The Biggest Ideas in the Universe | 24. Science

hello everyone welcome to the biggest ideas in the universe i'm your host sean carroll and you're here for idea number 24 which is science uh i think that this is not actually been guessed by anyone people have been trying to guess you know what other ideas are coming up along the series i don't think anyone guessed this one that's okay it's not surprising maybe someone didn't i forgot but uh it's a bit of a departure okay this is a meta topic the other things we've been talking about from space to symmetries to renormalization these are ideas within science mostly within physics and now we're going to be talking about science as a whole this is two things i really want to get across in this video one is going to be how science works what science actually is it's trickier to define and understand than what it is it's a whole area of intellectual thought just to figure out what science is i'm not going to go too deeply into it but i do want to get some of the basics on there because they're relevant to how science gets done you know scientists are not completely clueless about what science is they're they're often sometimes somewhat clueless uh but they do have opinions about it and so what we do in science is affected by how we think science is going what it's supposed to be doing and so forth the other thing i want to talk about is the current state and future prospects of fundamental physics you know even though i cheekily uh titled this series the biggest ideas in the universe there's plenty of ideas we didn't talk about that are equally big right we didn't talk about evolutionary biology dna we didn't even talk about various things in condensed matter physics or plasma physics or anything like that we focused mostly on what i would think of as fundamental physics not a great name but that's what it's called so that's what we talk about and fundamental physics is a little bit in a weird position these days as many people know progress is slower than we would like and so i'm going to talk about that you know why it's low what it means what we can do about it before we get there a couple of administrative announcements one is in case you don't know i wrote a blog post about how i make these videos in a previous q a video for force etc i did a little video explanation for how i make these videos but that's changed and so now i've done a more elaborate blog post you can check that out second administrative mention is that this is sadly the last video in the series of the biggest ideas in the universe it's idea number 24 in some base 12 system that's a nice round number but i'm not even going to be doing a q a video for this one because it is a different kind of thing we're not going to be talking about substantive physics or other scientific ideas but a little bit meta i think that this is an appropriate resting place this appropriate ending note for the series as a whole so this is the uh last video enormous thanks to everyone who's been watching encouraging so forth you know i'm very impressed that people followed along we went a lot deeper than i was planning to go when i started it was also supposed to be a quarantine project and the quarantine is going on a lot longer than we anticipated so the future it's hard to predict as wise people have said okay with those ideas out of the way let's dive in but even then before i get to the two things i want to talk about what is science and the future of fundamental physics let me do a little preamble uh to get in the mood here and by the way preamble i want to emphasize the fact that science oops science is man my handwriting is awesome now you knew that deserves an exclamation point now you knew that or you wouldn't be here okay but i really not just want to say like it's really cool doing science but i want to drive home how incredible it is that physics in particular science more generally is as good as it is okay in understanding the natural world by way of a couple of examples my favorite couple of examples about how kind of mind-boggling it is that science works as well as it does so examples um one is gravitational waves and ligo okay we found just a couple years ago i should keep writing gravitational waves uh just a couple years ago 2016 it was announced that the ligo and the virgo experimental collaborations uh detected gravitational waves um this is something we've been looking at for a long time and you know you can be cynical about it and say look we expected them to be there all along it would have been more surprising not to find them but let me let me drive home exactly how remarkable this is okay uh general relativity came along in einstein you know 1915 1916 so literally a hundred years before they found gravitational waves and einstein wrote down his field equation r mu nu minus one half r g mu nu equals eight pi g t mu nu and if you don't know what that means we did a video on it we did the video about gravity where we explained what this means not in tremendous detail but roughly speaking the left-hand side of the equation is a mathematical characterization of the curvature of space-time and the right-hand side is a mathematical characterization of the sources of space-time curvature the energy the momentum the heat and all that stuff okay pressure density and so forth and so this equation was arrived at by various arguments you know there's more than one way to get there we talked about the fact that david hilbert also derived this equation from an action principle from the principle of least action whereas einstein used other arguments about energy conservation and so forth but they they got the same equation and the point that i want to make is the equation is way smarter than we are even much smarter than einstein and hilbert which is saying something is the two of the geniuses of the 20th century okay there is stuff hidden implicit inside this theory that was not at all clear to the people who invented the theory okay so uh for one thing there's a prediction that there are a thing called black holes okay uh einstein thought that his equations were so complicated it would be very very difficult to find exact solutions to them he found several approximation schemes so he used them to make predictions for the orbit of mercury and so forth but as we mentioned soon thereafter carl schwartshield found a solution an exact solution for einstein's equation with no matter at all just empty space uh for a spherically symmetric curvature of uh nature and we used that immediately we the physicists used that solution immediately to understand the solar system and so forth but there were these mathematical obstructions to really understanding what happened when you went close to the center of this purported spherically symmetric mass distribution it took decades to figure out that what was really going on were these things called black holes regions of space and time where space-time was so curved that light itself cannot escape okay so it's a prediction of the theory that was not made by the people who invented the theory in the first place it took a long time it was until after einstein had died uh the people really understood that his theory predicted black holes it also predicted gravitational waves this was a little bit more readily apparent to einstein so einstein knew electromagnetism very well right he knew if you took an electron and you shake it electromagnetic radiation is emitted so he figured that probably if you take a massive object and shake it gravitational waves would be emitted but again the math was not at all clear remember we had a discussion about gage theories and in some sense general relativity is an example of a gauge theory there's a symmetry underlying what is allowed to happen in the world and you can interpret that as allowing for certain kinds of things including the curvature of space time not other kinds of things and this symmetry part of the symmetry group of general relativity is just the idea that you can change coordinates right you can have the same physical situation the same curvature of space-time but the metric which is the field that describes what's going on in gr can look very very different if you change coordinates from one system to another so even though einstein predicted gravitational waves he later started to wonder whether that prediction was correct and you know he wondered whether or not what he thought was a gravitational wave solution was really just a weird coordinate system in flat space and in fact he he submitted papers arguing that there are no such thing as a gravitational wave so he was wrong about that but eventually and you know different people wasn't just einstein many people argued back and forth eventually we realized yes gravitational waves are actually real okay so this set of predictions was there in the theory this is part of my point that you know physics is smarter than physicists are the theory knew perfectly well that there could be gravitational waves in black holes but it did you know the scientists took decades to figure that out and once they did you know they they invented the ligo project the laser interferometric gravitational wave observatory uh the first sort of glimmers of an idea were in the 1970s uh for interferometric gravitational wave detectors in the 1980s some steam began to build up in the 1990s we started spending real money on this kind of project and uh it in the end of the day it cost about a billion dollars in american money to make we don't need a plural there uh to build the ligo experiment and a lot of people were still skeptical that it would find anything they believe in gravitational waves but gravity gravity is very very weak okay and gravitational waves are very very tiny perturbations on any experimental apparatus you can build i mean the idea behind ligo is that you have two vacuum tubes that are perpendicular to each other and there is a beam splitter and so you send a laser and part of it goes this way part of it goes that way and there are mirrors and then they bounce off the lasers bounce off and come back to you and if there's a misalignment in the wavelength of the lasers caused by the mirrors moving then you detected something okay now maybe what you detected is a truck rolling by just vibrating the floor of the gravitational wave observatory but maybe what you've detected is a gravitational wave coming in and moving the mirrors and so the challenge was to make sure that you didn't that you were able to pick out the actual signal above the noise and a lot of people were skeptical that that would ever happen just to drive this home the amount by which the mirrors move the actual physical distance that the mirrors move in the gravitational wave detection that was announced in 2016 is less than the size of a proton okay it is a very very very short distance and you're saying yourself well how in the world can you detect like there's a big mirror that's made of many many protons well the answer is that you bounce many photons off the mirror and they remain collimate it's a laser right okay they're they're in lockstep and you build up a lot of statistics and you look for the tiny tiny deviation over and above the known noise sources not easy to do took decades to build etc billion dollars and what they found what they announced was that there were two black holes each of them was about 30 times the mass of the sun and they were about 1 billion light years away these two black holes had spiraled into each other and they gave off gravitational waves and there was a signal that lasted for seconds that was detectable by this observatory okay this was a prediction that was secretly embedded in this theory when einstein and hilbert for that matter wrote it down uh but they had no idea it was there later people worked it out they spent a billion dollars and decades and a lot of smart people's lives looking for it and they found it and it was exactly what the theory predicted there were no detected deviations so when einstein came up with general relativity it wasn't like you know okay this is a step and then we'll take you know it'll improve it along the way sometimes science is like that you know darwinian evolution is like that um biology is much messier than physics is but nature has this pristine structure somewhere embedded in it okay number one there is that structure in there and number two we can find it we human beings at least a little bit you know at least provisionally in some sort of haphazard way we can discover some of what structure is there in nature and that's pretty amazing to me uh it'll never stop being amazing another example along exactly the same lines is the higgs boson okay so in 1964 peter higgs and also a bunch of other people especially uh robert brow and francois anglair were thinking about not the mass of the electron or anything like that they were thinking about the question of why the nuclear forces were short range okay we knew that gravity and electromagnetism were long-range forces um we knew experimentally that there were short-range forces in the atomic nucleus what is going on there okay and we discussed this in the in the um various biggest ideas videos about symmetries and gauge theories and so forth if you fill space with this field that has a non-zero expectation value you can sort of cut off the lines of force that would want to extend out to infinity higgs and brow and anglair were all thinking that what they were explaining was the strong nuclear force what we now know thanks to steven weinberg and others is that what they were really explaining was the weak nuclear force and the funny thing was um brown angler actually wrote the first paper on this and higgs wrote a paper about it and then higgs wrote a follow-up paper that was rejected by the journal it's like you know why this is too speculative okay back in the 60s the standards for being speculative were a lot higher than they are now um so higgs you know thought about it and he actually just shifted to submit to another journal which is a very standard procedure in academia but he said you know i really he thought to himself i should beef up my paper i should put something in there that uh is more you know irresistible to the referees and so he realized that he could use this theory to predict the existence of a new particle the higgs boson okay so brown angler didn't mention the prediction of a new particle higgs only mentioned it because he was trying to beef up his paper after it was rejected okay so it's perfectly fair to give brown angler uh the id the credit for the original idea but it was higgs who discover who predicted the boson the actual particle and again they were predicting it for a different reason in some sense than we currently use it for they were thinking about the strong nuclear force not the weak nuclear force and for that matter the 1960s was a time when quantum field theory was in decline you know we think about the origins of quantum field theory in the 1930s and there were puzzles and neil feynman and schwinger and dominaga and others did renormalization it was all very triumphant and today of course we have the standard model and it's super duper triumphant but it wasn't a monotonic increase in the triumph of quantum field theory the 60s were a low point where there was a proliferation of particles that we couldn't explain there were mathematical problems with quantum field theory that did not get fixed by feynmanesque uh diagrams and renormalization techniques etc so people were trying to do things like called there's something called s matrix theory that they were trying so the fact that these folks were using quantum field theory and they were motivated by you know somewhat