Steven Weinberg, "Glimpses of a World Within" [2014]

well welcome my name is Matthew Murray and and I hope some will stop making funny faces I'm the current chair of the Department of Physics and it is my honor and pleasure to welcome you to this year's David Lee historic election in physics um but before we introduce our speaker for tonight there's one important tradition business we would like to attend to that is to present this year's morrison gertrude gold harbour prize so um is this this key I hope okay all right for some of you who have not heard of their work Gertrude and Maurice gold hubba-wha well what do you think physicists it may not become as too much of surprise food it really a pioneering research in nuclear physics in cirali to mid-2000 tree and they I believe they were born in 1911 Maurice was born in Austria and Gertrude was born in Germany and they started working together in the mid 1930s at Cambridge University then they moved to University of Illinois Maurice moved in 1938 and Gertrude moved in 1939 and there they did many very interesting experiment in nuclear physics in the 1940s and one of the experiments that rather well-known is for example the experiment in which they studied beta decays of radioactive nuclide and show that what people usually said beta ray is in fact exactly electrons the kind of thing that you see in textbook and then you don't wonder about it and those things have been found there and around 1950 they moved to Brookhaven National Laboratory in Long in Long Island the lab was relatively recently built I believe the lab was built in 1947 and they immediately started working on research in particle physics Wow they probably called it nuclear physics those days we recognized that as particle physics and among many many experiments and exciting discoveries they've done the most famous one probably is the experiment but in which Maurice showed the definitely that helicity of the neutrinos was words all always left-handed and that's that's the physical paper in 1958 and Maurice went on to become a large director at Brookhaven from 1961 to 1973 and you know they lived very long productive lives until very recently so tonight we are fortunate to have Maurice and Gertrude son professor Fred Gould Harbor if you could just raise your hands so that people could recognize you thank you very much for you for coming and to celebrate the legacy of Morris and Gertrude by we give Gold have a prize to the brightest graduate students in our department each year so each year we pick among the graduate students who pass the qualifying exam during the last academic year and you'll select one or two really good experimental physicists one or two really good theoretical physicists this year we are giving prizes for four of those brilliant young physicists and they are ok let's start from Dennis Dennis one expert then just please come if you don't stand up nobody can he's an experimental physicist are walking with with Jenny Hoffman next Xu Han Xiao Xuan is the theoretical physicist walking with G in and set up Beaulieu another theoretical physicist working with Rick Heller and the last one is there you go how do I close this one okay okay okay yes yes I can see you yeah okay oh good so that's science an experimental physicist and unfortunately he's too far away to get the check you will get it okay believe me you will I'll give it to Melissa okay he's he's he's far away because he's at he's in Geneva working at CERN on the Atlas experiment at the Large Hadron Collider so say something we can hear you we can hear you I have so many of the graduate students and the professors who I work with have such faith in me um I think that you know I don't know how this in theory and maybe other parts of physics but at least in particle physics there's actually very little that really you know you're the only one that can do it and in order to be successful people other people must believe in you and think that you know that you can do something and they will give you the job over giving it to someone else and I'm just very thankful for all the opportunity and the faith that my advisor melissa has had in me be several years and all the opportunity that she has given me so wonderful thank you Sam for calling in okay I can't I can't I can't disconnect to you thank you okay so it's my pleasure and honor to introduce Steve Weinberg tonight who's the ho Z recent hall professor at the University of Texas in Austin Steve is the recipient of many prizes the Nobel Prize the National Medal of Science the Franklin medal more interestingly he's an old friend here at Harvard and it's really great to welcome him back here Steve played a central role in our modern understanding of quantum field theory and quantum field theory is one of the crowning achievements of science in the 20th century it's a theory which has been tested to historically unprecedented accuracy 14 decimal places some of that here at Harvard in the final form of what's called the standard model of quantum field theory was written down by Weinberg in 1967 and within a decade experiments had revealed almost beyond any shadow of a doubt that this theory was correct and he was honored with the Nobel Prize along with glasha and and Salam and then proceeded to immediately leave Harvard I don't know why and but even though it was Nobel Prize came in the 70s the final piece of the standard model took 45 years to put into place and the final piece of that the Higgs boson was discovered only several years ago finally verifying the 1967 work in its entirety and Steve is now writing a book on the history of science and I don't know so much about it but in my impression it's pretty rare to