by Joshua Sokol
from QuantaMagazine Website
something orbiting just outside
the innermost possible orbit of
the supermassive black hole
Milky Way's center.
orbiting just outside the supermassive
black hole at the galaxy's center.
Their motions have given us the closest look
at that violent environment.
Their measurements suggest that this stuff - perhaps made of blobs of plasma - is spinning not far from the innermost orbit allowed by the laws of physics.
If so, this affords
astronomers their closest look yet at the funhouse-mirrored
space-time that surrounds
a black hole. And in time, additional
observations will indicate whether those known laws of physics truly
describe what's going on at the edge of where space-time breaks
From Earth, this black hole is a dense, tiny thing in the constellation Sagittarius, only as big on the sky as a strawberry seed in Los Angeles when viewed from New York.
But interstellar gas glows as it swirls into the black hole, marking the dark heart of the galaxy with a single, faint point of infrared light in astronomical images.
Astronomers call it
Sagittarius A* (pronounced "A-star").
Now, though, a team based at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, has measured not just this speck's brightness but its position with staggering precision.
When it flares, it also moves clockwise on the sky, tracing out a tiny circle, they find.
But one particular interpretation stands out, the team argues.
This wobbling likely comes from "hot spots," glowing blobs of magnetically heated plasma orbiting right above the black hole's gaping maw at almost one-third the speed of light.
As these hot spots circle, the black hole's immense gravitational forces twist space-time itself into something like a lens, one that flashes beacons of light across the cosmos like a galactic searchlight beam.
The idea, first proposed in 2005 by Avery Broderick, now at the Perimeter Institute of Theoretical Physics (PITP) and the University of Waterloo in Canada, and Avi Loeb of Harvard University, would explain why the black hole appears to flare.
If these rotating flares are due to hot spots in the way that Broderick and Loeb imagined, additional flares will help reveal the black hole's "spin," a measure of its rotation.
And it could also provide a new way to poke and prod Einstein's theory of general relativity in the flexed space-time at the mouth of a black hole.
Light from the four telescopes at
the Very Large Telescope array in Cerro Paranal, Chile,
can be combined to create, in effect,
a single, enormous telescope.
Earlier this summer, Genzel's team published a measurement of how general relativity is affecting the light of a star now passing close to the black hole; a similar paper by Ghez's team is now under review.
But since last year, the European team has had a unique tool - the power of four giant telescopes working together in a project called GRAVITY.
On a typical night, the European Southern Observatory's four 8-meter telescopes on Cerro Paranal, overlooking Chile's Atacama desert, loll in different directions on the sky.
GRAVITY pulls them
together using a technique called
interferometry that combines
observations from multiple telescopes to produce artificial images
that only a preposterously huge real telescope could make.
So on July 22, when Sagittarius A* flared, the light collected by each scope traveled through a Rube Goldberg-like setup of mirrors and fiber-optic cables that traced out a path with a total length that varies no more than 1/1,000th the width of a hair, said Frank Eisenhauer, a physicist at Max Planck in Garching and the leader of GRAVITY.
Then, inside a 3-ton freezing toolbox of optical tech, these light waves mixed together, their peaks and troughs combining and canceling to produce position measurements with impossible crispness.
Even after all that, GRAVITY still didn't have high enough resolution to make movies of the three flares it saw - the one on July 22 and two others.
But its measurements of
the faint speck wiggling on the sky promises to narrow down the
multiple options of what's causing Sagittarius A* to flicker in the
Or they could be hot
clumps out in the wide
Frisbee of gas draining into the black hole,
or other possible disk structures like spiral arms. In all these
cases, the flaring and dimming of light would come from the material
itself glowing hot, then cooling off.
They would form close to the black hole, not unlike what happens in a solar flare. Above the surface of our sun, a briar patch of magnetic fields snag together, spurting out flares of heated plasma when the fields snap into new shapes.
Something similar could
happen in the gas right around a black hole, which also hosts
strong, tangled magnetic fields.
And as that beam swept across Earth, we would measure the black hole flickering.
If jets caused the black hole's flickering, that motion would be linear, as blobs traveled outward and cooled, Eisenhauer said.
If clumps in the disk around the black hole were responsible, the motion wouldn't go in any particular consistent direction.
But the circular motion supports orbiting hot spots, the team argues.
GRAVITY also found that the light emitted during a flare shifts in polarization, following the same rough timescale as the apparent orbital motion. That fits, too.
The light emitted by a
hot spot would be polarized. As the spot traveled through warped
space-time, its polarization would twist throughout its orbit.
Now, though, theorists
hope the hot spots may be able to shine a harsh interrogation-room
lamp on Einstein's theory of gravity itself.
Reading the Horizon
As you approach, popular accounts say, you have one last chance to turn back - the event horizon that marks the black hole's edge. But perhaps a better place to rethink your approach would be earlier, at what astrophysicists call the innermost stable circular orbit (ISCO).
The hot spots around the
black hole at the center of the galaxy seem to orbit just a little
outside this boundary.
At some distance, going faster will only hasten your fall.
To Loeb, a light source flying around this fateful rim is a gift from Mother Nature.
A black hole's mass and its rotation speed determine where the ISCO is, plus how long a hot spot will orbit at a given radius. Beyond mass and spin, general relativity holds that nothing else determines how an object orbits an astrophysical black hole.
These two values should
be the only distinguishing characteristics.
The next horizon - quite literally - should come from the Event Horizon Telescope, or EHT, a separate effort now straining to resolve the space-time right around the Milky Way's central black hole.
The EHT team is currently
crunching through their data, with hopes to publish at some point in
2019, they say.
As Earth rotates, these
observatories sweep across space, collecting even more information.
But the hot-spotlike wobbles that GRAVITY found provide a new opportunity.
Provided the wobbles also occur in radio wavelengths, the EHT could also track them as tiny shifts in position.
And if they feel confident that something orbited the black hole during an EHT observation - say, after EHT and GRAVITY looked at the same flare on the same night - the EHT team could break their long exposure into sequential frames, using mathematical models to produce an actual movie of a circling hot spot.