Stiff challenge to spacetime

EINSTEIN'S THEORY of relativity tells us that gravity bends space. Now a physicist at the University of Portsmouth, U.K., has worked out that magnetic fields may smooth bent space, says a report in Physical Review Letters, casting doubt on the growth spurt that is proposed to have built the early Universe.

Magnetic fields can stiffen space, says Christos Tsagas, just as embedded metal wires can stiffen rubber. The lines of the magnetic field the invisible lines of force that are revealed by iron filings around a bar magnet push back against the bending. It is surprising that no one has deduced this fundamental fact before, given how central relativity is to the modern view of time and space. Most physicists have previously assumed that magnetism would be too weak to deform space-time.

Einstein's theory treats time as a fourth dimension that accompanies the three spatial ones. It presents space-time not as emptiness, but as a kind of fabric upon which the Universe is painted. In this theory intense gravitational fields can deform space and time.

The Sun's gravitational field distorts space-time very slightly enough to bend a light beam through a small angle as it flashes past. Some very dense astrophysical objects, such as neutron stars, cause larger deformations. Black holes leave such a dent in space-time that light, once inside the hole, can never escape.

Space is full of magnetic fields. The Earth and the Sun have magnetic fields, as do neutron stars and even entire galaxies. Until now, though, no one has merged relativity with magnetism to find out the effect that these fields might have on the shape of space. The researcher believes that magnetic fields are woven into space-time itself, becoming "an inseparable part of the space-time fabric. So distorting space-time has an effect on interactions with the magnetic field. They add that magnetic field lines don't like to be bent. When you bring together the north poles of two magnets, they repel one another because the field lines get squashed up. Likewise, if space is distorted by gravity, magnetic field lines try to straighten it out again. The researcher suggests that this could have implications for the way in which the Universe expanded just after the Big Bang. Many cosmologists favour the idea that very early on, when space was curved, the Universe grew fast.

If space was permeated by strong magnetic fields, however, such rapid growth might not have been possible. Magnetic fields could also alter the character of gravitational waves ripples in space- time that are predicted by Einstein's theory which can be triggered by very energetic processes in space such as collisions involving neutron stars.

If a gravitational wave passes through a magnetic region of space, Tsagas suggests, it may be eliminated by the magnetic field's flattening of space-time curvature. This could hinder attempts to detect gravitational waves on Earth using instruments that are currently being built.

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