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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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