One of the predictions in Einstein's general theory of relativity is that any rotating body shifts the very fabric of space-time around itself. This phenomenon is known as 'frame dragging'.
In everyday life, dragging and dropping frames is not essential as the effect is incredibly small. Detecting this effect, caused by the entire rotation of the Earth, requires satellites such as the $ 750 million 'Gravity Probe B', which has detected angular changes in gyroscopes equivalent to one degree every 100,000 years or so.
Luckily for us, there are many natural gravity laboratories in the universe where physicists can observe Einstein's predictions in all their glory.
Curvature of space – time. (Mark Myers / OzGrav ARC Center of Excellence)
A study by the team of scientists, published in the journal Science, reveals evidence of dragging and dropping frames on a much more prominent scale using a radio telescope and a unique pair of compact stars orbiting each other at breakneck speeds.
The movement of these stars could have puzzled astronomers in Newton's time, since they clearly move in curved spacetime, and Einstein's general theory of relativity is required to explain their trajectories.
General relativity is the foundation of the modern theory of gravity. This explains the precise movement of stars, planets and satellites, and even the passage of time. One of her lesser-known predictions is that rotating bodies drag space-time along with them. The faster the object rotates and the more massive it is, the more noticeable the shift in space-time.
One type of object is a white dwarf. These are the remnants of dead stars that were once several times the mass of our Sun, but have exhausted their hydrogen fuel.
What is left is similar in size to Earth, but hundreds of thousands of times more massive. White dwarfs can also rotate very quickly, completing a full revolution every minute or two, rather than 24 hours like Earth.
The dragging caused by such a white dwarf would be about 100 million times more powerful than Earth's.
That's all well and good, but we cannot fly to a white dwarf and launch satellites around it. Fortunately, nature is kind to astronomers and has its own way of allowing us to observe them through orbiting stars called pulsars.
Twenty years ago, the CSIRO Parkes radio telescope discovered a unique stellar pair consisting of a white dwarf (the size of Earth, but about 300,000 times heavier) and a radio pulsar (the size of a small city, but 400,000 times heavier than Earth).
Compared to white dwarfs, pulsars are generally on a different level. They are not made of ordinary atoms, but of neutrons pressed together, making them incredibly dense. Moreover, the pulsar rotates 150 times per minute.
This means that 150 times a minute a 'beacon beam' of radio waves emitted by this pulsar sweeps past our vantage point here on Earth. We can use this to plot the path of the pulsar as it revolves around the white dwarf, based on the time when its pulse reaches our telescope and knowing the speed of light. This method showed that two stars orbit each other in less than 5 hours.
This pair, officially named PSR J1141-6545, is an ideal gravity lab. Since 2001, scientists have traveled to CSIRO Parkes several times a year to map the orbit of this system, which exhibits many of Einstein's gravitational effects.
Even though PSR J1141-6545 is several hundred quadrillion kilometers away (quadrillion – million billion), we know the pulsar rotates 2.5387230404 times per second and that its orbit is in balance.
This means that the plane of its orbit is not fixed, but rotates slowly.
How did this system come about?
When pairs of stars arise, the most massive one dies first, often creating a white dwarf. Before the second star dies, it transfers matter to its companion.
A white dwarf that rotates while absorbing matter from its companion. (ARC Center of Excellence for Gravitational Wave Discovery)
A disk forms when this material falls towards a white dwarf, and it accelerates the white dwarf for tens of thousands of years.
On rare occasions like this, a second star can explode into a supernova, leaving behind a pulsar. The rapidly spinning white dwarf drags space-time with it, forcing the pulsar's orbital plane to tilt. This tilt is what we observed in our mapping of the pulsar's orbit.
Einstein himself thought that many of his predictions about space and time would never be discovered. But in the past few years, there has been a revolution in extreme astrophysics, including the discovery of gravitational waves and images of a black hole using a worldwide network of telescopes.
Matthew Bales, ARC Research Fellow, Swinburne University of Technology, Max Planck Institute Research Fellow.
This article was published by The Conversation.
Sources: Photo: Mark Myers / OzGrav ARC Center of Excellence / Swinburne University of Technology
