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• From far away, stars are tiny points of light. But up close, stars are massive, seething,

• fiery balls of burning gas. This fierce display does not last forever. Eventually, the nuclear

• fusion which powers the star will burn all its fuel. Gravity then collapses the remaining

• matter together. For very large stars, what happens next is a display of extremes. First,

• the star explodes in a supernova, scattering much of its matter throughout the universe.

• For a brief moment, the dying star outshines its entire galaxy. But once the light fades

• and darkness returns, the remaining matter forms an object so dense that anything that

• gets too close will completely disappear from view. THIS is a black hole

• The idea of a black hole originated hundreds of years ago. In 1687, Isaac Newton published

• his landmark work known as The Principia. Here he detailed his laws of motion and the

• universal law of gravitation. Using a thought experiment involving a cannon placed on a

• very tall mountain, Newton derived the notion of escape velocity. This is the launch speed

• required to break free from the pull of gravity. In 1783, the English clergyman John Michell

• found that a star 500 times larger than our sun would have an escape velocity greater

• than the speed of light. He called these giant objectsdark starsbecause they could

• not emit starlight. This idea lay dormant for more than a century.

• Then, in the early 20th century, Albert Einstein developed two theories of relativity that

• changed our view of space and time: the special theory and the general theory. The special

• theory is famous for the equation E=mc2. The general theory painted a new and different

• picture of gravity. According to the general theory of relativity, matter and energy bend

• space and time. Because of this, objects which travel near a large mass will appear to move

• along a curved path because of the bending in spacetime. We call this effect gravity.

• One consequence of this idea is that light is also affected by gravity. After all, if

• spacetime is curved, then everything must follow along a curved path, including light.

• Einstein published his general theory of relativity in 1915. And while Newton's theory of gravity

• could be expressed using a simple formula, Einstein's theory required a set of complex

• equations known as thefield equations.” Only a few months after Einstein's publication,

• the German scientist Karl Schwarzschild found a surprising solution. According to the field

• equations, an extremely dense ball of matter creates a spherical region in space where

• nothing can escape, not even light. A curious result, but did such things actually exist?

• At first, the idea of a black sphere in space from which nothing could escape was considered

• purely a mathematical result, but one which would not really happen. However, as the decades

• passed, our understanding of the lifecycle of stars grew. It was observed that some dying

• stars became pulsars, another exotic object predicted by theory. This suggested that dark

• stars could actually be real as well. These strange spheres were namedblack holes,”

• and scientists began the hard work of finding them, describing them and understanding

• how they are created.

• But how do you find an object in space that is completely black? Luckily, because black

• holes have a large mass, they also have a large gravitational field. So while we may

• not be able to SEE a black hole, we can observe its gravity pulling on its neighbors. With

• this in mind, astronomers looked for a place where a visible star and a black hole were

• in close proximity to one another. One such place is binary stars.

• A binary star is a system of two stars orbiting one another. We can spot them in several ways.

• You can look for stars that change position back and forth ever-so-slightly. Alternatively,

• if you observe a binary star from the side, the brightness will change when one star passes

• behind the other. So it's possible that somewhere in space, there's a binary star

• consisting of a black hole and a visible star. In fact, such binary systems have been observed!

• Astronomers have found stars orbiting an invisible companion. From the size of the visible star

• and its orbit, astronomers calculated the mass of its invisible neighbor. It fit the

• profile of a black hole.

• Since we can't see a black hole, is there a way to find its size? From Einstein's

• field equations, we know that given the mass of a black hole, we can determine the size

• of the sphere that separates the region of no escape from the rest of space. The radius

• of this sphere is called the Schwarzschild radius in honor of Karl Schwarzschild. The

• surface of the sphere is called the event horizon. If anything crosses the event horizon,

• it's gone foreverhidden from the rest of the universe.

• This means, once you know the MASS of a black hole, you can compute its SIZE using a simple

• formula. And it's actually quite easy to measure the mass of a black hole. Just take

• a standard issue space probe and shoot it into orbit around the black hole. Just like

• any other system of orbiting bodieslike the Earth orbiting the Sun, or the Moon orbiting

• the Earththe size and period of the orbit will tell you the mass of the black hole.

