Prof. John Baez: Unsolved mysteries of fundamental physics

[Music] thanks very much it's a great honor to give not just this talk but the Maxwell lecture because Maxwell was always one of my heroes he was actually a mathematics professor but is known for some work he did in physics namely figuring out the laws that govern electromagnetism and using them to predict the speed of light what better could you do than that so my talk is going to be about some unsolved mysteries of fundamental physics and I'll try to teach you a tiny bit of physics in the process because you sort of need to know where we are to know where know what we don't know so I mean something rather specific by fundamental physics I mean the search for a small set of laws which at least in principle although probably not so easily in practice determine everything that we can calculate about the universe so this is the dream of reductionism that dream that you could write on a postcard somewhat magical equation which from which you could derive you know the melting point of lead or the or the size of the average snowflake or anything like that if you're good enough at calculating this isn't always practical we know a lot now about the difficulties with reductionism we know for example that that we'll never be able to predict with certainty whether it's going to rain in Cambridge well certainly not within a week maybe not even tomorrow I don't know what the weather reports stand now because of the complexity of the motions of air and and water vapor that to build a computer that could simulate the the system accurately enough to predict what it does the computer would basically be about have to be as big as the system itself approximately and it would take about as long to run as the weather actually takes so fundamental physics is a limited pursuit I'm not saying that it's the best kind of physics I'm not saying that physics is the best kind of science but there's something specially attractive about the search for fundamental laws and so the questions arise where do we stand right now and searching for these laws and what do we know and what don't we know what are the mysteries so the most fundamental question of all as you probably already know is why so there are a lot of people who say that science doesn't address why questions it doesn't say why things happen it just says what happens and there's definitely some truth to that for example I think Newton really pushed that attitude when he wrote down his laws of classical mechanics and the inverse-square law governing gravity he said in some Latin phrase that I'm not making any hypotheses by which he meant that he was going to resist the temptation to it to say why these laws were true and it's very important that scientists have the right to do that so for example when Gelman invented the idea of quarks and posited that protons and neutrons were each made out of three quarks if somebody had said come on you have to tell me why this is the case before I'm gonna let you proceed with this theory why in the world should protons be made of three quarks I think it's crucial that Gellman had the right to do what he probably would have done which is to say sorry I don't know why and I don't care why really I'm I'm much more interested in figuring out the consequences of this theory then why and yet science often moves forward by asking why questions so the key thing about why questions is to know when it's good to ask them and when it's less productive to ask them of course children haven't been trained to resist asking why questions at certain points so they would start pretty much anywhere and asked why and it's very interesting to think about how these networks of why questions lead on to further questions so one very obvious question is why is the sky blue in here in the middle of a jungle it's quite commonly it would be the only blue thing there is that you see and so a standard answer now is well more than light of all other colors blue light is scattered in various directions by the Earth's atmosphere as it's coming in from the Sun and so you especially see like when the Sun is setting it's going through lots of atmosphere the Sun looks very red because most of the blue light has been scattered and what's left is more of the red type of light whereas the scattered light is what you see on a on a on a sunny day in the rest of the sky but then of course every why question when answered gives the potential for one or more further Y questions so the next question is why is blue light scattered more and then a standard answer might be well because blue light has a shorter wavelength than other kinds of light most other kinds of light already here there's some subtleties anyone who's really studied this question about about why the sky is blue we'll be able to like poke holes and things I'm saying of course violet light has an even shorter wavelength them than blue light so if if this is all there was to it the sky should be violet not blue but the point is there's more to it than that the Sun produces different amount of light of different frequencies and it doesn't put out enough violet light to have violet be the dominant thing you see blue so it's a more complicated situation that I'm letting on here but then you can ask okay why does blue light have a shorter wavelength well now this wouldn't make a physicist start getting nervous this isn't really the kind of question they want to answer so they'd say maybe like no reason we just happen to call the lot visible light with short wavelengths blue that's just sort of the definition of of blue so if you've ever tried explaining physics to non physicist you will often encounter this sort of situation where they're asking you questions and some of the questions you feel like oh good that's a great question I know the answer to that and then sometimes the questions will veer off in a direction that makes you intensely uncomfortable because it's not the kind of