The frequency boundaries of the radio spectrum are a matter of convention in physics and are somewhat arbitrary. Since radio waves are the lowest frequency category of electromagnetic waves, there is no lower limit to the frequency of radio waves. At the high frequency end the radio spectrum is bounded by the infrared band. The boundary between radio waves and infrared waves is defined at different frequencies in different scientific fields. The terahertz band, from 300 gigahertz to 3 terahertz, can be considered either as microwaves or infrared. It is the highest band categorized as radio waves by the International Telecommunications Union, but spectroscopic scientists consider these frequencies part of the far infrared band.