I sometimes imagine Claudio Maccone having a particularly vivid dream, a bright star surrounded by a ring of fire that all but grazes its surface. And from this ring an image begins to form behind him, kilometers wide, dwarfing him and carrying in its pixels the view of a world no one has ever seen. The dream is half visual, half diagrammatic, but it’s all about curving Einsteinian spacetime, so that light flows along the gravity well to be bent into a focus that extends into linear infinity.
My slightly poetic vision of what happens beyond 550 AU or so doesn’t do justice to the intrinsic beauty of the mathematics, which Maccone learned to unlock decades ago as he explored the concept of an ‘Einstein ring’ as fine-tuned by Von Eshleman at Stanford. When I met him (at one of Ed Belbruno’s astrodynamics conferences at Princeton in 2006), we and Greg Matloff and wife C talked about lensing at breakfast one morning. Even then he was afire with the concept. He’d been probing it since the late 1980s, and had submitted a mission proposal to the European Space Agency. He had written a short text that would later be expanded into the seminal Deep Space Flight and Communications (Springer, 2009).
Maccone said in his presentation at the Interstellar Research Group’s Montreal symposium that he was delighted to see the Sun’s gravitational focus moving into the hands of the next generation, citing the 2020 NASA grant to Slava Turyshev’s team at JPL, where a Solar Gravitational Lens mission is being worked out at the highest level of detail as an entrant into the sweepstakes known as the Heliophysics 2024 Decadal Survey. To see how far the concept has gone, have a look at, for example, Self-Assembly: Reshaping Mission Design, or A Mission Architecture for the Solar Gravity Lens, among numerous entries I’ve written on the JPL work.
Image: A meter-class telescope with a coronagraph to block solar light, placed in the strong interference region of the solar gravitational lens (SGL), is capable of imaging an exoplanet at a distance of up to 30 parsecs with a few 10 km-scale resolution on its surface. The picture shows results of a simulation of the effects of the SGL on an Earth-like exoplanet image. Left: original RGB color image with (1024×1024) pixels; center: image blurred by the SGL, sampled at an SNR of ~103 per color channel, or overall SNR of 3×103; right: the result of image deconvolution. Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II.
The astounding magnification we could achieve by using bent starlight was what drew me instantly to the concept when I first learned about it – how else to actually see not just pixels from an exoplanet around its star, but actual continents, weather patterns, oceans and, who knows, even vegetation on the surface? But at Montreal, after his praise for the JPL effort that could become our first attempt to exploit the gravitational lens if adopted by the Decadal survey, Maccone took a much more futuristic look at what humans might do with lensing, delving into the realm of communications. What about building a radio ‘bridge’?
The concept is even more audacious that reaching 650 AU with the payloads we’ll need to deconvolve imagery from another star. In fact, it’s downright science fictional. Suppose we achieve the technologies needed to send humans to Alpha Centauri. We have there in the form of Centauri A a G-class star much like the Sun (although we could also use the K-class star Centauri B). Both of these stars have their own distance from which gravitational lensing occurs, Align your spacecraft properly to look back towards the Earth from Centauri A and you can now connect to the ‘relay’ at the lensing distance from the Sun. You’ve drastically changed the communications picture by using lensing in both directions.
The consequences for contact and data transfer are enormous. Consider: If we want to talk to our crew now orbiting Centauri A and try to do so with one of the Deep Space Network’s 70-meter dish antennae using today’s standards for spacecraft communications, we’d have no usable signal to work with. Assume a transmitting power of 40 W and communications over the Ka band (32 GHz) at a rate of 32 kbps (these are the figures for the highest frequency used by the Cassini mission). The distances are too great; the power too weak. But if we factor in a receiver at the lensing point of Centauri A directly opposite to the Sun, we get the extraordinary gain shown in the diagram below.
This raises the eyebrows. Bit Error Rate expresses the quality of the signal, being the number of erroneous bits received divided by the total number of bits transmitted. Using a spacecraft at the solar gravitational lens distance from the Sun talking to one on the other side of Centauri A (alignment, of course, is critical here), we have a signal so strong that we have to go over 9 light years out before it begins to degrade. A radio bridge like this would allow communications with a colony at Alpha Centauri using power levels and infrastructure we have in place today.
Obviously, this is a multi-generational idea given travel times to and from Alpha Centauri. But it’s a step we may well need to take if we can solve all the problems involved in getting human crews to another star. Maccone told the audience at Montreal that in terms of channel capacity (as defined by Shannon information theory), the Sun used as a gravitational lens allows 190 gigabits per second in a radio bridge to Centauri A as opposed to the paltry 15.3 kilobits per second available without lensing.
Realizing that any star creates this possibility, Maccone has lately been working on the question of how a starfaring society of the future might use radio bridges to plot out expansion into nearby stars. He is in fact thinking about the best ‘trail of expansion’ humans might use to keep links being built and used between colonies at these stars. This turns out to be no easy task: The first goal must be to convert the list of nearby stars being studied (the number is arbitrary) into Cartesian coordinates centered on each star (their coordinates are currently given in terms of Right Ascension and Declination with respect to the Sun). Maccone calls this an exercise in spherical trigonometry, and it’s a thorny one.
A network of radio bridges between stars could evolve into a kind of ‘galactic internet,’ a term Maccone uses with an ironic smile as it plays to the journalist’s need to write dramatic copy. Be that as it may, the SETI component is intriguing, given that older civilizations may even now be exploiting gravitational lensing. It would be an interesting thing indeed if we were to discover a bridge relay somewhere at our Sun’s gravitational lensing distance, for its placement would allow us to calculate where the receiving civilization must be located. Using a gravitational lens for communications is, after all, extraordinarily directional. Might we one day discover at the lensing distance from the Sun an artifact that can open access to a networked conversation on the interstellar scale?
Human expansion to nearby stars would likely be a matter of millennia, but given the age of the galaxy, it would represent just a sliver of time. Whether humanity can survive for far shorter timeframes is an immediate question, but I think it’s refreshing indeed to look beyond the current work on reaching the solar gravitational lens to the implications that would follow from exploiting it. The radio bridge is great science fiction material – we might even call it the stuff of dreams – but solidly rooted in physics if we can find the tools to make it happen.