times larger than the force between tow electrons of saying this so that means that the mind

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that magnetic monopoles make large fields around them and do very very complicated things may come very complicated nests around there that complicated mess is extending its brought the same way that the king was broad and Mass very very heavy because so much going on in the vicinity of the sample and not really talking about the end of a bar magnet I'm talking about the end of the bar magnet without the bomb Big Mama polls they have been discovered in the laboratory because too heavy to make will ever be discovered perhaps but most serious believe in them but the point is more important point to the mathematics of quantum electrodynamics which contains and electronic is totally unclear about which of them is the elementary particles it depends on the fine structure constant depends on how for the fine structure constant defined structure constant which is basically square of electric charge of electric that's not much less than 1 electron is a simple 1 it's simple it's almost point like it has very little structure the charge is very weak the mom Paul has a very very complicated structure that limit the magnetic field is very powerful and those very complicated things to space around it and that's for its heavy why happens if you could imagine that you'll have a dial but you could start dialing redialing B fine structure constant start increasing 1 happens is stocked increase it the fields surrounding electron becomes stronger effectively electric charges getting larger field is getting stronger it stops to do more complicated things in the vicinity of electrons what happens the magnetic the main may not opposed us to shrink this is not something we can that I can explain easily but the fact is it starts to shrink it gets more more later in life that the electron gets heavier and heavier Zell forgets large where else I get bigger than warned they simply In the change the magnetic monopole becomes a light and small and fundamental be electron would become heavy complicated and full of all kinds of structures and basically they would just interchange himself so again which is the building block is electron booze a complicated structure here is making up the Apollo the which is simple and was complicated structures making appeal truck modern quantum field theory say call can't tell the difference of cost you may want to choose 1 of them is the starting point depending on the fine structure constant it may be more convenient to think of electrons is fundamental and less convenient think about among and that of course a stroll for reuse electrons as starting points for quite dynamics among models problem if you police while you can count on our country does not really redemption adjust his everything's moved smoothly varying ah but it doesn't in any way if flights a right now you chop up with things and the little boy will distances and U.S. Watson in those little distances and that tell you what fundamental things are crammed

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in quantum field theory is just an endless hierarchy of distance scales and so you never get to the point where you could decide which forms fundamental he bric-a-brac knew what was right for miles so far will fluctuate may fluctuate of every scale so the never smooth never smooth and some cents some of the great difficulties of understanding quantum field theory for reasons of mathematics mathematicians have so much trouble with it is sober because of trend since calculus of brick are studying fields they fought to a tomato very too much but the bloodsucker that now but no than theory say string theory haven't answer the what is the 1 are the fundamental building blocks up ominously string theory has an answer for the fundamental building blocks must be strengthened strike focus only tell you a little bit about we talked about be bring just right so I'll just very quickly just remind you be brain his we don't need an all-out about mathematics pictures of good enough brains are objects that were discovered Feuer a fairly indirect mathematical route they part of a few you can't get rid of them stuck with it their objects which are 1st of all very heavy they could be they come in different dimensionality the simplest 1 is called the D 0 brain and zeros stands for them major what's the what's the dimension of a point originality point with 1st widow dimensionality wine one-dimensional dimensionality of a membrane or a two-dimensional surface took less than Antonelli the . 0 so in that sense particles point particles are called Zero dimensional objects even if they are not absolute points is still regarded as 0 dimension dimensional they are not extended and higher dimensions of particle is a D 0 brains the word brain incidentally purity and eat brain comes from membranes and membrane surface is a tow brain to Wayne two-dimensional brain what's a string 0 1 rain what's a three-dimensional blocked over a solid stuff that's a free brain continue you picture of 4 brain not we honor not directions the proposed to red if we had more directions we could make for brings back so these Delo brain is a kind of particle the destabilized mathematician there's life some portent of the property of these D brains in string theory Is that they are places with strings can a fundamental strength what's called F string sisters have strained F standing fundamental the building blocks of things a grab at times made out of all the good stuff that we talked about the fundamental strings these no brains are just sites on which a fundamental string command and that a number of fundamental strings can and will be 0 brain and that means but these 0 brains can have strings attached to them and they typically do it you would probably 0 drained by scattering something offered you would find out but it has a collection of strings attached would like that and thank the hardy you probably the more string you would find closer and closer and you will eventually come to the conclusion that could be brains in some sense made up of strings but it's got a lot of strings and heavy it's much much heavier then the ordinary strings themselves I will call for its very massive and that court is place which brings can the mathematics at windy Mr. its discovery and so forth is less interesting and the fact that these things are necessarily forced with massive complicated

