How did scientists take the real picture of a black hole?

Source: EHT Collaboration, Image Link

10 April, 2019 will be a historic day in the science history. May be almost all of us have seen the real image of a black hole taken by EHT (Event Horizon Telescope). But how did the scientists take the real picture of a black hole? What are the technologies behind it? Why did we need to take the real picture of a black hole? If you want the answer of these questions, then read the full article below.

What is a black hole?

To understand the whole scenario, first we have to understand what a black hole actually is. Black hole is a favourite choice of any science fiction writer. Simply, black hole is a massive concentration of matter in the region of space and time. When a massive star collapses at the end of its live, then black hole forms. These are some places of the universe where gravity becomes so strong that nothing can escape from it nor even light. For the full article of black hole check this.

What is Messier 87?

Virgo Constellation

Messier 87 also known as Virgo A is a super giant elliptical galaxy in the constellation of Virgo. This galaxy was named after a French astronomer Charles Messier who discovered it in 1781. It is 53 million light year away from the earth and is also second brightest galaxy in the northern Virgo Cluster. It is ellipsoidal in shape and has no distinctive dust lanes. It has an active super massive black hole in the centre like our milky way galaxy. 

The Messier 87 (M87) is the super massive black hole which real picture was taken by Event Horizon Telescope.

Why black holes cannot be spotted by telescope?

How do we see actually? The light falls on the object and the object reflects some amount of light which then incidents on our eye's retina. Thus, we can see any object around us. Simply, to be able to see, the light should be reflected back by the object to our eyes. But, what if  the object does not reflect any light?

Taking picture of a black hole is like taking picture of darkness without any flashlight. Because, there is not any reflection of light from any black hole due to its intense gravity. It warps the space and time so that the light bends on itself and cannot come out from it. That is why, normal telescopes cannot spot the black holes.

How scientists captured the uncapturable?

Till now, whatever the images of black holes you have ever seen are based on scientific data and computer generated artist's illustrations. Those are not real photos. Because, they cannot be seen directly with normal telescopes. To make this possible, scientists used a group of telescopes named as EHT (Event Horizon Telescope).

Not any single telescope has the capability to take picture of any black hole. It can be compared with placing a ping pong ball at moon and taking photograph of it from earth. It is so hard because, the resolution of a telescope directly depends on its diameter. The larger the diameter, better is the resolution.

The accretion disk of a black hole emits strong and intense light which travels through the space and the wavelength of light is in radio wave range. Such a large wavelength of light cannot be spotted by any single telescope. But of course, it is possible if the size of the telescope is equal to the size of earth. It seems quite impractical. This leads to the born of Event Horizon Telescope.

What is EHT (Event Horizon Telescope)?

EHT network, Image Link

As, earth sized telescopes cannot be built, so a group of telescopes were used to produce a network which works like a giant earth sized telescope. Obviously, this virtual telescope was not as it is today now. In 2009, it was a combination of just four smaller observatories placed in Arizona, California and Hawaii. It captured an image of M87 black hole spewing a jet of plasma. But due to lacking of magnifying power of these telescopes, the picture of the black hole did not come out completely.

By 2017, they extended their number of radio observatories. Now, EHT consists of eight powerful radio observatories placed in North America, Hawaii, Europe, South America and South Pole. The ALMA (Atacama Large Millimeter/submillimeter Array), APEX (Atacama Pathfinder EXperiment) in Chile, the IRAM 30m (Institute for Radio Astronomy in Millimeter Range) in Spain, LMT (Large Millimeter Telescope) in Mexico, SMT (Submillimeter Telescope) in Arizona, the James Clark Maxwell Telescope and SMA (Submillimeter Array) in Hawaii and South Pole Telescope in Antarctica built up the whole team of EHT.

How EHT works?

By observing the image of the black hole, one might think that it is so easy to take the picture of a black hole; just roll, camera and action. No. it is not so easy. The wavelength of light coming from the M87 black hole is about 1.3 mm which comes in the range of radio waves. 

That is why, radio astronomers use a technique called Astronomical Interferometry, where a group of smaller radio dishes are linked together to form a giant radio dish. Each dish collects individual light from the target object and converts it to electrical signals. Then a computer called Correlator combines all the electric signals to a single one. For decoding the received signal a special kind of mathematics is applied which is called Fourier Transform.

EHT uses a technique called Very Long Baseline interferometry, in where the telescopes are placed in different continents and collect the data from each observatory. The telescopes uses precise atomic clocks, GPS and synchronised with each other. 

But, here is a question. If the telescopes are in different locations, then light should be spotted on those portions only. How did they actually take the whole photograph? Well, actually, EHT used the rotation of the earth. In different time, the locations of the telescopes are different. They took the individual pictures at different time and finally they combined the whole photographs.

EHT used four groups to analyse the picture. But why four? Why not one group? Because, they wanted to make it perfect. All four groups of the team worked very hard to make the picture best of best and now the result is here.

Now what?

For the very time, we are seeing the real image of a black hole (M87). As shown in the figure, the middle black portion is the shadow of the black hole and the surrounding coloured area is the Accretion disk. Accretion disk is that portion of a black hole where matter heats up to an extreme temperature so that it emits X-Rays. In the shadow portion there, there are two mysterious thing of black hole- Singularity and Event Horizon.

If you look closely then you will notice that the bottom portion of the accretion disk is glowing more compared to the upper portion. This means that the light of the bottom portion is coming towards us and light of the upper portion going away from us. But we do not have to worry about it. It will not destroy the earth. This is called Relativistic Beaming.

But what would be the benefits from such kind of pictures? Well, it has proved the theory of general relativity predicted by Einstein in 1915. Once again, Einstein proved him right. The concept of black hole works only when the object becomes infinitely massive and small in size. Thus, the gravity becomes infinite there and it squeezes the space and time around it. The bending of space and time is such that nothing can escape nor even light. So, this natural time machine actually exists in the universe.

Of course, it is not the clearer picture of a black hole like the Gargantua, but it is the real one. When Pluto was captured for the first time from a very far distance, then also we got a blurry image. But now, we have the clearest picture of Pluto. Obviously, black hole is a mysterious thing. We always had eyes on it. It teaches us how mysterious and interesting the universe is.