A solar gravitational lens will be mankind’s most powerful telescope. What are the best goals?

One of the central predictions of General Relativity is that a massive object like a star, galaxy or black hole can deflect light passing nearby. This means that light from distant objects can be focused by objects that are closer to us through gravitational lenses. Under the right conditions, gravitational lensing can act as a kind of natural telescope, brightening and magnifying light from distant objects. Astronomers have used this trick to observe some of the most distant galaxies in the universe. But astronomers have also considered using this effect a little closer to home.

One idea is to use the Sun’s gravity as a lens to study nearby exoplanets. Light coming from an exoplanet would be gravitationally focused by the Sun with a focal point in the range of about 550 AU to 850 AU, depending on how close the exoplanet’s light passes to the Sun. In principle, we could place one or more telescopes at that distance, creating a sun-sized telescope. This would give a resolution of about 10 square kilometers for objects 100 light-years away.

The current longest-range spacecraft we’ve built is Voyager I, which is only about 160 AU from the Sun. So it’s pretty clear that we still have a long way to go before this type of solar telescope becomes a reality. But it’s a project we could do in the future. It wouldn’t take any magical technology or new physics to pull it off. It’s just going to take a lot of engineering. And even then, another challenge is using all the data collected to put together an accurate picture. As is the case with radio telescopes, this solar lens telescope would not take a single picture at a time. It will require a detailed understanding of how the Sun focuses light to image exoplanets, and that’s where a recent study comes in.

No telescope is perfect. One of the limitations of optical telescopes has to do with diffraction. When light waves pass through a telescope lens, the focusing effect can cause the waves to slightly interfere with each other. It’s an effect known as diffraction that can blur and distort your image. The result of this is that for every telescope there is a limit to how sharp your image can be, known as the diffraction limit. While a gravitational lens telescope is slightly different, it also has a diffraction effect and diffraction limit.

In this study, the team modeled the Sun’s gravitational lensing effect to study the diffraction effects it would have on an image of extended objects such as an exoplanet. They found that a solar lens telescope would be able to detect a 1-watt laser coming from Proxima Centauri b, about 4 light-years away. They found that the diffraction limit is generally much smaller than the overall resolution of the telescope. We should be able to resolve details on the order of 10km to 100km, depending on the wavelength observed. The team also found that even at scales below the diffraction limit, there are still objects worth studying. For example, neutron stars would generally be too small for us to see features, but we could study things like surface temperature variations.

In particular, this study confirms that objects such as exoplanets and neutron stars would be good candidates for a solar telescope. It would be a revolutionary tool for astronomers in the future.

Relation: Sara Engeli and Prasenjit Saha. “Wave optics of the solar gravity lens.” arXiv form arXiv:2210.01568 (2022).