Paul Gilster's Blog, page 9

A Millisecond Pulsar Engine for Interstellar Travel

Suppose you want to migrate to another star, taking your entire civilization with you. Not an easy task given our technology today, but let’s remember that in the 13 billion year-plus history of the Milky Way, countless stars and their planets have emerged that are far older than our 4.6-billion year old Sun. If we imagine an intelligence that survives for a billion years or more, we can hardly put constraints on what it might accomplish. The idea of moving a star with its planetary system intact is out there on the edge of what science fiction can accomplish, if not yet science. There have even been SETI searches for such projects, though as with SETI at large, no hits.

Why would you want to move a star? Consider that if you are a long-lived species with a simple interest in exploring the universe, setting up a journey in which you can take your culture with you – all of it – could have serious appeal. For one thing, you are also taking your primary energy source with you, and can now settle into a traveler’s frame of mind, confident of reaching exotic destinations sooner or later. Slow boat to the heart of the Laniakea Supercluster, and imagine the sights to be had along the way.

Imagine a culture in which extraordinarily long lifetimes – perhaps ‘immortality’ for a machine civilization – is the norm. The accelerated expansion of the universe will continue rendering some destinations unreachable. Dan Hooper, in a paper called “Life Versus Dark Energy: How An Advanced Civilization Could Resist the Accelerating Expansion of the Universe,” considered this in 2018; we’ll be looking at his ideas in the near future. Perhaps migrating to a globular cluster or into the tightly packed center of a galaxy would suggest itself as a goal worth achieving. But let’s not dig too far into motivations. What could humans possibly understand about the motivations of a billion-year old culture? Let’s just look for observables. So how about this:

Image: This is Figure 3 from the paper by Clément Vidal we’ll be discussing today. Caption: The original black widow pulsar PSR J1959+2048. Left: the BW pulsar (in blue) is plotted in the RA-DEC plane, and its proper motion vector is displayed until it reaches a close encounter with a target star, in orange, whose Gaia DR3 ID is displayed (see Vidal et al. 2023 [60] for the method used to find close encounters). Middle: a Chandra X-ray view of the BW pulsar, displaying a comet-like tail; the candidate target star is also visible in the bottom right (visualization with ESASky). Right: The composite image on the right shows the X-ray tail (in red/white) and a bow shock visible in the optical (green). Credit: X-ray: NASA/CXC/ASTRON/B. Stappers et al. [38]; Optical: AAO/J.Bland-Hawthorn & H.Jones; scale: 1.2 arcmin on a side.

We’ll get to the terminology involved in that caption in a moment. For now, is this conceivably a stellar engine in motion toward a nearby star? Probably not, but I introduce this image because I suspect that SETI searches for stellar engines are not well known, although they have been performed and we’ll be looking at these soon. And it also introduces the possibilities of an entirely different kind of stellar engine.

It’s no surprise that it should emerge through the work of Clément Vidal, given the philosopher’s long-standing interest in deep time and societies that go back to the early eons of the universe. Vidal (Center Leo Apostel, Vrije Universiteit Brussel, Belgium) is the author of the essential The Beginning and the End (2014) which along with his papers explores the terrain of matter and energy as a means of increasing order, and the possibility of locating ancient civilizations by finding what appear to be natural objects conceivably being manipulated by engineering on scales that we can only imagine.

Vidal’s latest plunges into what he calls the ‘Spider Stellar Engine,’ a steerable design using binary star pairs as parts of a propulsive system that uses one star as payload and the other as fuel. If this reminds you of Olaf Stapledon, you’re right, as it was he who wrote about artificial stars created to support interstellar voyaging in the 1930s. But I’m glad to see Vidal also pointing to Fritz Zwicky, who has all too often been overlooked in the history of exotic concepts even though it turns out that on matters like dark matter, he was far ahead of the game. Here’s Zwicky on stellar engines:

“Considering the Sun itself, many changes are imaginable. Most fascinating is perhaps the possibility of accelerating it to higher speeds, for instance 1,000 km/s directed toward α-Centauri in whose neighborhood our descendants then might arrive a thousand years hence. All of these projects could be realized through the action of nuclear fusion jets, using the matter constituting the Sun and the planets as nuclear propellants.”

Various science fiction authors have explored the concept, and Stanley Schmidt even came up with an assist from aliens offering to boost the Earth to another stellar system to save it in Since of the Fathers (1977). But the most technically judicious treatment of moving an entire system like this is probably the one depicted in Greg Benford and Larry Niven’s Bowl of Heaven series, which in turn draws on the thinking of the Russian engineer and scientist Leonid Shkadov (1927–2003). As with Dyson spheres, here we are in the domain of engineering at a colossal scale.

Image: A Shkadov thruster at work. Credit: Greg School.

Let’s give Shkadov his due before exploring Vidal’s new concept. A Shkadov thruster works through a balancing act. A semi-spherical mirror is positioned in such a way that the pressure of solar photons balances the gravitational force of the star on the mirror. Achieve this and the radiation hitting the mirror reflects back toward the star, producing an imbalance that creates thrust. Here’s a diagram from a paper by Duncan Forgan (University of Edinburgh) that is the best I can find to illustrate the concept:

This is Figure 1 from Duncan Forgan’s paper “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves (citation below).” The treatment of Shkadov thrusters is useful, and I insert the diagram here because the Vidal paper only offers a short explanation of the original concept. Forgan’s caption: Diagram of a Class A Stellar Engine, or Shkadov thruster. The star is viewed from the pole – the thruster is a spherical arc mirror (solid line), spanning a sector of total angular extent 2ψ. This produces an imbalance in the radiation pressure force produced by the star, resulting in a net thrust in the direction of the arrow. Credit: Duncan Forgan.

Clément Vidal brings into the stellar engine mix the idea of using binary millisecond pulsars. You may recall Vidal’s stellivore concept, which we’ve discussed over the years in Centauri Dreams. Here’s how he describes it in the current paper:

…the stellivore hypothesis, that reinterprets some observed accreting binary stars as advanced civilizations feeding on stars… The scenario would be the following. For most of its time, a stellivore civilization would eat its home star via accretion. However, energy is never eternal, and instead of eating its star until the end and dying, a stellivore civilization would use its low-mass companion star as fuel not to be accreted, but to be evaporated, in order to create thrust and travel towards a nearby star. The fundamental accretion dynamics of close binaries would be reversed into an evaporation dynamics.

In the new paper, Vidal homes in on so-called spider pulsars, which are binary systems made up of one millisecond pulsar and a low-mass companion star. This in itself has always struck me as an extraordinary situation. We’re positing a massive star in a binary system that becomes a supernova and leaves behind a neutron star. In many cases, the binary system (i.e., the companion star) survives this event, allowing the neutron star thereafter to draw material out of it through accretion. It’s this transfer of mass that ‘spins’ up the neutron star – this is simple transfer of angular momentum – so that the neutron star can rotate at millisecond rates.

I haven’t seen any figures on what percentage of binary systems can survive a supernova event, but it’s telling that a high proportion of millisecond pulsars are indeed found in binary systems. Millisecond pulsars (MSPs) are detectable when the beam of electromagnetic radiation they emit sweeps past our observing apparatus.

Image: This diagram shows the steps astronomers say are needed to create a pulsar with a superfast spin. 1. A massive supergiant star and a “normal” Sun-like star orbit each other. 2. The massive star explodes, leaving a pulsar that eventually slows down, turns off, and becomes a cooling neutron star. 3. The Sun-like star eventually expands, spilling material on to the neutron star. This “accretion” speeds up the neutron star’s spin. 4. Accretion ends, the neutron star is “recycled” into a millisecond pulsar. But in a densely packed globular cluster (2b)… The lowest mass stars are ejected, the remaining normal stars evolve, and the “recycling” scenario (3-4) takes place, creating many millisecond pulsars. Credit: B. Saxton, NRAO/AUI/NSF.

A spider pulsar as described by Vidal is made up of a one millisecond pulsar and a relatively small companion, usually between 0.1 and 0.7 solar masses. And here we get the unusual evaporation effect that the above quote references. Rather than accreting matter from the companion star, a spider pulsar system evaporates the companion star. Hence the ‘spider’ metaphor, referring to female spiders that eat their male companion after mating. Get the right configuration and you have what Vidal calls a spider stellar engine. How it can be used to accelerate, decelerate and steer over interstellar distances takes up much of the paper. More on that next time.

