Astronomy

What does it mean for a black hole to “evaporate”?

What does it mean for a black hole to “evaporate”?


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It says in this Wikipedia's article:

Entropy, however, implies heat and therefore temperature. The loss of energy also suggests that black holes do not last forever, but rather "evaporate" slowly.

Would a black hole, given enough time, dissapear? Has this happened before?


It's due to Hawking radiation, a black body radiation due to quantum effects near the event horizon of black holes and named after Stephen Hawking that predicted this phenomena. From the same Wikipedia article:

Hawking radiation reduces the mass and the energy of the black hole and is therefore also known as black hole evaporation. Because of this, black holes that lose more mass than they gain through other means are expected to shrink and ultimately vanish. Micro black holes (MBHs) are predicted to be larger net emitters of radiation than larger black holes and should shrink and dissipate faster.

I don't think it was ever observed to actually lower mass of black holes that we observe as astronomy phenomena, the process would simply take too long for any measurable effects to manifest, but scientists did try to observe it experimentally. In September 2010, Belgiorno et al. claimed they've:

… produced Hawking radiation by firing an intense laser pulse through a so-called nonlinear material, that is one in which the light itself changes the refractive index of the medium.

But this is yet to be reproduced and confirmed by experiments of others.


What the Sight of a Black Hole Means to a Black Hole Physicist

The Event Horizon Telescope’s image of the black hole at the center of Messier 87, a large galaxy in the Virgo cluster. This black hole resides 55 million light-years from Earth and has a mass 6.5 billion times that of the sun.

Event Horizon Telescope collaboration et al.

Janna Levin

At this historic moment, the world has paused to take in the sight of humanity’s first image of the strangest phenomenon in the known universe, a remarkable legacy of the general theory of relativity: a black hole. I am moved not just by the image overwhelmingly I am moved by the significance of sharing this experience with strangers around the globe. I am moved by the image of a species looking at an image of a curious empty hole looming in space.

I am at the National Press Club, in Washington, D.C., a hive of excitement. Scientists with the Event Horizon Telescope aspired for years to take the first-ever picture of a supermassive black hole, so when they gathered journalists and scientists together today for a press conference, there wasn’t much doubt as to what we were here to see.

But still, there are surprises.

At the podium is Sheperd Doeleman, the director of the Event Horizon Telescope. He welcomes us, “black hole enthusiasts.” I have the strongest memory of standing at the chalkboard in an otherwise empty classroom at the Massachusetts Institute of Technology with Shep, my funny friend with his funny, unmistakable, burnt-mahogany hair. Covered in chalk dust, we acquired the hard-earned mathematics of Albert Einstein’s theory of relativity.

Video: Sheperd Doeleman, director of the Event Horizon Telescope, unveiled the first-ever photo of a black hole at a press conference at the National Press Club in Washington, D.C.

National Science Foundation

We knew the words already, the standard lore: All forms of matter and energy bend space and time, and light and matter follow those curves. The words have to be taken on trust. But the mathematics we could acquire. It would belong to us. When Einstein conceived of relativity, he gave us a gift that has been passed from person to person around the world. Relativity, defying its name, is true for all of us.

Maybe my memory of that particular board is so crisp precisely because that moment defines the cusp between before and after acquiring relativity. Now I cannot imagine my own mind without it. Relativity permeates my thoughts so that I think in relativity the way writers think in their natural language. Since that time at MIT, Shep and I have both found our way via relativity to the most remarkable of its predictions, black holes.

Black holes were conceived of as a thought experiment, a fantastical imagining. Imagine matter crushed to a point. Don’t ask how. Just imagine that. While enlisted in the German army during World War I, Karl Schwarzschild discovered this possible solution to Einstein’s newly published theory of relativity, apocryphally between calculating ballistic trajectories from the trenches on the Russian front. Schwarzschild inferred that space-time effectively spills toward the crushed center. Racing at its absolute speed, even light gets dragged down the hole, casting a shadow on the sky. That shadow is the event horizon, the stark demarcation between the outside and anything with the misfortune to have fallen inside.

Einstein thought nature would protect us from the formation of black holes. To the contrary, nature makes them in abundance. When a dying star is heavy enough, gravity overcomes matter’s intrinsic resistance and the star collapses catastrophically. The event horizon is left behind as an archaeological record while the stellar material continues to fall inward to an unknown fate. In our own Milky Way galaxy there could be billions of black holes.

Supermassive black holes, millions or even billions of times the mass of the sun, anchor the centers of nearly all galaxies, though nobody yet knows how they formed or got so heavy. Maybe they formed from dead stars that merged and escalated in size, or maybe they directly collapsed out of more primordial material in a younger universe. However they formed, there are as many supermassive black holes as there are galaxies — hundreds of billions in the observable universe.

We had never seen a black hole before today. No telescope had ever taken a picture of one. We have indirectly inferred the presence of black holes when they’ve cannibalized companion stars, powered energetic jets in twisted magnetic fields, and captured stars in their orbit. We have even heard black holes collide and merge, ringing space-time like mallets on a drum.

We had never taken a direct picture of a black hole before because black holes are tiny, despite their dramatic reputation as weapons of mayhem and destruction (yes, the Nova film I hosted was called “Black Hole Apocalypse”). A black hole the mass of the sun would have an event horizon a mere 6 kilometers across. Compare that to the 1.4-million-kilometer breadth of the sun itself. The supermassive black hole at the center of the Milky Way, dubbed Sagittarius A*, is 4 million times the mass of the sun but only about 17 times wider.

Consider the challenge of capturing a portrait of an entirely dark object only 17 times the width of an ordinary star at a distance of 26,000 light-years. Resolving an image of Sagittarius A* is comparable to resolving the image of a piece of fruit on the moon.

To resolve such a miniscule image requires a telescope the size of the entire Earth. Since those days in that chalk-dusted classroom at MIT, my funny, utterly unconventional friend has been determined to capture the image of a supermassive black hole all the same.

During our years in graduate school, Shep’s hair was an allegory for his mind — wild and spirited. I admired the freedom I sensed in the way he thought, always forging unexpected connections, sometimes at the expense of the required lesson. His shocked eyes would warn me that a crazy idea had struck him just at that precise moment, as though he was as surprised as I was by the thought.

The Event Horizon Telescope is a testament to bold ideas, as well as scientific ingenuity and collaboration. Exploiting large radio telescopes around the globe — relying on the newest, most sophisticated observatories and reviving some that were nearly defunct — EHT became a composite telescope the size of the Earth. As the planet spins and orbits, the target black holes rise into the field of view of component telescopes around the planet. To render a precise image, the telescopes need to operate as one, which involves sensitive time corrections so that one global eye looks toward the black hole.


The Perplexing Physics of Imaging a Black Hole

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NASA/CXC/Villanova University/J. Neilsen

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It's a big day for astronomy (and all humans, really). The first image of a black hole has been released. It was created using the Event Horizon Telescope—a collaboration of radio telescopes around the world. This image shows the material around a super massive black hole in the center of a galaxy some 55 million light-years away.

Yes, there is a ton of cool physics here, involving the crazy gravitational things that happen in extreme cases like a black hole. But that's not what I want to look at. Instead, I want to go over some of the more basic physics questions related to this image.

No—well, yes. It is true that black holes are black. We normally see things like stars and stuff because the light that they emit travels all the way to our telescopes (or straight into our eyes) and we detect them. Black holes are indeed black. They don't emit visible light (because of crazy gravity stuff) so you can't see them.

But that's not the big problem with a black hole. If we had one in our solar system, you could see it. You could see the warping of space due to its presence and you could see the stuff orbiting the black hole. If you've seen the movie Interstellar, you might have a feeling for what a black hole would look like up close. That visualization of a black hole was created with the help of astrophysicist Kip Thorne.