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and are there during the series now these at 0 Playing also D-1 brain be 1 brain it is like a Strang works Our to it came about because as much heavier than ordinary stranding much more mass per unit length but also has a property it's a place where ordinary strings fundamental strings can aid and again that's kind of a on object which is made up about every strings of Cable Cable a thick cable of thick cable much thicker than the ordinary things and much heavier heavier in the sense that preview Lane its masses much larger there much in the central at what strings next each other these great and he has an extra these ER D 1 brain can also be called a G-string is an ashtray are fundamental strength its its narrow its like these 2 objects are related in very much the same way he said this 1 this 1 on related in very much the same way and could you guess what was in it my polls electrons the fundamental strings alike electrons the building blocks the starting point for string theory these objects of big heavy calm positive structures which are made up of cables of lots of strain heavy and they obviously composite string theory I said it has no parameters I will never come back to where there will come back in a minute to the question whether it as parameters for practical purposes for the moment it does have a parameter parameter is the coupling constant what's called G G is a coupling constant a parameter of the theory which basically tells you the probability of Estrada's we're going around might Split in half its very similar in many ways probability that an electron accelerator will emit a photons strings wheeling around if you grab a piece of poet it might it can break the probability for that is called the coupling constant if the coupling constant is very small then the strings or light fins and they look very fundamental point like well my point like But lying like when she is very small be string is very heavy very complicated and full of extra guess what happens is you start increasing G as you start increasing GET the fundamental string here hence the break off little pieces those little pieces don't disappear and fly off the hang around and they form a kind of the atmosphere around the fundamental strain and a fundamental strength gets more and more complicated in the same way that electron complicated when you start increase the fine structure constant the fundamental strings starts to develop structures at the same heartened while at the same time the D string starts to get some fundamental string gets heavier because it gets heavier it gets harder and harder for the produce fundamental strain so these structures get beat as they did they now these drink its Finneran center eventually at some point with G is about 1 they stop the look exactly like each other and then if you go to even larger coupling constant these string gets very simple turns into a and line like object and the fundamental string as complicated as D string was originally which 1 describes or which 1 of these strange things when they formally loops create particles create Gravitt times create photons and so forth well G is much less than 1 than the fundamental strings when they close back on themselves form the particles the ordinary particles of string theory but as 100 larger these things get more and more massive the fact eventually they turned the black holes and what's left over is these strings which get wider and wider and the grammar made up of these strings this is called these strained fundamental string Rallidae it's not the usual may encourage easily called S 2 hours I do not know whether stands for I make up the terror about their or call strength duality strange yes strength of the coupling constant soul and again we find this pattern that you can't say it once and for all of which thing is more fundamental than which you just morphing into each other now they morph into each other as the coupling constant varies but only if you could say well at least even say that the coupling constant is more than profitable to think of the cover of fundamental strings is from has fundamental of the coupling constant is very large switch them the problem with that Is the coupling constant is something that can vary in space it's a field G In string theory is not constant it's a thing which can vary from place to place it satisfies field equations just like the gravitational field and sell you could easily have a situation in a mathematically idealized string theory where the coupling constant goes from very strong a very weak as you move across the universe in 1 place with these strings a fundamental and the other place we have strings of fundamental and in between and nothing is simple that's a character of cosmological scale is committee less Bassett with debt yup yup absolutely that's all we know that's 1 of the ways we know string theory mathematically precise urgent string theory is not world were get back alone what little more than we 5 out of the

