Some concepts in Physics can leave you completely befuddled while others will mesmerise you. In any case, whenever you deal with Physics you can’t help but be captivated by its ideas and principles and applications. Incidentally, have you ever pondered on the origin of the word ‘mesmerise’? A certain Franz Anton, German physician of the late 18th Century, postulated that there exists some sort of energy, which could be transmitted between animate and inanimate objects, that he termed ‘animal magnetism’. A charlatan, he used this made-up concept to con gullible aristocrats in part-taking in his experiments to cure them of their ailments with magnets. Franz claimed that a magnet could influence the internal ‘fluids’ of his patients and thus provide relief when the fluids would regain their natural balance. He claimed to be able to cure anything from muscular pain to blindness. Even though he managed to exploit his (mostly female) patients, some would even go as far as providing testimonies backing his ridiculous practices. These patients were completely enthralled, it seemed, by what Franz could achieve with his miraculous magnets. Or, as we might say, the patients were mesmerised by Franz Anton Mesmer.
One concept which has undoubtedly been the root of many a physicists’ insomnia is the wave-particle duality of matter. What that means is that the typical way of viewing the world as being composed of waves (light, sound, etc.) and particles (atoms) is not so clear anymore. The line between what is a particle and what is a wave is sort of fuzzy. Electrons whizzing around an atomic nucleus is not just a herd of individual tiny particles circling the dense nucleus like planets revolving around the sun. By exhibiting wave-like characteristics, these electrons can also be viewed as a ‘sea’ of electrons with the ripples or waves in that ‘sea’ corresponding to the individual electrons we speak of when referring to them as distinct particles. Is your head starting to split already? Wait, there is more.
Imagine a closed box with a small slit on its lid. Imagine, also, that the bottom of the box is transparent so that you can view underneath it and observe what accumulates at the bottom when bucket loads of sand is poured through the slit using a funnel. A simple enough experiment, right? What we expect to see is a hill of sand forming at the base of the box with the peak of the hill being just below the slit. That’s what we call a normal result, right? So far so good.
There is a similar experiment which is carried out using water flowing through a narrow slit into a basin. As the water waves pass through the slit, they come out of the other side spreading and overlapping with one another as they spread. This overlapping or interference of the waves is called diffraction. (Please see Compact for more on this.) What is then observed is a pattern formed from the diffracted wave. A pattern like the one below:
The two patterns are very distinct and relates to two distinct things. One is particulate in nature (sand) while the other is undulatory (water). If instead of sand we used discs, you could imagine the discs stacking one on top of each other as they fall, forming a nice column. You wouldn’t expect them to form multiple piles, one next to the other, with the tallest one in the middle. Yet, when we fire electrons from an electron gun onto a screen through a narrow slit, we don’t observe electrons piling on top of each other, so to speak, but we obtain a rather neat pattern resembling the one formed by diffraction. Not unlike the one woven by the water waves. Odd, isn’t it? How does that happen? In the diagram below, the black bands represent the bright area where electrons have accumulated whereas the white bands represent the portions where electron didn’t hit the screen.
Why should the electron display such an ‘unnatural’ characteristic? It is a particle, after all. Right? Well, no. It sometimes behaves as if it is a particle while at other times, it behaves as if it is a wave. It all depends, really, on how we observe the electron. When the electrons pass through the narrow slit, they essentially take all possible paths towards the screen as opposed to travelling in a straight line, like the falling grains of sand. As such, the electrons spread out all over (even outside the box!) as though they were waves spreading out while emerging from the slit. And like the waves they interact with themselves and, as a result of that interference, form this characteristic diffraction pattern.
What’s more interesting is that you could observe this behaviour with just one electron. What that means is that the electron (in its wave-like state) interacts with itself while simultaneously exploring all possible paths to the screen. Some paths are more likely to be taken than others. But generally speaking the electron tends to be mostly in the centre with some likelihood of whizzing to the sides. So, if we repeated this experiment with bucket-loads of electrons – well, not quite; you can’t fill a bucket with electrons but you catch my drift. If we were to flush a torrent of electrons through a slit then some will end on the side but most will be in the centre. And eventually, after millions and millions of electrons have been fired from the electron gun, this amazing pattern will form. Electron diffraction, thus, is a very real phenomenon just as a water wave or light wave or sound wave diffraction exists.
Yet there are cases where an electron behaves just as if it were a particle when it is fired onto a metal plate and knocks a particle of light off the metal! This phenomenon, it turns out, can only be explained if the electron is considered to be a particle. The emission of photons, or light, by impinging electrons onto certain metals is called the photoelectric effect. A rather curious phenomenon for which Albert Einstein’s explanation won him the not-so-insignificant Nobel Prize in 1921! I’ll go over this in more detail in another blog. It suffices to say that some things in nature display a rather baffling dual nature. We observe this quantum behaviour with elementary particles, mostly. (I touch on this in Quantum.) You wouldn’t expect to observe this double reality with more mundane objects such as bananas and books.
Physics is full of such perplexing yet amazing phenomena. And what is even more interesting is that we can make sense of it all. It might not be ‘common sense’ or straight-forward but with the right approach it all becomes clear. The world is not so mysterious after all. That doesn’t make it any less mesmerising…