Terahertz to Revolutions per minute

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.

Because the numbers are more human-friendly. An engine idling at 750 RPM sounds reasonable; saying 12.5 Hz just feels weird for a mechanical process you can watch. RPM also maps directly to what a mechanic cares about — how many times the crankshaft turns each minute. The unit stuck from the steam-engine era when counting revolutions per minute was literally what an engineer did with a watch.

These speeds balance data throughput against heat, vibration, and power draw. 7,200 RPM became the desktop standard because it moved the read/write head over data 33% faster than 5,400 RPM, noticeably improving access times. Server drives pushed to 10,000 and 15,000 RPM for even lower latency. Laptops favored 5,400 RPM for quieter, cooler, longer-battery operation. SSDs made the whole debate obsolete.

A typical front-loading washing machine spins at 1,000–1,400 RPM during the final spin cycle, generating enough centrifugal force to squeeze water out of clothes. High-end machines hit 1,600 RPM. Top-loaders usually max out around 700–1,100 RPM. Higher spin speeds mean drier clothes out of the washer (less dryer time), but they also increase wear on fabrics and make the machine vibrate more.

Divide by 60. One RPM means one revolution per minute, and there are 60 seconds in a minute, so 1 RPM = 1/60 Hz ≈ 0.01667 Hz. A 7,200 RPM hard drive spins at 120 Hz; a 33⅓ RPM vinyl record rotates at about 0.556 Hz. Going the other way, multiply hertz by 60 to get RPM.

In 2018 researchers at Purdue University spun a silica nanoparticle at over 300 billion RPM (5 GHz) using laser light in a vacuum — the fastest-spinning object ever recorded. At macroscopic scale, gas centrifuges for uranium enrichment spin at about 50,000–70,000 RPM, and dental drill turbines reach roughly 400,000 RPM. Turbomolecular vacuum pumps operate at around 90,000 RPM.