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Earth-sized telescope to photograph a black hole

Astrophysicists Heino Falcke, Luciano Rezzolla and Michael Kramer. © Dick van Aalst, Radboud University Nijmegen
Astrophysicists Heino Falcke, Luciano Rezzolla and Michael Kramer. © Dick van Aalst, Radboud University Nijmegen

European astronomers are planning to use a telescope as big as the earth to take the first ever photograph of a black hole – a task akin to photographing a mouse on the surface of the moon.

The proposed BlackHoleCam project, funded by the European Research Council, could provide new insights into one of the most enigmatic structures in the universe, and put Einstein’s ideas to the test, as black holes are a consequence of his general theory of relativity.

Many black holes are thought to be the remains of large stars that imploded when they ran out of fuel. However, others are so massive that no one really knows how they came to form.

Based on the laws of motion devised by German astronomer Johannes Kepler in the 17th Century, scientists believe there must be one 4 million times heavier than the sun lying unseen in the centre of our galaxy.

This theory gained strength when a mysterious radio source named Sagittarius A* was spotted at the heart of the Milky Way. According to Professor Heino Falcke, one of the project’s three principal investigators, based at Radboud University in the Netherlands, these radio waves are produced by material being sucked up by the supermassive black hole.

Wandering star

‘If Sagittarius A* does house a black hole, the pull of gravity around it would be intense enough to tear apart wandering stars and clouds of gas, grinding them into a cauldron of atoms at temperatures above a billion degrees,’ he said. ‘Because hot matter emits radiation, this accreting material would let out a distinguishable emission of radio waves before being sucked into oblivion.’

‘If this disk is inclined with respect to our line of sight, a black hole in its centre would absorb the radio waves emitted behind it, drawing its silhouette against the background of radiation.’

Professor Michael Kramer, Max Planck Institute, Germany

The BlackHoleCam project, which is expected to start on 1 October, intends to confirm the existence of the black hole by mapping the shadow it casts against this bright backdrop of radio waves.

‘The accreting material presumably orbits Sagittarius A* in a disk-like formation,’ said Professor Michael Kramer, from the Max Planck Institute in Bonn, another of the project’s principal investigators. ‘If this disk is inclined with respect to our line of sight, a black hole in its centre would absorb the radio waves emitted behind it, drawing its silhouette against the background of radiation.’

But because black holes are extremely compact, even the supermassive behemoth at the centre of our galaxy would cast a shadow no wider than the orbit of Mercury around the sun.

The BlackHoleCam team is therefore pushing the resolution of today’s radio telescopes to new limits, using a technique known as Very Long Baseline Interferometry. The objective is to run an international network of telescopes in unison so that they act as a single Earth-sized radio dish.

Observatories from around the world are supporting the project under the banner of the Event Horizon Telescope. Combining their measurements will require synchronisation to within a trillionth of a second and data processing algorithms which can account for perturbations as discrete as the drift of continental plates.

However, the results could challenge some of the fundamental assumptions of modern physics. ‘Maybe the most exciting result would be that Einstein was wrong,’ said Professor Luciano Rezzolla, the third principle investigator, who works at the Institute for Theoretical Physics in Frankfurt am Main. 

Very Long Baseline Interferometry

Very Long Baseline Interferometry (VLBI) is a means of using data from several radio telescopes in different locations on earth. By networking the collected data from astronomic radio sources such as quasars, researchers can create a higher resolution image than any single array can capture.

In a similar way to the Global Positioning System (GPS), the radio telescopes record the precise time they receive their data. By comparing the differences in the recorded times by different arrays, cosmic bodies can be mapped in three dimensions.

VLBI is currently used for tracking spacecraft and mapping distant sources of radio waves, but also, because the measurements are made at different points on earth, readings can be used to measure precise tectonic movements and the rotation of our planet.

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