The black hole is so hard to see because it's tiny. OK, it's not tiny in the sense of an ant. It's tiny in the sense that a human is tiny when viewed from a mile away. To visualize something, we need to consider not just its size but its distance. The better term to use is the angular size. If you turn your head all the way around in a circle, that would be a 360-degree angular view (but don't do that without also turning your body). If you hold your thumb out at arm's length, that is about half a degree of angular size. This is about the same angular size as the moon—which is why you can cover up the moon with your thumb.

So, what about the size of this stuff around the black hole? Yes, it's huge. But it's also about 55 million light-years away. That means it is so far away that light (traveling at 3 x 10 8 meters per second) would take 55 million years to get there. It's super far. But really, it's the angular size. The black hole (at least the part you can see) would have an angular size of around 40 microarcseconds.

What is a microarsecond? Well, a circle is broken into degrees (for ancient reasons). Each degree can be broken into 60 arcminutes and each minute is 60 arcseconds. Then if you break this arcsecond into a million pieces—you get a microarcsecond. Remember how the moon is 0.5 degrees in angular size (as viewed from the Earth)? That means the angular size of the moon is 45 million times greater than the size of the black hole stuff. The black hole is angular-tiny.

Wait. It gets worse. Because of diffraction, we can't see angular-tiny things. When light passes through an opening (such as a telescope or the pupil of your eye), light diffracts. It bends in a way that interferes with the rest of the light passing through the opening. In the case of the eye (with visible) light, this means that humans can resolve objects with an angular size of about 1 arcminute.

That means that something as angular-tiny (I'm going to keep using that phrase) as a black hole is pretty difficult to resolve to get an image.

Fine. Angular-tiny things are really difficult to see—then how do we see the stuff around a black hole? The angular resolution of a telescope really just depends on two things: the size of the opening and the wavelength of light. Using smaller wavelengths (like ultraviolet or x-rays) gives a better resolution. But in this case the telescope uses a wavelength of light in the millimeter range. This is a pretty large wavelength compared to visible light, which is in the the 500 nanometer range. So, that's bad.

That means the only way to overcome this diffraction limit is to make a bigger telescope. That's exactly what the Event Horizon Telescope does. It essentially makes a telescope the size of the Earth. That's crazy but true. By taking data from multiple radio telescopes in different parts of the world, you can combine the data to make them into one GIANT telescope. It's tricky, but that's what it does. Even with this, there are some problems. With just a handful of telescopes, the EHT group uses some analysis techniques to determine the most probable image from the data collected. But this will allow them to get the image of some super angular-tiny thing—like the stuff around a black hole.

If you look through a telescope and see Jupiter, you are actually seeing Jupiter. Side note: if you haven't done this before, you totally should do it. It's awesome. The light from the sun reflects off the surface of Jupiter and then travels through the telescope and into your eye. Boom. Jupiter. It's real.

That's not what is happening here with this black hole. The image that you see isn't even in the visible range. It's a radio image using wavelengths of light in the radio region. So, what's the difference between radio waves and visible light? Really, it's just the wavelength that's different.

Both light and radio waves are electromagnetic waves. They are a propagation of a changing electric field along with a changing magnetic field (at the same time). These waves travel at the speed of light—because they are light. However, since radio and visible light have different wavelengths, they interact differently with matter. If you turn on your radio inside your house, you can get a signal from a nearby radio station. These radio waves go right through your walls. Visible light, on the other hand, does NOT go through walls.

This also applies to images. If you have visible light from an object, you can see it with your eye and you can record this image on film (yes, that's old school) or with a digital detector (a CCD camera). This image can then be displayed with a computer monitor so that you are pretty much seeing what it actually looks like. This is what happens when you display a visible light image of the moon.

For the stuff around the black hole, it's not a visible light image. It's a radio image. Each pixel on the image you see represents some particular wavelength of a radio wave. When you see the orange parts of the image, that's a false color representation of a wavelength somewhere around 1 millimeter. The same thing happens if you want to "see" an image from infrared or ultraviolet. We have to convert these wavelengths to something we see.

So, that black hole image is not a normal photograph. It's not something you could see if you looked through a telescope—but it's still really awesome.


Does the Nearest Quasar Host a Black Hole Binary?

By: Monica Young September 16, 2015 2

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Astronomers are investigating a new technique for finding close pairs of supermassive black holes, and they might have found one in the nearest quasar.

A visible-light image of Markarian 231 (Mrk 231), which might host a pair of supermassive black holes.
NASA / ESA

Our universe should be teeming with waltzing pairs of supermassive black holes. Yet they’re incredibly difficult to find. A research team has proposed a new way to find these dancing duos, but whether the method actually works is up for debate.

At the center of every large galaxy (and many not-so-large ones too) sits a supermassive black hole millions or billions of times the mass of the Sun. And many of these galaxies have encountered and merged with at least one other galaxy. So it stands to reason that as galaxies meet and collide, their central black holes should meet, whirling around each other for at least a few hundred million years before they coalesce.

The trouble lies in finding these duos. To start, no current telescope can make out the finer details around a supermassive black hole in another galaxy. (Even resolving the dark beast lurking in our own galaxy proves difficult.) That means we can’t see directly the clouds of gas that orbit and feed the black hole or the wind that might arise from its accretion disk. Such intricate details are inferred instead from spectroscopic observations.

Telescopes can make out pairs of supermassive black holes as long as at least one of them is feeding. The gas funneling into the spacetime ditch heats up as it falls in, emitting brilliant radiation from optical to ultraviolet and beyond.

But even cutting-edge telescopes still have a hard time resolving black hole pairs if they’re too close together — one of the closest confirmed pairs that astronomers have spotted so far is still separated by 24 light-years. And it’s within 3 light-years that the black hole dance gets really interesting: there, the waltz might transform into more of a gravitational tango, providing astronomers with the perfect test bed for the finer points of general relativity.

Some astronomers have turned to spectroscopy to identify closer pairs. When a black hole feeds on gas, it produces many spectral lines. If two black holes are feeding on gas while simultaneously whipping around each other, they will each produce their own set of spectral lines. The Doppler effect shifts each set of lines according to the black holes’ motions: one set of spectral lines will shift redward as that black hole swings away from Earth in its orbit, while the other set shifts blueward as that black hole’s orbit brings it toward Earth.

But nature has ways of making double spectral lines without the presence of a second black hole. While some cases look quite convincing, they’ve proven difficult to confirm.

Now one team of astronomers, led by Chang-Sho Yuan (Chinese Academy of Science, Beijing), is taking a new approach (one that doesn’t involve time-consuming spectra). They put a ‘til-now purely theoretical idea into play in a paper published in Astrophysical Journal (arXiv preprint available here).

A Hole in the Data

This is what an orbiting pair of supermassive black holes might look like as they carve out the center of the gaseous disk that feeds them.
STScI

Theory says that, in a certain stage of their spiraling dance, the mutual orbit of two black holes will carve out a hole in the center of the accretion disk that surrounds and feeds them both. Since the center of the disk is the hottest part and produces the shortest-wavelength emission, if the center of the disk disappears, so does the hot gas and the short-wavelength emission it produces. In other words, a pair of feeding supermassive black holes will emit plenty of visible light, but very little ultraviolet.

Yan and colleagues applied this idea to images of the nearest quasar, Mrk 231, whose light takes 575 million light-years to reach Earth. Astronomers have long known of this quasar’s strange light distribution — it produces far less ultraviolet radiation than it should. That’s been difficult to explain in terms of obscuring dust. (Dust tends to scatter away ultraviolet light, while letting optical light pass through, but not in quite the right way to explain this quasar’s spectrum.)

This simplified spectrum shows what a quasar accretion disk's emission would look like if were solid or if it had a central gap. The simplified plot does not include any absorption or emission expected from the quasar's host galaxy.
NASA / ESA / P. Jeffries (STScI)

Instead, Yan’s team shows that Mrk 231’s output matches what’s expected from the gap scenario. To do that, one black hole would have to outweigh the other, with respective masses of 150 million Suns and 4.5 million Suns, and they would orbit closely, separated on average by just 590 times the Earth-Sun distance and completing an orbit every 1.2 years.

