18 January 2014

Gravitational Lens Telescopes

In reading Lee Billings's Five Billion Years of Solitude (which as a book has its strengths and weaknesses), I came across a really intriguing idea which apparently isn't at all new, but had escaped my attention to now. I knew that in 1919 E. R. Eddington had demonstrated the truth of Einstein's 1916 General Relativity Theory by measuring the displacement of star positions near the limb of the Sun due to gravitational refraction of the light.
There's that word: refraction. In the late 70s, several independent theorists conceived the idea that if the sun, or for that matter any star, is bending light, systematically and achromatically (without regard to the wavelength), then the light from any given direction should come to a focus at a point in the opposite direction from whatever is being observed. In other words, the sun's gravity could act in effect as a telescope with an enormous tube length (focal distance) and an aperture effectively equal to the Sun's 1.4 million kilometer diameter.

Turns out the focal length is about 550 AU (more, actually, due to electromagnetic effects; effectively it's about 150 billion kilometers). That's a long, long way... further than Voyager has yet reached. So we're talking about at least one order of magnitude leap in space technology before this could be attempted. But in principle, if you wanted to look at Alpha Centauri, for example, you could send a Hubble Space Telescope to the point opposite that star in the celestial sphere and 150 billion km. from the Sun. Then block out the disk of the sun and look at just the ring of space just beyond the sun's disk (filtering out coronal effects and flares; but that's easy enough). What would you get? A telescope that could image coastlines, mountains, rivers... cities. Really. This isn't science fiction. Out to a distance of at least tens of light years it would be just absolutely amazing; although of course you have to go to the right place to see any particular thing. But think of it. The focal-sphere around any star is automatically a fantastic telescope that can view the entire universe at magnifications we can only dream of using artificial instruments.

It's breathtaking. And we will do it, I believe. And even more strongly I believe that others, in the past, at other places, and in the future as well, have done it and will do it. Who knows? One of your ancient astronauts might have espied the Earth in just this way (although there's unfortunately no evidence for that as yet).

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