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this treaty signed a the lay not young let's go back to Parliament to them from there now my vertical axis time and he has an electron world aligned as a point that means it's a very thin worldwide not vertical remember vertical for physicist always means time horizontal means space that so here goes a point electrons Our moving through time or speak that point electrons in the those photons now when the fine structure constant is very small the emission and absorption of photons is very intimate means because part of that the orange very many of these photons being emitted what about the photons themselves well a photon To create and annihilate an electron-positron pair so that means that photons could create electron-positron parish but no each for these electrons here can also emitted from Times is assault on ending the hierarchy more and more distant scale of structure with an electron as warm and weak coupling constant is weak This is a relatively small effect fact to small effect and the electrons In a laboratory looks pretty darn point like you have to work very hard to scatter things off it and see the electron has this composite structure there is it's a it's a winner's the energy come from the energy the energy and the energy needed for this process is part of the Of the proton our missiles Elektra is part of the missiles electron it is a mess of it is a mess of electrons with the energy come from to create a corn field around your chance whoa appointed you don't start without the you the call Longfield was always there is the matter of fact Sepp Blatter that I see true electoral period produced that I don't see the Mets problem but was let go of their health if however what is that energy that energy is a kind of potential energy kind of potential energy our debts connected with weak interactions between electrons and photons the energy of a system and quantum electrodynamics is not just the energy of electrons plus energy of the photons social interaction energy between and that's that that interaction energy source but but but that's that's still and empirical fact that a few scattered things often electron high enough energy you will see this structure Emulex then well OK we can hope we can debate whether energy of experiment producing what you see or whether it with see honor we can debate that but the fact of the matter is but when you scatter things off electrons what happens is complicated for happens as cut while not so complicated most of the times you don't say anything about 1 per cent of the time on paper scores of fast you want would catch also yes just means the culture means the mess light stop now they are Smith the you link has called attention right exactly in a deeply worried by a mail bomb as I said as a as a fine structure constant starts to get bigger and bigger structure starts to proliferate and the result is electron develops very complicated structure it's no longer something which is very rare overeager look the electron begets big gets heavy or structure their very very similar thing happens to a string White what is this coupling constant g this coupling constant is the probability that the strained fluctuates it undergoes a quantum fluctuations of Sato a configuration like that then it's basically the probability of the strings puts off and forms a little less of a satellite string around the originals as the coupling constant gets larger and larger a very similar thing happens the satellite strings stop to proliferate and this form of structures surrounding the strain in the same way the electron-positron pairs of folk dance around the electron may get heavier figure the structure and the result is the F train stops look more could be strength in the meantime the these as more and more trouble in meaning or producing threatens why because Sosa strings are getting heavy with heavier and the result is a string gets less and less structure and becomes more clear so much that's another example are a duality he things a appalled duality use ours was called SQL worthy of the name is not important arm and interchanges fundamental strings would be mathematics of this but was very tight even Nora

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are not trying to halt due to the mathematics now August they all different versions of string theory the mathematics of them home port but bill are different versions In 1 version all of the D brains or on dimensional so that means there are 1 brain 3 brains brains heaviest sticker defied bring into space and we don't have enough dimensions of space well remember that string theory is fundamentally at 10 dimensional theory that a lot of extra dimension of string theory has enough dimensions that you could put things up to about 8 brains and the euro have directions to put brains and so 1 merger of string theory mathematics tells you that there must be V 1 b 3 B 5 he said in another version of string theory which is closely related but different there even dimensional brain null string theory has both odd Mitchell brains and even dimension is just a fact a version that has both not visit by no means transparent and new facts more than telling you explanation was it would take many hours of mathematics to explain why true but nevertheless this is a factor so let's go to the version of string theory which has even dimensional brains instead of odd that it doesn't have a string rules it has these 0 brings it has point like particles which are not quite point Light Florida very complicated and follow strengthened ended also has fundamental strings that make Gravitt palms so forth that's description when the coupling constant g is much much less than 1 this series does something very different as G starts to get large was a great puzzle that nobody the answer to what happens to these objects as GE starts to get large this he had become this this is a point like fingers a string which could be spread out all over the place different kinds of creatures that's not the case that 1 will go back and forth and the other that's not the the way it works something much more bizarre happen so I will tell you where before I tell you I have a column I am will tell you what happened and perhaps so the 2nd hour I can tell you why or what what it means for the happened what happens Is that a new dimension of says gee gets large he knew would they mention of space materializes that's crazy that's a so-so using ever heard no dimension of space materialize hokum dimensional space materialize will bowler can happen is a very small compact direction can start to expand I'm Richard now works to some extent but let's just begin with that West take ordinary space to start with the B two-dimensional just baby refer pictures really what talking about 10 dimensional string theory let's take nearly space coming directors of spaces the can they Mitchell string theory has no none of that can't dimensions time right OK source when somebody tells you can dimensional string theory that talking about theory with 9 dimensions of space OK so let's take a theory with 2 dimensions of space just because I could draw on a blackboard it's not the right theory but to draw on the blackboard here's the dimensions of space now maybe a secret way that show dimensional space is really three-dimensional how could really be three-dimensional it could be three-dimensional coast this because that could be a thickness to it we could imagine a world so nerve peanut butter sandwich With of bread on the side of piece of bread on the side of bread being infinitely thin and the peanut butter being real space in between but with a special rule the special rule is that when an object goes out here it reappears at the bottom that's the idea of compact efficacious period this city that there really isn't an edge to it but when you go out warning when you go out on top piece of bread you reappear on the bottom piece of bread that is no real edge somebody molding vertically would not notice the edge because wake up a copy so pretty which is reappear about peace of mind that's that's the idea Of and 1 compact the major now the compact dimensions small off it might not be noticeable to the physicist live in this world that the direction has moved up his extra dimension this direction has to know that there's some thickness of the world along the direction the notice so they call this world two-dimensional what happens as G starts to increase is that peanut but affair which a fickle and liquor and that and eventually g of very large or hollow them mention the space materializes where's the physicists a Libyan he would not be able to ignore any anymore they were discovered that they really live in a world where an extra dimension that is what happens to the theory that has D 0 brains and strings are OK now you say well what happens to the D 0 brains these you brains start getting a lighter and light just the same way that could be 1 brings to when she gets large we start getting wider and wider and wider more and more simple but what do they turned until they turn in the gravel ponds in this extra dimensional theory too steep ordinary Gravitt Collins the ordinary elementary particles that live In this