Mrk 231 does show signs of having been through a recent merger, so it could have a binary black hole. But not everyone’s convinced. Kelly Denney (Ohio State University) points out that Mrk 231 is also known to have all kinds of gas flowing out from the central region, as well as a burst of central star formation. These facts alone could potentially explain the quasar’s spectrum without a second supermassive black hole.

Unfortunately, if the binary is there, the orbit is too close to resolve with today’s telescopes, making it difficult to confirm. However, the authors suggest that future gravitational wave telescopes could listen for Mrk 231’s signal.

Meanwhile, the hunt for waltzing supermassive black holes continues. In today’s Nature, another team takes yet another approach, this time studying how a quasar’s light changes over time. And they find some pretty promising things . . . Take a look in our post on “New Evidence for Black Hole Binary.”

Why do we know there's a black hole in every large galaxy? How have black holes affected the evolution of the universe? Could we one day image a black hole's silhouette? Find out in our FREE Black Holes ebook.


Can black holes kill you?

You’d most likely be dead before you ever got close due to the immense gravitational pull of a black hole.

According to scientists, black holes are so strong your body would be pulled apart even before you got pulled in.

As you got closer, the difference in gravity between your head and your feet would stretch you out like a piece of chewing gum.

Scientists call this process "spaghettification".

You eventually become a stream of subatomic particles that swirl into the black hole like water down a plug.

According to TV physicist Neil De Grasse Tyson: "As you get closer and closer, the force of gravity grows astronomically. You stay whole until the stretching force exceeds the molecular bonds of your body's flesh.

"At that moment, your body would snap into two segments. Everything of you that ever was gets funnelled to the black hole's centre.

"Not only have you been ripped in half – you've been extruded through the fabric of space and time like toothpaste through a tube."

However, this isn't true in all cases – you see, the gravitational pull of a black hole depends on its mass the smaller the black hole, the higher the density, meaning a stronger gravitational force.

Therefore, if you somehow found yourself near a bigger black hole, the force of gravity would not kill you, in other words, you wouldn't die from spaghettification.

Avi Loeb, chair of astronomy at Harvard University, explained to Space.com: "You wouldn't "feel such forces to a significant degree."

However, he pointed out that while you wouldn't die before reaching the event horizon of the black hole, other hazards around it would kill you.

So, probably best to steer clear of black holes.


A New Math Shortcut Helps Describe Black Hole Collisions

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Last year, just for the heck of it, Scott Field and Gaurav Khanna tried something that wasn’t supposed to work. The fact that it actually worked quite well is already starting to make some ripples.

Field and Khanna are researchers who try to figure out what black hole collisions should look like. These violent events don’t produce flashes of light but rather the faint vibrations of gravitational waves, the quivers of space-time itself. But observing them is not as simple as sitting back and waiting for space to ring like a bell. To pick out such signals, researchers must constantly compare the data from gravitational wave detectors to the output of various mathematical models—calculations that reveal the potential signatures of a black hole collision. Without reliable models, astronomers wouldn’t have a clue what to look for.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.

The trouble is, the most trustworthy models come from Einstein’s general theory of relativity, which is described by 10 interlinked equations that are notoriously difficult to solve. To chronicle the complex interactions between colliding black holes, you can’t just use a pen and paper. The first so-called numerical relativity solutions to the Einstein equations for the case of a black hole merger were calculated only in 2005—after decades of attempts. They required a supercomputer running on and off for two months.

A gravitational wave observatory like LIGO needs to have a large number of solutions to draw upon. In a perfect world, physicists could just run their model for every possible merger permutation—a black hole with a certain mass and spin encountering another with a different mass and spin—and compare those results with what the detector sees. But the calculations take a long time. “If you give me a big enough computer and enough time, you can model almost anything,” said Scott Hughes, a physicist at the Massachusetts Institute of Technology. “But there’s a practical issue. The amount of computer time is really exorbitant”—weeks or months on a supercomputer. And if those black holes are unevenly sized? The calculations would take so long that researchers consider the task practically impossible. Because of that, physicists are effectively unable to spot collisions between black holes with mass ratios greater than 10-to-1.

Which is one reason why Field and Khanna’s new work is so exciting. Field, a mathematician at the University of Massachusetts, Dartmouth, and Khanna, a physicist at the University of Rhode Island, have made an assumption that simplifies matters greatly: They treat the smaller black hole as a “point particle”—a speck of dust, an object with mass but zero radius and no event horizon.

Scott Field (left) and Gaurav Khanna did not expect their approximation to work for black holes of relatively equal masses.

Courtesy of Scott Field & Gaurav Khanna

“It’s like two ships passing in the ocean—one a rowboat, the other a cruise liner,” Field explained. “You wouldn’t expect the rowboat to affect the cruise liner’s trajectory in any way. We’re saying the small ship, the rowboat, can be completely ignored in this transaction.”

They expected it to work when the smaller black hole’s mass really was like a rowboat’s compared to a cruise ship’s. “If the mass ratio is on the order of 10,000-to-1, we feel very confident in making that approximation,” Khanna said.

But in research published last year, he and Field, along with graduate student Nur Rifat and Cornell physicist Vijay Varma, decided to test their model at mass ratios all the way down to 3-to-1—a ratio so low it had never been tried, mainly because no one considered it worth trying. They found that even at this low extreme, their model agreed, to within about 1 percent, with results obtained by solving the full set of Einstein’s equations—an astounding level of accuracy.

“That’s when I really started to pay attention,” said Hughes. Their results at mass ratio 3, he added, were “pretty incredible.”

“It’s an important result,” said Niels Warburton, a physicist at University College Dublin who was not involved with the research.

The success of Field and Khanna’s model down to ratios of 3-to-1 gives researchers that much more confidence in using it at ratios of 10-to-1 and above. The hope is that this model, or one like it, could operate in regimes where numerical relativity cannot, allowing researchers to scrutinize a part of the universe that has been largely impenetrable.

After black holes spiral toward each other and collide, the massive bodies create space-time-contorting disturbances—gravitational waves—that propagate through the universe. Eventually, some of these gravitational waves might reach Earth, where the LIGO and Virgo observatories wait. These enormous L-shaped detectors can sense the truly tiny stretching or squishing of space-time that these waves create—a shift 10,000 times smaller than the width of a proton.

The designers of these observatories have made herculean efforts to muffle stray noise, but when your signal is so weak, noise is a constant companion.

The first task in any gravitational wave detection is to try to extract a weak signal from that noise. Field compares the process to “driving in a car with a loud muffler and a lot of static on the radio, while thinking there might be a song, a faint melody, somewhere in that noisy background.”

Astronomers take the incoming stream of data and first ask if any of it is consistent with a previously modeled gravitational wave form. They might run this preliminary comparison against tens of thousands of signals stored in their “template bank.” Researchers can’t determine the exact black hole characteristics from this procedure. They’re just trying to figure out if there’s a song on the radio.

The next step is analogous to identifying the song and determining who sang it and what instruments are playing. Researchers run tens of millions of simulations to compare the observed signal, or wave form, with those produced by black holes of differing masses and spins. This is where researchers can really nail down the details. The frequency of the gravitational wave tells you the total mass of the system. How that frequency changes over time reveals the mass ratio, and thus the masses of the individual black holes. The rate of change in the frequency also provides information about a black hole’s spin. Finally, the amplitude (or height) of the detected wave can reveal how far the system is from our telescopes on Earth.

If you have to do tens of millions of simulations, they’d better be quick. “To complete that in a day, you need to do each in about a millisecond,” said Rory Smith, an astronomer at Monash University and a member of the LIGO collaboration. Yet the time needed to run a single numerical relativity simulation—one that faithfully grinds its way through the Einstein equations—is measured in days, weeks or even months.

To speed up this process, researchers typically start with the results of full supercomputer simulations—of which several thousand have been carried out so far. They then use machine learning strategies to interpolate their data, Smith said, “filling in the gaps and mapping out the full space of possible simulations.”