write down some equations and have people spend 45 years and billions of dollars trying to verify them and then have it turn out to be correct down to the last detail now in addition to the standard model Steve has had many important contributions to physics over the last half a century those include the soft theorems that he discovered in the early 60s which have controlled everything that we study about infrared phenomena at accelerators and also are affecting modern developments in string theory in the 70s and 80s he shaped our understanding of quantum field theory as an effective field theory not a fundamental field theory and also I would though maybe this one isn't always mentioned I would mention his 1988 prediction using anthropic arguments of a cosmological constant which was subsequently experimentally verified within an order of magnitude to be a correct and unlike his other predictions this was an extremely annoying prediction because all good for this is disliked the anthropic principle but the correctness of this prediction has forced all of us to rethink that and/or the verification of this prediction forced all of us to rethink it and change some of our views about how the universe around us works now in addition to being a great physicist professor Weinberg is also a great communicator and an inspiration to many he has written a number of very influential and successful popular books the first three minutes dreams of a final theory which I'm now teaching to my Freshman Seminar class and he's also a great teacher I myself as an undergraduate took physics 210 from him a course which that general relativity taught from his book of course which I now myself teach it at Harvard so he was personally very inspirational to me and thank you for that Steve and so now I'll turn the podium over to Professor wine thank you Andy well thank you Andy for those kind words I wish I could say I remembered you in physics 210 I don't remember teaching physics 210 this is the first time part of it I'm anyway I'm very glad to be here to give the Lee historical lecture and grateful to Andy for inviting me I'm also glad that this is associated with an award to four students in the name of Gertrude and Morris goldhaber who were dear friends of my wife and myself it was nice just seeing their pictures on the screen that was an unexpected pleasure I'm also grateful to Andy for inviting me to attend his class this afternoon which all I can say is it's nice to see Harvard still get some pretty good students now I think I'll start by invoking the spirit of Isaac Newton I'm always a good start Newton in 1704 wrote a book the optics in which he described his theories of light and particular of color and the experiments that he had done on them I'm not going to be talking about that in the optics he took the occasion to look into the future and try to see some of the shape of future physical research he knew very well that in his great work a quarter century earlier the mathematical principles of natural philosophy and where he had laid out the theory of gravity and the theory of motion and for the first time really dynamically understood the solar system he knew that he had not succeeded in solving the problems of physics that then in particular the nature of matter and what gave matter the properties it has was completely on not understood at the time and so he he looked forward to what kind of theory would explain the properties of matter and he knew it was not just gravity also it was not gravity together with magnetism and electricity which were known since antiquity but that something else was needed some other forces of short-range that would act only within the particles of which matter was composed and here is what he said the attract I don't know if you can read this but i'ma read it to you the attractions of gravity magnetism and electricity reached a very sensible distances their long-range forces and so have been observed by vulgarize people in general and there may be other others other forces which attract us which reach to so small distances as to escape observation in other words there are forces that determine the properties of matter but which we don't know about from vulgar observation because the range is so short this turned out to be true in the at the turn of the 20th century it was discovered that matter wasn't discovered it was already suspected for a long time but it was confirmed at the beginning of the 20th century that matter does consist of atoms atoms themselves are mostly held together by the electro the electric force but then in 1911 in Rutherford's laboratory in Manchester it was realized that most of the mass of the atom most of the mass that makes up us is contained in a tiny nucleus around which the electrons revolve like planets around the Sun held in their orbits by electric forces but that the nucleus itself would have to be held together by some kind of other forces which as Newton anticipated were a very short range these forces that hold the nucleus together came to be known as strong nuclear forces also every once in a while a nucleus suffers a cataclysmic change into a nucleus of a different element emitting an electron this is what Maurice goldhaber worked on and this change of the nature of the nucleus which Rutherford understood explained radioactivity had to be due to some other kind of force which can change nuclear particles of one type into nuclear particles of another type which came to be known as the weak nuclear force it doesn't hold anything together but it acts it like the strong nuclear force it's short-range and acts only inside the atomic nucleus and allows nuclei to change their nature emitting