abstruse considerations and they made this prediction okay that there's going to be this particle and 48 years later the large hadron collider took about 10 billion dollars they found the higgs boson and you really should give credit here to stephen weinberg who in 1967 proposed a model using this mechanism to explain the masses uh and couplings of leptons electrons and and the muon etc but you know it was again spot on it was exactly what they predicted and you know it barely got in there on time for most of the original discovers uh robert brown sadly passed away before they discovered it but the but higgs and anglair and the others who discovered it were still around and um unlike gravitational waves which had not been discovered when it's uh which had not which took a lot longer to discover but again the point being that they were able to just use their brains and what we knew about particle physics to make this prediction decades ahead of time and we did this the most complicated experiment ever done and we found they were exactly right in some sense why because there is something true and real and rigid about nature that science can uncover that's what i mean when i say that science is awesome many of you will be thinking of uh the famous paper by eugene wigner called the unreasonable effectiveness of mathematical of mathematics in physics you know and the example he gives is you know a friend of his was doing some statistics and was using a gaussian normal distribution remember the normal distribution and when you we didn't actually normalize the normal distribution we didn't integrate it and set it equal to one but when you do the pre-factor involves pi and some friend of the friend said like that's ridiculous pi is the ratio over circumference to the diameter of a circle that has nothing to do with the statistics problem you're doing how does pi come in there and wigner's point is mathematics just suffuses all through the structure of physics in in remarkable ways and you know i'm not quite sure that it's as remarkable as vigner made it out to be like if the laws of physics were different i think we would still describe them mathematically even if what happened from moment to moment in the universe was completely random there would still be some distribution of what happened and we use statistics to describe it so the thing that is remarkable to me is not the effectiveness of mathematics but the simplicity and rigidity the unforgiving nature of the underlying laws that seem to be obeyed by nature and again it's not just physics i pick physics because it's a very wonderful set of examples very close to my heart old school physics also very good you know isaac newton's inverse square law of gravity is still what we use to get around the solar system if you're nasa and uh in biology and so forth those of you who are listeners to my podcast uh just released a podcast with theodore ornov who works on gene editing with crispr okay the idea that we can borrow techniques from bacteria to go in there and edit the human genome right this is kind of amazing stuff you know science science is pretty awesome that's what i'm that's what i'm getting at here and the sense in which it's awesome it's it's two things it's not just that it's true things you know as one i was once asked by an economist um who's little peeved because the point was how come physicists are so admired intellectually and economists are not that was that was the question so i'm not quite sure the premise of the question is exactly true economists appear on tv a lot more than physicists do for you know public policy recommendations and probably that's the correct thing but i got the point and so the answer i tried to give was physics not only describes and discovers true things about the world but they are number one demonstrably true you can test them you know they get unambiguously signals in the detector whether it's a large hadron collider or ligo or whatever and number two they're surprising results the idea that there are black holes and the idea that there is a higgs boson and you know there's this field pervading space that cuts off the weak nuclear force you wouldn't have guessed these things just sitting around the campfire right you know just using your brain power um physics is able to discover things about the world that are both true and surprising and uh you know economics finds out things that are true uh whether or not they're simultaneously true and surprising and explicable to the wider world is a harder thing right the number of surprising things i think i can say this with confidence the number of surprising things that physics has discovered and has demonstrated are true is larger and more impressive than that which economics has discovered this is absolutely no knock on economists economics is much harder right we started this set of videos with a spherical cow philosophy the idea that we can simplify the world and still make progress in physics that's why physics is great that's why physics is so is so possible to make so much progress because physics is easy right the world is a simple place okay thus that is my preamble spiel on why science is awesome let's think about what science is and you know i'm not uh completely clueless here what i'm going to say about science uh will is a little bit less rah-rah than many people often are i'm gonna you know i'm gonna admit that science is complicated okay um which i wanted to first get off you know get on the table the fact that science is awesome we should all agree on that but what science is is much more complicated than what a lot of people say so i'm going to try to make two points and in the next few minutes when i'm talking about this there'll be a continual sort of give and take i'm going to say a but also a little bit of not a right so this is this is a field once you get into sort of the philosophy or history or sociology of science where there are very few absolutes to say like you have to find your way in between a spectrum of different possibilities okay so what i want to say about science is on the one thing on the one hand it is something in other words uh there are things that are not science right there are activities you can do that are perfectly respectable activities and some disreputable ones that are not science there's a difference between being science and not being science okay that is clear but on the other hand it is hard to pin down what exactly counts as science and doesn't and in fact it is so hard to pin down that the problem of pinning down what is science and what is not science is a famous problem that has a name the demarcation problem demarcating here is science here is not science and a lot of people get it very very wrong and i'm not going to try to give you the once and for all final answer i have enormous respect for the people who are doing the hard work in philosophy and history and sociology of science to point out all of the ways in which science is a little bit messier than what you thought so i'm going to sort of lean into that messiness i'm going to talk about some of the aspects of science but not pretend to have the once and for all final explication of what it actually is so what is it what is going on when we do science um here's the way that i think about it we observe the world okay so we look at it that is one aspect of doing science we collect data we do experiments we look in our telescopes and our microscopes whatever we figure stuff out about the world even before we called it science we just look around and go huh there's a table right there's a chair things like that there's the sky there's rain falling from the sky we thought about it okay as soon as you start thinking about it you try to make sense you try to develop a model in your mind like okay it rained yesterday will it rain today right what are the chances can i predict ah you know like the sky is red does that mean it's going to rain you try to find patterns and regularities and develop a theory for what's happening in the world so that's another aspect so you theorize you sort of develop some kind of formal or informal system uh some sort of structure that says if this then that these two things are correlated these two things are related this is the correct vocabulary to talk about the natural world and so forth and clearly there's a give and take you theorize on the basis of what you have seen and you uh your theories suggest to you what you can look at right or what you should look at because you want to test your theories in fact i'm going to separately put down a little blob for compare by which i mean comparing theories with the observations because comparing theories with observations is not trivial it's not just automatic it doesn't just happen okay there's a lot of work going into saying what theories predict and going from the observations to a theoretical implication of them okay i mean this is way more obvious now than it used to be because these days we have computer simulations right um people will simulate the formation of a galaxy or the explosion of a supernova or something like that and there are they debate sometimes if i'm a person who writes a code to simulate a supernova explosion am i a theoretical astrophysicist or am i a computationally experimental astrophysicist am i creating new data the answer is you're not an experimenter you're using your computer to compare theory to data this this compare or analyze sometimes we call this analysis or something like that uh analyze oops i'm not in england analyze analyze the data compare it with the theory okay that's a a part of science all by itself and all these different aspects kind of work together and talk to each other in complicated ways some of you like me may have been subjected to the baconian scientific method when you were in high school okay francis bacon uh back in the day uh it's actually the history of the stuff is fascinating if you want to read about it because the bocconi and scientific method is this idea that you invent hypotheses you've got to collect data you test your hypothesis you come back and you know is it is it correct or is it is it false right so this didn't really arise organically and i'm not an expert in uh the history of this but my understanding is literally francis bacon and his friends sat around and said we have to invent something called the scientific method because we need to sort of gain some respectability for this new activity that we're all i don't know if they call it the scientific method but they wanted to they were very explicit about the need to sort of become more respectable and so they promulgated this idea of a scientific method it's entirely nonsense that sort of high school level scientific method there is no beginning or end to this process right you don't know what comes first maybe your theory came first maybe your observations came first i mean in some sense your observation some observations came first before you had the idea to invent theories of them but it's a constant give and take and you constantly change your theories in response to the observations and you choose what observations to make in response to the theories and so forth it's nothing clean and cut and dried like bacon would have said and in fact i want to i want to mention something else here which is all this stuff is done by human beings right by people and if we really care about what happens in the progress of science like where in here did i mention applying for grants where in here that i mentioned teaching phd students or you know building a laboratory or grading people or hiring people or firing people or giving people prizes or citing people or getting published right writing papers or not citing people or giving seminars i didn't mention any of those things but those things are crucially important to the progress of science to the practice of science so let's make another uh bubble here in a slightly different color for what i'll call socialize not socializing in the sense of uh sitting around having drinks but the human side of all of these things right the actual interactions between people and this is important for all of these different aspects okay whether you're i mean it's clearly important for theorists they need to talk to each other they need to understand what the data is talk to the experimenters for experimenters who need money for their experiments the socializing is even more important and likewise for the analysts so just to emphasize i'm not going to talk about that aspect in great detail but just to emphasize that i do want to get across the sort of truism that science is a human activity okay so let's let's note this is this is it this is my answer to what science is the science is the set of activities all in interaction with each other in an attempt to better understand what's happening in the natural world that is what science is and i want to note two things okay let me change back to white two things one thing is that whatever science is it is not revelation revelation or pure logic that is to say this is sort of the single most important thing that differentiates science from other ways that you might try to understand the fundamental nature of reality you might say well maybe god tells me the fundamental nature of reality right maybe it's just revealed to me that's what revelation means so revelation is often uh associated with a literally religious point of view theism or something like that but let's just take it in a broader meaning to say like somehow true information about the nature of reality appears in my mind in some way other than me going out and looking at it okay that's what science denies science says you know i can come up with ideas about the universe i can invent ideas and say maybe things are like this but in order to know whether or not those ideas are true i have to look at the universe okay that's the crucial thing science is about recognizing that things could have been different there is relativity but there's also classical newtonian mechanics these are different the universe is expanding but it could have been contracting in the sense that uh we mentioned before david lewis who is very famous the philosopher very famous for this taking seriously the idea of the set of all possible worlds okay in some sense you can think of the activity of science as saying here all the possible worlds let's find which one is our actual world that's what science is trying to do and you can't do it a priori so it's different than logic or mathematics right like you can prove logically or mathematically that from certain assumptions certain conclusions flow right from the axioms of number theory two plus two equals four from different axioms different things would flow we know this we we encountered this in geometry right from euclidean axioms the postulates of euclid you can derive certain theorems like parallel lines always remain exactly parallel but you could imagine different axioms right under which parallel lines diverge or converge or do weird things and science says you can go ahead and develop these mathematical theorems based on postulates but you're not going to know which postulates are applicable to the real world unless you go out and look at it right so it is a fundamentally empirical kind of thing and let me sort of drive this home a little bit um in a couple of more specific ways it is empirical so i actually put very explicitly revelation and logic as two different things because i don't want to denigrate either one you know like maybe there is revelation uh there certainly is logic but neither one of them is science okay science is specifically about letting various possibilities be true not knowing them a priori and