Centauri Dreams
Imagining and Planning Interstellar Exploration
What We’re Learning about TRAPPIST-1
It’s no surprise that the James Webb Space Telescope’s General Observers program should target TRAPPIST-1 with eight different efforts slated for Webb’s first year of scientific observations. Where else do we find a planetary system that is not only laden with seven planets, but also with orbits so aligned with the system’s ecliptic? Indeed, TRAPPIST-1’s worlds comprise the flattest planetary arrangement we know about, with orbital inclinations throughout less than 0.1 degrees. This is a system made for transits. Four of these worlds may allow temperatures that could support liquid water, should it exist in so exotic a locale.
Image: This diagram compares the orbits of the planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the planets found around TRAPPIST-1 orbit much closer to their star than Mercury is to the Sun, but as their star is far fainter, they are exposed to similar levels of irradiation as Venus, Earth and Mars in the Solar System. Credit: ESO/O. Furtak.
The parent star is an M8V red dwarf about 40 light years from the Sun. It would be intriguing indeed if we detected life here, especially given the star’s estimated age of well over 7 billion years. Any complex life would have had plenty of time to evolve into a technological phase, if this can be done in these conditions. But our first order of business is to find out whether these worlds have atmospheres. TRAPPIST-1 is a flare star, implying the possibility that any gaseous envelopes have long since been disrupted by such activity.
Thus the importance of the early work on TRAPPIST-1 b and c, the former examined by Webb’s Mid-Infrared Instrument (MIRI), with results presented in a paper in Nature. We learn here that the planet’s dayside temperature is in the range of 500 Kelvin, a remarkable find in itself given that this is the first time any form of light from a rocky exoplanet as small and cool as this has been detected. The planet’s infrared glow as it moved behind the star produced a striking result, explained by co-author Elsa Ducrot (French Alternative Energies and Atomic Energy Commission):
“We compared the results to computer models showing what the temperature should be in different scenarios. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.”
The TRAPPIST-1 work is moving relatively swiftly, for already we have the results of a second JWST program, this one executed by the Max Planck Institute for Astronomy and explained in another Nature paper, this one by lead author Sebastian Zieba. Here the target is TRAPPIST-1 c, which is roughly the size of Venus and which, moreover, receives about the same amount of stellar radiation. That might imply the kind of thick atmosphere we see at Venus, rich in carbon dioxide, but no such result is found. Let me quote Zieba:
“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds. If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn’t detect any coming from the planet.”
Image: This light curve shows the change in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c, moves behind the star. This phenomenon is known as a secondary eclipse. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)
What JWST is measuring is the 15-micron mid-infrared light emitted by the planet, using the world’s secondary eclipse, the same technique used in the TRAPPIST-1 b work. The MIRI instrument observed four secondary eclipses as the planet moved behind the star. The comparison of brightness between starlight only and the combined light of star and planet allowed the calculation of the amount of mid-infrared given off by the dayside of the planet. This is remarkable work: The decrease in brightness during the secondary eclipse amounts to 0.04 percent, and all of this working with a target 40 light years out.
Image: This graph compares the measured brightness of TRAPPIST-1 c to simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or a very thin carbon dioxide atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere with sulfuric acid clouds, similar to that of Venus (yellow line), is unlikely. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI).
I should also mention that the paper on TRAPPIST-1 b points out the similarity of its results to earlier observations of two other M-dwarf stars and their inner planets, LHS 3844 b and GJ 1252 b, where the recorded dayside temperatures showed that heat was not being redistributed through an atmosphere and that there was no absorption of carbon dioxide, as one would expect from an atmosphere like that of Venus.
Thus the need to move further away from the star, as in the TRAPPIST-1 c work, and now, it appears, further still, to cooler worlds more likely to retain their atmospheres. As I said, things are moving swiftly. In the coming year for Webb is a follow-up investigation on both TRAPPIST-1 b and c, in the hands of the system’s discoverer, Michaël Gillon (Université de Liège) and team. With a thick atmosphere ruled out at planet c, we need to learn whether the still cooler planets further out in this system have atmospheres of their own. If not, that would imply formation with little water in the early circumstellar disk.
