Timescales, Physical

The science of physics encompasses many orders of magnitude. This fact is witnessed impressively by the three most central entities of physical law—length, mass, and time. For example, the length or size of objects covered by experimental physics ranges approximately from 10−5 meter = 1 Fermi, the distance between two elementary particles in the atomic nuclei, to 3 × 1025 meters (m), the diameter of the [Page 1358]universe. Flanked by these cornerstones one finds characteristic lengths such as the distance between two atoms in a solid (a few 10−10 m), the average distance between two air molecules at normal pressure and room temperature (1 μm = 10−6m), and the thickness of a human hair, which is typically about 60 μm = 6 × 10−5 meter. In comparison, the diameter of the earth is about 13,000 kilometers (km), or 1.3×107m; the distance to the sun measures 150 million km (1.5×1011 m), and the distance to the next star, Proxima Centauri, is 4×1016 m.

The concept of mass comprehends an even wider scope: Between the mass of an electron (9×10−31 kg) and the estimated mass of the universe (10∼42 kg), there is an incomprehensible gap of 72 orders of magnitude. For example, a protein molecule weighs about 10−21 kilograms (kg), and a bacterium amounts roughly to 5×10−16 kg. One cubic centimeter of air at normal pressure and room temperature weighs 1 milligram (10−6 kg). Of course, the laws of motion also apply to very heavy objects such as airplanes (100 tons = 10−5 kg), or for the revolution of the earth (6×1024 kg) around the sun (2×1030 kg).

The range of the third fundamental entity of physics, time, is the subject of this article. Similar to length and mass, the concept of time obeys many orders of magnitude, or timescales, varying from the rapid electron dynamics in the atom (10−18 s) to the slow dynamics of cosmology (10−17 s). The 35 orders of magnitude in between—a factor of one hundred million billion billion billions!—host the full variety of microscopic and macroscopic phenomena.

The diversity of physical timescales is like that of physics itself. It cannot be the aim of this entry to describe every possible temporal dependency of physical interactions, as this would imply reducing the whole of physics to a few pages. Instead, it presents an overview of a selected number of characteristic timescales in the form of a viewgraph (Figure 1), where each time is representative of certain classes of physical events (e.g., the cosmo-logical scale or the mechanical scale). Following the time arrow in Figure 1 from the very slow to the very fast, every timescale is specified by important examples governed by physical law. This entry will furthermore explain how our perception of time is related to the perception of other physical entities, which allows us, for example, the use of the spatial coordinates of the clock's hands as a measure for time. Particular attention will be paid to the occurrence of periodic events in nature, which are the basis of any type of time measurement. It will turn out that our human definition and classification of time is in fact the natural result of our limited senses, determined by the physical conditions we are encountering on earth.

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