Star Lens: How we can use our sun as a telescope

Clemens Von Stengel 21 May 2018

Albert Einstein’s theory of General Relativity predicted that the path of light is bent by the gravitational effects of massive objects, such as the sun. This phenomenon, known as gravitational lensing, was observed during the total solar eclipse in 1919, and was the experiment which was used to verify Einstein’s theory, which is what propelled Einstein to his poster child status. Gravitational lensing is now a well-utilised technique in astrophysics. 

It is currently used to detect planets, using microlensing of far away stars by solar systems and measuring the change in brightness seen, and to estimate the amount of dark matter, using weak lensing of galaxy clusters to measure the average lensing effect by the dark matter on a set of galaxies. However our home star has not yet been used in this way.

In 1979, a physicist at Stanford University called von Eshleman applied the equations of General Relativity to find out at what point the gravitational lens formed by our Sun would come into focus. This yielded a minimum focal distance of approximately 550AU (Astronomical Units – the average Earth-Sun distance). At the point where the lens comes into focus the image received from the far side of the star – although distorted – would be massively magnified by the gravitational lens.

For this reason Eshleman was very interested in placing a receiver in the focal line of the gravitational lens. A focal line sounds unfamiliar to those who understand classical lens theory which has a well-defined focal point, however due to the variation in how far away the light waves are when they pass the sun, the amount of lensing that occurs will vary. This means the light will focus on a line radially away from the sun at distances ≥550AU.

Another scientist named Claudio Maccone, who works with the SETI (Search for Extraterrestrial Intelligence) Institute at the International Academy of Astronautics & Instituto di Astrofisica in Turin, furthered the work of Eshleman and designed a telescope that could make use of this phenomenon to investigate microwave frequencies, by placing two large radio antennae connected by a tether to the focal line and varying the distance between them to resolve microwave images of a small sector of the universe on the far side of the Sun.

This is particularly interesting to SETI, as they search for extraterrestrial communications in the microwave frequency. Furthermore the technology could then be utilised to send and receive signals from neighbouring solar systems. Furthering this, it does not take a great leap in logic to extend this to a larger variety of wavelengths. Although the positioning of the craft has to be more precise to view waves of smaller wavelengths accurately, the reward would be potential imaging of extrasolar planets directly in a variety of wavelengths (and in fact the resolution of the images only increases with smaller wavelengths). To do this a more complicated design would be necessary such as that outlined below.

All this being said, 550AU is an enormous distance, to put it into context, Pluto is 40AU from the Sun and the Voyager 1, the farthest man-made object from the Sun, is 141AU from the Sun, having operated for more than 40 years. Therefore getting to 550AU and beyond will take a long time with sophisticated propulsion techniques to get there. Arguably far more importantly, the accuracy to which the positioning of the probe must be to within approximately 1km at the very least (down to <100m to image visible and infrared radiation). Even in the most generous case an error to either side of 1km in 550AU is impossible to avoid with the technology we currently possess.

Thus, in the current day and age, it would impossible to place any such telescope on the focal line of the Sun’s gravitational lens. However, in the future, if/when a Solar System Positioning System is in place and the modelling of trajectories becomes ever more accurate we could one day image, detect life on, and communicate with a planet from another solar system, all with one telescope that uses the Sun as a lens.