The Event Horizon Telescope- the telescope that can can see beyond your imagination - Mystery of Galaxy

               The Event Horizon Telescope

On April 10th 2019, the very first image of a black hole will be released to the public and we're gonna preview it and understand this remarkable scientific achievement. Welcome back to my blog "Mystery of Galaxy" . Chances are by now you've already seen the very first image of a black hole. 

First image of M87 blackhole

Source: Event Horizon Telescope / CC BY

Anyway let's take a quick look at how black holes have been depicted traditionally. When we think about black hole images, they're usually surrounded by a disk of gas and dust that is circling at very high speed. Some of its falling into the black hole, some of it being accelerated out in bipolar jets. 

Jets of black holes


Source: ESA/Hubble / CC BY

Now these illustrations are very useful for understanding the geometry of the black hole and its accretion disk. However, if we were to look upon black hole and its accretion disk with our own eyes, we would see something necessarily different. And that's because black holes. like any massive object in the universe distort space-time around it and light follows a path around this distorted or curved space-time. And so it gives us a warped picture of the environment surrounding the black hole. 

accretion disk of black hole 

Source: NASA’s Goddard Space Flight Center/Jeremy Schnittman / CC BY-SA

                              

In the 1970s Jean-Pierre Luminet worked out what a black hole should look like. He envisioned a black hole surrounded by an accretion disk and using the technology available at the time - a photographic plate kind of looking down on the black hole and it's disk from a slight angle. The light from above the disk forms essentially the primary image, but light from underneath the disk would also be deflected by the black hole and would be redirected towards our photographic plate producing a secondary image. Lument then worked out the paths that light would take around a rotating black hole. He then plotted each photon on a piece of paper producing a virtual photograph; literally a graph of every photon. The first thing you notice is that there's a really bright region on the left-hand side. The black hole is rotating and so is the material in its accretion disk. As that material is rotating toward us however it is moving at nearly the speed of light. That means that the light gets stacked up and the intensity of that light coming toward us is increased. This is an effect called 'Doppler beaming' or 'relativistic boosting', and it's the reason why the left side of the disk appears to be so bright. Meanwhile on the right side of the disk the material is rotating away from us at nearly light speed, stretching out the wavelengths of light making them dimmer and dimmer so we get a very bright right side and a ghostly, very difficult to see left side. Now exactly how this disk appears to us is going to depend on our viewing angle. In the upper right-hand corner of this simulation, we can see a single hot spot in the accretion disk as we change our viewing angle. We can see how the black hole's gravity distorts and brightens the hotspot as it rotates into our field of view. But all of this is purely theoretical. In order to test relativity we need to make an image of a black hole and if it looks anything like what we expect, then that is pretty much the most strangest of relativity we can come up with. 

Fortunately our galaxy is home to a 4million solar mass black hole. It's in the direction of the constellation Sagittarius and so we call it Sagittarius A-star or Sag A*. We can't see Sag A* at visible wavelengths because of all the intervening gas and dust. But if we switch to infrared we can make out individual stars as they orbit the black hole. These observations allow us to make very precise measurements of Sag A star's mass as well as its overall size. The object turns out to be about 4 million solar masses and has a diameter of about24 to 44 million kilometers. However, at 26,000 light-years away Sag A star would be just five point three millionth of an arc second on the sky. Give you an idea just how tiny and angle that is let's consider a soccer ball. It's about 22 centimeters in diameter and our eyes can make out a soccer ball at a distance of about 3/4 of a kilometer. Now obviously that depends on your eyesight but on average the human eye can resolve down to about 60 seconds of arc. The Hubble Space Telescope can do much better - it can see down to five one hundredths of a second of arc. Even so that is about 10,000 times too low a resolution to image Sag A-star. In other words, We need a bigger telescope. We can link multiple telescopes together using a technique called interferometry. This however requires ultra precise timing so that the light from each telescope converges at the same time to form a single image and that's sets a practical limit on just how far apart our telescopes can be. However at longer wavelengths the timing requirement is a less stringent, so longer baselines are possible. However, the longer the wavelength, the larger the telescope has to be in order to achieve the desired resolution. Even microwaves which are about a millimeter across are so gigantic of a wave that in order to see down to 53 micro arc seconds you need a telescope the size of planet Earth. Interferometry at the planetary scale requires very long baselines we even call this technique VLBI for Very Long Baseline Interferometry. Interferometry uses a pair of telescopes separated by some baseline due to the angle of the target and the curvature of the Earth. Light is going to hit one receiver first; that means that the time difference in the arrival of the signals has to be accounted for. 

The Event Horizon Telescope and Global mm-VLBI Array on the Earth

Source: ESO/O. Furtak / CC BY

In the case of the Event Horizon Telescope, observations had to be conducted simultaneously and synchronized using atomic clocks. The Event Horizon Telescope consists of several telescopes around the globe - the Sub Millimeter Array and the James Clerk Maxwell telescope at Mauna Kea, the Sub-Millimeter Telescope at Mount Graham, Arizona, the Large Millimeter Telescope in Mexico, the IRAM 30-meter telescope in Pico Vieta, Spain, the Atacama Millimeter and Submillimeter Array or ALMA in Chile and the South Pole Telescope at the South Pole station in Antarctica. The 77-meter Greenland Telescope joined EHT for a second round of observations. EHT can be thought of as a mirror the size of Earth, but it's more accurate to describe it as fragments of a mirror scattered around the Earth. That means each fragment can only form a piece of the image. Luckily Earth's rotation allows those antennae to sweep out a wider swath of the sky so it does help to fill in the rest of the image.

but how do you reconstruct a true image when all you have are image fragments? 

Well let's imagine what we would see if we could only take images of a given subject. Well, if we look at this image, we can start to make out some familiar shapes, patterns colors, textures, and before long your brain has probably filled in that what we're really looking at is an image of an apple. Now that's easy for us because we know what an apple looks like. What exactly will Sag A star look like? Well fortunately we can run some theoretical computations using supercomputers to produce a virtual image. These are General Relativistic Magneto Hydrodynamics simulations. In this image, we're looking at the black hole at about a 45 degree angle. From that we can then take all the possible images that can be derived from the image fragments and rank them from least likely to most likely. The most likely image is the best representation of what it is we are actually seeing. These observations are extremely difficult to make. thank you for reading the article. hope you like it.