question you know the answer to or or maybe not even the question kind of question you think there should be an answer to so the like the person could persist and say yeah okay you call it blue but why does light with these short wavelengths actually look blue that's like this slide here is supposed to be I don't know how well it's projecting in my screen here let's just sky blue and at that point you would have to say uh well I don't know that's not a physics question in fact no one really even knows what kind of question that is it could be a psychology question but it's sort of so tough that they say no sorry that's a philosophy question and so don't make fun of philosophers for never solving these questions of philosophy the problem is their situation is that whenever there's a question that no one has a clue as how to answer they say it's philosophy so of course we can't answer the philosophy questions right so so there's a whole idea of qualia is there such a thing as blueness the blueness of blue it looks very apparent to me if you ask me to describe it I can't do it I could just say well it looks blue so anyway forget this this isn't that way we want to go so a better direction for physics is so why does light with short wavelength scatter more than with long wavelengths and one first answer is well what's really going on is the lights going through the air and it's actually hitting individual molecules it's molecules of air oxygen and nitrogen mainly that scatter the light and if the light gets scattered with enough intensity proportional to one over the fourth power of the wavelength so light with shorter wavelengths scatters much more and then of course the next question is well why why does life do that anyway and then you can say well it's nothing particular about nitrogen and oxygen it's just a general fact about electromagnetism that electromagnetic waves scatter off particles that are much smaller than the wavelength of the particle with an intensity that's proportional to one over the fourth power of the wavelength when the wavelength gets to be so small that it's about the same size as the particle then then it gets more complicated but but but visible light is much bigger than atoms and so this approximation is of thinking of the particles is much smaller than the wavelength is valid and you get this famous one over the fourth power law which is called rally scattering and this is a graph of how much how much the light gets scattered as a function of its wavelength but then of course the next question why will one over the way link to the fourth power why not the ninth power the third power and now this is a kind of question that physicists will get very happy about and they'll start like I could like spend the whole rest of the lecture talking about that but I won't so I won't do the calculation here but rally showed this in 1871 and he actually showed it in a rather simple way using what's called dimensional analysis so in physics you have to keep your units straight you you can't to say too if you're talking about how heavy something is you have to say two kilograms and by keeping track of the units and making sure that they work out properly that is you have equations that have the same units on both sides you can figure out lots of stuff and you can in particular figure out figure out this one over the wavelength to the fourth law from just some fairly simple assumptions and the main assumptions are one is that space is three-dimensional there's three XY and z coordinates and the other is that the energy density the amount of energy per cubic centimeter of an electromagnetic wave is proportional to the square of the amplitude so if you changed either of those facts you'd get a different law and in particular for example if space was just two-dimensional if we all lived on in flatland instead of three-dimensional space there would be a different law the intensity would go like one over the length cubed so this is a lot of fun for physicists because because you could do calculations and you can think about what would happen if you change different things and get different answers so this is a kind of why question that a physicist would consider very productive but of course every why question leads to another one so the next question could be why is face three-dimensional and that is a hard one no one has a clue really well there's ideas but that's too hard nobody I would say knows the answer to that one so I'll stop there so there are lots of really interesting questions in physics that right now they just seem too hard for us and and usually when you start out thinking about physics those are the first ones you think about and those are the ones you'll never know the answer to so you have to get interested in things that are a little easier why is phase three-dimensional why is time one-dimensional we do know by calculations what it would be like if time were two-dimensional those of us who care about such things and it would be really weird okay it would be really different we might not even think of calling it time it would be so different but you can nonetheless you can you can say what it would be like but we don't know why time is one dimensional another question that's very interesting in fundamental physics is really are there any fundamental laws we're looking around for these fundamental laws of physics we've been looking and looking and looking and never finding them maybe there aren't any fundamental laws of physics maybe there's only a series of better and better approximate laws there's nothing there's no guarantee on the universe that says there's gonna be some fundamental laws so that's an interesting question which I doubt I will ever know the answer to and then of course that was slightly more practical question is what are the fundamental laws well if there are fundamental laws what are they that's what we spend a lot of time digging away at but we don't know the answer to so we learn