going out and looking at the world that's what it means to be empirical recognizing that things could have been otherwise another aspect of this is that it is provisional so the thing about math or logic is that you can prove things and if you didn't make a mistake in your proof which sometimes happens but if you like want to prove that the square root of 2 is an irrational number you can do that and then you know with metaphysical certainty it's 100 true you know that square root of 2 is an irrational number because you have a proof of it in science there are never proofs that a certain theory is absolutely 100 correct it is necessarily provisional because there could always be an experiment tomorrow that is in conflict with your theory no matter how good your theory is so there are no proofs in science science does not prove things and so this is this is a thing where scientists get in trouble because they know this right even if they don't ever uh say it explicitly they know that science doesn't prove things so if you want to be anti-science and you want to sort of get a scientist to stumble in public conversation you say well have you proven that this is true and the scientists will say no because they know that they've never proven anything is true uh they've demonstrated that it's extremely likely that something is true but it's never quite the level of a proof that you get in math or logic and finally it's ground up not like ground up into pieces but science happens from the ground up is what i mean by this in other words just like there are no proofs there are no popes there's no no pope of science who just hands down the rules there's no emperor no king no queen okay there's no academy that decides okay here's the truth and here is not the truth okay you know i i sometimes compare and contrast science in democracy democracy and science are different okay they're trying to do different things people are very very very insistent that science is not a democracy and that is true you do not find the truth by voting but there is a crucially important similarity between them in that both bubble up from the bottom rather than being imposed from the top the idea of democracy is that rather than an autocrat who makes the decisions once and for all and the people obey the people ultimately through one way or the other one mechanism or the other have the authority to make decisions in your state okay likewise this is the similarity between science and democracy likewise science does not proceed by the smartest person in the room saying here's what's true now you all listen to me okay anyone can show that previous scientific beliefs were wrong so the cheeky way of putting it is if you're a theologian and you write a paper that shows the pope made a mistake you get excommunicated from the catholic church if you're a physicist who writes a paper that says that einstein was wrong in some important way you win the nobel prize science is not about uh bucking up the establishment and saying why it is correct uh now that's unfair to religion and to other aspects of life or even dictatorships because you can still have progress and change in those situations in those systems but science is built for progress and change it's what it's all about the excitement of science is showing that the accepted wisdom is wrong somehow now again you have to temper that as i said at the beginning of this i'm going to say this but also that the excitement comes from showing the accepted wisdom is wrong but usually the accepted wisdom is right right it's a rare wonderful event where you can truly show the accepted wisdom is wrong so you have to like take into account that you want to show the accepted wisdom is wrong and therefore that's a temptation to sort of lean into that a little bit too much and you have to say well how do i know you have to be very very careful you have to really understand the temptations to overthrow the system it's hard to overthrow the system in a paradigmatic science anyway okay um the other thing that was one thing i wanted to say the other thing is that every component of this little story observing the world theorizing about it comparing those two and all the different social aspects of science every component is messy so the the way to say this i think is that science just by its product right by what you get out of science you can do technology you can understand the world you can predict the existence of gravitational waves and so forth okay um it gives the impression of being pristine and algorithmic and objective okay uh and it's not it's really not it tries to be as objective as it possibly can be but every single one of these aspects of science is a mess and is a human activity that cannot be reduced to an algorithm so let me just mention you could go on for days you could write phd theses on all the different ways in which science is kind of messy let me just mention some of them so in other words what i'm getting at here is given this kind of structure what people will try to do is reduce the activity of science to an algorithm that's what bacon and his friends were trying to do it is doomed to fail that process it's not supposed to be reducible to an algorithm there's not an algorithm for figuring out how to make progress in science so you might say well look i observe the world i come up with a theory and my theory makes a prediction i make a prediction and i go out and test the prediction by making some observation to figure out whether my theory is true or not right that's what you might say if you were being a little bit naive and simple-minded about how science works but in the real world here's some things you have to take into account what do you observe we say okay you go out and look at the world you make some observations well what observations there are literally an infinite number of potential things you could observe about the world right when the large hadron collider uh reported its data that said that the higgs boson had been discovered they did not tell you what the humidity was in the room they did not tell you who had won the local football match that day right they did not tell you what color of socks the physicists were wearing there's an enormous amount of information that is left out when you do observations there are reasons why it's left out some information some data are relevant to the question of what theories are true and some are not we think but that's a judgment that the scientists make right the very and nowhere is it more clear than in this exam these examples of the large hadron collider and ligo billions of dollars are being spent why because we trusted that there were predictions being made by scientists that this precise really really difficult observation was worth it okay there's a process of deciding what observations are worth making you can put it crudely in terms of how to spend the money you know in some science budget but even if you had an infinite amount of money you don't have a amount of time some observations would be more useful than others and that's a that's a human judgment in some sense um are the observations correct so some of you might remember a few years ago when there was a press conference at cern home of large hadron collider but before the higgs boson that said that they discovered that neutrinos move faster than the speed of light okay it was later shown to be wrong there was a wire that was loose literally um but look if that had been true it would have been the most important scientific discovery of many many many decades um and yet scientists physicists did not get all that excited about it why because observations are sometimes wrong and this observation was so in conflict with our theoretical expectation that no one believed it everyone said that even without knowing even without being an expert in the experimental techniques everyone who was an expert in physics and yeah yeah it doesn't seem very likely i'm gonna wait for that one to go away right and you could be wrong about that maybe they'd they would do it again and other people would verify it and you would then eventually have to change your mind but in fact they were right to say that the experiment was wrong it was wrong so just because there is an experiment that says the theory is in trouble doesn't mean you declare victory you have to again have some judgment about what experiments to take seriously and which ones to discard um how to theorize so this is this is always the big mystery about the bocconi and scientific method and actually i first learned about this issue or is impressed upon me by reading of all things uh robert robert pierce book zen and the art of motorcycle maintenance uh incendiary motorcycle maintenance he makes a big deal of the fact that there's this step in the high school scientific method that says come up with a hypothesis and then test that hypothesis and he's like what do you mean come up with a hypothesis how do you do that what is it that what is the process you go through to invent a hypothesis it seems you know at the level we currently understand it all you can do is say things like it's fundamentally a creative thing right you need to be a smart physicist that has some spark of ingenuity to be able to say oh let's think of it in this new clever way einstein like we can tell ourselves stories about einstein's amazing physical intuition and his ability to invent thought experiments with elevators and rocket ships and trains and so forth but a little bit of that is x-pos facto you know like he did it so we tell a story about how it happened we don't know how to invent good scientific theories we just hope it happens and encourage people to do it it's not something that we can turn into an algorithm what do the theories predict okay so we say okay you get a theory you make a prediction you test it against the data right the real world and again we're saying that every component here is messy in the real world maybe your theory isn't done yet or maybe you just don't know what your theory predicts so again einstein just used examples that are familiar from our previous videos einstein wrote down his theory of relativity he said what if the universe is homogeneously filled with matter or radiation and he realized oh my goodness i predict that it will either be expanding or contracting then he talked to his friends the observers and they said it's not expanding or contracting because observations are not always correct right or at least they can be incomplete so did he just say oh i guess general relativity is wrong i guess my theory has been falsified by the data no of course he did not say that he said hmm i wonder if i could change my theory a little bit so he added the cosmological constant this was not cheating this was not fudging we now know the cosmological constant is actually there at least we think it is with high probability it wasn't a wrong thought that einstein had it just wasn't the right answer to that particular issue because the observations were misleading in that particular case but the this should be in principle the simplest part of science like you have a theory make a prediction right but turning that into you know knowing exactly what your theory predicts often requires a bit of judgment also um and then finally of the many many things we could say vague ideas often precede theories so einstein was lucky in the sense that when he invented general relativity well he wasn't lucky he was einstein he's very smart so he mentioned relativity it was a very very precise rigorous theory but you know he had the idea that space-time was curved years before he had the field equations for general relativity he made predictions for things like the gravitational redshift and things like that long before he had the full theory much more common is you have kind of a notion that that you should be moving in a certain direction theoretically long before you have the full theory like we talked in these in these videos about quantum mechanics and the foundations of quantum mechanics many worlds and bohmian mechanics and spontaneous collapse theories none of these theories are 100 finished yet okay the closest is many worlds it's just objectively true that many worlds at least has a clear statement of what the theory is but there's still a lot of work to be done connecting that theory to predictions for experiments the theory is not fully developed yet and the other theories you know pilot wave theories or spontaneous collapse theories are not completely compatible with quantum field theory or quantum gravity or anything like that which is not to say they won't be maybe they need to be developed into theories that are completely compatible so in the real world when you say okay i have a theory make a prediction your theory just might not be up to the task if you want to write in your face important example here many many modern astrophysicists think that the universe began or close to began with an inflation period of super accelerated expansion that explains the perturbation that they grow into galaxies and so forth but inflation is an idea it's not a certain specific theory so if you say well okay using inflation predict the exact level of anisotropy in the cosmic microwave background it can't inflation is a family of many different kinds of theories alan guth himself who invented it likes to call it the inflationary paradigm rather than inflationary theory it's sort of a set of many theories so even if you have something that is a good idea that might be true it might not yet be a theory and so all of this is just to say that all these components in the real world not in fake make-believe pristine algorithmic world in the real world these are a mess okay and that is not a bug it's a feature it's how science works um it's not that there's nothing objective okay what's objective is nature reality is there you know there's something there that is uh objective and unchanging and independent of human thought when it comes to what nature is at least that's the precondition of doing science that there is some objective reality out there that we are learning about but the process of learning about it is not objective so science is a non-objective technique for learning non-certain features of the objective certain reality underneath us all all that is true at the same time and you shouldn't be ashamed of it it's not it's not that you know we're not good at science yet and someday it will turn into an algorithm it need not be an algorithm it's a fundamentally human pursuit of developing the best theories that we have of nature so even though it's a mess it really does capture some truth and i want to say you know there's again here's another continuum there's a set of people who don't know that much about science but love it and think that science just gives you the truth right like if you just do science then you will get the truth there's another people who also love science but many of these people are working scientists and they prefer to emphasize the fact that you can't prove things in science right that was a one of the things we said no proofs right so they will say that the fundamental lesson of science is that you never know for sure anything okay anything could be overthrown and you know again strictly speaking sure you never know anything for sure but the most useful place to live is in between those two extremes where you think that science is just facts and immutable truths and when you think that science is just the statement that we never know anything okay we do know some things science does teach us some things but the things it teaches us are necessarily approximate and provisional um you know gravity pulls things together here on