Ripples in space-time
Ken Tapping, 23rd February, 2016
More than a billion years ago, two black holes spiralled into one another and combined. The energy of the event was high enough to distort the fabric of space itself, radiating ripples in space-time. Last September, these waves, known as gravity waves, arrived at the Earth and were successfully detected by two independent instruments. This discovery confirms Einstein’s prediction of their existence, and gives us a powerful, new tool for studying the universe.
Nature generates waves of many kinds. Anything that is stretchy, compressible, or wants to move back to where it was when displaced can carry waves. Throw a stone in a pond and surface ripples will radiate off in all directions. A boat moving across the pond will develop a bow wave and a wake. Measuring the properties of these waves as they arrive at some distant point will tell us the nature and location of their source. The sounds we hear are pressure waves carried through the air and captured by our ears.
Scientists living in the age of Isaac Newton and after, until the beginning of the 20th Century, believed that space is just an empty void in which stars, planets and galaxies move. Then Einstein proposed a new picture of the universe that better explained many phenomena that were puzzling us. His ideas have stood the test of decades of examination and testing. Einstein described space as being a something rather than a nothing. It is a fabric that can be stretched and distorted, and can therefore carry waves – ripples in space-time that have come to be known as gravity waves.
Planets moving in their orbits make bow-waves and wakes in space-time. This takes energy, which causes the planets to gradually spiral into their star. For planets the energy loss is very small. The energy the Earth puts into making gravity waves is only about 200W - the amount used by two incandescent light bulbs. This is nothing for us to worry about. The Sun will run out of fuel before the Earth’s orbit is changes enough for us to notice.
However, two massive black holes orbiting close to each other are a different issue altogether. They lose enormous amounts of energy by radiating gravity waves, and spiral inward toward each other. As they get closer together, and move faster and faster, they produce more and more intense gravity waves, making them spiral in even faster, until they collide. Supernovae, the huge explosions when old, giant stars run out of fuel and collapse would also produce gravity waves. We believe the beginning of the universe produced gravity waves, which if we can study them, will take us back closer to the beginning than we have yet been able to reach. This alone is a powerful incentive for searching for and studying gravity waves.
Detecting gravity waves is easy in principle but hard in practice. As gravity waves pass by, and space is stretched and compressed, distant objects will move closer and further away. For two black holes colliding over a billion light years away, the effect here at the Earth is tiny. The successful detections were made using laser interferometers, which are very good at measuring minute changes in the distance things lie from us. To measure such changes the laser light has to be in a vacuum to avoid distortions by passing air. Vibrations have to be avoided and temperature variations have to be cancelled out. In the recent experiment, the two identical instruments detected the same thing at the same time; a burst of gravity waves lasting a fraction of a second as the two black holes spun into each other and then collided.
This discovery has launched a new science; gravitational wave astronomy, which will show us things we have never been able to see before, and almost certainly, things about the universe and space-time of which we currently know nothing.
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