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Astronomers Have Caught a Star Literally Dragging Space-Time Around With It

One of the predictions of Einstein’s general theory of relativity is that any rotating body drags the fabric of space-time in its vicinity with it.

This is known as “frame-drag”. In everyday life, dragging frames is both undetectable and insignificant, since the effect is so ridiculously tiny. Detection of chassis drag caused by the entire Earth’s rotation requires satellites such as the US $ 750 million gravity probe B and the detection of angular changes in gyroscopes equivalent to a single degree every 100,000 years or so. Fortunately for us, the Universe contains many natural gravitational laboratories where physicists can observe Einstein’s predictions at work in exquisite detail. Our team’s research, published today in Science, reveals evidence of frame-dragging on a much more obvious scale, using a radio telescope and a unique pair of compact stars that follow one another at breakneck speed. The movement of these stars would have left astronomers perplexed in Newton’s time, as they clearly moved in a deformed space-time and required Einstein’s theory of general relativity to explain their trajectories.

An illustration of frame dragging. (Mark Myers/OzGrav ARC Centre of Excellence)

An illustration of frame dragging. (Mark Myers/OzGrav ARC Centre of Excellence)

General relativity is the foundation of modern gravitational theory. Explains the precise movement of stars, planets and satellites and even the flow of time. One of his lesser known predictions is that rotating bodies drag space-time with them. The more an object turns and the more massive it is, the more powerful the resistance is. One type of object for which this is very relevant is called a white dwarf. These are the remaining remnants of the dead stars that were once the mass of our Sun, but have since run out of their hydrogen fuel. What remains is Earth-like in size but hundreds of thousands of times more massive. White dwarfs can also spin very quickly, rotating every minute or two, instead of every 24 hours as Earth does. The dragging of the plot caused by such a white dwarf would be about 100 million times more powerful than Earth’s. It’s all right, but we can’t fly over a white dwarf and launch satellites around it. Fortunately, however, nature is kind to astronomers and has its own way of making us observe through orbits around stars called pulsars.

Twenty years ago, CSIRO’s Parkes radio telescope discovered a unique stellar pair consisting of a white dwarf (about the size of the Earth but about 300,000 times heavier) and a radio pulsar (about the size of a city but 400,000 times heavier). Compared to whites, pulsars are part of another league. They are not made of conventional atoms, but of neutrons put together tightly, making them incredibly dense. In addition, the pulsar in our studio rotates 150 times per minute. This means that, 150 times per minute, a “beacon beam” of radio waves emitted by this pulsar passes our observation point here on Earth. We use it to map the path of the pulsar as it orbits around the white dwarf, timing when its pulse reaches our telescope and knowing the speed of light. This method revealed that the two stars orbit one another in less than 5 hours. This pair, officially called PSR J1141-6545, is an ideal gravitational laboratory. Since 2001 we have walked three times a year on Parkes to map the orbit of this system, which shows a multitude of Einsteinian gravitational effects. Mapping people is not for the impatient, but our measurements are ridiculously accurate. If the PSR J1141-6545 is required to be a different number of quadrillions of kilometers (one quadrillion is one million billion), we know that the pulsar rotates 2.5387230404 times per second and that its orbit is plunging into space. This means that the plane of its orbit is not fixed, but instead is slowly rotated.


How did this system form?

When pairs of stars are born, the most massive dies first, often creating a white dwarf. Before the second star dies, he transfers the matter to his white dwarf companion. A disk forms as this material falls towards the white dwarf and over the course of tens of thousands of years it revives the white dwarf until it rotates every few minutes.

Illustration of a white dwarf being spun-up by the transfer of matter from its companion. (ARC Centre of Excellence for Gravitational Wave Discovery)

A white dwarf being spun-up by the transfer of matter from its companion. (ARC Centre of Excellence for Gravitational Wave Discovery)

In rare cases like this, the second star can then explode into a supernova, leaving a pulsar behind. The rapidly rotating white dwarf drags space-time with it, tilting the orbital plane of the pulsar as it is dragged. This inclination is what we have observed through our patient mapping of the pulsar orbit. Einstein himself thought that many of his predictions about space and time would never be observable. But the last few years have seen a revolution in extreme astrophysics, including the discovery of gravitational waves and the imaging of a black hole shadow with a worldwide network of telescopes. These discoveries were made by billion dollar structures. Fortunately, there is still a role in exploring general relativity for 50-year-old radio telescopes such as Parkes and for patient campaigns of generations of graduate students. Matthew Bailes, ARC Laureate Fellow, Swinburne University of Technology., Swinburne University of Technology and Vivek Venkatraman Krishnan, Scientific staff, Max Planck Institute.

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