When Newton speculated that light travels in the form of tiny particles, there was the obvious conclusion that the paths of these particles would be affected by gravitation.

By the early 19th century, this corpuscular theory of light, however, fell out of fashion. Instead, the wave theory prevailed, ultimately codified in the form of mathematics developed by Maxwell and perfected by Heaviside.

But then, Albert Einstein developed his general theory of relativity. In this theory, the universal gravitational field also determines the observed geometry of space and time. Because of this, not even a wave theory escapes being influenced by gravity: wave or not, light is deflected by gravity in Einstein's theory.

More than that, it is deflected more than it would be Newton's theory. General relativity introduces corrections to Newton's gravity. For things moving slowly, these corrections are imperceptibly small. But when something moves at the, well, speed of light, the corrections are as large as the Newtonian value. Consequently, Einstein's theory predicts that not only is light deflected by a gravitational field, the deflection is twice the amount calculated using Newton's theory.

How do you measure something like this? Gravity is exceptionally weak. Only very massive objects produce a discernible deflection. The Sun is obviously very massive, but unfortunately it is also very bright. Rays of light passing near the Sun are usually lost in all that solar brilliance.

Things are different during a solar eclipse. When the Moon blocks out the Sun, stars near the Sun become visible, and their sky positions can be measured. This is exactly the measurement carried out by the British astronomer Eddington in 1919. His results offered spectacular confirmation of Einstein's predictions: images of stars near the Sun were displaced in the sky by twice the amount that Newton's theory would have predicted.

The Sun deflects light "inward". That means that two parallel rays of light that pass the Sun on opposite sides are deflected towards each other and eventually meet. The Sun focuses light from a distant source.

Unfortunately, it does not do a very good job focusing light. A good lens focuses all light into a single point. Rays of light that are at a greater distance from the centerline of the lens are deflected more, to ensure that they arrive at the same focal point. The Sun has the opposite behavior: Rays of light that are at a greater distance from the centerline (or, as we say, have a greater "impact parameter") are deflected less. Consequently, such rays of light are focused at a greater distance. This imperfection of the lens is known as negative spherical aberration.

Even with this shortcoming, the properties of the Sun as a lens are impressive. Imagine for a moment placing a modest telescope, say, a telescope with a 1-meter mirror, in the path of this light. The light that reaches this telescope would be coming from a ring around the Sun (the Einstein ring) that has the width of the telescope but the circumference of the solar disk or greater. The Sun is gigantic: its radius is almost 700,000 kilometers, its circumference, almost 4.4 million kilometers. Multiply that by 1 meter: that means that light from 4.4 billion square meters is focused onto a telescope aperture less than 1 square meter in size. This is tremendous light amplification! This is what makes it possible to use the Sun's gravitational lens to image extremely faint objects.

The angular resolution of this lens is equally impressive. The so-called diffraction limit of a lens is directly proportional to its size. For the Sun, the size is the diameter of the Einstein ring, so 1.4 million kilometers. Taking the wavelength of light and dividing it by this value gives us the angular resolution of the lens. The result, for visible light, is less than one twentieth of a nanoarcsecond. Doesn't mean much? Well, how about a lens so powerful, if we had one readily available, we could use it to read a legal document with dense, small print... at the distance of Jupiter? It is this incredible resolving power that makes it possible for the solar gravitational lens to resolve detailed features of planets in other, distant solar systems.

These are all the great things about the SGL. There are some not so great things, unfortunately, which make using the SGL challenging. Here is a list:

• The focal region is incredibly far away; the useful focal region begins at a distance that is four times greater than the distance covered by our most distant spacecraft to date, Voyager 1.
• The "blur" due to spherical aberration makes reconstructing sharp images difficult.
• In addition to spherical aberration, the SGL also suffers from "astygmatism": The Sun is not perfectly spherical.
• That light from that Einstein ring? It filters through the solar corona, which is tens of thousands of times brighter.
• The Einstein ring appears very near the very bright disk of the Sun itself.
• Other sources of light (the host star of the target planet, other stars in the background) can contaminate observations.
• The image the SGL projects is very large, squares of kilometers in size, and must be scanned.
• The projected image of the SGL moves around as all the parts move, including our own Sun, "wobbled" about by the gravitational pull of the planets, especially Jupiter.
• As we collect light, the exoplanet does not stand still: it rotates, its phase changes, its surface features (e.g., cloud cover) change.

On the pages that follow, we provide details about these challenges and how to solve them. Our ultimate goal is to create a baseline design, a realistic concept for a mission to the SGL focal region, which can collect scientifically useful information, perhaps help confirm the presence of life on another planet, study its properties, and who knows? Maybe even detect the presence of an alien civilization.

Image formation »