This “surrogate modeling” approach works well so long as the interpolated data doesn’t stray too far from the baseline simulations. But simulations for collisions with a high mass ratio are incredibly difficult. “The bigger the mass ratio, the more slowly the system of two inspiraling black holes takes to evolve,” Warburton explained. For a typical low-mass-ratio computation, you need to look at 20 to 40 orbits before the black holes plunge together, he said. “For a mass ratio of 1,000, you need to look at 1,000 orbits, and that would just take too long”—on the order of years. This makes the task virtually “impossible, even if you have a supercomputer at your disposal,” Field said. “And without a revolutionary breakthrough, this won’t be possible in the near future either.”

Because of this, many of the full simulations used in surrogate modeling are between the mass ratios of 1 and 4 almost all are less than 10. When LIGO and Virgo detected a merger with a mass ratio of 9 in 2019, it was right at the limit of their sensitivity. More events like this haven’t been found, Khanna explained, because “we don’t have reliable models from supercomputers for mass ratios above 10. We haven’t been looking because we don’t have the templates.”

A visualization of a black hole merger with a mass ratio of 9.2 to 1. The video begins about 10 seconds before the merger. The left panel shows the full spectrum of gravitational radiation, colored according to signal strength—blue is weak, and orange strong. The right panels show the different components of the gravitational wave signal.

That’s where the model that he and Khanna have developed comes in. They started with their own point particle approximation model, which is specially designed to operate in the mass ratio range above 10. They then trained a surrogate model on it. The work opens up opportunities to detect the mergers of unevenly sized black holes.

What kinds of situations might create such mergers? Researchers aren’t sure, since this is a newly opening frontier of the universe. But there are a few possibilities.

First, astronomers can imagine an intermediate-mass black hole of perhaps 80 or 100 solar masses colliding with a smaller, stellar-size black hole of about 5 solar masses.

Another possibility would involve a collision between a garden-variety stellar black hole and a relatively puny black hole left over from the Big Bang—a “primordial” black hole. These could have as little as 1 percent of a solar mass, whereas the vast majority of black holes detected by LIGO so far weigh more than 10 solar masses.

Earlier this year, researchers at the Max Planck Institute for Gravitational Physics used Field and Khanna’s surrogate model to look through LIGO data for signs of gravitational waves emanating from mergers involving primordial black holes. And while they didn’t find any, they were able to place more precise limits on the possible abundance of this hypothetical class of black holes.

Furthermore, LISA, a planned space-based gravitational wave observatory, might one day be able to witness mergers between ordinary black holes and the supermassive varieties at the centers of galaxies—some with the mass of a billion or more suns. LISA’s future is uncertain its earliest launch date is 2035, and its funding situation is still unclear. But if and when it does launch, we may see mergers at mass ratios above 1 million.

Some in the field, including Hughes, have described the new model’s success as “the unreasonable effectiveness of point particle approximations,” underscoring the fact that the model’s effectiveness at low mass ratios poses a genuine mystery. Why should researchers be able to ignore the critical details of the smaller black hole and still arrive at the right answer?

“It’s telling us something about the underlying physics,” Khanna said, though exactly what that is remains a source of curiosity. “We don’t have to concern ourselves with two objects surrounded by event horizons that can get distorted and interact with each other in strange ways.” But no one knows why.

In the absence of answers, Field and Khanna are trying to extend their model to more realistic situations. In a paper scheduled to be posted early this summer on the preprint server arxiv.org, the researchers give the larger black hole some spin, which is expected in an astrophysically realistic situation. Again, their model closely matches the findings of numerical relativity simulations at mass ratios down to 3.

They next plan to consider black holes that approach each other on elliptical rather than perfectly circular orbits. They’re also planning, in concert with Hughes, to introduce the notion of “misaligned orbits”—cases in which the black holes are askew relative to each other, orbiting in different geometric planes.

Finally, they’re hoping to learn from their model by trying to make it break. Could it work at a mass ratio of 2 or lower? Field and Khanna want to find out. “One gains confidence in an approximation method when one sees it fail,” said Richard Price, a physicist at MIT. “When you do an approximation that gets surprisingly good results, you wonder if you are somehow cheating, unconsciously using a result that you shouldn’t have access to.” If Field and Khanna push their model to the breaking point, he added, “then you’d really know that what you are doing is not cheating—that you just have an approximation that works better than you’d expect.”

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.


NEWS CLIP: We are delighted to be able to report to you today that we have seen what we thought was unseeable…An international scientific team unveiling the first ever image of a black hole today.

GABRIELA: April 10, 2019. One of the most significant scientific accomplishments in history.

that's when we knew that we had done something extraordinary.

It's the environment that humanity came to be in and it's the backdrop to our existence. It was that good that it just blew our minds.

GABRIELA: Maybe you’ve seen it, the very first photo of a black hole. It’s a blurry, glowing, orange ring on a black background. But in case you’re thinking that astronomers just pointed a telescope at a certain part of the sky and snapped a picture… well it’s the result of almost two decades of work. And an international group of hundreds of people. A team called The Event Horizon Telescope Collaboration.

I think this was . really a testament to how large groups can function and work together and do extraordinary things. But at the same time. you'd never envision all the problems that you run up against.

GABRIELA: I’m Gabriela Cowperthwaite and this is Teamistry, an original podcast from Atlassian. This show is all about the chemistry of teams. and what happens when people are so open to new ideas of working, innovating and expressing themselves together, they end up doing something amazing.

GABRIELA: When I was a kid the scariest things in the world were. tidal waves, quicksand, I thought there were just like little pools of quicksand everywhere, and black holes. was it really like a hole that you could fall into for like all eternity in space. What would happen if you came close, would it be like Stephen Hawking said and stretch you into spaghetti? Just so, so terrifying. The funny thing is now I have two boys and they are always asking me the same questions like “How long would you last in a black hole? Would you be conscious for long enough to realize you were in a black hole? Would you freeze? Would you cease to exist completely? Like, what does that even mean?” so being able to photograph a black hole, and be able to understand one a little better that’s something I feel I need to be able to know at some deep primal level so that I can assuage one of my biggest fears but also to be a cool mom.

GABRIELA: So first things first. what the heck is a black hole, anyway?

EMILY: a black hole is a place in the universe where gravity is so strong that nothing can escape, not even light.

GABRIELA: That’s Emily Conover, she’s a physics reporter for Science News and the thing is, it’s really hard to wrap our heads around what exactly a black hole is. First of all, it’s not really a hole like we think of it. In fact, we only call it that because as a joke, back in the 60s, physicist Robert H. Dicke compared them to this horrible jail that was known as a “black hole,” because if you went in, you never came out. And the name stuck. But instead of being a hole, it’s actually a tiny, infinitely dense spot in space. And because of all that gravity, light can’t escape it, so we can’t actually see a black hole.

EMILY: The event horizon is what we think of as the edge of the black hole, this sort of surface that surrounds it. And if you were to be so unlucky as to fall into a black hole, once you passed the event horizon, you would not be able to escape, even if you were traveling at the speed of light

GABRIELA: And that’s why it’s called the “Event Horizon Telescope” because since we can’t see anything that gets too close to the black hole, all we can see is the stuff that just skirts around it, on the edge of the event horizon.

GABRIELA: Ok, ok, so that’s what we’re talking about when we say “black hole” and “event horizon.” But until very recently, most scientists agreed that even if black holes existed like we thought they did, we’d never be able to photograph them, for two major reasons. First, even the biggest black holes are so far away, we don’t have telescopes big enough to capture them. And second, even if we did have some supermassive telescope, it would produce so much data. well, we just didn’t have the computer power to process it all. Until this century, when a group of scientists came together.

it was really around 2006, 2007 that we staged some experiments that showed us beyond a shadow of a doubt that we could potentially image the supermassive black hole in the center of our galaxy . So, that really launched the beginnings of the project.

GABRIELA: That’s Shep Doeleman. He works at the Smithsonian Astrophysical Observatory and is one of the founders of the Event Horizon Telescope collaboration. And even though his team was confident, all they could hope to see was that thin ring around a black hole.

the Event Horizon that we're looking at is equivalent to seeing an orange on the moon.

