Happy 2023! Amazingly enough, the blog machinery still works after several idle years. This post and the next are about a preprint posted on arxiv, back in October: Andrew Gould’s “Exploration of M31 via Black Hole Slingshots and the ‘Intergalactic Imperative”. This paper is a fascinating mix of garden-variety astrophysics and wild speculation that I think is worth further discussion.

Arxiv is a ‘preprint server’, where academic papers are posted pre-publication: sometimes pre-peer review, sometimes not. I try to skim the listings of new papers regularly, both to keep up with literature for work and to spark new SF story ideas. This one caught my eye because the title involves the Andromeda galaxy (M31), which has been the topic of much of my own research, and because “intergalactic imperative” is not a phrase you typically see in the professional astrophysics literature!

Arxiv postings are moderated to some degree, such that not every garage cosmologist can post their alternative theory of the universe there. As far as I can tell, this is a serious paper, written by a serious astrophysicist, even if the topic is not what serious astrophysicists usually write about. (The arxiv listing doesn’t state whether the paper has been submitted for publication.) Gould asks what it might take to send a spacecraft to another galaxy, arguing that

such “impractical” intellectual exercises can have quite practical effects, both in stimulating ideas to achieve more prosaic aims and in framing our perspective on how to focus present resources.

The paper discusses the specific example of travel to M31, which at a distance of about 2.5 million light years is the nearest large galaxy to our own Milky Way.

Gould begins by reminding the reader about the slingshot mechanism (also called a gravity assist), commonly used to speed up and redirect spacecraft. If you really want to speed up a spacecraft, you want to slingshot it not around a planet but around a black hole, and in particular the biggest one that we know of nearby, namely the one at the centre of the Milky Way, Sgr A*. To get your spacecraft going in the right direction, you have to send it toward the black hole at exactly the right angle, and to do that with Sgr A* requires having the spacecraft first encounter another, smaller, black hole to fine-tune its orbital motion. Lots of small black holes with masses comparable to those of the Sun (compared to supermassive black holes which are millions or billions of times larger) are known in our galactic neighborhood, so it’s reasonable to assume there are some near Sgr A*.

If one wants to slingshot a spacecraft around a black hole, though, there’s the problem of tidal forces: the spacecraft walls nearest to and farthest from the black hole won’t feel exactly the same gravitational forces, because they’re at slightly different distances. If the difference between the forces is too high, the spacecraft will be torn apart. Gould calculates how close a hypothetical spacecraft can get to a black hole (its “peribothron”, analogous to “perigee” or “perihelion”) before tidal forces would pull steel apart. This minimum distance depends on the size of the spacecraft and the mass of the black hole, and the distance and mass determine how fast the spacecraft will be going after the encounter. The calculation leads to a phrase in the paper’s abstract that joins two concepts I’ve never seen together before:

0.1c, a speed that is ultimately limited by the tensile strength of steel and the BH [black hole] mass

Gould’s calculations show that approximately one-tenth of light speed (c) is the limiting resultant speed for a spacecraft that slingshots around a stellar-mass black hole. With the orbital mechanics sorted out, the next problem is that no stellar mass black holes near Sgr A* are currently known. Gould describes how one could find such black holes via their gravitational microlensing. He’s an expert in this topic; in my opinion this section contains more detail than most readers will care about. He concludes that dark stellar-mass black holes near the Galactic center aren’t visible from Earth. They would have to be found en route and would not be detectable more than a few weeks before the Sgr A* encounter. He proposes that the M31-bound spacecraft be preceded by a “shepherd spacecraft” which could do the black hole mapping required for a successful encounter. After the encounter, the shepherd craft could send a report back to Earth and/or itself slingshot around Sgr A* to return to Earth.

Section 7 of the paper sketches out a complete intergalactic mission. After launching from Earth, the spacecraft slingshots around a nearby binary white dwarf system to speed it up to about 0.2% the speed of light. The resulting journey to the Galactic center takes about 13 million years. After the spacecraft encounters the small and massive black holes at the Galactic center to send it toward M31, it’s accelerated to a tenth of light speed, so the intergalactic journey takes about 25 million years. At journey’s end, the spacecraft must slow down in an anti-slingshot around M31’s central black hole, guided by a “daughter probe” similar to the shepherd spacecraft. Gould discusses various options for exploring M31 (about which more later) that altogether take about 33 million years, with an optional return to Earth that would take an additional 25 million years. Slowing the spacecraft down at the end of the return journey isn’t required since it could report its results during a flyby; a return mission would not even be required if transmitting information over intergalactic distances is achievable.

The next section discusses other options for sending a spacecraft to M31. As a thought experiment, Gould considers a spacecraft that accelerates for the first half of the trip and decelerates for the second half, reaching a speed very close to the speed of light. Time dilation gets involved, so onboard the spacecraft only about 30 years pass. Besides “prodigious fuel requirements” – he calculates that the non-fuel spaceship mass would be a trillionth of the fuel mass (!!) – Gould notes that

problems (besides inventing the engine) include keeping teratons of fuel (likely antimatter) stable during the 30-year voyage

(No kidding.) The point of this calculation and of the gravitational slingshot calculation is to demonstrate that travel to M31 is, if not currently technically possible, then at least physically feasible, and it could occur over a timescale that’s short compared to the age of the Earth and of Sun-like stars. This timescale leads to the ‘intergalactic imperative’ that Gould discusses at the end of the paper.

This is a clearly-written paper whose tone and technical level indicate to me that the intended audience is people with at least some undergraduate or education in astrophysics. There’s quite a change when Gould reaches the much more speculative “intergalactic imperative” sections at the end, and I’ll discuss these sections in a future post.