by Prasad Ganti
In January 2019, I wrote an article on radio astronomy in which I mentioned a recent book titled “Einstein’s Shadow” by Seth Fletcher. The book talked about photographing black holes and why it is very difficult to do so. The spectacular pictures of a black hole came out recently. It has been one of those major scientific victories of this decade like the discovery of Higgs Boson or the gravitational waves. It is another validation of Einstein’s theory.
It is now widely believed that every galaxy has a supermassive black hole in its center. The very first picture of the black hole is from Messier 87 (M87), a galaxy located more than 53 million light-years from Earth. It has a mass 6.5 billion times that of the sun.
Why is it difficult to see a black hole ? Why was the first black hole pictured not from our own galaxy ? Black hole by definition is black. Because it absorbs all the light and radiation and matter falling into it. It does not reflect any radiation back to us to capture and hence see. The size of the black hole is much much smaller compared to the galaxies harboring them. We can try to look at the gas surrounding a black hole. The gas and matter close to the black hole, but has not yet reached the point of no return, which is called the Event Horizon. The gas in this region can get heated up to billions of degrees (at such temperatures it does not matter if it is fahrenheit or centigrade). And emits radiation of its own as any hot body would do so.
The radiation coming out of the halo surrounding the black hole, spans across different frequencies. Lower frequencies are radio waves and microwaves. Higher frequencies are infra-red, light, ultra-violet, x-rays and gamma rays. These different frequencies then start their journey across the universe. Across the vast space to reach us. The space between us and the source of radiation is not truly empty. Other than possibly objects like stars and galaxies, there is lot of dust and clouds of organic molecules which can act as potential obstacles in our line of sight. Most of the higher frequencies are absorbed by the such noise in the space. What manages to escape are the radio waves. Which are very similar to our AM and FM radio waves, except that they are not intelligible like our radio stations are.
Since it is the radio waves we are trying to capture, an optical camera will not do. Certainly not the ones which are there in our phones. We basically need a radio receiver. Instead of optical telescopes, we need the radio antennas – dish shaped or a set of rods. This combination of equipment leads to radio astronomy instead of lens and mirror based optical astronomy. Lens and mirrors can be thought of as antennas for light, or antennas can be thought of as lenses and mirrors for radio waves.
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. The bigger the telescope, the larger is the collection area. Better it is to capture pictures of distant objects. There is a limitation on how big the telescopes can grow. Another option is to combine telescopes across the globe to effectively form a big telescope. Such a giant telescope to picture a black hole is called an EHT (Event Horizon Telescope).
EHT combined measurements from radio observatories on four separate continents – namely North America (Large Millimeter Telescope), South America (Atacama Large Millimeter Array), Europe (Institute for Radio Astronomy in Millimeter range), and Antarctica (South Pole Telescope). And a few other telescopes were involved. Since they are all spread out geographically across multiple time zones, synchronizing them and co-ordinating among different teams was a himalayan effort. The collected data was so massive that it could not transported over the internet. Hard drives containing vast amount of data were physically transported back to MIT and Max Planck Institute of Radio Astronomy in Germany. There were delays in getting data from South Pole due to the Antarctic winter.
The book “Einstein’s Shadow” has lot more details about the EHT and the team which did all the conception and the co-ordination of this massive exercise.
I am not reproducing any pictures here. They are there on the internet, particularly on the site space.com. A note that the pictures of the black hole or any other astronomical object like the galaxies and the distant stars we are shown, is not how they would be appearing even if we happen to be in the neighborhood. The human eyes are simply not enough to perceive that kind of reality. Based on the data collected by the telescopes, computer algorithms are used to assign colors based on factors like the temperature. This is the best we can do with our limited senses.
Our Universe the Milky Way also has a supermassive black hole in the middle. This black hole is closer to us, but we cannot be looking from the above, since we are in the same plane. We will be looking at it from the side. That makes it difficult to view. But I am expecting more black holes to be pictured in the future. More radio telescopes will be joining the EHT in the hunt. I am also expecting a Nobel prize in physics in the coming years for this effort.