In this kind of engine, the payload is a neutron star of about 1.8 solar masses. The propellant is the low-mass companion star somewhere between 0.1 and 0.7 solar masses. And where are the passengers; i.e., the civilization that is transporting itself around the galaxy? We have a lot to discuss here, and I also want to dig into the Hooper paper referenced above as well as the subject of hypervelocity stars, which are possible instances of stellar engineering technosignatures. We continue next time, with an email exchange I’ve been having with Vidal and the fine-tuning of stellar engines.

Clément Vidal’s new paper is “The Spider Stellar Engine: a Fully Steerable Extraterrestrial Design?” Journal of the British Interplanetary Society Vol. 77 (2024), 156-166 (preprint). Leonid Shkadov’s original paper on the Shkadov thruster is “Possibility of controlling solar system motion in the galaxy,” 38th Congress of IAF,” October 10-17, 1987, Brighton, UK, paper IAA-87-613. The Forgan paper is “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves,” accepted at the Journal of the British Interplanetary Society (preprint).

Habitability and a Variable Young Sun

Given our intense scrutiny of planets around other stars, I find it interesting how little we know even now about the history of our own Sun, and its varying effects on habitability. A chapter in an upcoming (wildly overpriced) Elsevier title called The Archean Earth is informative on the matter, especially insofar as it illuminates which issues most affect habitability and how the values for these vary over time. It’s also a fascinating look at changing conditions on Venus, Earth and Mars.

We know a great deal about the three worlds from our local and planetary explorations, but all too little when it comes to explaining the evolution of their atmospheres and interior structures. But it’s important to dig into all this because as Stephen Kane, director of The Planetary Research Laboratory at UC-Riverside and colleagues point out, we seem to be looking at the end state of habitability on both Mars and Venus, meaning that our explorations of these worlds should yield insights into managing our own future. Powerful lessons are likely to emerge, some obvious, some obscure.

The chapter title is “Our Solar System Neighborhood: Three Diverging Tales of Planetary Habitability and Windows to Earth’s Past and Future” (citation below), from which this:

Resolving the numerous remaining questions regarding the conditions and properties of our local inventory of rocky worlds is… critical for informing planetary models that show how surface conditions can reach equilibrium states that are either temperate and habitable, or hostile with thick and/or eroded atmospheres.

Given the challenges of working at the Venusian surface, Mars is particularly valuable as a case study, a world that has evolved through a period of habitability (is this true of Venus?) and undergone interior changes including cooling, not to mention loss of atmosphere. The more we learn about the evolution of rocky worlds, the more we will be able to apply these discoveries to planets around other stars. We’re in search of models, in other words, that show how planets can reach equilibrium as habitable or hostile.

The Archean eon (roughly 4 to 2.5 billion years in extent) takes us through the formation of landmasses, Earth’s early ‘reducing’ atmosphere (light on oxygen but rich in methane, ammonia, hydrogen carbon dioxide) and all the way to the emergence of life. So we have plenty to work with, but for today I want to focus on one aspect of habitability that we haven’t discussed all that often, the changes in the Sun since Earth’s formation. Our G-class star had to progress through a protostar stage lasting, perhaps, 100,000 years or more, then a T-tauri phase likely lasting 10 million years. There’s a lot going on here, including shedding angular momentum as the young Sun expels mass, slowing its rotation rate and producing a powerful solar wind.

The Sun’s arrival on the main sequence follows, and we’re immediately involved in a singularly vexing astronomical puzzle. The consensus seems to be that the early main sequence Sun was only about 70 percent as bright as the star we see today. That being the case, how do we account for the fact that water appears early in Earth’s history in liquid form on the surface? The expectation of a frozen world is a natural one, yet liquid water at the surface seems to have been available for some four billion years.

Image: The landscape during the early Archean could have looked like this: small continents on a planet mostly covered by oceans, illuminated by the much fainter young Sun, and with a Moon that appears larger in the sky because of its smaller distance to Earth. Credit: Potsdam Institute for Climate Impact Research.

Infant stars in the T-tauri phase are quite bright, getting their energy primarily from gravitational contraction. When they reach the main sequence and hydrostatic equilibrium sets in, they are considerably dimmer. The Sun enters the main sequence undergoing stable hydrogen fusion, which drives its luminosity. The notion that the Sun was dimmer early in the main sequence derives from models showing a solar core that is less dense and contains less helium. As core density increases as hydrogen is converted to helium, the resultant heating accounts for the steady brightening since.

Carl Sagan was an early voice in exploring the ‘faint young Sun’ problem in relation to liquid water and the matter is still controversial, although a number of possibilities exist. The authors believe the logical way to explain why a fainter star could provide conditions for liquid water on the surface is through the presence of greenhouse gases like methane and carbon dioxide, although the influence of methane is problematic unless, as in recent papers, scientists invoke planetary impacts or a primitive photosynthetic biosphere that amplifies the methane cycle. Whatever the case, mechanisms to regulate habitability are clearly crucial. From the chapter:

It is remarkable that Earth seems to have become habitable within its first few hundreds of millions of years and maintained that state seemingly without interruption to the present. This persistence is a testimony to the power of feedbacks that drive and regulate climate evolution with notable success on Earth in comparison with our Solar System neighbors Venus and Mars. Most important is the silicate weathering negative feedback, whereby rising temperatures coinciding with a warming Sun are offset by increasing rates of CO2 consumption through continental weathering, which is enhanced under warmer, wetter conditions. And we can thank diverse plate tectonic processes and their roles in such carbon cycling for this sustained thermostatic capacity over much of our history.

But let’s return to that very early Sun emerging onto the main sequence. Factors affecting a young star’s luminosity include the rotation rate of the star, and here we have little information (the Sun’s rotational rate in this era is uncertain, although some evidence suggests that it was a slow rotator). But the earliest planets could have been pummeled by radiation in this early state for tens or even hundreds of millions of years, which would have had a dire effect on any atmospheres they had formed. Note this:

…the intensity of the emission depends on the rotation rates of the stars, which are highly variable (from observation of stars in <500 Myr old stellar clusters). “Fast rotating” young stars can reach about a hundred times the rotation rate of the modern Sun, whereas moderate rotators could rotate at about ten times that rate and slow rotators would reach a few times the Sun’s rotation rate (Tu et al., 2015). Corresponding luminosities for young stars in the EUV and X-ray range of the spectrum can reach on the order of a 1000 times present mean solar luminosity at similar wavelengths for fast rotators, a few 100 times for moderate rotators and between a few tens to a hundred times the reference for slow rotators.

We’ve looked before at both the solar wind and solar flares as factors influencing planetary atmospheres and interacting with their magnetic fields, perhaps stripping atmospheres entirely in the case of M-class planets orbiting tightly around their primary. Atmospheric escape takes two forms, the first being thermal loss, where atoms are accelerated to the point where they reach escape velocity. Intense radiation is one but not the only way of doing this, but the effect on an atmosphere can be dire.

Other processes that can adversely affect an atmosphere include ions being accelerated along magnetic field lines by the solar wind or photochemical reactions. You would think that these effects would vary with distance from the star, but as the authors note, the loss estimates for Mars, Venus and Earth are relatively similar, and this despite the major differences in the makeup of the atmospheres on these planets and their magnetic field situation. Mars lacks a protective magnetic field today but seems to have had one in the past. Venus likewise lacks an intrinsic magnetic field like that produced on Earth through its convective dynamo, although interactions between the solar wind and the CO2-rich atmosphere do produce a weak magnetic effect.

Image: Diagramming a solar flare encountering Earth’s magnetosphere. Credit: Jing Liu (Shandong University, China and National Center for Atmospheric Research in the U.S).

As to the Sun, variations in the solar magnetic field over time are assumed given that a faster spinning star would have produced higher levels of magnetic activity. Today’s solar cycle, eleven years long, is the result of the magnetic reversal cycle of the Sun. The twists and tangles of magnetic field lines during this cycle cause sunspot activity to peak and then decline as the north and south magnetic poles reverse. The Sun’s magnetic field strengthens and weakens during the cycle, with effects apparent in the solar wind itself and in spectacular events like coronal mass ejections.