to sort of cut cut our expectations down and try to tackle that are a little bit easier but it's important to realize that even these questions that I just listed it may seem hopelessly hard to answer we should never I think completely give up hope because there are a lot of questions that once looked way too hard to answer that have actually been answered and I'll just pick one why is time so darn different from space so we've got three coordinates that we call space we've got one that's called time they're incredibly different right so you can you can go back to your dormitory but you can't go back to last Wednesday there's all sorts of things that are different but there's an answer to this question which Einstein came up with when they invented special relativity this is an answer that applies in special relativity it gets fancier in general relativity but it's the same basic answer the answer is the back when you learn to calculate distances using coordinates you learned about the Pythagorean theorem that said that the distance from here to there is the change in x squared plus the change in Y squared and then take the square root at the end and in three dimensions it would work the same way you'd have the change in x squared the change in Y squared and the change in Z squared is the distance squared but if you include time Einstein realized you can include time in this formula but it comes with a minus sign you get minus the change in x squared plus the change in x squared plus the change in Y squared plus the change in Z squared is the square of some thing that we don't usually call distance anymore we call it interval it's a kind of unification of the notion of the distance in space and the distance in time but with this funny minus sign and I'm working in units where the speed of light is equal to one so that time shows up on an equal footing with space more or less but that minus sign is the answer to the question everything comes from that darn minus sign you can do calculations in physics in special relativity and you can see all the stuff about our world that makes time being different from space all arising from that darn minus sign it turns out it's not so much the minus sign as the relative minus sign I couldn't use a different convention and people often do where I put minus signs on all three of the space guys and I put a plus sign on the DT squared delta T squared and that would also make time act the way it does so really what matters is just that it comes in with a different sign so that that of course would again be the potentially the start of a long lecture where I explain why that's really how that really works I mean this is a cryptic answer that needs six expounding to be clear but this is great it really is the answer as far as I'm concerned so so that's of course why Einstein is a sort of well known guy he could do stuff like that but it's what it means for me is that it's very hard to know for sure which questions will answer next and which ones are hopeless but it certainly helps to stand to know where we stand now before you you you can make a guess as to which questions will answer now so let me just give you a quick tour of where we stand in fundamental physics we have two best theories of fundamental physics today one of which describes all the forces and all the particles except for gravity and one of which describes gravity the standard model describes three kinds of forces electromagnetic which is the thing that's making the light in this room the weak force and the strong force which you only notice when you do experiments with atomic nuclei or subatomic particles and it describes them all using quantum mechanics it it it's easy to do experiments with these kind of forces on very tiny distance scales and on very small distance scales quantum mechanics becomes incredibly important because you learn that what you thought were just point particles are actually wave functions they're blurry things general relativity only talks about gravity and how gravity is affected by my matter but it doesn't give a detailed description of matter and gravity is very hard to do experiments on small distant scales because it takes something pretty big like a planet to generate a significant amount of gravity so we don't know we haven't done any experiments any serious experiments connecting general relativity and quantum mechanics because it's just too hard for us right now and so general relativity does not take quantum mechanics into account so our right now our picture of the universe is sort of split in this horrible way that we have one theory for everything except gravity and we understand how it works with quantum mechanics and then something else for gravity but ignoring quantum mechanics no one in physics that I know of thinks that this is the last word in the situation in fact if you try this you could say like well maybe gravity doesn't care about quantum mechanics maybe the Right theory of gravity doesn't involve quantum mechanics but it turns out if you try to do that you get in even more trouble it's bad it does not work so we know that general relativity should be unified with the others at least insofar as they should all be talked about using quantum mechanics but we have no we don't know how there's been a lot of work trying to do that but we would we don't really know how these two fit together so let me talk a bit about the standard model for it so I'll talk about them separately so the standard model describes particles and their interactions and it uses special relativity and it uses quantum mechanics and so some of the first particles to be thought about this way were the electron which are those ease there and the photon the carrier of light which is written gamma because sometimes some kinds of light it's called gamma rays and here I add left we see one kind of thing that can happen and that right we see something else that can happen but the basic thing that can happen is always