The World According to Physics - with Jim Al-Khalili

um so I'm delivering this lecture not from the RI where I had hoped to deliver it a few weeks ago but from my study at home in South Sea in Portsmouth and I don't know if you know but Portsmouth is the most densely populated city in the UK so hopefully not everyone in Pompey is going to be streaming Netflix or whatever at the moment and this this line keeps us going but for the next hour and a half so this talk really is my ode to physics my love affair with the subject that I have spent most of my life working on thinking about and of course you know I have I have a a new book the world according to physics in which I get across some of these ideas so essentially there are a lot of books that have come out in recent years that are huge thousand page tomes covering the whole of physics all the way back from the ancient Greeks how we got to know what we know and so on sort of the history of physics as well as bring you up to date I wanted to have something so wrong as pocketbook sized that just gives you the status of the subject today so if you think about the whole of our knowledge of the physical universe of reality itself as an island that's our knowledge beyond it is the vast ocean of the unknown we don't know how much more we need to find out about the universe but there's certainly mysteries out there so those big books will be an exploration of the whole island everything we know this book is a is a is a journey around the shoreline okay so this is a to give the the lay reader an introduction to what is most exciting at the cutting edge of fundamental physics today and then maybe you know later on the book we roll out a power trouser legs and take her a paddle in the ocean of the unknown and explore the some of the mysteries that that we hope to be able to resolve in in the coming years and I'll say something about that in this talk now because this book is also somewhat personal to me in terms I'd I get a bit polemical but you know I you know I'm it it my case certain issues a colleague of mine my the head of the physics department at the University of Surrey Justin Reed who actually gave me some some advice when I'm writing about dark matter so obviously I gave him a copy of the book when it came out and he suggested a better title to the book would be this the world according to Jim al-khalili you know I'm I'm obviously such a modest chap that I I recall in horror that the thought but it's there those who know me probably know that isn't true so um physics for me I mean lots of people have many ways of understanding their world understanding reality many people turn to religion some people meditate others have philosophies and and and other ideologies that that give their life meaning that tell them about the world around them meaning for me science and physics in particular is the only real way of getting to the ultimate truth about the nature of reality and I don't um I try not to say this in an arrogant way you know because people say oh there are different kinds of truths you know for me there's an objective reality out there the universe is the way it is there are laws of nature that govern the behavior of everything in the universe physics is about understanding those fundamental laws and in a way to give you an example of just how profound it is because of course we don't know everything and and and you know our theories in science you know are only as good as you know they they continue to it's results of experiments until then a better theory comes along but many of them are as close as possible to truth yes there is not going to change we're not going to do anything better I'll give you the simplest possible example I can think of if I were to drop a ball from a roof of a house say five meters high if I drop a ball from a height of 5 meters physics will tell me it'll hit the ground after one second not this is on earth not half a second not two seconds but one second Galileo was the person who first came up with that formula to tell us how long it would take for an object to fall so dropped four five meters it'll hit the ground after one second no religion no ideology no no amount of meditation or prayer nothing would tell you that that ball will hit the ground after one second but physics can do that and I fell in love with the subject because it helped answer some of these deepest questions about the nature of reality now this is a the front page of a paper written by Stephen Hawking back in 1980 and so you can see the so he says is the end in sight in theoretical physics in fact if I highlight the first paragraph he sought to discuss the possibility that the goal of theoretical physics might be achieved in the not-too-distant future say by the end of the century by this I mean that we might have a complete consistent and unified theory of the physical interactions which describe all possible observations what Stephen Hawking was talking about was what we now call a theory of everything a theory that describes all physical phenomena so not a theory that describes human consciousness or human behavior or economics no no a theory it describes the physical universe and that is the holy grail of physics to find this theory of everything a an equation that you can stick on a t-shirt if that were possible and at the end of the twentieth century and this is 1981 it really was thought that we're getting close to finding this this deep or ultimate theory and all we had to do was dot the i's and cross the t's in fact I don't think that is the case anymore I think we are realizing that we are further away from finding the ultimate answers then we certainly then we thought we were a few decades ago in fact that our attitude by many theoretical physicists at the end of 20th century repeated its similar attitude at the end of the 19th century by 1890 it was thought that most of physics was solved we had Newton's law of gravity his equations of motion mechanics we had thermodynamics and statistical mechanics we had Maxwell's theory of electromagnetism the theory of the nature of light and most physicists thought that was it that's pretty but that's everything right that's the whole you know physical reality all the laws of physics are there and understood then in the mid 1890s we discover x-rays we discover the electron we discover radioactivity and that kick starts a whole revolution in physics by 1900 we come up with quantum