Twisting space-time

Twenty or so years ago, the radio telescope at Parkes, Australia, found something unusual: a white dwarf star and a neutron star in close orbit around one another, taking only five hours to complete each orbit. 

In addition, the white dwarf is spinning very rapidly - once every 20 seconds. This has provided a unique opportunity to check out another strange aspect of Einstein's General Theory of Relativity: "frame-dragging."

Once upon a time there were two stars, one more massive than the other. As the more massive one aged, it expanded, and its neighbour made use of its gravity to capture some of its material. This meant the star losing material would not end its life by exploding, which would otherwise have happened, and it ended up as a white dwarf, which is what the sun will become when it runs out of fuel. 

However, increasing the mass of the second star meant it did end its life by exploding, with such an intensity that the atoms in its core were crushed into neutrons. The white dwarf is similar in size to the Earth, but the neutron star is only a few kilometres across. When the greedy star exploded, its material was thrown off into space and the white dwarf got some of it back, fortunately not enough to make it explode. Instead, just as when a ballerina spins faster when she pulls in her arms, the star spun faster, until it was spinning once every 20 seconds, almost fast enough to fly apart. This unusual "dynamic duo" of a neutron star and a rapidly-spinning white dwarf star provides a very good test bed for measuring frame-dragging.

All the objects in the universe are immersed in a multidimensional, stretchable and twistable medium called "space-time." When a planet or any other object moves through it or rotates, it drags space-time along with it. For small objects, like the Earth, this process is almost negligible. Precise measurements made using the Gravity Probe B reveal the Earth's frame-dragging. The effect is tiny; it would take about 36 million years to drag a full circle. However, it gets larger for more massive objects, like rapidly spinning white dwarf stars.

When a star shrinks to become a neutron star, it spins faster, and the magnetic flux in the original star is concentrated in this much smaller body. The result is powerful interactions in the magnetic fields linking the star to its surroundings. These drive a beam of strong radio emissions, which sweeps round like the light beam from a lighthouse.

Every time that beam sweeps in our direction, we receive a pulse of radio waves, which is why we often refer to neutron stars as pulsars. Since they are locked to the rotation of a very massive body, the spacing between the pulses is very stable, providing a very accurate clock. The neutron star in the dynamic duo is rotating 150 times a minute.

A rotating neutron star alone in space would give us a pulsar with a pulse timing that varies only slowly with time. When orbiting another object we will get changes in timing as it approaches and recedes, and when the rapidly-rotating white dwarf star gets anywhere near the line of sight, its frame-dragging will affect the timing too, in a measurable way, proving another test of Einstein's ideas.

For centuries Isaac Newton's idea of a uniform space and time ruled. Even today we can use Newton's ideas to navigate the solar system and understand much of what we see out there in space. Unfortunately, they break down in extreme circumstances, such as where there are very large masses, extreme speeds or temperatures, or very long periods of time. Einstein's ideas work better there. However, in what seems to be an increasingly bizarre universe, we cannot conclude they will continue to account for everything. Can they be improved? That's why the testing and exploration continue.

• Venus shines brightly in the southwest after sunset and Mars rises in the early hours, Jupiter a bit later, with Saturn very low in the dawn glow. 
• The moon will reach last quarter on the 15th. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.