the same it's just an electron is moving along and it emits a photon or the same picture could be used for other things you could also say an electron comes along and it absorbs a photon we're basically reading the picture from bottom to top here but but depending on the slant of the that horizontal line you could say that the somebody is admitting or absorbing a photon and even to make it even more fun you notice that these electrons lines have little arrows on them which are sort of going forwards in time but you notice some of them are going backwards in time like this one over here at the at the left of the right-hand picture that's called an anti particle that's a positron the opposite of an electron and here what's happening is in an electron and a positron are colliding turning into light turning into a photon and then that photon is breaking back into an electron and a positron again those things really do happen and the beauty of this is that these two pictures are just the same picture just turned around so the same theory predicts these various processes in a unified kind of way but now the standard model has a bunch of particles and here they are here are all of them there are particles that carry forces one of which I mentioned the photon carries the electromagnetic force so like if you have a magnet and it's attracting a piece of iron there are actually photons involved doing that the weak nuclear force is carried by particles called the W and the Z there's actually a positively charged W and a negatively charged one the strong force is which holds for example a proton together protons are very hard to bust so there we we think they're held together by an extremely powerful force and it's carried by particles which are jokingly called the gluons and there are actually eight of them eight different kinds of them for some nice math Nicolle reason and then they're particles that constitute matter and the strange mysterious thing about them is they come in three generations each generation being very similar to the two the red to the other generations the first generation is sort of the one that that has all the particles that are in this room in large quantities the electron is in the first generation the house skip the next guy for a second the down quark and the up quark are the two kinds of quarks that make up protons and neutrons for example a proton is two ups and down a neutron is two downs in and up and then there's one ghostly thing called the electron neutrino nu sub e there that is a particle that's very hard to detect and yet has been detected and for example when a neutron decays into a proton it will emit among other things well it will emit a anti neutrino of the electron type so they're involved with their very hard hard to detect and then the really weird thing is that there's a second generation of particles that's very much like the first one and a third and the main difference between these three generations is just the masses of these particles so for that new thing is a muon it's a lot like an electron but it's almost exactly two hundred and seven times heavier when they first discovered it they weren't expecting it and this physicist I I Robbie said who ordered that they were looking around for particles that would help solve the problems that they were dealing with this one showed up it's like a waiter brings you a particle on a tray is not what you ordered and and we still don't know really why there are these other generations but they're definitely there and then finally there's one more which you may have heard about called the Higgs boson which was the last of these to be discovered and it interacts with all the rest that have mass although particles that have mass get their mass by interacting with the Higgs boson that's what there is and they interact in various ways and this is just like a cute little chart for how they interact where we draw a blue line between two particles if there's some interaction between them and so for example the Higgs there is interacting with all the particles that have mass and it has mass so it interacts with itself the W and Z bosons have mass so it's interacting with them the photon does not have mass so there's no line between the Higgs and the photon and the gluons are also massless the gluons interact only with the quarks and with themselves so the gluons mainly hold quarks together to form things like protons and neutrons and stranger' particles of the same ilk the the other kinds of particles forming matter are called leptons so they elect the electron and the electron neutrino are called leptons they don't interact with the gluon so they don't feel the strong force but they do feel the weak force so there's a line from them down to these weak particles the W plus minus and the Z and so on so you have to write down a formula that says exactly how all these particles interact with all these other particles and it's pretty long and complicated because because it must involve all of all of these things and there's no over underlying principle from which we can derive all this stuff at present it's just stuff that we've observed to be true so in fact you need 25 fundamental constants 25 numbers to describe the strengths of these various interactions these are numbers that are dimensionless constants like one of them because it's called the fine-structure constant tells you how strong the electromagnetic force is it's one over a hundred and thirty-seven point o blah blah blah we have no idea why it's that 22 of these 25 numbers involve the Higgs boson because it's interacting with lots of other things and giving them mass that's why people were so why it was such a big deal to actually really discover for sure the Higgs boson we knew it was essential but it was good to actually see the darn thing so this should make you have all sorts of questions you should be able to ask me like at least a month's worth of questions about that last slide now this