Gravitational Lenses

At the close of the previous chapter I mentioned Charlie Chaplin and his magnum opus Modern Times, a movie first shown in 1936. In the same year Albert Einstein published a short note in the journal Science concerning how starlight might be focused by gravitational fields. The gist of his paper was as follows.

Consider again Figure 4-1 and imagine the light beams being extended to the right until they meet. Then you could think of the Sun as having acted as a lens: a gravitational lens. Light passing the Sun at top and bottom is brought to a common focus, well off the page compared to the scale of that diagram.

Astronomers like to use big telescopes for two distinct reasons. One is that a larger mirror or lens collects more light, making fainter objects detectable. The other is that better resolution or acuity is, in principle, possible when a large aperture is employed. In reality, however, the turbulence of the terrestrial atmosphere limits the resolution achievable with ground-based telescopes; this movement causes the scintillation (the technical term for twinkling) of stars. This is one of the reasons for putting devices like the Hubble Space Telescope into orbit, above the blurring effect of the atmosphere.

Suppose we positioned a satellite at the focus of the solar gravitational lens, the extended Figure 4-1. With an occulting disk obscuring the Sun, an artificial eclipse would be produced. In a ring around the edge of the disk, the light coming from some hugely distant star or planet would be focused by the solar gravity. The width of the aperture produced by this "solar gravitational lens" would be phenomenal, the Sun being about 865,000 miles in diameter. This would give a resolution—a measure of the smallest detail possible—totally outstripping anything we can achieve either from Earth or using satellites like Hubble.

This solar lens concept all sounds very nice, but is it practical? Actually, when one puts the relevant figures into the equations one calculates a focal length for the solar gravitational lens (the distance to where the lines extrapolated to the right in Figure 4-1 meet each other) of about 500 times the Sun-Earth distance (the astronomical unit or AU). This would mean that your imaginary satellite would need to be located out beyond all the planets, a dozen times as far away as Pluto. So it doesn't appear to be a feasible proposition, at least within the next several decades.

Might we, though, see the gravitational lens effect produced by some other star? The nearest stars are about 260,000 AU away (this is equivalent to about 4.2 light-years). Other stars have different masses and sizes from the Sun, and so produce all sorts of focal lengths. It could happen that the Solar System is close to the focus produced by some relatively nearby star (nearby on the cosmic scale that is).

This is what Einstein discussed in his 1936 paper: the possibility of other stars producing gravitational lenses. It is a nice idea, but for us to see anything in this way some object of interest must lie very close to the extrapolated line from the Earth to the star acting as a lens, and then beyond, and the probability of such a coincidence occurring is miniscule. For that reason Einstein considered his note only of theoretical interest; "Of course, there is no hope of observing this phenomenon directly," he wrote.

Here, though, the great man's imagination had failed him. He was thinking only of the chance of individual stars within our own galaxy, the Milky Way, acting this way. Single stars are of comparatively small mass, cosmically speaking, and so produce little deflection of light beams. Whole galaxies, made up of hundreds of billions of stars, can produce greater effects though. In the 1930s the cosmic distance scale and the characteristics of galaxies were only just beginning to be comprehended, so Einstein can hardly be blamed for his comment. But it was wrong.

A year later another astronomer suggested that galaxies might produce such a lensing effect, but it was four more decades before the first example was uncovered. Several more examples followed, and in the 1990s the search for gravitational lenses became a major pursuit of astronomers, with detection becoming commonplace. The basic idea is shown in Figure 4-7, with interstitial masses such as the spiral galaxy sketched there producing distorted images of

FIGURE 4-7. How an intervening galaxy may cause gravitational lens-ing of some distant object, producing multiple images that may be amplified in brightness. The deflection angles shown here are greatly exaggerated.

FIGURE 4-8. This image shows the effect of a gravitational lens. The bright smudge in the center is a massive galaxy, but arrayed about it are four other spots, those at the top and bottom being especially bright. These are separate images of some more distant quasar, focused by the lens action of the intervening object. In essence the galaxy is eclipsing the quasar, but paradoxically its gravitational lens effect brightens the light received from the latter. These quadruple images form what is known as an Einstein Cross. It is also possible for other slight misalignments to produce bright images that are double, triple, arcuate, or other distorted forms. In the case of a precise alignment, a circular image, called an Einstein Ring, is formed.

FIGURE 4-9. This Hubble Space Telescope image shows the gravitational focusing effect of a huge cluster of galaxies known as Abell 2218. The many arcs spread across the photograph are distorted images of other galaxies and quasars five to ten times as far away from us as the cluster causing the lensing.

FIGURE 4-9. This Hubble Space Telescope image shows the gravitational focusing effect of a huge cluster of galaxies known as Abell 2218. The many arcs spread across the photograph are distorted images of other galaxies and quasars five to ten times as far away from us as the cluster causing the lensing.

more-distant light sources. An example is shown in Figure 4-8, a focusing galaxy producing four images of a distant quasar (that is, a quasi-stellar object; the true nature of such sources is still unknown, but they seem to be very distant but extremely luminous objects). The effect is similar to that obtained by looking through the bottom of a wineglass, where a variety of distorted images form as you move your eye around. This is more obvious in Figure 4-9, a photograph of a cluster of galaxies whose combined gravity leads to many arcuate images of more distant galaxies and quasars that cannot otherwise be seen.

Is the phenomenon seen in Figure 4-8 an eclipse?Yes, because the focussing galaxy is blocking our direct view of what is behind it. Paradoxically the eclipse is amplifying the brightness of the quasar, in the same way a magnifying glass enhances the intensity of sunlight such that a piece of paper may be ignited. Without that amplification the quasar might well have been too faint to detect, so that, rather than simply hiding it, the galactic eclipse has made possible the detection of this light source at the periphery of the universe.

It happens that Einstein was also wrong in the context of observing gravitational lenses within the Milky Way. The gravitational lens formed by a single star is extremely narrow, and so even with about 400 billion stars in our galaxy he reasoned that the chance of getting two stars aligned with the Earth at the focal position must be exceedingly small. That is based on the assumption of a static situation though. In fact, all the stars are moving, in orbit around the galactic center and also shifting relative to each other with their own peculiar velocity components. Every so often two will align with the Earth, and major astronomical research projects now automatically monitor thousands of stars each night. As an alignment occurs, the focussing through the transient "lens" produces an increase in intensity (like the magnifying glass again), and this brightening may persist for days or months. The computers scanning the images are programmed to draw attention to such intensity enhancements. Using the results, astronomers are also searching for the so-called "missing mass" that seems to hold our universe together.

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