And the way we did this was we turned the whole planet into a telescope. It sounds crazy. It is a little crazy because it takes so much work to do.

GABRIELA: It’s kind of mind boggling how much work this would take. I mean, something like this had never been attempted before, so they were writing the manual as they went along. Plus, the financial investment was massive: some estimates have put the project at 50 to 60 million dollars. If it doesn’t work, it will shake the confidence of institutions and governments around the world in these kinds of mega projects. But if they manage to photograph a black hole, it could lead to a wave of new cosmic discoveries. Like watching new planets form in distant galaxies. Or testing the limits of the laws of physics.

GABRIELA: It all starts, as Shep said, with the challenge of turning our planet into a telescope.The way they did that was by networking together telescopes. In other words, by getting telescopes in Hawaii and Mexico, Spain and Chile, Arizona and the South Pole to all focus on one spot in the sky.

Yeah. So, they certainly don't teach you how to build a big earth sized telescope . in graduate school.

GABRIELA: One of the first things Shep and his initial small team of astronomers had to do was put aside the astronomy and develop their diplomatic skills

. that is what took a lot of political maneuvering and a lot of convincing of directors of facilities that normally would do other things, that this was scientifically worthwhile. And that took a lot of effort. We cashed in a lot of brownie points.

GABRIELA: Here’s an example of that early stage of the project.

GABRIELA: We’re in the high desert of Chile, the home of the “Alma Array” telescope. Well, it’s one site but it’s actually made up of over 60 dishes. Shep and his team knew that if they could combine all these dishes, they could increase the sensitivity of the overall Event Horizon Telescope 10 times. But the Alma Array isn’t set up for that sort of thing.

And we had to approach the Alma administration and the management team very gingerly. Because they did not want to let us rummage around inside the very sensitive electronics of their facility.

GABRIELA: But eventually, through diplomatic negotiations, calling in favours, making promises, the folks in Chile came around to the idea. And that’s when the black hole team realize they had another big challenge, long before they started focusing on capturing the photograph

So, one of the things that was absolutely essential for doing this. was that we had to build a large international team. We had to have experts in engineering, in instrumentation, but also in theory, interpretation, data analysis, from all over the globe.

GABRIELA: Have you ever tried to set up a meeting between folks in a couple of different time zones? It’s a headache. Now, imagine that that’s how you have to work every day, with people all over the world, with different backgrounds, languages, time commitments, everything. The bigger challenge, though, was ensuring that everyone saw how their long work hours contributed to the final results of the project. This is a collaboration superpower I’ll call “Universal Frequency,” not only organizing your international team, but keeping them motivated, so that the project can continue to move ahead.

GABRIELA: In the case of the Alma Array, Shep’s team was joined by Paul Ho, Director of the East Asia Observatory in Taiwan. Paul was tasked with rejigging the tech at the Chilean telescope so it would jive with the Event Horizon project. Universal Frequency was essential because he had to do it all from Taiwan

This involved bringing the equipment that are being built in various places like in Japan, in Europe, in the US, and then it comes to an integration center in Taiwan. and then test them and calibrate them, and then ship them to Chile. So there's a huge amount of coordination between the various regions involved in the project in order to deliver these products.

GABRIELA: After six years of work, the Alma Array joined the Event Horizon Telescope. And how else was “Universal Frequency” at play here? Not only Paul Ho’s international group’s success, but seeing the overall project’s power jump ten times. But again, getting these telescopes online was just the start. After all, now they had to actually collect data about the black hole. So Shep’s team reached out to experts like black hole astrophysicist Avery Broderick. He had been putting together simulations of what a black hole event horizon could look like. And then one day Shep walks into his office.

the vivid memory I described is him coming into the office. and saying, "I think these pictures that you keep making and putting in your papers, I think we might be able to make those for real." And that was the moment that I signed on….. And, we never looked back since.

GABRIELA: Avery might sound pretty relaxed about it now, but this was all a huge gamble when they started to build the project. All it would take is for some of these telescopes to not join up or end up pulling out and the entire thing could collapse. Even bouts of bad weather would mean certain telescopes could go offline. And then the team just wouldn’t have the power, the resolution, to gather enough data for the photo. And there had already been so much money, and so much time poured into it… personally, yeah, there’d be frustration and disappointment, but professionally, the ramifications could be worse. as Shep says

. people had to accept that they were putting their careers at risk because you were really chasing something that we didn't even know existed. You have to get some people who are willing to lay it all on the line in order to push the field forward. And if it hadn't worked out, then we'd be laying brick

GABRIELA: He’s kind of joking about it, but reputations were on the line here. Failure could be a major stumbling block in people’s careers. Part of the risk they were taking, and not just Shep but folks like Avery, was moving out of their comfort zones to build an international team.

people like me, as an astrophysicist, I'm not necessarily trained for is, is collaboration building. It's not just building a small team, but it's building a large group that brings all of those facilities together. And that was the part that I think, at least for me, was the most difficult, is understanding how to navigate a large collaboration of people problems.

GABRIELA: And those people problems weren’t always just about managing schedules and roles. Sometimes the problems got a lot more serious. And dangerous.

I got a call at 4:00 in the morning saying some of our postdocs were accosted by gunmen on their way to one of our sites. And we made the command decision to shut down that site because the safety of our people was absolutely paramount. But it did cause us to stop observing at that site a few days early.

GABRIELA: Despite the risks they were taking, with millions of dollars, thousands of hours of research and work, and their own careers, they now they faced the biggest challenge of them all: photographing what might not be possible to photograph

GABRIELA: It’s 2017 and the Event Horizon Telescope is up and running. The thing is, they’re not actually photographing the stars. All the telescopes are radio telescopes, so they’re capturing radio waves. Each one collects as much data as it can. The more data, from as many points as possible, the higher resolution the final image can have. And they are harvesting a huge amount of data, as Avery says.

. We had petabytes of data, literally tons of data. There were about half a ton of hard drives on which this data ended up being stored.

GABRIELA: Just to be clear, a petabyte is one million gigabytes. So, yeah, a new challenge. You see, all that data is spread out around the world, from Hawaii to Spain, Mexico to the South Pole. But it needs to be brought together for a whole other team to process and put it all together

The internet doesn't really work for this amount of data.

GABRIELA: That’s Event Horizon Telescope team member Sera Markoff, a professor of theoretical astrophysics at the University of Amsterdam.

. You can't just send it off, especially since some of the stations are in places that are very remote, like the South Pole. And in that case, of course you have to wait actually, until winter is over and planes can get out and you can pack a crate with the discs, put them on a plane and fly them to the computers, which are in either the Boston area or Germany.

GABRIELA: Yeah, even getting the data from place to place is a challenge. Just think about it: so many hours of work have been invested in setting up one of these telescopes, then hoping the weather and technology will cooperate, and then you see the data pouring in. If you lose that data because of a messed up delivery, months of work are gone, setting back the entire project. Imagine the anxiety if something goes wrong, as Shep explains.

We sent some data back to one of our processing centers in Bond, Germany, and what showed up was a shipment of bolts of fabric. So somewhere there was a bunch of disc drives showing up at a clothing manufacturer, and we got the fabric.

GABRIELA: Oh my god, the wave of nausea I’d get if I opened up that box and the disk drives weren’t there. Like literally looking at months of work lost forever.

Thankfully, we were able to switch these around and we didn't have to make matching jumpsuits for everybody in the Event Horizon Telescope Team

GABRIELA: In this facility in Bond, Germany, there are dozens of mega computers. This is one of the two centres where the petabytes of data collected by the telescopes will be processed…

And when you bring them together and you compare those recordings, you can create a dataset as though you had a telescope that's as large as the distance between the telescopes. But the cool thing is that those data don't mean anything until they're compared with one another. It's that magical act of combining the data that gives us the image.

