A Long Distance Call
E.T. managed to call home with a Speak and Spell, buzzsaw blade, and an umbrella. The reality of interstellar communication is a bit more complicated. Space is really, really big. The power needed to transmit a signal across the void is huge. However, rather than using super high power transmitters, recent research by Stephen Kerby and Jason T. Wright shows that we could make use of a natural signal gain boost built into solar systems – the gravitational lensing of a solar system’s star. Networking a series of stars as nodes could get signals across vast tracts of the Milky Way. And we may be able to detect if our Sun is already part of an alien galactic communication network.
Like a heavy ball placed on a trampoline, a massive object such as a star will cause space itself to curve creating a “gravity well.” Both mass and energy travelling through curved space will follow that curve. For example, our orbit around the Sun is literally the Earth following the curve in space made by our star’s mass. As light travels through space, its path also follows these curves causing the light to bend. The effect is similar to how light is bent by a glass lens which is why the bending of light due to gravity is called “gravitational lensing”. Like a lens, stars can focus a distant source of light, such as a radio signal, greatly boosting signal gain or likewise focus an outgoing signal for better transmission. Gravitational lensing is also visible to our telescopes called “Einstein Rings” as it was Einstein’s work on relativity which demonstrated mass curves space.
As our Sun’s gravity focuses light, a receiving or transmitting craft can be placed along an axis that runs between a distant target star where a signal may originate, the Sun, and a focal line where the Sun focuses that signal from the target star. The target star then is directly opposite the spacecraft on the other side of the Sun which the spacecraft sees not through but around the Sun as the Sun bends light around itself due to gravity. Imagine it like an eyeball – the Sun is the lens of your eye with the spacecraft the back of your retina (but the light is going around the glass of the “lens” rather than through it).
As signals are received, the craft could relay information to the Earth or send the signal to another transmitter/receiver stationed around the Sun aligned with a different distant target star to forward the signal onward. A connection to another star system would require another craft stationed at the distant target star. We have yet to establish a network like this, but perhaps other civilizations have.
Using relativity, a measurement of our Sun’s minimum focal distance out in space can be determined…550AU about thirteen times the distance to Pluto. Presently, our most distant probe is Voyager 1 launched in 1977 which, after 44 years of flight, is 154.7 AU (about 21 and a half hours at the speed of light) out in space. And 550AU only represents the minimum focal distance at which the light from a target curves around the Sun’s surface rather than being lost in the Sun itself. Some targets may be focused by the Sun even farther out into space. With our present technology, we could park a spacecraft out that far, but it would take a long while to get there. As our propulsion technology improves, such a mission becomes even more possible.
How much signal gain could be achieved by the Sun’s gravitational lens? A lot. Estimates in the research show that an incoming transmission focused by the Sun could increase gain by 120db (decibels). A range of ten decibels represents an increase of ten fold strength…so 10 trillion fold increase in signal gain – the equivalent in going from a barely audible whisper to a live rock concert. The increased gain means not having to deploy super powerful transmitters to brute force messages across the void or likewise ultra sensitive receivers. We can use the more efficient natural signal gain created by the Sun’s gravity.
“Stars work like lenses, meaning that they provide a natural and powerful way to boost signals across interstellar distances. An analogy might be hilltops and cell towers: sure you could build a cellular network without putting any towers on hilltops, but if the hills are there anyway, why wouldn’t you use them? So if there is a “Galactic Internet” out there, it would be surprising if it didn’t take advantage of these lenses.”
Thrusters at Station Keeping
Maintaining focus is a challenge. Using thrusters, a spacecraft transmitting or receiving signals at the ideal focal location will need hold position relative to the Sun to within a hundred meters. Think about that – sub kilometer positioning accuracy while billions of kilometers away. Precise alignment will require automated adjustment by the spacecraft against two major causes of misalignment. The first is the inward pull of gravity from the Sun itself. At the extreme focal distance, the Sun’s gravity is relatively weak compared to where our solar system’s planets orbit. But, loosing thrust would still cause the craft to fall into the solar system in an eccentric “comet like” orbit eventually plunging into the Sun itself in a few thousand years.
The second major cause of misalignment is the wobbling motion of the Sun . The orbit of a star’s planets, especially massive gas giants like Jupiter, causes a star to wobble as its planets exert a gravitational pull. Our Sun’s wobble is the largest disruption of alignment between a distant target star, the Sun, and a communication spacecraft. The craft could be fitted with an optical scope that would relay position information to the craft’s guidance computer using the relative position of other stars. Alternatively, the telescope could point toward to Sun to ensure the target star from which a signal is being received is fixed in the Sun’s Einstein Ring. Just as the Einstein Ring picture of LRG 3-757 above shows how a distant background galaxy shines around a closer foreground galaxy, a distant target star from which the spacecraft is receiving a transmission would appear as a ring of light encircling the Sun from the craft’s perspective.
