After more than four decades of thought and development, in the late summer of 2015 the LIGO detectors were finally sensitive enough to detect plausible astrophysical sources. On 14 September 2015, serendipity favoured physicists again: almost as soon as the detectors were turned on, they gave a signal that was strong enough to be an unmistakable source12 (see Fig. 2)—although given the importance of the discovery, the LIGO team spent months validating their analyses. Thus began the flood of results from the direct detection of gravitational waves, which have already had major impacts in fundamental physics, astrophysics and nuclear physics.

Fig. 2: Representation of gravitational waves emitted by a merging black hole binary. a, b, Two black holes several orbits before their merger (a), and at the point of merger (b). c, The aftermath of the merger; the remnant has settled down into its final state as a single black hole. On the vertical axis, strain is the fractional change in the arm lengths of the LIGO detectors. Figure adapted from ref. 91 (Springer Nature). Full size image

The LIGO detectors were actually designed as a physics experiment, and the avalanche of fundamental physics discoveries did not disappoint. Perhaps the most remarkable one is the 2017 measurement of the speed of gravitational waves28. According to Einstein, this speed should be equal to that of light, and indeed this is what the LIGO/Virgo and Fermi collaborations inferred, to an accuracy better than one part in 1015. As we describe later, this measurement was possible thanks to the gravitational-wave observation by LIGO and Virgo at the time of a neutron star binary merger, in coincidence with the short γ-ray burst that followed less than two seconds later and was detected using the Fermi observatory29. This single observation sufficed to place stringent constraints on violations of Lorentz invariance30,31, on violations of the equivalence principle32 and, in particular, on theoretical models that attempt to explain the late-time acceleration of the Universe through modified gravity instead of a dark-energy fluid33,34.

But that is not the only gold nugget hidden in the gravitational-wave mine. The very first gravitational-wave detection by the LIGO instruments in 2015 was generated by the merger of two black holes12. The signal-to-noise ratio of this event was so high (at roughly 25), and the event was so far away from Earth (at roughly 400 Mpc), that this single observation led to some of the most stringent model-independent constraints (at less than 10−22 eV) on the mass of the particle that is supposed to be responsible for mediating the gravitational interaction: the graviton35. Moreover, this single event confirmed that gravitational waves travel at the same speed irrespective of their frequency, just as predicted in the general theory of relativity36. This observation begins to dig into the parameter space of massive-graviton theories, which attempt to go beyond the early work of Fierz and Pauli37 through the inclusion of nonlinear interactions that evade certain otherwise unavoidable instabilities38,39.

The astrophysical implications of the direct detection of gravitational waves were equally important. Prior to the detections, the only category of LIGO-detectable astrophysical sources known to exist was binary neutron stars. As a result, the detector sensitivity and much of the science case for LIGO were built around predictions for the rate of detectable double neutron star coalescences. Some theoretical models of binary evolution predicted a high rate from double black holes with several tens of solar masses (for example, ref. 40), but the lack of any clear evidence for the existence of such black holes (the highest mass identified for any stellar-mass black hole in our Galaxy is only about 15M ʘ ; M ʘ , solar mass) meant that the LIGO case could not be staked on such hypotheticals. The very first event seen with LIGO (which was called GW150914 because the waves reached Earth on 14 September 2015) was a coalescence between two black holes with masses of about 29M ʘ and 36M ʘ , leading to a final mass of about 62M ʘ (meaning that roughly 3M ʘ of mass energy was radiated in gravitational waves). Thus, the LIGO detections immediately doubled the mass range of known stellar-mass black holes, and then the merger doubled the range again (see Fig. 3)! The additional nine double black hole coalescences that have since been discovered have led to progressively tighter constraints on their formation rates, as well as to additional puzzles. For example, the inferred angular momenta for stellar-mass black holes seen in our Galaxy are typically sizeable fractions of the maximum allowed41, and alignment of those spins with the orbit in a binary black hole system is expected in the most popular scenario. Yet there are growing indications that the black holes observed in mergers usually have either low or misaligned spins (for example, ref. 42).

Fig. 3: Masses of black holes and neutron stars inferred from gravitational-wave detections and from electromagnetic (EM) observations. For the gravitational-wave detections, the masses of the original objects (black holes or neutron stars) and their final products are shown. Image credit: Frank Elavsky, LIGO-Virgo, Northwestern; adapted with permission from ref. 92. Full size image

An even greater astrophysical return was realized from the gravitational-wave detection and associated electromagnetic observations of what is so far the only double neutron star event29 seen: GW170817 (see Fig. 4). For this, the European Virgo detector made a key contribution to source localization, and the electromagnetic observations from radiofrequencies to γ-rays qualify as perhaps the most intense electromagnetic campaign ever focused on a single astronomical event. The associated short γ-ray burst validated a long-standing belief that double neutron star events can produce such bursts, and the subsequent optical and then infrared glow from this source corroborated predictions from the previous few years for the radioactive-decay-powered material expanding from such a merger43. The nature of the emission, particularly in the infrared, also supports the growing consensus that most of the elements considerably heavier than iron are produced by neutron star mergers, rather than by supernovae, as had been previously thought.

Fig. 4: The final stages of the merger between two neutron stars, as detected in the gravitational-wave event GW170817. As the stars merge, they produce a relativistic jet along the original orbital axis, and a fraction of the matter emerges quasi-isotropically as an outflow. Image reproduced from the cover of Nature 551, November 2017 (Springer Nature). Full size image

The gravitational waves from neutron star mergers may also open a new window of knowledge into the properties of the dense matter in neutron star cores. These properties are highly uncertain because (1) the microphysical conditions inside neutron stars (density of a few times the nuclear density, thermal energy much lower than the Fermi energy, and far more neutrons than protons) cannot be attained in terrestrial laboratories and (2) although neutron star mass measurements provide solid astrophysical constraints (for example, ref. 44), radius measurements are currently bedevilled by systematic errors (for example, refs 45,46).

To understand the potential contribution of gravitational-wave data, we note that as two neutron stars spiral towards each other, they act as point masses for most purposes when their separation is many times larger than their radii. However, at sufficiently close separations, tidal effects can cause the orbit to deviate from that of two point masses; in particular, orbital energy starts to be used to tidally deform the stars, which means that two neutron stars will spiral together roughly a millisecond faster than would two point masses of the same mass. The initial constraints on the properties of dense nuclear matter from GW170817 show the potential of gravitational waves for probing the interior of neutron stars (for example, refs 47,48). In particular, this one observation enabled the first attempt at a measurement of the tidal deformabilities of neutron stars, which has provided preliminary information about their equation of state. Future observations will improve these measurements, both because sensitivities at high frequency will be improved and because with more observations will come rare, very strong events that can be measured with exquisite precision.

When additional, somewhat uncertain, astrophysical assumptions are added, one can extract even more information about dense nuclear matter. For example, before the LIGO/Virgo detection of the double neutron star merger, a few groups argued that if the final product of the merger collapsed to a black hole shortly after merger, then it would be possible to use the measurement of the total mass of the neutron star binary to place an upper limit on the maximum mass of a non-rotating star. This upper limit could, in some cases, be much tighter than the about 2.8M ʘ upper limit allowed by current nuclear experiments49,50,51. Interpreted in this framework, the GW170817 data improve this limit greatly52, to about 2.2M ʘ , but this inference depends on astrophysical assumptions and thus the resulting constraints on neutron star matter are not as reliable as those following from, for example, the highest neutron star mass yet observed53 (of 2.01M ʘ ± 0.04M ʘ ).