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extra dimensional theory no theory is now 11 dimensional we saw a string theory with only 10 dimensions wrong we started increasing the coupling constant we discovered another dimensions basis then these D 0 brains just turned into a graphic arms the exist in him another words again very very complicated objects turned into a very simple light objects they were happens the strings on the stand the strings go back to the beans at which provide Griffin Sarah wench now remember I told you that this theory has even dimensional brains zeros even so as D 0 brains but also has veto brains before rain 6 frames concentrate in particular R&D toe brains was a veto bringing it means a memory it means a memory you could have To what kinds of membranes stuck into this sandwich a membrane simply means a surface surface of

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energy you could stick in a membrane in between the floor and ceiling in between 2 pieces of bread sort of embedded in the peanut butter or if you like you could think of the year copies of bread ceiling about piece of bread as the floor and then the brains stuck in between would be a floating magic carpet Batista but just covered halfway between us 1 kind of D brain to a deed to bring you could have as another kind it too could have the other kind is a kind of written sale said and dropped it goes from the floor to ceiling but some remained so stretched between the floor-to-ceiling vertically which is expanded and the other direction 1 happens where we Florida assuming it closer and closer what happened was ribbons stocks still look more and more like a string it starts to look more and more like a point like string so we see is very small point like malign like straining the line like strings I just these membranes which are oriented vertically in this direction he asked end there also be 0 brains 0 brains being very heavy now start to increase the coupling constant the floor ceiling stop a seperate 0 brains just turning to graphic pounds it turned into just grab upon is moving in the extra dimension Mary simple adjustable elementary particles and higher dimensional theory What about the membranes or were about to strengthen the strain things now become very very on the street light very very unstring light they have all lot of membrane in stretched a long ways they're very heavy why they have so heavy because so much material them vertically here they turn into something very heavy d brains turned into something very like this is an extremely bizarre story is 1 which is what the mathematics was discovered by a bunch of people I love and the consistency of it the requirement for it the need of it was so it was a lot of heavy mathematics which went into it the need for it to be consistent involved conjectures mathematical conjectures which the physicist did not approve they were conjectures about mathematical structures which mathematicians knew about the mathematicians had no idea about these conjectures they never heard of these conjectures physicist all them look for these ideas to make any sense of such and such a mathematical fear has to be true the mathematicians went back and yeah those members steers turned out to be true soul this is really very little about the role of this together as bizarre is sound but it says the whole collection of examples on the the break down of what I call reductionism beginning no the other hate ingredient that makes it especially bizarre user all these things like coupling constants can vary in space so you could have a variation from place to place where in some places the world looked very very close depend I mentioned with very light strings Heavy D 0 brains and a few more a couple of a leaders or whatever we theory will change its character developed by large direction in this direction strings will turn into a very heavy membranes and D 0 brains were trying to grab these things of called do our relationships between what happens at 1 end of a parameter space and the other in the parameters space and in many ways Every magical surprising then nothing like or was originally expected from this theory only expect those simple theory strings makeup particles and the story about what turned out was much much more complex and much more elusive than certain

