In the 1960s Canada was the first country to successfully use a radio telescope thousands of kilometres in diameter. A technique was developed to make multiple radio telescopes, separated by thousands of kilometres, act as though they were joined together. This technique, known as very long baseline interferometry, or VLBI, was developed for one main reason: to find out what quasars actually are. However, it also became very useful for something completely different: measuring the changing shape of the Earth.
The level of detail in an image is dictated by how big the mirror or lens used to make it is compared with the length of the waves being imaged. Light waves are very short, so our eye lenses, with a diameter of a few millimetres, are good enough to see details as small as one per cent of the area of the lunar disc. To record the same detail for waves that are centimetres or metres long, the lenses or mirrors have to be huge. For example, our Synthesis Radio Telescope, which can yield radio images with levels of detail achievable with our eyes, consists of a row of antennas 600 metres long.
We can increase our eyes’ ability to discern fine detail by using a telescope or binoculars. These have the effect of making the pupils of our eyes larger. However, this is hard to do with radio telescopes. Modern engineering science and materials can provide us with radio dishes up to about 100 metres in diameter, but no larger.
In the 1960s a strange class of cosmic radio sources called quasars were detected. They were very small, looking like stars through our biggest optical telescopes, and they lie millions or billions of light years away. Most of their strangeness was in the radio emissions they produced, so we wanted to make radio images of them. We now believe they are driven by black holes.
Cosmic radio sources appear so small in the sky that most of them lie beyond the imaging ability of dish radio telescopes, even the largest. This led to the development of techniques whereby we can combine groups of small radio telescopes into arrays bigger than any possible single-antenna radio telescope. As time passed, most radio sources were imaged, but the quasars remained unimagable. We needed even bigger arrays. However, the telescopes in an array have to be connected together, which limits these arrays to maybe a few kilometres across. Even the biggest arrays were inadequate for dealing with quasars. Something completely new was needed. Could we make arrays without having to connect the antennas together? There was an international race to achieve this. Canada got there first.
The idea was to record on videotape the signals received by different antennas. The tapes from the various antennas would then be brought together later for processing. To make this possible extremely precise timing signals were added to the tapes. Now the antennas could be anywhere on Earth. An array the size of the Earth gave us some usable images. This is the technique that recently gave us our first ever images of a black hole.
One really useful by-product of these VLBI experiments is a measurement of the positions of the antennas, including the distances between them, accurate to a few millimetres. By putting antennas on different continents we can measure the speed the tectonic plates are moving and how landmasses are stretching, twisting or otherwise changing shape. This works well because the small size of quasars, which makes them extremely hard to image, makes them also ideal reference sources for measuring the positions of the antennas. Our obsession with quasars has given us the ultimate ruler for measuring our world.
Mars lies very low and inconspicuous in the west, sinking slowly into the twilight. Jupiter, shining like a searchlight, rises around midnight and Saturn at 2 a.m. The moon will be full on May 18.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory. E-mail: email@example.com