on October 4th our land aspect John clauser and Anton zeilinger were awarded the Nobel Prize in physics for proving that the Universe isn't locally real what I love about this story is that it's a story of quite literally some of the smartest people who have ever lived being confused and how ultimately Einstein was proven wrong which doesn't really happen all that often that idea of locally real is made up of two concepts locality is the idea that things are only affected by their local environment you can't flick a switch in another galaxy hundreds of light
years away and instantly see the results here because nothing not even information can travel faster than light realness though is much harder to explain and that is the focus of the Nobel Prize foreign close to the beginning around the 1930s where there were two paradigms two ways of thinking about the physics of small things like particles atoms electrons photons Etc the view of Einstein and many others was that the universe is real the particles atoms and electrons have definite properties that are inherent to them regardless of if they are being measured essentially the if a
tree falls in the forest and no one is around to hear it it does in fact make a noise and then there was a counter group The anti-realists championed by people like Bohr as well as many others the particles have properties that haven't really made up their minds until you actually go out and measure them that they exist in a wave function of possible States and only when you actually take a measurement do they really make up their minds the famous example here is that of Schrodinger's cat that is both alive and dead until you
look in the box and then ultimately you go to prison foreign [Music] culminated in a famous paper called the epr paper where Einstein podolki and Rosen put forward a thought experiment that they thought perfectly highlighted the quantum mechanics at best was incomplete and at worst may be totally wrong their thought experiment focused on an idea in quantum mechanics called entanglement that the property of two particles can be inherently related their line of reasoning for their thought experiment started with well we know that energy is conserved things don't suddenly start moving in a direction unless someone
or something pushes or pulls them neither also do they suddenly start spinning rotating jumping up and down or doing anything else unless someone else is directly affecting them number two if we start with a small quantum mechanical particle that isn't spinning moving or doing anything else let's imagine that particle spontaneously breaks into two if we look at one one of those pieces it breaks into and find that it's moving to the right we know instantly that the other particle must be moving to the left to conserve momentum or if we had looked at it and
found that it was spinning one way maybe clockwise we would know instantly that the other particle must be spinning counterclockwise to conserve angular momentum you'd be confused if you looked at this system and saw both particles suddenly moving to the right your intuition would tell you that some outside force maybe must have hit them or for the same reason though may be less intuitively because most people don't think about angular momentum or spin you'd be equally surprised if you saw both particles rotating in the same direction you'd assume that something must have hit them and
caused them both to start spinning step 3 quantum mechanics here says these states are impossible to know before you go out and measure them if you separated these particles light years apart and measured one say and found that it was spinning clockwise you'd know instantly even if that particle was a universal way a that its counterpart must be spinning counterclockwise but how could this be if they really only take on a definite value when you measure them and the other one always needs to be the opposite of what you've measured then the particle that you
did measure would need to instantaneously communicate to its partner and tell it to adopt the opposite value permanently Einstein argued that this was impossible because it would have violated locality and meant that information had traveled instantly faster than the speed of light to tell the other particle to collapse its wave function and make up its mind which way it was spinning instead he argued it must have made up his mind at the very beginning when it was created we just didn't know it or we weren't smart enough yet to find a good way of measuring
it he called this unknown knowledge hidden variables and said that this was the piece in quantum mechanics that was yet to be completed and for around 30 years physicists really kind of split into two groups either they sided with Einstein or they sided with cool mostly because no one had really come up with a good theoretical or experimental counter argument to Einstein's epr paper that was until about 1964 when John Bell an Irish physicist on sabbatical from working at CERN started to do some more theoretical work of his own these Works which were later called
Bell's theorem or Bell's inequalities of which there are many different forms but the underlying idea is to try and get the universe to pick a side tell us whether those hidden variables and Einstein's right or tell us whether truly there is a wave function and that quantum mechanics is real now this hinges a lot on what happens to Quantum objects when you actually go out and measure them so let me introduce you to the idea for a particle of light called a photon photons have a property called polarization which describes which way the wave of
