Updated: Dec 8, 2020
We humans have come a long way with regards to experiments on various fronts, one experiment in particular was very fascinating to most of us, so much so that it transcended our ways of understanding reality, it was the famous double slit experiment. But, before we talk about the ways in which the double slit experiment changed our views, it's necessary to point out one key factor regarding atoms, which has a very different explanation when looked at, through the lenses of quantum mechanical aspects.
Me, being an undergraduate student, I have a rough time trying to comprehend any of it, nevertheless it is a huge leap for me from something like Newtonian mechanics. The classical model of the atom, as most of us know it, brought forth, showings us that the atoms consists of electrons that revolve around the nucleus, just like planets revolve around the sun in the solar system. This very model also tells us that most of the space in the atom is just empty, with most of the mass being concentrated at the center of the atom. But, it's a completely different interpretation of the atom when it comes to quantum mechanics. The quantum physicists tells me that, no, most of the atom is basically a cloud and it's described with the help of a wave function, since electrons are basically waves. Now, again the wave function has a weird feature to it, when you are not looking at it, it's a wave its all spread out or it is localized somewhere and it obeys the Schrodinger equation and all of it fits like the classical ways of predicting things, the problem comes when you measure it, because there is some level of you interacting with the electron. That was one of the outcomes of the Copenhagen interpretation, fields i.e., waves is what we are all made of and particles are what we see.
Coming to the famous double slit experiment, if you have two slits and you let marbles go through it, you get two lines on a screen placed right behind the slits. Whereas, if you let a wave go through two slits you get an interference pattern, one wave, from one of the slits cancels out the other and you get a band of lines depending on the regions where the two waves cancel each other and respectively where the crests are formed. And so what happens when you let an electron go through those very slits, the answer is you get an interference pattern , inferring that the electron is more like wave than a particle.
But, the real weird thing is, if you let an electron go through two slits and you place little detectors on the slits, in order for you to predict or say which slit did the electron passes through. Then the experiment always says that the electron goes through one or the other, and never goes through both, as a consequence the interference pattern on the other side disappears. You only see two lines that you would have seen if the electrons were marble like, so the point is when you're not looking the electron is acting like a wave and when you look at it the electron acts like a particle that's the conclusion of the double slit experiment. When you put the detectors on the slits, you have interacted with the electron and you have in some ways localized it, There's no such thing as the position of electron, there's no answer to the question, did electron go through one slit or the other?, there's only a cloud, there's only a wave going through, but you affected it with the detector, it affected the electron when looking through the slits to see, whether it i.e., the electron went through one or the other slit and that reason transposed the electron from going through both slits, to only going through one of the two.
That phenomenon is called quantum entanglement, the detector gets entangled with the electron and this is where physicists came up with a version of quantum mechanics, which is the many world's interpretation. The right way of thinking about that electron was that cloud like atom which is a wave going through the slits, that's the natural representation by quantum mechanics. The weird part in this view would be that, when you look at the different slits you see the electron go through one or the other and it acts like a particle. In other words, our natural intuitive way of thinking about electrons is as particles, little marbles and quantum mechanics says that's not the precise picture, the electron is naturally a wave, the gizmo in this case is when it acts like a particle. The many world's theory would say, when you look, did the electron go through one slit or the other, you and the detector have become entangled with the electron. What that means is that the wave function of the whole universe, the wave function of both you and the detector and the stars and galaxies and so forth, splits in two and so there's now one branch of the wave function that acts like its own separate world which says the electron went through the left slit and your detector saw the electron go through the left slit and as a result, it made a little band on one side, there's another branch of the wave function, which says the electron went through the right slit and it made a band or line on the other side and so they're both still there, the world split in two and you're in one of them, Both results take place, the only reason you don't see both is because you don't see the whole world anymore.