earth right if you if you jump out the window with very high confidence you will fall down you will not fall up under the force of gravity you can't know for sure but it is something that is worth taking very seriously so i don't know exactly what words to attach to this but you know in a legal context we would say that some things are established beyond reasonable doubt and science never establishes anything beyond metaphysical doubt right it never says that anything is a hundred percent certain but certain things can be 99.9999 certain right and then you go like i don't want to waste my time thinking about these other possibilities that is also very possible so not only do we actually learn things but we we learn them to the point where it is not worth paying attention to alternatives that's a very very high standard i don't wouldn't want to leap there too quickly there are certainly cases in the history of science where people have left there too quickly thought they were more certain than they should be but nevertheless we can eventually get there so science does teach us something even if it's not perfect it's tricky okay that's that's the point it's not like cut and dried and that is one of the one of the things that people fail at is they try to turn science into something a bit more cut and dry than it really is so just to drive this home i want to drive home this message even even further uh by comparing different ways of logically reaching conclusions okay different uh logical techniques there's the technique called deduction abduction there we go deduction is you know something that you can put in the form of a syllogism for example right you're all familiar with this right uh a implies b a is true therefore b um socrates is a man all men are mortal therefore socrates is mortal right that's very nice and logical and uh this is a perfectly good form of logical argument and it's it's very admirable because if you're sure that a implies b and you're sure that a is true then you can safely say that b is true you've proven it to be true the problem is that science is never sure about either of the premises of the argument so these are the premises and this is the conclusion okay so in science you're never going to be sure about the conclusion of a deductive argument because you're never sure where the premise is you could always be wrong about some of the premises so deduction although it's it's seductive as a way of learning true things it is not the way that science works and this is pretty clear people sort of knew this for a long time so there was this idea that maybe we should think about science in terms of induction okay a different method logical method of learning new true things and the very very short version of induction let me see how i wrote it down here let's imagine we have a sequence of statements a sub n a1 a2 a3 etc right so if you if you know that if a n is true then a n plus one the next statement in the list is necessarily true if a n then a n plus one a n implies a n plus one if you know that and you know a1 is true okay so you have some starting point then therefore all a n are true for all n okay then you've proven a whole list of things right so um uh if n is even then n plus two is even two is even therefore four is even six is even eight is even et cetera you can prove inductively that all those numbers are even numbers there's probably better examples of inductive proofs but this is also a absolutely valid way of doing logic and proving things from the premises you see the problem which is that in the real world in in science we never know if these premises are true and it was david hume our old buddy david hume who pointed this out he says yeah you know i see why you think that science works this way you see something happening you see it again and you see it again uh you know you see the pen fall you see it fall again you see it fall again and you're like i bet next time it's going to fall oh i was right okay that's kind of like induction right like you're collecting a lot of data you see that a series of things are true and then you say okay it's going to continue to be true but hume points out it could change tomorrow right you don't know with metaphysical certainty that just because the pen fell every time it will also fall tomorrow and indeed there are possible worlds i could invent laws of physics where the pen falls every day except tomorrow it falls up right or it push flies up i guess so this is hume's problem of induction you can never be sure in the real world so you need to separate out mathematical or logical induction which is very valid from sort of the attempt to make induction empirical or scientific where it fails okay induction is not the way that science works so there's another way that it's worth thinking about that is much much closer to the real world and this is called abduction it's kind of like kidnapping somebody but not exactly and this is associated with charles sanders purse his name is spelled kind of like pierce but it's pronounced purse uh if you're not already a huge fan of cs purse i recommend looking online or buying a book and reading about him he was a polymath a philosopher a linguist a mathematician an inventor um he was a weird guy he lived you know about a little over a hundred years ago so uh end of the 19th centuries when he really flourished bertrand russell said he was the greatest american intellect of all time there have been other american elects since then but he was a founder of pragmatism as a school of philosophy made major contributions to semiotics he was a cartographer he invented new projections of the globe and things like that okay and but he was also slightly crazy and hard to get along with um he spent the last couple decades of his life in poverty and after he died it was discovered he left behind over a hundred thousand pages of unpublished manuscripts about everything in the world which most of them are still unpublished we don't even know but anyway so purse is looking is worth looking into he's enjoying a bit of a renaissance uh lately so the idea behind abduction is the idea that rather than being purely a hundred percent logical and proving things what we do is we try to infer what is the best explanation for something in fact there's there's a debate in philosophy circles there's something called inference to the best explanation which is very very similar to abduction but people wonder whether or not it's the same or not so i don't care i'm not i'm not drawing those fine distinctions but the basic idea is that rather than these sort of premises and conclusions kind of thing you have a large set of data and i'm putting this in my own words i'm not sure if purse would have recognized this so you have a set of data a1 a2 a3 so data or you know observations things you think are true about the world facts in some sense or another by the way interestingly you know i called these data and i said you could call them facts do you know that scientists almost never use the word facts i actually checked this because you know back in the day where we're debating creationism um you know science and facts was a big deal and so uh i i thought about it in my my daily routine as a scientist no one ever talks about facts even though those are associated with science what they talk about are data and the reason why is because you don't know what the facts are you know what the data are but like we said observations can be wrong so i literally like went through um titles in the particle physics archive and looked for the appearance of the word facts and like they were almost never there there's like two papers then you look for words like data or evidence there's huge piles of things okay so deep in their bones no matter how much they talk when they try to play philosophers scientists know how provisional what they do is and they are actually a little bit careful about understanding that what they have to deal with are data and some of those data could be wrong it's a mistake to promote a datum to a fact without putting little footnotes saying well unless we're wrong about this so let's call this data a set of data and then what you have are a set of hypotheses about the data b1 b2 b3 you know maybe the universe is expanding versus contracting dark matter is axions the dark matter are wimps dark matter or black holes whatever it is set of hypotheses then you ask yourself which bi best explains all those data all the a n and you say that bi is probably true if it's the one that best explains all the data okay now you might be outraged by this you're like this is not the same as deduction or induction because and you're right deduction and induction give you 100 true results if you start from 100 true premises okay whereas this seems very fuzzy like what do you mean best well what counts is the best explanation uh is it the simplest explanation is the one with you know you can state is it sort of the smallest common role of complexity uh does it is it you know the the easiest to explain to other people does it make the most predictions like what do you mean by the best explanation yeah that's a good question that is hard to answer um you know thomas kuhn a later philosopher of science he wrote the structure of scientific revolutions and he got accused by a lot of people for being a relativist about science because of things he said in structure about how if you believe in one theory you can't even understand what people who believe in another theory are saying right you do that paradigm shift in your mind uh so he later wrote another paper where he said like no no there are it's not an algorithm choosing between scientific theories but it is not irrational either there are criteria people use simplicity fruitfulness you know predictability predictiveness etc and that's right like i don't know if his list was right but the idea that it's not an objective algorithm but nevertheless it is not arbitrary either that's what is at the heart of this process of abduction we can do our best to explain why one hypothesis or another is the best explanation some people are going to disagree but as we accumulate more and more data the idea is we get better and better at understanding which explanation is better and this picture of abduction in purse's terms really fits much better with this messy set of interconnected activities than the idea of logical induction or deduction ever did you know there's an example that that purse gives of looking out his window and seeing azaleas into the garden and he says you know i would say i see that there are azaleas out the window and he goes well no you don't what you see in in modern terms what you see is some light hitting your eye right some photons hitting your retina you infer the existence of azaleas because that is the best way to explain that particular pattern of light falling in my eyeball you might be you might re remember that this is more or less exactly the same logic we used when we talked about the past hypothesis and the low entropy early universe we see the cosmic microwave background it seems pretty uniform to us and so you say look the early universe had low entropy because gravitational forces would have pulled things together and made it higher entropy if it were lumpier rather than smooth but you don't see the early universe you see the light you see the radiation field at your eyeball now you use the past hypothesis to infer that there was probably uh that it was consistent to say there was a low entropy initial condition so it's this sort of consistency this back and forth what is the simplest explanation what is the most powerful what is the best that's how science really works and that's what purse was really getting at you will also notice a very close resemblance between these ideas and bayesian reasoning it's basically the same thing to be honest you can think of bayesian reasoning as a formalization of this abduction procedure but it makes it formal and therefore look more systematic but all of the loosey-gooseyness that i mentioned in abduction really is also there in bayesian inference it's just hidden so let me explain what i mean by that so in bayesian reasoning you start with some set of theories that could be true right t i some theories about the world your theories could be oh i left my keys in my car i left them in my bedroom whatever like they don't need to be grandiose physics theories statements propositions about the world and you assign to them some prior probabilities right priors some probability measure on the space of all these theories so that adds up to one okay and these are your prior probabilities that is to say you're imagining in this little fake scenario you have not yet looked at the world all of us have looked at the world so it's a little bit fake so what account is a prior and something that is informed already is a little bit tricky but that's okay and then you use your theories to make predictions and you collect data d and so then you uh update update your theories to update your probabilities so you get posterior probabilities based on your prior probabilities and your data via phases rule which says that the posterior probability for theory i given the data is the probability of the data given the theory that is to say the likelihood times the prior probability for that theory divided by the net probability for getting the data in all the theories combined okay bayes's rule right there so you can update your theory so the idea behind bayesian reasoning is um in some sense you're doing abduction you're saying i have a set of hypotheses i want to figure out which is the best explanation well what's the best explanation the one that predicted the data that i'm going to get next okay so if newton's theory says that mercury should be doing one thing and einstein's theories has issue doing something else i then go and look at it and i figure out which theory is more likely and less likely i chose that example explicitly because it already brings up a problem the motion of mercury was known about the discrepancy of the motion mercury was on about before general relativity came on the scene okay so what you call here a prior probability is already colored by the fact that you know things about the world so bayesian reasoning is correct you know it's the right way to go update your prior probabilities but it hides a lot of the loosey-gooseyness like i said so let me just bring some of that to the four okay um let me just note some things about this process and compare it with what we've already said about science so one is that the probability the prior probabilities for your theory set of theories and even the posterior probabilities p of t i given the data should never [Music] be exactly zero or one okay because if you thought like some theory was just so dumb i'm going to rule it out 100 percent okay so it can't be right what that would mean by baze's rule is that no experiment you could ever do could change your mind you have to just good scientific practice this is not built into bayesian analysis and in a score as far as baze's rule is concerned you could by all means set your priors equal to zero or one and effectively if the prior is point zero zero zero one then maybe you just set it to zero to be simple about it but as a rule of thumb to be a good scientist you should never set these exactly to zero or one because you can always make mistakes and you can always be surprised by data in the future um that's that's kind of obvious but here's something less obvious the likelihood that's the probability of the data given the theory this is the thing that is the most objective supposed to be the most objective about bayesian inference if you tell me