Monday, May 2, 2016

Black Holes

On February 11, scientists at the Laser Interferometer Gravitational-Wave Observatory announced that they had detected gravitational waves from one of the most powerful events in the universe: the collision of two black holes. In light of this event, I thought I'd post a basic overview of black holes.

Black holes exist in that fuzzy realm on the border between theory and fact. A surprising amount of information regarding these mysterious objects has been determined through mathematical proof, and yet there are still dozens of questions to answer and apparent contradictions to explain. We don't completely understand how they work, but we know they exist.

Because of its strange nature, the idea of the black hole plays a large role in the average person's concept of the universe. Everybody knows about black holes. The problem is, most people don't understand what a black hole is. Some people imagine black whirlpools sucking everything in. Other people imagine powerful vacuum cleaners, from which nothing can escape. The truth is a little different.


The idea of a black hole began 100 years ago when Einstein published his general theory of relativity. According to this theory, gravity bends spacetime, with the result that light will be bent around massive objects. Arthur Eddington verified this prediction a few years later when he photographed stars close to the sun - he found that their apparent positions were distorted due to the sun's mass. The bending of space is what makes a black hole possible; before scientists knew that space was bent, the idea of a black hole would have gone nowhere.

The next big step in deducing black holes' existences came a year later, when Karl Schwarzschild published the solution to Einstein's equations for gravity around a spherical mass. In Schwarzschild's resulting equation, there was a distance from the object at which the solution contained a division by zero, and became undefined. Although it wasn't realized immediately, this is the Schwarzschild radius, or the radius at which a compressed mass becomes a black hole. Within this region, light heading outwards will be curved back around towards the center, never to escape. The idea of the black hole was born.

The problem? It's impossible to compress an object that far. At least, that's what the physics stated at the time. Anybody who understood physics considered black holes to be a scientific impossibility. This idea continued until Subrahmanyan Chandrasekhar worked on the problem in 1930, and calculated that there was a mass above which a dying star cannot exist in a stable state, but instead will collapse to within the Schwarzchild radius. This critical mass is now known as the Chandrasekhar limit. Interestingly, scientists initially overlooked Chandrasekhar's discovery, because it necessarily implied the existence of black holes. It wasn't until 1939 that his work was generally accepted, and black holes changed from myth to reality.

(For more information on the history of black holes, see this article from Scientific American.)

Current Views

Scientists now understand that black holes are a scientific reality. So what exactly is a black hole? A black hole is defined as a region of space in which the gravity is so intense that light cannot escape. Schwarzschild's equation shows that a region like this exists around material compressed within its Schwarzschild radius, and Chandrasekhar showed that a ball of electron degenerate material will compress below this size if it has a high enough mass. So a black hole is essentially a massive dead star, crushed to a very small size.

Gravitational equations still apply to black holes, so a non-spinning black hole acts just like an ordinary star would with the same mass. It doesn't "suck" anything in; it acts like a normal spherical mass. In fact, a black hole has the same gravitational field as the star that formed it; the only difference is that the object is smaller, making it possible to get closer to its center and experience a stronger gravitational force. Its small size allows for distances so close that nothing can escape (not even light).

This strong gravity also affects its appearance. Because a black hole bends spacetime, distant objects behind it would appear warped. If we could visually observe a black hole from close up, it would look kind of like a really weird lens that was distorting everything.

The image at the top of this post depicts a black hole with an accretion disk. The odd rings above and below the black hole are also the accretion disk - the light is bent, so we can see objects that are directly behind the black hole. Note that the dust is not being sucked in; it is only rotating around the black hole.

Will we ever see a black hole?

We already have, in a sense. When two black holes merge, they release a massive gravitational wave that passes at the speed of light through space, unhindered by the interstellar gas and dust that would block out visible light. Scientists knew this, and built LIGO, the Laser Interferometer Gravitational-Wave Observatory, which is a pair of special facilities that are designed to observe cosmic gravitational waves. Although most people would tell you that the observatory was built for the advancement of scientific knowledge, I suspect the people running it were mostly motivated by the coolness factor of detecting what Einstein thought was undetectable.

In September last year, LIGO finally measured the gravitational waves from a black hole collision (they announced their achievement in February). This was one of the biggest leaps in science that mankind has ever made, pushing our technological capability to its utter limit. Not only have we verified the existence of black holes, we've also confirmed one of the predictions of Einstein's theory of relativity in a way he himself hadn't thought possible - over a time period of a few microseconds.

Our scientific understanding of the universe is greater than it has ever been before. In a period of a hundred years, we deduced the existence of the black hole, and then produced the technology to directly observe some of its effects on the fabric of space.

What really amazes me is that science will only get better from here.

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