One of the first and most effective tricks of teaching any subject is making analogies. A concept that initially doesn’t click may make more sense when compared to a common experience or narrative. In the sciences this can come up a lot, where the mechanical motion of balls can be compared to things we’ve seen in sports, or laws of heat transfer compared to experiences cooking food. Scientific inquiry is, among other things, an inquiry into the rules of the world around us, so it makes sense that while exploring those rules, we try to place them in the context of that world.
But as we examine the world at a smaller and smaller scale, looking at components much tinier than we have any normal experience thinking about, the rules begin to change. As you approach the scale at which the constituents of matter become indivisible, the behavior of these constituents is nothing like what we are used to at our scale, the macro scale, with bouncing balls and boiling water. This is the quantum world, so-called because many aspects of reality are quantized and discrete rather than continuously variable . And the trouble with learning about the quantum world is that there are very few analogies to the world we know, even though the quantum world is what underlies our own!
One example of weirdness in the quantum world is particle-wave duality. This terms stems from the history of research on light: from the 17th century through the early 20th, many of the best scientific minds were divided on whether light was a particle or a wave. Initially light was assumed to be an indivisible particle, observed to travel along straight lines. But the way that light can bend, or refract, around corners seemed more similar to the behavior of a wave. An experiment was devised to test which theory was true, with two narrow slits cut into a thin plate, in front of a screen. Classical particles would be expected to form an image of the two slits on the screen, whereas a wave traveling through the two slits would form a complicated interference pattern on the screen due to overlapping wavefronts. When light is sent through the double slit, the interference pattern is seen, implying that light is a wave. But, if you set up a detector to determine which slit each light particle is passing through, then the screen shows an image of the two slits. Thus light has both a particle and a wave nature, and in fact this is true for many other “particles” such as the components of atoms: protons, neutrons, and electrons. Each of these particles has wave properties as well.
But something about the double slit experiment may have struck you as even stranger than light being both a particle and a wave. It appears that whether we see evidence for the wave or the particle nature of light depends on the measurement we do: if we measure which slit the light passes through, we see particles, and if we do not, we see waves. It turns out that another example of strange quantum behavior is the importance of measurement.
Within quantum mechanics, the mathematical system devised to describe quantum behavior, it turns out that in addition to discrete quantum states, it is possible to have superpositions of states. So, in addition to having an atom aligned to the left or to the right, we can have a state which is “left plus right”, that is not just zero. There is no analogue for this idea in the macro world, and the whole strange setup leads to the question: If we measure the state “left plus right” for alignment, what do we get? Left or right? It turns out that we have an equal probability of measuring either one, but what’s even stranger is that before we perform the measurement, the system is not really in either state. It is in both, and neither. The measurement itself causes a fundamental physical property of the system to change, meaning that the traditional scientific idea of observing a physical system without affecting it must be discarded!
Both of these ideas, particle-wave duality and the importance of measurement, have a lot of tricky implications and can be explored in much more detail than I’ve given so far. And they take awhile to wrap your head around! For many people, their first reaction to these concepts is a creeping unease, an inability to place these in any “real world” context and a sense that something must be wrong. To me, that is precisely what is so fascinating about the quantum world. It’s completely unlike anything that we have practical experience with, and yet it has been verified by experiment after experiment that the quantum world is a reality that underlies everything we see, a bizarro-land hidden behind all the things with which we are familiar. What a place to explore!