Radio telescopes and the black hole

by Prasad Ganti

Astronomy traditionally have been confined to seeing the light emitted by heavenly bodies. Nineteenth century discoveries proved that light is only a small part of the spectrum of what is known as electromagnetic radiation. It was found later that infrared (heat), x-rays, ultra violet rays, gamma rays, microwaves, radio waves are all part of this same spectrum. In the twentieth century, Karl Jansky of AT&T Bell Labs found that radio waves are also emitted from outside of our solar system. Gradually, radio astronomy emerged where telescopes were built to receive radio waves coming from all over our Universe.

I watched a very interesting collection of video lectures on radio astronomy. It was part of a course conducted by Dr. Felix Lockman and produced by the company “The Great Courses”. What is surprising is that the pictures of the distant stars and galaxies can be different than what is observed using optical telescopes. In multi messenger astronomy, such pictures from radio and optical telescopes are combined to produce a composite picture. The invisible pictures captured by the radio telescopes, which are a bunch of radio antennas connected to a radio receiver, reveals a different aspect of astronomy.

Light gets blocked by the interstellar dust, whereas radio waves just pass through. Due to this reason, clouds of hydrogen, ammonia, water vapor, organic chemicals like formaldehyde and acetic acid have been detected in interstellar space using radio telescopes, which would not have been possible using the optical telescopes. The presence of organic chemicals in space, although no DNA or proteins have been found yet, means that life could be present in other parts of our Universe.

Examples of radio telescopes are ALMA (Atacama Large Millimeter Array), a collection of 66 radio dish antennas in Chile. A 300 foot dish radio telescope is in Green Bank, West Virginia. A 500 foot dish radio telescope was recently built in China (called FAST). A single dish radio telescope is in the South Pole. When multiple antennas exist, they combine together (called interference pattern) and form effectively a larger telescope. Huge telescopes like the 300 foot dish cannot be made as a single piece. Multiple panels driven by motor actuators to correct for factors like gravity, wind, heat etc. Special white paint keeps the dish cool. Heat produces noise. That is why the radio receivers are cooled to a few degrees above absolute zero to eliminate most of the noise.

Picture below is of ALMA. By looking at the vehicle going on the ground, you can estimate the size of the telescope. Cables from all the antennas are connected to very sensitive radio receivers in a control room.

ALMA

ALMA in Chile

Now let us turn to Black holes. These are very interesting cosmic objects in our Universe. Massive amounts of matter gravitationally compressed very densely so that not even light can escape from them. With high level of confidence, astronomers state that supermassive black holes exist at the center of every galaxy, including our milky way. The problem is we cannot see a black hole. Even if we try to detect some form of radiation around it, there is too much dust as we get closer to the center of our galaxy. By the way, our Sun is about two thirds distance away from the center of our galaxy. Radio astronomy to come to our aid to view the center of our galaxy.

I recently read a book called “Einstein’s Shadow” by Seth Fletcher. The radio telescopes we have currently are not good enough to peep into the center of our galaxy. The book talks about a great idea of building something called EHT (Event Horizon Telescope) to view the black hole at the center of our galaxy. It is an extension of the idea of combining multiple antennas. It combines radio telescopes in different parts of the world to make one huge telescope called the EHT. EHT is a combination of ALMA, LMT (Large Millimeter Telescope), JCMT (James Clerk Maxwell Telescope), SPT (South Pole Telescope) from different parts of the world, to name a few. The eventual goal is to photograph the black hole at the center of the galaxy. That is what the author calls as “Einstein’s Shadow”.

A note that the pictures of galaxies and distant stars we see are not really the colors they have. Based on the data collected by the telescopes, computer algorithms are used to assign colors based on factors like the temperature. Please don’t expect to see a black hole in red !

The first set of experiments have been conducted using such combinations of widely scattered telescopes. The data will be analyzed for the next few years. And hoping to get a glimpse of the black hole in coming years. The revolution which Jansky sparked has taken over the universe in bringing light to otherwise dark areas.

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This entry was posted in January 2019, Sidereal Times and tagged . Bookmark the permalink.

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