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elusive in the sense that just became a cult only unclear what the starting objects of the fury of what the stunning objects what the things which with but the bricks are nor the houses are and that this day we really still simply don't know what what if anything the fundamental objects of the theories built out of our if they are such objects perhaps as justice way perhaps it is not so objects which are more fundamental than the strings of brains and so so forth just transform into each other perhaps there's something underlying all things are that's more fundamental than any of them and can rearrange itself can rearrange itself into different these different patterns and form different kinds of structures so we don't know what we don't know the answer those questions but let's take a break for a her minute but just tell you a few more interesting connections Iris talking about a few Michael a couple of other people on visit an enormous web of interconnected ideas and interconnected theories that string theory brings together so sure you were 2 more interesting ideas that take a D 3 burning remember of those theories that have ot brains and those theories that have even brighter let's take the case Our odd brain theories which have D-1 brains fundamental strings and also be 3 brains not be 3 brain is like ordinary space has 3 dimensions which move around and if you are stuck on his brain you couldn't get off it perhaps because you were attracted to it by you might think you're living in a world of three-dimensional is just as if there was a two-dimensional membrane your little creature that around on the membrane you would think it would dimensions and might not under certain circumstances you might really describe your world was being pretty similar tour for quantum theory or theory in 2 dimensions to space adventure same thing in on 3 brains 3 brains in many ways just behave as if they were just ordinary space now the 3 itself can have all sorts of wiggle all sorts of motions endeavor also has hours among other things yes is a picture of 3 break because should drive the other way a strut this way Mrs. rein to rein in Tibet akin to arms and are not good at drawing three-dimensional figures imagine that its three-dimensional two-dimensional where NDB other dimension for additional dimension so I don't want to use a full three-dimensional OK that's a D 3 brain and and do things it can with goes on and so forth can also have strings attached to it like that he 2 strings attached to be 3 brain ordinary strings those 2 strings could come along here they are 1 of he 1 over here they can come along Lake enjoying and form a single string Israel is a picture come along with join a form of single string and then they go back out to do something else fish cannery which gathered the interact we do all the things of particles and so I on this world of 3 dimensions which is not really the full space time but just a brain and also to things go gone which are in many ways similar the things which happened in the in ordinary quantum field theory in 3 dimensions while they give you 1 interesting thought so pulsing from outside the D 3 brain a fundamental strings calm and ends on a D 3 brain how about the people who live on the 3 brain 1 made up about the strings we're going up and a string what they call the subject over here the call a particle right Wilson a particle to them so little point tho he

1:01:24

somebody he sees a little particle over here somehow it coming from other some of their actions and directions only Caesar

