Double Slit Experiment explained! by Jim Al-Khalili

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

Channel: The Royal Institution

Views: 2,279,085

Rating: 4.8382916 out of 5

Keywords: Double slit, Two slit, Double slit experiment, Physics, Light, waves, particles, Science, Ri, Royal Institution, Wave particle duality, quantum theory, quantum mechanics, Jim Al-Khalili, Scrodinger's cat, Obeserver

Id: A9tKncAdlHQ

Length: 9min 8sec (548 seconds)

Published: Fri Feb 01 2013

Reddit Comments

[u/A40]

Dammit! I was hoping it was finally explained in this video!

[u/necluse]

You should see he quantum eraser experiment.

They find a way to detect which photon goes through which hole through quantum entanglement in a double slit experiment when they fire photons one by one. But, even without interfering with the wave function, the interference pattern is still destroyed.

But the photons still "know" they are being observed. The thing is, the experimenters then find a way to scramble the data coming out of the detector, essentially "erasing" the data, and the photons revert to making the interference pattern

The photons somehow know that the observer can no longer interpret the data correctly, and then returns to it's standard interference pattern wave function

Fucking weird. Like the universe is self conscious

Edit: added words for clarity

[u/Benny-Gutmensch]

Other comments mentioned the "Delayed Choice Quantum Eraser" experiment. It''s the double slit experiment with lots of extra gear. Here the clearest video on the topic I know

• https://www.youtube.com/watch?v=U7Z_TIw9InA

It comes down to: if there is no residual of information about the path of the photon in our universe, then it behaves like a wave. If our universe knows about the path, it behaves like a particle. And NOBODY knows why.

[u/______-____________]

inexplicable

[u/0LucidMoon0]

If you guys want to see what the next step of this experiment is, look up: Quantum Eraser. There's also, the delayed choice Quantum Eraser (but spoiler, both have the same results.)

Here's my horrible attempt at summarizing and explaining the above: Essentially the Quantum Eraser and the factors of running the experiment in this fashion eliminates the need for a 'conscious observer' (as in the human mind is not necessary; just having a measurement machine present is enough) and provides proof that it is probably not the machine doing the 'observation' that is causing the results to vary (from being two piles or multiple piles).

It's too complicated for me to explain but it basically means the fact that the second measurement machine (making physical measurements of where the particles have landed on the wall) does not affect the results of whether the particles end up as two piles or multiple piles because it is 'ON' at all times. Meaning 'if' in both results the machine is manipulating the particles, it is doing so in the same way, every time. Machine one (refer to the 'sensor' in the above video) doesn't matter because it is not physically touching the particles. --> The given explanation in the video I watched explaining this phenomenon, which I assume is correct.

In any case, the conundrum becomes why does measurement kill a quantum particles 'superposition'? or pretty much,

"How does 'measurement' work in quantum mechanics?"

Answer this question and you'll get a Nobel Prize.

[u/MaxxBreak]

I don't get it. Is it the fact that they are simply being measured, regardless of how they're being measured? Or is the apparatus somehow influencing the result i.e. some sort of field which makes the atoms behave differently? For instance, when measuring radio waves, a device emitting radio signals as part of the apparatus is going to influence the result, not simply because you're measuring them but because the tool used to measure it is influencing the result?

Edit: My lack of understanding stems from me asking the question:

How can we be so sure that it's not the WAY we're measuring or observing the data and it is simply the fact that we HAVE observed it.

[u/bonrmagic]

I don't understand bow ties.

[u/InformedChoice]

Feynman's lecture on the subject was pretty definitive as far as I remember.

https://www.youtube.com/watch?v=2mIk3wBJDgE&t=4s

[u/favix]

For the people that don't understand this video, this one is way clearer.

Edit: Heard from many people that this video might be somewhat false?..