electrons and neutrinos the challenge was to understand these forces and that challenge was met largely in the 1960s in the 1970s and by the 1970s we had a perfected theory the standard model of strong weak and electromagnetic forces which accounts for everything we observe in our I should I should be a little bit more modest which is consistent with everything we observe in our laboratories in the behavior of elementary particles a good deal of the work of putting together this standard model was done here in the gate Greater Cambridge area and in particular I at Harvard where I was in the 1970s there was a brilliant galaxy of stars of theoretical physicists physics who I mentioned in alphabetical order Apple Quist Coleman Georgie Glashow Politzer Quin Witten and me I'm proud to say now Newton and eating the existence of structures at various of forces that would only reach to very short distances and hence a new world of physics that was only accessible when we could study very tiny scales had no idea how tiny that scale had to be he was he anticipated that there was some very tiny range forces which were needed to explain matter but he didn't know what that size would be today we know that Newton was a long way away from it that is this his scale let's say the height of Isaac Newton himself was 10 to the 15 that's a million billion times larger than the size of the radius of a typical nucleus and so he had a 15 orders of magnitude to look forward to exploring before he could get to his short range forces he didn't know that of course we today like Isaac Newton are in a position to say yes there must be scales of distance whether it's the range of a force or the Compton wavelength of a particle whatever there are scales of distance which play a role in physics at a very fundamental level which we have only bare clues to but which but we have the advantage of a Newton that we can anticipate the scale at which these new this new physics will be found and that's what I mean by glimpses of a hidden world we have three glimpses I mean by three I don't just mean speculation but actual experimental evidence that although it doesn't reveal what is going on in this hidden world at least strongly suggests that it exists and what its scale is the three clues which I'll be talking about are the convergence of the intrinsic strengths of the various interactions of the standard model the weakness of gravitation and the masses of neutrinos and all three agree that the new scale of physics which we have to reach at least intellectually in order to make real progress toward a fundamental theory is of the order of magnitude compared to an atomic compared to the nucleus of an atom compared to the radius of a typical nucleus about 17 orders of magnitude smaller smaller by a factor that is of 10 to the minus 17 and I've written down here what 10 to the minus 17 is possibly 10 even smaller 10 to the minus 19 somewhere in that range but when you're talking about 10 to the minus 17 or 10 to the minus 19 what's a couple of zeroes it's in that range that we expect to find really new physics so let me take these in order the first has to do with the convergence of the interaction strengths of the standard model this really brings me back to the 1970s when I was at Harvard I was sitting in the faculty room the same room where I met with MD sera majors class this afternoon and I was listening to a talk by a student of Sidney Coleman's you David Pulitzer and he drew a curve I've tried to this is all freehand drawing it's not to be taken literally but he drew a curve something like the blue curve on the top of the figure in this curve the strength of interaction increases upward distance decreases to the right so as you go to the right you're talking about smaller and smaller scales the left-hand part of the picture shows the great discovery that Pulitzer and also gross and we'll check at Princeton had made which was the breakthrough that led to our understanding of the strong nuclear forces that the strong nuclear forces are very strong at large distances that is not the force between neutrons and protons which are not really elementary but the forces between the quarks of which neutrons and protons are made are extremely strong if you try to pull them apart which is why we can never see them in isolation this was a great breakthrough because the idea of a quark was very attractive but why weren't we seeing quarks well this work explained it but that as you go to shorter and shorter distances the strong force becomes weaker and weaker and that had been suggested by experiments on high-energy scattering that if you probe an elementary particle like a neutron or a proton at very small scales within the neutron or proton it looks like the quarks are not interacting very strongly with each other this this phenomenon of becoming weaker at a decreasing distance is called asymptotic safety pictured me asymptotic freedom the asymptotic safety is something else the as I was sitting there listening to David Pulitzer and showing this curve I suddenly thought of the other forces of the standard model II weaken electromagnetic forces which we lump together as electro weak forces they have two independent characteristic strengths that is numbers that describe the strengths of the forces and it was known that they very slowly one of them increasing a little bit the other decreasing the curves here not to be taken seriously and I imagined in my mind these curves near the bottom in the red and orange the electroweak curves and what would happen if you projected these curves all the way to the right many many orders of magnitude and distance to the right and the same thought must have