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Event Horizon Telescope Ready to Image Milky Way's Central Black Hole

03/10/2017 2:26 AM

Scientists believe they are on the verge of obtaining the first ever picture of a black hole.

They have built an Earth-sized "virtual telescope" by linking a large array of radio receivers - from the South Pole, to Hawaii, to the Americas and Europe.

There is optimism that observations to be conducted during 5-14 April could finally deliver the long-sought prize.

In the sights of the so-called "Event Horizon Telescope" will be the monster black hole at the centre of our galaxy.

Although never seen directly, this object, catalogued as Sagittarius A*, has been determined to exist from the way it influences the orbits of nearby stars.

These race around a point in space at many thousands of km per second, suggesting the hole likely has a mass of about four million times that of the Sun.

But as colossal as that sounds, the "edge" of the black hole - the horizon inside which an immense gravity field traps all light - may be no more than 20 million km or so across.

And at a distance of 26,000 light-years from Earth, this makes Sagittarius A* a tiny pinprick on the sky.

The Event Horizon Telescope (EHT) team is nonetheless bullish.

"There's great excitement," said project leader Sheperd Doeleman from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

"We've been fashioning our virtual telescope for almost two decades now, and in April we're going to make the observations that we think have the first real chance of bringing a black hole's event horizon into focus," he told BBC News.

The EHT's trick is a technique called very long baseline array interferometry (VLBI).

This combines a network of widely spaced radio antennas to mimic a telescope aperture that can produce the resolution necessary to perceive a pinprick on the sky.

The EHT is aiming initially to get down to 50 microarcseconds. Team-members talk in analogies, describing the sharpness of vision as being the equivalent of seeing something the size of a grapefruit on the surface of the Moon.

They emphasise the still complex years of work ahead, but also trail the prospect of an imminent breakthrough.

The scientists certainly have an expectation of what they ought to see, if successful.

Simulations rooted in Einstein's equations predict a bright ring of light fringing a dark feature.

The light would be the emission coming from gas and dust accelerated to high speed and torn apart just before disappearing into the hole.

The dark feature would be the shadow the hole casts on this maelstrom.

"Now, it could be that we will see something different," Doeleman said.

"As I've said before, it's never a good idea to bet against Einstein, but if we did see something that was very different from what we expect we would have to reassess the theory of gravity.

more...

Testing general relativity

The no-hair theorem of general relativity predicts that the spacetime around a black hole can be expressed in terms of only three parameters: the black hole mass, spin, and charge. Since it is hard to see how a real astrophysical black hole could sustain a large electric charge, the no-hair theorem predicts that the black hole can be characterized by its mass and spin alone.

The strong curvature of spacetime near a black hole produces a dark shadow surrounded by a bright photon ring. The shape of this shadow is roughly circular. Detecting the shadow of a black hole and establishing that it is indeed circular would constitute an observational test of general relativity.

The diameter of the shadow is proportional to the mass of the black hole and is mostly insensitive to the value of the black hole spin. Detecting the shadow would also allow astronomers to obtain a direct estimate of the ratio of the mass of a black hole to its distance from the observer.

General relativity predicts that the shadow of a black hole should be circular (middle panel), but a black hole that violates the no-hair theorem could have a prolate (top) or oblate (bottom) shadow.

EHT website

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#1

Re: Event Horizon Telescope Ready to Image Milky Way's Central Black Hole

03/10/2017 11:12 PM

That "no-hair theorem" is quite offensive.

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#2

Re: Event Horizon Telescope Ready to Image Milky Way's Central Black Hole

03/11/2017 1:31 PM

Great post, Andrew. I have no doubt they'll see the shadow someday.

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