1:01:33

directions you lives in and overseas a particle over here what kind of particle this is now a fundamental strength what kind of particle electrically charged particles well if we can have a fundamental strings in the eat with on breaking news we also have these string was so here's a dish these can also Our free reign as part of a mathematical story of D brings fundamental strings a D strings committee aide on referred to be sure that was these string will quite somebody here it also looks at a particle cut off particle wisest Fatah because these string of his Fatah and the fundamentals of coupling constant is the launch of course the coupling constant very large they might get changed but I'm a world now with the coupling cars small fundamental strings are and threadlike D strings off that rope like these string ending on he would look like a bigger heavier particles this 1 looks like electric charge could you guess what this what looks like a magnetic monopole so it the duality that connects the fundamental strings with these strings is nothing but the duality which connected electric charges With the magnetic charges of ordinary field theory that that's us From a spectacular a connection that something we already know from long time it has a new interpretation In terms of D strings and fundamental strings duality between these strings and fundamental stray or do you want more examples 1 more example which also involves G strings and strings and I want to go back to his 11 dimensional theory that we had over here 11 mentions the horizontal axis is change is 9 dimensions of space 1 extra dimension space plus time was 4 back to this theory over here but I I I unfortunately I can't draw enough mentioned the the do satisfactory in this picture over here only 1 dimensional space as compact the sandwich as only 1 direction along which sandwich like Al Uncle imagine taking tool dimensions of space and compact defined how can I while the maximum number of dimensions I can draw on blackboard 3 I can't do more 3 3 dimensions of a blackboard if I compact apply to all of them that only gives me 1 more left think others all the other dimensions so I'm just gonna have to make do with a world which has only 1 lodged an ancient Norway's 1 visible large dimension and 2 small ones was not look like it looks like a visit to a small dimensions and he is the 1 big which is a stand-in for harmony but set right OK now imagine this is the same ahead detour brains all that just fireworks draw artist but as a rather asymmetric drug asymmetrically so low that 1 of the Act is shorter than the the other make a point theory also has a detour brings the brains were these strip where these things which formed strings when Nissan which was small those strings so stretched from the ceiling to the floor is a ceiling is the flora and we can draw strains over some political this happens to be a strain which is stretched out along the 1 visible direction the 2nd here are to draw more directions I could make a string wiggle in and so forth our but it's just stretch it out along the direction it say straight it was quite a strenuous but if these directions are very small their looks orchestra right for 1st right what happens areas the sandwich which in this directions it's Finneran than it gets more and more string like of course but in fact it also gives lighter and lighter because less material in the vertical direction here but we can also put a string and other oriented the other way How about the shrill but also from the point of view of the large them space it also looks at a one-dimensional object as monitors this as long both a small wears is 1 called not the same thing In fact which 1 of these 2 strings would you think is heavier the bottom warned because more material that were right Chris Ontario the one's light the bow strings could guess what they are Have strings and be strings so and this is another way to think about don't have to think about the relation between strings and be strings their membranes which are wrapped around tool told different directions to different small directions 1 of the directions being larger than the other now what happens if we imagine taking the space and distorting it so we started squeezing this direction this way and stretching when that happens the 2 kinds of strings ES strings get heavier and heavier occurrence that more and more material With these strings get lighter and life so we see again this is another way to think about b strings a veterans brawl mathematical you court them it's just a small part of the network of ideas that emerged out of string theory connecting sometimes connecting 1 string theory with another string theory sometimes connecting string theory with quantum field theory connecting ideas like polls in charges with strings and other things and most of it having very little to do with what we see in the laboratory but for pictures out at a ship that brought it you that which we now know that that exactly when the ratio when this direction

1:10:32

small we would normally call it them more and they say the quite well they had a by name and expand but this a quick so so the bottom here is the same point as the top so if you walk along the river in nude came to here it would reappear here likewise when allocating view that used to look what go ahead as the same kind of infinity is if the the man Is this the same kind of infinity as a circle you could go around the Noranda Noranda but never come to to begin but nevertheless you would say this surrounding know this case is it would discourage people like this and that they fear what will always all all them all and this is going around and around and around it's just it's taking a circle was idea of going up the top and reappearing in the bar is nothing but taking a circle wages go around and In order to mathematically exhibited you cut it open the up drama black is lying but remember that this point at this point I'm really the same pouring so that when you go from here crossed you jump across two-year Euopean Yukon blazes is that this will be this is we were going along this way so what if you don't have to go Is there any course useful story out here next year career while remaining I would like there articles From death from the 50 that's a little bit hard to see on it is useful it is useful but you need a little bit more a geometric emerging nation to see how or were Howard D. brain can be connected up with that bring in the reaching saturation point complexity with we're dropped the move saying it violated their with what Nathalie aspects of all over jostling 1 of my short possibly time what will you just but Bush after the possibly you can't just not may you just just switched out of there will now that that that you could imagine that as you move along space in this direction squish is horizontally and broadens vertically right so then you would say it already here the fundamental strings or light over here the fundamental strings a heavy now fundamental System name is quite clear that which is fundamental in with his no no preference for which ones fundamentalists is the word call the ones which are arranged vertically a fundamental we call the ones which are arranged horizontal D and the threat as you as you move along here you command Nagin going back and forth always kind you could also mentioned that with time all these and I can also happened so you could go back and forth between the 2 of them absolutely not really just a sort of finish up the Fox pressing the East precise features Our features Our on an idealization the idealization is the idealized supersymmetric string theory supersymmetric means as follows on firm eons of exactly the same as end it's an idealization in the sense that it's not really nature and some centers likely at idealization of studying perfectly circular orbits white study circular orbits easy to study done undoubtedly the 1st animal Newton's studied circular orbits they have extra symmetry there while highly symmetric and easy to study the mathematics was easy they warrant the real planetary orbits but of course many of the features of real planetary orbits Our in simplified form where there a circular orbits again the kinds of string theories that we really know how to deal with Mathematica string theories infinitely more a complex the theory of a orbits the kind of theories it we know how to deal with her also especially symmetric symmetric meaning to say that they have some mathematical properties which are idealization makes them easy to understand easier to understand and our but it is not real nature what they show us is the kind of things that can happen the kind of things all of these things which happened he had expected have a reflection of 1 kind or another in a more realistic theory but it will be more complicated and there will be no less tractable and not so easy to visualize certainly not so easy to prove anything about but does show with the kind of things that can happen in a string theory and very likely the kinds of things that can happen more generally just just string theory so as as string theory as we understand it now peace that the things that we really understand mathematically are Over idealization insolent people arguing that Preston books and silly books for the most part about whether string theory is the right theory of the world or not issues is not this is not a in doesn't have an enormous amount teach us about the way the quantum mechanics gravity microstructure fits together probably yes that's my opinion in any case on many of the things I really don't depend on the details of string theory but offer sermon from the objects besides debate Ramon I think probably saturated you with ideas for tonight there are other objects in the theory there are things called fluxes things called order folds things called orient the folds things all kinds of structures our which they had to leave elements but you could put together to create a world Yucatan take a geometry for example this room with floor and ceiling being thought of as the top and bottom of the peanut butter sandwich maybe even periodically identified with take that won't allow it take that this rule think of it really as 1 Compaq direction and true not Compaq directions and then we can put a debate beetle

