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.