The paper is Zieba et al., “No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c,” Nature 19 June 2023 (full text). The paper on TRAPPIST-1 b is Greene et al., “Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST,” Nature 618 (2023), 39-42 (abstract).
Abundant Phosphorus in Enceladus Ocean Increases Habitability: But is Enceladus Inhabited?
Finding the right conditions for life off the Earth justifiably drives many a researcher’s work, but nailing down just what might make the environment beneath an icy moon’s surface benign isn’t easy. The recent wave of speculation about Enceladus revolves around the discovery of phosphorus, a key ingredient for the kind of life we are familiar with. But Alex Tolley speculates in the essay below that we really don’t have a handle on what this discovery means. There’s a long way between ‘habitable’ and ‘inhabited,’ and many data points remain to be analyzed, most of which we have yet to collect. Can we gain the knowledge we need from a future Enceladus plume mission?
by Alex Tolley
There has been abundant speculation about the possibility of life in the subsurface oceans of icy moons. Europa’s oceans with possible hydrothermal vents mimicking Earth’s abyssal oceans and the probable site of the origin of life, caught our attention now that Mars has no extant surface life. Arthur C Clarke had long suggested Europa as an inhabited moon in his novel 2010: Odyssey Two. (1982). While Europa’s hot vents are still speculative based on interpretations of the surface features of its icy crust, Saturn’s moon, Enceladus, showed visible aqueous plumes at the southern pole. These plumes ejected material that contributes to the E-Ring around Saturn as shown below.
While most searches for evidence for life focus on organic material, it has been noted that of the necessary elements for terrestrial life, Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, and Phosphorus (CHONSP), phosphorus is the least abundant cosmically. Phosphorus is a key component in terrestrial life, from energy management (ATP-ADP cycle) and information molecules DNA, and RNA, with their phosphorylated sugar backbones.
If phosphorus is absent, terrestrial biology cannot exist. Phosphorus is often the limiting factor for biomass on Earth, In freshwater environments phosphorus is the limiting nutrient [1]. Typically, algae require about 10x as much nitrogen as phosphorus. If the amount of available nitrogen is increased, the algae cannot use that extra nitrogen as the amount of available phosphorus now determines how large the algal population can grow. The biomass-to-phosphorus ratio is around 100:1. When phosphorus is the limiting nutrient, then the available phosphorus will limit the biomass of the local plants and therefore animals, regardless of the availability of other nutrients like nitrogen, and other factors such as the amount of sunlight, or water. Agriculture fertilizer runoff can cause algal blooms in aqueous environments and may result in dead zones as oxygen is depleted by respiration as phytoplankton blooms die or are consumed by bacteria.
While nitrogen can be fixed by bacteria from the atmosphere, phosphorus is derived from phosphate rocks, and rich sources of phosphorus for agriculture were historically gleaned from bird guano.
A recent paper in Nature about the detection of phosphorus in the grains from the E-ring by the Cassini probe’s Cosmic Dust Analyzer (CDA) suggested that phosphorus is very abundant. As these grains are probably sourced from Enceladus’ plumes, this implies that this moon’s subsurface ocean has high levels of dissolved phosphorus.
The authors of the paper have modeled, and experimentally confirmed the model, and make the claim that Enceladus’ ocean is very rich in phosphorus:
…phosphorus concentrations at least 100-fold higher in the moon’s plume-forming ocean waters than in Earth’s oceans.
around 100-fold greater than terrestrial phosphorus abundance levels. They show that the CDA spectrum [figure 1) is consistent with a solution of disodium phosphate (Na2HPO4) and trisodium phosphate (Na3PO4) (figure 2) The source of these salts on Enceladus is likely from the hot vents chemically releasing the material from the carbonaceous chondritic rocky core and the relatively alkaline ocean. Contrary to intuition, the greater CO2 concentrations in cold water with the hydroxyapatite-calcite and whitlockite-calcite buffer system maintain an alkaline solution that allows for the high phosphate abundance in the plume material that produces the grains in Saturn’s E-ring.
