Centauri Dreams

A friend and I were sitting in a diner some time back talking mostly about old movies (my passion is for black-and-white films from 1927 to the death of Bogart in 1957). Somehow the topic of gravity came up, I suspect because we had homed in on early 50’s science fiction films. Anyway, I remember his eyebrows raising when I mentioned how puny a force gravity was. I can understand why. We think about massive objects when we think about gravity, but of course it takes a lot of mass to get a little gravity.

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.

What an intriguing thing to find Freeman Dyson’s “Gravitational Machines” paper popping up on arXiv. This one is yet another example of Dyson’s prescience, for in it he examines, decades before the actual event, how gravitational waves could be produced and detected, although he uses neutron stars rather than black holes as his focus. Fair enough. When this was written, in 1962, black holes were far more conjectural than they appear in most of the scientific literature today.

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.