Is Gargantua real? The physics is. In May 2013 Christopher Nolan asked the visual effects company Double Negative and the physicist Kip Thorne to build a realistic spinning black hole for Interstellar, and Thorne wrote out, step by step, how light travels from a glowing disk or a distant star through the curved space around a black hole to a camera. The team turned that into a renderer that bends every ray the way Einstein's general relativity says it bends, so the black hole you see in the film was calculated, frame by frame, from the same equations astronomers use.
Then they took one true thing out of it on purpose. A real disk of gas around a black hole is lopsided, one side bright and bluish and the other dim and red, because half of it is rushing toward you near the speed of light and half is rushing away. Gargantua glows evenly because the filmmakers judged the true version, so lopsided that the shadow almost disappears, too much for a mass audience. The first real black hole anyone photographed, M87* in 2019, is brighter on one side, and the astronomers who made the picture put that down to the same effect the film left out.
So yes, and no. Gargantua itself is invented, a hole of 100 million suns that Thorne picked so Miller's planet could survive next to it, and its light is real.
How Kip Thorne and Double Negative drew Gargantua
Thorne's part came first. He put together a prescription for mapping a light ray, and a whole bundle of rays, from where the light starts to the camera's own sky, and handed it to the Double Negative team, who built a fast renderer around it and called it DNGR, the Double Negative Gravitational Renderer. In 2015 Oliver James, Eugénie von Tunzelmann and Paul Franklin of Double Negative published how it works with Thorne in Classical and Quantum Gravity. The paper opens with the claim that matters here: “Interstellar is the first Hollywood movie to attempt depicting a black hole as it would actually be seen by somebody nearby.”
The clever part is the word bundle. A renderer usually fires one thin ray backwards out of each pixel and asks what it hits. Stars are points, so a thin ray almost never hits one, and as the camera moves the stars would blink on and off: “If we were to treat these rays as infinitely thin, there would be zero probability of any ray intersecting a star.” So DNGR sends back a narrow cone instead, follows how gravity squeezes and stretches it into an ellipse, and collects whatever light falls inside, so a moving star flickers by 2 percent at most, too little to see.
The paper gives the scale: an IMAX frame of 23 million pixels, every one of them a beam, and Double Negative's London render farm of 1,633 blade servers, several hundred of them usually running DNGR.
The lie Interstellar told on purpose: no Doppler beaming
The disk around Gargantua was drawn by Double Negative's artists, and they made it gentle on purpose. The paper calls it “very anemic compared to the disks that astronomers see,” so that the people flying near it “will not get fried by X-rays and gamma-rays.” It is thin, it isn't falling in, and it has cooled to one temperature everywhere, 4500 kelvin, which glows a warm gold.
Let the physics run on that disk and the two sides split. The left side is moving toward the camera and the right side away, both at roughly 55 percent of the speed of light. Light from the left gets squeezed to higher frequencies and light from the right gets stretched, and climbing out of the hole's gravity takes about 20 percent off both. Put together, the paper says, the frequencies change “by multiplicative factors of order 1.5 and 0.4.” The left turns blue and blinding, the right turns red and goes dim. Their verdict on the fully realistic version is in the paper too: “A fully realistic accretion disk, Figure 15c, that is exceedingly lopsided, with the hole's shadow barely discernible, was obviously unacceptable.”
So Nolan and Franklin kept the bending and dropped the shift. They changed one more thing. The black hole needed to spin almost as fast as a black hole can to explain the time lost on Miller's planet, and at that spin the shadow gets a flattened left edge with extra slivers of disk beside it. They decided that this, with the off-centre disk, “would be too confusing for a mass audience,” and slowed Gargantua down to 0.6 of the maximum spin for the pictures. Last, they asked for the soft glow a real IMAX lens throws around anything bright, the veiling flare, so the black hole would sit in the same world as everything shot on film.
Move the hole around and watch what happens to the galaxies behind it. The ones straight behind it spread into a ring, the Einstein ring, and every galaxy near it shows up twice, once on each side. The band of disk across the middle is the near side. The arc over the top is the far side of the same flat disk, its light bent up over the hole toward you, and the thin ring under the shadow is the underside, bent around the bottom. None of that is drawn by hand: it falls out of the same equation for every pixel. Switch to As physics and the left side flares white and blue while the right sinks into dull red, the version the paper calls “what the disk would truly look like to an observer near the black hole.”
The team didn’t hide the cheat. They had the real image, and they published it next to the one they used, with the reason, so anyone can check.
Interstellar's black hole vs the real ones: M87* and Sgr A*
On 10 April 2019 the Event Horizon Telescope showed the first image of a black hole. Eight radio telescopes, from Hawaiʻi to the South Pole, were linked into one telescope the size of the Earth and pointed at the centre of the galaxy Messier 87, 55 million light-years away, where something with the mass of 6.5 billion suns sits in the dark. The ring they found is about 40 microarcseconds across, which ESO compares to the length of a credit card on the surface of the Moon, and the event horizon inside it measures just under 40 billion kilometres, about two and a half times smaller than the shadow it casts.
