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SESK: Stuff Everyone Should Know · sesk.info/astro-ligo-arm-length

How long is each perpendicular arm of an Advanced LIGO detector?

Each arm is 4 kilometers long.

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  1. 400 meters
  2. 40 kilometers
  3. 4 kilometers ✔

Why: LIGO compares laser light traveling along two perpendicular arms. Long arms turn an extraordinarily small fractional strain into a larger absolute length change.

Huh, didn't know that: The GW150914 arm-length difference was about 4 times 10 to the minus 18 meters.

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LIGO has long arms because spacetime changes are hard to reach.
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References

Answer: 4 kilometers. LIGO compares laser light traveling along two perpendicular arms. Long arms turn an extraordinarily small fractional strain into a larger absolute length change.

  1. GW150914 Detector ligo.org/science-summaries/GW150914Detector
    The L-shaped Advanced LIGO detectors consist of two perpendicular arms, each two and one-half miles (4 kilometers) long.
    Checked 2026-10-10.

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