Published online 15 February 2005 | Nature | doi:10.1038/news050214-5

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Black holes bend light the 'wrong' way

Refraction effect may be distorting astronomers' results.

The galaxy Centaurus A has a supermassive black hole at its heart – but could its gravity be fooling astronomers?The galaxy Centaurus A has a supermassive black hole at its heart – but could its gravity be fooling astronomers?© NASA/JPL-Caltech

Astronomers could be misinterpreting their observations of distant stars, suggest mathematicians.

Starlight may be bent in odd directions when it passes close to a rotating black hole, the researchers say, unexpectedly shifting its source's apparent position in the sky. The cause is a recently discovered phenomenon called negative refraction, which physicists are still struggling to understand.

Astronomers already adjust their observations to account for the fact that light is bent by massive objects such as black holes, an effect called gravitational lensing. But Akhlesh Lakhtakia, a mathematician at Pennsylvania State University in University Park, has studied what happens when a black hole rotates. In this case, light is bent in the direction opposite to that predicted by conventional theory.

"Astronomical measurements, particularly those relating to black holes and other massive stellar bodies, need careful reinterpretation," says Tom Mackay of the University of Edinburgh, UK, who worked with Lakhtakia on the analysis, published online in Physics Letters A1.

Material fact

Negative refraction is new to astronomy, but has been causing a stir in materials science in recent years. When light crosses a boundary, it is bent in a characteristic way; this is why an oar dipped in water looks as though the submerged part is angled towards the surface.

But in 2001, US researchers showed that certain artificial materials bend light in the opposite direction2. If water had this property, the submerged oar would appear to angle away from the surface.

The revelation prompted a flurry of research, most of which has focused on understanding and developing negative refracting materials. "But this is exactly the same phenomena," Mackay points out.

Last year, Mackay and Lakhtakia demonstrated that negative refraction could occur in a vacuum, provided that the gravitational field in the region had the right properties. Now, they have identified something that meets these requirements: a rotating black hole. Very large rotating stars would have the same effect, adds Mackay.

Strong field

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This might force astronomers to rethink some of their observations. "The deflection of light could be significant," says Mackay. In theory, starlight could even turn through a 90° angle, apparently putting the star in a completely different part of the sky. "And the further away the object is, the more likely it is that these effects are interfering with observations," adds Mackay.

However, some researchers question how much influence the effect will have in practice. Matthias Bartelmann, a theoretical astrophysicist at the University of Heidelberg in Germany, describes Mackay and Lakhtakia's paper as very interesting. "But I'm in doubt as to the astronomical relevance," he says. Bartelmann points out that the effect will be limited to small regions of space, as it can only occur in regions where the gravitational field is extremely strong.

The effect could find other uses, however. Theoretical astronomers are currently debating whether the cosmological constant, a key number in the equations that describe the evolution and growth of the universe, is positive or negative. Mackay says that negative refraction can only take place if the constant is positive, so experimental verification of such refraction could help to settle the debate. 

  • References

    1. Lakhtakia A., Mackay T. G. & Setiawan S. Phys. Lett. A, 336. 89 - 96 (2005). | Article | ChemPort |
    2. Shelby R. A., Smith D. R. & Schultz S. Science, 292. 77 - 79 (2001). | Article | PubMed | ISI | ChemPort |