theory few years later Einstein develops his special theory of relativity and we have the whole of what we still call today modern physics even though it's it's over a century old so we thought we were at the end of physics at the end of the 19th century we thought we were too to the end of physics at the end of the 20th century but we have a long way to go in fact and I find that actually quite reassuring I quite I think that's it's fun I'm not only would I be out of a job if we had all the answers but it's the mystery the discovering miss mysteries are out there to be solved and there's still stuff we're trying to understand yes we want to reach the answer but the journey in looking for those answers is in itself gratifying and enjoyable now if I look back over my my career in physics and the time that I've been studying physics what have been the big the big Discover's what have been the most important let's say in the in the 21st century what have been the big advances in physics while the two obvious ones of course was confirmation of the existence of this elementary particle the Higgs boson by the Large Hadron Collider in 2012 and you see there's a there's a Peter Higgs after whom this this particle was named and then a few years later another big science story hit the news the detection of gravitational waves in 2016 by in fact twin laboratories in America on either sides of the continent that picked up these very very weak ripples of space itself that have been traveling for over a billion years and they came they originated from the collision in a merger of two black holes now you might argue that these are two wonderfully exciting discoveries in physics and yes I guess yes they are they certainly make health headlines but another way of looking at it is that both these discoveries were predicted many many years ago the Higgs boson was predicted by Peter Higgs and others back half a century ago and gravitational waves were in fact predicted by Einstein's general theory of relativity a hundred years ago so in a sense both of these discoveries were ticking boxes they were confirming what physicists mostly suspected was the case you know we weren't surprised to discover gravity gravitational waves we expected it we've been looking for gravitational yet waves for many many years and different designing ever more intricate ever more sensitive instruments to pick them up so none of these were really surprises in physics so why is it that we think we still have a long way to go in uncovering the ultimate laws of the universe well in my lifetime I would say the only real surprise that we weren't expecting the only discovery in in physics that we weren't expecting happened in 1998 and I'm sure many of you watching this lecture will will will know this but for the benefit of those who doubt this is what we now call dark energy I quite like this slide so I wrote a ladybird book on gravity and and the artist who provided the the the illustrations allows me to use these these these these these images for my talk so I like this dark energy mysterious force dark energy essentially what was discovered in 1998 completely unexpected was that while we knew that the universe is expanding and has been expanding ever since the Big Bang 13.8 billion years ago until 1998 it was thought that that expansion would be slowing down it was set in motion at the Big Bang by the initial conditions of the universe and ever since then we thought that gravity the combined gravity of all the matter all the stuff in the universe would be putting the brakes on this expansion it's a bit like um stretching a spring you know you can stretch it stretch it and stretch it and then at some point it's something something will happen it could collapse in on itself we thought if there's enough matter if the spring would collapse in on itself but maybe the universe is like blue tack that you stretch it and stretch it and then it's sort of stir ask to get easier to stretch and it just carries on stretching steadily so I thought the universe could collapse in on itself it might slow down and stop or it might just slow down to a steady accelerating rate um in 1998 it was discovered that in fact the universe's expansion isn't slowing down it's speeding up and that was what led to this idea that there's this mysterious stuff out there we call dark energy for want of a better name although we're starting to get an inkling as to what its origin and something called the quantum vacuum but the idea that dark energy even existed was a complete surprise it was the equivalent of the discovery of x-rays and radioactivity at the end of the 19th century and therefore much more surprising than the discovery of the the dissection of the Higgs boson or the the detection of gravitational waves which we were expecting and so dark energy should be added to a growing list in fact if you think about it growing list of things we have yet to resolve in physics closely related to dark energy in name if not in in in the physical properties is something called dark matter we've known that Dark Matter exists for many years but we still don't know what it's made of so dark matter dark energy I should stress up very different things dark energy is some mysterious something there is providing a force of anti-gravity it's stretching space ever more quickly it's winning the battle against gravity in the vast expanses of space between galaxies it's stretching space ever more quickly so it's for a force of anti-gravity dark matter from dark matters but not not a very good name for it I think it would have been better if we called it invisible matter because we can't see it but it has a gravitational influence so dark matter has positive gravity and it holds galaxies together in fact that matter turns out to have be very important in the the formation of galaxies and they were in the very early universe and so we know that Dark Matter exists out there but like dark energy we don't really understand what dark matter is made of there are a number of hypothesized particles that it could be made of but we don't know what they are certainly not made of anything that we already know about the the particles that we know exist so dark matter we don't know about we don't know about dark energy we don't know where all the antimatter in that must have we were formed in the early verse has disappeared too there are other we you know how to use by various theories in physics which