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Category Archives: 14. Black holes: the end of space and time

The European Research Council (ERC) has awarded 14 million euros (around $19 million) to a team of European astrophysicists to construct the first accurate image of a black hole. The team will test the predictions of current theories of gravity, including Einstein’s general theory of relativity. The funding is provided in the form of a synergy grant, the largest and most competitive type of grant of the ERC. This is the first time an astrophysics proposal has been awarded such a grant.

The team, led by investigators at the University of Nijmegen, the Max Planck Institute for Radio Astronomy, and Goethe University in Frankfurt, hopes to measure the shadow cast by the event horizon of the black hole in the center of the Milky Way, find new radio pulsars near this black hole, and combine these measurements with advanced computer simulations of the behavior of light and matter around black holes as predicted by theories of gravity. They will combine several telescopes around the globe to peer into the heart of our own galaxy, which hosts a mysterious radio source called Sagittarius A* which is considered to be the central supermassive black hole. (See p. 383 and Section 15.5, p. 391.)

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Credit and © M. Moscibrodzka & H. Falcke, Radboud-Universität Nijmegen

Black holes are notoriously elusive with a gravitational field so large that even light cannot escape their grip. The team plans to make an image of the event horizon – the border around a black hole which light can enter, but not leave.  The scientists want to peer into the heart of our own galaxy, which hosts a mysterious radio source called Sagittarius A*. The object is known to have a mass of around 4 million times the mass of the Sun and is considered to be the central supermassive black hole of the Milky Way.

As gaseous matter is attracted towards the event horizon by the black hole’s gravitational attraction, strong radio emission is produced before the gas disappears. The event horizon should then cast a dark shadow on that bright emission. Given the huge distance to the center of the Milky Way, the shadow is equivalent to the size of an apple on the Moon seen from Earth. By combining high-frequency radio telescopes around the world, in a technique called very long baseline interferometry (VLBI), even such a tiny feature is, in principle, detectable.

In addition, the group wants to use the same radio telescopes to find and measure pulsars around the very same black hole. Pulsars are rapidly spinning neutron stars, which can be used as highly accurate natural clocks in space. While radio pulsars are found throughout the Milky Way, surprisingly none had been found in the center of the Milky Way until very recently.

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A recent article in the New York Times highlights the ongoing debate among theoretical cosmologists about what happens when you enter a black hole, the so-called “Firewall Paradox”. At stake are some of the basic tenets of modern science, in particular Einstein’s general theory of relativity, the theory of gravity, on which our understanding of the Universe is based.

The traditional view holds that an astronaut falling into a black hole would not be physically aware of crossing the point of no return, known as the event horizon. (Of course, he or she will inevitably be crushed by the monstrous gravitational forces…)

In 1974, British cosmologist Stephen Hawking theorized, using general relativity and quantum theory (the laws which govern behaviors on the smallest, subatomic scales) that black holes, could leak particles and radiation back into space. This in itself generated a 30-year debate, and a famous wager with Caltech physicist John Preskill, on whether such escaping particles would carry some quantum information with them or not. In 2004, Hawking conceded that such information could survive.

A group of researchers at the University of California, Santa Barbara studying how information escapes a black hole’s clutches have presented the “Firewall Paradox”: that having information flowing out of a black hole is incompatible with having an otherwise smooth space-time at its boundary, i.e. the traditional event horizon. Instead there would be a discontinuity in the vacuum that would manifest itself as energetic particles — a literal “firewall” — lurking just inside the black hole.

If the firewall argument proves to be correct, one of three ideas that lie at the heart of modern physics, must be wrong. Either information can be lost in a black hole after all; Einstein’s principle of equivalence is wrong; or quantum field theory, which describes how elementary particles and forces interact, is wrong and needs fixing.

To find out more about possible solutions to this problem, read Dennis Overbye’s full New York Times article here.