FIGURE 1. Noise from the Earth's atmosphere and the minimum noise from our Galaxy, versus wavelength.

The vertical axis is in kelvins, which is proportional to power per unit bandwidth. The plot illustrates the low noise floor of the region between 3–60 cm, explaining its popularity among proponents of SETI. The curve marked 'minimum receiver noise' is ten times the theoretical minimum for radio-type receivers; the noise of many radio-type receivers approaches this value. The 3-K microwave background is the remnant radiation from the Big Bang¹⁴. Interstellar dust absorbs optical radiation, but not radio waves, so in the plane of our Galaxy radio signals can penetrate to great distances¹⁴.