Which stars would make ideal nodes in a hypothetical network? The authors recommend looking for spherical stars requiring less focus adjustment and therefore less fuel consumption by the spacecraft. Less spherical stars distort signals. Even our Sun is not perfectly spherical. Stars with faster rates of rotation bulge toward their equator and are less ideal. More massive stars with more gravity exert more pull on the spacecraft. Stars with more planets, or with gas giants in close orbits – hot Jupiters – have a more pronounced wobble requiring more thrust to maintain position. Stars with companion stars, binary or trinary systems, will have even more pronounced wobbles.
A communication spacecraft with a more efficient propulsion system could maintain position and focus for a longer time. With the Sun as an example solar system, a spacecraft using our chemical rocket propulsion could maintain focus position for a few hundred years. Given the light travel time between stars within our galaxy – dozens, hundreds, or even thousands of years – this isn’t a significant amount of time. With ion thrusters, also used by some of our present day satellites and probes, you could maintain focus for nearly a millennia. But what if you were an alien civilization with more advanced propulsion technology?
Are We In The Loop?
We are already experimenting with fusion based rockets that could stabilize a craft for tens of thousands of years. Beyond our current technology, but hypothetically possible, is an antimatter propulsion system that could stabilize a craft for millions of years. Exotic thrusters like antimatter may be more easily detectable than other forms of propulsion meaning if there is a communication craft already in our solar system placed there by another civilization we might be able to see it.
What do I mean by “already?” The authors note it’s possible the Sun is presently a member of a communication network – one node among many that is hosting an alien communication craft. How wouldn’t we already be aware of that? Well, if an alien spacecraft is using our Sun’s gravitational lens, it will be difficult to detect as it would be hundreds of AU away – a future area of interest for artifact SETI research (Search for Extraterrestrial Intelligence searching for alien artifacts). An exotic propulsion system may give it away. Furthermore, the signals focused by gravitational lensing are shaped into a narrow cone that the Earth may not orbit through. If Earth doesn’t pass through this cone of signal, a network of alien communication cloud pass right through our solar system undetected– an explanation to the quietness of the galaxy if there are alien civilizations using gravitationaly lensed signals.
“If an ETI can overcome the engineering challenges we examined, they could use gravitational lenses to send transmissions across the galaxy in a vast network of communication nodes. They could overcome the huge expanses of space and communicate more reliably. While we need to conduct observations to see if the Sun or another star is being used for this purpose, it also gives a blueprint for how humanity might communicate across the galaxy in the distant future.”
-Stephen Kerby, Lead Author
Assuming that gravitational lensing is being used for interstellar communication, and that some star systems may make better receiving/transmitting points than others, we could narrow radio SETI (Search for ETI using radio transmissions) searches to these ideal systems. A more complete survey of neighbouring star systems would reveal if they are better network candidates – spherical stars with less wobble. We could then search for outbound/inbound signals from a region opposite of their location in our own solar system where their light would be focused by our Sun toward a potential transmitting/receiving craft.
The authors also propose two other station keeping alternatives. First, you could place a fuel depot near the ideal focus location that the communication spacecraft uses to refuel. Secondly, a whole constellation of communication craft could be placed in orbit of the Sun. Each would maintain position for a brief time and then allow itself to fall out of place, orbit the Sun, and then return to position. The craft would repeat this orbit in sequence with other probes so that each spacecraft reduced the overall fuel cost required remain in place while at least one craft always remained in focus. If an alien civilization is using multiple craft in a system, then there’s a better chance of detecting an individual craft.
After years of delay, the highly anticipated James Webb Space Telescope is launching soon (tentatively Dec 18th of this year). James Webb recently completed a journey by sea to its launch site in French Guiana. This next gen scope will provide unprecedented views of the Universe. But just as the Sun’s lensing effect boosts signal gain, so too would the effect magnify distant star systems and other cosmic objects creating a giant super telescope. A gravitational scope would be far more powerful than anything we’ve created capable of viewing expolanets with a similar clarity as we see the planets in our own solar system!
In the meantime, we eagerly await those first JWST images. And if we ever do launch a gravitational lens scope/transmitter, perhaps we’ll find another one already out there from someone else! It’s incredible to think that maybe, MAYBE there’s already a highway of comms traffic passing through our system. Who knows what conversations we’re missing out on – ancient planetary surveys, new advanced technologies…interstellar takeout orders? Maybe we’ll find out!
“The search for extraterrestrial intelligence is so multidisciplinary that scientists from all fields can contribute. It’s a vision of diverse and open-minded exploration that should be a goal for the scientific community, and it’s very rewarding for me to learn from astronomers from different academic and personal backgrounds. SETI also captures the public’s imagination and helps everyone reflect about humanity’s place in the cosmos.”
-Stephen Kerby, Lead Author
Feature Image: Fig1 from Kerby and Wright 2021 “A schematic of a on-axis stellar relay transmission system, opening angles, distances, and sizes not to scale. The initial unfocused transmission beam may even have an annular pattern to prevent flux from being lost to the disk of the Sun. A reversed arrangement can be used to receive signals from a distant star by focusing rays onto the spacecraft.” c. Kerby and Wright 2021
More to Explore
[2109.08657] Stellar Gravitational Lens Engineering for Interstellar Communication and Artifact SETI (arxiv.org) (Originating Open Access Research Article by Kerby and Wright)