Speed of light they appeared 13 billion years ago, when the universe was less than a billion years old. The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the evolution of stars, of galaxies, and of the universe itself. Astronomical distances are sometimes expressed in light-years, especially in popular science publications and media. A light-year is the distance light travels in one year, around 9461 billion kilometres, 5879 billion miles, or 0.3066 parsecs. In round figures, a light year is nearly 10 trillion kilometres or nearly 6 trillion miles. Proxima Centauri,
3-j symbol and formula_21 integers. Similar relations exist for the spin-weighted spherical harmonics if formula_22: For formula_25 a non-zero 3-"j" symbol has where formula_27 and formula_28 is a Wigner function. Generally a better approximation obeying the Regge symmetry is given by where formula_30. The following quantity acts as a metric tensor in angular momentum theory and is also known as a "Wigner 1-jm symbol", It can be used to perform time-reversal on angular momenta. Wigner 3-j symbols are related to Racah -coefficients by a simple phase: 3-j symbol In quantum mechanics, the Wigner 3-j symbols, also called 3"-jm" symbols, are an alternative