If we back out to the largest timescales, the variations in the Sun’s magnetic field indicate a much stronger field early in the Sun’s evolution, coupled with a (presumed) faster stellar rotation rate. From the standpoint of habitability, that tells us that ultraviolet and X-ray flux along with the solar wind has been decreasing with time. We’re probably talking about orders of magnitude difference in UV and X-ray radiation, but the authors point out that the intensity of emission depends upon the rotation rate of stars, and there is much we still have to learn about both this and luminosity.

The high degree of uncertainty on the Sun’s rotation rate and temperature affect our estimates of the ultraviolet at X-ray emissions our star would have produced, and thus the impact of this flux on the atmospheres of the terrestrial planets. Enlarging our field of view to exoplanets, it’s clear that the more we can piece together about the Sun’s early history, the more we can learn about the atmospheres and habitability of planets around the various spectral types of stars. Kane and team see the Sun as a template for planetary evolution, which makes heliophysics crucial for the study of astrobiology.

This study, then, homes in on the factors that need to be clarified as both heliophysics and exoplanetary science move forward. Even the closest planet to us presents formidable challenges, and the authors’ discussion of Venus is well worth your time. Here we have a world whose young surface – relatively uniform in the range of 250 to 1000 million years old – makes digging into its more remote past all the more difficult. Evidence can only be indirect at this stage of planetary exploration. We’re lucky to have Mars within reach as we widen the study of planetary formation to worlds other than our own.

The chapter in the Elsevier book is Kane et al., “Our Solar System Neighborhood: Three Diverging Tales of Planetary Habitability and Windows to Earth’s Past and Future,” available as a preprint. Two Sagan papers on the faint young Sun paradox are notable: Sagan & Mullen, “Earth and Mars: Evolution of Atmospheres and Surface Temperatures”, Science 177 (1972), 52 (abstract) and Sagan & Chyba, “The early faint sun paradox: Organic shielding of ultraviolet-labile greenhouse gases,” Science 276 (1997), 1217 (abstract). You might also want to look at Ozaki et al., “Effects of primitive photosynthesis on Earth’s early climate system,” Nature Geoscience 11 (2018), 55-59 (abstract), which offers up a methane solution for the faint young Sun.

Vega’s Puzzling Disk

Over the weekend I learned about Joseph Haydn’s Symphony No. 47, unusual in that it offers up some of its treasures in perfect symmetry. Dubbed ‘The Palindrome,’ the symphony’s third movement, Minuetto e Trio, is crafted to play identically whether attacked normally – moving forward through the score – or backwards. You can check this out for yourself in this YouTube video, or on this non-auditory reference.

The pleasure of unexpected symmetry is profound, and when seen through the eyes of our spacecraft, can be startling. Consider the storied star Vega. We see this system from our perspective at a very low inclination angle relative to its rotational axis, as if we were looking down from above the star’s pole. This face-on perspective is profoundly interesting when examined through our space-borne astronomical assets. In the image below, we get two views of Vega’s disk, from Hubble and then JWST.

Image: The disk around Vega as seen by Hubble (left) and Webb (right). Hubble detects reflected light from dust the size of smoke particles largely in a halo on the periphery of the 160-billion-kilometer-wide disk. Webb resolves the glow of warm dust in a disk halo, at 37 billion kilometers out. The outer disk (analogous to the solar system’s Kuiper Belt) extends from 11 billion kilometers to 24 billion kilometers. The inner disk extends from the inner edge of the outer disk down to close proximity to the star. Credit: NASA, ESA, CSA, STScI, S. Wolff (University of Arizona), K. Su (University of Arizona), A. Gáspár (University of Arizona).

We don’t, of course, have the perfect symmetry of the Haydn minuet and trio here, but do notice how smooth this disk appears, and particularly take note of the lack of any embedded planets, the sort of large objects we observe in formation around stars like Beta Pictoris. This bright star, a summer object in Lyra for those of us in the northern hemisphere, was well studied in the infrared by astronomers using the Spitzer Space Telescope, and now we have the clearest view ever, the subject of two upcoming papers in The Astrophysical Journal. Co-author Andras Gáspár (Steward Observatory, University of Arizona, calls the Vega disk “ridiculously smooth.”

The view shows layered structure, with Hubble detecting dust on the outer regions of the disk and Webb’s infrared view resolving the warmer dust in the inner disk, the disk in this region being sand-sized as compared to the outer halo, whose particles are of the consistency of smoke. These are useful observations because they offer information on dust movement in circumstellar disks, and also point out the stark difference among the planetary disks thus far observed. The hot, A-class Vega at 450 million years old remains a relatively young star and we would expect interactions among debris in the disk to keep replenishing it, a process that continues in our own much older system.

But where are the planets in formation? Lead author Kate Su (University of Arizona), says that the lack of them forces astronomers to rethink the way they’ve looked at forming planetary systems. It’s possible to tease out a gap at about 60 AU, but everywhere else the disk appears smooth, which rules out planets larger than Neptune in outer orbits. Adds Su:

“We’re seeing in detail how much variety there is among circumstellar disks, and how that variety is tied into the underlying planetary systems. We’re finding a lot out about the planetary systems — even when we can’t see what might be hidden planets. There’s still a lot of unknowns in the planet-formation process, and I think these new observations of Vega are going to help constrain models of planet formation.”

In the second of the two papers, lead author Schuyler Wolff (University of Arizona) and colleagues home in on the star Fomalhaut by way of comparison, this being a star that is a close twin to Vega in terms of luminosity, distance and age. As the paper notes, “[T]he Vega disk morphology differs significantly from Fomalhaut,” going on to say:

Vega’s fellow archetypical disk, Fomalhaut, has been extensively studied in scattered light (Kalas et al. 2005; Gáspár & Rieke 2020) and shows a narrow cold belt from 130-150 au (possibly confined by an undetected planet e.g., Boley et al. 2012) with a scattered light halo extending outwards from the belt. The parent planetesimal belt is clearly detected with ALMA (MacGregor et al. 2017), with an estimated dust mass of 0.015±0.010M⊕. Fomalhaut has also recently been the target of groundbreaking JWST observations (Gáspár et al. 2023) showing the same narrow cold belt (albeit with a slight radial offset indicative of grain size stratification) and halo.

Indeed, Fomalhaut appears to be building a planet within its debris disk, and possibly more than one, these objects acting as ‘shepherds’ shaping the disk. Any such worlds in the Vega system are clearly much smaller, if indeed they exist. Vega’s dust distribution is broad, whereas at Fomalhaut the outer debris ring is the dominant source of light available for our observation. Three nested debris belts appear at Fomalhaut.

Nothing better than a new astronomical puzzle, especially when it challenges our notions of planet formation. You may remember Vega’s role in Heinlein’s Have Spacesuit Will Travel (1958), where its civilization acts as a kind of overseer for Earth, and perhaps the ‘Vegan Tyranny’ from James Blish’s Cities in Flight series. For that matter, Vega has an important commercial role in Asimov (the source of ‘Vegan tobacco’ in the Foundation books), appears prominently in Jack Vance’s work, and is of course of great interest to Ellie Arroway in Carl Sagan’s book and film Contact. How enjoyable that it now throws a new mystery at us.

The papers are Su et al., “Imaging of the Vega Debris System using JWST/MIRI,” accepted at The Astrophysical Journal (preprint) and Wolff et al., “Deep Search for a scattered light dust halo around Vega with the Hubble Space Telescope,” accepted at The Astrophysical Journal (preprint).

Monkeying Around with Shakespeare

Bear with me today while I explore the pleasures of the Infinite Monkey Theorem. We’re all familiar with it: Set a monkey typing for an infinite amount of time and eventually the works of Shakespeare emerge. It’s a pleasing thought experiment because it’s so visual and involves animals that are like us in many ways. Now we learn from a new paper that the amount of time involved to reproduce the Bard is actually longer than the age of the universe. About which more in a moment, but indulge me again as I explore infinite monkeys as they appear in fictional form in the mid-20th Century.

In “Inflexible Logic,” which ran in The New Yorker‘s February 3, 1940 issue, Russell Maloney tells the tale of a man named Bainbridge, a bachelor, dilettante and wealthy New Yorker who lived in luxury in a remote part of Connecticut, “in a large old house with a carriage drive, a conservatory, a tennis court, and a well-selected library.” He has about him the air of an English country gentlemen of the 18th Century, interested in both the arts and science. An eccentric.

