Astronomia da fantascienza 👉 -8

Sci-fi Astronomy, edited by Camilla Pianta

Diaspora, the twilight of Earth in the echo of gravitational waves 💫

What if a neutron star collision happened in our galactic backyard?

COUNTDOWN TO APRIL 2026, THE CENTENARY OF SCIENCE FICTION: -8

Clicca qui per la versione italiana di questo articolo

By one o’clock in the afternoon, the gravitational waves from Lacerta had intensified to a hundred times their usual strength. There was no need to wait for the final data from the outermost TERAGO detectors to filter out interference: the signal was arriving directly from the Bullialdus crater in real time. The pulse from Lac G-1, accelerating steadily, was now strong enough to overwhelm every other celestial source of gravitation. The waves were visibly shortening — each cycle becoming narrower than the last — and the two most recent peaks were separated by only fifteen minutes, a clear indication that the neutron stars had already crossed the two-hundred-thousand-kilometre threshold. Within an hour, the distance between them would be halved; a few minutes later, it would vanish entirely. Yatima had hoped that the system’s evolution might slow, but the gleisners’ extrapolations, growing ever steeper, proved accurate.

Diaspora by the Australian writer Greg Egan is one of the most ambitious and radical novels of hard science fiction, a narrative that combines scientific rigour and philosophical speculation in a complex post-human future. Egan imagines a humanity fragmented into three distinct evolutionary strands: the statics, beings still bound to traditional biological bodies; the gleisners, minds transferred into mobile robotic bodies; and the polises, virtual cities that host disembodied intelligences such as Yatima, the protagonist — an artificial consciousness emerging from stochastic mutations in a pre-existing digital genotype. For the inhabitants of the polises, science is an immersive, first-hand experience, because they literally enter the mathematical models they construct, perceiving spacetime and extra dimensions as though they were real landscapes. In this way, knowledge of the universe becomes an integral part of their identity.

However, the estimates produced by artificial intelligences, based on an incomplete understanding of gravitational mechanisms in extreme regimes, prove tragically wrong when, in 2996, a catastrophic and unexpected event devastates the Earth: the binary neutron-star system Lac G-1, located about one hundred light-years away, collapses into a single hypermassive star one million years earlier than predicted. Accompanying the merger is the release of a gamma-ray burst powerful enough to strip the ozone from the stratosphere and ravage the entire terrestrial ecosystem, marking the onset of an unprecedented crisis. TERAGO, the lunar observatory designed to detect gravitational waves with the highest possible precision, manages to record the event in real time, but still too late to allow intervention or preparation. Egan writes that, in the coalescence phase of the two neutron stars, gravitational waves of increasing frequency and intensity are emitted, reaching peak at the moment of merger in line with the findings of actual operational detectors — such as LIGO, Virgo and KAGRA.

The cover art for the Italian edition of *Diaspora* by Greg Egan was created by Franco Brambilla, one of the contemporary masters in this field. Source: https://www.anobii.com/it/books/diaspora/018ab2669a64c840d7

According to Albert Einstein’s general theory of relativity, gravitational waves are ripples in spacetime — the fabric that permeates the universe and can be deformed under the influence of mass and energy —, perturbations that travel at the speed of light, compressing and stretching spacetime as they pass. They are produced by the merger of compact (enormously massive) objects, such as neutron stars or black holes, in tight binary systems (systems in which the separation between the two components is very small). For a binary system of compact objects to emit gravitational waves, its overall mass distribution must be asymmetric, the total moment of inertia must not be conserved, and the rotational motion of its components must be accelerated. In practical terms, the two bodies must rotate in a manner that is not perfectly symmetric with respect to their common centre of mass, with motion that accelerates and continually alters the mass distribution within the system.