Figure 1. CDA cation spectrum co-added from nine baseline-corrected individual ice grain spectra. The mass lines signifying a high-salinity Type 3 spectrum are Na + (23 u) and (NaOH)Na + (63 u) with secondary Na-rich signatures of (H2O)Na + (41 u) and Na 2+ (46 u). Sodium phosphates are represented by phosphate-bearing Na-cluster cations, with (Na3 PO4)Na + (187 u) possessing the highest amplitude in each spectrum followed by (Na2HPO4 )Na + (165 u) and (NaPO3)Na + (125 u). The first two unlabelled peaks at the beginning of the spectrum are H + and C +, stemming from target contamination 3 (source nature paper). a.u., arbitrary units.
Figure 2. Spectrum from the LILBID analogue experiment reproducing the features in the CDA spectrum. An aqueous solution of 0.420 M Na2HPO4 and 0.038 M Na3PO4 was used. All major characteristics of the CDA spectrum of phosphate-rich grains (Fig. 1) are reproduced at the higher mass resolution of the laboratory mass spectrometer (roughly 700 m/?m). Note: this solution is not equivalent to the inferred ocean concentration. To derive the latter quantity, the concentration determined in these P-rich grains must be averaged over the entire dataset of salt-rich ice grains. (source Nature paper).
Fig. 3: Comparison of observed and calculated concentrations of ΣPO43– in fluids affected by water–rock reactions within Enceladus. a, Relation between ΣPO43– and ΣCO2 at a temperature of 0.1 °C for the hydroxyapatite-calcite buffer system (solid lines) and the whitlockite-calcite buffer system (dashed lines). Constraints on ΣCO2 obtained in previous studies are indicated by the blue shaded area. The area highlighted in pink represents the range of ΣPO43– constrained in this study from CDA data. b, Dependence of ΣPO43– on temperature for the hydroxyapatite-calcite buffer and different values of pH and ΣCO2. A similar diagram for the whitlockite-calcite buffer can be found in Extended Data Fig. 11.
The simple conclusion to draw from this is that phosphorus is very abundant in the Enceladan ocean and that any extant life could be very abundant too.
While the presence of phosphorus ensures that the necessary conditions of elements for habitability are present on Enceladus, it raises the question: “Does this imply Enceladus is also inhabited?”
On Earth, phosphorus is often, the limiting factor for local biomass. On Enceladus, if phosphorus was the limiting factor, then one would not expect it to be detected as inorganic phosphate, but rather in an organic form, bound with biomolecules.
But suppose Enceladus is inhabited, what might account for this finding?
1. Phosphorus is not limiting on Enceladus. Perhaps another element is limiting allowing phosphates to remain inorganic. In Earth’s oceans, where iron (Fe) can be the limiting factor, adding soluble Fe to ocean water can increase algal blooms for enhanced food production and possible CO2 sequestration. On Enceladus, the limiting factor might be another macro or micronutrient. [This may be an energy limitation as Enceladus does not have the high solar energy flux on Earth.]
2. Enceladan life may not use phosphorus. Some years ago Wolfe-Simon claimed that bacteria in Mono Lake used arsenic (As) as a phosphorus substitute. [2] This would have been a major discovery in the search for “shadow life” on Earth. However, it proved to be an experimental error. Arsenic is not a good substitute for phosphorus, especially for life already evolved using such a critical element, and as is well-known, arsenic is a poison for complex life.
3. The authors’ modeling assumptions are incorrect. Phosphorus exists in the Enceladan ocean, but it is mostly in organic form. The plume material is non-biological and is ejected before mixing in the ocean and being taken up by life. The authors may also have wildly overestimated the true abundance of phosphorus in the ocean.
Of these explanations allowing for Enceladus to be inhabited, all seem to be a stretch that life may be in the ocean despite the high inorganic phosphorus abundance. Enceladan biomass may be constrained by the energy derived from the moon’s geochemistry. On Earth, sunlight is the main source of energy maintaining the rich biosphere. In the abyssal darkness, life is very sparse, although it can huddle around the deep ocean’s hot vents.
