My favourite quote from American physicist Richard Feynman is "Physics is like sex: Sure, it may give some practical results, but that's not why we do it".
You see an apple fall from a tree. You think "Aha gravity!" Isaac Newton did not discover gravity - it's obvious that something makes objects fall to the ground.
Fifty years earlier, Johannes Kepler worked out the rules of planetary motion. The big jump Newton made was to show that whatever kept the planets in their orbits was the same as whatever pulled apples to the ground. But he had no idea what caused this force.
And 250 years later a few odd things cropped up that could not be explained using Newton's ideas. Accurate measurements found that the orbit of Mercury changes and light was shown to be a wave. But a wave needs something to travel through (a water wave without water is nonsensical). All efforts to find the "ether" that carried light through the vacuum of space failed.
Einstein linked the three dimensions of length, breadth and height with the fourth dimension of time into a coherent whole which he called spacetime. What we experience as gravity are distortions of spacetime caused by things that have mass. So, mass affects length, breadth and height and time.
His ideas explained perfectly the orbit of Mercury and the bending of light when it passes near to massive objects. The software in your GPS device allows for the high speed of the GPS satellites through spacetime making their clocks run more slowly relative to clocks on Earth. Without this fix your GPS would be inaccurate.
Unfortunately, this does not explain what "mass" actually is.
Artist s illustration of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the bursts of gamma rays that are shot out just seconds after the gravitational waves. Credit / NSF/LIGO/Sonoma State University/A. Simonnet
When an electric charge accelerates in a magnetic field it generates radio waves. This is what happens in your cellphone or your Bluetooth speaker. This effect was predicted by James Maxwell in 1867 and demonstrated to be true by Heinrich Hertz in 1887. In 1893 Oliver Heaviside thought "If this works for magnetic fields, why should it not work for gravitational fields? Will objects with mass, accelerating in a gravitational field cause gravity waves?"
In 1915 Einstein figured there should indeed be gravity waves but he reckoned they would be so weak we could never detect them.
Each of the billions of billions of stars in the visible universe is running through a life cycle that starts with a cloud of gas collapsing under its own gravity. With sufficient compression the temperature becomes high enough for the fusion of hydrogen atoms into helium to happen causing outward pressure.
When the gravity and fusion reach a balance point the star settles to a stable state until the hydrogen runs out. For relatively small stars like the Sun this might take 10 billion years. For very large stars the life span is much shorter. Perhaps as little as 10 million years.
When the hydrogen fuelling the Sun runs out (in about 5 billion years) it will collapse under gravity to a white dwarf. The density of a white dwarf is such that your family car would be compressed to about the size of a sugar cube.
Larger stars have correspondingly greater gravity and so when their fuel runs out the compression is greater. The end result of this is called a neutron star. A neutron star has the mass of about 1.5 suns in a sphere that would fit between Whanganui and Palmerston North. A sugar cube sized piece of a neutron star has the same mass as the entire human race (all 7.5 billion of them).
Part of the process of formation of a neutron star is the blowing off of a huge amount of material in what we call a super-nova. If enough of this material falls back to the neutron star the resulting object may have such a high density that a black hole is formed.
The idea of a black hole is based upon the idea that matter deforms spacetime (which we see as gravity). In a black hole the deformation is so extreme that even light (or anything else) cannot escape which means we cannot see them. This is a problem.
Their presence can be inferred from circumstantial evidence such as the way their gravity affects the path of other objects and the deviation of light beams but we cannot see them.
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Einstein suggested an answer. The motion of a planet in an orbit is accelerated motion in just the same way you accelerate towards the earth when you jump off a chair. So this motion should radiate energy as tiny ripples in spacetime. These ripples are what we call gravity waves. So how do you get a black hole to move in an orbit? Simple, get two black holes to orbit around each other.
Okay. We now have our source of gravity waves. How do we detect them? Considerthis: Measure the average depth of Lake Taupō. Now drop one drop of water into the lake, then measure the change in the average depth of the lake. This is the magnitude of the problem. Now you see Einstein's point.
Pushing laser technology to its (present) limits is the answer. If you take two light waves of exactly the same wavelength and brightness and impose one over the other, with the wave peaks matching, you get a brighter light. If the peak of one wave is imposed on the trough of the other wave you get darkness. So, moving the starting point of one of the waves will change whether you see light or dark or somewhere between. The theory is easy. The practice has made the LIGO and VIRGO gravity wave detectors the most sensitive scientific instruments ever built and earned part shares in a Nobel Physics Prize for Barry Barish and Kip Thorne working at the two detectors.
The detectors measure the changes in the path length of two laser beams in two 4km vacuum tubes to an accuracy equivalent to one tenth of the diameter of a proton. Before you can see the gravity waves in the measurements you need to filter out traffic noise from the road several kilometres away, the farmer cutting hay on the other side of the valley, sea waves from the beach and every little earthquake around the world.
After 30 years of development the astrophysicists decided that, although the machines were already running, September 15, 2015 would be the official date that the search would begin. On September 14, 2015 both of the LIGO detectors picked up the same signal. The scientists were so amazed they took six months to do exhaustive analysis of the signal which had lasted all of half a second before announcing they had seen a gravity wave from a pair of orbiting black holes as their orbits collapsed and then merged into a larger black hole. The signal came from 1.3 billion light years away. When this event happened, multicellular life was just coming into being on Earth. Its signature chose the day before the official opening to arrive on Earth. The energy released by the merging of the black holes is equivalent to converting the mass of three suns into pure energy in less than a second. Compare this to the mass conversion in a nuclear bomb which is about the mass of a 10c coin. This was, by some margin, the most energetic event ever detected in the universe.
The merging of neutron stars is the only known event that is sufficiently energetic to make gold by the fusion of lighter elements. Your gold jewellery is actually debris from the collision of two neutron stars.
We are seeing here a scientific beginning equivalent to Galileo looking at Jupiter through his home-made telescope. Developments in this field will allow us to "see" objects which have previously been invisible - black holes.
