How to take pictures of black holes? Seven questions to solve the puzzle- “Black Hole Catcher” -PanSci

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Before joining the adventure of the “Black Hole Catcher” universe, you may have many questions on the subject of black hole photography. This chapter may satisfy your curiosity.

Q1. Why is this photo of the black hole, so sensational from all walks of life, exposed? Hasn’t the black hole been confirmed a long time ago? I have also seen many photos!

Many people have simulated photos and animations of black holes, which makes many people think, “Is the existence of black holes taken for granted?”

However, while there is plenty of indirect evidence (such as gravity waves) that makes scientists generally believe that black holes exist, it wasn’t until April 10, 2019 that the first visual evidence confirmed the existence of black holes.

Q2 If a black hole absorbs light, how can there be a halo in a photo of a black hole?

The ring of light in the black hole photo is not a black hole, but the radio waves emitted by the material around the black hole.

A black hole is a celestial body of great mass, and the light will be absorbed when it approaches the black hole, reason why the black hole itself does not emit any light. However, what we are looking at is the area near the black hole. Because the surrounding gas is attracted to the black hole, it orbits around the black hole; The closer the gas is to the black hole, the greater the speed of movement. The collision and friction between the gases will produce high heat and release energy.

Some of the released energy will be absorbed by the black hole, but some will be able to escape the gravity of the black hole. The depleted energy contains components of radio waves, and astronomers can observe the escaped radio waves and learn the shape and structure of the black hole.

Q3. So many black holes, which one to choose to observe?

There are black holes everywhere in the universe. But those black hole targets seen from the ground must be large enough and not too far from us. Sagittarius A * in the southern sky and M87 * in the northern sky are two observable targets.

Q4. Why does the news say you have built a telescope “the size of the earth”?

If you want to see things further, you need a telescope with a larger aperture. The M87 black hole we photographed this time is 55 million light years from Earth. According to the projections, a telescope with a diameter of 8,000 kilometers is required to clearly see the appearance of the black hole.

A telescope with a caliber of 8,000 kilometers! How could it be built?

In fact, you don’t need to cover such a large telescope. Scientists use the “extra long baseline interference technique” to allow telescopes around the world to observe black holes at the same time, and to organize the data from their observations, which can achieve the same effect as building a telescope as “big as the earth”.

Q5. Why can the telescope see far away?

There are two necessary conditions for the human eye to see one thing clearly: the light must be strong enough and the angle at which light enters the eye must be large.

For example, if you have a donut in front of you, it’s easy to see what the donut looks like. If you turn off the light, you won’t see it. This is the need for light. Then move the donut farther and farther away from you, and you will see it getting smaller and smaller, so small that it becomes a point, and you can no longer identify the hole in the middle.

When light reflected from both ends of the donut enters the eyes, the angle between them is called the “viewing angle.” When something is too far away, the viewing angle is too small and the human eye cannot recognize the object. The telescope relies on the combination of concave and convex lenses to achieve magnification and condensation functions to strengthen the above two conditions, so that the human eye can recognize distant objects.

Q6. Why do you need so many telescopes to observe the universe?

Matter in the universe emits various electromagnetic waves, such as visible light (red orange yellow green indigo blue visible to human eyes), ultraviolet rays, infrared rays, X-rays …

Electromagnetic waves in the universe are easily absorbed and dispersed by the Earth’s atmosphere; Along with very limited human eyes, only visible light can be seen in a narrow band. Therefore, if humans only rely on visible light to judge what the universe looks like, they would be too blind to touch the elephant. There are two ways to improve the above situation.

• First, don’t just rely on visible light telescopes, use different types of telescopes to get various types of signals in the universe.
• Second, find a way to launch the telescope out of the atmosphere to avoid atmospheric interference and receive a clearer signal.

Let us first analyze the first method. In addition to the more common visible light telescopes, there are a variety of telescopes with different wavelengths that can observe the universe (as shown on the right). Scientists have discovered that the “submillimeter wave telescope”, whose working band is between infrared and microwave, is best for observing interstellar gas and the evolution of stars. It is the best weapon for observing black holes and the biggest contributor to this black hole photograph.

However, waves such as millimeter and sub-millimeter waves are easily absorbed by moisture in the atmosphere, so such telescopes must be built in dry, high-altitude locations. For example, the Alma matrix in northern Chile is covered in the Atacama desert at an altitude of 5,000 meters.

The second point is to find a way to launch the telescope into outer space, to avoid atmospheric interference. Hubble telescopes are telescopes that operate in space.

Q7. What is the Event Horizon Telescope?

EHT (Event Horizon Telescope) is the “event horizon telescope”, in reference to an international astronomical project, the objective is to obtain the first photo of a black hole in history. The “event horizon” refers to the boundary of the black hole.

In 2012, astronomers from around the world held a meeting and organized the “Event Horizon Telescope” project, calling radio telescopes around the world to connect and take pictures of black holes.

As of March 2020, there are 11 telescopes joining EHT. Taiwan participated in the production and operation of 4 of them. (Represented by a Taiwanese icon in the picture) However, the black hole photo exposed in April 2019 is based on observation data in late 2017. At the time, only eight telescopes joined EHT and participated in the shooting.

These telescopes were dispersed in various places, after cooperation, equal to the super large diameter telescopes. The white arc between the telescopes in the image represents the baseline. The more baselines, the greater the amount of observable data and the clearer the photos taken.

-This book is taken from “Black Hole Catcher: The Story of Taiwan First Black Hole Photography in History”, March 2020, World Culture.

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