occurred at the same time although I never quite discussed with them how they came to it but the same thought must have occurred to Howard Georgie and to Helen Quinn because the three of us got together in a collaboration where we calculated what these curves would do and by God they all three came together at an energy which was well at a distance which was like the distance that I was referring to about 10 to the minus 17 a 10 million billion of a nuclear radius this was extremely exciting because you know a lot of the the payoff we were seeking was unification a theory we had already achieved the kind of unification of the weak and the electromagnetic forces then there were the strong forces which are obviously much stronger a number of brave theorists particularly Patty and Salam in 73 in Georgia and Glashow later but in a more attractive theory in 1974 had speculated about some kind of union of all the forces of the standard model strong weak and electromagnetic but they faced the obvious problem the strong forces are stronger than the others so how can they be unified they're very much stronger especially if you go to large distances and this now was a suggestion that in fact although it didn't particularly tell us that either party and Salam or Georgie and glasha were right it did strongly support the idea of a theory that would combine the strong weak and electromagnetic forces where the apparent differences between them arose from a breakdown of some kind of symmetry and the breakdown was associated with some other forces whose range was about 10 to the minus 17 nuclear radii and what seemed like almost an absurdly small distance well why don't we just do experiments probe those small distances and see what's going on it's not so easy if you want for example to have a photon with a wavelength like this its energy would have to be about 10 to the 16 GeV that's ten million billion GeV a GeV is is the energy contained in the mass of a proton or in other words ten trillion times the highest energy that we can reach now you know in if somehow physicists convinced the world as a whole that all the economic resources of the human race should be devoted to this we would not have any idea how to reach such energies so in the words the famous words of Tony Soprano forget about it the second that is forget about a direct experimental exploration of these scales I'm afraid we must forget that for the foreseeable future this kind of a distance 10 to the minus 17 nuclear radii was not entirely unfamiliar to us and it rang a bell because we had seen it before in a different context having to do with the extreme weakness of the force of gravity you can suppose you want to consider the group well between two protons or two electrons there was an electrical repulsion suppose you asked how heavy would they have to be for the gravitational attraction to balance their electrical repulsion it doesn't of course in the real world in the real world world we never see effects of gravity within the atom the gravity can be totally ignored when you study the mechanism of atomic physics but how heavy would the proton or the neutron have to be in order for their gravitational attraction to balance their electrical repulsion well the energy would be not 10 to the 16 GV which is what I mentioned earlier that's the energy of a photon with a wavelength of the scale I was talking about where the couplings come together but a little bit higher two orders of magnitude higher but still in the same general ballpark this energy we can calculate precisely it's 1.0 4 times 10 to the 18 GeV it's known as the Planck energy it actually was first brought into physics not in connection with quantum mechanics but by Stoney I think it's George Johnstone Stoney I'm not sure in 1881 based on the estimates which were then current of the charge of the fundamental unit of charge it wasn't known to be the charge of the electron but it was believed that there was a fundamental unit of charge that was transferred for instance in electrolysis and Stoney actually calculated this number called the Planck energy not as accurately as I'm giving it here so there is a strong suggestion that gravity is somehow unified with the other forces at these small scales of in fact I remember that Georgie and Quinn and I were relieved that the scale we had found where the couplings came together was somewhat lower in energy or larger in distance than the scale of the Planck scale because it meant that our calculations which ignored gravity were not thereby wrong I mean that in fact it was legitimate us for us to ignore gravity although not by much now the last clue to the existence of a hidden world is the mass of the neutrino but in order to explain this I first have to provide a brief digression on fundamental constants when you have a fundamental theory that contains a fundamental constant it relates different scales which otherwise would be independent for instance the speed of light relates the scales of energy and mass because of Einstein's relation e equals MC square a certain amount of mass is equivalent to a certain amount of energy and you find out what that is by multiplying by the square of the speed of light not an unfamiliar fact and more even more obviously it relates distance and time the distance light travels in a certain times the speed of light times the time in the same way Planck's constant H together with the speed of light relates energy and distance I've already mentioned that there's a relation between energy and wavelength the energy of a photon with a definite wavelength lambda is Planck's constant times the speed of light divided by lambda now theorists today I'm afraid it may be a