1:19:31

Brentwood debate until it that fills up like a flying carpet halfway between the floor-to-ceiling and that's structure that could be there in space shrink the size of the of the Florida the ceiling but would that be a flying carpet always stuck in between a changes the character of physics in net two-dimensional world William Poel once flying carpet in weaker but to flying carpets in Wigan but 3 flying carpets in liquor make a whole variety of different worlds by changing the debating content that we put in the world as I said rather objects 0 fluxes we could change the nature of the microscopic world by aiding those things subtracting those things what string theory gives us it a Budget can go toy parts a lot of tin guitar parts that we comply with a range rearrange the complicated things with and in the process change what the microscopic physics is like change what particle physics is like change what what the parameters of nature or it gives rise to huge numbers of possibilities permutations and combinations of how you put these think dog pieces together how many possibilities a huge number of Cantor 500 is a numbers often thrown around the sailors have a hard to get into the 500 suppose Inc we are really 500 million 500 this message big number right supposing you have a space which has which is like a doughnut except that has 500 holes in handles now 500 holes and handles is not a large number for the kind of spaces that there that string theory uses lobby spaces 6 dimensional spaces that complicated 500 holes and handles patch there is not very complicated now we can around each 1 of what called cycles we can wrap bringing we can wrap fluxes if around each 1 of 500 cycles we can wrap a few brains 3 brains moral less Walleck and put in a few fluxes Oliver fluxes and modify a geometry for each candle if we could modify the structure in 10 different ways than we would have came to the 500 possibilities cut by DNA taken DNA chain if the DNA chain was 500 units along comedy different creatures humming different possibilities are there for a creature what the 500 not 500 the of the sold with the permutations and combinations of 500 objects being Meech being rearranged and maybe 4 5 10 ways you could make huge numbers of possibilities and that's what happened that's many keeper capoeira given apology said how many different constructions could you make well you have harmony and enables the wheels and each wheel you now put the burden on the numbers given very very large solved the same is true with string theory and the number is a large and 80 giving construction is likely b rather complicated the generic construction you make is likely to be rather complicated just like the generic construction you can make with the DNA chain our DNA chains of more than 500 you want 100 million units along million base pairs long still only about 20 thousand our genes how many Creek kind of creatures could you make a huge number are any of them simple wool some of them are simple but the generic a creature that you make is very complicated his trunks in years and all sorts of where all sorts of appendages it could very well be that that's the reason the particle physics is so complicated because a lot moving parts go in constructing a particular string theory construction stirring fears believe this by now that the world was complicated because as a lot of moving parts but not because the rules are complicated rules of DNA are not complicated the long-chain NEW permit TUG Anania whenever I will ever be base pairs are put him in an arbitrary sequence of base pairs and then with at formalistic PGC in now predicament Parizeau you realize is yeah impetus to wear this to strengthen too strands of focus on 1 stranded move longer there any point along to be 1 of 4 Seselj fortified 100 aware of before the 30 thousand armed and none of particularly simple but the rules the basic underlying rules for putting things together a simple soon as 2 of string theory very very much the same way that the basic rules for what you're allowed to do pretty simple the number of arrangements that you could make for a particular set up is very large and most of them will Lead a fairly complicated the particle physics that's the situation with facing earned a string theory if string theory is correct I think we should expect that the on the particle physics will be more complicated probably them will ever able be able to 1 rival is thought of before the fire really only a few 500 employers under arrest of a reserve that he had only but only 1 of them not the rather do our views prepare broad you will used to you divide out of the rallies the Cantor of 500 years with the number of and stop and that is only a few that we knew would only 1 presumably selves presumably so but there may be many which will very similar in many many which will very very restorer maybe a lot of junk DNA and stuff get what has left a like the son of the IRS that I think you're asking whether there especially symmetric pouring them especially symmetric points pl there especially symmetric pouring all world does not seem to be normal especially symmetric . com especially symmetric point would be 1 in which the mine up old man and electron mess with the same apparently we don't live in that world we live in a lopsided world more like this the reader White House it is a long ago that he at that I will talk about the case with that almost all of my we then they don't give you just

