The Sun as Gravitational Lens: A Breakthrough Technology?

In the previous post on colonizing the Oort cloud, I pointed out that with sufficiently cheap transportation technology any place within range will be inhabited to some extent by recreational-touristic populations or populations that deliberately seek out isolation. This outermost part of the solar system is unlikely to be economically or demographically important at any point in the remotely near future, but there is a possibility for jump-starting development in the region that went unmentioned and will be elaborated on here: gravitational lensing. Specifically, the use of the sun as a gravitational lens for observation of and even communication with other solar systems.

A mass situated between an observer and a light source bends the light as it passes toward the observer through the force of gravity. This bending is similar to a lens, hence the term “gravitational lens” for such masses. The most famous cases of this phenomenon tend to be galaxy clusters that bend the light of other galaxy clusters behind them (as seen from Earth) so that the clusters are magnified. This is scientifically important because it enables our telescopes to observe distant galaxies at a resolution that would otherwise be impossible.

This is usually accomplished through so-called “strong lensing”, where the distortion is strong enough to render the object behind the mass as a ring or series of arc segments. This was the first type to be observed, starting in 1979 with the discovery of the “Twin Quasar”. “Weak lensing” is where the image is distorted but not to the point of forming a ring or arc segment, which is much less useful for magnifying distant objects but has recently been used to map dark matter concentrations across wide swaths of the universe. Gravitational lensing is particularly helpful in this case since, as the name suggests, dark matter does not emit enough light to be observed and can only be detected through gravitational effects.

Where this has implications for space colonization is the fact that every object that has mass bends light around it through its gravity, not just massive galaxy clusters. This effect is too small to be significant around, say, asteroids, but stars have more than enough gravity to act like magnifying glasses. “Microlensing” is a third class of gravitational lensing we observe that is generated by stars passing in front of (relatively massive) objects; because stars are far smaller than galaxy clusters the images produced are too small to be resolved by our telescopes, but a great increase in light can be observed as the star passes in front of a large object. This light can provide clues as to the nature of the object. Perhaps most prominently, a significant number of extrasolar planets have been discovered through microlensing.

Breakthroughs with Gravitational Lensing

On Earth we are at the mercy of chance alignments of stars and other objects to gain the benefits of microlensing, but the sun is a star itself and has more than enough gravity to act as a lens. If you could position the sun between you and a distant object the sun’s gravity would magnify the image and enable far greater resolution to be obtained than would otherwise be possible. If you move yourself so that the sun is directly in front of an object you wish to magnify, the light from the object that the sun bends comes into focus starting at around 542 AU away from the sun.

542 AU is where you will start to see a ring, arc, or other sort of image (depending on your exact position) of the background object around the sun, greatly magnified. Interestingly, there is no focal point in gravitational lensing; it continues to work no matter what the distance is, as long as it is beyond the minimum distance. This is rather convenient, since as some people (most recently Geoff Landis) pointed out the solar corona will interfere with the lensed image until you’re significantly further away (shrinking the apparent size of the corona). Of course, for observations it may be possible to adjust for such interference and keep one’s telescope at closer range; this would be more convenient if it could be done, since 542 AU is already 75 light-hours away from Earth. This means that it would take over six days to send a signal to the telescope and have it send a signal back to Earth, even at the minimum distance.

How much magnification could be achieved? According to this excellent blog post at Centauri Dreams on the same topic, at these sort of distances the light from a lensed object would be magnified by a factor of 100 million. That’s a lot of free magnification for just locating your telescope a few light days further out. Effectively, the object will appear as it if is ten thousand times closer than it actually is. A planet orbiting Alpha Centauri, for instance, would appear as large and bright as it normally would at 27 AU through lensing, even though it is actually 270 000 AU (4.3 light-years) away.

The Hubble Space Telescope, if placed in such a location, could image any planets of Alpha Centauri as well as it currently images Neptune. This would enable very detailed observations, to a degree astronomers and scientists can only dream about today. Space telescopes of far higher resolution than Hubble could be engineered even today (indeed, the soon-to-be-launched James Webb Space Telescope will have higher resolution), and would be trivial to build for any civilization that can manufacture mirrors and telescope assemblies in space, which will include any spacefaring near-future human civilization.

