Cary Chin, of Wellington, asks :-

What stops an electron, say in hydrogen atom, from being drawn into the nucleus since it is attracted by the proton charge? (Is it only speed momentum like planets' centrifugal forces, and the centre of the nucleus is so small that it misses the target and spins around the nucleus? or is it something else?)

Daniel Schumayer, a physicist at the University of Otago, responded.

Every day we make predictions, intentionally or unintentionally, on how objects behave, e.g., where the ground is when we step forward, or how long it takes for a car to reach our location. A commonality of these phenomena is their temporal and spatial scale, i.e., they are measured in seconds and metres. Let us call the physics of these kind of phenomena "human scale physics." We got used to this physics so much, however, both at much larger and smaller scales, e.g., in galaxies and elementary particles, nature does not behave as we would anticipate. This is the very case in your question.

Your question is not only intriguing, but quite important in the history of the physics of microcosm. After electrons and protons have been discovered, physicists asked the same question. Since they knew about electrostatic attraction between unlike charges, it seemed logical to assume that this attraction keeps the electrons whizzing around the much heavier protons. Thus a microscopic version of the solar system has been hypothesised; electrons are orbiting around the nucleus on fixed paths.

According to "human scale physics" an electron moving on such an orbit behaves as a small antenna and radiates its energy out to the world. Thereby it loses energy and must fall into the nucleus in well within a second. However, we are still around and relatively stable elements form our everyday objects. We can resolve this apparent contradiction by concluding: the solar system model is incorrect. We cannot apply our "human scale physics" rules to this microscopic world.

Other experiments had indicated that small particles produce similar phenomena as waves do. These observations gave the idea to physicists to think of very small particles, such as electrons, protons and neutrons, as small wave packets. In this picture the electron forms a diffuse cloud around the nucleus (which is also a diffuse cloud itself, but smaller and denser). As an electron and nucleus come closer together due to the electrostatic attraction, the wavelength of the electron becomes shorter and shorter. However, the shorter the wavelength the larger the kinetic energy. At some point the kinetic energy of the electron becomes sufficiently large that the electrostatic attraction cannot pull the electron closer to the nucleus.

Finally, I mention an exception. Above we imagined an electron as a diffuse cloud around the nucleus. This cloud penetrates the nucleus, meaning that there is some small, but not zero, likelihood of the electron being “inside� the nucleus. Such event, under certain circumstances, can happen and it is called "electron capture". In this sense the electron can indeed "fall" into the nucleus.

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