Exploring the Biggest Unsolved Mysteries of the Quantum Realm with Physicist Jim Al-Khalili

hello and welcome to instant genius a bite-sized masterclass in podcast form every Monday and Friday you'll hear World leading scientists and experts talking about the most fascinating ideas in science and technology today I'm Jason goodger commissioning editor at BBC Science Focus in today's episode we're lucky enough to be joined by Professor Jim alkal a theoretical physicist based at the University of su author of several bestselling books and the longtime Center of BBC Radio fors the life scientific Jim thank you so much for joining us my pleasure glad to be here so today we're going to delve into the Mysterious World of quantum mechanics so these days uh the word

Quantum is sort of thrown around a lot for washing powders running shoes and all sorts I have Quantum deodorant exactly yeah it seems to be everywhere first off we what does it actually mean well Quantum goes back uh long before the quantum mechanics came on the scene the quantum is the smallest discret jump you can have the origin in physics goes back to Max plank who in 1900 suggested that Warm Bodies radiate energy not as a continuous flow but in discrete chunks there are small the smallest indivisible chunks of energy which he called Quant and from then that kicked off the quantum Revolution and led a couple of decades later to the fully blown quantum mechanics so

let's have a look at quantum mechanics then versus classical mechanics or Newtonian mechanics so first off there's the schinger equation that people might have heard of that sort of describes the strange property of in quantum mechanics of waves behaving like particles and particles like waves so what can we say about that well we I mean we are celebrating the centinary this year of quantum mechanics and indeed of or rodinger coming up with his famous equation it's fair to say that back this is back in the mid 1920s there was a lot of confusion about what it means to say that particles behave like waves and wave behave like particles and physicists were arguing amongst themselves as to you know who had the

the correct way of explaining what was going on down at this microscopic level Schrodinger's equation is a wave equation in the sense that it describes the behavior of waves he believed that electrons trapped within atoms don't orbit around as particles along specific trajectories but are as spread out waves so he really was of the view that there weren't particles they were all waves right and of course there were there were other views um Vera Heisenberg who also developed a version of quantum mechanics that same year 1925 hated the idea that you could describe atoms electrons as physical things he said no it's their abstract quantities we can't really say what they look like what they

are we can describe them mathematically and we can predict the results of experiments but they are not real physical waves which is what Schrodinger believed so we've carried out some really strange experiments investigating this phenomena I think probably the most famous one is the double slit experiment so could you explain that yes I some years back I gave a talk at the Royal Institution in London about this subject weirdness of quantum mechanics and I came to explain the Tess experiment in all its weirdness and at the end I made the mistake of saying if anyone has a logical Common Sense way of explaining what's happening here give me a shout because you know the king of Sweden might want to call you up and off

your El price so of course I forgot that I wasn't just talking to the few hundred people in the audience there but the RO institution record their lectures and they go online to this day more than 10 years later I probably get one or two emails a week saying something like I don't have a background in physics however I think I figured out what's going wrong in the two slet experiment to the extent that even on my website I've said look if it's two slet experiment explanations please don't email me but the long story short the idea is if you fire a subatomic particle at a screen with two slits and then there's another screen behind it that would capture the arrival of that particle then you would imagine if they

behave according to new Ian mechanics that particle if it gets through the middle screen with the slits it'll either go through the upper slit or the lower slit and you'll get an accumulation of particles on the back screen adjacent to the two slits two sort of piles of particles but we know if you send light through this goes all the way back to the beginning of the um 19th century Thomas Young explaining the wave nature of light uh saying that light travels as a wave and so it passes through both slits simultaneously and then the other side of each SLP slit acts as a new source of light and the two waves interfere and interact you get an interference pattern if quantum particles behave like waves then they

should also give an interference pattern and indeed they do and we've tried to understand how this is you could even send a particle say an electron one at a time at the screen with the two slits you'd think that it would either go through one slit or the other but after many electrons pass through and by the way on the back screen it hits as a DOT of light there you can the back screen can be a fluorescent a screen that shows a flash of light when an electron arrives so you see it arriving as a particle but somehow as it goes through the two slits you don't see individual electrons whether they go through one or the other but cumulatively they build up an interference pattern and if you try and catch the electron out to see which

slit it goes through it knows you're watching and behaves like a particle it's only when you look away that it behaves like a wave so I mean the physicist Richard fan said the tus experiment is the central mystery of quantum mechanics if we could understand that logically we do understand it but it's you don't entirely get rid of the weirdness so another weird sort of weird aspect of quantum mechanics is superposition so what do we mean by that and how does that work well the idea I mean again this is a property of waves when you have two waves interfering they we've can talking about them as superposing on top of each other in the two slit experiment you the waves coming from each slit will superpose on the other and so