is something I'm gonna come to in a moment so I think we're we're some way off from Stephen Hawking's dream that fundamental physics is almost at at an end one of the the Holy Grails of physics certainly for theoretical physics has been this idea of unification unifying our theories into this one theory of everything so I wanted to give you in one slide the story of unification in fact I might even temporarily turn off my video so I don't interrupt this I'm hoping as you're watching this on your laptop's on your computers that the resolution is going to be enough for you to make this app because I have to fill it up with words I have to say I really enjoyed making this PowerPoint slide lots of very exciting anyway you'll see right so this this is the the history of unification of physics in one slide it starts with the very first two disparate phenomena that were unified by Isaac Newton Newton supposedly sitting under the apple tree on his mother's farm whether or not the Apple actually fell on his head I don't know but he tells that story but essentially what he realized was that the invisible force that pulls the Apple down to the ground is exactly the same force as the one that keeps the moon in orbit around the earth and the earth in orbit around the Sun that I mean that is obvious to us now that's gravity right but back then there was no reason to think that the laws of physics that govern the behavior of objects here on earth would be the same laws that govern the motion of heavenly bodies Isaac Newton was the first person to unify those two rather very different phenomena into one with his law or gravity the equation that we all learn at school the force is the the product of the two masses divided by the square of the distance between them that that was different I guess the first big that step in our journey of unification but as you can see I wrote those words very good you think Jim why are you writing these words so tiny on a PowerPoint slide her--her that's because I'm gonna fill it up right next one middle of the 19th century James Clark Maxwell had been following Michael Faraday's experiments with the electricity and magnets and and and messing with them and seeing what you know induced magnetic fields and so on James Clark Maxwell developed the theory that showed that electricity and magnetism are part of the same more general field called the electromagnetic field in fact for me one of the most profound and enjoyable moments during my undergraduate physics degree was when my lecturer started with Maxwell's equations of electricity and magnetism run through lines of algebra and arrives at a new equation called the wave equation in which there's a number a constant which is actually the speed of light and he shows just from that algebra that light is itself oscillating electric and magnetic fields it was a walk shiver down the spine moment for me so Maxwell unified electricity and magnetism what next heat and energy thermodynamics so that combined with with with statistical mechanics the the the theory you learn at school was called kinetic theory that you know things like pressure and and and temperature are really just vibrations oscillations of molecules in a gas that was unified together into what we now called thermodynamics okay back up here beginning of the 20th century Einstein unified space and time and he gives us in 1905 his special theory of relativity again I mean that I could give a whole talk just on that combination how do you define sire to give you four-dimensional space-time special relativity that's the that's the theory that tells us equals mc-squared it's the theory that says nothing can go faster than light no such thing as warp drive by the way just in case you're wondering um by the let's say I'm trying to think what comes next I can't even remember right so the late 1920s oh how come no that's right yes so no so I'm not a good date like Swiss what am I talking about 1915 Einstein unifies the special relativity he broadens it to give a more general theory general relativity because he gives us a new picture of Newton's law of gravity so Einstein says gravity isn't what Newton told us it was it's not an invisible rubberband that pulls masses together this force of attraction between all bodies I stars picture of gravity is the curvature of space-time his general theory Livity many regard is the most beautiful theory in physics it it proved it combines his special theory with Newtonian gravity and of course general relativity then leads on to another new whole new field of science cosmology which tells talks about the the evolution size shape and behavior of properties of the entire universe okay now I'm gonna jump down can you see down there near the bottom atoms the existence of atoms is really probably only proved mathematically in fact by Einstein in 1905 by May the end of the first decade of 20th century Ernest Rutherford has looked inside atoms for the first time and discovers the atomic nucleus and inside it are these nuclear forces which I'll come to in a moment right now we hit the mid-1920s we have we have quantum theory that's first develops in the beginning of the 20th century by people like Max Planck who says radiates heat radiation thing is lumpy ultimately it's made of time lumps of energy called quanta as where the word quantum comes from in physics Einstein and others developed their idea and by the 1920s we see the emergence of what we now call quantum mechanics it's the theory that replaces Newton's mechanics because down at the level of atoms and below the the behavior of matter is very different so it needs quantum mechanics to explain it right and with me I'm just I'm just laying here I'm still here I haven't I haven't changed into my pajamas or anything yet I feel I'm going to work even fill that little corner up so I'll disappear again for a moment so late 1920s quantum mechanics is then combined with special relativity this is the work of the English physicist Paul Dirac and we arrive at what we call quantum field theory so you can see when I'm talking about unification I mean lots and lots of different phenomena that we didn't think we're connected to each other and we're seeing them part of a broader more general idea that describes a combination of different or they're both aspects of the same thing quantum theory quantum field theory then is combined with electromagnetism I don't know if people can see my might my cursor