Key science projects

> Cradle of Life
> Probing the Dark Ages
> The origin and evolution of Cosmic Magnetism
> Strong field tests of gravity using pulsars and black holes
> Galaxy evolution, cosmology and dark energy

The phenomenal success of Isaac Newton's theory of gravity in explaining the motions of bodies on the earth and in our solar system meant that the theory went unchallenged for about 300 years. Only slowly did astronomers realize that certain deviations from the predicted motion of the planet Mercury could not be understood with Newton's theory.

It took the genius of Albert Einstein to replace the traditional idea of the gravitational interaction between two bodies by the effects of curved ``space-time''. Suddenly, the orbit of Mercury could be explained, and Einstein's simultaneous prediction of the deflection of star-light at the Sun was confirmed spectacularly during the solar eclipse in 1919. Ever since, scientists have thought of new experiments to test Einstein's ``General Theory of Relativity'', as he called his theory of gravity, to ever better precision. So far, general relativity has passed all these tests with flying colours and not a single deviation from the theoretical prediction has been found in the experiments. Was Einstein right? Is his theory the last word in our understanding of nature's most fundamental force?

Quantum mechanics, developed at the same time as Einstein's general relativity, is known to describe nature successfully at the sub-atomic level. It has been proposed that the ultimate theory of the Universe will involve the unification of general relativity with quantum mechanics. Although such a marriage is some way off, we can expect that this theory of "quantum gravity" produces somewhat different predictions that general relativity. But how can we measure such deviations? How can we test when - if at all - general relativity fails? We have to assume that so far we have not probed gravitational fields that are strong enough to show deviations from general relativity's prediction.

Tests in the solar system are made under ``weak-field'' conditions. ``Strong-field'' tests have only been possible using pulsars, which provide some of the most stringent tests ever made. A pulsar is a highly magnetized rotating neutron star -- a gigantic nucleus consisting mainly of neutrons with a mass of 1.4 times the mass of the Sun and the size of a big city (20 km diameter).

A pulsar emits a radio beam along its magnetic axis. As the star rotates it acts like a cosmic lighthouse emitting an apparently pulsed signal when the beam is pointed in our direction. Since a pulsar concentrates a huge amount of mass in a very small volume, it represents a massive flywheel in space which rotates very steadily. The observed pulses act therefore as the ticks of a natural clock which can be as precise as the best atomic clocks on Earth.

As general relativity describes the effects of gravitational fields on clocks, we have a superb laboratory when a pulsar moves in the gravitational potential of a massive binary companion. Observations of pulsars orbiting another neutron star or pulsars have thus provided us with most of the very sensitive tests of general relativity mentioned above -- which the theory has so far passed
without problems.

In order to test general relativity further, it seems that we have to try harder. What we need to find is a pulsar around a really massive companion -- a black hole! Although these are expected to be very rare objects, with the unique sensitivity of the SKA we will almost certainly be able to find them in the disk of our Galaxy. We may also find pulsars in orbit around the super-massive black hole in the centre of the Galaxy.

By studying the regularity of the received pulsar ticks we can not only test general relativity under very strong-field conditions, we can also study the black hole properties at the same time. General relativity makes clear predictions about the nature of a black hole. Observations with the SKA can measure these properties and hence provide the ultimate test for general relativity. Finally we will know whether Einstein was right in his description of nature of black holes and space-time in the presence of strong gravitational fields.

Einstein's theory also predicts the existence of waves propagating in space-time by the motion or collapse of massive objects. The existence of gravitational waves was indeed confirmed about 30 years ago - again with radio pulsars. While this discovery, awarded with the Nobel prize in 1993, showed the existence of gravitational waves due to the motion of stellar-sized objects, we can expect gravitational waves on a much greater, cosmic scale.

Both the birth of the Universe in the Big Bang and the collision of super-massive black holes in the centre of galaxies much later should produce gravitational waves that still propagate through our neighbourhood. Our accurate pulsar clocks provide us again with the unique opportunity to find these elusive gravitational waves. No other instruments currently planned will be able to detect them. With the SKA, we have a unique opportunity to discover these waves and test yet another bold prediction of Einstein's theory.

Pulsar observations with the SKA address some of the most fundamental questions in our understanding of nature. Gravity is all around us and eventually this force will decide the fate of our Universe. So far, Albert Einstein was always right, but we would be surprised if we would not finally find a flaw in his theory. Rather than worrying about such a possibility, we should relish it.

If we find deviations Einstein's predictions, we will ultimately understand much more about the Universe. The example of Mercury's orbits which required the genius of Einstein's theory demonstrates that very clearly. Whatever the answer will be, Einstein's achievement cannot be overestimated. We would surely make him proud when pulsar observations with the SKA will finally provide the answer to the burning question ``Was Einstein right?''-- even if he were wrong at the end.

Read " Strong field tests of gravity using pulsars and black holes " for scientists.
Animations:
> Multibeam
> Pulsars

©copyright ska - contact webmaster: www@astron.nl