Gravitational Waves are Not the Ultimate Test of General Relativity

Last year’s ground breaking gravitational wave detections generated some of the widest media coverage of a scientific discovery to date. Many articles and reports described the detection as the “ultimate” test of general relativity, the “final” test of general relativity, or confirmation of Einstein’s “last” prediction. For a theory that is 100 years old, that was developed by one of the most celebrated physicists of all time, and has survived every experiment thrown at it, you could be forgiven for thinking that general relativity is done and dusted. However, general relativity is far from facing its “ultimate” test.

Gravitational Waves — another success for General Relativity, but not the final test.

Gravitational Waves — another success for General Relativity, but not the final test.

Over the past century, general relativity has been put through its paces with some of the brightest minds on the planet developing new ways to test it. General relativity explains anomalies in the planet Mercury’s orbit around the Sun, why the universe cannot be static, and the way star-light is deflected by massive bodies like the Sun and distant galaxies. Predictions of general relativity have been tested using high-precision gyroscopes on satellites orbiting the earth and atomic clocks, by timing the orbits of distant neutron stars, and now, by the direct detection of gravitational waves. General relativity has survived every test, passing with flying colours. Even the fact that GPS functions properly is evidence of the success of general relativity.

But the nature of the scientific method means that no test that general relativity survives will be its ultimate test. Scientists will never stop testing general relativity until it fails. We know general relativity is not the final solution to a theory of everything, and we are reasonably certain that general relativity must fail at some point. While general relativity has been spectacularly successful in describing the large-scale workings of the cosmos (such as solar systems, galaxies, and the entire visible universe), it does not work on the small scale (atoms, nuclei, and fundamental particles), for which we need quantum mechanics. General relativity does not tell us what goes on inside a black hole, or at the moment of the Big Bang, because, in those situations, we are dealing with much smaller scales where quantum mechanics comes in to play. While we are very aware of general relativity’s ultimate limitations, this does not give us a very convenient starting point for developing a replacement theory. Like an engineer investigating the collapse of a bridge, if we can find the exact point of failure, we are better able to develop a solution. Because of this, many tests of general relativity today seek to determine the exact point at which general relativity breaks.

Most of the current approaches to breaking general relativity assume the devil is in the detail, and aim to make more precise measurements of previously tested phenomena. As more and more gravitational wave events are detected and studied, they will provide ever more stringent tests of the predictions made by general relativity. One research group wants to measure the precise orbits of different types of metal around the Earth, while another wants to time the fall of different types of atoms, things that general relativity says should show no difference between the different materials and atoms. The ESA’s ACES mission aims to make the most precise measurements ever of gravitational time dilation and gravitational redshift. If general relativity passes these tests, physicists will celebrate another success for a brilliant theory and for human intellect, and will then set about designing the next test. However, if, for any of these experiments, the predictions of general relativity do not match the data, physicists will celebrate the discovery of the breaking point, and the dawning of a new era in our understanding of the nature of the universe.

In order to get to space, throw yourself at the planet, and miss.

High precision space- and ground-based experiments aim to test General Relativity in minute detail

Rest assured, articles claiming “[Some experiment] is the ultimate/final test of Einstein’s greatest theory” are not yet a thing of the past.

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