bad habit but we all have it that we often use natural units where the speed of light is 1 and a reduced version of Planck's constant H over 2 pi is 1 so in other words energy is equivalent to mass which is equivalent to the reciprocal of distance and is reciprocal to the is equivalent to the reciprocal of time for example the electron mass if you converted the electron mass into energy and gave that energy to a photon the wavelength of that photon would be three point eight six times 10 to the minus 11 centimeters and if you had 10 times the mass then the photon wavelength would be ten times shorter now in the standard model as in the earlier theories of Maxwell and Dirac all the field equations are simple and in particular in natural units which I've just described there are no constants appearing whose units are length or length square or any positive powers of length and this imposes well this is a way of describing the simplicity of the equations because it limits the number of powers of fields or rates of change appearing in the equations now generally speaking physicists from Einstein certainly Maxwell Dirac physicists in general when they're trying to understand the new phenomena assume the equations are as simple as possible as simple as consistent with known principles and with known data that's a good strategy for physicists first try something you can work with easily before mucking it up with complications but even if you're successful as Einstein and Maxwell and Dirac and others were and as we were and we did the same thing in the standard model we took the equations to be as simple as possible even when you're successful you have a responsibility after the fact to explain where that simplicity came from just to say that God prefers simple equations somehow isn't entirely satisfying although some people have perhaps felt it was now starting in the late 1940s in the context of electromagnetism we thought we had an answer to the question the equations of electromagnetism are very simple and in particular there are no quantities there are no coefficients of the various terms in the field equations which have the units of length or power or length square or any positive power of length if we allowed such terms the theory could be arbitrarily complicated but we don't allow such terms we impose simplicity now starting in the 1940s we thought we had a reason for that and it had to do with the appearance of infinities infinities in simple theories theories which satisfy this criterion about the units of the constants in the theory can be canceled by a proper definition or renormalization of constants like physical charges physical masses and so on and such theories this is called renormalization and such theories are called renormalizable but in other theories where there are constants with the units of length always in natural units where h-bar and C are one where there are constants with the dimensions of length of some positive power when we sum over the wavelengths of photons or other particles in intermediate States we usually encounter contributions that become infinite when we allow the wavelength of the particles to be arbitrarily small if we cut off the sum over intermediate wavelengths at a certain minimum wavelength say we simply won't consider any wavelength less than that minimum we find we get contributions whose constants are powers of length divided like divided by those powers of the minimum wavelength so that when the minimum wavelength is allowed to go to zero these infinities blow up and it appeared that these infinities are not of the kind that we can't can be cancelled by renormalization this was a problem for early theories like Fermi's theory of the weak interactions in natural units the constant that governs the strength of the weak interactions has the units of length square and this was a great bone in our throat until these theories were replaced by the renewal izybelle electroweak theory which doesn't have any constants whose dimensions are positive powers of length but there are other successful field theories that obey symmetries that require there to terms in the field equations that are proportional to constants that in natural units are positive powers of the legs and among these theories are gravity itself the constant of gravity Newton's constant although he never had it in the Principia but we call it Newton's constant has the units of length square and that means that ordinary the new einstein's theory of general relativity is not R anomalies able the infinities can't be eliminated by being absorbed into constants and also a theory of low-energy pi mesons which I worked extensively on in the early 60s also does not allow any renormalizable theory it the basic constant in this theory has like Einsteins constant two dimensions of length square so what do you do well I think there is a different modern viewpoint which has gradually matured over recent years by the way I should say that I was happy to give lobe lectures here on this theory of low-energy pians and ideas of gravitation in the early 90s in 92 93 and then I presented what I regard as a modern view point in my Loeb lectures in 95 96 the modern view point is that all terms in the field equations that are consistent with symmetry principles like Lorentz invariance or gauge invariance do appear in the field equations if it's if it's allowed it's compulsory but the lengths that appear is coefficients of these complicated terms which to destroy the simplicity of the theory are very small in such theories where all possible interactions appear all infinities are absorbed in a redefinition of the constants in the field equations and in fact the