A Quantum Century D. Indumathi, The Institute of Mathematical Sciences, Chennai We have all learnt about Newton and his laws of mechanics. They decide how apples fall from trees, how you can balance on your cycle, and how space ships can reach Moon and Mars. But there are very different laws that govern physics on a very small scale, on scales that are even smaller than a virus; these are the atomic scales. For instance, a hydrogen atom has a radius of about 1 Angstrom; if you divide a metre into 1 billion parts, each piece is 10 Angstroms and can fit 10 hydrogen atoms in a row! These are such small objects that you cannot see them with your eyes, or even a microscope. But there are even smaller objects in the Universe. These are the protons and neutrons that are in the nucleus of atoms, and are 100,000 times smaller than atoms themselves! The laws that govern the motion of objects at such small scales is determined by a new kind of mechanics called Quantum Mechanics. The traditional understanding of Newton is called Classical Mechanics. You must have learned about atoms and their nuclei in school. And that the electron lies outside the nucleus. Perhaps you've even learned that the electron goes around the nucleus. This picture is convenient but not totally correct. In college you will learn that you should actually think of the electron as a kind of fuzzy cloud surrounding the nucleus. You cannot really tell exactly where it is at a given point in time. This is because of a property of quantum mechanics that was pointed out by Werner Heisenberg, called the Heisenberg Uncertainty Principle. Briefly, it states that there are pairs of variables, the most common among which are the position and momentum (magnitude, not direction) of a particle. In quantum mechanics, both these pairs of variables cannot be measured simultaneously and accurately. If one is measured more accurately, the other can only be measured approximately. Let us take an example of a car going on the road. You can measure its position and its velocity (and hence momentum) easily enough. But as the car becomes smaller and smaller, and shrinks to the size of an atom (now stretch your imagination!) if you pin-point its position, then its momentum will be unknown and vice versa. In fact, if you measure the position of an electron exactly*, then Heisenberg's principle says that its momentum is fully unknown. Hence we have the notion of an *electron cloud* around a nucleus rather than an electron going around and around it. In 1927, two years after Quantum Mechanics was born, physicists gathered to discuss its impact at the famous Solvay Conference on Physics in Brussels. This picture shows many important physicists who were in this meeting, including Werner Heisenberg (back row, third from right) and Erwin Schrödinger (back row, sixth from right.) Albert Einstein is front row center. The Birth of Quantum Mechanics Beginning around 1900, many people contributed to the birth of Quantum Mechanics. It started with trying to understand what was called Black Body Radiation: a perfect black body (literally black) absorbs 100 % of the light falling on it. It then proceeds to emit most of it; in fact, more than other coloured objects can emit. Although it emits that radiation in all frequencies (visible to invisible infra-red and ultraviolet, etc), the peak* of the frequency depends only on its temperature. In trying to understand this, Max Planck came up with a new law which gave new insights into the phenomenon. He said that light (or radiation) can only be emitted in fixed energy units called a *quantum*. Around 5-10 years later, a variety of experiments by Rutherford and colleagues showed that the atom has a central, very dense nucleus. Can you imagine that about 100 years ago, no-one knew that nuclei existed?! Neils Bohr proposed a model which could explain how an electron is bound to a nucleus (a hydrogen atom is a positively charged proton with a negatively charged electron, isn't it?). You will learn about it in Class 12. He not only explained atomic binding (and hence ionisation energy) but also explained how atoms radiate energy. However, this left a deep puzzle in the minds of scientists. Since electron and proton are oppositely charged, and attract each other, how come the electron stays bound to the proton in a Hydrogen atom without "falling in" to the nucleus? This was the problem that was solved 100 years ago by Heisenberg and signalled the birth of Quantum Mechanics. While Heisenberg's approach used mathematics of matrices, Erwin Schroedinger independently came up with a different forumation of Quantum Mechanics using wave theory. In this approach, you think of the electron as a wave spread out in space. If you measure its momentum exactly, then the wave is spread over infinite space. As you squeeze the wave in space and localise it more and more, the measurement of the momentum gets worse and worse. While Schroedinger's approach is more physical, it was soon established that both Heisenberg and Schroedinger were solving the same problem. However, the situation was very messy, because it was a radically new theory and an entirely new way of looking at our world. Over the years and in fact decades, many scientists clarified the essential aspects of Quantum Mechanics. Many reformulations have taken place and improved our understanding of physics at small scales. There is yet another complication: that of measurement. Quantum mechanics states that the very act of measurement can affect the state of the object. This is called the measurement problem and is closely related to how the wave function of the electron (or object) is used to determine its properties such as position and momentum. According to Quantum Mechanics, the electron in the hydrogen atom can have several possible values of position and/or momentum. Only when we measure it will the electron "collapse" into one of the possible values. This was known as the Copenhagen interpretation of Quantum Mechanics. While this remains very popular, many scientists have challenged this view point because it can lead to aburdities. One of the early examples that highlighted the problem with wave function collapse is called the Schroedinger's cat example. Read about it in the next article. The Impact of Quantum Mechanics in today's world Quantum physics has since permeated a wide range of scientific disciplines. For instance, it can explain the periodic table, which you would have studied in Chemistry. It allows us to understand how stars are made, and how they die in massive supernova explosions. It also touches our lives closely because it has enabled all kinds of technology from the laser to the smartphone. And the new buzzword of this decade of course is Quantum Computing. But more on this in a later issue of JM. Sources: Sciences News Quantum Physics (also for image of scientists from the Solvay Conference), Wikipedia