Using the Sun’s Gravitational Lens for Interstellar Communications
by Pat Galea
In a previous article we took a broad look at the problems involved in interstellar communications. In this article, we will take a closer look at one of the ‘exotic’ techniques mentioned: Gravitational Lensing. Einstein published his General Theory of Relativity in 1915. This theory considered gravity to be the result of the curvature of space (or, more precisely, spacetime, though the distinction will not trouble us here). One of the consequences of this is that a massive object such as the Sun will bend the path of a light beam that passes it. Indeed, this effect predicted by Einstein provided one of the first verifications of General Relativity. During a total eclipse of the Sun it is possible to see the small distortion caused by the Sun’s gravity on the apparent location in the sky of distant stars. Once it was realized that gravity could bend light this way it was also noted that massive objects can act as huge lenses, focusing light to a point, just as a glass lens does. The gravitational bending of light is much weaker than you can get from a glass lens, so despite using a huge lens like the Sun, the focus is still very far away. Interestingly there is an important difference between the optical focusing that we are familiar with and gravitational focusing. A normal glass lens has a specific focal distance. If you want to get a sharp image you have to hold the lens at exactly the right distance from your focal plane (which could be film or some electronic image detector). With a gravitational lens there is no specific focal length. Beyond a minimum focal distance at which the distant light rays are brought together, all the points in the line away from the Sun are foci.
The fact that we don’t have to be at a specific distance from the gravitational lens in order to use it has an interesting consequence for astronomy, We can use a distant galaxy as a lens and observe the bending of the light that it causes in the image. For example, in this image of Einstein’s Cross we are seeing the same distant quasar four times, brought to a focus by an intervening galaxy which is acting as a lens. The quasar is 8 billion light years from Earth, while the galaxy is only 400 million light years away. So we know that this isn’t just some theoretical quirk of General Relativity with no practical effect. We can observe this lensing in reality. But can we exploit the Sun in a controlled way to focus on specific targets? First of all, we need to know how far away from the Sun our camera needs to be in order for the distant light to be focused. It turns out that if you consider the Sun to be a perfect sphere, then you need to get at least 550 AU from the Sun. (1 Astronomical Unit (AU) is the distance from the Earth to the Sun.) However, there is a complication. The Sun is not a perfect sphere. There is a corona on the outside surface that tends to deflect light away from the Sun. This is working against the gravitational lensing effect, and serves to push the focus even further away from the Sun. The result is that we actually need to place our camera about 700 AU from the Sun.
So we fly our camera out to 700 AU, and start snapping pictures of the planets around Alpha Centauri. Is it that easy? No, of course not! First of all, 700 AU is an enormous distance. By comparison, Voyager 1 was launched in 1977 to take a tour of the gas giants of our solar system. This is one of the fastest craft mankind has ever produced, and yet it is (at time of writing) only 113 AU from Earth. There are options for getting craft out to these vast distances relatively quickly, but they don’t need to concern us here. Let’s stipulate that we can get our camera to 700 AU.
We can now use our camera to spy on the comings-and-goings on the surface of a planet at Alpha Centauri. (That’s no idle boast; the resolving power of a gravitational lens telescope is phenomenal.) However, we’ll leave such activities for the astronomers and xenobiologists. Here we are concerned with exploiting the gravitational lens for interstellar communications. Well, it’s really quite simple… in principle! We have our receiver craft sitting at the Sun’s focus, and the distant probe at some point in interstellar space or around another star. We use the Sun to focus the transmissions from the distant probe onto the receiver. This gives us an enormous antenna gain compared to what we would be able to achieve if we were trying to receive the signals directly, without using the Sun. What this means is that we can use much lower transmitter power on the probe without impacting the bandwidth that we can transmit. And this means that we do not have such hefty power supply requirements on a probe that may have been flying for a hundred years before reaching its destination. These seemingly mundane resource constraints should not be underestimated. Like the old military saying goes: “amateurs study strategy; professionals study logistics.” It’s all very well having a gleaming state-of-the-art fusion drive to get your probe to the target, but it’s all for nothing if you don’t have the power to get any data back to Earth.
So we’ve got our receiver out to the focus, and the distant probe is beaming data back to us via the Sun’s gravitational lens. Have we solved all the problems? Unfortunately, no. The big problem remaining here is that the receiver has to stay very closely aligned with the transmissions from the probe to a phenomenal degree of accuracy. We are talking about a tolerance on the order of tens of metres, for a craft which is over 700 AU from the Sun. There is plenty more work to be done in this area, and the Icarus team is studying some ideas to see if they might help to make this a practical system in the near future. Let’s assume that we have solved the positioning accuracy problem, and we can keep the receiver exactly in line with the distant probe. Is there anything else we can do with the system to improve communications even further? Indeed there is. We can exploit two gravitational lenses: the Sun, and the distant star. So we have our receiver craft at the Sun’s gravitational focus, and the distant probe at the focus of the target star. If we can keep both the probe and the receiver exactly in line with each other, then the antenna gain we achieve is beyond enormous; it’s simply phenomenal. Using trivial amounts of power, we can achieve perfect communications between the Sun and (say) Alpha Centauri. (Read the Centauri Dreams article referenced below for the fascinating details.)
Clearly the positioning accuracy problem is greater again when we are trying to keep two craft, separated by over four light years, exactly in line with each other. However, we also get another great bonus: we are now able to send decent amounts of data from Earth back to the probe. Remember that the probe may have left Earth up to a hundred years earlier. While it has been en route, our algorithms for data processing may have improved, and we may have obtained fresh data by other means that the probe could usefully take advantage of. It would be incredibly productive to be able to update the probe with the new information. If we can make this system work, especially with the double-lens communications, we will have taken the first steps in creating an interstellar internet. Imagine the possibilities that a network of transceivers around all the local stars will open up for the next generation of probes. They will no longer need to send signals directly back to Earth. They can just “log on” to their local node which will relay data back to Earth for them. With the establishment of an interstellar communications infrastructure, humanity will truly be a starfaring civilization.
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References: General Relativity [Wikipedia] Gravitational Lens [Wikipedia] Einstein’s Cross [Wikipedia] Voyager 1 distance from the Sun [Wolfram Alpha] The Gravitational Lens and Communications [Centauri Dreams] Antenna Gain [Wikipedia] Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens, C. Maccone, Springer-Praxis, 2009 [Google Books Preview]