ask like a few of the ones that instantly come to mind like why are there three forces in this picture why not five why not just one we don't know we have no idea why does each generation include two leptons and two quartz there's this strange thing that the quartz are a lot like the leptons but they're also not right so the quarks feel the strong force the leptons don't but they still in each generation you get two quarts and two leptons and there are a lot of other similarities between them that are a little fancier to describe why are there three generations why are not just one it would be so much simpler if there was just one why do these various numbers I mentioned have the values they do so there are some there is definitely progress on some of these questions there are attempts to unify these three forces into a single force and those are called grand unified theories and some grand unified theories offer what seem to me pretty tempting answers to the first two questions in other words this you can make there's a theory called the Esso ten grand unified theory in which you can see that you're gonna get three forces from it even though it starts out being one single unified force and even better you can see that each generation of particles is going to have two leptons and two quarks in quite a bit of detail so quarks are weird quarks have charges like 2/3 and negative 1/3 whereas an electron has charged negative 1 but this grand unified theory says why they've got to have those weird charges that fits in in a certain way and you just can't change it it's got to be that way if the theory is true there are problems with this theory what is why not everyone believes it yet but it's there's so much that seems to work it's it's it's hard to get escape the impression that it's on the right track on the other hand the third question about why are there three generations I don't think that I don't personally believe there's any really good theory about that of course there are theories about it theorists it's their job is to make up theories of stuff like this so you know at least 100 theories of it but but but that doesn't mean that I'm convinced of any of them and the fourth one why the constants have the values they do that's so hard that a lot of physicists have just said forget it we're never gonna answer that don't ask it if you want to know more about these grand unified theories it was very nice that you just happen to mention a paper that I wrote with a grad student of mine with the title the algebra of grand unified theories if you know if you know some some physics and some math you you could take a look at this and it's our attempt to explain in relatively simple terms you may disagree when you look at the paper or relatively simple terms what's going on here so I think I think there's definitely some progress in this on these questions but I wouldn't say that they're answered and in fact it seems easier to make progress on certain other puzzles so for example what's up with these neutrinos the neutrinos are weird and they've always been weird so the first people thought they were just like made up for bookkeeping purposes that energy wouldn't be conserved unless some mysterious particle is shooting out of a neutrino of a neutron when it decayed but then they found them they actually detected them but very hard to detect them but they they did but then our opinions about them keep sort of shifting as we learn more and more about them for a long time people thought they were massless they have very small mass but now we know that they do have a non-zero mass I guess if you're really technical we know that all that there's at most one that's massless because we can mainly measure massive differences or actually differences of squares of masses so there couldn't be one that's massless but we don't think that two probably all have a mass and then the other thing people learned when they saw that about one-third as many electron neutrinos were coming out of the Sun as we expected was that these guys can turn into each other and so those electron neutrinos are actually turning into muon neutrinos and tau neutrinos before they reach us and we can now do experiments on earth where you make neutrinos of one kind over here and it'll look for neutrinos of another kind over here and you see yeah we're getting them now when you turn this on when you turn it off they're not there so they're turning from one kind into another as they zip through space and the poor all this to describe how all this works actually eats up ten of those 25 constants I mentioned in the standard model they're all about talking about how neutrinos are doing these things and they are all due to its interaction with the Higgs so it's sort of mysterious already but what's more mysterious is that the standard model may not be really fitting the data we see there are anomalies weird things in the data so here's the kind of thing that physicists do you build an 800 ton spherical tank of mineral oil you bombard it with new on neutrinos that you make from a particle accelerator that's 500 meters away and you watch it very carefully for 16 years with all these photomultiplier detectors here all those little yellow dots are things that detect photons that are formed when for example a neutrino is absorbed by one of the atoms in your mineral oil or if for other reasons so you have to sift your way through the noise you carefully watch this and you discover or they discovered at this experiment which is called mini boon that if you hit it with muon neutrinos if you make new muon neutrinos 500 meters away that you seem to be detecting electron neutrinos in this in this during this big vat of mineral oil and and what's weird about it is that they've only gone 500 meters and so they've had to have turned from one kind into another pretty quickly and we have the standard model makes predictions for how long it takes for them to do that and that they shouldn't be able