the theory i should be able to predict the likelihood i will get some data right your priors are not objective everyone can have their own priors right you can argue about what is more simple what is more elegant what is more likely to be true but then the idea is you collect your data and that forces you to the truth that's the hope and part of that process is saying well even if i don't know what my prior probability on the theory is at least i know what it predicts but often you don't this is also a messy thing remember einstein and the expanding universe or contracting universe if we find some data that disagrees with our theory one absolutely legitimate option is to tweak our theory like who knows whether we got it right the first time or not that's an absolutely okay thing to do furthermore as we said very often you have a really good idea whether it's inflation or curve space time or the many worlds interpretation or string theory or whatever or natural selection if you're darwin right but you haven't yet articulated the theory fully so you have a vague idea that you would like and hope it grows into a fully blown theory someday but it's not there yet so it's hard to predict the data given that theory that doesn't mean your theory is wrong right it just means i don't know what my likelihood functions are that's perfectly legit that can that can happen um then of course we have the question of the priors where do they come from so we already mentioned the zen in the art of motorcycle maintenance problem uh where do the theories come from so even if i know the theories when i had these credences prior to doing experiments on different theories being true where did those come from like why do i think that a certain theory is more likely another one a priori that's the reason why a lot of people don't like bayesian analysis at all because these priors play a really crucial role in it and they seem arbitrary so what i'm trying to emphasize is yes they are well sorry they're not arbitrary but they're subjective okay do you see the difference there they're not objectively set by the world there's not an algorithm that is absolutely deterministic for telling you what your prior should be but they're not arbitrary either it makes sense given a set of theories that predict the same things to prefer the simpler ones for a set of theories that you know are not yet fully developed yet prefer the ones that seem to be more developable and you know more fruitful and make more powerful predictions down the road you can always rule them out by collecting data but there are reasons and this is what was getting at there are reasons to have big or small priors for your theories um finally an issue with that sort of the the formula basis formula here it sort of hides the messiness right another thing that it hides is what about the fact that you did not invent every theory possible so what about ti equals something else that is to say let's say we're doing quantum gravity okay or whatever quantum gravity is a good example and you say well it could be string theory or it could be loop quantum gravity or it could be causal set theory or be immersion space time entropic gravity so the set of things i'm going to assign prior probabilities to all of these okay but surely you have to admit that maybe the correct theory of quantum gravity is one that nobody has thought of yet okay you need to assign some prior to that how do you do that number one and when that is your theory my theory is it's some theory i haven't invented yet how in the world do you make a prediction for the data on the basis of that right maybe you can approximate it or something like that but this factor down here the probability of getting the data depends on these likelihood functions for all the theories and if one of your theories is a theory i haven't yet invented that can be hard to specify so in very clean crisp situations where you know what every possibility is you can do this in principle but in science when you're when you're confronting unknown features of reality you have to admit none of these equations none of these factors none of these numbers is absolutely objectively precisely determined i need to be a little bit more open about the reality of it okay so what i'm saying here in yet other words um is um science is hard to tidy up it is very very difficult maybe impossible but at least much harder than it looks to invent a theory of science so to invent this algorithm that tells you what happens when you do science uh people try so here is i'm not sure if this is a helpful diagram or not but i'm going to draw a diagram anyway here's how i think about it so imagine that this is actual science okay and science let me say science i don't know what the blob represents but the contours of it are you know inside is what you're doing is science outside what you're doing is not really science maybe it is revelation or logic or something else okay maybe it's just you know you're playing poker you have nothing to do with science but there's some set of human activities which we call science some other set which we don't and what people do is they try to understand what this is you know what counts the science and what doesn't that's the demarcation problem so what they do is they uh you know invent a theory of what science is and their theories inevitably you know are nice and tidy right and they sort of end up looking like this they have nice sharp well-defined edges and so this is your your idea about science it's much more crisp and clean and easy to say what is and is not science the problem with that is that sometimes you'll hit some thing that is really should count as science and it does not fit into your criteria right you try to invent a theory of science and there's someone doing science but it doesn't count according to your criteria what you should say in that case is oh i bet my criteria were not perfect that my theory of science was not exactly right i should update it but in fact what people say all the time is oh that's not science what you're doing what you're doing over there that's not science i know you think it's science but it's not because i have figured out what science is and it's not that okay i think you know as much as i am a fan of philosophy in general this is something that that happens at the boundary of philosophy and science itself either philosophers or scientists try to come up with the correct once and for all we're done with it philosophy of science and rather than updating that philosophy when they understand something i didn't understand before they exposed facto excluded from counting as science i think that's a mistake okay so let me give you a couple of examples of what i mean one is maybe you're too young to know much about this but there's this idea called methodological naturalism it's a mouthful methodological this again is a holdover from [Music] the days of fighting against creationism in public schools which is a big thing in the united states of america in the 80s 90s into the 2000s okay so the idea of methodological naturalism it's different than naturalism so what naturalism is is the idea there's only one universe one reality okay the natural world the world that we discover by science that's what naturalism says and you might call that metaphysical naturalism or ontological naturalism it's a statement about the nature of reality reality is fully described by the natural world there's no separate supernatural world for example methodological naturalism was the idea that was completely invented by people who were trying to fight against creationism so with the best intentions they said science proceeds under the assumptions of methodological naturalism which is the assumption ahead of time that when we go to explain some observations we do so in a naturalistic way in other words that the kinds of explanations that science can come up with are just in principle just a priori just because we said so naturalistic explanations so the idea of methodological naturalism was even if the true explanation for a phenomenon was supernatural science would never allow you to contemplate that okay and the reason why they came up with this cockamamie scheme was they wanted to be able to say intelligent design should not be taught in public schools so it was an entirely political motive okay not a philosophical or a scientific one they wanted to say that this attempt to bring religion to schools which is what it was like intelligent design was clearly just an attempt to smuggle religion into public school teaching um and they wanted to argue against it by saying well we can't allow supernatural explanations in science okay but that's just wrong like nothing that we said up here had anything to do with what kinds of explanations are allowed we come up with the best explanations that's whatever that's what abduction says whatever explanation fits the data the best is the one that we like if if you could convince me that there is some data for which the best explanation is to assume that naturalism is wrong then any good scientist will say therefore naturalism is probably wrong okay if an angel comes down you know with a flaming sword and wings and so forth and says all right i'm gonna you know perform some miracles for you now and this happens regularly and it can't be explained by illusions or trickery or laws of physics or anything like that or if you know all sorts of things happen simultaneously throughout the world to bring true some predictions that were made by some religious tradition okay a good scientist would say well we should at least consider those explanations so the reason why you should keep intelligent design out of schools is not because it's not science it's because it's bad science it's because they're not explaining anything better than darwinian natural selection ever would have explained okay but that argument seems a little bit more loosey-goosey right like you're not saying that something is cut and dry disallowed you're saying it's not very good and then people can make an argument a counter argument against that what i'm here to say is yes that is correct you need to bite the bullet and make that hard argument make the case that this bad science should not be taught because it is clearly just an attempt to smuggle religion into your schools okay you should not invent weird philosophical principles after the fact to justify your choice you should be honest about how science works science is not methodologically natural it is methodologically so this is bad let's make this bad that's not what science is what science is or a feature of science it is methodologically oops methodological empiricism in other words the the philosophical presupposition of science is not what kinds of explanations we will allow but how we decide which explanations are correct namely we do not appeal to revelation we appeal to the data to looking at the world we say like which of our explanations is the best fit to the empirical information we get about the world and we could always be wrong and we could always try to do better a more famous and more contentious example is of course karl popper's idea of falsifiability this was very explicitly an attempt on part of karl popper to solve the demarcation problem to separate what is science from what is not science okay um and just to be super duper clear here i'm sure i will be misunderstood because i've been down this road before i do not know or care what karl popper thought i am not trying to say what popper said i'm not trying to analyze the ideas in karl popper's brain or his writings okay i know that karl popper has a lot of real fans out there and i don't want to get them upset it's a weird thing in philosophy people care a lot about what the great figures of history really truly thought whereas in physics or even science more generally nobody cares like if modern physicists knew what was going on in isaac newton's mind they'd be appalled but fortunately they don't care what they care about is the idea okay so i'm not trying to do an exegesis about what karl popper thought what i'm trying to talk about is the version of popper's idea that has become popular among working scientists which is a literally a bumper sticker slogan which says that a theory is scientific only if it could be falsified that is to say uh not if it's false right like if a theory can still be scientific if it's true but the idea is you could imagine in this set of all possible worlds you could imagine a set of experiments you could do that would collect data that would conflict with the theory and this was a very good idea that popper had and what he was trying to distinguish between were theories like einstein's theory of general relativity okay which made a prediction for the deflection of light by the sun which had it been wildly wrong you would have said okay general relativity is not right versus something like freudian psychoanalysis which popper thought no matter what the patient said freud could come up with some explanation after the fact so again i'm also not offering any um opinion about the usefulness or correctness of freudian psychoanalysis but that was the distinction that popper had in mind and now you know from what we've said already that that's not exactly right right because einstein did face examples of observational data that he thought were in conflict with the theory and he didn't say oh my theory is wrong he changed the theory to fit the data and maybe he would have done the same thing with a deflection of light or something like that and that's perfectly okay you're trying to look for the best explanation not playing some purity game where if new data comes in that doesn't fit your theory you're not allowed to change your theory to update it and improve it in any way the real progress of science is much messier than that and it's a funny thing where modern philosophers of science who for their professional lives think about what is science and what is not science none of them it's an exaggeration almost none of them think that popper got the right answer for the demarcation between science and non-science they think that he said something important and true this is very often the case in the history of philosophy right someone says something interesting and important captures some part of what we're trying to get at without getting the final answer but scientists many many scientists think that he just got the right answer right and honestly it's a little bit unfair for me to say a little bit uncharitable but i think that they just like it because it's simple right the falsifiability criterion fits on a bumper sticker and they can you know it can help them understand what is science and what is not but it completely uh is subject to this problem that there are edge cases uh general relativity is not an edge case but there are other cases that we'll talk about just in a second where it's less clear how to apply this idea of falsifiability um what so what popper did get right so the good part of this criterion was he was getting at something real and true okay if you think about psychoanalysis versus gender relativity popper was trying to say the reason why gr is a really good theory is because i could in potentially hypothetically if it were wrong i could show that it was wrong whereas a theory that is so vague and malleable doesn't really count as scientific that was his idea so the the the things that he was getting at which are true is that a good scientific theory is definite in other words it's not infinitely flexible right there are things that it would explain or would account for would predict and things that it would not allow for if this were true then the theory would