moving along a lot of the screen maybe maybe not anyway so quantum field theory just with the electrodes ism gives us something called quantum electrodynamics right then oh right now I've combined you see how why I'm so proud of this the slide I basically had the full the full picture and lots of white blocks covering things and every click animation removes from other blocks it took me hours to put this together right so now we have quantum mechanics applied to the atomic nucleus and we understand that there are new forces inside the nucleus holding its constituents together and those two forces are rather unimaginative li in my view call the strong nuclear force and the weak nuclear force those two forces are ad to the other two known forces at the time electromagnetism and gravity to give us the four forces that we know of in the universe right the next thing is that the strong nuclear force also is described by a constant field theory to give us quantum chromodynamics so we have the electromagnetic force electromagnetism put into a quantum field theory to give us quantum electrodynamic we now have one of the nuclear forces combined with quantum field theory to give us quantum chromodynamics what happened what about the weak nuclear force well that was then combined with quantum electrodynamics to give us an even more general theory called the electroweak theory now we arrive at sort of the late 20th century where we have three of the four forces of nature described by quantum field theories we have quantum chromodynamics that describes the strong nuclear force and we have the electroweak theory which describes electromagnetic force and the weak nuclear force together these form what we call now the standard model of particle physics so the standard model is everything we know about the three forces of nature that can be described within the quantum realm it's it's an all-encompassing framework the standard model of particle physics it may not be the end of the story in fact we're pretty sure it's not the end of the story because we would really like to combine the electroweak theory and quantum chromodynamics into one really solid theory called a grand unified theory we're not there yet but the standard model of particle physics is pretty much everything we know now about the world of the very small right we next jump back up to the top remember I left you with cosmology which is the new theory from general relativity that gives us what we call the standard model of cosmology that the the the letters there that's the Greek letter lambda lambda CDM so lambda is is quantity called the cosmological constant that that crops up in Einstein's equations of general relativity and CDM stands for cold dark matter because within our standard model of cosmology we know we have to include an explanation of dark matter and of course also of dark energy and so we don't really know entirely how dark matter dark energy fit into the standard model of cosmology because we don't really understand what their nature is we don't understand what they're made of but we now have to we've arrived from all these different Cena from falling apples all the way down to statistical mechanics all the different phenomena in the physical universe we've got two models the standard model of cosmology and the standard model of particle physics what we really want to do is combine the two into a theory of everything a theory which physicists refer to as quantum gravity and you can see where the word the name comes from quantum mechanics from from the the the three forces that are described by the rules of the the world of the very small and gravity because cosmology essentially comes from Einstein's theory of relativity which is a theory of gravity it's the fourth force what we don't yet know how to combine that fourth force with the other three forces in a standard model of particle physics so what else is there well I think my view is that thermodynamics is the third pillar of physics which whist we tend to forget about and I think we're only going to arrive in the theory of quantum gravity when we understand how to fit thermodynamics into the picture as well likewise there are new areas so quantum mechanics has led to a whole new field called quantum information theory quantum computing and that may have some bearing on our final Theory similarly it down in this corner I've just added lots of our stuff where we're still trying to get our heads around nonlinear dynamics complexity theory non equilibrium thermodynamics there are missing ingredients essentially is what I'm trying to say before we can reach a theory of everything and we don't know how far off that is so where are we at the moment with with candidates for a theory of everything well here is a nice cartoon this is a I'd depict a struggle between the two oh I see my pictures frozen on my screen I'm not sure whether let me just toggle it on and off No I'm not quite sure if you can see my picture moving or not maybe the audios fine for us so when the pictures are still moving for me so I think it's still okay I shall continue right so yeah so it's just the broadband here in Portsmouth it looks like it's slow so I so I'm looking at a frozen picture of myself with my mouth gaping gormless Li open but luckily for the rest of you that isn't the case hopefully so here we are a struggle an arm-wrestle between two superheroes one of them represents what we call super string theory and that's the theory that's trying to unify the four forces of nature and the other is a rival approach called loop quantum gravity which is trying to quantize space and time themselves both come at this from different directions but both are trying to achieve the same goal to to to be the the theory of everything we don't yet know where the string theory or loop quantum gravity is the true the correct theory of everything that's something we haven't we haven't figured out yet but both theories have are being worked on have been worked on fought for for some years now and we're realizing that actually the job is harder than we thought it will be and we're we're we don't yet know there are many very very smart physicists working in string theory working loop quantum gravity and indeed in other approaches but progress is slow the mathematics is very hard very difficult they're very powerful mathematical theories but it's one thing to have beautiful maths and quite another to say that that beautiful