to do it that fast we other experiments would suggest that they should you have to go a lot further before this happens so so some so in other words the experiment is not fitting the standard model apparently and so people are very interested in this an earlier experiment had detected the same effect and and that's why they replicated it in a different way at mini-moon so it could be that there's a fourth kind of neutrino that's making things more complicated or it could be that the experiments are wrong in some way and I would I would give that a pretty hefty probability because the it's there's as I said there's various other sources of little photons in your tank of mineral oil and you have to be very careful to - to correct for these other sources of noise or it could be something else so next they're gonna do micro Boone I don't I think this is sort of a jokey terminology it's not exactly small it's 170 tons of liquid argon a noble gas and they're going to replicate the experiment with higher sensitivity and see if this effect is real so stay tuned in another 5 or 10 years we'll know it takes a long time to do these experiments because it's very hard to detect neutrinos you have to wait wait wait every so often you got one okay so that's an example of something that we're gonna actually find out in our lifetime at least if we're young enough let's switch over to the other side the gravity side general relativity is much simpler than the standard model it's mind-blowing but it's simple in comparison it's says that if you have any freely falling object it will trace out a path that's as straight as possible but space-time itself is curved so so so it's like if you're like walking around on a sphere you could be trying to go as straight as possible but you may like why not back where you started from or something funny like that and you blame it on the earth being not flat but matter is what's making space-time be curved and the meat of general relativity is this equation called Einstein's equation which is usually written in a fancy way but I'm trying to write it in a simpler way that says the same thing and it tells you how matter curves space-time and it says if you've got a imagine like a little ball of particles so like imagine like you're floating around in the space station and you like spill your your your Nestle coffee powder and there's a little ball of of coffee grounds floating freely only you know not feeling any force except gravity and and the gravity of each other as well and the rate at which this volume begins to shrink of this little ball of particles if it starts out that they're initially at rest relative to each other so there the ball starts out not expanding or shrinking but then it it it starts yeah it starts accelerating inwards due to the its own gravity and the rate at which its volume begins to shrink is proportional to well the volume but also the energy density at the ball in the middle of the ball plus the sum of the pressures in all three directions you may be used to situations like air where the pressure is the same in all three directions but like if I step on a book there's pressure more this way than the other directions so pressure really you have to say which direction you're talking about pressure and so it's an equation you'd say the second or time derivative of the volume of the ball divided by the volume of the ball initially at time zero is minus 1/2 that energy density plus the pressure in the three different directions and I'm using units to make some constants one so basically it tells you exactly how how how matter curves space in such a way that all these guys trying to go straight nonetheless start converging in on each other if the if the energy density is and the pressure are positive so from that one principle and tons and tons of work you can derive Newton's law of gravity believe it or not in the limit when you're only studying slowly moving objects much slower than the speed of light and weak gravitational fields but you can also figure out the formulas for black holes gravitational waves and the expansion of the universe and so I want to just sort of scrap idli sketch out the exhale the expansion of the universe works and if you want to know more you could type the meaning of Einstein's equation into the all-knowing Google and you'll be led to my paper where I start Ted bun and I start with this formulation of Einstein equations which believe me is tons simpler than the usual one and try to just use that to figure out well-known stuff about about gravity well known in did you read image of relativity textbook that is so did for a long time people thought the story was that in intergalactic space there's not much stuff there so the pressure is negligible so we'll only except in the very early universe when things were really scrunched up so so we'll just throw away those pressure terms in the last equation and just write this simplified one where all that matters is energy density Rho remember e equals MC squared so you can convert mass density into energy density if you prefer to think about mass density so if you imagine I was talking about a small ball of particles but now the universe is very large so I can't imagine us quote small ball containing our galaxy and a few hundred other galaxies and apply this equation to it and we can simplify it a little bit because the the volume is proportional to the radius cubed and the mass of all the galaxies in this ball will be the density Rho x times the volume cubed I guess times 4 PI over 3 so you can get rid of volume and get rid of get rid of volume and write it in terms of radius and you get an equation like this you have to do some algebra to do that and this says that the second time derivative of the radius of this ball of galaxies is proportional to 1 over or minus 1 over the R squared so it's like the 1 over R squared force law which is just what you did get for studying a rock thrown up from the earth if you