You Spin Me Round Like a Rotating Black Hole


Disney’s 1979 film, The Black Hole, holds a special place in the pantheon of bad science fiction films.

It’s about a team of astronauts who encounter a mad scientist orbiting a black hole in his spaceship — being oh-so-careful not to cross the event horizon (at least not until he’s ready). It’s campy, with cheesy special effects, stilted dialogue, a silly plot, an annoying R2D2 wannabe robot, and some pretty spectacularly bad science.

In the film’s defense, the whole concept of a black hole was a relative newcomer to the popular imagination back in 1979. As recently as 1990, most scientists agreed with Einstein that black holes were theoretical entities — an intriguing mathematical anomaly, to be sure, but largely the stuff of science fiction.

SEE ALSO: Casting Call: ‘The Black Hole’ Remake

Spinning Black Holes, Warped Spacetime

However, several hundred such objects have since been identified, and as many as 300 million might exist in the visible universe. Astronomers now believe that most of the galaxies in our universe have black holes at their centers; a black hole was identified at the center of our own Milky Way.


Furthermore, there is indirect evidence — obtained by measuring the velocity distributions of stars within a given galaxy — that many of the black holes observed thus far appear to be rotating, some as fast as a million miles per hour, thereby giving a black hole angular momentum. And when they rotate, they drag space-time into a vortex, pretty much forcing any objects — including light particles — to follow the same path of rotation.

(Here’s an interesting bit of trivia. Technically, a rotating black hole has two event horizons: the outer horizon that marks the point of no return for objects falling into the black hole, and an inner horizon closer to the center, arising from the hole’s angular momentum.)

WIDE ANGLE: Black Holes, Big and Small

Twisted Lasers

But ideally, physicists would love to be able to measure that rate of rotation directly, in part because that telltale distribution of stars might be influenced by other factors — exotic stuff like dark matter in particular.

That’s the gist of a new paper appearing in Nature Physics this week, by a team of physicists who looked at how light behaves in experiments with lasers and lenses.

They quickly zeroed in on the fact that while light generally travels in a straight line, if you send it through just the right kind of lens, the beams will twist into a shape looking for all the world like fusilli (corkscrew) pasta.

It turns out that spacetime around a black hole can twist the light emitted as a result of matter falling onto the object’s accretion disc. The twisty spacetime would have the same effect as a lens, and that effect should be observable via a property of light known as orbital angular momentum — how a photon (like planets orbiting the sun) orbits around a fixed point.

The team conducted a series of computer simulations, which revealed that how much the light twists will depend on how fast the black hole is rotating, giving scientists a much more precise means of measuring that rate of rotation. Those new measurements, in turn, could shed light on how black holes form, and could even help scientists detect Hawking radiation: the glow emitted by black holes as they evaporate over time (the bigger they are, the more slowly they evaporate). This is something Stephen Hawking predicted in 1974, but has not yet been directly observed.

Choose Your Supermassive Black Hole

Could today’s telescopes be able to detect that signature twisting? It is, after all, a very slight effect. Then again, even the Earth drags spacetime a little as it rotates, which has a noticeable effect on satellites each year, and the Earth is far less massive than a black hole.

Lead author Fabrizio Tamburini of Italy’s University of Padova thinks it could be possible in as little as two years, using radio telescope arrays like the Very Long Baseline Array in the U.S. or Europe’s LOIS-LOFAR telescope array. You just aim the telescope array to the center of your chosen galaxy, with different telescopes positioned to observe from different angles, and then superimpose those piecemeal images to get a complete picture of the wavefront of the light as it moves through spacetime. And then you repeat the process a few times, pointing to different section around the black hole each time.

If the scientists are correct, we’d have a handy new technique for detecting and measuring the rate of rotation of different black holes. If they’re wrong — well, we might need to take another look at Einstein’s theory of general relativity, and that, in itself, would be pretty darn interesting.

“The nice thing is when you find there is a contradiction between exiting theories and reality,” co-author Bo Thide (Swedish Institute of Space Physics) told Wired. “This is what everybody is hoping for, including myself.”

Image credit: NASA

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