Reality → Scale → Micro world → Subatomic
The weird and fantastic subatomic world of 'elementary' particles is the world of the very small and the very fast, the world in which the rules of Einstein's theory of special relativity apply, a frontier in the search for answers to ultimate questions about essence and workings of the physical world [1] . Proceeding from the atom's outer boundary to its nucleus involves already a scale of four orders of magnitude (if we imagine an atom blown up to the size of a stadium, then the nucleus would be the size of a hazelnut). Depending on the element, a proton or neutron may be about one order of magnitude smaller than the nucleus. Quarks, gluons and other elementary particles may still be orders of magnitude smaller or have no size at all. Nobody knows where or how the micro world ends [2] .
From studying the world of the ultra-small, astrophysicists gain at the same time insight into the world of the ultra-large, the origin, life and death of stars, and the structure and history of the universe.
The string theory of theoretical physics assumes incredibly tiny lengths (down to the Planck length, about 10-35 m). However, nuclear and particle physicists do not determine the size of elementary particles in fractions of a meter, but instead measure energy expressed in mega- or teraelectronvolts (see Mass-energy equivalence). Concepts of particles, waves, and fields are used to understand the world at the most fundamental level. The many worlds interpretation of quantum mechanics is compatible with the string theory and the fantastic, unprovable idea of a multiverse (see also Electromagnetic radiation, Note 3 and Particles, Note 4).