where you get a Crest and a trough they cancel out two crests will magnify the amplitude in quantum mechanics Quantum objects particles waves whatever you might call them like electrons or protons or neutrons they have this property that of superposition in that you can talk about them as not being in one place or having a particular energy or momentum but rather a combination of being in lots of places at once or being having lots of energies at once so we say they are in a superp position of different states and only when you measure them for example measure what energy they have do you kill off that superp position and you select just one outcome same with position it could be

in a superp position of different places only when you get a detector to locate the position of the electron do you see it somewhere else and that kills it that's what's is often called collapse of the wave function killing off the superp position again it's a property of waves which is not so strange but when it comes to subatomic particles in the quantum world seems weird yeah so you mentioned there measurements there's a fundamental problem in quantum mechanics that was with measurement that was um discovered by Heisenberg the yes the idea of measurement brings in the role of the Observer and that's led to a lot of confusion among students of quantum mechanics and The Wider public when

they learn about the theory when a Quantum object or system let's let's say an electron is in a superposition of different states say uh an electron that has can be in a supposition of different energies at the same time it doesn't have a precise energy it has it can have a ability of being in different energies at the same time when you measure it you pick out just one how does that happen schinger himself actually was very unhappy with the idea and he developed his famous Paradox Schrodinger's cat Put The Cat In The Box and you close the box and in with the cat is a radioactive material that can emit a particle that can release poison that can kill the cat but until you open the box and measure the cat because it's

also made of atoms is in a superp position of being dead and alive at the same time today we know how to resolve the measurement problem to a large extent something called decoherence um a Quantum system isn't alone interacts with its surroundings but there's still an issue as to what happens to all the other possibilities that you don't see when you measure if it had several energies or was in several positions at once and you measure it you find it in one it's not like you've got something closed in a box and you don't know what it is or you put a um you have two boxes one with a left glove and one with a right glove until you open the box you don't know which one's left and which one's right so of course you open the

box and you see a left glove you immediately know the other one is a rightand so it's not just our ignorance that leads to these probabilities in quantum mechanics they really are in all having all possibilities at once and this leads to issues in philosophy about how to interpret what's going on one of the most logical ways although also weird uh is the idea that all possibilities happen when we measure something we measure the open the box to see the cat is alive there is another parallel reality in which we opened the box and found the cat is dead that solves the measurement problem but it means you have to buy into this idea that there are multiple realities this

is called the many worlds interpretation yeah so you mentioned there um the glove analogy so that's a bit like quantum entanglement so can you tell us about that so quantum entanglement uh we are coming to realize is one of the most fundamental aspects of quantum mechanics very often even students at University studying physics don't get taught about quantum entanglement it wasn't it was deemed as rather extreme uh sort of uh weirdness of even Einstein didn't like entanglement but we're realizing it's quite fundamental the basic idea is that you have two particles now a single particle can be in a superp position of two states let's say an electron can be spinning in two different ways we call

it spin up and spin down the vague the rough classical analogy is to say it's spinning clockwise and anticlockwise if it's in a superp position it's doing both at the same time don't even try to figure out what this means but to for a particle to spin clock and anticlockwise at the same time just doesn't make sense at all but that's what a Quantum superposition would mean now if that particle is interacted with another that electron with another electron then the fate of the second one becomes intertwined with the first so if the first one is in a superposition of spinning both ways at once the second one's also spinning both ways at once and measuring one will instantaneously change the state of the other one so we

talk about those two electrons as being Quantum and entangled and they would hold this quantum entanglement provided they're not Disturbed and the entanglement destroyed however far apart do you separate them and this is being tested absolutely I mean this is something that's it's it's well established now I mean may be weird Einstein may not have liked it but it's it's uh one of the fundamental features of modern Technologies in the in the quantum world that we're developing at the moment Quantum Computing Quantum encryption and so on they all rely on this idea of quantum entanglement so not just with like us humans understanding physics and trying to figure it out it also happens in the animal kingdom so um something called

Quantum biology so I've heard that some birds use quantum mechanics in order to navigate when they're migrating so we think I mean this is this I should say quite clearly that we don't yet have the experimental confirmation that this is what happens but it does seem to be the this is really this is the only theory in town that explains what happens so I should first of all say Quantum biology is basically the idea that life has evolved the ability to make use of the quantum world uh in a way that inanimate matter doesn't do so it's not saying we are made of atoms and atoms behave Quantum mechanically then of course quantum mechanics plays a role in life that's that's goes without saying no some of these ideas like quantum

entanglement they might play a role in life it was known since the 1970s that certain anim animal uh Birds marine mammals and so on consense the Earth's magnetic field and even that in itself is weird you know how can something as weak as the Earth's magnetic field affect uh an organism's chemistry it's one thing sticking you into an MRI scanner that's a very powerful magnetic field but the Earth's magnetic field is very weak but these animals seem to have evolved a chemical Compass of some form and at the moment the current theory suggest that well first of all we believe this Compass is based some somewhere in the animal's retina so the European Robin is the classic example uh that uses Magneto reception