GABRIELA: Magical, yes, but not easy, or quick. This is going to be months and months of more work, as another group of people reconstitute the data. Which really means putting a jigsaw puzzle together with no guide. And, where all the pieces don’t fit together perfectly. Plus, it’s not just one group doing it, as Sera explains:

You always want to do lots of double checks, and there's no better way to do this than have completely independent teams do their own analysis with different tools.

GABRIELA: So, yeah, that massive amount of data has been copied so that different groups, in different locations, can process it and put it together into an image

we purposely did not want to be fooled by group think. If we were all in the same room working on the data, it would have been very easy for us to be lulled into this sense that yes, we were seeing this ring, when it could have been something else. So we split the teams up into four separate groups and each group worked on the data completely independently. They were not able to share any data. They were not in communication with others.

GABRIELA: This is another superpower the team used, splitting into subgroups who could take different paths to solve the same problem. Shep calls this “Constructive Tension.” But he does point out a potential danger with this approach: that people might feel their results could be discarded at the end, when everything’s narrowed down and finalized. The answer is to report on all the methods used, so that each group can see their work in the final manuscript.

GABRIELA:The groups that are being formed to work on the data are yet another new cohort, as Sera Markoff says, mostly graduate students and postdoctoral researchers

I think the key to the success was how many brilliant, especially young people, were involved and helped put this all together. The collaboration had a huge range of expertise, but also a huge range in life stages

GABRIELA: These young people are bringing a new energy to the project. They are also helping the overall team to evolve, by bringing new perspectives, from a scientific point of view. I mean, some of them were still in high school when the Event Horizon Telescope project started. To them, photographing a black hole is not impossible, it’s inevitable

Like most scientific projects, you have kind of the visionary aspect, and the top leadership of course tends to be a bit older, and then the people who are actually pathfinding it and working it out on the ground tend to be more at the PhD and the postdoc level. That was the people who did the majority of a lot of the really hardest part of this project, and they were just amazing.

GABRIELA: They were also recognized for their work, to make sure they knew their contributions mattered.

GABRIELA: And so finally, after a decade and a half of work, Shep remembers when things started to fall into place.

there was an evening banquet in May of 2018 and some of the postdocs and graduate students came to me and they showed me just the raw data in a plot, not an image, but a plot. And we looked at this and we realized right away that there were some key signatures in the structure of this plot that showed us that there was something very interesting there. We didn't know if it was a ring yet, but we saw right away that there were some ring-like tendencies to the data. That got us very excited.

GABRIELA: It’s important to keep in mind here that for many years, based on astronomers’ understanding of light and gravity, people had been guessing and building simulations of what an event horizon might look like. And they all kind of look the same: a bright orange ring around a black circle. So as the image is constructed, if it looks totally different from these simulations that could mean something has gone really wrong, either with the Event Horizon Telescope—over a decade of work—or the decades of theorizing about black holes. But as the processing of the data is coming together, there’s a growing sense of excitement on the team, as Avery says.

And we were watching them go through this process, and at the same time getting ready. Because we knew they were close, we knew that there was an image to be seen, and we had some feeling already. There were some clues that it was going to look amazing and it was going to look like what we kind of thought it should look like. And so we were already well underway years underway in getting ready to interpret that image. And so we were watching this with bated breath wondering how good is it going to be? Are we going to see a ring? Is it just going to be a slight dimple in the center?

GABRIELA: The anticipation is building that any day now they will finally see what scientists thought, until only recently, was unseeable. A black hole. But other than the steps they have taken so far and all that real, tangible hard work, what else is at play here that has led to their success? One word keeps coming up, “trust,” first from project leader Shep Doeleman.

when you have something this complicated, something this distributed, you really have to trust in the team that you've built. And you're trusting that you have vetted procedures that will allow this huge complicated machinery to produce something new and true. We didn't know that we could really pull this off, but we had confidence that we had the right people and that we had the right team. What I like to say is that …we jumped off cliffs and invented parachutes on the way down, but there's no other team I'd rather jump off a cliff with than these guys.

GABRIELA: After months of work processing unimaginable amounts of data, the four separate groups have come up with their results. And, thankfully, incredibly, they all arrive at the same image. But before we all get to see the black hole, the entire Event Horizon Telescope team had to share it amongst themselves. And it happened with folks all over the world on a video conference call.

I can't remember if I was in my pajamas, it was definitely in the evening in European time

GABRIELA: But Sera does remember her reaction, the first time she saw the image pop up on her screen:

there was that moment of like, "Is this really the black hole or is this just another simulation of a black hole?” Because it looks so much like these theoretical simulations that we'd been doing

GABRIELA: Avery had the same reaction:

I was asking them, "Did you guys put this in there? Is this real?"

GABRIELA: But, of course, it was real.

and I was like, "Holy crap, you know, it is really a black hole", I'm actually seeing this thing that I've been studying since I was 20 or whatever. It was extremely, yeah, it was very emotional and it was very powerful moment I think for everybody, and very exhilarating as well.

GABRIELA: It certainly was for Shep who, like Sera, had been working on this for the better part of his life.

I've been doing this since the beginning, 20 years or more. And the only thing that's driven me through this has been a sense of optimism, the sense that it was possible and that we had the right group to do it. And then we had the resources, and if nature just cooperated that we could pull it off and to see it, one of the great predictions of modern physics confirmed so clearly. And that was really emotional.

EMILY: So I was at the press conference in Washington DC where they announced the photo and it was very suspenseful

GABRIELA: That’s Emily Conover, the science writer from the top of the show, remembering April 10, 2019, when the photo was first unveiled to the world.

EMILY: There was definitely wonder, I mean, to imagine being able to take a picture of a black hole. I never would have predicted that we could have been able to do that. When I was a kid imagining ‘what are these weird black holes?’, I never would have imagined seeing a picture of one one day. I think that is the case that, you know, you're looking at the real thing. That is really what the universe is telling you. That is what a black hole is.

GABRIELA: For Shep Doeleman, it took another day for the significance to sink in.

When we saw this image on the front page of all the newspapers around the world, that's when it hit us that it meant as much to other people as it did to us.

GABRIELA: And that, of course, is the bigger picture. Not only what this photo means to the world of science, but what capturing it means to the world, as Sera says..

it's really important to remember that people from different nationalities and different countries can get together, and work together, and do something that is just. amazing to have achieved

GABRIELA: Despite the challenges of building and working with a huge, international team, this group managed to do something that until recently was considered impossible. Something so amazing that it brought the whole planet’s attention together, and as Shep points out, that just doesn’t happen much any more

in a time when things are dividing us as people. This was something that by its very nature drew us together and it really couldn't have been done I think without the combined efforts of a global team. And that's important because we're going to be facing a lot of challenges in the coming decades, even centuries that are going to require a global effort to solve. and if this can be any kind of indication that we can work together to solve outrageously hard problems that we didn't even know if we could solve, then it's been a good day's work for us too on that score.

GABRIELA: It’s been a year since the black hole photo was revealed to the world. And what is the team working on now? An even bigger challenge: capturing video of a black hole. If they can do that, we can see what happens to matter as it’s being sucked into the Event Horizon. Spaghetti, anyone?

GABRIELA: There were some key superpowers used to motivate a huge international team and to recognize that team’s efforts. To find out more about those strategies, check out the “extras” page at atlassian dot com slash Teamistry. And please subscribe so you don't miss what’s coming up next - how a team at the car company that revolutionized how cars were made. managed to do it again seventy years later. That’s next time on Teamistry, an original podcast from Atlassian.


What is a black hole?

  • A black hole is a region of space from which nothing, not even light, can escape
  • Despite the name, they are not empty but instead consist of a huge amount of matter packed densely into a small area, giving it an immense gravitational pull
  • There is a region of space beyond the black hole called the event horizon. This is a "point of no return", beyond which it is impossible to escape the gravitational effects of the black hole

Prof Falcke had the idea for the project when he was a PhD student in 1993. At the time, no-one thought it was possible. But he was the first to realise that a certain type of radio emission would be generated close to and all around the black hole, which would be powerful enough to be detected by telescopes on Earth.