math is the correct description of the way the universe works another issue of course is with these theories is that we don't know how to test them we don't know how to care at experiments to see whether what they predict is correct and I don't want to get to the details of either of these theories but only to say that in reefs years there's even been almost a backlash against these ideas that you know they haven't led to the success that they suggested and so some physicists saying is it really proper physics is it proper science to come up with a theory that you can't test clearly won't you know these are very powerful ideas they're very useful mathematics and we shouldn't give up on them but we should try and find ways of testing we don't know how to do that but maybe it means we need to have a fresh look at where we might be going wrong do we really understand the nature of of matter and energy in space and time and that's something I talk about in the book so let me give you a quick example what is space well for the millennia scholars have have tried to understand you know all the way back to people like Aristotle they try to understand what the nature of space is so for example if you have a box an empty box containing nothing at all but there's some there's some volume of empty space inside it does that space still exist if you remove the walls of the box was that space only real because it was within confines you remove the walls is there any empty space at all what if that box were in a larger box and now you remove the walls of the small box does it space is it space still in existence because it's now part of the empty space of the larger box you know the these are almost philosophical questions but physicists and thinkers throughout the centuries have really wondered what is the nature of space so Isaac Newton believed the space was a thing space was was where stuff happened it was the stage in which events took place he said space has to be they're in existence even when there's no stuff no matter to put in it you have to have space in the first place and then stuff can be put in it so that sounds reasonable but other thinkers getting all the way back to Aristotle and they're more later than the mush legs and that day carts they argued that when you remove the walls of the box the empty space inside doesn't exist anymore it's only given meaning as the distance between things you remove those things and and there is no space what do we know now then what what is the current view well the current view of course the nature of space and time yes you've probably guessed it was down to Albert Einstein now Einstein in 1916 so only a year after he came up with a German Thea relatives he published his lovely book relativity the special and general theory published in German first but then got translated into other languages and what is so beautiful about it is that it's around the years Einstein would add appendixes to this book you know wait wait it's time you know what over the years when he'd clarified his thinking a bit more he wouldn't go back and rewrite the book in a new edition he just tagged on a bit more and the most famous appendix 5 appeared just one year before he died and in it even physicists probably aren't so familiar with this but in it he gives his best understanding of what space itself is so he says if we imagine the gravitational field to be removed there does not remain any flat space-time but absolutely nothing because in Einstein's general theory of relativity there's an equation it's called an Einsteins field equation on one side and so an equation has an equal sign ok so I won't give you the equations lots of you know Greek symbols it's very pretty prison system I get but everyone else thinks what what does what the hell does that mean on one side of the equal sign is space and time the shape of space and time on the other side is matter and energy and so one way of talking about relativity or certainly this equation is that matter and energy curves space and time tell space and time how to bend and space and time as it's curved tells matter and energy how to move and how to behave they're inextricably linked matter energy on one side space and time on the other and as far as I understand is concerned space-time is the gravitational field itself due to matter and energy so he also said he says there's another quote from him space-time does not claim existence on its own but only as a structural quality of the gravitational field so in a sense he's agreeing with with Aristotle and Descartes he says you know without matter and energy to fill the universe there would be no universe without matter and energy there would be no space and time but he also agrees with Newton that space thus let's leave time out of the picture for now I'll come back to in a moment space Einstein agrees with Newton in saying is the thing it's a it's a it's a it's a physical something because it can be influenced by matter and energy it can be curved it can be bent space is a is the fabric of the universe itself but unlike what Newton thought Einstein says you remove all the stuff from space or the matter and energy space doesn't exist anymore space is the structural quality of the gravitational field due to matter and energy D is a profound profound and ideas I'm not getting them across to try and you know so you'll understand them you know exactly what Einstein was going on about but simply to give you a notion that some of these deep concepts very fundamental causes what are the nature of space what is nature of time we're still trying to figure out we're still trying to answer we don't have the full answer yet let me come very briefly let's see how am i doing for time I've got about 15 minutes good ok I'm doing well we talk about space and time and matter and energy and ice dynasty of relativity essentially we're talking about the world at the very large we're talking about the you know the entire universe of course the other theory that describes the other three forces in the universe is quantum mechanics here's my depiction of quantum mechanics right so if you know anything about quantum mechanics there's that on one side is the Greek letter psi which is the mathematical symbol for a central quantity in quantum mechanics called the wavefunction on the other side you've got the cat's Schrodinger's cat in the box I'm not gonna go there now you can ask me about that if you want