Empirical Earth · Gravity, Buoyancy & the Air

LIGO — catching ripples in spacetime with a giant Michelson interferometer

The claim

“Gravity is unproven and 'gravitational waves' are just physicists seeing patterns in noise.”

What is true

Detecting a length change smaller than a proton is an extraordinary claim, and demanding hard evidence for it is the right instinct.

What is measured

LIGO’s two detectors 3,002 km apart catch the same gravitational wave within 10.0 ms, the light-travel time between them. GW150914 came in at 6.9 ms, and where the delay falls inside that bound fixes the source on the sky.

What would show this is wrong

a gravitational-wave signal reaching the two detectors with a lag longer than their light-travel separation, which is 10.0 ms for the 3,002 km between Hanford and Livingston. No detection has exceeded it.

Sources

  1. LIGO & the first gravitational-wave detection. Twin 4-km Michelson interferometers at Hanford (WA) and Livingston (LA), ~3,002 km apart, detected GW150914 on 14 Sep 2015 (two black holes of link
  2. GW170817 — the multi-messenger neutron-star merger. On 17 August 2017 LIGO and Virgo detected a binary neutron-star merger ~130 million ly away; Fermi caught the gamma-ray burst 1.7 s later link
  3. The LIGO–Virgo–KAGRA network & catalogue. Advanced LIGO (US), Virgo (Italy) and KAGRA (Japan) observe jointly; multiple detectors localise sources by arrival-time differences, and hundreds o link
  4. LIGO built straight through Earth’s curve. Over each 4 km arm, a straight line in vacuum departs from the Earth’s surface by about 1.25 m. LIGO’s beam tubes were aligned to the light’s strai link
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