The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation. Electromagnetic radiation can be divided into octaves — as sound waves are — adding up to eighty-one octaves. Physicists have studied electromagnetic radiation with wavelengths from thousands of kilometres down to fractions of the size of an atom. It is commonly said that EM waves beyond these limits are uncommon, although this is not known to be true. The short wavelength limit is likely to be the Planck length, and the long-wavelength limit is the size of the universe itself, though in principle the spectrum is infinite. 