Einstein was relatively correct, new test shows
Why was ‘one of the classic experiments in the history of physics’ ignored by the media, asks CORMAC O'RAIFEARTAIGH
EARLIER THIS YEAR, a team of US researchers announced that a Nasa satellite experiment provided dramatic support for Einstein’s general theory of relativity. “Einstein survives!” declared Francis Everitt, principal investigator on the experiment known as Gravity Probe B, at a press conference at Nasa headquarters in Washington DC. Curiously, this important test of a famous scientific theory received little publicity, possibly because the experiment was complex. So what exactly was measured?
Most readers will have heard of Einstein’s theory of relativity, which predicts that nothing can travel faster than the speed of light in vacuum, and that bodies travelling at speeds close to this will exhibit strange effects. The theory is most famous for its postulate that mass, the amount of matter in a body, is simply a form of energy (E = mc2). Underpinning these predictions is the extraordinary idea that space and time are not absolute, but can be affected by the motion of bodies. Now known as special relativity, Einstein’s theory was a great surprise when it was first postulated in 1905, but its predictions have been verified time and again in measurements of the tiniest particles of matter moving at extremely high speeds. (Special relativity is a favourite target of science sceptics, but relativistic effects are in fact observed daily in modern particle accelerators such as the Large Hadron Collider at Cern in Geneva).
Less well-known is that Einstein soon extended his theory of relativity. The extended version, now known as the general theory of relativity, predicts that space and time are also affected by matter. A massive object like the Sun is predicted to cause a warping of space and time in its vicinity, much as a heavy object sitting on a trampoline distorts the trampoline surface. According to Einstein, the force of gravity is the motion of objects following a path in warped spacetime. (“Matter tells spacetime how to curve while spacetime tells matter how to move,” as the relativists say.)
For many years, general relativity was considered a highly mathem- atical theory of little relevance to the world about us. However, nowadays it underpins a great deal of modern physics, from our understanding of black holes to the Big Bang model of the origin of the universe. Yet direct evidence in support of the theory remains difficult to obtain. One reason is that the force of gravity is only measurable in the case of extremely large bodies: another is that the predictions of the theory deviate only slightly from those of classical physics in most circumstances. The best-known pieces of evidence in support of general relativity are the bending of distant light by stars, and a correction factor necess- ary for the synchronisation of the earthbound and satellite clocks used in GPS navigation.
The Gravity Probe B experiment was designed to test directly two spectacular predictions of general relativity; a warping of space caused by the mass of the Earth (the geodesic effect) and a twisting of space caused by its rotational motion (known as the frame dragging). In the experiment, four purpose- built gyroscopes were kept in suspension in a satellite 650km above the Earth, and minute changes in their direction of spin monitored. Free from disturbance, any stretching or twisting of space could be measured as a change in direction of gyroscope spin.
The satellite was launched on a Delta II rocket on April 20th, 2004, and the flight lasted until 2005. After years of analysis, the results announced in May 2011 agreed exactly with the predictions of general relativity, within the margins of experimental error.
It is interesting to note that this successful experiment was dogged by controversy for many years. First mooted by Everitt and colleagues in the 1960s, many other physicists felt that the experiment was too grandiose in design and it suffered many delays and funding shortages in consequence. Most of the pricetag ($760 million, €555.8m) was eventually covered by Nasa, but substantial funding was provided by a private donor.
In addition, Everitt’s team were to some extent scooped in 2004, when a team of scientists at the University of Maryland measured the frame dragging effect by monitoring the orbits of media satellites. However, Gravity Probe B is unique in that it simultaneously measured two distinct relativistic effects. “This is an epic result,” said Clifford Will, a well-known relativity expert at Washington University. “One day, it will be written up in textbooks as one of the classic experiments in the history of physics.”
Dr Cormac O’Raifeartaigh lectures in physics at Waterford Institute of Technology and writes the science blog Antimatter