The evolution of a compact binary system can ideally be divided into three phases: inspiral, merger and ringdown. During the inspiral phase, the two components slowly spiral towards one another as they lose energy in the form of gravitational waves. This process causes a gradual orbital decay: the orbit shrinks, the orbital velocity increases, and so do the frequency and amplitude of the gravitational signal. The typical emission frequency ranges from a few tens to a few hundreds of hertz, and its exact value depends on the orbital frequency of the binary system — that is, how quickly the two objects orbit around the centre of mass: the closer and faster they are, the greater the higher the oscillation rate. The gravitational signal therefore acquires a characteristic profile known as a chirp, since the resulting sound exhibits the rising frequency typical of a chirping noise. For the mathematical description of the inspiral phase, post-Newtonian approximations are employed: a series of corrections to the equations of classical physics that progressively introduce relativistic effects into the system’s dynamics.

The situation changes dramatically during the merger phase, when the two bodies come into contact and then coalesce: this is a strongly non-linear regime in which post-Newtonian approximations fail, and Einstein’s field equations must be solved numerically through hydrodynamical simulations. This stage represemts the point of maximum gravitational-wave emission, with the signal attaining its peak amplitude exactly as Egan predicted. Notably, the matter ejected following the merger of neutron stars undergoes the r-process (rapid neutron capture), promoting the synthesis of heavy elements such as gold, platinum and uranium. The radioactive decay of the nuclei of these elements initiate a kilonova, a thermal glow visible in the infrared and optical bands. Other electromagnetic phenomena associated with the merger of binary neutron stars, induced by the amplification of their magnetic fields to intensities of the order of 10¹⁶-10¹⁷Gauss, include relativistic jets and short gamma-ray bursts.

A typical gravitational wave signal produced by a merging black hole binary. The inspiral phase can be described by a post-Newtonian series expansion, while the final part of the ringdown phase can be described using linear perturbation theory (the blue parts of the signal). The merger and the initial ringdown, however, exhibit non-linear spacetime dynamics (the orange part of the signal). Credit: Top, Kip Thorne; bottom, B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Observation of gravitational waves from a binary black hole merger”, *Phys. Rev. Lett.* 116, 061102 (2016); adapted by APS/Carin Cain. Source: https://physics.aps.org/articles/v16/29

Depending on the total mass and the type of stars in the system, the outcome of the merger may be either a black hole surrounded by a hot, dense accretion disc, or a hypermassive and unstable neutron star, which may collapse into a black hole after a few milliseconds or persist for longer, while still emitting gravitational waves at an almost constant frequency. Specifically, if the sum of the masses of the two neutron stars exceeds the critical threshold of approximately 2.5–3 solar masses — considering that the mass of a single neutron star lies between 1.2 and 2.3 solar masses —, the formation of a black hole is inevitable, and the system enters the ringdown phase, dominated by quasi-normal modes (a set of exponentially decaying oscillations characteristic of the newly formed black hole).

The gravitational waves radiated during all these phases contain valuable information about the physical parameters of the objects involved, including mass, spin (intrinsic angular momentum), distance, orbital inclination, and, in the case of neutron stars, even their internal structure.

The detection of gravitational signals entails substantial technical difficulties. To this end, interferometers such as LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo and KAGRA (Kamioka Gravitational Wave Detector) make use of laser beams that travel along two perpendicular arms extending for several kilometres. Gravitational waves imperceptibly alter the lengths of these arms, changing the phase difference between the two beams. These variations are of the order of 10-21 m and require seismic isolation, thermal control and ultra-high vacuum conditions for accurate measurement, without interference from background noise. LIGO, with two twin facilities located in the United States — Livingston, Louisiana, and Hanford, Washington —, has conducted three observing runs: O1 (September 2015 – January 2016), O2 (November 2016 – August 2017) and O3 (April 2019 – March 2020). From the O2 run onwards, LIGO operated in synergy with Virgo, the European interferometer situated near Pisa. This collaboration improved localisation of gravitational-wave sources via triangulation of signals recorded simultaneously at multiple points on Earth.