be wrong and that it is judged by empirical criteria that is to say ultimately we want to look at the data so these features that popper was getting at are true features of science a definite theory is better than a fuzzy theory and you do want to compare your theory with the observations it's just that in both cases the actual application to real world examples of science in progress are not nearly as cut and dried as popper would have us believe or rather sorry as modern scientists cartoon version of popper that they carry around in their minds would have us believe it's messier than that right it's more complicated so uh persian explanation is a better model for what science does because that's what popper was sort of getting at um the real problem with again popper's view of freud not necessarily with freud which i have no opinion about but popper's view of freud was it doesn't explain anything because it could explain anything right there's nothing that could happen that the theory would say no no that that can't happen that that doesn't make sense at all right if that's true if your theory really could be bent and twisted to explain anything then it doesn't explain anything that act of explanation distinguishes things that could happen from things that couldn't happen and that's already there in abduction in the persian model of inference to the best explanation because if you just predict anything could possibly happen you haven't explained the actuality of why certain things happen at all so this this you know abduction model is less algorithmic less cut and dried less crystal clear okay how we draw the dividing line but it's more accurate it's more close to what actually happens and let's uh let's fill in some of the details here by looking at a couple of famous examples okay let's look at the many worlds interpretation of quantum mechanics here is a famous example where people say this is not a good scientific theory because it's not falsifiable it's not hard to find people who say that now popper didn't say that popper knew about the many world's interpretation of quantum mechanics he didn't like it as a theory he didn't promote it he had his own interpretation of quantum mechanics which really never got off the ground um but he vastly preferred everett's theory to the copenhagen interpretation because he said look ever it makes sense you know it's a definite theory whereas copenhagen is kind of the mess it's not a definite theory at all and definiteness was part of what he was trying to get at the issue is i think it's with people not who misunderstand the philosophy of science but with people who misunderstand the many world's interpretation they think that the many worlds interpretation starts by saying there are a lot of worlds and they're all non-interacting and you cannot ever know that they're there okay these worlds are constantly coming into existence and therefore there there's a whole bunch of stuff in the universe which we can never observe and what they say is i cannot imagine how to falsify that claim right that is the argument i hope i hope i'm doing it justice the claim that there are many many other worlds out there seems to be falsifiable but if you understand the many world's interpretation you know that the claim that there are many many worlds out there is not the many world's interpretation it is a prediction based on the many world's interpretation many worlds is just there's a wave function psi which is an element of hilbert's base and psi obeys the schrodinger equation h psi i'll make it a capital psi equals i d by dt psi that's the many worlds interpretation right and then you use this mathematical formalism to predict that there are other worlds out there could you possibly falsify the many worlds theory of course you can in fact there are experiments going on right now that would do it namely experiments looking for violations of the schrodinger equation looking for experiments that see wave functions explicitly evolve in ways that are not the schrodinger equation if any of those experiments come true many worlds is falsified so i don't see what they're complaining about but more importantly i mean i get i get the uh sort of psychological um what should i say the the motivation behind their claim they they know that many worlds predicts things we can't see and they don't like that that seems to be in violation of the spirit if not the letter of the law as far as falsifiability is concerned but again that's not a philosophy of science problem that's just a quantum mechanics problem everett claims that under this very simple set of rules you predict the existence of other worlds now maybe he's wrong maybe there's something that we got wrong about the interpretation of these equations uh which says that there is not another world in which the cat is awake and another world in which the cat is asleep okay if that's true by all means explain why we have misunderstood the schrodinger equation and coherence and so forth but the only other option is that these postulates of many worlds are wrong one or the other of them either there's other stuff in the world in addition to the wave function or the wave function is not the world at all it's just a completely different kind of thing or the wave function does not obey the schrodinger equation so if you're truly bothered by these other worlds the the the result the the suggestion is not that you say well it's not falsified falsifiable therefore it's not science the suggestion is you come up with a theory that explains why the other worlds aren't there and then i could ask about how to test that theory right but right now since we don't have any other well we have other theories that do try to do exactly that the grw theory tries to do exactly that and guess what we're trying to test it so honestly you know i'm i'm being a little bit uncharitable here a little bit a little bit uh rhetorically overheated but the point is many worlds is not in any sense an example of a non-falsifiable non-scientific theory that's just bad physics to think that you might not like many worlds for other reasons that's perfectly okay there are good objections to many worlds but this is not one of them the other example closely related to this is the cosmological multiverse okay a little bit different worth rehearsing why it's different the multiverse is the idea that for some reason or another there's different uh reasons why this could come to pass but there are different regions of the universe different regions of space-time they're just far away they're not like the everetti and multiverse where they are appearing literally simultaneously existing and with no spatial relationship between them there's just a place far away maybe impossible to get to if you can't go faster than the speed of light where conditions are very different okay and if that's true a bunch of things follow this the thing that follows most strongly is that would change your opinion about whether or not certain features of our observable universe are natural okay so it doesn't make a strong prediction this cosmological multiverse idea it doesn't predict something falsifiable in the strict sense right it doesn't unlike everett that does if you see the wave function evolve in a non-schrodinger way you falsified it the cosmological multiverse is a set of ideas that you cannot directly falsify now just like everett the cosmological multiverse is not really by itself a theory just in the sense that the many worlds are not a theory they are a prediction of a theory however in this case the theories that predict the cosmological multiverse are much less well defined than ever reading quantum mechanics is you talk about inflation and eternal inflation and string theory in the landscape evacuate all these things all these are in the category of those vague ideas that are not yet fully fleshed out so even those ideas unlike the schrodinger equation those ideas of inflation and string theory those are not themselves falsifiable but the multiverse in that version of the multiverse is a prediction of those theories and what you would want to falsify are those theories not the multiverse but what i want to say is that this is just an example of where the idea of falsifiability is leading us astray it's a perfect example of perfectly good science that people are criticizing not because it's bad science but because they tried to invent in their heads what counted as good science and this didn't fit so they decided after the fact that this was not good science uh the multiverse the question from the point of view of abduction or bayesian inference or whatever you want to call it the question is is it conceivable that the best explanation of the universe we do observe invokes a cosmological multiverse right that's the question we should be asking and you might say well even if it is the best explanation how would we ever know for sure you'll never know for sure you never know anything for sure that's how science works right not proving things for sure the question is is it the best explanation that we have and so once you ask it that way in principle the answer is clearly possibly yes so i'm not saying the multiverse is true or false but it's absolutely conceivable that it's true and that we decide that it's true down the road so it can play an explanatory role it can even let me finish writing it can even make predictions for example for the cosmological constant this is the cosmos constant okay capital lambda um stephen weinberg 10 years before we discovered the acceleration of the universe sat down and thought about the cosmological multiverse and he said you know if there is a multiverse out there then there'll be places where the cosmos constant's really big places where it's negative but we can't live with those places if it's big and positive it blows us apart if it's big and negative the universe re-collapses therefore i make a prediction namely that number one the cosmological constant is small because otherwise i couldn't be here but number two there's no reason for it to be zero okay that was the crucial part before then physicists theoretical physicists knew that the cosmological constant was much smaller than it should be on the basis of quantum field theory and renormalization group arguments and effective field theories um so they said well i don't know why it's so small but it is much much smaller than it should be therefore there's probably some symmetry or some principle that sets it equal to zero this is the example that we talked about earlier what about one of when what about when one of your theories is a theory i haven't yet invented okay physicists put a large prior probability for the cosmos constant on a theory we haven't yet invented that sets it equal to zero when weinberg analyzed the cosmological multiverse he realized that here is a theory that can explain why lambda is small but it predicts something different than that theories i haven't yet invented but have some symmetry principle idea namely that it should be not quite zero there's no reason for the cosmological constant to be exactly zero just for us to be here it can be small and not quite zero so guess what in 1998 when we discovered that the universe is accelerating that was predicted by the cosmological multiverse in this sense so it is just true from the bayesian perspective that the probability of the multiverse went up when we discovered well in 1998 yeah when we discovered i should write it out the acceleration of the universe why because we learned a new fact about the universe that has small but not zero cosmological constant that one could argue was better explained by the cosmological multiverse then the big bucket of ideas that just set lambda equal to zero in other words you had a likelihood function big bucket of theories that says lamb equals zero says that it predicts that lambda is zero multiverse says that it predicts with a certain probability that lambda is not zero lambda is not zero therefore your bayesian probability your posterior probability on the multiverse went up likewise probability of the multiverse could go down if we invent a better explanation a better theory so that's also completely allowed if i tomorrow uh now that i'm not making videos anymore if i finally sit down and write down the correct theory of the cosmological constant and i publish it and it predicts on the basis of you know the mass of the electron what exactly the cosmological constant should be and it's no multiverse it's just a formula that gets the right answer then people's prior people's credence that the multiverse is real will go down not to zero because it should never go all the way to zero but it should go down so i want to mention these two things because look the first one discovering the acceleration of the universe this is extremely empirical this is data this is an experiment that was a surprising experiment that changed our opinions on the credences for the multiverse or for not the multiverse that you don't get more scientific than that you don't get more real world empirical data-driven than that your credence has changed because of a new experimental discovery okay um but the other one is also important your credences can also change just because of a non-experimental discovery right um there's a there's a controversy going on in this talk about string theory in the multiverse uh richard david who is uh uh he used to be a string theorist he turned into a philosopher of physics and he's been writing about you know don't think about falsifiability and he coined this term non-empirical theory confirmation or non-empirical confirmation and this was it's too bad when he uh coined this term for two reasons one is because the word confirmation is used in a technical sense by philosophers that is different than what it means to everybody else to everyone else when you say you've confirmed something you mean you've shown that it is true right you've confirmed it it's true i've shown it right in philosophy of science literature confirmation is any kind of consideration that changes your credences for something up or down right so what david means by non-empirical confirmation is not that you can prove a theory right just by thinking about it what he means is that by thinking about it might change your credences one way or the other and look this is obviously true okay if we think about a theory and discover that it predicts even more things uh that it's even simpler and even more robust than we thought or contrary wise that there's a much better theory that explains all the same things that is much more simple and elegant and more fruitful than the theory we have of course our ideas about the credences that we put on these theories are going to change so the one so i said there's two things that bugged me about it one is it's a bad word confirmation he didn't mean confirmation in the usual sense he just meant changes in your credences from non-empirical means and inventing a better theory is definitely an example of that um so for the example i like to give is um you know we had in i said in 1960s quantum field theory wasn't even very popular right and um one of the reasons why there were all sort of mathematical puzzles so higgs and brown and anglair had come up with the higgs mechanism weinberg had already said well it could help explain the masses of the particles and their symmetries etc but no one paid any attention you literally looked the stephen weinberg's paper for a long time was the most highly cited paper in all of particle physics in the history of particle physics but in the first like three years after he published it there were maybe one or two citations of it no one paid any attention and the