LISA Pathfinder in the context of great physics experiments
In the sixteenth century, the Italian physicist Galileo Galilei began his investigation of falling objects. By rolling spheres down inclined plains he showed that the action of gravity is to constantly accelerate falling bodies. He then showed that objects of different mass and composition are accelerated by gravity in exactly the same way.
He is said to have performed the experiment to prove this by dropping differently-sized objects off the leaning tower of Pisa. The famous location is probably not true, however, and was an embellishment of Galileo's student Vincenzo Viviani, who included the detail in a biography of his master. But in 1971, the experiment was performed in a spectacular setting.
American astronaut Dave Scott took a hammer and a feather to the Moon during the Apollo 15 mission. In the absence of air pressure, both fell to the floor at exactly the same time. "How about that," said Scott to the camera, "Mr Galileo was correct."
In 1687, British polymath Isaac Newton produced a mathematical masterwork when he showed that the gravitational force generated between two objects depended upon the combination of the two masses and their distance apart. If the distance between them doubled, the force dropped to a quarter of its original value – a behaviour known as an inverse square law.
One hundred and eleven years later, British physicist Henry Cavendish calculated the density of the Earth using an incredibly precise set of scales, known as a torsion balance. It worked by measuring the force of gravity between two small masses, then comparing this to the weight of one of those masses, which gave the gravitation force between it and the Earth. The torsion balance was a large piece of apparatus, spanning almost two metres in width. It had to be placed in a large crate, in a shed, to stop the measurement being ruined by air currents. Cavendish found that Earth’s average density was 5400 kg/m3 (5.4 times that of water), a remarkably accurate result – the accepted value today is 5500 kg/m3 – even by today's standards.
In the 19th century, physicists recast Newton's work as an equation that required a mathematical constant to work. Known as Big G, the constant quantifies the intrinsic strength of gravity. It can only be measured, rather than calculated theoretically. It is set during the Big Bang by means we do not yet understand.
Using Cavendish's density of the Earth, they calculated the value of this constant to be 6.74 × 10−11 m3 kg−1 s−2. The accuracy to which we know Big G hasn't changed much ever since. Modern measurements are within one percent of this first value because it is incredibly difficult to measure in the Earth's overwhelming gravitational field.
In the early 20th century, British astrophysicist Arthur Eddington made his own great gravitational observation. He was on the African Island of Príncipe, in the midst of a total solar eclipse, to test Albert Einstein's General Theory of Relativity.
According to the German-born physicist, the gravitational field of the Sun would bend light rays as they passed by twice the value predicted by Newton's theory. During the eclipse, Eddington photographed nearby stars and compared their positions to night-time shots when the Sun was nowhere near.
He found changes in the positions of the stars as predicted by Einstein's theory. It was a stunning vindication of the esoteric theory that described gravity as an invisible landscape of hills and valleys.
The technology being tested in LISA Pathfinder will lead to new great experiments in gravity as modern researchers look for ways to extend Einstein's theory so that it can be sed to understand the moment of the Big Bang and the internal workings of black holes.
