What is quantum physics? Why should we care about knowing what is quantum physics? Do we NEED to know what quantum physics is? How does quantum physics affects our daily lives? These questions, and many more, come to mind when one thinks of quantum physics. Which questions should we try to answer first? We will first settle the issue of know what we are talking about: quantum physics. Understanding the story of the birth of quantum physics will help us understand what quantum physics is.
Quantum physics is the physics of the microscopic world; the world of molecules, atoms, and sub-atomic particles. It is a branch of physics that was born in the early decades of the twentieth century, with its seeds germinating in the late decades of the nineteenth century. Quantum physics was born out of a mixture of failure to understand certain experiments, and also out of non-sense results when applying classical physics to atoms. Yes, physicists were already familiar with the notion that matter is composed of microscopic packets, the atoms. This notion was initially put forth by the Greek philosopher Democritus who postulated that everything is built from atoms and the void. Later on, chemists starting in the eighteenth century with English chemist John Dalton, and after carefully studying chemical reactions and the proportions of reagents that go in and the proportions of the resultant chemicals, correctly inferred that matter is built of individual packets, which he called atoms! Dmitri Mandeleev in the nineteenth century classified atoms by their chemical properties and arranged them into the periodic table of elements that we are familiar with today. We will see as go on with our story that the chemical properties of atoms are a direct consequence of quantum physics!
Physicists knew that matter is overall neutral: made up of equal of positive and negative charges. So after verifying that matter is made up of atoms, experiments focused on understanding the architecture of the atom. Basically, understanding how the negative (electrons) and positive charges (protons) are arranged in atoms. Enter New Zealander physicist Ernest Rutherford in the first decade of the twentieth century who shot alpha particles, helium atoms that that have lost their two electrons, onto thin gold foil. Rutherford observed that the alpha particles (positively charged) were deflected from positively charged point particles, of much higher charge. The picture became clear now: atoms are made up of dense positively charged nuclei surrounded by orbiting negatively charged electrons.
All this is fine and nice and a source of confidence, so much so that many physicists at the beginning of the twentieth century were predicting the deat* of physics! Their opinion went something along these lines: “It is only a matter of a few years when we know all that we need to know about the fundamental physics of Nature. We know everything. It is now just a matter of applying our physics to the world around us.” And that is exactly what they did, but nasty surprises awaited them! Follow our Youtube channel, Insane Curiosity, to learn about more nasty surprises awaiting scientists even today!
Just a decade before Rutherford conducted his experiments, German physicist Max Planck was studying radiation emitted by the so-called blackbody. In physics a blackbody is an object whose peak emission wavelength, or in other words color, is related to its temperature. Imagine a piece of iron that you throw in a fire. As it gets hotter it first glows red (long wavelength), and when it gets even hotter it glows even brighter but its color will become blue (short wavelength). This piece of glowing piece of iron is referred to by physicists as blackbody.
What Planck was pondering was what happens if one were to increase the temperature of a blackbody even further. The relevant physics known at the time, thermodynamics, predicted that the blackbody should glow yet brighter but in the ultra-violet. The color of the blackbody should not only shift toward shorter and shorter wavelengths, but its brightness should become increasing higher very fast, so fast that it reaches infinity! But this is non-sense. How can heating an object, basically pumping a finite amount of energy into it, make it emit an infinite amount of energy? This is forbidden by the law of conservation of energy. Planck’s success was postulating that radiation, or light, is QUANTIZED in individual packets of energy, called photons.