However, if life is not extant, then the abundance of inorganic phosphorus salts is simply the result of chemical equilibria based on the composition of Enceladus rocky core and abundant frozen CO2 where it formed beyond the CO2 snow line.
While the popular press often conflate habitability with inhabited, the authors are careful to make no such claim, simply arguing that the presence of phosphorus completes the set of major elements required for life:
Regardless of these theoretical considerations, with the finding of phosphates the ocean of Enceladus is now known to satisfy what is generally considered to be the strictest requirement of habitability.
With this detection, it would seem Enceladus should be the highest priority candidate for a search for life in the outer solar system. Its plumes would likely contain evidence of life in the subsurface ocean and avoid the difficult task of drilling through many kilometers of ice crust to reach it. A mission to Enceladus with a suite of life-detecting instruments would be the best way to try to resolve whether life is extant on Enceladus.
The paper is Postberg, F., Sekine, Y., Klenner, F. et al. Detection of phosphates originating from Enceladus’s ocean. Nature 618, 489–493 (2023). https://doi.org/10.1038/s41586-023-05987-9
References
1. Smil, V (2000) Phosphorus in the Environment: Natural Flows and Human Interferences. Annual Review of Energy and the Environment Volume 25, 2000 Smil, pp 53-88
2. Wolfe-Simon F, et al (2010) “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. SCIENCE 2 Dec 2010, Vol 332, Issue 6034 pp. 1163-1166
DOI:10.1126/science.1197258
Tightening our Understanding of Circumbinary Worlds
I’m collecting a number of documents on gravitational wave detection and unusual concepts regarding their use by advanced civilizations. It’s going to take a while for me to go through all these, but as I mentioned in the last post, I plan to zero in on the intriguing notion that civilizations with abilities far beyond our own might use gravitational waves rather than the electromagnetic spectrum to serve as the backbone of their communication system. It’s a science fictional concept for sure, though there may be ways it could be imagined for a sufficiently advanced culture.
For today, though, let’s look at a new survey that targets highly unusual planets. Binaries Escorted by Orbiting Planets has an acronym I can get into: BEBOP. It awakens the Charlie Parker in me; I can almost smell the smoky air of a mid-20th century jazz club and hear the clinking of glasses above Parker’s stunning alto work. I was thinking about the great sax player because I had just watched, for about the fifth time, Clint Eastwood’s superb 1988 film Bird, whose soundtrack is, of course, fabulous.
On the astronomy front, the BEBOP survey is a radial velocity sweep for circumbinary planets, those intriguing worlds, rare but definitely out there, that orbit around two stars in tight binary systems. Beginning in 2013, BEBOP targeted 47 eclipsing binaries, using data from the CORALIE spectrograph on the Swiss Euler Telescope at La Silla, Chile. This is intriguing because what we know about circumbinary planets has largely come from detections based on transit analysis. Radial velocity work has uncovered planets orbiting one star in a wide binary configuration but until now, not both.
Image: Artist’s visualization of a circumbinary planet. Credit: Ohio State University / Getty Images.
The new work adds data from the HARPS spectrograph at La Silla and the ESPRESSO spectrograph at Paranal to confirm one of two planets at TOI-1338/BEBOP-1. Thus we have radial velocity evidence for the gas giant BEBOP-1 c, massing in the range of 65 Earth masses, in an orbit around the binary of 215 days. A second world, referenced as TOI-1338 b because it shows up only in transit data from TESS, complements the RV find, making this only the second circumbinary system known to host multiple planets. TOI-1338 b is 21.8 times as massive as the Earth and as a transiting world could well be a candidate for atmospheric studies by the James Webb Space Telescope.
But BEBOP-1 c is the planet that stands out. I think I am safe in calling a co-author on this paper, David Martin (Ohio State University), a master of understatement when he describes the problems in extracting radial velocity data on a circumbinary world. After all, we’re relying on the tiniest gravitational effects flagged by minute changes in wavelength, and now we have to factor in multiple sets of stellar spectra. Here’s Martin:
“When a planet orbits two stars, it can be a bit more complicated to find because both of its stars are also moving through space. So how we can detect these stars’ exoplanets, and the way in which they are formed, are all quite different. Whereas people were previously able to find planets around single stars using radial velocities pretty easily, this technique was not being successfully used to search for binaries.”