He also recalled reading a scientific paper from 1973 that suggested that because of their enormous gravity, black holes appear 2.5 times larger than they actually are.

These two factors suddenly made the seemingly impossible, possible. After arguing his case for 20 years, Prof Falcke persuaded the European Research Council to fund the project. The National Science Foundation and agencies in East Asia then joined in to bankroll the project to the tune of more than £40m.

It is an investment that has been vindicated with the publication of the image. Prof Falcke told me that he felt that "it's mission accomplished".

He said: "It has been a long journey, but this is what I wanted to see with my own eyes. I wanted to know is this real?"

No single telescope is powerful enough to image the black hole. So, in the biggest experiment of its kind, Prof Sheperd Doeleman of the Harvard-Smithsonian Centre for Astrophysics led a project to set up a network of eight linked telescopes. Together, they form the Event Horizon Telescope and can be thought of as a planet-sized array of dishes.

Each is located high up at a variety of exotic sites, including on volcanoes in Hawaii and Mexico, mountains in Arizona and the Spanish Sierra Nevada, in the Atacama Desert of Chile, and in Antarctica.

A team of 200 scientists pointed the networked telescopes towards M87 and scanned its heart over a period of 10 days.

The information they gathered was too much to be sent across the internet. Instead, the data was stored on hundreds of hard drives that were flown to central processing centres in Boston, US, and Bonn, Germany, to assemble the information. Katie Bouman a PhD student at MIT developed an algorithm that pieced together the data from the EHT. Without her contribution the project would not have been possible. Prof Doeleman described the achievement as "an extraordinary scientific feat".

"We have achieved something presumed to be impossible just a generation ago," he said.

"Breakthroughs in technology, connections between the world's best radio observatories, and innovative algorithms all came together to open an entirely new window on black holes."

The team is also imaging the supermassive black hole at the centre of our own galaxy, the Milky Way.

Odd though it may sound, that is harder than getting an image from a distant galaxy 55 million light-years away. This is because, for some unknown reason, the "ring of fire" around the black hole at the heart of the Milky Way is smaller and dimmer.

How to see a Black Hole: The Universe's Greatest Mystery can be seen the UK at 21:00 on BBC Four on Wednesday 10 April.


What does it mean for a black hole to &ldquoevaporate&rdquo? - Astronomy

How do you observe something you can't see? This is the basic question of somebody who's interested in finding and studying black holes. Because black holes are objects whose pull of gravity is so intense that nothing can escape it, not even light, so you can't see it directly.

So, my story today about black holes is about one particular black hole. I'm interested in finding whether or not there is a really massive, what we like to call "supermassive" black hole at the center of our galaxy. And the reason this is interesting is that it gives us an opportunity to prove whether or not these exotic objects really exist. And second, it gives us the opportunity to understand how these supermassive black holes interact with their environment, and to understand how they affect the formation and evolution of the galaxies which they reside in.

So, to begin with, we need to understand what a black hole is so we can understand the proof of a black hole. So, what is a black hole? Well, in many ways a black hole is an incredibly simple object, because there are only three characteristics that you can describe: the mass, the spin, and the charge. And I'm going to only talk about the mass. So, in that sense, it's a very simple object. But in another sense, it's an incredibly complicated object that we need relatively exotic physics to describe, and in some sense represents the breakdown of our physical understanding of the universe.

But today, the way I want you to understand a black hole, for the proof of a black hole, is to think of it as an object whose mass is confined to zero volume. So, despite the fact that I'm going to talk to you about an object that's supermassive, and I'm going to get to what that really means in a moment, it has no finite size. So, this is a little tricky.

But fortunately there is a finite size that you can see, and that's known as the Schwarzschild radius. And that's named after the guy who recognized why it was such an important radius. This is a virtual radius, not reality the black hole has no size. So why is it so important? It's important because it tells us that any object can become a black hole. That means you, your neighbor, your cellphone, the auditorium can become a black hole if you can figure out how to compress it down to the size of the Schwarzschild radius.

At that point, what's going to happen? At that point gravity wins. Gravity wins over all other known forces. And the object is forced to continue to collapse to an infinitely small object. And then it's a black hole. So, if I were to compress the Earth down to the size of a sugar cube, it would become a black hole, because the size of a sugar cube is its Schwarzschild radius.

Now, the key here is to figure out what that Schwarzschild radius is. And it turns out that it's actually pretty simple to figure out. It depends only on the mass of the object. Bigger objects have bigger Schwarzschild radii. Smaller objects have smaller Schwarzschild radii. So, if I were to take the sun and compress it down to the scale of the University of Oxford, it would become a black hole.

So, now we know what a Schwarzschild radius is. And it's actually quite a useful concept, because it tells us not only when a black hole will form, but it also gives us the key elements for the proof of a black hole. I only need two things. I need to understand the mass of the object I'm claiming is a black hole, and what its Schwarzschild radius is. And since the mass determines the Schwarzschild radius, there is actually only one thing I really need to know.

So, my job in convincing you that there is a black hole is to show that there is some object that's confined to within its Schwarzschild radius. And your job today is to be skeptical. Okay, so, I'm going to talk about no ordinary black hole I'm going to talk about supermassive black holes.

So, I wanted to say a few words about what an ordinary black hole is, as if there could be such a thing as an ordinary black hole. An ordinary black hole is thought to be the end state of a really massive star's life. So, if a star starts its life off with much more mass than the mass of the Sun, it's going to end its life by exploding and leaving behind these beautiful supernova remnants that we see here. And inside that supernova remnant is going to be a little black hole that has a mass roughly three times the mass of the Sun. On an astronomical scale that's a very small black hole.

Now, what I want to talk about are the supermassive black holes. And the supermassive black holes are thought to reside at the center of galaxies. And this beautiful picture taken with the Hubble Space Telescope shows you that galaxies come in all shapes and sizes. There are big ones. There are little ones. Almost every object in that picture there is a galaxy. And there is a very nice spiral up in the upper left. And there are a hundred billion stars in that galaxy, just to give you a sense of scale. And all the light that we see from a typical galaxy, which is the kind of galaxies that we're seeing here, comes from the light from the stars. So, we see the galaxy because of the star light.

Now, there are a few relatively exotic galaxies. I like to call these the prima donna of the galaxy world, because they are kind of show offs. And we call them active galactic nuclei. And we call them that because their nucleus, or their center, are very active. So, at the center there, that's actually where most of the starlight comes out from. And yet, what we actually see is light that can't be explained by the starlight. It's way more energetic. In fact, in a few examples it's like the ones that we're seeing here. There are also jets emanating out from the center. Again, a source of energy that's very difficult to explain if you just think that galaxies are composed of stars.

So, what people have thought is that perhaps there are supermassive black holes which matter is falling on to. So, you can't see the black hole itself, but you can convert the gravitational energy of the black hole into the light we see. So, there is the thought that maybe supermassive black holes exist at the center of galaxies. But it's a kind of indirect argument.

Nonetheless, it's given rise to the notion that maybe it's not just these prima donnas that have these supermassive black holes, but rather all galaxies might harbor these supermassive black holes at their centers. And if that's the case — and this is an example of a normal galaxy what we see is the star light. And if there is a supermassive black hole, what we need to assume is that it's a black hole on a diet. Because that is the way to suppress the energetic phenomena that we see in active galactic nuclei.

If we're going to look for these stealth black holes at the center of galaxies, the best place to look is in our own galaxy, our Milky Way. And this is a wide field picture taken of the center of the Milky Way. And what we see is a line of stars. And that is because we live in a galaxy which has a flattened, disk-like structure. And we live in the middle of it, so when we look towards the center, we see this plane which defines the plane of the galaxy, or line that defines the plane of the galaxy.

Now, the advantage of studying our own galaxy is it's simply the closest example of the center of a galaxy that we're ever going to have, because the next closest galaxy is 100 times further away. So, we can see far more detail in our galaxy than anyplace else. And as you'll see in a moment, the ability to see detail is key to this experiment.