1:28:32

or just turned out in many years that bag that must exists and that its met them structure with it that structure all but looked on us anything you could name is too simple to be embarrassed like almost every numbers small because in number you need almost all of them are much bigger than anything you could describe as probably to add to your described it as a small number of bits is the others probably true and I think what it means if true if these things neutral Is regret learn Espino class of questions the question is not exactly what it is set up chair 1 of the common features 1 of the generic features work and bill with people are wrestling with those questions Yum yum yum now will it turned out that way we forces but it doesn't do anybody any good to have ideal logical warfare in the press over these things get me out of my head seeing is said to be a day of their love us not a man of doing their job is that media was standard old-fashioned cosmological theories of the 3 dimensions former sphere growing sphere to be sure but 3 sphere of wall compact or compact but growing so no we don't we can only say that it bigger than this or that very early measured flatness of the University of your big earth was around Yost going out and the farmer's field which is already kilometer by a kilometer and looking around all the could do was put a bounty on how big the earth from looking at triangles and things no small distances because state well the earth is so flat that I suspect that must be at least 60 miles there's a little bit better measures a little more accurately triangles at the service of the assuming the surface of the earth was nice and flat in the hills and if he might be able the Sabres 500 miles Beagle Baker and so we can go until he starts to detect the curvature of your family the curvature of Europe that is hot Europe looks like 24 thousand miles in circumference same thing when we look out cosmological space looks very flat from very large scales so all we can say is it's at least as big as at least on 40 billion light years Summerside because that non or 400 light years at least a minimum then the but we can't we can't say you 1 way or another whether it's in Thetford he faced back real world it can't real-world which is not symmetrical as simple as it appears now know it is it may be able to vary but it will vary in steps we gotta talk about this we talk about cosmology what other kinds of other kinds of variations that the laws of physics can have from place to place consistent with both theory and what we know experimentally and from what we know experimentally is that these changes take half the take place in discrete jumps moralist job along with that thing we we know with pretty much

1:33:42

certainty that came continuously smoothly vary but we don't know is whether it can vary in jumps we're good reasons to think that there is a job for lack of is some kind of black coal other kind of ordinary Swatzell Michael now now wrapped earlier horizon of which were chilled Oracle rise and what black or was a very ordinary place with nothing special like that happens I'm losing my voice for more please

1:34:23

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### Metadaten

#### Formale Metadaten

Titel | Topics in String Theory | Lecture 1 |

Serientitel | Lecture Collection | Topics in String Theory |

Teil | 1 |

Anzahl der Teile | 9 |

Autor | Susskind, Leonard |

Lizenz |
CC-Namensnennung 3.0 Deutschland: Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen. |

DOI | 10.5446/15113 |

Herausgeber | Stanford University |

Erscheinungsjahr | 2011 |

Sprache | Englisch |

#### Technische Metadaten

Dauer | 1:34:27 |

#### Inhaltliche Metadaten

Fachgebiet | Physik |

Abstract | (January 10, 2011) Leonard Susskind gives a lecture on the string theory and particle physics. In this lecture, he begins by describing the theory of reductionism and then goes on to tell why string theory and other modern theories spell the end of reductionism. In the last of course of this series, Leonard Susskind continues his exploration of string theory that attempts to reconcile quantum mechanics and general relativity. In particular, the course focuses on string theory with regard to important issues in contemporary physics. |