as it's flying light enters the retina it hits one of a pair of entangled electrons so they're both two electrons sitting on one atom they're entangled in the sense that one is spinning one way the other one has to be spinning in the opposite direction the photon comes in knocks an electron off the atom they're now still sitting within this protein called cryptochrome but they're not on the same atom anymore they're Quantum entangled that means their spin their spins are still correlated they're still interconnected in some way and the way these two electrons spin is very sensitive to the orientation of the bird in the earth magnetic field and so the Earth magnetic field can be sensed by

the bird through these entangled electrons that and that sort of Cascades a signal through to the bird's brain that tells it what direction to fly so quantum entanglement might help the European Robin migrate every Autumn down to the Mediterranean from Scandinavia it's a lovely idea that the robin uses a theory that even Einstein didn't like because it was so wacky that doesn't make it right doesn't make it you know the correct answer to how the birds navigate but we don't have yet another uh explanation for this Magneto reception in an animal so you often hear um quantum mechanics called the sort of physics or the science of the very small so what is the sort of scale limit and you know is are we pushing further

and further against that in experiment um there is still to this day a very vibrant area of research that examines the boundary between the quantum world and our everyday macroscopic world what we call the classical World classical mechanics compared with quantum mechanics it's a very vague broad area and it depends on what we're what experiment we're doing it depends on how careful we're examining a particular system certainly you get down to the level of atoms and molecules you're in the quantum domain you should not expect them to behave classically and certainly in the our everyday worlds of tables and chairs and balls and humans and so on you don't see Quantum Behavior unless you dig down into the atomic structure but in between

it's it's it's difficult to know so we're for examp example developing Quantum sensors quantum computers these are objects that we can you know use to carry out certain tasks and they rely on ideas like quantum entanglement but they're they're large systems course the bigger a system gets the harder it is to retain the quantumness qu the quantum effects are very delicate very sensitive to surrounding environment so the more complic to the system is the less likely it is that we're going to be able to maintain any Quantum behavior for very long on the sort of other side of the coin then is the science of the very big the physics of the very big which is Einstein's theory of relativity and quantum mechanics and

relativity don't get on do they don't like each other why is that and you know what will we ever be able to get them to the hope is that we will and many physicists are working in this area of quantum gravity but mathematically the mathematics of quantum mechanics is very different from the mathematics of the very large Einstein's general theory of relativity general relativity is all about fields and geometry and the curvature of space time whereas quantum mechanics is all about the discreetness of particles and probabilities and so on and mathematically the theories don't don't mesh together but we sort of know they have to there are certain um uh environments or situations where we need both Quant mechanics and general relativity

to understand them you know for example the nature of the Big Bang the birth of the universe we can the Big Bang was predicted by general relativity and later confirmed that is really how our universe was born by lots of experimental evidence but we can't explain that very moment the initial what's called The Singularity the beginning of time where time and space and matter all first appeared without quantum mechanics because quantum mechanics also describes the very small and the universe was very small at the Big Bang so we feel we need a theory of quantum gravity sometimes people call they talk about this as a Theory of Everything not everything as in including psychology and sociology but everything within physics but

we're no nearer I think than we were four five decades ago Stephen Hawking famously wrote an article back in the early 80s saying we're almost at the end of theoretical physics we've almost got to our Theory of Everything just dot some eyes and cross some teas back then the idea was there were ideas uh like um uh super string theory was just emerging on the scene super gravity was another idea people have been working on string theory now for decades there are other ideas rival theories that could become the theory of quantum gravity there's something called Loop quantum gravity ah yeah but we don't know which of them is the correct one if any of them are indeed the correct ones uh and it's

frustrating because you know we thought we were getting close and they all they all have their problems string theory is probably the one that's most popular in the sense that most physicists theoretical physicists are working in and slow advances being made it turns out it's a very powerful mathematical construct that might be very useful might even help us um ANW other questions in other areas of physics but we don't yet know whether it's the correct theory of quantum gravity is it the one that's going to finally bring quantum mechanics and general relativity together it's uh it's frustrating and of course all one of the problems is we don't yet have a way of experimentally testing some of these ideas so many physic will say well that's not even real

science then physics is an empirical discipline andless you can test your theories there might as well be you know theology rather than proper science so it can be frustrating I don't quite subscribe to that but I do think that uh you know we're a long way yet from finding a way of merging quantum mechanics with relativity great so thanks very much for joining us uh been a pleasure speaking to you my thank you for watching this episode of instant genius brought to you from the team behind BBC Science Focus that was Professor Jim alkal if you liked what you just saw then please do consider liking and subscribing to this channel also why not check out the audio only version of instant genius podcast that's

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