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Cosmic Scales of Electromagnetic Waves

A terametre (Tm) is equal to 1 trillion metres (10¹² m), an enormous unit used to describe wavelengths on an interplanetary or even interstellar scale. Such colossal wavelengths correspond to extremely low frequencies in the picohertz to femtohertz range and are primarily relevant in astrophysics, cosmology, and gravitational wave studies. At this scale, electromagnetic or gravitational waves can span distances comparable to the size of the solar system or beyond.

For example, a frequency of 1 femtohertz (10⁻¹⁵ Hz) corresponds to a wavelength of approximately 300 terametres, or 300 billion kilometres — about twice the distance from the Sun to Pluto. These wavelengths are far beyond practical terrestrial communication but are important for understanding phenomena like primordial gravitational waves, cosmic microwave background fluctuations, and large-scale cosmic structures.

Using terametres to express wavelength helps scientists conceptualize and study the vast, slow oscillations that shape the universe over billions of years. These extreme wavelengths offer insight into the very fabric of space-time, the origins of the universe, and processes occurring on the grandest cosmic scales.

The Realm of Extremely Slow Oscillations

The nanohertz (nHz) is a unit of frequency equal to 10⁻⁹ hertz, meaning one cycle occurs every 1 billion seconds—about 31.7 years. This incredibly low frequency is important in fields like astrophysics, cosmology, and geophysics, where slow periodic phenomena unfold over decades to centuries.

Nanohertz frequencies are often associated with gravitational waves produced by supermassive black hole binaries orbiting each other over many years. These ultra-low-frequency waves have immense wavelengths, spanning light-years across space. Pulsar timing arrays, which monitor the precise arrival times of pulsar signals, are used to detect such nanohertz gravitational waves, offering insights into galaxy evolution and cosmic structure.

On Earth, nanohertz frequencies can describe long-term oscillations in the geomagnetic field or climate cycles. Studying these slow frequencies helps scientists understand gradual changes in planetary environments and the universe.

Although nanohertz waves are far below everyday human perception and technological applications, they are critical for unraveling the universe's slowest dynamics. Using nanohertz as a unit helps researchers quantify and analyze these grand-scale processes, linking time scales from decades to cosmic evolution.