One night at a party in the city, Bainbridge enters into conversation with literary critic Bernard Weiss, who he overhears saying of a lionized author: “Of course he wrote one good novel. It’s not surprising. After all, we know that if six chimpanzees were set to work pounding six typewriters at random, they would, in a million years, write all the books in the British Museum.”

Impressed, Bainbridge learns that the experiment has never been tried. He acquires six chimpanzees and provides them with paper and typewriters. Some weeks later, he is with James Mallard, an assistant professor of mathematics at Yale, whom he has asked to his estate to discuss the ongoing experiment. Showing him the monkeys at work, he points to tall piles of manuscript along the wall, containing in each complete works by writers such as Charles Dickens, Anatole France, Somerset Maugham and Marcel Proust.

Image credit: Amazingly generated by Gemini AI. Note that the three foreground monkeys have their typewriters facing the wrong way, but I suppose it doesn’t matter, as they can still reach the keys.

Indeed, since the beginning of the experiment over a month before, not a single monkey has spoiled a single sheet of paper. Great literary works continue to pile up. After Mallard leaves, the weeks go by and the monkeys never cease their labors. They produce Trevelyan’s Life of Macaulay, The Confessions of St. Augustine, Vanity Fair and more. Bainbridge keeps passing this information on to Mallard, who grows increasingly confounded. And worried.

Finally, when leafing through a manuscript of Pepys’ Diary produced by Chimpanzee F (named Corky), a work that contains material not in his own abridged edition, Bainbridge is again visited by Mallard at his home. Taken back to the scene of the experiment, Mallard pulls out two revolvers and shoots the chimpanzees. Both men end up, after a fight, shooting each other and both die. Mallard’s last words: “The human equation…always the enemy of science… I deserve a Nobel.”

And so the story concludes:

“When the old butler came running into the conservatory to investigate the noises, his eyes were met by a truly appalling sight. A newly risen moon shone in through the conservatory windows on the corpses of the two gentlemen, each clutching a smoking revolver. Five of the chimpanzees were dead. The sixth was Chimpanzee I. His right arm disabled, obviously bleeding to death, he was slumped before his typewriter. Painfully, with his left hand, he took from the machine the completed last page of Florio’s Montaigne. Groping for a fresh sheet, he inserted it, and typed with one finger, “UNCLE TOM’S CABIN, by Harriett Beecher Stowe. Chapte…” Then he too was dead.”

Maloney was a Harvard grad who seeded ideas for many of The New Yorker‘s cartoons; he became editor and writer of the Talk of the Town section. Here he shows us what happens when something absurd becomes true. The story was reprinted in Clifton Fadiman’s Fantasia Mathematica (Simon and Schuster, 1958).

What would actually happen if we set up Bainbridge’s test? The new work exploring this is out of the University of Sydney, where Stephen Woodcock and Jay Falletta considered the problem within a more spacious context. Bainbridge was dealing with a tiny cadre of six monkeys. But most versions of the thought experiment involve infinity. Woodcock explains:

“The Infinite Monkey Theorem only considers the infinite limit, with either an infinite number of monkeys or an infinite time period of monkey labour. We decided to look at the probability of a given string of letters being typed by a finite number of monkeys within a finite time period consistent with estimates for the lifespan of our universe.”

This is the kind of thing mathematicians do, and I have often wished I had the slightest gift for math so I could join the company of such a jolly group. Or maybe that’s only in Australia, because I’ve known a few grim mathematicians as well. Whatever the case, the new paper appears in a peer-reviewed journal called Franklin Open. The authors assume a keyboard with 30 keys, which allows for all the letters of English along with the most common of the punctuation marks. They assumed that one key would be pressed every second until the end of the universe in 10100 years.

The latter are bold assumptions considering monkey finger dexterity as well as attention span, and I’ll also note that the end of universe calculation is very much up for grabs, although excellent books like Fred Adams and Greg Laughlin’s The Five Ages of the Universe deal with numbers like this. Still, we’re not exactly sure that the accelerating expansion of the universe is stable, or what it might do one day.

But enough of that.

Get this: There is a 5% chance that a single chimpanzee might type the word ‘bananas’ in its lifetime. But Woodcock and Falletta worked out two sets calculations, the second involving a population of 200,000 chimpanzees (200,000 is apparently the current global population of chimpanzees, although that number seems low to me). Anyway, if you throw the entire 200,000-strong retinue at Shakespeare, the Bard’s 884,647 words will not be typed before the end of the universe. As the authors point out:

“It is not plausible that, even with improved typing speeds or an increase in chimpanzee populations, monkey labour will ever be a viable tool for developing non-trivial written works.”

AI is another matter…

Of course, Mr. Bainbridge used only six monkeys, and look what he got. I think we can take the ending of the Russell Maloney story to be saying something about our attitudes toward science. Our essential understanding of probability had better be right. If it turns out we unleash monkeys who begin typing out For Whom the Bell Tolls, we are confronted with not just an improbability, but an assault on the structure of the cosmos. We can see why professor Mallard lost his wits and blew the monkeys away, an outcome that, had the story been written in our more animal-considerate times, would not have been allowed by the editor.

Mallard thought an impossibility could not be allowed to exist. He had saved science.

And I have to add the delightful conclusion from the paper:

Given plausible estimates of the lifespan of the universe and the amount of possible monkey typists available, this still leaves huge orders of magnitude differences between the resources available and those required for non-trivial text generation. As such, we have to conclude that Shakespeare himself inadvertently provided the answer as to whether monkey labour could meaningfully be a replacement for human endeavour as a source of scholarship or creativity. To quote Hamlet, Act 3, Scene 3, Line 87: “No”.

The paper is Woodcock and Falletta, “A Numerical Evaluation of the Finite Monkeys Theorem,” Franklin Open Vol. 9 (December, 2024) 100171 (full text).

Deep Space Implications for CubeSats

The Hera mission has been dwarfed in press coverage by the recent SpaceX Starship booster retrieval and the launch of Europa Clipper, both successful and significant. But let’s not ignore Hera. Its game plan is to check on the asteroid Dimorphos, which became the first body in the Solar System to have its orbit altered by human technologies when the DART spacecraft impacted it in 2022. Hera is all about assessing this double asteroid system to see first-hand the consequences of the impact, which shortened the smaller object’s orbit around asteroid Didymos by some 32 minutes.

That’s a pretty good result, some 25 times what NASA had defined as the minimum successful orbital period change, and we’re learning more about the ejecta, which involve tons of asteroidal rock. The collision occurred at 6.1 kilometers per second, to be more fully assessed by Hera’s twin CubeSat craft, which will make precise measurements of Dimorphos’ mass to analyze the efficiency of the impact. All this factors into planning for asteroid impact missions if an object ever threatens Earth.

Image: Astronomers using the NSF’s NOIRLab’s SOAR telescope in Chile captured the vast plume of dust and debris blasted from the surface of the asteroid Dimorphos by NASA’s DART spacecraft when it impacted on 26 September 2022. In this image, the more than 10,000 kilometer long dust trail — the ejecta that has been pushed away by the Sun’s radiation pressure, not unlike the tail of a comet — can be seen stretching from the center to the right-hand edge of the field of view. Credit: CTIO/NOIRLab/SOAR/NSF/AURA/T. Kareta (Lowell Observatory), M. Knight (US Naval Academy).

I want to draw your attention to those two CubeSats aboard Hera. My thinking is that these miniature spacecraft, built upon standardized 10-cm boxes, are a story as big as the results they’ll gather. There are two of them: Juventas is the product of GOMspace (Luxembourg), designed to make the first radar probe of the interior of an asteroid. Milani was produced by Tyvak International, an Italian operation. Its job is to perform multispectral mineral prospecting. Their recent activation for a check of on-board systems was in both cases completely successful. Says ESA engineer Franco Perez Lissi:

“Each CubeSat was activated for about an hour in turn, in live sessions with the ground to perform commissioning – what we call ‘are you alive?’ and ‘stowed checkout’ tests. The pair are currently stowed within their Deep Space Deployers, but we were able to activate every onboard system in turn, including their platform avionics, instruments and the inter-satellite links they will use to talk to Hera, as well as spinning up and down their reaction wheels which will be employed for attitude control.”

So this is good news for the Hera mission, but in the larger context we are seeing the continuing growth of miniaturized technologies that will improve the efficiency and capability of missions to much more distant targets. Juventas was activated on October 17 at a distance of 4 million kilometers from Earth; Milani’s turn came on October 24, with the craft 7.9 kilometers out. Both will be deployed from their ‘mothership’ to make close approaches to Dimorphos upon arrival in 2026.

