Mark Reid
Harvard-Smithsonian
“A Telescope the Size of the Earth”
Abstract
Interferometers have allowed astronomers to image the sky with angular resolutions greatly exceeding that possible with a single telescope. At radio, unlike at optical, wavelengths one can use amplifiers to split the received signals with minimal loss and mix them with similar signals from all other telescopes. In this manner, large arrays of telescopes can be used to image radio sources, and resulted in a Nobel Prize for the technique. Furthermore, by recording the incoming voltage on tape or disk, one can mix the signals long after the observations, allowing telescopes across the Earth to work together as a single interferometer, truly expanding on radio’s “secret weapon.” This technique, called “Very Long Baseline Interferometry” yields images with the angular resolution of a telescope the size of the Earth. At wavelengths of ~1 mm, this results in an angular resolution of ~20 micro-seconds of arc. With this resolution one could read my presentation from the Moon! More interestingly, with this incredible angular resolution we have been able to directly measure the acceleration of the Sun as it orbits the Milky Way (informing tests of General Relativity with binary pulsars), surveying the Milky Way’s spiral structure, directly measuring the angular rotation and motion of galaxies across the sky, and measuring the gravitational Brownian motion of the super-massive black hole at the center of the Milky Way and even imaging emission around its event horizon.