we have half an hour of questions after the lecture um con su mechanic developed in the 1920s describes the this subatomic world the world of atoms and the particles that make up atoms and like relativity theory describing space and time at the larger scales quantum mechanics is incredibly successful at depicting an explaining phenomena within the world of the very small but to give you an example despite the success of quantum cast and without I mean without quantum mechanics I wouldn't be giving this talk not because I would be out of a job but because the electronic devices that we are using for me to communicate this talk to you wouldn't exist because quantum mechanics allowed us or helped us to understand the nature of matter at the tiniest scales and how electricity is transmitted quantum mechanics explains how semiconductors work so without quantum mechanics who wouldn't have developed silicon chips we wouldn't have developed the whole of modern electronics we wouldn't have computers we wouldn't have laptops we wouldn't have smartphones all of modern electronics only exists thanks to our understanding of the rules of quantum mechanics the way that these particles at the tiniest scales behave so tremendously successful but again I want to give you an idea of why we haven't got to the end of the journey even within quantum mechanics one of the things that's absolutely bugged me throughout my career an increasing number of physicists are also quite rightly perturbed by this is that quantum mechanics is the only theory in the whole of science that has got away with not having a single unique interpretation now by interpretation I mean an explanation of what the mathematics tells us about the physical world Einstein when he developed a special theory of relativity he the mass was already in place other other physicists before him and mathematicians had already developed the maths of special relativity the reason we would give our stein credit for special relativity is because he gave the explanation the interpretation of the mathematics the story the narrative the the thing that has to run alongside the Greek symbols and the algebra to tell us what they mean about the real outside world quantum mechanics has at least half a dozen different ways or we interpreting what the mathematics tells us about the world and that's not good enough physicists for the last almost a century have managed very well without having an interpretation because the maths works the maths is powerful is accurate and it helps us understand phenomena so in a way many physicists said have said over the years well look if you're gonna worry about what it all means how does an atom how can I be in two places at once how can two entangled particles communicate instantaneously across the vastness of space if you're gonna it works you're happy just accept it and if you're gonna worry about it go and do philosophy you know it's it was almost said as an insult to anyone who said hang on a minute how how does that work I start certainly was concerned famously about the meaning of quantum mechanics he had long-running arguments with with other physicists about it and the history of science sometimes rather lazily tells us that Einstein lost their argument he didn't he lost part of the argument but there's still an important aspect of Einstein's way of thinking which i think is is is something we should hang on to which is that even though we have lots of different ways of explaining quantum mechanics we have yet defined the way that nature behaves so for example one interpretation is called the many worlds theory and it says that every time down at the quantum level some thing is faced with a choice it doesn't do this all that it does both but each one happens in a separate reality in a separate parallel universe as it were hence them the many worlds in the name there's another interpretation called the the hidden variables interpretation which says that there's some deeper mechanisms going on that we haven't yet uncovered quantum mechanics isn't the end of the story we've got to dig deeper to find some deeper meaning there there are other interpretations that say that you know there are signals moving backwards and forwards in time we may have this embarrassment of ways of explaining quantum mechanics but Nature doesn't care the universe was doing what it does long before humans and particularly quantum physicists came on the scene and worried about this stuff either there are parallel worlds or they're not in other their hidden variables are they're not they can't all be right and so for me certainly one of the other mysteries to add to be resolved in in fundamental physics is to find the correct interpretation of quantum mechanics we may never find it because so far every experiment we've done to test quantum mechanics comes out with flying colors and tells us quantum mechanics is correct but it doesn't help us discriminate between the different interpretations they all make the same predictions that that we see when we cannot experiments we're not able to sort of rule any out just yet but it's not good enough you know that you know we have to we have to work harder at it so I wanted right I wanted to give you an idea of just a few more slides now or to give you an idea of the way different physicists approached mechanics and how important they felt it was that we find an interpretation or you know whether we should we just do worry about you know that whether that's just we leave it to the philosophers well here's one famous quote it's wrong to think that the task of physics is to find out how nature is physics only concerns what we can find out about nature now you might think what what what is what is the difference between those two alternatives this is a quote from Niels Bohr Niels Bohr Niels Bohr's view and he's one of the founders of quantum mechanics in fact probably the father figure of quantum mechanics his view was that not just quantum mechanics but that the whole of physics he isn't about finding out how the world actually is that ultimate objective reality there because all we can ever do is carry out experiments come up with theories and test them against experiments so all we can do is learn about what we can say about the universe what we can see the properties of the universe are not how it actually is that's a philosophical view but it's one that isn't shared by many physicists here's another famous