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Understanding Infrared and Thermal Radiation

A micrometre (µm), also known as a micron, is equal to one millionth of a metre (1 µm = 10⁻⁶ m) and is commonly used to express wavelengths of electromagnetic radiation, particularly in the infrared (IR) region of the spectrum. Wavelengths in this range are crucial for understanding heat, thermal imaging, remote sensing, and optical communications. The infrared spectrum typically spans from 0.75 µm to about 1000 µm, with specific regions divided into near-IR (0.75–1.4 µm), mid-IR (1.4–8 µm), and far-IR (8–1000 µm).

Many natural processes, including thermal emission from objects, occur in the micrometre wavelength range. For example, the human body emits peak thermal radiation at around 9–10 µm. Materials scientists, astronomers, and engineers use these wavelengths to study heat flow, detect gases, and design sensors. Optical fibers used in telecommunications also operate efficiently in the near-IR range around 1.3 to 1.55 µm. Using micrometres to describe wavelength offers a practical and precise way to work with electromagnetic waves that are too long for nanometres but still far shorter than those measured in millimetres.

The Scale of Interstellar and Cosmological Waves

A petametre (Pm) equals 1,000 terametres (10¹⁵ metres), representing unimaginably vast distances that describe the longest electromagnetic wavelengths in the universe. These wavelengths correspond to frequencies in the attohertz (10⁻¹⁸ Hz) and lower ranges, which are mostly relevant in cosmology, astrophysics, and the study of gravitational waves and large-scale cosmic phenomena.

For context, a frequency of 1 attohertz (10⁻¹⁸ Hz) corresponds to a wavelength of approximately 300 petametres. This scale is far beyond any human-made signals and reflects waves that stretch across entire galaxies or even clusters of galaxies. Such waves help scientists study the cosmic microwave background (CMB) fluctuations, the large-scale structure of the universe, and primordial gravitational waves created shortly after the Big Bang.

Using petametres to measure wavelength allows researchers to grasp the vastness of these cosmic oscillations and the slowest processes influencing the universe’s evolution. These extreme wavelengths provide crucial insight into the origins, expansion, and ultimate fate of the cosmos.