It was back in 2011 that NanoSail-D2 demonstrated successful sail deployment, to be followed by The Planetary Society’s LightSail-a in 2015. We were learning that a spacecraft as small as a CubeSat could carry a solar sail, leading to The Planetary Society’s subsequent LightSail missions. Sara Seager at MIT has investigated CubeSats as exoplanet research platforms, the notion being that a fleet of CubeSats could be deployed with each monitoring a single star. The first detection of an exoplanet by a CubeSat occurred in 2017 with the ASTERIA (Arcsecond Space Telescope Enabling Research In Astrophysics) 6U CubeSat space telescope.

NASA’s Mars Cube One (MarCO) CubeSats demonstrated multiple CubeSat operations beyond Earth orbit in 2018, and there have been a host of CubeSat projects in the hands of private companies and universities ranging from 2009’s AeroCube-3 to LunaH-Map in 2022, the latter aimed at mapping the distribution of hydrogen at the Moon’s south pole. QubeSat is a project out of UC-Berkeley to study quantum gyroscopes in low Earth orbit and explore precision control for small satellite navigation. CubeSats are cheap. Lose one at launch – and early failures abound, as witness Lunar Flashlight – and the impact on your budget is minimized.

Image: The first image captured by one of NASA’s Mars Cube One (MarCO) CubeSats. The image, which shows both the CubeSat’s unfolded high-gain antenna at right and the Earth and its moon in the center, was acquired by MarCO-B on May 9, 2018. Credit: NASA/JPL-Caltech.

Most CubeSat missions have been designed for low-Earth orbit but some scientists are aiming to go much farther. In a 2023 paper Slava Turyshev (JPL) and colleagues investigated small spacecraft (smallsats) with solar sails in missions to the outer system. In general, a ‘smallsat’ refers to a spacecraft that is both small and lightweight, usually less than 500 kilograms, and sometimes much less, as when we get into the realm of CubeSats.

Smallsats with solar sails are a combination of miniaturization and efficiency, for a sail requires no on-board propellant and can be sent on ‘sundiver’ trajectories that could achieve, in the view of the authors, velocities of 33 kilometers per second (7 AU per year). By comparison, Voyager 1’s pace is 17.1 kilometers per second. Using chemical propulsion, we would need 15 years to reach Uranus, so much faster travel times are welcome.

So let’s add to Sara Seager’s ideas on exoplanet constellations of CubeSats the idea of solar sail smallsats on fast trajectories for flyby missions, impactor missions like DART, and formation and swarm operations at targets like the ice giants, about which we know all too little. The challenge here will be the need for lightweight instrumentation and continuing miniaturization, both trends that seem to be accelerating. Two-years to Jupiter and three to Saturn at these velocities are tantalizing prospects as Turyshev and team continue to study sailcraft designs to reach the Sun’s gravity lens beginning at 550 AU.

How does a fleet of future smallsats hardened for deep space and using modularized components stack up against, say, a single enormous (by comparison) orbiter to Uranus or Neptune? The two could, of course, work together, but given the costs, flyby missions on the cheap in their tens or hundreds could offer priceless scientific return. We always return to the practicalities of prying money out of political entities, so finding ways to lighten the budget while accomplishing the mission will continue to be a priority. Will we see an ice giant orbiter off in the 2030s? Somehow I doubt it.

The exoplanet paper via CubeSat constellation mentioned above is “Demonstrating high-precision photometry with a CubeSat: ASTERIA observations of 55 Cancri e,” The Astronomical Journal Vol. 160, No. 1 (2020), 23 (full text). The Turyshev paper is “Science opportunities with solar sailing smallsats,” Planetary and Space Science Vol. 235 (1 October 2023). Full text. For more on all this, see Building Smallsat Capabilities for the Outer System, published last year in these pages.

Do You Really Want to Live Forever?

Supposing you wanted to live forever and found yourself in 2024, would you sign up for something like Alcor, a company that offers a cryogenic way to preserve your body until whatever ails it can be fixed, presumably in the far future? Something over 200 people have made this choice with Alcor, and another 200 at the Cryonics Institute, whose website says “life extension within reach.” A body frozen at −196 °C using ‘cryoprotectants’ can, so the thinking goes, survive lengthy periods without undergoing destructive ice damage, with life restored when science masters the revival process.

It’s not a choice I would make, although the idea of waking up refreshed and once again healthy in a few thousand years is a great plot device for science fiction. It has led to one farcical public event, in the form of Nederland, Colorado’s annual Frozen Dead Guys Days festival. The town found itself with a resident frozen man named Bredo Morstøl, brought there by his grandson Trygve Bauge in 1993 and kept in dry ice by Trygve’s mother when her son was deported for visa violations. Bredo Morstøl remains on ice in Nederland, where the festival includes “Frozen Dead Guy” lookalike contests, although Covid caused several recent cancellations. I am not making any of this up.

I have it on good authority that Robert Heinlein was once asked by a cryonics enthusiast why he shouldn’t sign up for cryopreservation, and Heinlein replied that he would not because he was too interested in what would happen to him after biological life ended. My source, a writer who knew Heinlein for decades, raised his eyebrows when telling me this. Does anything ‘happen’ after biological life ends? No one knows, of course, and even near-death experiences can’t tell us because we don’t know how to interpret them. Getting into a religious answer is something left to the preference of the reader.

Image: The Triumph of Death, Peter Bruegel the Elder, oil on panel, 1562, Prado, Madrid.

Given these musings, I was interested to see that science fiction author Ted Chiang has explored a related question: Should we even pursue the study of immortality as a desirable goal? The Chiang talk was titled “Do You Really Want to Live Forever?” held at Princeton as part of a lecture series that drew 200 attendees to Chiang’s talk. I want to look at what he said, but only after a few notes on my own preferences.

First, as to why I would never choose cryopreservation: If I wanted to live forever, I would balk at using what have to be considered rudimentary and questionable methods to do so. I’ve heard the argument that as time passes, techniques will improve, and any damage to the body can be mitigated along the way to reviving it, but even if this is so, I would fear some kind of weird consciousness emerging in my frozen brain, sort of akin to what Larry Niven’s astronauts on Pluto experienced in the story “Wait It Out.” Stuck on Pluto and with relief decades away, they expose themselves to the elements, only to find that their flash-frozen minds are still active through superconducting effects. Imagine that extended over centuries…

Well, it probably couldn’t happen, but it’s a grim thought, and it alone would keep me from dialing the Alcor number. But would I accept if given a credible way to stay alive forever, perhaps a new discovery in the form of a simple injection guaranteed to do the job? I’d like to hear from readers on the pros and cons of that choice. Because of course ‘forever’ only means as long as something doesn’t happen to take you out. ‘Eternity’ gets snuffed through simple accident somewhere along the line, and it’s inevitable that after a few tens of thousands of years, something is going to get me.

So it’s a bedeviling personal choice and we should be thinking about it. After all, research into life extension in forms other than cryopreservation continues. Advanced AI may even tell us how to do it within a decade or two. Chiang, whose fiction is monumentally good (I consider “Story of Your Life” one of the finest pieces of writing ever to appear in science fiction), notes the concerns society faces over such decisions. Writers Sena Chang and Christopher Bao, covering the Chiang lecture event for The Daily Princetonian, say that Chiang waived away any moral judgments on eternal life (what would these be?) but noted: “The universe, as we understand it, does not enforce justice in any way. If it turns out that medical immortality is impossible, that, by itself, will not mean that immortality is a bad thing to want.”

But as we follow this path, we have to ask what the individual would experience with a ticket to immortality. Chiang isn’t sure it would be a desirable life. For one thing, a person sated with life expectancy and no fear of death would probably not be motivated to accomplish anything interesting. Life might get, shall we say, dreary. Moreover, if immortality becomes a viable option, we face issues of overpopulation that are obvious, and the likelihood of seriously exacerbated wealth inequalities. Here Chiang settles on something I want to quote, as drawn from the article:

“…the relationship between what is sustainable and what is ethical is not simple. The desire to live forever is fundamentally in conflict with the desire to have children. Allowing people to pursue one of these goals will inevitably entail restrictions on people to pursue the other.”