threw it up so high that you actually noticed that gravity was a weaker far away because gravity gets weaker like 1 over R squared so what we know right away is that is that this expanding universe is gonna collapse back down on itself unless it's expanding fast enough to exceed escape velocity unless it's expanding fast enough so there are three basic possibilities for what the universe can do in this simple model which is of course oversimplified in some ways but it's widely studied so one is that it could Rico it could expand starting from very small radius expand and shrink down this radius is not the radius of the whole universe it's the radius of any particular ball of galaxies in the universe so they could spread out and then crunch back down this is called a Big Bang over here at left this would be called a Big Crunch over here at right for awhile people thought that was really a serious option it's mathematically a serious option but it turns out that we're more like in the open scenario where the universe keeps on expanding forever and right at the brink of the - there's a critical situation where it occurs to expand forever but to just barely so those are the three abilities and that was what people were very interested in in the 50s and 60s and 70s which is which is it and the answer seems to be that it was the universe was going to expand forever but it then when they got to be much better at measuring things about galaxies using all these wonderful satellites we have they found that the universe is expanding faster and faster so the big question is why what's making the expansion accelerate one possibility would be that the vacuum has negative pressure that believe it or not is the most popular explanation but if so why and if not what else is going on maybe our laws of gravity are wrong but the most popular option it's called dark energy you've probably heard that what it really means is that the vacuum has negative pressure and actually positive energy and you just need a tiny amount of this dark energy to match what we see but we don't know why it's there if it's there and there are a lot of other mysteries about gravity which are hard to solve like the one I mentioned before how do we combine it or reconcile it at least with the standard model what really happened at the Big Bang some people will say like there was nothing before the Big Bang but we don't know that you know we don't know either happened that we know some stuff that happened shortly after the Big Bang like even like microseconds after the Big Bang we have a pretty good picture of what was going on that I believe but not at the Big Bang so that's a big puzzle similarly sort of on the other side of things what hat what really happens to stuff that falls into black holes Hawking came up with convincing calculations that say that heart by coal should eventually radiate away evaporate away but there are a lot of questions about that that people love to argue about and while we're at it no point in thinking small here what's the ultimate fate of the universe is it really just going to keep on expanding expanding if so it's gonna be given the physics we know now it could be sort of boring in a way it will turn out eventually there well it's if we just use the theories we have now and act like we know what's going on but we'll eventually reach a state where every atom is completely far away from every other atom and so nothing much fun can happen which is sad but maybe that's the way it is another one why is the future so different than the past well if you have this cosmology which starts at the Big Bang and then expands forever then of course you the future is incredibly different than the past and you might just say that's it that's just all there is that that's the way it is and then we'll try to lay everything every other difference between the future in the past on that so if you ask most physicists like how come you can remember the past but not the future they will try to tell you a story which I think is a pretty convincing story that it's ultimately due to this Big Bang versus eventual eternal expansion of the universe that's a long story to get into that obviously it's not obvious that they're connected but we think they are but these are hard puzzles easier ones are the ones where you have data coming in new measurements and dark matter is the buzzword for for this thing that I'm talking about here which is it seems that most of the matter in the universe is some invisible sort of matter that's not like any of the kind of matter we know about there's lots of evidence that something like this is true galaxies are rotating much faster that then can be explained by all them understood forms of mass though heavier something something is the more the gravity there is a faster thing should be orbiting but galaxies are are not spinning the way we wish think they should be unless there's an extra stuff galaxies don't even form at all in our simulations of galaxy formation unless we have this extra stuff which is called cold dark matter in the sense that it's like cool enough that it will collapse down and help seed galaxy formation the fluctuations in the microwave background radiation the leftover glow from the Big Bang fit a model that has cold dark matter and not a model without so most visits believe that this dark matter is real it simultaneously explains a lot of mysterious things but then the question is what is this stuff and luckily we're getting new data about from astronomy quite a bit quite quite rapidly that may shed new light on this also people are just doing experiments to look for weird kind of particles so these these enormous vats of argon or mineral oil that we could use to detect neutrinos you could also build similar detectors or even the same detectors to detect other kinds of particles that hardly ever interact with anything and so people are using the same experiments and also other experiments to look for dark matter