quote from a hero of mine John Bell Irish physicists quantum physicist see he came up with famously what we now call bells inequality which we should actually stuff highlights that the arguments between Niels Bohr and Einstein john bell says to restrict quantum mechanics to be exclusively about piddling laboratory operations is to betray the great enterprise so he's not agreeing with Niels Bohr's idea he says it's not enough just to say your theory predicts the results of experiments and therefore you should be happy because there's no more you can do about it you need to try and understand how the world at chile is and then the third quote is let me put my my picture to make the third quote is not surprisingly from Albert Einstein who gives me this opposing view to Niels Bohr the job of physical theories is to approximate as closely as possible to the truth of physical reality now I lay my cards on the table and although Niels Bohr is a great hero of mine and III spent a lot of time working at the Niels Bohr Institute in Copenhagen and with many of his his colleagues although he died the year I was born so I never actually worked with Niels Bohr at all um nevertheless I signed with Einstein on this I think there is an objective reality out there and the job of physics is to approximate as closely as possible we don't know if we would ever reach that that ultimate truth their ultimate reality but we should try and get closer and closer to it now I'm using words like I think and I believe and those aren't words that we should really be using in physics so it's it's a personal preference the job of physics is not about you know what what beliefs you have or what preferences you have you have to be objective about it and so this this sort of strays a little bit into into philosophy but to give you a final example of what I find actually second to last example how are we doing for time or five minutes let's see if I can whiz through what is the nature of time now you I've talked about space-time is also fundamental earth I mean over the over the millennia I guess scholars have tried to understand the meaning of time but just give you an example of how far we are away from an ultimate theory we have three different definitions of what time is from our three pillars of physics remember we have general relativity which led to the standard model of cosmology we have quantum mechanics quantum field theory we leave to the person to the standard model of particle is and then we also have thermodynamics so what are they each a general relativity says that time is part of the physical fabric of the universe I tired or II said time as the fourth dimension but in general activity he says space-time this four-dimensional thing gets warped by gravity so time according to general relativity is a dimension it's an axis it can be stretched it can be warped by gravity literally time can be influenced and affected by gravity and that's not just theory we know that that's you know I asked me the question if you want then I can say little bit more about it later on quantum mechanics on the other hand says time is just a number it's a parameter it's something you plug into an equation probably a Schrodinger equation of quantum mechanics the most famous equation in quantum mechanics so say plug in time now and say you and you want and you know the state of some system saying electron now Chuck crank the handle solve the equation and work out what that electron will be doing what it's constant state will be at a future time okay but you could equally well crank the handle backwards and work out the state of electron in the past so time is simply a number you plug in time 1 and you work and work out the state of something at another time in its past or future it's not a dimension it's just a number and then you have thermodynamics which says that time is an irreversible arrow pointing from the past to the future the the one of the most famous ideas and physics is called the second law of thermodynamics which says things evolve they they wear out they they become more disordered stuff changes and unwinds we get older we don't get younger thermodynamics describes all of this so it says that time is not a dimension it's not number is an arrow pointing from past to future there's no such arrow in quantum mechanics there's no such arrow in general relativity and so three very different ways of describing what time itself is from those three theories that we'd like to merge together to unify into one theory of everything and we're not there yet um what do I want to say about the history of the universe over this there's a lot to say here I don't know if oh let me just say one quick thing another to highlight another unknown when I was studying physics we learnt that you know the universe began space and time began with the Big Bang and then we think there was a period of very rapid expansion of the universe expansion of space something called inflation in which the universe started very small and expand into very little you know within a fraction of a second it's a very large volume very large space and then it slowed down to a more sedate expansion that we see today so Big Bang was the first thing that happened and then there was inflation now many cosmologists would argue that they they saw Porter and another idea called eternal inflation in which these things are now reversed in which there is inflation first in what's called the multiverse and within it bubble universes pop into existence so our universe our Big Bang was the formation of our universe within this inflaton field which is the height that the multiverse where all the bubble universes existing and so inflation happened before the Big Bang not ask the Big Bang now which which way round is it well we don't know yet this is his bubble universe is for you I like this picture as well and guess why um so in vanilla cosmology we don't know answers to questions like is there a multiverse are there other universes and like theories like string theory they're difficult because we don't know how to test them we don't know how to design an experiment or carry out an observation that would check whether or not they're right and which is why people working in this field are criticized and they're told you're not even doing proper physics you're not doing proper science because your theories on you can't falsify them you can't test them and that's the only way you know that's