A 2017 article in Air & Space Magazine estimates that near-future space telescope technology ( detailed by NASA at their website , an interesting read by itself) could, using gravitational lensing from the sun, image a planet 100 light-years away at 10 kilometers per pixel. This would yield a (roughly) 1300×1300 pixel picture of an Earth-sized planet. This opens up the mind-boggling possibility of future scientists studying and children gazing in wonder at galleries of pictures of an alien solar system 100 light-years away that are of comparable quality to the ones taken by our probes in our own solar system. Consider that there are 4088 stars located within 100 light-years of Earth, implying that there could very easily be tens of thousands of planets that could be imaged at such high resolutions.

Toward a Telescope Cloud

The impact of such a powerful observational tool on our knowledge of alien solar systems would be nothing short of revolutionary. There is only one serious obstacle to obtaining this bounty: such a telescope can only observe one target at a time, because the telescope, the sun, and the target have to all be in a line. To observe another target, it must change its position so that it is aligned with the sun and the other target. To observe a target on the opposite end of the sky, the telescope will have to travel to the opposite end of the solar system, which since it needs 542 AU or more of distance from the sun, requires a trip of at least 1100 AU, or six light-days.

This distance would take months for even the most advanced near-future propulsion technology (nuclear pulse) to traverse. Instead of accepting months of downtime, it would be far more efficient to have multiple telescopes stationed at different ends of the solar system, perhaps starting with a pair at opposite ends of the sun and filling in different slices of the sky from there. In this way the maximum travel distance could be cut to hundreds of AU, then down into tens of AU, which as outlined in my post on colonizing the Oort cloud is a manageable distance with nuclear pulse propulsion. At 1g of constant acceleration 20 AU can be traversed in twelve days; the required maximum speed of 1.8% of light is easily attained by nuclear pulse. Keep in mind that this would be the maximum travel distance; most repositionings would, when the sphere of space telescopes at >542 AU is fully built out, take only from a few hours to a few days to complete.

Repeated and long-term observations of distant targets are possible and would surely be done extensively when this infrastructure is created. In fact, it isn’t hard to imagine a time when this region of the solar system may be the preferred place to conduct astronomical observations of not only extrasolar planets, but an array of distant objects. Thus, the innermost parts of the Oort cloud may not become economically or demographically important, but they may well become scientifically important for this reason alone.

Considering that the concept for a mission of this type is already being promoted by Claudio Maccone and being talked about at NASA, the innermost Oort cloud may be penetrated by space telescope arrays sooner than almost anyone even suspects. The build-out of such an infrastructure (one might call it the “telescope cloud”) is hardly imminent, but it wouldn’t be surprising if it was built up at roughly the same time humans establish colonies in the outer solar system. The same level of propulsion technology that can get you from Earth to Saturn in a few weeks can also get you to the gravitational lensing distance in a few years.

An Oort Cloud Gold Rush?

Gravitational lens telescopes may not be a driver of human settlement like the reasons explored in the previous post on colonizing the Oort cloud, since telescopes can be operated autonomously, especially with the sort of technology likely to be available in the near future. The fact that a round-trip transmission takes a week, however, may compound any errors that occur, making human servicing so frequently necessary that it may be easier to simply station people on-site or on a more mobile spaceship nearby that roams the region checking up on the telescopes. Visits for servicing will likely be needed from time to time in any case, though this wouldn’t be a driver of permanent settlement.

The very fact of week-long round-trip transmission times, however, might stimulate some scientists and scholars to station themselves on-site in order to access and analyze observational data faster than would otherwise be possible. In a scenario where detailed observations are needed with multiple-step decision-making processes as to what, how, and when to observe, transmitting all the steps from the inner solar system could take months (at one week per message round-trip). Without the light-speed lag the same could be accomplished in days, accelerating by a factor of ten the gathering of scientific data in these sort of contexts. If these scenarios prove to be commonplace (as they have to some extent in the probes we’ve sent out already), if the scientific data is valuable enough, and if the launch cost of sending humans and their habitation module is low enough (it’s likely to be cheap by the time the telescope cloud is built out), it may actually be worthwhile to permanently station people at these telescopes.

