By Gregory Benford
I’ve published over 200 short stories and over 200 scientific papers, reflecting a symmetry of sorts.
My career as a professor of physics at UC Irvine has taken most of my working life, with writing as a hobby that has surprised me by success. So I see SF through a scientific lens, focused on plausible futures. But sometimes I just wing it, and speculating on physics a century hence is a grand leap, indeed.
The mock future news report in the current Analog issue [“Physics Tomorrow: A News Item of the Year 2116,” March/April 2018 Analog, on sale now] came from a contest the journal Physics Today ran in 2016: to devise an entry for that journal in a century. I took the challenge, and produced this “story” because the physics intrigued me.
Physics Today did not select my essay, from 230 others. They published much more pedestrian stuff. Since then, I’ve worked with an old friend and general relativity physicist Al Jackson, to calculate in detail how to in fact make a “gravwave transmitter.”
Then I thought, why not try Analog? As a physicist and SF writer, both avenues are natural. Indeed, maybe writing future news items is a new way to think of SF.
Perhaps, for motives we cannot well imagine, other smart beings will even encode signals in the gravitational waves that wash through our space-time every moment.
The LIGO and the VIRGO detectors, abuilding then, now see gravwavs from black holes and neutron stars merging, using software templates derived from detailed, strong relativistic calculations. Pulling a good signal out of the vast sea of noise effects demanded filtering from the many sources of noise. In future, gravitational wave astronomy will combat such noise problems with ever-more detailed methods to tease out even fainter signals. Perhaps, for motives we cannot well imagine, other smart beings will even encode signals in the gravitational waves that wash through our space-time every moment.
A similar possibility emerged in the 19th century, after Maxwell predicted electromagnetic waves moving at the speed of light and Hertz, in a simple experiment using electrical circuits, detected them in radio wavelengths. Hertz thought sending signals would never happen; his waves were too weak and diffuse in spectrum, he thought. An Italian teenager heard of Hertz’s remarks and thought of sending messages with the waves and made it so, along with others. That resolve by Marconi provoked a world we now enjoy. We now listen for electromagnetic signals from other minds across vast distances, using technologies similar to ours.
Perhaps intelligences elsewhere have mastered a complex energetic instrumentality and can send possible messages to civilizations such as ours. Their motives we cannot know. Perhaps they have reason to prefer to speak to those who have mastered the far more difficult task of sensing gravitational waves, compared to the vastly simpler detection of electromagnetic signals in a myriad of possible wavelengths.
This study stands in the tradition of Dysonian ideas. More than a half-century ago Freeman Dyson proposed that SETI agendas should look at technologies for harnessing an entire star’s energy, building on ideas of the legendary writer Olaf Stapledon. Dyson suggested that we should look not only for signals, but for side effects like infrared emission, on scales that do not contradict physical law, but are beyond conceivable human engineering. We do similarly here for signals in gravitational waves emitted for a purpose, following on the SETI ideas of the 1950s.
Gravitational Waves and Kardashev Civilizations
If one supposes that a civilization sends signals using gravitational waves there are two problems to be solved, the transmitter and the receiver. The LIGO receivers have seen gravitational radiation from natural objects. As a gravitational wave passes through matter it can change its geometry, namely a characteristic length. If one measures a length L and it responds to a gravitational wave by ΔL, the “strain” is measured by h= ΔL/L. This dimensionless amplitude is very small due to the weakness of gravitational waves. LIGO can measure h to the value of 10-22, or in approximate physical terms 1/1000 the diameter of a proton.
Physically, h is related to the transmitter by h~ΔE/r where ΔE is a burst of gravitational radiation energy and r is the distance from the transmitter. Take ΔE as the amount of energy produced in annihilation of a mass m, namely mc2, and take the distance of the transmitter to be 100 light years. The amount of energy produced can be related to the quantity of energy by a parameter specified by the Russian scientist Nikolai Kardashev, which relates the amount of energy available to a civilization. Energy types are characterized by scales such as Type 1 “planetary;” Type 2 “stellar;” and Type 3 “galactic.” Let K 1, 2, and 3 denote these civilizations.
Take the energy radiated to be enough to excite the LIGO non-dimensional amplitude at 1000 light years. The giga-hertz frequency is far higher than any current gravitational wave observatory. Background noise in such frequencies is far lower than in the LIGO region, so it is a preferred region to send signals. It’s so quiet, we could measure signals seven orders of magnitude more faint, so h of the value of 10-29.
The process would be, using K3 technology, inject a small mass m into an “unbound” orbit about M, deep in the non-Newtonian region of the black hole m orbits make it orbit ~10 times and return to far away, a few meters. To keep the mechanism going, the small mass m has to be “artificially” returned to orbit about the big mass. The energetics are enormous.
A unique feature of this system is how tiny it is! Black hole M of an Earth mass is almost 1 centimeter and black hole m is less than a centimeter. The whole orbital configuration is about one meter. The civilization must array an instrumentality that can precisely aim and hit the right impact parameter, monitor and “trim” the stability of the “operational” orbit. The energy radiated will cause decay of the circulating orbit and the small mass m cannot be allowed to fall into the big mass black hole. The small mass must be cycled back to the big mass. The physics is allowed but the engineering physics is beyond comprehension! Hertz thought the same of electromagnetics.