slogan that I provided my Loeb lectures are was that non-normalizable theories of Justice Radames able as renormalizable take that now the coming back to neutrino masses eventually the the if you allow a theory like the standard model to contain arbitrarily complicated terms simply suppressed by higher and higher powers of some very small length then any symmetry of the standard model that's a mere accident that's there only because the equations of the similar model are too simple to violate it any such symmetry is bound to be violated but by very small effects because we're talking about very small lengths one of these well and we'll check and Z and I both adopted this viewpoint and cataloged some of the new kinds of effects that could be produced by higher terms in the field equations that are suppressed by small lengths one of them was proton decay the field equations with these higher terms in it contain terms that allow three quarks to turn into an electron or a neutrino with a rate proportional to some new length to the fourth power and that length doesn't have to be very small for proton decay to be unobservable although i think there still is a good prospect that eventually we will see proton decay produced by such terms neutrino masses are different in the sense that we have observed them the interaction energy according to the modern viewpoint must contain terms that are allowed by the symmetries of the standard model these terms involve a pair of electron neutrino fields a pair of Higgs fields and a constant with the units of length not length to the four but just one power of length we now know that the Higgs fields well fact we've known since 1967 that the Higgs fields have a vacuum expectation value of 247 GeV and if we assume that there are no very large a very small dimension full dimensionless constants in this interaction then the fact that we observe neutrino masses in the general neighborhood of point O 1 volts tells us that this length is well the reciprocal of the length the car is 10 to the 15 GeV in other words the length is like 10 to the minus 16 times the size of an atomic nucleus in the same ballpark and of course this is very crude because we don't yet know the neutrino masses in detail we don't know the theory here that this interaction may contain constants that are a tenth or hundredth or 10 so this is a very crude estimate but the masses of the neutrinos which are being observed are in the same ballpark that you would expect from new physics associated with a fundamental length like the length at which the for the couplings the interaction strengths of the standard model come together and the the length converted into an energy at which particles attract each other gravitationally as much as they repel each other electrically it all seems to hang together now oh by the way I should say these effects of proton decay were seen earlier in models of georgi and Glasgow and neutrino masses in models of gell-mann Raymond and slansky but the point that we'll check and I will check in Z and I were trying to make I if I can guess what their I their way of thinking was is that these are endemic this is what you expect in any theory because the symmetries that keep protons stable and heat Trino massless in the standard model are just accidents they're just accidents of the simplicity of the theory and as soon as you allow the theory to contain more complicated terms as I think you must then these symmetries are violated and protons can decay and neutrinos get masses likewise there's no reason to suppose that Einstein's equation for gravitational fields are as simple as he thought terms involving constants with extra powers of length must appear in astronomical calculations divided by powers of the very large lengths of astronomy and therefore they're undetectable they're undetectable in ordinary astronomy they may play a very important role in black hole collapse or in the very early universe you know there are theorems that say what you're certain stage has been reached in the collapse of a black hole the further collapse is inevitable I don't think those theorems are right because they are proved within the context of ordinary general relativity I think that all you can say is that once certain conditions have been reached in a black hole the black hole collapse will continue until general activity itself in the simple form that Einstein proposed breaks down and you begin to be sensitive to the more complicated terms which must be there in the field equations of general relativity by the way talking about things that we only know about because they violate symmetries we only know about gravity because we that is we should say we can only observe Lapp gravity directly because we happen to live on a sphere with about containing about 10 to the 51 protons and neutrons all of which are pulling at us and if it weren't for that that we happen to live on such a large mass we would never be able to detect gravity in the laboratory loved every proton and neutron of these 10 to the 51 is attracting this bottle of water to the center of the earth and yet I with my feeble muscles can hold it up purely electrical forces and you know I win but it's only because I can do that because gravity is so incredibly weak in fact I'm going to quickly I deserve that and so now the big question that we face I'm out of towns at time so I have to get to it is that can we find a truly fundamental theory uniting all the forces including gravitation whose structures are characterized by tiny lengths like 10 to the minus 17 to 10 to the minus 19 nuclear radii is it a string theory we don't have I mean that seems like the most beautiful candidate but we