It is well known that a theory is only correct until it is disproved. Albert Einstein's general theory of relativity is no exception, even if the predicate Einstein always seems to have stood for scientific quality. A system of two neutron stars now allows the researchers to carry out another test - in several disciplines.
Whether a stone falls to the ground, the planets revolve around the sun or stars come together in galaxies - the cause is always the same: gravity. Isaac Newton recognized this and attempted to formulate gravity. He was quite successful, but also far from perfect. For he viewed gravity as a force acting in a rigid backdrop of space and time between masses. About 200 years after him, Albert Einstein also tried his luck by breaking new ground with his general theory of relativity.
In his imagination, he linked space and time into a four-dimensional structure called space-time, which now actively participated in what was happening. Galaxies, stars and other matter in the universe should cause a curvature in it - comparable to marbles lying on a rubber membrane and denting it under their weight. The heavier the mass, the greater the curvature. Conversely, in his depiction, space determines the movement of matter. With this, gravity loses its character as a force and is now a property that follows from the geometry of space.
Thanks to the revolutionary view of the general theory of relativity, many physical processes in the universe could finally be described more precisely than was the case with Newton's equations. On the other hand, the theory also produced previously unknown phenomena. For example, light from a star passing close to the sun should be deflected by its gravity. This effect could actually be observed during a solar eclipse in 1919 - the first proof of Einstein's theory.
But light can not only be deflected, it can even be delayed or shifted to longer wavelengths if it were influenced by gravity. A number of exotic things are also prophesied by General Relativity, such as gravitational waves. These periodic warps of spacetime are said to arise when masses experience accelerated motion. One can imagine the movement of such a wave as a stretching and compression of the space through which it passes. So far, however, nobody has been able to prove it directly.
Because all of these effects have one major disadvantage: they are so unbelievably small that they only become measurable and verifiable with enormous masses. For this reason, the interest of researchers, such as Michael Kramer Jodrell Bank Observatory, is focused on very extreme events and objects in the cosmos. Over a period of about three years, he and his team studied a rapidly rotating binary system of neutron stars – the densest objects in the universe after black holes.
In addition to the enormous mass, the actual special feature of the system PSR J0737-3039, which is 2000 light years away, is that the neutron stars are so-called pulsars. Similar to lighthouses, they emit highly focused radio waves that astronomers detect as a brief signal as their beams sweep across the Earth. Here, the radiation pulses usually appear so regularly that pulsars are often referred to as cosmic clocks.
Any wrong going due to the enormous gravitational pull that the two stars exert on each other should therefore catch the eye of the astronomers. Especially when, as in the case of PSR J0737-3039, the attention of three of the world's largest radio telescopes is drawn to the system.
If Einstein was right, the radio pulses from one pulsar should be delayed by the so-called Shapiro effect when they pass close to the other. The travel time of the electromagnetic radiation is extended by the curvature of space-time in the area of the massive object - so the idea.
In fact, there was a minimal delay - around ninety millionths of a second. The variations measured in the arrival times of the radio pulses agreed exactly with the predictions of general relativity. With a measurement uncertainty of just 0.05 percent, the scientists are providing the most accurate test that has been carried out in such strong gravitational fields to date.
But other aspects of the general theory of relativity could also be checked using the pulsars. For example, by observing the orbits on which the pulsars orbit each other within about 2.4 hours at speeds of one million kilometers per hour. Kramer and colleagues were able to show that the stars move exactly as Einstein predicted.
And they noticed something else: the distance between the two pulsars decreased by about seven millimeters every day. The resulting loss of rotational energy could be explained by gravitational waves, which, according to Einstein, should be emitted by such a system. Now separated by a distance of about a million kilometers, the neutron stars could one day merge as a result.