Double Slit Experiment explained! by Jim Al-Khalili

Double Slit Experiment explained! by Jim Al-Khalili

October 23, 2019 100 By Kailee Schamberger


I’m going to explain [to] you. What’s Known as the central mystery of quantum [when] I was a [richard] [Fineman] the American physicist said this is the central mystery of Quantum Mechanics there’s lots of weird stuff [that] goes on in the quantum world Hit you with this and it basically tells you what it’s all about. It’s called the two-slit experiment I’ll start with this imagine you have a source of light shining against the screen with two slits now for dependence in the audience this source of light has to be Monochromatic light light of a particular Wavelength well where’s of course a light bulb is white light and that’s made up of all the colors And [spectrum] lots of different wavelengths, but imagine this is just a single wavelength of light And you can see the like is coming out in in waves like like Ripples in a pond that’s the nature of wave-like behavior as the light hits the screen it Squeezes through the two slits and each slit in turn on the other side becomes almost like a new source of light and the light spreads out it diffracts and as the waves of light overlap They will interfere with [each] other. So where a crest hits a trough. They will cancel where crest hits a crest they will amplify and so on and so [on] the back screen you end up with what’s called an interference pattern a series of light and Dark Fringes where the waves have either canceled out or Worked together in Phase that’s fine. That’s not quantum mechanics. That’s a Property of light that goes back over [200] years that we’ve known about since the early 19th century Imagine doing the same experiment again, but doing it not with waves, but with particles Do it with grains of sand so this is the same experiment, but I’ve tipped it 90 degrees Rather than waves that are spread out that wash up against the two slits and squeeze through Here you’ve got individual Particles of said and each particle will either go through one slit or the other and so you see there will sort of drain through And you get two bumps underneath each of the slits so the two peaks is reminiscent of Particle-like, Behavior, Whereas the the multiple Pattern of interference is wave-like behavior What if we do the same experiment with atoms? Well, so imagine we have an atom gun something can fire Atoms as a stream of atoms you can’t see them because [it’s] very small Let’s block off one of the two slits So these two slits are you know the the dimensions and separation of the slits is chosen appropriately to show us how atoms do things and so far so [good] nothing strange here, you’ll see a lot of atoms hitting the back screw, so this will now have to be some sort of photosensitive screen where Whereby when an atom hits it they’ll it will give off a little flash of light to say the atom has arrived here so the atoms are arriving as these little pinpricks of light that we see of Course a lot of the atoms will be blocked by the first screen. They won’t go through [that] slit But those that do get through to the other side you can [see] there’s a bit of spreading of the atoms But if we didn’t know anything about atoms you say well, that’s fine We can understand that some [a] lot of the atoms are going clean through the [slits] some are sort of maybe bouncing off the edge of the slit and so They’re sort of being deflected a bit. Which is why you get a bit a bit of a spread? The first mystery of Quantum mechanics comes when we open the second slit Because now we see something that’s very much like the interference pattern we got with light rather than having two bands of Spots where the atoms have gone through the two slits it’s as though the atoms have gone through the slits behaving like waves and And into [and] and you get interference of the waves and you get these Bands if we know nothing about atoms or quantum mechanics You could try and rationalize and say well, you know maybe atoms behave in a very strange way and Only a certain number of them are allowed to all sit together And so you know me and my gang we all going to go on this slit, no Sorry no room for you. You go next lit above and by the way. There’s this rule that no one can go in Between the two met [bands], but a few naughty atoms do so there’s a bit of a scatter. Yeah, we don’t there could be some forces between atoms that make them coordinate their actions in a way to give this pattern That’s not mysterious. That’s just we just don’t know how atoms do things, but we can be clever and we can force the issue What if we were to not send the atoms all through at once, but send them through one at a time Leave enough of a gap for the atem to get through to hit the screen of course as I say some atoms will Hit the hit the first screen and not get through but those that get through will hit the back screen so let’s run the experiment again slowly and Gradually you’ll see as the atoms go through they’ll be Look like they’re just randomly arriving on the other side you keep sending atoms through one at a time and gradually That same pattern appears so each atom by itself is Somehow contributing its small part to the overall Wave-like behavior that we see in the interference pattern How does it do it how does? We know the atoms are tiny? Localized particle. We can’t say it’s too small to even see under a microscope. [we’re] firing it at the screen with the two slits Some moment later you see a flash of light on the back screen. It’s arrived in a localized point It’s not spread itself out you Don’t get so like a wash of a sort of a faint light across the whole screen so a little point the atom is localized arriving in a certain location and yet It somehow seems to have been aware of there being two slits not one Because it’s given [rise] to this interference pattern how does one Assam do that does it split in half does it become like a cloud [that] goes through both well we can try and be Even cleverer, what if we were to spy on the atom and see where it goes? We’re going to gently just observe which slit it goes through So you put a detector? Just above the upper slit that will flash or beep Whenever it sees an atom go through that top slit Sure enough you fire the atoms through one at a time 50% of the time the detector will beep The other 50% of time it doesn’t the assumption being that the atoms has gone through The lower slit but of course I’ve been cheeky here. I haven’t shown you [the] results of the experiment That’s where you get 50% of the time it beeps, and you see a spot arrive Adjacent to the upper slit the other half of the time It doesn’t beep but you see a spot arrive at the lowest it. So yeah it’s picked out the atoms that have gone through the upper slit and Not the ones that go until each atom does go through one slit or the other But that’s a different result to what we had earlier So here’s the last bit of sneakiness that we can play with atoms surely now. You know we’re going to get to grips with it Leave the detector there, but just very quietly go and unplug it Don’t let the atoms know that you’re not spying on them Make them think that you’re still detecting them [someday]. Yeah, okay. We’re gonna run experiment a soms, okay get ready one at a time We’re going to be checking on you Alright, so run the experiment again now if you can explain this using common sense and logic Do let me know Because there’s a nobel prize for you Quantum [entanglement] is the idea that particles however far apart? They [are] Still somehow their fates remain intertwined they they are still aware of each other’s existence