Nice work indeed. Circumbinary planets are what the paper describes as ‘harsh environments’ for planet formation given the gravitational matrix in which such formation occurs, and thus we should be able to use the growing number of such systems (now 14 including this one) in the study of how planets form and also migrate. BEBOP should be a useful survey in providing accurate masses for planets in systems we’ve already discovered with the transit method.
Image: This is Figure 3 from the paper, offering an overview of the BEBOP-1 system. Caption: The BEBOP-1 system is shown along with the extent of the system’s habitable zone (HZ) calculated using the Multiple Star HZ website. The conservative habitable zone is shown by the dark green region, while the optimistic habitable zone is shown by the light green region. The binary stars are marked by the blue star symbols in the centre. The red shaded region denotes the instability region surrounding the binary stars as described by Holman and Wiegert. BEBOP-1 c’s orbit is shown by the red orbit models…shaded from the 50th to 99th percentiles. TOI-1338 b’s orbit is shown by the yellow models, and is also based on 500 random samples drawn from the posterior in its discovery paper. Credit: Standing et al.
Learning more about how planets in such perturbed environments emerge should advance the study of planet growth around single stars. It’s likely that the increased transit probabilities of circumbinary planets should play into our efforts to study planetary atmospheres as well. And while transits should provide the bulk of new discoveries in this space, radial velocity follow-ups should expand our knowledge of individual systems, being less dependent on orbital periods and inclination. BEBOP presages a productive use of these complementary observing methods.
The paper is Standing et al., “Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system,” Nature Astronomy 12 June 2023 (full text). See also Martin et al., “The BEBOP radial-velocity survey for circumbinary planets I. Eight years of CORALIE observations of 47 single-line eclipsing binaries and abundance constraints on the masses of circumbinary planets,” Astronomy & Astrophysics Vol. 624, A68 (April 2019), 45 pp. Abstract.
Building the Gravitational Machine
In fact, gravity is some 1038 times weaker than the strong force that holds atomic nuclei together, easily illustrated by pointing out to my friend that I was overcoming an entire planet’s worth of gravity by lifting the salt shaker on the table. I learned from Greg Matloff and Eugene Mallove’s The Starflight Handbook that despite Freeman Dyson’s early interest in using the gravitational force to capture energy from astronomical objects, it was Stanislaw Ulam who first pondered the idea in print.
Now Ulam is an interesting figure, a name that resonates on Centauri Dreams in the context of nuclear pulse propulsion, which he first analyzed as far back as 1947 in a report for Los Alamos Scientific Laboratory. This grew into the Project Orion concept, with nuclear bombs exploded behind a flat steel plate, the crew protected by the mother of all shock absorbers. Ulam’s work on gravitational machines, however, analyzed how much energy could be extracted in a three-body system, one of which was a rocket, and what kind of velocities such a rocket could attain.
Image: Stanislaw Ulam (1909-1984), well known for his work on Orion, but also an early analyst of the extraction of gravitational energy.
Freeman Dyson’s notion, explained in the paper “Gravitational Machines” that we looked at last week, was to extract energy from a binary star system, as shown in the figure from the paper below. The ever imaginative Dyson, remember, was captivated by the possibilities of engineering on the part of advanced civilizations, whose works we might observe in the form of technosignatures. Here, the idea involves two stars of mass equal to the Sun revolving around a common barycenter. A spacecraft can be injected into an orbit that maximizes the gravitational effect, as Dyson explains:
The exploiters of the device are living on a planet or vehicle P which circles around the double star at a distance much greater than R. They propel a small mass C into an orbit which falls toward the double star, starting from P with a small velocity. The orbit of C is computed in such a way that it makes a close approach to B at a time when B is moving in a direction opposite to the direction of arrival of C. The mass C then swings around B and escapes with greatly increased velocity. The effect is almost as if the light mass C had made an elastic collision with the moving heavy mass B. The mass C will arrive at a distant point Q with velocity somewhat greater than 2V.