• If you don't have a space probe handy, then compute the mass and orbit of a companion

• star and use that to find the Schwarzschild radius.

• Black holes come in many sizes. If it was made from a dying star, then we call it a

• stellar massblack hole, because its mass is in the same range as stars. But we

• can go bigger - much bigger. And to do so, we are going to visit the center of a galaxy.

• Galaxies can contain billions and billions of stars, all orbiting a central point. Scientists

• now believe that in the center of most galaxies lives a black hole which we call a “supermassive

• black hole,” because of its tremendous mass. The size can vary from hundreds of thousands

• to even billions of solar masses. For example, at the center of our own Milky Way galaxy

• is a supermassive black hole with a mass 4 million times that of our sun.

• Black holes have another property we can measure - their spin. Just like the planets, stars

• rotate. And different stars spin at different speeds. Imagine we can adjust the size of

• this star but keep the mass constant. If we increase the radius, the spinning slows down

• If we decrease the size, the spinning speeds up. But while the rotational speed can vary,

• the angular momentum never changes - it remains constant. Even if the star were to collapse

• into a black hole, it would still have angular momentum. We could measure this by firing

• two probes into opposite orbits close to the black hole. Because of their angular momentum,

• black holes create a spinning current in spacetime. The probe orbiting along with the current

• will travel faster than the one fighting it, and by measuring the difference in their orbital

• periods we can compute the black hole's angular momentum.

• This spacetime current is so extreme it creates a region called the ergosphere where nothing,

• including light, can overcome it. Inside the ergosphere, nothing can stand still. Everything

• inside this region is dragged along by the spinning spacetime. The event horizon fits

• inside the ergosphere, and they touch at the poles. So in one sense, black holes are like

• whirlpools of spacetime. Once inside the ergosphere, you are caught by the current. And after you

• cross the event horizon, you disappear.

• One final property of black holes we can measure is electric charge. While most of the matter

• we encounter in our day-to-day lives is uncharged, a black hole may have a net positive or negative

• charge. This can easily be measured by seeing how hard the black hole pulls on a magnet.

• But charged black holes are not expected to exist in nature. This is because the universe

• is teeming with charged particles, so a charged black hole would simply attract oppositely

• charged particles until the overall charge is neutralized.

• There are 3 fundamental properties of a black hole we can measure - mass, angular momentum,

• and electric charge. It is believed that once you know these three values, you can completely

• describe the black hole. This result is humorously known as theno hair theorem,” since

• other than these 3 properties, black holes have no distinguishing characteristics. It's

• not a blonde, brunette, or a redhead.

• We now have a good idea of a black hole from the outside, but what does it look like on

• the inside? Unfortunately we can't send a probe inside to take a look. Once any instrument

• crosses the event horizon, it's gone. But! Don't forget we have Einstein's field equations.

• If these correctly describe spacetime outside the black hole, then we can use them

• to predict what's going on inside as well.

• To solve the field equations, scientists considered two separate cases: rotating black holes,

• and non-rotating black holes. Non-rotating black holes are simpler and were the first

• to be understood. In this case, all the matter inside the black hole collapses to a single

• point in the center, called a singularity. At this point, spacetime is infinitely warped.

• Rotating black holes have a different interior. In this case, the mass inside a black hole

• will continue to collapse, but because of the rotation it will coalesce into a circle,

• not a point. This circle has no thickness and is called a ring singularity.

• Black hole research continues to this day. Scientists are actively investigating the

• possibility that black holes appeared right after the big bang, and the idea that black

• holes can create bridges called wormholes connecting distant points of our universe.

• We know a great deal about black holes, but there are many mysteries still to be solved.

• It's a little known fact that all YouTube videos are stored in a special fabric called

• playtime. When you watch a video, it sends ripples of energy throughout playtime. And

• when you subscribe to a channel, it creates a teeny, tiny black hole. So if you like

• Black Holes, then you know what to do...

From far away, stars are tiny points of light. But up close, stars are massive, seething,

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# What is a Black Hole? -- Black Holes Explained

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joey joey posted on 2021/04/11
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