So, how do astronomers prove that there is a lot of mass inside a small volume? Which is the job that I have to show you today. And the tool that we use is to watch the way stars orbit the black hole. Stars will orbit the black hole in the very same way that planets orbit the sun. It's the gravitational pull that makes these things orbit. If there were no massive objects these things would go flying off, or at least go at a much slower rate because all that determines how they go around is how much mass is inside its orbit.

So, this is great, because remember my job is to show there is a lot of mass inside a small volume. So, if I know how fast it goes around, I know the mass. And if I know the scale of the orbit I know the radius. So, I want to see the stars that are as close to the center of the galaxy as possible. Because I want to show there is a mass inside as small a region as possible. So, this means that I want to see a lot of detail. And that's the reason that for this experiment we've used the world's largest telescope.

This is the Keck observatory. It hosts two telescopes with a mirror 10 meters, which is roughly the diameter of a tennis court. Now, this is wonderful, because the campaign promise of large telescopes is that is that the bigger the telescope, the smaller the detail that we can see. But it turns out these telescopes, or any telescope on the ground has had a little bit of a challenge living up to this campaign promise. And that is because of the atmosphere. Atmosphere is great for us it allows us to survive here on Earth. But it's relatively challenging for astronomers who want to look through the atmosphere to astronomical sources.

So, to give you a sense of what this is like, it's actually like looking at a pebble at the bottom of a stream. Looking at the pebble on the bottom of the stream, the stream is continuously moving and turbulent, and that makes it very difficult to see the pebble on the bottom of the stream. Very much in the same way, it's very difficult to see astronomical sources, because of the atmosphere that's continuously moving by.

So, I've spent a lot of my career working on ways to correct for the atmosphere, to give us a cleaner view. And that buys us about a factor of 20. And I think all of you can agree that if you can figure out how to improve life by a factor of 20, you've probably improved your lifestyle by a lot, say your salary, you'd notice, or your kids, you'd notice.

And this animation here shows you one example of the techniques that we use, called adaptive optics. You're seeing an animation that goes between an example of what you would see if you don't use this technique — in other words, just a picture that shows the stars — and the box is centered on the center of the galaxy, where we think the black hole is. So, without this technology you can't see the stars. With this technology all of a sudden you can see it. This technology works by introducing a mirror into the telescope optics system that's continuously changing to counteract what the atmosphere is doing to you. So, it's kind of like very fancy eyeglasses for your telescope.

Now, in the next few slides I'm just going to focus on that little square there. So, we're only going to look at the stars inside that small square, although we've looked at all of them. So, I want to see how these things have moved. And over the course of this experiment, these stars have moved a tremendous amount. So, we've been doing this experiment for 15 years, and we see the stars go all the way around.

Now, most astronomers have a favorite star, and mine today is a star that's labeled up there, SO-2. Absolutely my favorite star in the world. And that's because it goes around in only 15 years. And to give you a sense of how short that is, the sun takes 200 million years to go around the center of the galaxy. Stars that we knew about before, that were as close to the center of the galaxy as possible, take 500 years. And this one, this one goes around in a human lifetime. That's kind of profound, in a way.

But it's the key to this experiment. The orbit tells me how much mass is inside a very small radius. So, next we see a picture here that shows you before this experiment the size to which we could confine the mass of the center of the galaxy. What we knew before is that there was four million times the mass of the sun inside that circle. And as you can see, there was a lot of other stuff inside that circle. You can see a lot of stars. So, there was actually lots of alternatives to the idea that there was a supermassive black hole at the center of the galaxy, because you could put a lot of stuff in there.

But with this experiment, we've confined that same mass to a much smaller volume that's 10,000 times smaller. And because of that, we've been able to show that there is a supermassive black hole there. To give you a sense of how small that size is, that's the size of our solar system. So, we're cramming four million times the mass of the sun into that small volume.

Now, truth in advertising. Right? I have told you my job is to get it down to the Schwarzchild radius. And the truth is, I'm not quite there. But we actually have no alternative today to explaining this concentration of mass. And, in fact, it's the best evidence we have to date for not only existence of a supermassive black hole at the center of our own galaxy, but any in our universe. So, what next? I actually think this is about as good as we're going to do with today's technology, so let's move on with the problem.

So, what I want to tell you, very briefly, is a few examples of the excitement of what we can do today at the center of the galaxy, now that we know that there is, or at least we believe, that there is a supermassive black hole there. And the fun phase of this experiment is, while we've tested some of our ideas about the consequences of a supermassive black hole being at the center of our galaxy, almost every single one has been inconsistent with what we actually see. And that's the fun.

So, let me give you the two examples. You can ask, "What do you expect for the old stars, stars that have been around the center of the galaxy for a long time, they've had plenty of time to interact with the black hole." What you expect there is that old stars should be very clustered around the black hole. You should see a lot of old stars next to that black hole.

Likewise, for the young stars, or in contrast, the young stars, they just should not be there. A black hole does not make a kind neighbor to a stellar nursery. To get a star to form, you need a big ball of gas and dust to collapse. And it's a very fragile entity. And what does the big black hole do? It strips that gas cloud apart. It pulls much stronger on one side than the other and the cloud is stripped apart. In fact, we anticipated that star formation shouldn't proceed in that environment.

So, you shouldn't see young stars. So, what do we see? Using observations that are not the ones I've shown you today, we can actually figure out which ones are old and which ones are young. The old ones are red. The young ones are blue. And the yellow ones, we don't know yet. So, you can already see the surprise. There is a dearth of old stars. There is an abundance of young stars, so it's the exact opposite of the prediction.

So, this is the fun part. And in fact, today, this is what we're trying to figure out, this mystery of how do you get — how do you resolve this contradiction. So, in fact, my graduate students are, at this very moment, today, at the telescope, in Hawaii, making observations to get us hopefully to the next stage, where we can address this question of why are there so many young stars, and so few old stars. To make further progress we really need to look at the orbits of stars that are much further away. To do that we'll probably need much more sophisticated technology than we have today.

Because, in truth, while I said we're correcting for the Earth's atmosphere, we actually only correct for half the errors that are introduced. We do this by shooting a laser up into the atmosphere, and what we think we can do is if we shine a few more that we can correct the rest. So this is what we hope to do in the next few years. And on a much longer time scale, what we hope to do is build even larger telescopes, because, remember, bigger is better in astronomy.

So, we want to build a 30 meter telescope. And with this telescope we should be able to see stars that are even closer to the center of the galaxy. And we hope to be able to test some of Einstein's theories of general relativity, some ideas in cosmology about how galaxies form. So, we think the future of this experiment is quite exciting.

So, in conclusion, I'm going to show you an animation that basically shows you how these orbits have been moving, in three dimensions. And I hope, if nothing else, I've convinced you that, one, we do in fact have a supermassive black hole at the center of the galaxy. And this means that these things do exist in our universe, and we have to contend with this, we have to explain how you can get these objects in our physical world.

Second, we've been able to look at that interaction of how supermassive black holes interact, and understand, maybe, the role in which they play in shaping what galaxies are, and how they work.

And last but not least, none of this would have happened without the advent of the tremendous progress that's been made on the technology front. And we think that this is a field that is moving incredibly fast, and holds a lot in store for the future. Thanks very much. (Applause)


Physical and philosophical

&ldquoNo physicist is going to travel into a black hole and measure it. This is a maths question,&rdquo Hintz said.

&ldquoBut, from that point of view, this makes Einstein&rsquos equations mathematically more interesting. This is a question one can really only study mathematically, but it has physical, almost philosophical, implications, which makes it very cool.&rdquo

Even if someone were to make it to a small black hole to try the theory, the tidal forces close to the event horizon &ndash or point of no return &ndash are enough to &lsquospaghettify&rsquo anything.

However, this work has encouraged other researchers to examine the idea, one of whom suggests that most well-behaved black holes will not violate determinism. Hintz insists that one instance of violation is one too many.

&ldquoPeople had been complacent for some 20 years, since the mid-1990s, that strong cosmological censorship is always verified,&rdquo he said. &ldquoWe challenge that point of view.&rdquo