Gravity’s Engines by Caleb Scharf - Meet the most mysterious and fantastical phenomena in the universe.
Black holes really are like something out of a fairy tale. They are the most mysterious and fantastical phenomena in the universe. Black holes are the densest objects in existence. They have a strong influence on the cosmic environment that we find ourselves in.
The story began in 1783, in the idyllic West Yorkshire. The Reverend John Mitchell was a brilliant man and a renowned scientist. Before running the little Michael’s Parish Church in West Yorkshire, he studied at Cambridge University. For some years he taught Hebrew, Greek, mathematics, and geology. He helped establish seismology as a science and together with Henry Cavendish he developed an apparatus to measure the earth's gravitational field.
While in Yorkshire, he became interested in cosmology, particularly with the idea that objects like stars could imprint some information about themselves on the light that came to us from them. It was a revolutionary idea. If a star has gravity and if light, - the way people thought about light at the time, based on Newton’s theory that light consists of small material particles - slowed down by the effect of gravity, then big stars ought to slow light down.
He deduced that by measuring the reduction in the speed of light that was being emitted by stars, it would be possible to calculate the mass of the star. Not quite right, but it was a clever idea. But then, he asked himself, what will happen if the star was so enormous that could slow light altogether to a stop? We won’t be able to actually see it. He called this star, ‘the dark star’.
We had to wait more than one hundred years for someone to pay attention to John’s Mitchell’s idea. More specific until 1915. What happened then was Einstein and his general theory of relativity. Einstein realized that what we thought as gravity wasn’t quite right. Gravity is a consequence of what mass does to space around it. Mass distorts, it stretches space and time down toward itself.
It was a beautiful and revolutionary theory and immediately scientists started applying it. One of these scientists was the physicist Karl Schwarzschild. While serving in the imperial German army, during World War I, Schwarzschild calculated the radius of the event horizon surrounding a non-rotating black hole. From his calculations became apparent that an object can create a distortion so extreme, that an event horizon would be formed around itself, a place where the distortion of space is so extreme that light cannot escape. Schwarzschild contracted a fatal illness and died in the Russian front, but he had already sent his calculations to Einstein, who, despite what the equations said, thought that this was an absurd and inconceivable idea. It was also an accurate idea.
And so, John’s Mitchell’s dark stars transformed into black holes which are not exactly black, or to be more precise, the surroundings of a black hole are not black. Matter does not go into the black hole quietly. Just before it reaches the event horizon around a black hole, matter gets torn apart and gets accelerated to close to speed of light. Sometimes, it collides with something else, releasing energy. This energy, which can be in the form of light or clouds of subatomic particles, is spewed back to the universe.
Black holes are the most efficient way in the universe to convert matter into energy. It is even better than nuclear fusion. This raises a few interesting questions. Where’s all this energy produced by black holes is going? Does is it doing anything?
Scientists have learned a great deal about black holes in the past few decades. This knowledge has revolutionized our understanding not only about black holes themselves but also about the role they have played in shaping the universe the way we see it today. We could think black holes as the universe regulators. Without the black holes the galaxies would grow to be enormous things, so probably black holes stunted their growth, kind of keeping things in line. The presence and behavior of black holes in the universe asserts Caleb Scharf, could very well be connected to the origins of life.
A truly fascinating, intelligent and well-written book.
Feature image: Animated simulation of gravitational lensing caused by a black hole going past a background galaxy. A secondary image of the galaxy can be seen within the black hole Einstein ring on the opposite direction of that of the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole. The surface brightness of the two images remains constant, but their angular size varies, hence producing an amplification of the galaxy luminosity as seen from a distant observer. The maximum amplification occurs when the background galaxy (or in the present case a bright part of it) is exactly behind the black hole.
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