Making Black Holes
To make black holes requires collisions of large masses at very high velocities, perhaps in a spherical implosion resembling the physics of fission plutonium detonation. Perhaps also, small-mass black holes can be made by high-energy collisions. Then we could feed ordinary matter into the hole, growing it. We deal with such below.
To get the ordinary mass in the first place means collecting many small masses, probably from the Kuiper Belt or the Oort cloud, to get a planetary mass. The harder way is to take apart a large body, even a planet. “One can think of several feasible methods of disassembling a planet,” Freeman Dyson wrote in a 1966 paper describing one such technique, using Earth as an example. Dysonian SETI looks at such mammoth-scale technologies, Dyson swarms around a star being a classic example, that do not contradict physical law but are beyond conceivable human engineering.
To make black holes requires collisions of large masses at very high velocities, perhaps in a spherical implosion resembling the physics of fission plutonium detonation.
“One can think of several feasible methods of disassembling a planet,” Freeman Dyson wrote in a 1966 paper describing one such technique, using Earth as an example. Dysonian SETI looks at technologies, Dyson swarms around a star being a classic example, that do not contradict physical law but are beyond conceivable human engineering.
Dyson proposed accelerating the planet’s rotation about its axis until centrifugal forces become greater than its internal cohesive forces. Then the whirling world will break up, slinging material into space. The sling point for Earth comes when the rotation period is about one hour. To spin up a planet or asteroid, Dyson suggests wrapping around it a metal grid carrying into a powerful electric current. This torques the world with an electromagnetic force, accelerating its rotation. Centrifugal force rises most strongly at the equator, where the first fragments fly off. This enormous spinning top turns ever faster, metallic chunks arc out into space, captured by a gigantic system of magnetic nets.
Dyson calculates that the earth’s rotation could be doubled (or halted) in a mere 2,500 years. For plausible (non-superconducting) circuitry, it would take 40,000 years for Earth to begin spinning apart. To drive all this, Dyson envisioned a massive array of solar energy collectors large enough to capture 300 times the solar power intercepted by the earth.
Is this plausible? Not for us, now, but Dyson later wrote about the galaxy as a whole, “there is nothing so big nor so crazy that one out of a million technological societies may not feel itself driven to do.”
Ripping apart a planet is not needed for the transmitter we envision. Just a planetary-sized mass and a smaller companion is enough. What masses could we use? Earth’s mass is 5.8742 x 1024. Our Oort cloud total mass is not known, but, assuming that Halley’s comet is a suitable prototype for comets within the outer Oort cloud, roughly the combined mass is 3×1025 kilograms, five times that of Earth.
The collective mass of the Kuiper belt is relatively low. The total mass is estimated to range between 1/25 and 10 times the mass of the Earth. Models of the Solar System’s formation predict a collective mass for the Kuiper belt of about 30 Earth masses. The Kuiper belt seems to show that there are 8 times more objects in the 100—200 km range than in the 200—400 km range, and for every object with a diameter between 1000 and 1010 km there should be around 1000 objects with diameter of 100 to 1000 km. These and Oort cloud masses could be spun up with Dyson’s spinner method to yield the mass for making black holes. So the debris cloud of a solar system like ours may be enough to make black holes in that mass range. But how?
The easiest way to get them is to find primordial black holes from the Big Bang, for free. Gravitational collapse to a black hole requires great density. Today such high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing black hole creation. For primordial black holes to form in a primordial dense medium, there must be initial density perturbations which grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses. Primordial black holes could thus account for the creation of any type of black hole. The task, then, is to find black holes with masses useful in a transmitter, such as the range within an order of magnitude of an Earth mass, 5.8742 x 1024 kg. This task we leave to others!
Al Jackson and I have explored qualitatively the possibility that truly advanced societies of Kadarshev numbers of 2 or 3 would use gravwave methods. Why? To avoid detection by societies with only electromagnetic methods? (Mature, nonaggressive civilizations may prefer the gravwave method.) To reach very large distances, far beyond the scales of galaxy clusters? Other motives we cannot now know? The Search for ExtraTerrestrial Intelligence has envisioned many motives for messages from civilizations that want to convey their art, religion, or philosophy, or even as “funeral pyres” leaving a heritage. Others might ask for help or seek converts to their passing faiths or philosophies. This may be true of electromagnetic messages, but the colossal energies required to radiate gravwaves suggests other motives beyond our present knowing.
In any case, as our gravwave detection improves and perhaps moves to far higher frequencies, these ideas should be kept in mind. Al Jackson and I have not yet finished the paper on detailed calculations of making gravwave emitters. When we do, we’ll publish. After all, much of the research cited here was published, because physics leads to such ambitious ideas.
Other minds may have thought this way, too—and in the huge expanse of the Universe, done something about them. So let’s listen. . . .
My latest novel is out in trade paperback, after its hardcover: THE BERLIN PROJECT from Simon & Schuster. It’s an alternative history of the Manhattan Project, one that very nearly happened. Historians now know that had the best method of separating U235 from U238 been chosen, centrifuges, we would’ve had a working bomb in 1944—and thus a very different World War II. In THE BERLIN PROJECT I show how that woulda-coulda-shoulda happened. I knew nearly all the scientists in it. My father in law, Karl Cohen, is the protagonist. It was great fun to write, too!—the most physicky fiction I’ve ever written.