don't have any direct evidence that it is a string theory the only handle we have aside from things I've mentioned like the neutrino mass possible proton decay the weakness of gravity and the convergence of the couplings the only handle we have on this to do further experiments is in cosmology because in the very early universe in the period of inflation the energies may very well have been as high as this energy of 10 to the 18 GV per particle and there are actually experiments that can tell this because if the energy density in the very early universe is that large then gravity which couples to energy would have grabbed gravitational radiation would have been strongly produced in addition to the purely scalar waves of pressure like sound waves that resulted in the large scale structure we see galaxies and clusters of galaxies the very early universe the period of inflation would have generated gravitational waves which could leave their imprint on the microwave background which is ordinary electromagnetic radiation but is affected by the charged particles when the universe was 380,000 years old which in turn were affected by gravity could have been attracted by gravitational waves there is an experiment called bicep2 which indicates a discovery of just such gravitational waves and it does suggest that the density of energy in in the early universe during the period of inflation was characterized by this scale of the order of 10 to the 18 GeV or 10 to the minus whatever it is I forgotten now nuclear radii 1560 this experiment is not definitive there's a big question hanging over it whether or not the effects they are seeing are produced by dust in the relatively recent universe having nothing to do with the very early universe or with gravitational waves and we simply don't know we're waiting to hear from experiment from observers whether or not there are ways of distinguishing them whether or not the bicep2 results really do represent gravitational waves from the early universe but if they do and I think there's a 50% chance then it will be another piece of concrete evidence that there is something fundamental about the scale of 10 to the minus 15 nuclear radii at which we can expect a true unification of all the forces of nature thank you I'm happy to answer questions well you know this is as I understand it the question is could different civilizations come to quite different understandings about nature I think the answer is no not that not to try to pussyfoot about this clearly the the timing the location of advances in physics depends on all kinds of societal effects Robert Merton thought that the great blossoming of physics in England in the 17th century was due to the triumph of Protestantism and he had arguments I don't know whether that's true but I do think I know why Newton's theory takes the form it does and would take that form wherever it was discovered and that is that's the way the world is the world to a good approximation really does obey Newton's laws now what evidence do I have for this obviously there have been many civilizations which did not come to these conclusions but modern physical science which developed in Europe in the 16th and 17th centuries has completely conquered the world as far as far as scientists who provide reliable knowledge on which you can base technology or government policy all in every country in the world science is what developed in Europe in the 16th and 17th centuries which in turn has roots in among the Arabs and the medieval Europe would and which in turn depended on the precocious science of the Greeks but once we learned what the solar system was and what makes the planets go around the Sun the way they do we never will unload and no other society will ever replace that with a different picture if I'm not a you know I don't know if I would bet my life on this but in a sense I have bet my life on this because if in fact this is not true what's the point dr. Weinberg I remember the first Bragg brown-bag lunch that you had and at UT shortly after Greene and Schwartz had discovered the anomaly cancellation and you I think you described string theorists up to that point as being something like psychiatrists there was a new sense of optimism however at that time in physics and a whole new world of algebraic topology in calabria manifolds and all sorts of things opening up could you update us in a few words on where that state of optimism is at this well not really I I did study string theory intensively in the 80s and gave a course on it and wrote a couple of really grossly unimportant articles whose function was just Auto didactic I was just writing articles in order to try to learn the subject and then I saw was getting very mathematical that you mentioned some of these things khalaby Yau manifolds and so on and it was either do that or and give up everything else I might do or do other things like cosmology and I chose the latter course but not because I thought string theory was doomed I think it still is the most attractive direction to go toward a truly unified theory it has a kind of rigidity that I look for in a beautiful theory that is as I understand it string theory becomes nonsense if you change it in little ways and that's it it it is believed that there is one string theory although there are many approximate solutions unfortunately it has not pointed to a particular solution that looks like the real world and string theory has had qualitative success like requiring the existence of gravity I mean there's no string theory without gravity because one of the modes of vibration of a string is the quantum of gravitational radiation but it has had no quantitative successes and it's the