What is a black hole, why is it important to study it?

Black hole – Read more about the Event Horizon Telescope in the questions and answers column.

What is a black hole?

A black hole is an area of space-time in which gravity is so strong that no particle or even electromagnetic radiation can escape from it. The boundary surrounding the black hole, behind which escape is impossible, is called the event horizon. Within the event horizon, the curvature of space-time caused by the gravity of a black hole is so extreme that the hole seems to exclude itself from the surrounding universe.

A black hole is created when a sufficiently large mass is compressed into a very small volume – our own sun would make a black hole by compressing it to a radius of about three kilometers. Astronomers have found two types of black holes in space: star-shaped black holes created by the collapse of massive star interiors in a supernova explosion, and so-called supermassive black holes located in the centers of galaxies. Supermassive black holes can weigh up to several billion suns.

Do black holes need to be feared?

There is no need to fear black holes. They are so far away from us that we cannot accidentally get close to a black hole.

What is Event Horizon Telescope?

Event Horizon Telescope (EHT) is an international collaborative project established to produce the first image in history of a black hole using Very Long Baseline Interferometry (VLBI). It is a technology that combines several radio telescopes around the globe to get the most accurate picture possible of a distant object. Together, these telescopes are as resolute as one earth-sized telescope. In addition to Aalto University, EHT has sixty research organizations around the world, and more than two hundred researchers. There are dozens of financiers. In Finland, the financier is the Academy of Finland.

Event Horizon Telescope has studied a black hole in the center of our Milky Way called Sagittarius A *, weighing four million suns and located 26,000 light-years from Earth. However, the now published image was taken from another object studied. The black hole in the center of the M 87 galaxy was chosen as the object to be photographed because, due to its larger mass (six billion solar masses), its apparent horizon across the sky is of the same order as the Sagittarius A *, even though it is much farther away, 55 million light-years away.

Black hole
” The extreme gravitational field of the black hole provides an opportunity to test Einstein’s general theory of relativity ”

How can a picture be taken of a black hole when the black holes do not emit light?

As the name implies, the black hole itself does not radiate. However, the area around the black hole may radiate strongly if gas falls into the hole. As the gas circulates through the orifice at an oblique speed, it heats up strongly and radiates. The gravitational field of the black hole bends the passage of this light, causing some of the photons to revolve around the hole. The result is a bright ring with a dark shadow in the center – the silhouette of the event horizon. The shadow is about two and a half times larger than the event horizon itself and its size depends mainly on the mass of the black hole and to a lesser extent on how fast the hole rotates.

What is the event horizon?

The event horizon is the boundary of the black hole, from the inside of which the black hole absorbs everything, including light, i.e. we talk about the event horizon of the black hole.

Why explore black holes?

The extreme gravitational field of the black hole provides an opportunity to test Einstein’s general theory of relativity – one of the cornerstones of modern physics – under conditions not found in terrestrial laboratories or even in our own solar system.

The general theory of relativity describes gravity as the curvature of space-time, and for the last hundred years it has survived clearly from all experimental tests. For the average person, general relativity is most easily seen in GPS positioning, which is familiar from mobile phones: if relativity were not taken into account in GPS satellite clocks, the system’s positioning error would increase significantly in just one day.

However, it is not clear that the theory of relativity also applies to very strong gravitational fields such as near a black hole. A possible deviation from the predictions of the theory of relativity could lead physicists even deeper into the theories of the universe.

In addition to tests of general relativity, the Event Horizon Telescope provides an opportunity to study the behavior of a gas falling into a black hole just near the event horizon. This is expected to provide answers to questions such as how the huge plasma jets generated by supermassive black holes originate.

How could an image of such a distant object be obtained?

The image now obtained has been made possible by the development of technology, equipment and methods of analysis. Very Long Baseline Interferometry VLBI has been used to obtain the image. It is a technology that combines several radio telescopes around the globe to get the most accurate picture possible of a distant object. Together, these telescopes are as resolute as one earth-sized telescope. This virtual telescope is almost the size of the earth and so accurate that, with its resolution, one could read, for example, a newspaper in the Canary Islands from Helsinki.

The image of the Event Horizon Telescope has required very precise synchronization of these eight radio telescopes, which is why high-precision atomic clocks have been installed on each telescope. The signals from the telescopes are stored on a thousand specially made hard disks, which after the observations have been transported to two supercomputers for combining the signals. After this, an image of the combined signals has yet to be calculated. This was not easy either, as an interferometer like the Event Horizon Telescope does not form an image in the same way directly as a camera, for example. Since there are only a few radio telescopes in the network, it has the detection of trying to take a picture with a telescope whose mirror is intact from only a few small places. Thus, advanced algorithms are needed to form the image and researchers have had to be extremely careful to ensure the accuracy of the image.

Black hole – 10 Mind-Blowing Scientific Facts About Black Holes

Black holes are the only objects in the Universe that can trap light by sheer gravitational force. Scientists believe they are formed when the corpse of a massive star collapses in on itself, becoming so dense that it warps the fabric of space and time.

And any matter that crosses their event horizons, also known as the point of no return, spirals helplessly toward an unknown fate. Despite decades of research, these monstrous cosmological phenomena remain shrouded in mystery.

They’re still blowing the minds of scientists who study them. Here are 10 reasons why:

Black holes do not suck.

Some think that black holes are like cosmic vacuums that suck in the space around them when, in fact, black holes are like any other object in space, albeit with a very strong gravitational field.