The first direct detection of gravitational waves came in September 2015, coinciding with the start of the O1 run. The event, GW150914, saw two black holes of roughly 36 and 29 solar masses merge to form a final black hole of about 62 solar masses. In August 2017, close to the end of the O2 run, LIGO and Virgo recorded GW170817, the first observed merger of two neutron stars. Initially spotted through gravitational waves, the event was soon associated with the short gamma-ray burst GRB 170817 and with electromagnetic signals across the entire spectrum, from X-rays to radio waves, thus inaugurating the era of multi-messenger astronomy.

In 2020, the activation of the Japanese interferometer KAGRA, built in an underground laboratory in the Kamioka district and equipped with cryogenically cooled mirrors, provided significant support to the O3 run. The LIGO–Virgo–KAGRA network was therefore able to identify a large number of compact-object mergers with great reliability.

The gravitational wave detectors of the LVK collaboration. From top left, clockwise: LIGO (Hanford); KAGRA; LIGO (Livingston); Virgo. Source: https://www.ego-gw.it/blog/2023/05/24/i-rivelatori-di-onde-gravitazionali-stanno-iniziando-un-nuovo-periodo-osservativo-in-cui-esploreranno-i-segreti-delluniverso/

Future projects for gravitational-wave detection include the Einstein Telescope and the Cosmic Explorer, terrestrial interferometers — the former European and the latter American — tasked with observing sources at very high redshift and testing general relativity and cosmology. These will be complemented by LISA (Laser Interferometer Space Antenna), a space-based interferometer engineered by ESA and NASA, which will enable even more detailed investigation of the primordial gravitational universe. Our researcher Matteo Calabrese is working on preparatory studies for the LISA mission.

Although we cannot be certain of its global effects, a gravitational wave originating one hundred light-years from Earth would probably not bring about disastrous consequences for the planet’s climate or geology: its interaction with matter would in fact be minimal, and the caused spacetime distortion measurable only by the most sensitive interferometers. By contrast, a gamma-ray burst at the same distance could inflict irreparable damage, releasing an energy of approximately 10⁵⁰ erg — that is, about 10⁸times the Sun’s annual output — focused into a narrow cone over just a few seconds. If, as hypothesised by Egan in Diaspora, the Earth lay along the jet axis, it would be fully exposed to the gamma rays, which would ionise the stratosphere and destroy almost all of the ozone present. This would allow the immediate penetration of solar ultraviolet radiation, particularly harmful to DNA and biological organisms, against which ozone normally acts as a shield; without this natural filter, the Earth would hence be rendered uninhabitable for living beings. Only the artificial intelligences of the polises could survive, given that they are devoid of physical bodies and isolated from the external world, with which they interact solely on a digital level.

Gravitational waves are silent messengers of remote cataclysms; nevertheless, if one were to occur “just” one hundred light-years from us, that silence could become deafening: only a handful of well-collimated gamma rays could annihilate life as we know it. Thankfully, this threat is mitigated by the absence of any binary system of compact objects in the Earth’s immediate vicinity. By comparing observational data from events like GW170817 with the conjectures formulated by Egan in Diaspora, one can appreciate the power of science fiction literature to anticipate scenarios that have not yet occurred, but remain physically plausible. It is in this space of possibility, between calculation and imagination, that hope is nurtured for discoveries that could revolutionise our vision of the cosmos.

Nus, 2 August 2025 – English version published on 18 May 2026

Astroglossary

m: metre
s: second
hz: hertz, unit of measurement for frequency
G: gauss, unit of measurement of the magnetic field in the CGS system, corresponding to 10^-4 T, the symbol for the tesla
erg: unit of measurement of energy in the CGS system, equivalent to 10⁻⁷ J, symbol for the joule
light-year: distance traveled by light in a year in a vacuum, corresponding to 9,46 × 10¹² km
solar mass: massa of the star Sun, corresponding to 1,9885 × 10³⁰ kg