kind of theory that you'd like to believe in but you have no firm ground for that and I I find it all very disappointing I think but that isn't to criticize the people who do it I think because it's the most attractive direction to go I'm very glad that there are people who immerse themselves in it but I hope that experiments either in cosmology or at very high energy or in the quieter mode of looking for rare things like proton decay I hope that experiments will give us a kick in the backside and get us moving in the right direction again even though we don't have a good string theory so I don't know what the future is going to bring I don't I think the fundamental principle of progress in physics is to keep working and if your tastes are in string theory that's a good direction to work on if your tastes are in particle phenomenology fine if it's in cosmology fine that's very exciting right now but I remember the tremendous excitement of elementary particle physics as I started out talking about in the 1960's and 1970's you know most Americans if you say what were the 1970s like would say it was a period of inflation and unemployment view of the US cat elementary particle physics physicists what was the 1970s like he would say it was great we had experiments that were relevant to theory and theories that were relevant to experiment and always coming together and we just and all the problems of the standard model like for instance the thing that Adler Bell and Jacques Yves worked on that PI zeroes did decay the way they were supposed to all that got settled in the 1970s we understood it all so it was a wonderful time we don't have that kind of wonderful time in elementary particle physics these days we do in cosmology and maybe who will again in elementary particle physics we'll see yeah yes I haven't even touched on that that's and of course an embarrassment I said that you should allow every term in the field equations that is allowed by symmetry principles and that includes terms with positive powers whose interaction strengths of positive powers of length there are terms with negative powers of lengths like masses which of course have to appear in the field equations but if you're really going to take that point of view consistently then you should allow a term in the field equations that like Einsteins cosmological constant which corresponds to a an energy density hence goes like a minus fourth power of length and if you take that length to be a tiny length like the links we've been talking about today or even perfectly normal astronomical lengths like the size of the solar system it gives an enormous energy density which would wreck the universe and would cause the universe to expand so fast that galaxies and stars could never reform so there's about a hundred and twenty two order magnitude discrepancy between that estimate and the actual value that we observe the dark energy which is the modern way of looking at the cosmological constant and nobody really has a good idea of why that that one term is so incredibly small the only idea that seems to work is something Andy referred to in his introduction of me but is based on very wild speculation about multiverses and certainly is not a firm enough success to justify those speculations although those speculations may in fact be turn out to be right on other grounds so we don't know I gave a series of lectures here at Harvard I think in 1978 maybe 76 I'm not sure I've given a number of love lectures and in that I analyzed every theory that had been proposed for explaining why the cosmological constant or the dark energy was as small as it was many of them by the way predicted that it was zero which of course would now we know it isn't true and I found that none of them worked I mean they all had inconsistencies in some of them it wasn't so much that they were logically inconsistent as they've simply amounted to saying let it be very small and the only one that seemed to work was this multiverse idea but of course that's going way out on a limb and I don't know if that's true physics what principles ask something about a physics principles like the sum principle like the minimum function function notes or like the research on a different structure on manifold of the topological structure like this kind of fundamental fundamental principles so what's your opinion what is the most important or most basic funding support in in physics nowadays or in the future I'm not more interesting in the future yeah and actually in innovate from the purview of the theoretical physics no that's my question well the deepest principles I think we know are the principles of quantum mechanics I personally find quantum mechanics unattractive in its present form and there are various interpretations of quantum mechanics all of which seem to me to have drawbacks it would be a whole other lecture to explain that so I think quantum mechanics will eventually be replaced by some other theory but the trouble is that quantum mechanics has just that quality of beauty I referred to earlier that if you change its equations in just a little bit you get nonsense and so I don't know what could possibly replace quantum mechanics in addition to quantum mechanics itself we have symmetry principles and we don't know why nature obeys just the symmetry principles they do but aside from quantum mechanics itself those symmetry principles are the deepest things we know and I suspect that will continue to be true well thank you very much you

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Keywords: Steven Weinberg (Academic), Physics (Field Of Study)

Id: S2jVqJDZIck

Length: 68min 44sec (4124 seconds)

Published: Fri Jul 24 2015