A view From the Bridge

Imaging and imagining black holes

Posted on behalf of Davide Castelvecchi

Until several years ago, most cinematic and artistic depictions of black holes — including many in the pages of Nature — failed to match the known facts. A black hole (the remnant of a runaway gravitational collapse) often looked like a space whirlpool, or perhaps a simple black sphere representing the event horizon — the surface that constitutes a point of no return for anything that falls inside. This would be pictured either against a background of stars, or surrounded by an ‘accretion disk’. (Think Saturn’s rings, but made of superheated plasma and spiralling in at close to the speed of light.)

Thanks in part to physicist Kip Thorne’s involvement,Christopher Nolan’s 2014 film Interstellar was the first one to show what you would actually see if you were to fly near a black hole (see image here). And as I wrote last week in Nature, an ambitious radio astronomy project now aims at taking the first snapshot of an actual black hole. In other words, a real-life picture of Interstellar’s black hole Gargantua, if a highly pixelated one.

Between accurate art and actual observation, it might finally begin to sink into our collective imagination just how weird these objects must look. Gravitational lensing, a consequence of Albert Einstein’s theory of gravity, makes light rays curve around a black hole — some light rays do so multiple times. This means that ironically, even though a black hole forever hides what has fallen into it, it cannot hide anything that lies behind it. In particular, if there is an accretion disk, gravitational lensing produces multiple images of it, which appear to wrap around the black disk of the event horizon like a halo (see the infographic accompanying my article).

A black hole cannot hide another object (in this case another black hole) that passes directly behind it. Instead, the object in the background will appear like a ring surrounding the one in the foreground.

A black hole cannot hide another object (in this case another black hole) that passes directly behind it. Instead, the object in the background will appear like a ring surrounding the one in the foreground.

Alain Riazuelo/Institut d’Astrophysique de Paris

Theoretical physicist John Wheeler famously made the term ‘black hole’ official in 1967 to describe the phenomenon. Fewer realise that around a decade after that, an astrophysicist accurately portrayed a black hole, as Thorne relates in his splendid companion book to the film, The Science of Interstellar. In 1978 at the Paris Observatory, Jean-Pierre Luminet became the first to make a detailed computer calculation of a black hole’s appearance. He did so, he told me, by programming a (by then already obsolete) 1960s IBM 7040 computer, using punch cards.

Because Luminet had no way to print out the resulting image or visualize it on a screen, he used the data to draw an image by hand, putting individual dots of India ink onto a photographic negative. He published it that year in the French magazine La Recherche, and then with more detailed technical results in the journal Astronomy and Astrophysics in 1979. (On his blog, Luminet explains how calculating the appearances of black holes is technically similar to understanding the optics of glories, atmospheric phenomena similar to rainbows.)

Given that Gargantua is an accurate simulation using twenty-first-century knowledge and computing, it is uncanny to see how Luminet’s hand-drawn picture made from a punch-card computer’s data already had all the crucial ingredients. In fact, in one respect it was even more accurate. In Luminet’s image, one side of the accretion disk (the one rotating towards the observer) looks much brighter than the other — a consequence of its extreme speeds. As Thorne notes in his book, the Interstellar team considered including this effect in their renderings, but director Christopher Nolan decided it would be too confusing for viewers. This was possibly the only aspect in which the Gargantua sequence strayed from scientific accuracy.

The first accurate image of the appearance of a black hole (India ink on Canson negative paper).

The first accurate image of the appearance of a black hole (India ink on Canson negative paper).

Jean-Pierre Luminet

That realism was a long time coming. From the 1970s at least, most popular-science renderings of black holes lacked the effects of gravitational lensing. “I was a little bit upset to see that in many popular magazines, they more or less systematically used artistic views with no scientific accuracy at all,” Luminet recalls. Starting in the late 1960s, science-fiction had also battened onto black holes, but under an intriguing array of names. A 1967 Star Trek episode had a ‘black star’. A 1975 episode in another TV series, Space: 1999, involved a ‘black sun’. Films, too, began to feature black holes, including  Disney’s 1979 The Black Hole.

Meanwhile, the rise of powerful computers in the decades after Luminet’s efforts meant researchers made ever more realistic simulations, and began to craft colour animations. In the early 1990s, the late astrophysicist Jean-Alain Marck, also at the Paris Observatory, created the animation at the top of this piece, which Luminet later used in the documentary Infinitely Curved. Even more spectacular animations were created by Alain Riazuelo at the Paris Institute of Astrophysics and by Andrew Hamilton at the University of Colorado in Boulder. (Hamilton also rendered what happens when you fall inside a black hole.)

However, none of these outreach efforts had the same impact as Interstellar. The film has begun to affect the way artists represent black holes, says Eugénie von Tunzelmann, who led the 200-strong team of computer-graphics experts at London-based company Double Negative, which created the special effects. Stylized icons now often look like a strip crossing a circle – suggestive of the accretion disk and its lensed image. “The first thing that comes to mind when people say ‘black hole’ might have changed.”

Even in relatively inaccurate sci-fi representations, black holes still provided inspiration for young minds – including for many kids who grew up to become researchers and perhaps work on projects such as the Event Horizon Telescope (EHT), the radio astronomy project that plans to image real black holes. “A lot of scientists, and maybe especially astronomers, always carry that little flame within them,” says Sheperd Doeleman, an astrophysicist at Harvard University in Cambridge, Massachusetts, who helms the EHT. “It really gets you thinking about what’s possible.”

Davide Castelvecchi is senior physical sciences reporter at Nature. He tweets at @dcastelvecchi.

Notes on the animations:

Colour Animation of a Black Hole with Accretion Disk (top): this shows the gravitational lensing around the event horizon (Jean-Alain Marck; from the documentary Infinitely Curved).

A Journey into a Black Hole (bottom): a simulation of what an observer would see while falling into a black hole (Andrew Hamilton).

 

For Nature’s full coverage of science in culture, visit www.nature.com/news/booksandarts.

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