Quantum mechanics is supposed to be our best theory of physics That's because it can, tell us what will happen in any experiment that we've tried But science is also meant to explain, why things happen Quantum mechanics doesn't give us why because, we still don't understand how, to interpret its meaning at all For example when an object starts at. A and ends up at b does quantum mechanics say that it goes all possible ways at once Or that there are many copies of each object in many different universes each going a different, way Or, does it say that it doesn't even make sense to talk about something when we're not looking at it These are all possible ways to interpret quantum mechanics at least on face value In this video we're going to look at the basic rules of quantum mechanics to understand why its meaning is so evasive We're, about to look at an experiment that showed that nature has a very strange rule Objects act differently when they're not being measured Before we get to that though? We need, to understand when objects are and aren't being measured so let's look at This, familiar situation, you've lost an object but, you're sure you left it either here or here You're, not currently, measuring its position yourself because you're, not looking to see, where it is However consider that there are many other things that are measuring it for example think of all the light particles bouncing off it right now they're essentially taking a photo of where your object is What about the air molecules that would have gone straight through Where your object is if it wasn't there but are now bouncing off that position they're also, measuring your particles real position, and that's how it usually is Objects are constantly being measured, by other things so then this statement that objects act differently when they're not being measured Doesn't apply for everyday objects which is why, we didn't notice this rule for such a long time But then scientists started to look at very small things like electrons Things like air molecules and light aren't constantly hitting them because they're small enough to sort of fit in the gaps so what exactly Do objects. Like that do when no one is watching let's get back to the scenario, where our object so in this case our electron is Definitely in this spot or this spot but, we now know. That nothing is measuring it? We expect that objects can, only be in one place at a time Even if you, don't know, where that place is you know it's in one of the places you just don't know Which and this is usually true? But in the case, where nothing at all is measuring our object like our electron the following experiment will show Us that this isn't true the electron can't be at one place at. A time, while it's not being measured This experiment is called the double slit experiment. And here's how. It goes we're. Going to fire electrons one at a time from here When they get to this back, wall the place where the electron hits lights up how Do, we make a wall that lights up on contact with, electrons the physics is a two-step process First find one of those clever experimentalist people and to Ask, them to use their experimental physics magic to help you simple The point is we know, where the particle starts and somehow, we measure where it ends? But in the middle, we don't measure it and hopefully nothing else is measuring it either then put in a barrier with, two tiny doors in it Obviously there's a good, chance that an electron fired towards this barrier doesn't make it but we're going to ignore any that don't get through The electrons that do get through must have gone through via the doors We'd expect that it went through one of these, doors it's just that, we don't know, which one? But we'll see that, while the electron isn't being measured. By anything this isn't the case it doesn't go through a single door To show, this we're going to predict Where the electrons would have landed if they acted as we expected and see that it's different from the actual result of this experiment To get our prediction, we need to know, what an electron that definitely went through only one door would do So let's close one door and actually do this experiment As you can see electrons going through
The store end up in a pile behind the door and of course if we had done it with the other door open instead then Similarly electrons would end up behind that door in a bunch So if we know an electron goes through one of the doors then, we know approximately, where it will land Now, say both doors are open, and we predict, each electron has to go through one door We're going To, make sure that the electrons are fired slowly enough so that different electrons can't go through together and then interact with, each other Now, we predict that some of our electrons will go through, this, door but we know electrons going, through this? Door end up here because that's what happened in the single door experiment and Similarly others will go through the other door and end up here So if each electron goes through just one door then We must get two piles and yet this prediction is completely wrong? We have to conclude that the electrons didn't go through just one door each But i know, what some of you are thinking? Then they must have gone through both in other words the electron quite literally gets cut Into two part of its, mass goes one way and the other part the other way Then these two parts of the object. Bump into, each other causing them to ricochet wildly and the weird pattern Or in some versions of this explanation the particle splits into two and then flattens out into two waves that interfere with each other Unfortunately there are many problems with, these sorts of explanations Firstly if it splits into two bits or if it spreads out Then when you measure it at the wall you'd expect to get two dots or a spread out smear That's not what happens though? one electron through, gives you one crisp dot But, some people believe a more complicated version of this wave story to get around This, that goes it's a wave when you're not looking but when you measure it it becomes a particle again That would mean that as soon as the electron hits the back, wall it has to immediately pick up all of its Mass from everywhere it's spread out to and gather into a particle again But moving, mass around takes energy Where's the electron supposed to get that from There is a sense that waves are involved in this experiment but it's much much more subtle than you, may have been led to believe Anyway, the point is the double slit experiment showed us some important things, where nothing Watches what an object, does it doesn't just do one of the options, we can't see that directly but we can infer that from the experiment and The object doesn't physically split up and have parts of itself to do each of these options So, what exactly, is the object doing behind our backs and how are, we supposed to predict the results of this experiment they're? Crazy, and it isn't just the double-slit experiment that had unexpected results by the way loads of other experiments showed that particles are doing something really fishy, when we're not looking and That's why, scientists invented quantum mechanics it's some nice maths that you can tell an experiment to and it tells you, what you can expect and So far it's always been exactly right which is a grand achievement But we still want to know, what's the particle actually doing But this quantum mechanics doesn't tell us it's just a bunch of maths after all In fact some physicists say that it doesn't even make sense to talk, about an object that isn't being Measured that it doesn't exist This, seems crazy, to me and thankfully to many others too Recently, more physicists have started to tackle this question, again, and they've even started to tease meaning out of this mathematics Amazingly, by using more mathematics it's a really exciting time to be asking questions about this theory That's why i'm making a series of videos on quantum mechanics The first set of videos will teach you the rules of quantum, which are mathematical but they're really not hard and you don't need Any background in maps or physics already to understand them After that we'll cover fun topics including quantum computing eventually which, is what i'm doing my phd on? When the next video is up you can click here to see it feel free to subscribe if you, want to know, when that happens Meanwhile you can try these homework problems to see how Much you understood from this video Write your answers in the comments if you'd like i read them and it helps me a lot to understand which bits were the trickiest First question, we talked, about doing the double slit experiment with, small things, like, electrons but Is there any reason in theory that we couldn't Have done it with bigger things, and what would you need to do to make it work Second question, we didn't measure, where the electron Went in the middle of the experiment but It is possible and so if we were to measure each door to see, which one the electron went through what would we find and what would happen to the pattern on the back wall Finally there is another way to interpret the double slit experiment, which i think is really cool it's called pilot-wave theory And i've made a video about it In that theory the particle goes through just one door even though our argument said that that's not possible find the floor in our argument, also comment on why particles still act strange in the pilot-wave theory i Hope you have a go at Some of these questions and asked anything that you didn't understand below i'll see you in the next video of the Understanding quantum mechanics series you
"If you think you understand quantum mechanics, you don't understand quantum mechanics."
Maybe Richard Feynman should have watched this 11 minute video
Thanks, i can now cross dimensions.