reason why is because we didn't think it was renormalizable we didn't think you could make mathematical sense of that spontaneously broken symmetry in the same way you could have quantum electrodynamics okay so in the early 1970s gerard tuft showed that the spontaneously broken symmetry of the electro weak theory was renormalizable it's a mathematical proof and suddenly people's credences in the higgs mechanism and the electric weak theory increased by an enormous amount okay you can see the you can literally see the credences affect the number of citations for weinberg's paper and for higgs's paper etc nothing wrong with that but it was not new data it was non-empirical changes in credences because we understood the theory better right and that's exactly this is so the second thing that bugs me about david's formulation is there's nothing new here it's not like we're applying the thing that i disagree with about philosophers who try to defend string theory by saying it's a new way of doing science that we need to have in the this modern age it's the old way of doing science there's nothing new about it the only thing new it's not really new but the only thing that might think make you think it's new is that it's hard when we talk about the foundations of quantum mechanics or the cosmological multiverse or theories of quantum gravity like string theory it is very often the case that the kinds of predictions that they make are hard to test so that means that life is tough for us right that means that we can't get the answer quickly or easily that doesn't mean the answer is not gettable it doesn't mean it's not science in some sense okay so it's hard but it's not a fundamental paradigm shift in how science is done it's just that you know and look maybe we'll never be finished maybe a million years from now scientists when they are asked is there a cosmological multiverse or not we'll just say i don't know 50 50 40 60 something like that okay there's no guarantee that we will converge and there's certainly no guarantee that we will converge on any short time scale that's not part of the scientific method but that doesn't mean it's not science it just means it's hard that's okay all right i've been going on a long time losing my voice i'm going to have something to sip here but um i'm not going to stop because i did want to talk just a little bit about that's all i have to say about what is science let's talk about fundamental physics and its uh potential what's going to happen in the future what is the prognosis for the future as well as the president of fundamental physics and i don't as i've said before i don't love the phrase fundamental physics but you know what i mean the kinds of ideas the kinds of concepts the kinds of physics that we've been talking about in the biggest ideas series right um elementary physics basic down to earth reductionistic physics okay so we're in this really weird position when it comes to that stuff when it comes to the stuff that i do for a living we have good theories very very very good theories of gravity right we have general relativity gr uh we have good theories of cosmology vaguely speaking the hot big bang model and not bottle uh as we talked about in the cosmology video there's a hot big bang event there's the moment the singularity and forget about that that's not what we're talking about the scenario under which the universe starts in a hot dense state expands and cools and galaxies form that's the hot big bang model you can embed it with some by fixing some parameters right about you know what the density of certain things is what the perturbation spectrum looks like stuff like that that's the hot big model that we meet that we mean when we say the standard model of cosmology and then of course for particle physics we have the boringly named standard model and it looks like huddle um so that's good i mean it's not bad to have good theories that you're trying to um invent when you're trying to explain the world this is a triumph this is an amazing fact right uh you know a hundred and 20 years ago the year 1900 we didn't have any of these things right we have all these very very good theories of really deep facts about nature that we developed in the last 120 years okay the bad news is that the theories are too good that the predictions made by these theories have raced ahead of our ability to do new experiments and undermine these theories remember how do you win the nobel prize in physics not by showing your theory is correct right not by propping up the establishment but by overthrowing the theories so the question is not how do we how can we possibly confirm these theories over and over again the question is how do we undermine them how do we go beyond them okay so there haven't been that many surprises in fundamental physics in my lifetime there was a big surprise i counted as a big surprise in 1998 when we found the universe was accelerating obviously it was known that could happen i was the co-author on a review article on the cosmos constant came out years before we discovered it um but most of us didn't think that it was actually true that there was vacuum energy in the universe so i count 1998 as a big surprise there haven't been big surprises in gravity theory in the theory itself you know do we modify general relativity since it was invented general relativity is the same today that it was when einstein wrote it down over 100 years ago in particle physics the last big surprising things were in the 1970s right that we were still putting the standard model together quarks were still sort of a little bit uh not yet established protons inside asymptotic freedom uh the later generations you know the tao and the top and the bottom quarks these all came about in the 1970s big surprises since then we've had plenty of discoveries in all of these areas we found gravitational waves evidence for black holes cmb perturbations and the beautiful spectrum of temperature fluctuations in the microwave background evidence for dark matter and dark energy the w and the z bosons the top quark and higgs boson found all those things okay but they were all part other than the acceleration of the universe they were all kind of part of what we expected of what we had predicted okay and that's good except nobody thinks that we're done you know it would be one thing if we thought we had the final theory of any of these areas but we know we don't you know in gravity we have a theory that is not compatible with quantum mechanics um in cosmology we have a theory that doesn't explain the initial conditions we don't yet know what the dark matter is some people think we don't even know what the dark energy is right okay so there's plenty of open questions there in particle physics obviously there's a whole bunch of problems why do we have the particular set of masses and couplings and so forth that we do let me um yeah so anyway standard modern particle physics doesn't include gravity it's not completely unified there's a whole bunch of things that we think are not quite there yet so when you're in a situation where you have a bunch of theories and they all fit all the data like there's literally no data that is wildly in conflict with all these theories like you can argue around the edges you know neutrinos have masses so do you count neutrino masses as being part of the standard model or not who cares okay i think you can just stick them in there it's not a big deal you have theories that fit the data but they're not right they're not the final answer in the sense of that sense of being right so how do you make progress scientific progress is usually driven not by brilliant theoretical proposals but by experimental data that doesn't fit the theories the there are brilliant theoretical proposals but usually after you've done the experiment and shown that it doesn't fit your theoretical prediction so the question is are there hopes for future discoveries that would be surprising and lead us somewhere so what are the possible future discoveries of course we're looking right we're not done and no one is sitting on their laurels so let's think about what they could be um for gravity you know uh i'm just gonna say it's unlikely that's that's all i'm gonna say and i say this as someone who has written papers suggesting modified theories of gravity okay so nobody would be happier than me if einstein were wrong if there is some way uh that gravity beyond general relativity was really really important in astrophysics or cosmology i would love that idea it's an awesome idea uh i'll tell you why it's unlikely number one it's not at all what you should expect from first principles remember if you go back to the video we did on renormalization where we talked about effective field theories um in field theory generally speaking what you expect is that given knowledge at some energy scale or length scale you can extrapolate that knowledge safely to lower energies and larger distances you cannot always extrapolate that knowledge to shorter distances or higher energies but the fact that we figured out general relativity here in the solar system makes us think we probably understand it on cosmological scales and so far after decades and decades and decades of trying all of the data that we have says yes we do understand it so you can work hard as a theorist you can like really stretch your brain to try to come up with ways to modify gravity at large distances in ways that would not show up here in the solar system but it's hard and it's not natural uh very often people say all right i've invented new theory i've done this i've invented a new theory um that changes gravity in the infrared at long distances and someone else says up here's an experiment that is already in conflict with it okay that's what happens over and over again so it's not what you would expect there's no evidence for it and there's no need for it there's no data that says you can't explain this without modifying gravity right it's still possible we should keep it open as a possibility that gravity is different on astrophysical scales than gr but i'm just going to tell you the truth and judge it to be unlikely i'm glad that some people are trying to develop those theories and test them against data because that's the only way to know for sure but my prior is small that that theory that that approach is going to work uh in cosmology there's a lot more hope um for discovering new things you know uh we can find things there's a whole bunch of things we could discover like cmb anomalies i'll just say what i mean by that is we have the cosmic microwave background the leftover radiation from the big bang and the kobe satellite in 1992 show that there were slight variations in temperature from place to place on the sky which are primordial which came from the very very early universe you know by the way this is another example of the progress of science when i was your age there were a whole bunch of people who took very very seriously the idea that the perturbations in density that lead to galaxies came from something like cosmic strings okay and cosmic strings made a prediction for what you should see in the microwave background which was utterly different than the prediction you get from inflationary cosmology and inflation was spot on and cosmic strings were ruled out there could still be cosmic strings but they are not the origin of the density perturbations that we see in the universe today progress on the basis basis of data okay we have a model a sort of phenomenological model with a small number of free parameters that fits the cmp data perfectly well an anomaly is something that wouldn't fit into that model so what we say is there are perturbations on all scales it's interesting because you know we talked in the last video about power laws and scale-free behavior and the universe the perturbations in the universe seem to be scale-free to a very good approximation does that speak to some generative model that produces them in some interesting way i don't know that's a very good question inflation might be an example of that but that's the kind of thing that's worth thinking about anyway um are there the the it's not exactly flat the spectrum so when i say the spectrum here i mean well i can draw it i have a pad here so here are perturbations in density delta rho over rho as a function of scale in the early universe and roughly in the early universe we think that they were just flat okay i'll just draw the straight line but not exactly it can be tilted a little bit in fact the best fit to the data has it tilt a little bit but if there were a bump or something like that that would count as an anomaly that would be something very very different other things that would count as anomalies are gravitational waves from the early universe we can hope to find them but we haven't yet that's a prediction of inflation but the amount of them is not predicted by inflation we could find what are called non-gaussianities which means that the different modes of fluctuation somehow conspire like all their peaks are in one area and or other troughs in some other area not what is predicted by the simplest models but you could mock up more complicated models that predict them maybe there are what are called power asymmetry in the cosmic microwave background um there's another thing i've worked on myself so you imagine that not only is the temperature different in one direction versus another but the amount of fluctuations is different in one direction or another there's a little bit of hint uh that that might actually be true in the data there's something called the axis of evil that you can look up online if you want sadly it's hard to google that because there's another meaning for access of evil but people have talked about this anyway the point is there's a lot of possible ways that future observations of the microwave background could lead to discoveries that are a little bit different than what we find that what we have predicted in our current best model the bad news is that none of them are smoking none of them are sure things there it is we have a model that predicts what we see now that model might continue to be true as we just get better and better and better data it's not like the higgs boson where we had a missing piece of the standard model so we could really say with confidence we'll find something at the large hadron collider even though we don't know what it is for the microwave background we might just keep finding what we've already found over and over again we might not so be optimistic but we might and then of course there are is dark matter what is it that's a big looming question is it weakly interacting with particles is it black holes is it axions is it neutrinos if we find that that'd be great if we actually detect directly the dark matter that's a huge another surprising discovery no matter what it is that we find um the standard model is not up to the task of explaining the dark matter but the bad news is we have to admit there's a certain credence that says there is dark matter and we'll never detect it directly right we'll just do not have the experimental reach to find out exactly what it is i hope that's not true and i don't actually think it's true that's something i don't think is very likely but it could be hard to find if there's e it's easy to invent theories of dark matter that fit all the data they'll be very very hard to test experimentally there you go um and then dark energy you know the simplest idea for dark energy is the cosmological constant but it could be not quite constant it could be slightly variable again i've written papers about this i've constructed models i've thought about this myself i'd be very happy if it were