By Nicole Wu 胡欣蕾
Imagine if we were casted a spell and shrank to the size of an atom, we would see a completely different “quantum world” that cannot be explained by common sense. How bizarre it could be? Let’s discard our common sense and dive into the realm of quantum mechanics.
Double-Slit Experiment & Wave-Particle Duality – Is It Wave or Particle?
Whenever we talked about quantum mechanics, we must mention the iconic double-slit experiment because it directly demonstrates the strangeness of quantum mechanics.
Let’s assume that we use an electron gun to shoot electrons one at a time to a wall at random angles, and the electrons have to pass through any of the two narrow slits before arriving at the wall. On the wall, there is a screen to accumulatively record the locations where the electrons hit it. If we let the electron gun operate for a while, what pattern will form on the screen?
According to our common sense, we should expect the pattern shown in Figure 1:
The electrons that pass through the slits form two straight bands on the wall – perhaps this is what we have expected. This prediction is still correct if we shoot objects with the size of a tennis ball or a bullet. However, this is not what actually happens if we shoot tiny particles like electrons and photons.
The pattern that actually forms (Figure 2):
The electrons seemed to have curved around the two slits and arrived at the positions that we didn’t expect. An orderly stripes pattern formed. This pattern is super familiar to scientists – it is created when waves (e.g. water waves) pass through two narrow slits due to its property of interference, like in Figure 3.
This implies that the gunshots that passed through the double slit were waves instead of particles. However, we clearly know that electron is a particle – when one electron was shot, we could only see one signal on the wall. But when the signals accumulated, an interference pattern was shown – did the electrons suddenly turn into wave when passing through the double slit but become particles again before they hit the wall?
Scientists at that time were confused by this ambiguous phenomenon because it could not be explained by classical mechanics. Later, they realized that electrons can simultaneously possess the properties of wave and particle. In other words, they show wave-particle duality1. Electrons may display the characteristics of wave and/or particle depending on the experimental setting, but both kinds of characteristics can’t be fully shown in a single experiment, e.g. the patterns in Figure 1 and 2 can’t be shown simultaneously.
To further add mystery, observation itself can affect the experiment. The pattern shown in Figure 2 only appears when we are not “observing”. If we monitor the path of each electron and know exactly which slit they pass through, the pattern in Figure 1 forms. On the other hand, if we are not “monitoring” and don’t know which slit will the electrons pass through, all the possibilities will superpose to form the resulting pattern in Figure 2. This is like a classroom – when the teacher is absent, the students can either be studying seriously or messing around; but when they realize that the teacher is watching, they will all be good kids.
This is bizarre, isn’t it? Not really, some explain this phenomenon with the analogy of the ambiguous image, “My Wife and My Mother-in-Law”. In this drawing we can see either a young lady or an old woman depending on our perspective. The drawing itself has both the young lady and the old woman drawn on it, but it is our observation that obliterates the existence of either one of them.
Superposition & Wave Function Collapse
Imagine if the students are not intelligent enough to play and study simultaneously, they can only be 100% playing or 100% studying. Then, no matter whether the teacher exists, they are either playing or studying – this is our common sense in everyday life. However, if the students are extremely tiny particles like electrons, you’ll have to abandon all your common sense and embrace a totally different reality. Before we observe these “tiny students”, we can’t say whether they are playing or studying but they are in a state of “superposition” of playing and studying – perhaps they are a piece of dark cloud, an enigma, a pixelated image, or whatever strange things that is difficult to put into words. It is our observation that forces them to display the state of either studying or playing.
In quantum mechanics, this phenomenon is called “wave function collapse”. In the microscopic world, since each particle has the characteristics of wave, we cannot specify the locations of them. “Wave” is throwing a dice with many faces: each face of the dice is labelled with a position where the quantum may appear when wave “transforms” into a complete particle. If we add up the possibilities of all the locations where an electron may exist in the next second, the sum is always equal to 1, i.e. 100 percent.
Let me explain it with another analogy. In 2 p.m. (which is within the school hours), at a school in the quantum world, students always appear simultaneously in the classroom, laboratory, playground, medical room, washroom, etc. However, if we keep an eye on a student, the “clones” of that student will disappear and the student will be forced to only appear in one place. As it’s within the school hours (and the students are supposed to be having a class), the student is likely to be in the classroom: say, 70%. But they can also be having a laboratory session or PE class, at a lower frequency: say, 20%. Other uncommon places that they may exist are the medical room or the toilet, which may add up to a possibility of only 10%. So, when we start to “observe”, the dice stops, and there will only be one possibility – although the original possibility may only be 10%, if the dice stops at “medical room”, the fact that the student is receiving medical treatment becomes the 100% truth. And this is the collapse of the wave function.
Here, we hope to cast light on the famous thought experiment, “Schrödinger’s cat”. In this thought experiment, a cat and some radioactive substance are put into a black box such that nothing can be seen from outside. Due to the wave-particle duality of the reactive substance, the substance is actually tossing a coin with the words “decay” and “remain unchanged” on either side. Eventually, if “decay” is shown, the cat dies; if “remain unchanged” is shown, the cat survives. The coin keeps flipping in midair and the cat is staying between the boundary between “alive” and “dead” – a state resulting from the superposition of them. Only when we open the black box to observe does the coin land, and shows either “decay” or “remain unchanged” to decide the fate of the cat. It’s our observation that forces the wave function to collapse. “Curiosity killed the cat”, isn’t it? – we opened to observe the black box curiously, but it forced the cat to be either live or die.
If you are still with us, then congratulations, you have entered the world of quantum mechanics. What is fascinating is the abrupt difference between this quantum world and the everyday world we know. This is just the beginning. If you continue to explore, you will discover more surprises and possibilities.
1 About wave-particle duality: Actually all objects possess wave-particle duality, including large objects like tennis balls and soccer balls. However, only the wave-particle duality of objects in atomic sizes (like electrons and photons) is apparent to us.