how we define a scientific theory so we have a long way to go I think I want to end with this slide just giving you an idea in fact six lessons on how we know what we do know now remember I gave the idea of the ball dropping and if we know that it takes one second to fall five meters how do we know what we know about the universe how does science work first of all I would say a science isn't just another ideology or belief system because science progresses based on what we call the scientific method the scientific method requires us to our theory to be falsifiable to be testable to be repeatable that experiments has to be repeated in order to prove a phenomena not at all - to confirm a particular theory a good scientific theory has to make predictions it can't just say yes you know that's the number that you should get you should have got when you got that experiments after you've done the experiment it should predict that if you do this experiment or you care about that observation this is what you'll get so there are certain rules and criteria that are what we now call a scientific method and that lifts the the the edifice of science and physics in particular above other ideologies ideologies and belief systems how do we know what we know what is because we it's okay for us to make mistakes can you imagine a politician admitting a mistake saying well I was wrong about that I've changed my mind because I've given some new evidence and now I'm going to do this they will try and make it sound and I never thought that I always said this in science we are not embarrassed to admit our mistakes because that's how we move forward if we arrogantly stuck with what we believed in if we didn't have have an open mind we would never developed different countries will never replace all theories with newer accurate ones so admitting our mistakes is valuable in helping us persuade people that we this is how we know what we know then a scientific theory as I mentioned I think had there has to be falsifiable I identify those of you who follow me on Twitter I had a bit of a Twitter stream wasn't a rant I think it was linking you know talking about these conspiracy theories linking Crona virus to 2g5 networks I'm not even gonna go there [Music] rubbish obviously so so I was trying to make the comparison between scientific theory and a conspiracy theory a scientific theory is falsifiable you can you know within the light of new evidence you can prove it's wrong a conspiracy theorist no matter what evidence you give them they will twist it and use it to make to indeed to support so their their theory or they will dismiss it altogether a proper scientific theory is one where you can prove it wrong you can knock it down and kill it and replace it with something better fourthly it's no good coming up with a theory and saying well that's it well you know you have to repeat it many years ago when I was doing my PhD there was a discovery of something called cold fusion the idea that to electrochemist had discovered nuclear fusion on a tabletop that would have been the holy grail of giving us fusion energy unlimited energy solving the world's energy problems well no one could repeat those experiments and so that idea died today not because there was a conspiracy against them it wasn't so right the Goliath of you know big you know the poor David fighting against Goliath and you know big corporate interest islands these two heroic scientists they made a mistake and no one else could repeat the experiment because actually what they had done was wrong there was no such thing as cold fusion so performance and and results have to be reproducible by others before we believe them so that adds credibility credence to how we know what we know choosing out over certainty scientists unlike politicians again um always have doubts now you love her heard in the news when in the daily press conferences when when the journalists are asking the science advisors to the politicians you know when is this going to happen when is when are we going to know when it were tested and they say we don't know we don't know yet we don't have enough evidence that is the essence of good science to have doubts to have uncertainty not to say I am sure now a lot of people think that's a weakness you know oh if you're not sure about this how can I believe anything you say well science isn't about being sure because then you know you would never want to change your mind again you know you wouldn't you wouldn't want to admit your mistakes you wouldn't want to allow your theory to be falsified to be rigorous enough that if you come up with new evidence you remove that fear and place it with a better one my final point is one that is an interesting one because I don't know whether all just because a mathematical theory is beautiful or it's simple its elegant doesn't mean it's the truth now over the past century or more we actually have found a lot of fundamental truths about the universe from beautiful theories people like Paul Dirac and others develop beautiful mathematical structures mathematical theories mathematical equations and they trusted them and they said that this speak how the world works because look at my equation look how pretty it is and sure enough nature behaves in that elegant way but it's dangerous to follow that line too far not everything in the workings of the universe is simple and beautiful sometimes is just hard a lot of problems in physics today and some of the mysteries there still remaining I suspect it's going to take some effort to resolve them because we're not going to say ah yes of course that's how it is now this is my fear of everything we were missing this I don't think another Einstein is going to come along and say this is the simple thing you're missing here you are here's your theory of everything your theory of quantum gravity I think it's more complicated than that it may even be that we require artificial intelligence to help us solve some of these problems because artificial intelligence will be able to see patterns in in the complexity of our equations or our mathematics that we humans are unable to see [Applause]

Channel: The Royal Institution

Views: 304,401

Rating: 4.8416142 out of 5

Keywords: Ri, Royal Institution, physics, jim al-khalili, thermodynamics, general relativity, quantum theory, physics 101, space and time, space

Id: gKrjOD3ZeO0

Length: 61min 57sec (3717 seconds)

Published: Thu Apr 23 2020