Science News - October 9, 2010

in stable orbit. By revealing those orbits, the Event Horizon Telescope can tell what the black hole’s spin, or angular momentum, is. With zero spin, the closest stable orbit should have a period of about half an hour — while if the black hole has its maximum permitted spin, hot spots could race around it in as few as four minutes.

Many other black holes are nearer than Sgr A*. But all are leftovers of collapsed, individual stars and are far too tiny— perhaps 30 kilometers across — for even the new telescope to see. Far beyond the Milky Way, however, a monstrous black hole seems ripe for inspection. It is in the heart of the M87 galaxy, a blob of stars more than 50 million light-years away—about 2,000 times farther than Sgr A*. M87’s core ejects a powerful beam of matter and radiation, a jet that extends for thousands of light-years. It can be seen going in only one direction, like a rocket that seems to have shoved the entire black hole itself slightly away from dead center. The spectacle makes it a natural laboratory for studying the full panoply of black hole physics. A primary hope is to get a look at how some of the material and energy approaching its equator turns 90 degrees and jets out of the system’s pole at near light speed.

Astronomers in the 1990s calculated from the speeds of gas clouds near M87’s core that it has a mass of about 3 billion suns, 750 times that of Sgr A*. That was already enough to make it among the most massive black holes known. Then last year Karl Gebhardt of the University

Einstein to the test Some scientists are already angling for time on the new instrument. Among them are astrophysicist Dimitrios Psaltis and graduate student Tim Johannsen, both of the University of Arizona in Tucson. They want to watch photons — particles of light — that go into orbit around black holes and then eventually spray away into space, scattering off material falling toward the event horizon. The exact shape of the ring of light created around the black hole’s shadow should allow a check on one of the odder implications of Einstein’s general theory of relativity: the

Meeting expectations

the shape of the photon ring around a black hole allows a check on general relativity. the rings (red) at left and middle both fit with theory, though they belong to black holes with different spins. the photon ring at right does not fit theoretical predictions.

5

y’ (M)

0

y’ (M)

0

- 5

0

x’ (M)

5
a=0.0
=0.0

- 5

of Texas at Austin and a German colleague took into account the effects of unseen dark matter surrounding the galaxy. The team reported in June 2009 in Pasadena at a meeting of the American Astronomical Society that the mass of M87’s core is more likely to be a whopping 6. 4 billion suns, well over 1,000 times that of Sgr A*.

The event horizon diameter of the black hole in the middle of M87 would be correspondingly larger, too — perhaps twice Pluto’s farthest distance from the sun. Knots of material orbiting Sgr A* are likely to circle it every hour or less. By contrast, visible changes around gigantic M87’s black hole will probably take days to weeks to transpire, permitting more leisurely, detailed study.

no-hair theorem. And the ring’s appearance may tell science that once again Einstein got it right — or not.

The no-hair theorem states that a black hole can be entirely described in the outside universe, no matter what has gone into it, by its mass and the accumulated angular momentum, or spin, of all it has absorbed. ( Technically, it may also have electric charge, but physicists see no way for large black holes to accumulate significant net charge from galactic gas and dust.) “No hair” means nothing else, whether material or force field, sticks out beyond the event horizon. And spin and mass imply a specific distortion of space and time — in turn defining what the radius of the light ring around the black hole should be and how the ring’s appearance will be distorted to outside view.

“At a black hole, the fields are like nothing anywhere else. It is the breaking point for physics as we know it.” 

“General
relativity has
passed all tests
before with
flying colors.
But only in weak
fields, like near
the sun.”
DiMitrios Psaltis

If the ring looks exactly as Einstein’s theory says it should — almost perfectly circular when one corrects for the warping of dimensions near a black hole, and influenced only by the mass and the spin of the black hole — then general relativity will have continued its string of triumphs. If not, says Psaltis, “we will have exciting things to think about.”

5

Asked if he really wants to disprove Einstein, he paused. “General relativity has passed all tests before with flying colors. But only in weak fields, like near the sun. At some level, everyone believes Einstein must not be correct. His theory says that inside the black hole, everything collapses to nothing, to zero. Other physical theory,” he said, referring to quantum mechanics, “says this cannot be correct. Something is clearly wrong. Something must give way.