If you replaced the Sun with a black hole of equal mass, Earth would not get sucked in – it would continue orbiting the black hole as it orbits the Sun, today.

Black holes look like they’re sucking in matter from all around, but that’s a common misconception. Companion stars shed some of their mass in the form of stellar wind, and the material in that wind then falls into the grip of its hungry neighbour, a black hole.

Einstein didn’t discover black holes.

Einstein didn’t discover the existence of black holes – though his theory of relativity does predict their formation. Instead, Karl Schwarzschild was the first to use Einstein’s revolutionary equations and show that black holes could indeed form.

He accomplished this the same year that Einstein released his theory of general relativity in 1915. From Schwarzschild’s work came a term called the Schwarzschild radius, a measurement of how small you’d have to compress any object to create a black hole.

Long before this, British polymath John Michell predicted the existence of ‘dark stars’ so massive or so compressed that they could possess gravitational pulls so strong not even light could escape; black holes didn’t get their universal name until 1967.

Black holes will spaghettify you and everything else.

Black holes have this incredible ability to literally stretch you into a long spaghetti-like strand. Appropriately, this phenomenon is called ‘spaghettification’. Look it up.

The way it works has to do with how gravity behaves over distance. Right now, your feet are closer to the centre of Earth and are therefore more strongly attracted than your head. Under extreme gravity, say, near a black hole, that difference in attraction will actually start working against you.

As your feet begin to get stretched by gravity’s pull, they will become increasingly more attracted as they inch closer to the centre of the black hole. The closer they get, the faster they move. But the top half of your body is farther away and so is not moving toward the centre as fast. The result: spaghettification!

Black hole – Black holes could spawn new universes.

It might sound crazy that black holes could spawn new universes – especially since we’re not sure other universes exist – but the theory behind this is an active field of research today.

A very simplified version of how this works is that our Universe today, when you look at the numbers, has some extremely convenient conditions that came together to create life. If you tweaked these conditions by even a miniscule amount, then we wouldn’t be here.

The singularity at the centre of black holes breaks down our standard laws of physics and could, in theory, change these conditions and spawn a new, slightly altered universe.

Black holes literally pull the space around them.

Picture space as a stretched rubber sheet with criss-crossing grid lines. When you place an object on the sheet, it sinks a little.

The more massive an object you put on the sheet the deeper it sinks. This sinking effect distorts the grid lines so they are no longer straight, but curved.

The deeper the well you make in space, the more space distorts and curves. And the deepest of wells are made by black holes. Black holes create such a deep well in space that nothing has enough energy to climb back out, not even light.

Black holes are the ultimate energy factories.

Black holes can generate energy more efficiently than our Sun.

The way this works has to do with the disk of material that orbits around a black hole. The material that is nearest to the fringe of the event horizon on the inner edge of the disk will orbit much more quickly than material at the very outer edge of the disk. This is because the gravitational pull is stronger near the event horizon.

Because the material is orbiting and moving so rapidly, it heats up to billions of degrees Fahrenheit, which has the ability to transform mass from the material into energy in a form called blackbody radiation.

To compare, nuclear fusion converts about 0.7 percent of mass into energy. The condition around a black hole converts 10 percent of mass into energy. That’s a big difference!

Scientists have even proposed that this kind of energy could be used to power black hole starships of the future.

There is a supermassive black hole at the centre of our galaxy.

Scientists believe there is be a supermassive black hole at the centre of nearly every galaxy – including our own. These black holes actually anchor galaxies, holding them together in the space.

The black hole at the centre of the Milky Way, Sagittarius A, is more than four million times more massive then our sun. Although the black hole, which is almost 30,000 light years away, is pretty dormant at the moment, scientists believe that 2 million years ago it erupted in an explosion that may have even been visible from Earth.

Black holes slow down time.

To understand why, think back on the twin experiment that is often used to explain how time and space work together in Einstein’s theory of general relativity:

One twin stays on Earth while the other one zooms out into space at the speed of light, turns around, and returns home. The twin that travelled through space is significantly younger because the faster you move, the slower time passes for you.

As you reach the event horizon, you are moving at such high speeds due to the strong gravitational force from the black hole, that time will slow down.

Black holes evaporate over time.

This surprising discovery was first predicted by Stephen Hawking in 1974. The phenomenon is called Hawking radiation, after the famous physicist.

Hawking radiation disperses a black hole’s mass into space and over time, and will actually do this until there is nothing left, essentially killing the black hole. This is why Hawking radiation is also known as black hole evaporation.

Anything can become a black hole, in theory.

The only difference between a black hole and our Sun is that the centre of a black hole is made of extremely dense material, which gives the black hole a strong gravitational field. It’s that gravitational field that can trap everything, including light, which is why we can’t see black holes.

You could theoretically turn anything into a black hole.

If you shrunk our Sun down to a size of only 3.7 miles (6 km) across, for example, then you would have compressed all of the mass in our sun down to an incredibly small space, making it extremely dense and also making a black hole. You could apply the same theory to Earth or to your own body.

But in reality, we only know of one way that can produce a black hole: the gravitational collapse of an extremely massive star that’s 20 to 30 times more massive than our Sun.

Article source: https://www.aalto.fi/fi/uutiset/mika-on-musta-aukko-miksi-sen-tutkiminen-on-tarkeaa-miten-metsahovi-otti-osaa-hankkeeseen and https://www.sciencealert.com/10-mind-blowing-scientific-facts-about-black-holes

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