true having done that and having seen there's no evidence for it and having seen that theories that try to predict it often run afoul of data elsewhere my credence that that is true is small these days i think that the dark energy is probably just the cosmological constant i think there's nothing more experimentally to be discovered about it i hope i'm wrong but that is my prediction so you get the you know you get the uh the feeling for what i'm saying here namely that you could find things but i'm not quite sure how likely it is and then just to finish that off for particle physics well we have this huge looming problem in particle physics we had real hopes for the large hadron collider because we have the hierarchy problem which we talked about before um the hierarchy problem in the most naive way of saying it is that there's an energy scale associated with the electro weak theory right with the weak interactions and how they have a symmetry that gets split off from the electromagnetic interactions at around has an energy scale of around 10 to the 2 billion electron volts gev right near the mass of the higgs and the top and other particles and this number is just much much less than the planck scale which is around 10 to the 18 gb so you might say well gravity has nothing to do with particle physics but the philosophy of effective field theory that we talked about predicts that the electro weak scale should be as high as possible it doesn't want to be low it wants to be big so this is not a direct conflict between theory and observation it's just a violation of our expectations okay it just doesn't seem right doesn't seem natural so people tried very hard to invent theories that would explain the hierarchy problem and for better for worse they did there were lots of explanations super symmetry was the most popular one but there were large extra dimension models uh infinitely big extra mentioned models a whole bunch of other different models that purported to explain the hierarchy problem what they had in common was they predicted particles new particles at the lhc or at lhc energies and we didn't find any i mean there were people who said you know with some trepidation but with also some seriousness we will turn on the lhc and gluino's supersymmetric partners of the gluon will just come spewing out so it'll be impossible to ignore that did not happen right um maybe it will you know the lac is not turned off yet it's going to run for years decades more maybe we'll find something but it hasn't yet and we have to at least step up to the possibility to the prospect that it won't uh that those are all the particles there at the electroweak scale maybe there are particles beyond there maybe not and so where that leaves us with is these theories that do a very good job of fitting the data we don't think they're right we need to move beyond them an experiment is not telling us how [Music] you know in some sense what i'd like to say is you know we're really spoiled by the first half of the 20th century i think i've already mentioned this in these videos but um in the first half of the 20th century we invented quantum mechanics we invented all these theories right we invented quantum mechanics quantum field theory general relativity special relativity we even invented the big bang right the expansion of the universe and all that stuff first half of the 20th century it was so good so anomalously good for physics that in some rough sense is a bit unfair but you get the point the second half of the 20th century was spent cleaning up all the discoveries we made from the first half of the 20th century okay trying to understand them at a deeper level and that leaves us here in the 21st century not knowing exactly how to move forward so how do we move forward what are the strategies given the situation and again it's not a bad situation it's no one no one's failure no one has made a mistake right it's just that nate we our our theories are too good we've explained what we observed in nature this is not some crisis because physicists lack imagination right it's because physicists or imagination is too good we've imagined enough to fit the data what more do you want than that it's nature that is at fault here nature is not giving us any experimental clues so what do we do next um i'll mention three strategies these are literally strategies that might be followed by actual physicists and i'm imagining that maybe there's some physicists listening they're not going to listen to my advice but maybe there are some young people listening who want to know what they should do if they want to go into physics the first and most obvious strategy is to press on in other words we have a set of strategies that has been productive for the last hundred years okay we write down models of particle physics and cosmology you know here's a model for the dark matter the dark energy the hierarchy problem the grand unification we make predictions and we try to do experiments that test those predictions so we build an experiment uh that collides particles together at ever higher energies or we build a better telescope that looks at the microwave background or or what have you okay and we hope that we discover something new and surprising let me be very very clear this is a very good strategy okay this is not a bad strategy this is not a failure of imagination this is the most sensible thing to do it is the thing that has worked empirically over the course of time but we also have to be fair in saying that there is something different now than there used to be the thing that is different is that we don't have a target to shoot for okay when we had the higgs boson we had gravitational waves or we had the cosmic microwave background anisotropies when those things had not yet been discovered we in some sense had a pretty good idea what to look for and what we would find when we looked there unless there was some even more surprising thing that we would find right um we have no strong predictions for what will come next what is the thing that will surprise us so if you want to say like should we spend i don't know 25 billion dollars building a next generation particle experiment um in a world of infinite money that is absolutely a worthwhile thing to do because you might discover something that changes everything that overthrows all of known physics and changes things in a really wild way but you might also discover literally nothing at all you might do nothing more than measure some parameters that we've already measured but measure them to higher precision okay and i can't tell you what the chances are so in a world of a finite amount of money the answer is my answer is i don't know what to do i would prefer if we did it i would prefer if we spent a lot of money doing next generation particle physics number one if we don't we will lose all that expertise we will forget how to build giant particle accelerators okay number two every time we build a giant particle accelerator it comes with unintended benefits you know people have done this like this when i wrote my book the particle the end of the universe people did an economic cost-benefit analysis and they discovered that by putting money into building giant particle accelerators it increases the economic output by much more than you spent on building the accelerator not because you discovered the higgs boson but because you build better computers better communication equipment you know better radiation safe detectors and things like that it's a good payoff to build very high quality physics experiments um on the other hand it's going to be an international experiment you're going to need cooperation between different countries the united states of america sadly has proven over the last few decades that we are terrible international partners that we are not capable of following through on our commitments so if there is a next generation giant particle accelerator it is very very unlikely we built here in the united states different locations in asia are thought of or europe are thought of so we'll have to see what happens so i hope it happens i'm rooting for it but i'm very very cognizant of the fact that there's no guarantees it might not find anything still worth doing and that's worth doing not only experimentally but theoretically as well building new models of beyond standard model particle physics inflationary cosmology of dark matter it's worth doing it's just hard to know where to look like as a working theorist i am no longer motivated to do that kind of stuff because the chances of success just seem to be small for any one theory someone's going to get it right someone's going to move me on the standard model and figure out what the right thing is but the chance that it's any one person is pretty small because a lot of people trying to do it so the another strategy is to dig deep or dig deeply dig deeply deeper by which i mean rather than building yet another model for inflation or for particle physics step back take a breath think about the underpinnings of these theories that we have quantum mechanics right foundations of quantum mechanics quantum field theory is quantum field theory the be all and end all like how necessary is it what are the theorems we can prove about the relationship of quantum field theory to underlying structures um cosmology the big bang what happened before inflation is inflation really a solution to the problems it proports to be a solution to you know i talked about inflation in the q a for what we did a cosmology video i talked about inflation in particular in the q a video for the cosmology video um you know the multiverse raises some interesting philosophical questions i don't just mean like what does it all mean with if there's a multiverse i mean at a down and dirty calculational level i said that weinberg made a prediction was that prediction justified how do you make predictions in a multiverse right like what is the philosophy of probability in this context so all of these are foundational questions which if we were if we had successful theories but were confronted with a bunch of new experimental data that was in conflict with them maybe the underlying foundational questions would be less urgent right maybe the most high priority thing would be to explain the data that we found but we're not in that situation and we're in a situation where the things that the theories that we have fit pretty well but we know they're not the final answer maybe taking a step back and thinking more deeply about where those theories come from what their underlying philosophical presuppositions and how they could be changed how you could predict different things maybe that's the thing to do and that's actually you know you know listening to me that's my view personally this is my personal idiosyncratic favorite way of moving forward given the situation we're in right now not for everyone i hope it's not for everyone like let a thousand flowers bloom in this i'm glad different people have different opinions about what to do and the final strategy is to just change direction in other words if what you care about is fundamental physics start caring about something other than fundamental physics right a lot of people do this a lot of people who were trained in quantum field theory are now thinking about quantum computers or biophysics or machine learning or something like that um i personally have interested in complex systems which we talked about in the last video which is not fundamental in the same sense although there's something fundamental-ish about it perfectly valid way of moving forward like maybe a thousand years from now historians of science will look back and say yeah 20th century man they really overturned the foundations of physics in a really important way and then things more or less settle down for the next 500 years before we came up with new fundamental discoveries in physics maybe i don't think so i don't think that's very very likely but it's possible and there's certainly plenty to be discovered and learned in other areas of physics or science or intellectual activity more broadly so if what you want to do is something that isn't fundamental physics at all uh godspeed more power to you i think that's a perfectly valid uh way to go anyway uh we're closing up we're we're shutting down we're i'm going to turn off i'm going to get rid of my green screen i'm going to turn off my lights um the videos are have come to an end here uh i think that science is in great shape you know this science is the topic of this video in the in the series science is doing great let's not be uh let's not forget that science is awesome as we said at the beginning of the video physics is awesome there's a lot of new discoveries coming down this specific subset of really deep questions in fundamental physics uh progress recently has been slow the problems are really really hard there's no guarantee that progress will always be fast or will always be at the same rate in my eyes these problems are worth working on right worth thinking about in a very deep way worth spending our intellectual effort because it's the most fundamental questions that we have the nature of the universe the nature of reality what is space what is time what are we made of right i even think as i argued in my book the big picture that what we learn about fundamental physics has an impact on our lives and self-conception as human beings right it's an indirect effect but i think that it's there so i think thinking about these things working to make progress is important i especially think that it's crucially important not to become armchair about it right like i'm an armchair physicist i don't do experiments but the field needs experiments we even if we don't have a direct target to shoot for the field is kept vibrant by trying to find new experimental results and trying to overthrow the current theories we have to keep moving on that so what i want to do is just to work hard at understanding the world um see what happens and you know i'm sure that our descendants many generations from now will look back at what we're doing right now and say both that we were brilliant and that we were very silly right because that's what we always say like every generation you look back at you know 1900 plunk and einstein and all those people on the one hand you can say all the brilliant things they did boltzmann and uh boar and so forth they so many brilliant ideas but in some ways they were deeply silly like they missed some things that were right in front of their noses they just couldn't see them i'm sure we're missing something that is right in front of our noses and that's one of the reasons why i want to make videos like this spread the excitement spread the ideas and the open questions to as many people as possible you know i think that um the the professional activity of getting paid to do a job of being a theoretical physicist is and always will be available to only a small number of people but appreciating physics thinking about it talking about it having a deeply seated conviction about what is the best interpretation of quantum mechanics or your favorite theory of dark matter is something that everybody can have so thank you for following along i'm sure you've all seen every minute of every video it's been a lot of fun for me it's not the end of me talking about physics or talking about other things so i'm not going to go away entirely but the the video project was a special thing i'm very glad i did it i hope you enjoyed it nearly as much as i did thanks

Channel: Sean Carroll

Views: 210,760

Keywords: science, philosophy, cosmology, physics

Id: ZqphkIO7yt4

Length: 130min 42sec (7842 seconds)

Published: Tue Sep 01 2020