University of California - Double Slit Experiment lyrics

For the next few videos, we'll focus on a very simple experiment that demonstrates the strange behavior of nature at the atomic level. This is the double-slit experiment that many of you are probably familiar with from high school physics where it's used to illustrate the wave nature of light. Now, the nature of light was a source of great confusion for physicists over the centuries. Newton believed that light was a rain of particles, which he called corpuscles. Later there was a lot of evidence that light actually travels as waves. But in the beginning of the 20th century Einstein discovered the photoelectric effect which showed that light is transmitted in discrete packets, which we now call photons. Now, there was a similar kind of confusion about electrons, which were first believed to be particles. But then there was evidence that they behaved like waves in phenomena such as electron diffraction. And so there was a growing confusion about the nature of atomic particles. Were they wave like or particle like? And this confusion did not get resolved until the mid '20s when the laws of quantum mechanics were discovered, which actually showed that atomic particles are neither waves nor particles. They behave in their own strange quantum mechanical way. And it's this strange counter-intuitive quantum mechanical behavior of atomic particles that we'll try to describe through the double-slit experiment. Now, this experiment really illustrates many of the features of quantum mechanics. And since it does so in a very simple context, many of you will find this intuition to be very useful as we go along. So many times in the future we'll be able to point back and say, remember this double-slit experiment? It's just like that. On the other hand, some of you might find this intuition not very useful because you might feel that it takes too many strides. Some of you might find it difficult to relate to the double-slit experiment. And some of you, especially with a computer science background, might prefer the very simple description of quantum bits that we'll start with in the next lecture. That's going to be entirely self-contained. So if you have trouble following the intuition from this lecture, don't worry. We'll get to qubits soon. OK, so let's start with the double-slit experiment. So in this experiment we have a source of either light or electrons. So either a source of photons or of electrons. But now the source, its intensity is turned down so low that it emits these photons or electrons as a discrete particles once every so often. So let's say once every second or once every 15 seconds. Now, the apparatus also has--a long way from this source--it has a screen with two slits in it. And then a long way from this screen, there's a backstop along which we place a detector at some point on the screen, which we'll measure by this variable, x. And so what we are interested in--let's say we are doing the experiment with electrons. So what we are interested in is what's the probability that this electron is detected at a point, x, on this back screen? So now we'll first do this experiment where we block one of the slits. So if you block slit number two, then the probability we see the electron at point x varies according to this curve, P1 of x, which has this behavior because when what we'd expect. Because when the electron goes through slit one, it gets scattered by the slit and so it goes in some direction. But it's most likely to go along the straight line path. And so you get this peak here, and then the probability drops off as you go further out. There's a similar such curve, P2 of x, when we have only slit two open and slit one is closed. Now, what happens when we open both slits? Well, what we'd expect to see--since the electron can go either through slit one or through slit two--is to sum of these two curves, so something like this. What we actually observe is this interference pattern. So let me call this P12 of x. This was P1 of x. And that's P2 of x. The mystery is how could it possibly be that when both slits are open, we got this strange interference pattern? Let me put it to you more succinctly. So let's look at a particular point on this screen. Let's say we look at this particular point on this screen. So this is a point where there was a substantial chance that the electron got here when only slit one was open. There was a substantial chance that the electron was detected at this particular point. So now we would say to ourselves, if the electron went through slit one, how could it possibly matter to it whether slit two was open or not? And if it did not matter to it, how could it be that the probability of detection of that electron went down from a substantial value to being very close to zero? This is really, in a nutshell, this encapsulates the mystery of atomic particles. How could they possibly behave, what could nature possibly be doing behind the scenes that makes atomic particles behave in this way? This mystery is illustrated by this quote from the great physicist, Richard Feynman, who once said, I can safely say that no one understands quantum mechanics.