To take this further, immortality disrupts the process that leads to the very advances in medicine and technology that make it possible in the first place. Chiang sees this process as a ‘social instinct’ and identifies this need for ‘collective scholarship’ through the social impulse as underlying our science. Immortality breaks the bond. Thus the billionaires who try to extend their lifetimes who seem to follow a cultural muse based on individuality and egotism as opposed to what benefits society at large.

What an intriguing thought. Yet it’s probable that if a key to immortality is achieved by science, it will start out by being fantastically expensive and in the hands of a tiny coterie of people whose wealth is beyond the imagination of almost all of us. As I see it, they would then have a choice. Do I share this information? Would they factor into the question the welfare of society at large, or make a personal choice based on their own fear of death? Would fantastically wealthy immortals become a cadre of rulers over a society that otherwise continues to face the everyday dilemma of the end of life?

Chiang pokes around in questions like this in much of his fiction. Remember, this is a guy for whom awards are routine, including four Nebulas, four Hugos and six Locus awards. Stories of Your Life and Others is the best way to get into his work, especially in times when the AI question is becoming acute and the very meaning of advanced intelligence is under scrutiny. Structures of language, Chiang understands, undergird all our perception, and they play against the philosophies by which we describe ourselves. It can be said that Chiang has few answers – who does? – but no one asks the questions and draws out the ineffability of human experience with more eloquence.

Does Artificial Intelligence Explain the Fermi Question?

Science fiction has been exploring advanced machine intelligence and its consequences for a long time now, and it’s now being bruited about in service of the Fermi paradox, which asks why we see no intelligent civilizations given the abundant opportunity seemingly offered by the cosmos. A new paper from Michael Garrett (Jodrell Bank Centre for Astrophysics/University of Manchester) explores the matter in terms of how advanced AI might provide the kind of ‘great filter’ (the term is Robin Hanson’s) that would limit the lifetime of any technological civilization.

The AI question is huge given its implications in all spheres of life, and its application to the Fermi question is inevitable. We can plug in any number of scenarios that limit a technological society’s ability to become communicative or spacefaring, and indeed there are dozens of potential answers to Fermi’s “Where are they?” But let’s explore this paper because its discussion of the nature of AI and where it leads is timely whether Fermi and SETI come into play or not.

A personal note: I use current AI chatbots every day in the form of ChatGTP and Google’s Gemini, and it may be useful to explain what I do with them. Keeping a window open to ChatGTP offers me the chance to do a quick investigation of specific terms that may be unclear to me in a scientific paper, or to put together a brief background on the history of a particular idea. What I do not do is to have AI write something for me, which is a notion that is anathema to any serious writer. Instead, I ask AI for information, then triple check it, once against another AI and then against conventional Internet research. And I find the ability to ask for a paragraph of explanation at various educational levels can help me when I’m trying to learn something utterly new from the ground up.

It’s surprising how often these sources prove to be accurate, but the odd mistake means that you have to take great caution in using them. For example, I asked Gemini a few months back how many planets had been confirmed around Proxima Centauri and was told there were none. In reality, we do have one, that being the intriguing Proxima b, which is Earth-class and in the habitable zone. And we have two candidates: Proxima c is a likely super-Earth on a five-year orbit and Proxima d is a small world (with mass a quarter that of Earth) orbiting every five days. Again, the latter two are candidates, not confirmed planets, as per the NASA Exoplanet Archive. I reported all this to Gemini and yesterday the same question produced an accurate result.

So we have to be careful about AI in even its current state. What happens as it evolves? As Garrett points out, it’s hard to come up with any area of human interest that will be untouched by the effects of AI, and commerce, healthcare, financial investigation and many other areas are already being impacted. Concerns about the workforce are in the air, as are issues of bias in algorithms, data privacy, ethical decision-making and environmental impact. So we have a lot to work with in terms of potential danger.

Image: Michael Garrett, Sir Bernard Lovell chair of Astrophysics at the University of Manchester and the Director of the Jodrell Bank Centre for Astrophysics (JBCA). Credit: University of Manchester.

Garrett’s focus is on AI’s potential as a deal-breaker for technological civilization. Now we’re entering the realm of artificial superintelligence (ASI), which was Stephen Hawking’s great concern when he argued that further developments in AI could spell the end of civilization itself. ASI refers to an independent AI that becomes capable of redesigning itself, meaning it moves into areas humans do not necessarily understand. An AI undergoing evolution and managing it at an ever increasing rate is a development that could be momentous and one that poses obvious societal risks.

The author’s assumption is that if we can produce AI and begin the process leading to ASI, then other civilizations in the galaxy could do the same. The picture that emerges is stark:

The scenario…suggests that almost all technical civilisations collapse on timescales set by their wide-spread adoption of AI. If AI-induced calamities need to occur before any civilisation achieves a multiplanetary capability, the longevity (L) of a communicating civilization as estimated by the Drake Equation suggests a value of L ∼ 100–200 years.

Which poses problems for SETI. We’re dealing with a short technological window before the inevitable disappearance of the culture we are trying to find. Assuming only a handful of technological civilizations exist in the galaxy at any particular time (and SETI always demands assumptions like this, which makes it unsettling and in some ways related more to philosophy than science), then the probability of detection is all but nil unless we move to all-sky surveys. Garrett notes that field of view is often overlooked amongst all the discussion of raw sensitivity and total bandwidth. A telling point.

But let’s pause right there. The 100-200 year ‘window’ may apply to biological civilizations, but what about the machines that may supersede them? As post-biological intelligence rockets forward in technological development, we see the possibility of system-wide and even interstellar exploration. The problem is that the activities of such a machine culture should also become apparent in our search for technosignatures, but thus far we remain frustrated. Garrett adds this:

We…note that a post-biological technical civilisation would be especially well-adapted to space exploration, with the potential to spread its presence throughout the Galaxy, even if the travel times are long and the interstellar environment harsh. Indeed, many predict that if we were to encounter extraterrestrial intelligence it would likely be in machine form. Contemporary initiatives like the Breakthrough Starshot programme are exploring technologies that would propel light-weight electronic systems toward the nearest star, Proxima Centauri. It’s conceivable that the first successful attempts to do this might be realised before the century’s close, and AI components could form an integral part of these miniature payloads. The absence of detectable signs of civilisations spanning stellar systems and entire galaxies (Kardashev Type II and Type III civilisations) further implies that such entities are either exceedingly rare or non-existent, reinforcing the notion of a “Great Filter” that halts the progress of a technical civilization within a few centuries of its emergence.

Biological civilizations, if they follow the example of our own, are likely to weaponize AI, perhaps leading to incidents that escalate to thermonuclear war. Indeed, the whole point of ASI is that in surpassing human intelligence, it will move well beyond oversight mechanisms and have consequences that are unlikely to merge with what its biological creators find acceptable. Thus the scenario of advanced machine intelligence finding the demands on energy and resources of humans more of a nuisance than an obligation. Various Terminator-like scenarios (or think Fred Saberhagen’s Berserker novels) suggest themselves as machines set about exterminating biological life.

There may come a time when, as they say in the old Westerns, it’s time to get out of Dodge. Indeed, developing a spacefaring civilization would allow humans to find alternate places to live in case the home world succumbed to the above scenarios. Redundancy is the goal, and as Garrett notes: “…the expansion into multiple widely separated locations provides a broader scope for experimenting with AI. It allows for isolated environments where the effects of advanced AI can be studied without the immediate risk of global annihilation. Different planets or outposts in space could serve as test beds for various stages of AI development, under controlled conditions.”

But we’re coming up against a hard stop here. While the advance of AI is phenomenal (and some think ASI is a matter of no more than a few decades away), the advance of space technologies moves at a comparative crawl. The imperative of becoming a technological species falls short because it runs out of time. In fact – and Garrett notes this – we may need ASI to help us figure out how to produce the system-wide infrastructure that we could use to develop this redundancy. In that case, technological civilizations may collapse on timescales related to their development of ASI.

Image: How will we use AI in furthering our interests in exploring the Solar System and beyond? Image credit: Generated by AI / Neil Sahota.

We talk about regulating AI, but how to do so is deeply problematic. Regulations won’t be easy. Consider one relatively minor current case. As reported in a CNN story, the chatbot AI ChatGTP can be tricked into bypassing blocks put into place by OpenAI (the company behind it) so that hackers can plan a variety of crimes with its help. These include money laundering and the evasion of trade sanctions. Such workarounds in the hands of dark interests are challenging at today’s level of AI, and we can see future counterparts evolving along with the advancing wave of AI experiments.