Image: This is Figure 1 from the Dyson paper. Caption: The solid line indicates the orbit of A and B; the dashed line indicates the orbit of C. Credit: Freeman Dyson.
Two options open up as we reach point Q:
At Q the mass C may be intercepted and its kinetic energy converted into useful form. Alternatively the device may be used as a propulsion system, in which case C merely proceeds with velocity 2V to its destination. The destination might be a similar device situated very far away, which brings C to rest by the same mechanism working in reverse.
Why call this a ‘machine’? Dyson speculated that if the advanced civilization would create ‘a whole ring of starting points P and end points Q’ around the same binary system, masses dropped into the system would emerge as a continuous stream of payloads, or cargo, or whatever. The point is that the energy source for this system is simply the gravitational potential between the two stars.
Then we can go further and extrapolate what happens as the machine continues to function. Over large timespans, the two stars will be drawn closer together, with the effect that their orbital velocity will necessarily increase. Thus the machine continues to operate, extracting energy from the system until the stars close to such a tight distance that no passage between them is possible. Dyson thinks this would be a point where the distance between the centers of the two stars is 4 times the radius of each star.
Dyson calculated that the luminous energy radiated by Sun-like stars in a three-body system like this would be a more practical source than gravitational energy, but white dwarfs, far less luminous than the Sun, would ramp up the gravitational energy by a factor of a hundred. So there’s an interesting technosignature for you, a search for white dwarf binaries with the parameters defined by Dyson, marking a system that could accelerate objects to 2000 kilometers per second without any propellant.
The ever imaginative Dyson thought such a system of white dwarf binaries scattered around the galaxy could serve as a long-haul freight transportation network. More significantly, he went on to consider the more condensed form of star known as the neutron star, which as the time of writing was still a theoretical concept. “[T]he fact that none has yet been observed does not argue strongly against their existence.” And of course, it would not be long before Jocelyn Bell and Antony Hewish found the first pulsar in 1967.
If we were to choose a pair of white dwarf stars as our binary system, Greg Matloff notes in The Starflight Handbook, we might reach velocities of 0.009 c. This is roughly 2700 kilometers per second, not bad given our Voyager 1 travel speed of a mere 17.1 kilometers per second. Even so, it’s a long way to Proxima Centauri. If we could work with a pair of neutron stars, according to the calculations Dyson made, we might reach 0.27 c,or almost 81,000 kilometers per second. Now we’re moving out, reaching Proxima in a couple of decades. Then, of course, we’ve got to slow down.
Adds Dyson:
…it may be said that the dynamics of stellar systems, under conditions in which gravitational radiation is important, is a greatly neglected field of study. In any search for evidences of technologically advanced societies in the universe, an investigation of anomalously intense sources of gravitational radiation ought to be included.
What an extraordinary thinker Dyson was! I look forward to the recent essay collection “Well, Doc, You’re In”: Freeman Dyson’s Journey through the Universe (MIT Press, 2022), just arrived here and placed at the top of my stack of necessary reading. Meanwhile, it’s intriguing to take the subject further still. Although Dyson didn’t push into this direction, Greg Benford has examined how truly advanced civilizations might create a different kind of gravitational machine to enable communications systems that would make using the electromagnetic spectrum seem quaint. More on that soon.
Inadvertent Test Post
Those of you who follow Centauri Dreams through email probably received an inadvertent test post this morning. My apologies. The post was triggered by work on the site’s internals and was generated automatically by the email software module. Work on the site continues, but I think the email issue is fixed, so I anticipate no more of these. Thanks for your patience.
Freeman Dyson’s Gravitational Machines
But what a tangled history this paper presents. First of all, how does a 1962 paper get onto arXiv? A quick check reveals the uploader as David Derbes, a name that should resonate with Dyson purists. Derbes (University of Chicago Laboratory Schools, now retired) is the power behind getting Dyson’s lectures on quantum electrodynamics, first given at Cornell in 1951, into print in the volume Advanced Quantum Mechanics (World Scientific Publishing, 2007). He’s also an editor on Sidney Coleman’s Lectures on Relativity (Cambridge University Press, 2022) and has written a number of physics papers.