M87* · 2019 · 6.5 billion suns
Sgr A* · 2022 · 4 million suns
Look at M87*, on the left of the pair. Its ring is heavier along the bottom, and the collaboration's first paper says why: “The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole.” The Doppler beaming Interstellar took out shows up in the very first real picture.
On 12 May 2022 came the one in our own galaxy, Sagittarius A*, about 27,000 light-years away and four million times the mass of the Sun. From here it looks about as big as a doughnut on the Moon. It was harder to catch: gas takes days to weeks to go around M87*, and around Sagittarius A* it goes around in minutes, so the picture kept changing while they watched, and the image you see is an average of many.
The orange is a choice. Radio waves have no colour, so the team chose a colour map to show how bright each part is. The astronomer Derek Fox explained it when the first image came out: “The yellow is the most intense emission, the red is less intense, and then black is little or no emission at all.” In visible light, he said, the ring would probably look white, perhaps tinged with blue or red. Gargantua's gold and the EHT's orange look like the same fire, and they come from two different decisions: one an artist's 4500 K, the other a scale for brightness.
And size. Thorne gives Gargantua 100 million suns and says its horizon circumference comes out about the same as the Earth's orbit, about a billion kilometres. That makes it 25 times heavier than the black hole at the centre of our galaxy and 65 times lighter than M87*.
Webb bends light too: the arcs in the First Deep Field
The idea that gravity bends light was tested a century before any of this, when two expeditions photographed the 1919 solar eclipse from Príncipe and from Sobral in Brazil and found the stars near the Sun had moved, the way Einstein said they would. What Gargantua does to the galaxies behind it in the instrument above is the same effect, near a single black hole where it's strongest.
The James Webb Space Telescope's first deep field, released on 12 July 2022, shows it at a much bigger scale. The picture covers a patch of sky about the size of a grain of sand held at arm's length, took 12.5 hours of exposure, and holds thousands of galaxies. In the middle sits the galaxy cluster SMACS 0723, as it looked 4.6 billion years ago, and ESA's caption describes what its mass does: “The combined mass of the galaxies and dark matter act as a cosmic telescope, creating magnified, contorted, and sometimes mirrored images of individual galaxies.” Those orange arcs are galaxies far behind the cluster, bent into crescents: the physics of Gargantua's Einstein ring, done by a whole cluster of galaxies.
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The whole field
A patch of sky about the size of a grain of sand held at arm's length, 12.5 hours of exposure, and thousands of galaxies in it. The cluster in the middle is SMACS 0723 as it was 4.6 billion years ago.
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The lens
The bright white elliptical galaxy at the centre and the smaller white ones around it. Their combined mass, with the dark matter you can't see, bends the light of everything behind them.
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An arc to the right
A galaxy far behind the cluster, stretched into an orange arc. ESA's caption says each galaxy in these arcs is shown twice, mirrored inside one arc.
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Arcs to the left
More of the same on the other side of the brightest galaxy: long thin orange streaks that are whole galaxies, pulled along a circle around the lens.
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The ladybug
Near the bottom of the bright star's vertical spike, a galaxy flecked with star clusters, mirrored top to bottom. Draw a line between its wings and the clusters match up.
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A star in our galaxy
The six long spikes are Webb's own optics. This star sits in the Milky Way, right in front of everything else, and bends almost nothing.
A lensed image depends on where you stand. An Einstein ring only closes when you, the lens and the galaxy behind it fall on one line; move a little and the ring breaks into arcs, move more and one galaxy becomes two images, which is what happens every time you drag the black hole above. Andrea Pozzo’s painted sky on the vault of Sant’Ignazio in Rome is the opposite case, a fake sky that holds together only from one marble disc in the nave floor. Gravity builds real ones, true from wherever you happen to be, a different sky for every place you could stand.
Miller's planet: one hour, seven years
Thorne tells this one himself in The Science of Interstellar. “When Christopher Nolan told me how much slowing of time he wanted on Miller's planet, one hour there is seven years back on Earth, I was shocked. I didn't think that possible and I told Chris so. ‘It's non-negotiable,’ Chris insisted.” So Thorne went home and worked it out. It can be done if the planet sits about as close to Gargantua as anything can orbit without falling in, and if Gargantua spins within about one part in 100 trillion of the fastest a black hole can spin. He also says he expects real black holes to spin no faster than about 0.998 of that maximum, so Miller's planet needs a black hole the universe almost never makes.
Thorne's book puts the planet's speed in its orbit at 55 percent of the speed of light. In 2016 Nathan Steinle and Donald Olson of the physics department at Texas State University ran the numbers from the film and the book through general relativity, and called their result somewhat surprising: “multiple choices of the black hole's mass and spin parameters are required to match all the details of the film's plot and the numerical values in the book.” No single Gargantua fits every number in the story. The pictures use a spin of 0.6, the time loss needs a spin a hair under 1, and the mass that saves the planet from its tides is Thorne's 100 million suns, with a footnote that 200 million might be more reasonable.
