A civilisation that could build a Dyson swarm might want something more ambitious still: to move its star. Stars drift through the galaxy, and over millions of years they can pass dangerously close to each other or towards supernovae. A stellar engine is a structure that uses a star’s own energy to do work, and in its most dramatic form, to steer the star itself, taking its planets along with it.
Zwicky’s jets
The first proposal came from the Swiss astronomer Fritz Zwicky. In a lecture in 1948 he imagined firing fusion jets from the surface of the Sun to accelerate it, with the whole Solar System, towards Alpha Centauri at about 1 000 kilometres per second.
A mirror the size of an orbit
In 1987 the Soviet scientist Leonid Shkadov proposed a gentler method. Place an enormous curved mirror on one side of the star, at the distance where the star’s gravity pulling it in and its light pushing it out exactly balance, so the mirror hovers in place. The mirror reflects light back towards the star on one side. Because the star now shines more strongly in one direction than the other, it is pushed very slowly the opposite way, and the mirror, held by gravity, comes along.
The push is tiny but endless. Reflecting half of the Sun’s output would give a thrust of about 1.28 × 10¹⁸ newtons. After a million years, the Sun would be moving at only 20 metres per second and would have travelled 0.03 light-years. After a billion years it would reach 20 kilometres per second and have moved about 34 000 light-years.
In 2000 Viorel Badescu and Richard Cathcart sorted stellar engines into classes. The Shkadov thruster is Class A. A Class B engine is a Dyson-like shell that uses the star’s energy to drive machinery, and Class C combines the two.
Faster
In 2019 the physicist Matthew Caplan designed a more powerful engine. It would collect material from the star itself and fire it back out as a fast fusion jet, while a second jet holds the engine in place. By his calculation, such an engine could move a star about 10 parsecs, around 33 light-years, in as little as a million years.
Could we see one?
In 2013 Duncan Forgan suggested that if an alien civilisation had built a Shkadov thruster, the mirror might betray itself by distorting the light curve of a planet passing in front of the star. The chance of catching one is small, but the method would work with telescopes we already have.