It could be said that SETI is a useful exercise partly because it forces us to examine our own values and actions, reflecting on how these might transform other worlds as beings other than ourselves face the their own dilemmas of personal and social growth. But can we assume that it’s even possible to understand, let alone model, what an alien being might consider ‘values’ or accepted modes of action? Better to think of simple survival. That’s a subject any civilization has to consider, and how it goes about doing it will determine how and whether it emerges from a transition to machine intelligence.

I think Garrett may be too pessimistic here:

We stand on the brink of exponential growth in AI’s evolution and its societal repercussions and implications. This pivotal shift is something that all biologically-based technical civilisations will encounter. Given that the pace of technological change is unparalleled in the history of science, it is probable that all technical civilisations will significantly miscalculate the profound effects that this shift will engender.

I pause at that word ‘probable,’ which is so soaked in our own outlook. As we try to establish a regulatory framework that can help AI progress in helpful ways and avoid deviations into lethality, we should consider the broader imperative. Call it insurance. I think Garrett is right in noting the lag in development in getting us off-planet, and can relate to his concern that advanced AI poses a distinct threat. All the more reason to advocate for a healthy space program as we face the AI challenge. And we should also consider that advanced AI may become the greatest boon humanity has ever seen in terms of making startling breakthroughs that can change our lives in short order.

Call me cautiously optimistic. Can AI crack interstellar propulsion? How about cancer? Such dizzying prospects should see us examining our own values and how we communicate them. For if AI might transform rather than annihilating us, we need to understand not only how to interact with it, but how to ensure that it understands what we are and where we are going.

The paper is Garrett, “Is artificial intelligence the great filter that makes advanced technical civilisations rare in the universe?” Acta Astronautica Vol. 219 (June 2024), pp. 731-735 (full text). Thanks to my old friend Antonio Tavani for the pointer.

Streams of Stars and What They Tell Us

A quick follow-up to yesterday’s post. The idea of a stream of debris or even large objects like comets or asteroids from another star continues to resonate with me. The odds on identifying such a stream in terms of origin seem stupendous, but the benefits of doing so would be obvious. I notice that another kind of stellar stream is in the news, one involving not debris but entire stars. The Icarus stream is a grouping of stars that seem to have been tidally disrupted by the Milky Way, probably from an earlier encounter between the parent galaxy and a dwarf galaxy.

Digging a bit, I learned that we can carry the idea of stellar streams back to the work of Donald Lynden-Bell, who in 1995 proposed the stream concept to explain the long structure or filament of stars evidently tidally stripped from the Sagittarius Dwarf Spheroidal Galaxy, the latter being a satellite galaxy of the Milky Way. The Sgr dSph, as it is known, actually contains four globular clusters within it. It travels a polar orbit around the Milky Way at a distance of some 50,000 light years from galactic center, and has evidently passed through the plane of the galaxy at least several times.

Image: The Sagittarius dwarf galaxy, a small satellite of the Milky Way that is leaving a stream of stars behind as an effect of our Galaxy’s gravitational tug, is visible as an elongated feature below the Galactic center and pointing in the downwards direction in the all-sky map of the density of stars observed by ESA’s Gaia mission between July 2014 to May 2016. Scientists analyzing data from Gaia’s second release have shown our Milky Way galaxy is still enduring the effects of a near collision that set millions of stars moving like ripples on a pond. The close encounter likely took place sometime in the past 300–900 million years, and the culprit could be the Sagittarius dwarf galaxy. Credit: ESA/Gaia/DPAC,CC BY-SA 3.0 IGO.

Stellar streams, like the debris streams we considered yesterday, are the result of tidal interactions that are now well studied, resulting in the identification of further structures like the Icarus stream (the Gaia observatory is frequently relied upon for the relevant data). Such stretched out streams of stars contain clues about the Milky Way’s gravitational potential and the distribution of mass, which accounts for the continuing interest in the formations. The Icarus stream apparently came from a dwarf galaxy with a stellar mass of about one billion solar masses in a low-inclination orbit.

We now have a new paper from a team of scientists led by Paola Re Fiorentin (Observatory of Turin), again using Gaia data along with results from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the Galactic Archaeology with HERMES (GALAH). 81 members of the Icarus stream have been identified, upon which the group has managed to conduct chemical analysis and investigate the dynamics of the stream, learning that it is at least 12 billion years old. Everything about this work supports the notion of the disrupted galaxy remnant scenario.

But we’re early in the game when it comes to these probes of galactic interactions and what they tell us about the earliest days of the Milky Way. The paper adds this:

…even though we cannot exclude the contamination of unevolved low metallicity in situ stars, we argue that the Icarus stream is consistent with debris of a low-inclination prograde dwarf galaxy with a stellar mass ∼ 109M⊙. In any case, we remark that this scenario assumes the existence of a protogalactic disk formed in situ at ages > 12 Gyr (cf. Xiang & Rix 2022). A similar scenario was suggested by Carter et al. (2021) that proposed the accretion of low-α stars from a co-rotating dwarf galaxy onto a primordial high-α disk.

Image: The Milky Way and its halo of stars, many of which have been cannabalised via collisions with other galaxies (Image credit: ESA/Gaia/DPAC, T Donlon et al. 2024; Background Milky Way and Magellanic Clouds: Stefan Payne-Wardenaar).

In the quotation above, the term low-α stars refers to stars low in the elements formed through the fusion of helium nuclei in what are known as alpha-capture processes. These would include oxygen, magnesium, sulfur, silicon, etc. Low-α stars are considered to have formed in regions where star formation was relatively slow. Studying this variable can offer insights into star formation histories. What all this means is that the Icarus stream seems consistent with the scenario of galaxy disruption.

But the paper adds:

However, the origin of metal-poor stars with disk kinematics is currently a matter of lively debate in the astronomical community. We think that such controversial interpretations could derive from the different selection criteria that may generate kinematically or chemically biased samples.

Such studies probe the formation history of the galaxy, and are in their early stages. The Icarus stream does appear to have formed outside the Milky Way, which would be consistent with everything we’re learning about the formation of stellar streams. All of which points to the complex interactions that move material among the stars, but reinforces the difficulty of deducing the origin of material from any single star.

The Lynden-Bell paper on the Sagittarius stream is “Ghostly streams from the formation of the Galaxy’s halo,” Monthly Notices of the Royal Astronomical Society, Volume 275, Issue 2 (July 1995), pp. 429–442 (abstract). The Paola Re Fiorentin paper is “Icarus revisited: An Ancient, Metal-poor Accreted Stellar Stream in the Disk of the Milky Way,” accepted at The Astrophysical Journal and available as a preprint.

An Incoming ‘Stream’ from Alpha Centauri?

Here’s an interesting thought. We know that at least two objects from outside our Solar System have appeared in our skies, the comet 2I/Borisov and the still enigmatic object called ‘Oumuamua. Most attention on these visitors has focused on their composition and the prospects of one day visiting such an interloper, for it is assumed that with new technologies like the Vera Rubin Observatory and its Legacy Survey of Space and Time (LSST), we will be picking up more of the same.

But consider origins. Extrapolating backward to figure out where either object came from quickly exhausts the most patient researcher, for it only takes the slightest changes in trajectory to widen the search field so broadly as to be useless. That’s especially true since we don’t know the ages of the objects, which may span hundreds of millions of years.

Enter Cole Gregg (University of Western Ontario), who has embarked on a project to study the question from a different perspective. Gregg asks how likely it is that material from another star could be passing through Sol’s neighborhood. As presented at the American Astronomical Society’s Division for Planetary Sciences meeting in Boise, Idaho earlier this month, his calculations explore the gravitational interactions such objects would experience. And he studies whether ‘streams’ of such material could identify by their common characteristics the likely system of origin.

The question is intriguing given how difficult it is to get a probe to Alpha Centauri at present levels of technology. Material from that system, if identified as such, would have interesting implications, especially with regard to the dispersal of chemical elements and organic molecules. The idea of panspermia rides on the possibility that life itself might move between planets and even stars in such material, so getting our hands on something from another star would offer obvious benefits.

