2. Weak and strong field measurement

Clearly, the use of the terms "weak fields" and "strong fields" is arbitrary and relative. To understand this context, let's take the example of the measurement of electromagnetic phenomena of natural origin. Let's start with radio astronomy, where the observation of clouds of interstellar matter is usually carried out by measuring the emission or absorption of the 21-centimeter wavelength line of the hydrogen atom. These electromagnetic events from the cosmos are akin to incoherent noise, exposed to such amplitude dispersion that the electric fields collected on terrestrial antennas are below µV/m. In these extreme circumstances, measurements can only be made using high-gain receiving antennas. Like antennas, the reception of sub-µV voltages requires the use of low thermal noise amplifiers. Although the 21 cm wavelength is carefully protected from the risks of anthropogenic pollution,...