Millihertz to Terahertz

After a magnitude-9 earthquake the entire planet vibrates like a struck gong, with its deepest mode at about 0.3 mHz — one oscillation every 54 minutes. The surface rises and falls by fractions of a millimeter. You cannot hear it (human hearing starts at 20 Hz), but gravimeters and seismometers worldwide pick it up. The 2004 Sumatra quake kept Earth ringing measurably for weeks.

Ocean swells, tidal constituents, and seiches (standing waves in harbours or lakes) all oscillate in the millihertz band. A 10-second ocean swell is 100 mHz; a harbour seiche with a 10-minute period is about 1.7 mHz. Monitoring these frequencies helps coastal engineers predict resonance in ports and design breakwaters that don't amplify destructive wave energy.

Not directly — our senses are far too fast. But some physiological rhythms operate here: the Mayer wave, a ~0.1 Hz oscillation in blood pressure, sits at the high end of the millihertz scale, and slower vasomotion (tiny blood vessel contractions) can dip below 10 mHz. You don't feel them as vibrations, but they show up clearly on a continuous blood-pressure monitor.

Infrasound is sound below the ~20 Hz threshold of human hearing. The lowest infrasound blends into the millihertz range — the International Monitoring System for nuclear-test detection listens down to about 20 mHz. Sources include volcanic eruptions, meteor airbursts, severe storms, and ocean microbaroms (standing pressure waves between ocean swells and the atmosphere).

Instruments record a time series (pressure, acceleration, displacement) over hours or days, then apply a Fourier transform to extract frequency content. Superconducting gravimeters can resolve Earth's free oscillations below 1 mHz by measuring gravity changes of 10⁻¹² g. The trick is not a fast sensor but a patient, ultra-stable one and enough data to separate signal from drift.

For decades, electronics could generate frequencies up to ~100 GHz and optics could work down to ~10 THz, but the range between 0.1 and 10 THz was hard to reach from either direction. Electronic oscillators became too slow and lasers too low-energy. Only in the last 20 years have quantum cascade lasers and photoconductive antennas started closing this gap, opening new applications in imaging and spectroscopy.

Active scanners illuminate passengers with millimeter or terahertz waves (typically 0.1–1 THz), which pass through clothing but reflect off skin and dense objects. The reflected signal creates a body outline showing concealed items without ionising radiation. Because terahertz energy is about a million times weaker than an X-ray photon, it cannot break chemical bonds or damage DNA.

No. Terahertz photons carry far less energy than visible light photons and are non-ionising — they cannot knock electrons off atoms or damage DNA. At extremely high power they could heat tissue (like a microwave), but every practical terahertz imaging system operates at power levels thousands of times below any thermal threshold. You are bathed in more terahertz radiation from your own body heat than from an airport scanner.

Red light starts around 430 THz (700 nm wavelength) and violet reaches about 750 THz (400 nm). So the entire rainbow occupies roughly 430–750 THz. Infrared sits below red at 0.3–430 THz, and ultraviolet begins above violet at 750+ THz. When someone says "terahertz imaging," they mean the far-infrared end below about 10 THz — well below anything your eyes can detect.

For some applications, yes. Terahertz imaging can distinguish cancerous from healthy tissue based on water-content differences, and it does so without ionising radiation. It is already used experimentally during skin and breast cancer surgery to check tumor margins in real time. The limitation is penetration depth: terahertz waves are absorbed by water within millimeters, so they cannot image deep organs the way X-rays or MRI can.