Gregg is working on this project with colleague Paul Wiegert, who is perhaps best known for pioneering work with Matt Holman back in the 1990s on planetary orbits possible in the Alpha Centauri system. That work established that rocky terrestrial worlds could remain on stable orbits around Centauri A or B within 3 AU of the host stars, and a bit further out if the planetary orbit was retrograde. As these orbits were stable for billions of years, this finding was an incentive for deeper investigation into Alpha Centauri as a possible home for life.

Gregg and Wiegert now turn their attention to what they call ‘interstellar meteor streams,’ analyzing the development of such streams as they form and “as the material evolves in time through a time-independent, asymmetric potential model for our galaxy.” Their method is to conduct simulations with existing galactic models that involve ejected material in the galactic bulge, the halo and the disk, with the direction of ejection oriented randomly and at speeds of up to 15 kilometers per second.

We’re early in the process here (remember that this work has just been presented), but quoting from a brief write-up made available for a 2023 conference:

As expected, since the physics of their motion is identical, we see the ejecta travel through the disk of the Galaxy in much the same way that stars do. However, through the progression of the simulation we see complicated and interesting effects on the ejecta swarm, hinting at the complicated evolution of interstellar meteoroid streams. This talk will also discuss the implications of the modeled motion on the identification of interstellar meteoroid streams.

For stars in the galactic disk (which is where we are), material ejected from their systems evolves in a way similar to meteoroid streams we see in the Solar System, undergoing ‘orbital shear’. These streams can be long-lived and may include a flux of material passing through our Solar System that, while small, would be of great interest. Gregg sees his ongoing work as refining the process of interstellar transfer and producing population estimates for interstellar visitors currently nearby.

Image: A hypothetical interstellar meteoroid stream after 185 Myr of evolution from a burst ejection originating in the Alpha Centauri system. Credit: Cole Gregg.

As to the Centauri system, Gregg told Sky & Telescope in an article published this month that his calculations using a simplified model of the Milky Way show that about 0.03 percent of material ejected from Alpha Centauri could reach the Solar System, and perhaps most important, be recognized as coming from that source. We’re talking about material that could be as small as dust grains and as large as a comet or asteroid, but moving within a stream that has similar orbital velocities within the galaxy and similar positions, the latter traits making their source identifiable.

Gregg’s work is intriguing and also preliminary, for he intends to produce a full follow-up study looking to refine the galactic model and include better estimates on the likely size of the material that would have made such a crossing. So at this point what we have is only a possibility that may be excluded if the further analysis rules it out. But it’s a worthwhile study given the implications of finding such a stream.

The Sky & Telescope article is from October 18, 2024, titled “Are Objects from Alpha Centauri Streaming by Earth?” (available here). Gregg’s writeup for a 2023 conference is “The Development of Interstellar Meteoroid Streams” (full text). A PowerPoint presentation can be found here. The Wiegert and Holman paper, a key paper in Alpha Centauri studies, is “The Stability of Planets in the Alpha Centauri System,” Astronomical Journal 113 (1997), 1445–1450 (abstract).

SETI: Learning from TRAPPIST-1

Given our decades-long lack of success in finding hard evidence for an extraterrestrial civilization, it hardly comes as a surprise that a recent campaign studying the seven-planet TRAPPIST-1 system came up without a detection. 28 hours of scanning with the Allen Telescope Array by scientists at the SETI Institute and Penn State University produced about 11,000 candidate signals for further analysis, subsequently narrowed down to 2,264 of higher interest. None proved to be evidence for non-human intelligence, but the campaign is interesting in its own right. Let’s dig into it.

The unique configuration of the TRAPPIST-1 planets allowed the scientists involved to use planet-planet occultations (PPOs). A cool M-dwarf star, TRAPPIST-1 brings with it the features that make such stars optimal for detecting exoplanets. The relative mass and size of the planets and star mean that if we’re looking for rocky terrestrial-class worlds, we’re more likely to find and characterize them than around other kinds of star. True, they’re also orbiting a class of star that is dim, but another beauty of TRAPPIST-1 is that it’s only 40 light years out, and we see its seven planets virtually edge-on.

Planets e, f and g can be squeezed into the star’s habitable zone (liquid water on the surface) if we tweak our numbers for possible atmospheres. The edge-on vantage means that planets can pass in front of each other from our viewpoint, with the additional advantage that this well-studied system has planetary orbits that are sharply defined. This raises intriguing possibilities when you consider our own space activities. The Deep Space Network sends powerful signals to communicate with distant craft like the Voyagers, signals wide enough to propagate beyond them and into deep space. The right kind of receiver, if by chance aligned with them, might make a detection, producing evidence for a technology by the nature of its signal.

At TRAPPIST-1, there are seven planet-planet occultations, with two of them involving a potential transmission-source planet within the star’s habitable zone. But we have to consider that transmissions between planets might not be this limited, for radio traffic could move through relays placed for communications purposes on worlds that are uninhabitable. This would obviously be traffic never intended for interstellar reception, the kind of ongoing activity that marks a society communicating with itself, but perhaps leaving a technosignature that would reach the Earth through the width of its beam.

Image: A look from above at the communications line of sight between two worlds in the TRAPPIST-1 system, illustrating the PPO method used in this study. Credit: SETI Institute/Zayna Sheikh.

The possibilities of a detection using this PPO technique vary, of course, with the orbital parameters of the planets in any given system. We must also account for the drift rates produced by orbital motion. The paper explains the recent search’s technique this way:

…it is assumed that the TRAPPIST-1 planets are tidally locked due to their proximity to their host star and will have a negligible rotational contribution to the drift rate of a transmitter on their surfaces. Additionally, their orbital parameters are well constrained, making it possible to calculate the drift rate contributions from their orbital motion. Satellite transmitters in circular orbit around each planet could produce much higher drift rates, up to an additional ∼45 nHz on top of the contribution from the planet’s orbit around the star. However, we have chosen to limit our scope to analogues of our deep space communications, the strongest of which are surface transmitters to deep space probes.

The seven planet-planet occultations studied during the 28 hours of observation ranged from 8.6 minutes to 99.4 minutes. And it turns out that widening that window of observation through simulations produces numerous PPOs with a similarly large range of duration, making this strategy still more interesting. The animation below shows the TRAPPIST-1 system in motion and the possible communications opportunities. Credit: Tusay et al., citation below.

http://www.centauri-dreams.org/wp-content/uploads/2024/10/concatenated_gifs.mp4

Animation: This is Figure 10 from the paper, the caption of which reads: Simulated potential PPO events during our observations. Online viewers will see a concatenated video of the orbital configuration of the system during each of the observations, including any potential PPO events that we found to occur during those windows. A still image of a PPO event during the observation on Oct 29, 2022 is included where the animation is not accessible. The top panel shows a bird’s-eye view of the system with planet radii scaled up for better viewing. The distances and beam sizes are to scale, assuming a beam created with a 3.4m dish at 3.3 GHz (the maximum frequency observed during this particular session) from the surface of planet g aimed at planet e. The bottom panel shows the edge-on view with planet sizes scaled with distance, showing how much of the beam spills over the planet toward the direction of Earth in the negative z-direction. The red dashed lines in the illustrated beam is the inner angle blocked by the occulting planet, e. The blue dashed lines show the outer angle of the beam that would spill over the planet. The window for this event lasted roughly 95 minutes. Credit: Tusay et al.

Is this method the longest of longshots? SETI itself might be described that way, depending on your views of life in the cosmos. But our steadily growing capabilities at signal detection can’t be ruled out when we consider the possibilities. From the paper:

The analysis of the observations presented here demonstrates that precise characterization of ideal systems, like TRAPPIST-1, enabling orbital dynamical modeling and prediction of PPO events offer practical application for leaked emission searches. This provides SETI a powerful new observational tool and search strategy. As signal detection and RFI mitigation pipelines improve, the inclusion of PPOs to provide narrow search windows may make it more feasible to increase time resolution and sensitivity at higher drift rates.

What beckons most strongly about technosignatures is that they assume no intent (which in any case would be unguessable) on the part of a hypothetical alien civilization. We would essentially be eavesdropping on their activities. Grad student Nick Tusay (Pennsylvania State), lead author of the paper on this work, adds this: “[W]ith better equipment, like the upcoming Square Kilometer Array (SKA), we might soon be able to detect signals from an alien civilization communicating with its spacecraft.” And that would be a SETI detection for the ages.

The paper is Tusay et al., “A Radio Technosignature Search of TRAPPIST-1 with the Allen Telescope Array,” currently available as a preprint.