According to the International System of Units, the modernized form of the metric system, a GHz is a gigahertz, meaning a cycle operating at a billion times per second, while a Mg is a megagram, the official designation for a tonne, also called the metric ton, equivalent to 1000 kilograms. So a GHz and a Mg refer to units measuring fundamentally different things: the speed of a cycle and weight.
The unit GHz is commonly used in the study of electromagnetic spectra, which light with a frequency of a GHz falls into the microwave portion of the spectrum, with a wavelength of about 20 cm, and in measuring the clock speed of computers, which is often a few GHz. Contrary to popular belief, the clock speed of a computer is not directly related to its computational performance. This misconception has been referred to as the Megahertz Myth. As of 2008, the best personal computers operate at a clock speed of about 3 GHz. This is sure to increase to 10 GHz and beyond in coming years, as faster computers are built to meet consumer demand. Some molecular vibrations are in the GHz range.
The megagram, or metric ton, is 1000 kilograms or approximately 2205 lbs. This unit is used to measure quantities in agriculture, industry, petrochemical extraction, structural engineering, and many other areas. The Empire State Building has an estimated weight of 336,000 metric tons, while the International Space Station will have a weight of 471 metric tons upon completion. A car weighs about 1-2 tonnes, while an elephant weighs 3-12. The Hubble Space Telescope weighs 11 tonnes. Launching a tonne of material into space costs about $14 million USD.
The "giga" in gigahertz and the "mega" in megagram both come from Greek words meaning "giant" and "great" respectively. When used in the SI system of units, "mega" denotes a million, while "giga" denotes a billion. Other even larger SI prefixes include tera-, which signifies a trillion, and peta-, which indicates a quadrillion.