This would only support Antarctica-style research station populations at first, but with a bit more imagination one could see these telescope arrays and research stations evolving into full-scale research centers for all fields relating to their observations, particularly astronomical fields. If trainees, apprentices, and students are added on at a later date it turns into something like a college. Together with support personnel and the families of all those who live there, one now has a full-fledged space colony in the outermost solar system.

One could see it expanding even further, but any good science-fiction writer, worldbuilder, or futurologist can see the possibilities of a more purely research-focused colony as a setting for stories or other artistic purposes. Both in real life and in fiction, it isn’t that much of a stretch to see it evolve into a more diversified colony eventually, but unless the population were interested in investing in the construction of a full-fledged city Dubai- or Las Vegas-style it is very easy to imagine a college-town-like setting as the end point of its development.

Communicating Through Gravitational Lenses

In addition to passive observation, gravitational lensing can be used for active transmission of information. Claudio Maccone has proposed that the magnification power of gravitational lensing could also be used to receive radio transmissions from the target solar system that would otherwise be too faint to detect. This is especially interesting because the maximum range the Arecibo radio telescope would be able to detect our own radio transmissions is one light-year. This isn’t enough range to hear anything, so only much more powerful transmissions, probably deliberately sent to us, are detectable by SETI. The same sort of telescope at the gravitational lensing distance would have much greater range and sensitivity.

The same technology used to receive transmissions from aliens could also be used to create information links between human colonies. Although theoretically these telescopes, in this case also equipped with radio or laser transceivers, could be used to link opposing points in the Oort cloud, their primary usage will likely be for interstellar communication. Any probe sent to, say, Alpha Centauri would have a much easier time transmitting and receiving information if the transmissions were magnified by gravitational lensing. A pair of transceivers, one beyond the gravitational lensing point of the Sun, one aligned with it beyond the gravitational lensing point of Alpha Centauri, could use gravitational lensing to communicate with each other without needing extremely powerful transmitters or having to hope that less-powerful transmissions don’t get lost in the interstellar noise. These transceivers, in turn, could use more ordinary methods to relay the interstellar transmissions to and from the inner parts of each solar system.

Although transmissions to even the nearest star, Alpha Centauri, would still take over four years to travel one-way, these relays in the “telescope cloud regions” do seem to be by far the most effective way of transmitting between star systems. The same transceivers used by probes can also be used by future human colonies, theoretically even to the point of maintaining an interstellar internet of sorts, though with years of lag time it would necessarily be more like transmitting an internet archive with “messages in a bottle” between systems every so often than any remotely real-time communication. Pairs of transceivers can be set up in other solar systems as well, thus forming a network connecting all the nearby stars.

The amount of amplification that can be achieved is quite large; as this New Yorker article points out, one particular frequency that SETI has identified as interesting could be enhanced by a factor of up to 1.3 quadrillion using gravitational lensing. This is powerful enough to open up the possibility of detecting signals across intergalactic distances. Of course, any radio transmission would be distorted, and an alien transmission would be much harder to reconstruct than a message from one of our own transceivers we know the position of (and thus can calculate precisely how to adjust for the distortion), but the idea that such a feat may be possible with near-future technology is certainly quite interesting.

Conclusion

Indeed, after considering all these possibilities, the sun’s gravitational lensing effect appears to be one of the greatest natural endowments of our solar system as far as human use is concerned. Any alien civilization in any star system would have the same advantage, and the same techniques discussed here for human use in the future may already be in use by any aliens that may be out there. Aliens might be sending transmissions to other solar systems (including our own) that are designed to be receive using gravitational lensing, perhaps intended for only a more advanced civilization to receive, or because they assume anyone they would want to talk to would be using it, or because of convenience for them if they already have such transceivers or telescopes.

The many possibilities of using gravitational lensing for astronomical observation and transmitting information, along with the implications for human scientific discovery and even space colonization, is an area that to my knowledge is hardly ever explored in science fiction or speculation about our real-world future. Despite that, the chances that it will prove important in the real world or in a near-future-level hard science fiction setting are very high, providing very fertile ground for visionaries to speculate on the future and create hard-science fiction stories, worldbuilding projects, and other works of art.

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