light is oscillating through space either vertically or horizontally or potentially somewhere in between if you wanted to measure which state a photon was in you would put it through a polarizer that lets through either vertical light in one orientation or it will only let through Horizon gentle light in another orientation but then at least your detector which you place behind that polarizer knows what sort of light it's detecting if you fired in randomly polarized light some of it vertical some of it horizontal some of it in between by placing a vertical polarizer first then a
horizontal polarizer after you would expect correctly to see that no light reached your detector because all polarization angles had been blocked you can see this effect when using polarizing films which are the same stuff you find on a pair of polarizing Sunglasses by orienting two polarizing films at 90 degrees to each other you see no light emerges through them what is interesting here is that if you place a third polarizer between these two you suddenly and I think quite counter-intuitively start to see more light this is because fundamentally measuring a particle changes its state allowing
light to slip through the final polarizer where usually it wouldn't be able to so you start to see more light than you would otherwise expect so let's jump back though into the story at this point in history Belle's work was still more Theory and thought experiment than anything else and that classically is the problem with theoreticians if you look at them from a distance it just looks like a wizard trying to have an argument with you [Music] physicists really needed to find a way of actually completing an experimental measurement one of the early and most
elegant and now heavily evidenced extensions of Belle's theorem work is the chsh inequality by John clauser the Nobel Prize winner that we're talking about here Michael Horn Abner shimini and Richard Holt and their work here makes this theorem that Bell developed actually experimentally testable the scenario the describe is similar to what we've talked about before two entangled photons are sent in opposite directions to Two observers Alice and Bob both Bob and Alice get a polarizer to play with into their setup that helps them determine which polarization the light is actually in and finally Alice and
or Bob at random are told to rotate their polarizers over time and record where the photons arrive successfully or not at their detectors what we're interested in doing here is counting how frequently that Alice and Bob agree on whether they've seen or not seen a photon if Alice and Bob perfectly anti-aligned their polarizers then they should always both either see a photon or neither one should see a photon they should always agree on whether photons arrived or photons didn't arrive if however Alice and Bob both align their polarizers either Alice should see a photon and
Bob shouldn't see one or Bob should see a photon and Alice shouldn't see one the interesting part happens at angles between these positions if the universe is real and photons are truly independent Alice and Bob's rate of agreement should linearly move between full agreement and full disagreement if however the universe is not real we should expect to see a higher rate of coincidence than otherwise expected just like in our three polarizer setup except now our polarizers are on the other side of the Universe from each other and the same Photon isn't going through both or
is it this can only be true if the particles are really still connected to let each other know which polarization state to be in so that measurement on one truly does affect measurement on the other and in 1972 it was John clauser who built the first experimental setup capable of conducting this measurement in the paper that he released that year this was the figure that he displayed exactly matching the prediction of quantum mechanics proving that the Universe isn't locally real that Einstein's deterministic view was incorrect and a story from this point goes I'm not sure
if it really happened or not that excited by proving the result clauser runs into Richard feynman's office to tell him the news and in classic Feynman fashion Feynman throws him out of his office for ever doubting quantum mechanics and tells him good now get on with some real physics the other Nobel Prize recipients Elaine aspect and Anton Zollinger closed important loopholes that remained within this experiment as well as showing that quantum entanglement can be transferred to other particles in a process called quantum teleportation all this to say that without a doubt the universe showed itself
to be Stranger Than even Einstein had imagined [Music] all of these phenomena are really the backbone of what is driving the modern Quantum Computing Revolution the idea that at some point in the hopefully near future quantum computers will outperform classical computers because they have this inherent baked in advantage that down at their core their particles that run them are communicating to each other while this doesn't prove and I can absolutely forgive anyone for on first hearing thinking it that people can now communicate faster than light although it would be possible to derive some fast and
light communication that unfortunately is where the field pretty unanimously says no this is ultimately because the core to this phenomenon is inherent Randomness at the source of that Photon or entangled particle generation and then also at those measurement Points each Photon will either make it or not make it through a polarizer each Photon will either be created in some combination of states that ultimately those combination of states will be impossible to know until measured there's no way of loading information onto that Communication channel and as a result there is no way of violating Einstein's fundamental
contribution which is that the speed of light is a fundamental limit in the universe that absolutely stands correct and I think with Keith Einstein reasonably happy [Music] thank you foreign [Music]
