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 realise 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 predicting 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 by the notable English scientist Arthur Eddington. 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.
Quantum mechanics, developed at around the same time as Einstein’s general relativity, is known to describe nature successfully at the sub-atomic level. It has been proposed that a grand unified theory will involve the unification of general relativity with quantum mechanics.
‘Classical’ general relativity, which is the theory developed by Einstein in 1915, is a theory where gravitational fields are continuous entities in nature. They also represent the geometric properties of 4-dimensional spacetime. In quantum mechanics, fields are discontinuous and are defined by ‘quanta’. So, there is no analogue in conventional quantum mechanics for the gravitational field, even though the other three fundamental forces have now been described as ‘quantum fields’ after considerable work in the 1960-1980s. Quantum mechanics is incompatible with general relativity because in quantum field theory, forces act locally through the exchange of well-defined quanta.
- For something theoretically the size of the Earth to become a black hole, it would need to shrink to the size of a large toy marble.
- The Sun, when it uses all of its nuclear fuels, will expand and most likely form a planetary nebula. It is too small to become a supernova and then a neutron star or black hole.
- The first pulsars were found in the UK in the 1960s by PhD student Jocelyn Bell Burnell and were known at the time as LGM – Little Green Men.
Although such a marriage of these two great scientific fields is potentially some way off, we can expect that this theory of quantum gravity may produce somewhat different predictions than 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 of gravity that will be carried out using pulsars and the SKA will provide some of the most stringent tests ever made.
Strong-field tests of gravity
The SKA will investigate the nature of gravity and challenge the theory of general relativity. Pulsars, the collapsed spinning cores of dead stars, will be monitored to search for gravitational waves – ripples in the fabric of space-time. The SKA will also use pulsars to test general relativity in extreme conditions, for example, close to black holes.
A pulsar is a highly magnetised rotating neutron star – a gigantic nucleus consisting mainly of neutrons with a mass of 1.4 times the mass of the Sun but only as wide as a big city – about 20 km in 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 – the SKA will be able to detect this signal. Because a pulsar has a huge amount of mass concentrated in a very small volume, it acts like a massive flywheel in space and rotates very steadily. The observed pulses act as the ticks of a natural clock which can be as precise as the best atomic clocks on Earth. It is this accuracy, and the SKA’s ability to detect even the most subtle variations in this, which will hopefully enable this breakthrough in science.
About 50 years after the discovery of pulsars marked the beginning of a new era in fundamental physics, pulsars observed with the SKA have the potential to transform our understanding of gravitational physics.
How will the SKA detect gravity, when it is a radio telescope?
The SKA will be able to indirectly measure the effects of gravity on objects in the Universe. It will scan for pulsars near to black holes and look at the gravitational influences of these two objects, looking for the minuscule perturbations in the fabric of space-time itself.
These perturbations, known as gravitational waves are the subject of extensive research by both ground and space-based telescopes. They have been detected for the past 30 years from observations of pulsars in the radio frequency, but up to the early part of the 21st century, larger scale gravitational waves, permeating through all of space, possibly caused by the collision of two black holes, were still only theoretical.
The SKA’s sensitive instruments will attempt to look at these huge scale gravitational events in an attempt to refine and test Einstein’s theories to the absolute limit.
Pulsars and black holes
In order to test general relativity further, we need to find a pulsar orbiting something really massive, like a black hole. These systems are expected to be very rare, but with the unique sensitivity of the SKA we expect to 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 gravity conditions, but we can also study the black hole properties at the same time. General relativity makes clear predictions about the nature of black holes. Observations with the SKA can measure these properties and hence provide the ultimate test for general relativity.
Finally, we will know whether Einstein was correct in his description of the nature of black holes and space-time in the presence of strong gravitational fields.
How do pulsars relate to black holes?
Pulsars are the fast spinning remnants of stars that have exploded, leaving behind just the gravitationally collapsed central region. They are super dense objects comprised of neutrons, a teaspoonful of which would weigh trillions of kilos.
Black Holes take this concept even further; they are the result of a core collapse of larger stars after they have exploded in a supernova, to sizes and densities so extreme that not even light can escape their gravitational forces. It is believed that the centre of our, and many other galaxies contain super-massive black holes.
Pulsars and gravitational waves
Einstein’s theory also predicts the existence of waves propagating in space-time as a result of the motion or collapse of massive objects. The existence of gravitational waves was confirmed about 30 years ago 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 expect to find 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 would be expected to produce gravitational waves that still propagate through our neighbourhood.
The accurate pulsar clocks, together termed a pulsar timing array (PTA), in combination with the SKA telescope, will provide us with a unique opportunity to find these elusive gravitational waves at wavelengths that no other planned instruments will be able to detect.
The SKA will significantly enhance current efforts using PTAs, ensembles of millisecond pulsars that act as multiple arms of a cosmic gravitational wave detector, by detecting many more millisecond pulsars than is currently possible.
The SKA will allow the pulsars to be timed to very high precision (~< 100 ns), making them very sensitive to the small space-time perturbations of gravitational waves. This ‘device’, with the SKA at its heart, will be sensitive to gravitational waves at frequencies of nHz thereby complementing the much higher frequencies accessible to Advanced LIGO (~100 Hz) and the proposed and possible European Space Agency spacecraft LISA (~mHz).
The SKA will provide crucial answers to questions about the existence, nature and composition of the gravitational wave background predicted by Einstein.
Further information [external links]
Strong-Field Tests of Gravity Using Pulsars and Black Holes – M. Kramer, D. C. Backer, J. M. Cordes, T. J. W. Lazio, B. W. Stappers, S. Johnston – Science with the Square Kilometre Array, 2004.
Click here for animations showing a pulsar in action.