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In modern physics, many concepts have their origins in everyday life; the deepest and most famous example being Symmetry. This talk will discuss another example – Size. Everybody knows what "size" means. For example, in general, an adult is larger than a child. One common observation – when an adult and a child both stand under the sun, they cast shadows of well-defined sizes. The shadows have well-defined sizes because all the visible colours in the white light of the sun have wavelengths much smaller than the dimensions of the bodies. It is actually remarkable that the sizes of the shadows are independent of the wavelengths in the visible range, something usually taken for granted.
In the language of physics, the areas of the shadows are proportional to the bodies' total cross sections. Total cross sections can be defined for many objects, including nuclei, proton and electron. The total cross section of a given object changes as we go from microwaves to visible light to X-ray, etc. The reason is because it depends on the wavelength. In quantum mechanics, both light and particles have wavelengths, with shorter wavelengths corresponding to high energies. A fundamental question is this: how does the total cross section for a proton, for example, depend on energy? Such questions and other related issues will be discussed.
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