Cosmic Space

Black hole’s shadow boosts the case for relativity

The first-ever picture of a black hole is the gift that keeps on giving — in the form of new insights into the dynamics behind the mysterious phenomenon and new evidence that Albert Einstein was right.

The validity of Einstein’s theory of general relativity has been proven time and time again over the course of the past century. But physicists keep coming up with new ideas for tweaking the theory’s equations in unorthodox ways.

To figure out how much leeway there could be for variations on Einstein’s theme, researchers took a closer look at the supermassive black hole at the center of the galaxy M87.

M87’s black hole, which lies about 55 million light-years from Earth, was featured in a history-making close-up last year, produced by a radio astronomy collaboration known as the Event Horizon Telescope. The achievement is likely to win the EHT collaboration a Nobel Prize as soon as next week.

The team behind the relativity-checking research, published this week in Physical Review Letters, measured the size of the black hole’s shadow — that is, the dark central region from which light rays can’t escape, due to the gravitational pull of a singularity that’s 6.5 billion times as massive as our sun.

The predicted size of the shadow could vary, depending on which theory of gravity you go with. But in M87’s case, the size matched up precisely with Einstein’s theory.

“Using the gauge we developed, we showed that the measured size of the black hole shadow in M87 tightens the wiggle room for modifications to Einstein’s theory of general relativity by almost a factor of 500, compared to previous tests in the solar system,” the University of Arizona’s Feryal Özel, a senior member of the EHT collaboration, said in a news release.

“Many ways to modify general relativity fail at this new and tighter black hole shadow test,” Özel said.

The Event Horizon Telescope’s findings add to a bonanza of black hole data from the Laser Interferometer Gravitational-wave Observatory, or LIGO, and Europe’s VIRGO detector.

“Together with gravitational-wave observations, this marks the beginning of a new era in black hole astrophysics,” said lead study author Dimitrios Psaltis, a University of Arizona astronomer who recently finished his stint as the EHT collaboration’s project scientist.

And the EHT isn’t stopping with last year’s image. Just last month, the collaboration unveiled a “movie” that shows a wobbling pattern of emissions from the surroundings of M87’s black hole. The analysis of black hole dynamics over time, published in The Astrophysical Journal, was created by feeding more than a decade’s worth of observations into a computer model.

Eight observatories around the world contributed to the initial round of observations for the Event Horizon Telescope project. For the EHT’s next campaign in 2021, there’ll be three more observatories on the case, in Arizona, Greenland and France.

The added capacity should result in higher-fidelity images — not only of M87’s black hole, but also of Sagittarius A*, the supermassive black hole at the center of our own Milky Way galaxy.

Up-close views of black holes could well shine a light on another prediction made by general relativity, known as the no-hair theorem. This theorem states that the characteristics of black holes are completely determined by their mass, spin and electrical charge.

If the theorem is correct, all black holes with the same values for those three attributes would be identical to each other. Any other distinguishing characteristics for black holes and their history — their “hair,” metaphorically speaking — would disappear forever inside the black hole’s event horizon.

In a theoretical paper that was published months after the death of British physicist Stephen Hawking, he and three co-authors argued that black holes might be surrounded by a distinctive kind of “soft hair” that’s left behind as they evolve. This would have deep implications for general relativity, and for a long-running, almost metaphysical debate over what happens when something falls into a black hole.

Was Einstein right, or was Hawking? Thanks to gravitational-wave detectors and the Event Horizon Telescope, we could well find out.

Flash interactive: Putting Einstein to the test

Relativity rules star near our galaxy’s black hole

A 26-year-long observational campaign provides clear evidence of the effect that general relativity has on the motion of a star known as S2 as it boomerangs around the supermassive black hole at the center of our Milky Way galaxy.

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LIGO witnesses another black hole crash

Image: Gravitational waves
An artist’s conception shows gravitational waves emanating like ripples in space time as two black holes approach each other in their orbits. (Credit: T. Pyle / LIGO)

t looks as if gravitational-wave watchers are in for a bumpy, beautiful ride. Scientists using the Laser Interferometer Gravitational-Wave Observatory, or LIGO, have confirmed the detection of another merger involving two faraway black holes.

The observations, which were made last Christmas and reported today in a paper published by Physical Review Letters, support the idea that LIGO could open up a whole new branch of astronomy focusing on gravitational disturbances and black holes.

“It is a promising start to mapping the populations of black holes in our universe,” Gabriela Gonzalez, a Louisiana State University astrophysicist who serves as the spokesperson for the LIGO Scientific Collaboration, said in a news release.

She and her colleagues say this smash-up was smaller than the first black-hole merger, which was observed in September and reported by the LIGO team in February. That clash involved black holes that were 29 and 36 times as massive as the sun. This one brought together black holes that were eight and 14 times the sun’s mass.

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LISA Pathfinder blazes trail to test relativity

Image: LISA Pathfinder liftoff
The LISA Pathfinder probe lifts off from French Guiana. (ESA photo)

The LISA Pathfinder probe is heading for a vantage point a million miles from Earth to help look for gravitational waves and add a missing piece to the evidence for general relativity.

The European Space Agency said an Italian-built Vega rocket sent the spacecraft into low Earth orbit from ESA’s spaceport on the South American coast, at Kourou in French Guiana, at 04:04 GMT today (8:04 p.m. PT Wednesday).

Over the next two weeks, LISA Pathfinder will go through a series of maneuvers to set a course for L1, a gravitational balance point between Earth and the sun. The spacecraft is due to reach L1 in mid-February and begin its scientific mission in March.

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General relativity gets a 100th birthday party

Image: Albert Einstein
Albert Einstein works at the blackboard during a lecture in Vienna in 1921. (F. Schmutzer via Wikipedia)

This week’s 100th anniversary of Albert Einstein’s general theory of relativity is a geeky cause for celebration, but what’s arguably the concept’s toughest test has just gotten under way.

General relativity was a follow-up to special relativity, Einstein’s big idea from a decade earlier. Back in 1905, he worked out a way to explain why the speed of light is constant, regardless of an observer’s point of view: It’s because space and time are not inflexible metrics, but interrelated dimensions that are measured differently depending on your perspective.

Special relativity explained a lot of the weirdness that physicists were puzzling over at the time, but the theory applied only to “special” conditions that didn’t involve acceleration – for example, how things fall in a gravitational field. On Nov. 25, 1915, Einstein laid out how the interplay of space and time gives rise to gravity and the fabric of the cosmos.

The theory passed its first big test in 1919, when observations during a total solar eclipse were found to be more consistent with Einstein’s view of gravity than with Isaac Newton’s. General relativity has been passing tests ever since. For example, if we didn’t take relativistic effects into account, our GPS readings would seem out of whack.

This week is prime time for centennial retrospectives on the theory and its implications. Here are a few to keep you entertained.

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