Dust: the Dust Bowl inside Interstellar
Before there's any black hole, Interstellar is about dust. The film opens on old people remembering it, and some of those interviews come from Ken Burns's 2012 documentary The Dust Bowl. The end credits thank “Ken Burns, Dayton Duncan, Florentine Films, PBS, and those interviewed for The Dust Bowl,” and the Los Angeles Times reported that Nolan took inspiration from the documentary for the dust storm sequences.
The picture everyone knows from the real Dust Bowl is this one. Arthur Rothstein was 20 and photographing for the federal Resettlement Administration when he stopped in Cimarron County, Oklahoma, in April 1936. A farmer, Arthur Coble, heads for a shed with his two sons, bent into the wind, and the fence posts around them are nearly buried.
The film’s colour lives between these two places. On Earth everything is the beige and brown of Rothstein’s dust, a sky with no horizon in it, and out past Saturn the world goes black with one warm gold thing in the middle of it. The same warm colour hangs over a dying farm in one half of the story and over a hole in space in the other.
The print to hang beside Gargantua is No Colder Place. A small red figure hangs above a ridge of snow beneath a burning ring of light that drops blue shafts through the stars: Cooper’s fall toward the disk, with a frozen world like Mann’s planet below, and the same single warm thing in the dark.
Frequently asked questions
Is Gargantua a real black hole?
No. Gargantua is a fictional black hole of about 100 million solar masses. What's real is how it bends light: every image of it was rendered by tracing light through the equations of general relativity, with Kip Thorne working on the science.
Is the black hole in Interstellar scientifically accurate?
The lensing is. Two things were changed on purpose: the Doppler and gravitational shifts that would make one side of the disk bright and blue and the other dim and red were removed, and the spin was lowered to 0.6 of the maximum for the images, while the time dilation on Miller's planet needs a spin almost at the maximum. Lens flare was added so it would look filmed.
What does a real black hole look like?
The Event Horizon Telescope images of M87* (2019) and Sagittarius A* (2022) show a ring of glowing gas around a dark shadow. The ring of M87* is brighter on one side, which the collaboration explains by relativistic beaming. Their orange is a colour map for radio brightness; in visible light the ring would probably look white.
Why does the disk go over the top of the black hole?
It doesn't: the disk is flat. Light from the far side of the disk is bent up over the top of the hole and down under the bottom on its way to you, so you see the back of the disk wrapped around the shadow.
Read next
- The Star Gate in 2001, how light became a corridor, made by hand one frame at a time.
- Why Dune: Part Two goes black and white, the infrared sun of Giedi Prime.
- Paintings of space, from Elsheimer's Milky Way to Webb, painters and telescopes looking at the same sky.
- Kubrick's lighting, the f/0.7 lens from a batch Zeiss made for NASA, and the candles in Barry Lyndon.
References
- Oliver James, Eugénie von Tunzelmann, Paul Franklin and Kip S. Thorne, “Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar,” Classical and Quantum Gravity 32 (2015) 065001: arxiv.org/abs/1502.03808
- Kip Thorne, The Science of Interstellar (W. W. Norton, 2014), Chapter 6 excerpt: space.com
- Nathan Steinle and Donald Olson, “The Black Hole(s) of Interstellar,” Joint Spring 2016 Meeting of the Texas Sections of the APS, AAPT and SPS Zone 13: meetings-archive.aps.org
- ESO, “Astronomers Capture First Image of a Black Hole,” 10 April 2019: eso.org/public/news/eso1907
- Event Horizon Telescope Collaboration, “First M87 Event Horizon Telescope Results. I,” ApJ Letters 875 L1 (2019): arxiv.org/abs/1906.11238
- ESO, “Astronomers reveal first image of the black hole at the heart of our galaxy,” 12 May 2022: eso.org/public/news/eso2208-eht-mw
- Mindy Weisberger, “Why Is the First-Ever Black Hole Photo an Orange Ring?” Live Science, 11 April 2019, via space.com
- ESA/Webb, “Webb's First Deep Field (NIRCam Image),” weic2209a, and release weic2209, 12 July 2022: esawebb.org
- NASA Scientific Visualization Studio, “Black Hole Accretion Disk Visualization,” 13326, 25 September 2019: svs.gsfc.nasa.gov/13326
- Federico Giannini and Ilaria Baratta, “Andrea Pozzo's masterpiece: the Glory of St. Ignatius in Rome,” Finestre sull'Arte, 14 November 2024: finestresullarte.info
- AFI Catalog of Feature Films, Interstellar (2014), credits and notes: catalog.afi.com
- Arthur Rothstein, Farmer and sons walking in the face of a dust storm, 1936, Library of Congress (ppmsc.00241), via Wikimedia Commons
