Gravitational waves

One prediction of general relativity took a century to be verified: gravitational waves, ripples in the spacetime metric that propagate at the speed of light, generated by masses undergoing accelerated motion — in particular by binary systems of black holes or neutron stars spiralling towards each other until they merge.

The first direct detection was on 14 September 2015, obtained by the LIGO interferometer with its two detectors (at Hanford and at Livingston). The event GW150914 (Abbott et al. 2016) corresponds to the merger of two black holes of about $30$ solar masses each, which occurred about $1.3$ billion light-years from us. The relative deformation of spacetime measured is of order $h \sim 10^{-21}$: over a distance of $4$ km (the length of the interferometer’s arms), this means a displacement $\Delta L \sim 4\cdot 10^{-18}$ m, about a thousandth of the diameter of a proton. The precision required seems miraculous, and indeed the 2017 Nobel Prize was awarded to Weiss, Barish and Thorne for achieving it.

General relativity in three key results to remember

1. time slows down in gravitational depths (Pound–Rebka, GPS);
2. below $r = r_s = 2GM/c^2$ black holes form (M87, Sgr A*);
3. spacetime propagates at $c$ via waves (LIGO 2015).

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Topics: Relatività ristretta Concepts: Legge di gravitazione universale

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