A Cosmic Telescope
Ken Tapping, August 26th, 2015
It is not clear exactly when glass was invented, although it was probably by accident, when someone lit a particularly hot fire on sand. It was certainly a long time ago. It is also not clear when one of our ancestors, when playing with a piece of glass that happened to be thicker in the middle and thinner at the edges found he or she could catch the sunlight and burn themselves or their friends, or set fire to things. Those early experimenters put us on the road to modern telescopes, cameras and other optical equipment. These early lenses and their modern, more precise descendants all make use of an important fact; light travels more slowly in glass than it does in air. This principle is what gives us modern telescopes and other optical equipment
Imagine a long line of skaters, with their arms linked, skating in a long line abreast. If we slow down the skater in the middle, and the ones either side a little less so, and the others by decreasing amounts as we get further from the middle of the line, that line will become a curve. That curve will get smaller and tighter until all the skaters end up in a heap. We have collected all the skaters and brought them together at one point. In the same way we can use lenses and mirrors to collect and concentrate light. This is instrumental in astronomy, because most cosmic objects are very faint. This is why so much of our efforts in telescope development goes into making bigger and bigger light collectors. The biggest currently in our plans will have a light collector thirty metres in diameter. However, there is a way we can achieve a far bigger light collector. How about one that is bigger than a galaxy? It sounds incredible, but this is something that is now a standard research tool.
We have achieved that thanks to Albert Einstein and the other physicists who have analyzed the properties of space and time. They showed us that large, massive objects, like a planet, star or galaxy, distort space, rather like the way a big, iron ball dropped in the middle of a rubber sheet would pull it downwards into a depression. Now imagine a skating rink with such a depression in the middle. Once again we have our line of skaters, holding hands, all resolutely skating at the same speed. The skaters who encounter the hole will have further to travel than those who don’t, because they have to go down one side and up the other. The skater going through the bottom of the hole will be most affected, and others less so. The result will be the skaters who met the hole will lag behind the others, and the line will be pulled into a curve, and once more the skaters will all end up in a heap. That hole is acting like a lens.
The distortion of space by a massive galaxy affects light the same way as the hole affected the skaters. It bends light, bringing it together at one point; it is focussed. We call this process gravitational lensing. The galaxy acts as an enormous light collector, concentrating the light of very faint objects lying beyond. Without that concentration of their light we would never be able to see them. Of course we can only do this when there is a large galaxy conveniently located in the direction we want to look, and these gravitational lenses are not perfect, so it sometimes requires a lot of image processing to make sense of what we are seeing. However, without the process we would not be seeing anything; it would be too faint.
Such cosmic telescopes will never replace good ground and space-based telescopes. However, they provide a valuable complement to those instruments in addressing important questions about the early history of the universe and that time long ago and far away, when the first stars and galaxies were forming.
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