“Gravitational Machines” has been hard to find. Dyson wrote it, according to my polymath friend Adam Crowl, for the Gravitational Research Foundation in 1962; Centauri Dreams regular Al Jackson corroborates this in an email exchange, noting that the GRF was created by one Roger Babson, who offered a prize for such papers. Astrophysicist Alastair G. W. Cameron added it to his early SETI tome Interstellar Communications: A Collection of Reprints and Original Contributions (W. A. Benjamin, 1963). The paper, a tight six pages, does not appear in the 1996 volume Selected Papers of Freeman Dyson with Commentary (American Mathematical Society, 1996).
So we can be thankful that David Derbes saw fit to post it on arXiv. Al Jackson noted in his email that Greg Benford and Larry Niven have used Dyson’s gravitational concepts in their work, so I suspect “Gravitational Machines” was a paper known to them at this early stage of their career. A recent phone call with Jim Benford also reminded me of the Dyson paper’s re-emergence. Listen to Dyson’s familiar voice in 1962:
The difficulty in building machines to harness the energy of the gravitational field is entirely one of scale. Gravitational forces between objects of a size that we can manipulate are so absurdly weak that they can scarcely be measured, let alone exploited. To yield a useful output of energy, any gravitational machine must be built on a scale that is literally astronomical. It is nevertheless worthwhile to think about gravitational machines, for two reasons. First, if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future when engineering on an astronomical scale will be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kinds of observable phenomena a really advanced technology might be capable of producing.
There’s the Dysonian reach into the far future, sensing where exponential technology growth might lead a civilization, and speculating at the most massive scale on the manipulation of matter as a form of engineering. But here too is the Dyson of ‘Dyson Sphere’ fame, tackling the question of whether or not such a project would be observable if undertaken elsewhere in the cosmos, just as he would go on to bring numerous other ideas on ‘technosignatures’ to our consciousness. Hence the term ‘Dysonian SETI,’ which I’ve often used here on Centauri Dreams.
Dyson goes on to speculate on the nature of eclipsing white dwarf binaries and their output of gravitational radiation, working the math to demonstrate the strength of such systems in terms of gravitational wave output, and finding that the output might be detectable. However, what catches his eye next is the idea of neutron star binaries, although he notes that at the time of writing, these objects were entirely hypothetical. But if they did exist (they do), their gravitational output should be “interesting indeed.”
…the loss of energy by gravitational radiation will bring the two stars closer with ever-increasing speed, until in the last second of their lives they plunge together and release a gravitational flash at a frequency of about 200 cycles and of unimaginable intensity.
It’s interesting that at the time Dyson wrote, Joseph Weber was mounting what must be the first attempt to detect gravitational waves, although he seems to have found nothing but instrumental noise. The LIGO (Laser Interferometer Gravitational-Wave Observatory) team would go on to cite Weber’s work following their successful detection of GW170817 in 2017, produced just as Dyson predicted by a neutron star binary. Calling such waves “a neglected field of study,” the 1962 paper adds this:
…the immense loss of energy by gravitational radiation is an obstacle to the efficient use of neutron stars as gravitational machines. It may be that this sets a natural limit of about 108 cm/sec to the velocities that can be handled conveniently in a gravitational technology. However, it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible.
At the end of the paper posted on arXiv, David Derbes adds a useful note, pointing out Dyson’s prescience in this field, and adding that he had secured Dyson’s permission to publish the article before the latter’s death. But as typical of Dyson, he also stressed that he wanted Weber’s contribution to be noted, which Derbes delivered on by inserting a footnote to that effect in the text. We can all thank David Derbes for bringing this neglected work of a masterful scientist back into wider view.
In the next post, I want to talk about how these gravitational wave energies might be exploited by the ‘machines’ Dyson refers to in the title of the paper. The paper is Dyson, “Gravitational Machines,” now available on arXiv.
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