particles and so far no success at all which makes people wonder if maybe we have something there's some other explanation of all this mystery so I gave a talk sort of similar to this around 1995 and one thing that strikes me is how little this my talk has changed since then oh I made a different in some ways on purpose but basically by the 1980s theorists were had worked out the standard model general relativity had been worked out much earlier and so they could have told the story of general relativity in the standard model that I did way back then and they were very happy that they'd gotten this far and they thought we must be pretty close to figuring everything out we just have to figure out how the standard model and general relativity fit together which may be just something that we could do just like sitting down and thinking about it and they guessed that they were pretty close to the final theory of fundamental physics and around the 1980s is when people had invented string theory which has a nice ability to get gravity and other forces into a single picture but it has the not-so-nice ability of saying that's extra feature that it says that space-time is ten dimensional and that every particle that we see has another particle that we don't see of the exact same mass and so on and so on there are lots of unfortunate features of string theory so they decided first to unify the forces other than gravity so I'm going back a little bit come up with these grand unified theories and then unify them with gravity and that was where people tried theories like string theory and they hoped that the mathematical aesthetics that is coming up with a really beautiful Theory combined with what we already knew could quickly finish the job but that just did not happen that's not what has happened since 1980s what has happened since the 1980s is that we found the Higgs boson and and and you may have read these exciting accounts in the newspapers but but but physicists were all going oh no that's just what we knew what we're gonna see we were hoping to find something different or more other particles but no no nothing new from the Large Hadron Collider yet although I was talking to a student and and she was says the Thursday discovering some interesting anomalies with B Maison so maybe we'll find something but they haven't yet let's say and and and the other things that have happened are that we've gotten more and more information about the mystery of cold of dark matter but we still don't know what it is we've definitely detected gravitational waves so last time when I gave this talk in 1995 we hadn't detected gravitational waves so that was still sort of interesting although everyone knew that they were gonna happen because Einstein was right you know so so so it's been a sort of frustrating period I'd say an extremely frustrating period in physics partially because it had been such rapid progress from 1900 to 1980 Einstein wrote of paper arguing that atoms existed in his youth so back when he was young they hadn't weren't even sure that atoms existed by the time he died we were studying subatomic particles and making up the standard model it's incredible changed during that period it's just not happening that fast now so I think it's experiments that we need to look for at and luckily experiments keep on revealing new clues we don't know what to do with these clues yet but there keep accumulating so just last week a paper came out on results from of the tournament called Anita the Antarctic impulse transient antenna it's this big thing which they actually lift up on a on a rate on a balloon and it looks for neutrinos but it looks for neutrinos in an interesting way so this is like 1 to 4 kilometers thick of ice in Antarctica some neutrinos skim the Earth's surface coming in from neutrinos coming from outer space due to wild crazy things happening with stars far away and black holes far away and and some of these neutrinos will skim through the ice like this big new here and because they're actually going faster than the speed of light in ice they produce radiation called Cherenkov radiation which is sorta like a sonic boom but for light and when some of that radiation is in the form of radio waves and then some of it gets refracted when it comes up into the air and then Anita floating up in the air there will detect them so it can detect neutrinos in this indirect way by the radio waves that they produce but if they seem to have in the last bunch of years seen two incredibly high-energy neutrinos coming up actually having must have gone through the ground not not just going through the ice but coming up from a steeper angle and they're true these super high-energy neutrinos actually do interact with matter fairly strongly so we don't think they couldn't make it through the ground neutrinos that are low entered can go through anything for a long long time they can go through a light year of lead without any trouble but these are super high-energy neutrinos and they interact a lot so they should not have been able to do this so in other words we're seeing some things happen that doesn't we don't see how they could happen if the standard model is all there is of course the we could just be like neglecting something on some clever way that things could happen but that got people excited and that's the kind of thing that we have to keep looking for little cracks in the armor of our existing theories we've there are plenty of them we need to find more of them and someday someone or some group of people no doubt are just going to put together these clues and make them just take a little step closer towards answering these why questions that I'm talking about Thanks [Music] [Applause] [Music] [Applause]

Channel: CUPS - Cambridge University Physics Society

Views: 12,438

Rating: 4.8915253 out of 5

Keywords: cambridge, university, physics, society, talk, lecture

Id: Stn1FoXuX9A

Length: 54min 13sec (3253 seconds)

Published: Wed Jan 02 2019