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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.

The Scale of Ultra-Low Frequency Waves

A megametre (Mm) equals 1,000,000 metres (10⁶ m) and is used to describe extraordinarily long wavelengths found in the ultra-low frequency (ULF) and extremely low frequency (ELF) bands of the electromagnetic spectrum. These wavelengths correspond to frequencies less than a few hertz, often in the range of millihertz to a few hertz. At this scale, wavelengths span hundreds to thousands of kilometres, extending into the megametre range.

Waves with megametre-scale wavelengths are critical for studying natural phenomena such as Earth’s magnetospheric oscillations, geomagnetic pulsations, and seismic electromagnetic signals. These frequencies and wavelengths are also important in geophysical research, allowing scientists to monitor changes in the Earth’s magnetic field and space weather effects. For example, a frequency of 0.1 Hz corresponds to a wavelength of about 3,000,000 metres, or 3 Mm.

Because of their immense scale, megametre wavelengths are not used for typical communication systems but are crucial in understanding planetary and space environments. Using the megametre unit helps researchers conceptualize and quantify these gigantic waves, linking electromagnetic theory with geophysical observations and space science.