The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. In coordinated press conferences across the globe, EHT researchers revealed that they succeeded, unveiling the first direct visual evidence of the supermassive black hole in the centre of Messier 87 and its shadow. The shadow of a black hole seen here is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across. While this may sound large, this ring is only about 40 microarcseconds across — equivalent to measuring the length of a credit card on the surface of the Moon. Although the telescopes making up the EHT are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm during a 2017 global campaign. Each telescope of the EHT produced enormous amounts of data – roughly 350 terabytes per day – which was stored on high-performance helium-filled hard drives. These data were flown to highly specialised supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration. Credit: Event Horizon Telescope Collaboration
The event horizon of a black hole is the boundary (‘horizon’) between its ‘outside’ and its ‘inside’; those outside cannot know anything about things (‘events’) which happen inside.
What an event horizon is – its behavior – is described by applying the equations of Einstein’s theory of General Relativity (GR); as of today, the theoretical predictions concerning event horizons can be tested in only very limited ways. Why? Because we don’t have any black holes we can study up close and personal (so to speak) … which is perhaps a very good thing!
If the black hole is not rotating, its event horizon has the shape of a sphere; it’s like a 2D surface over a 3D ball. Except, not quite; GR is a theory about spacetime, and contains many counter-intuitive aspects. For example, if you fall freely into a black hole (one sufficiently massive that tidal forces don’t rip you to pieces and smear you into a plastic-wrap thin layer of goo, a supermassive black hole for example), you won’t notice a thing as you pass through the event horizon … and that’s because it’s not the event horizon to you! In other words, the location of the event horizon of a black hole depends upon who is doing the observing (that word ‘relativity’ really does some heavy lifting, if you’ll excuse the pun), and as you fall (freely) into a black hole, the event horizon is always ahead of you.
You’ll often read that the event horizon is where the escape velocity is c, the speed of light; that’s a not-too-bad description, but it’s better to say that the path of any ray of light, inside the event horizon, can never make it beyond that horizon.
If you watch – from afar! – something fall into a black hole, you’ll see that it gets closer and closer, and light from it gets redder and redder (increasingly redshifted), but it never actually reaches the event horizon. And that’s the closest we’ve come to testing the theoretical predictions of event horizons; we see stuff – mass ripped from the normal star in a binary, say – heading down into its massive companion, but we never see any sign of it hitting anything (like a solid surface). In the next decade or so it might be possible to study event horizons much more closely, by imaging SgrA* (the supermassive black hole – SMBH – at the center of our galaxy), or the SMBH in M87, with extremely high resolution.
The Universe Today article Black Hole Event Horizon Measured is about just this kind of black hole-normal star binary, Black Hole Flares as it Gobbles Matter is about observations of matter falling into a SMBH, and Maximizing Survival Time Inside the Event Horizon of a Black Hole describes some of the weird things about event horizons.
NGC 6240. Image credit: X-ray: NASA/CXC/MIT/ C.Canizares, M.Nowak; Optical: NASA/STScI
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If black holes could communicate, there would likely be a lotta in your face trash talkin’ going on between these two merging black holes. This image of NGC 6240 contains new X-ray data from Chandra (shown in red, orange, and yellow) that has been combined with an optical image from the Hubble Space Telescope originally released in 2008. The two black holes are a mere 3,000 light years apart and are seen as the bright point-like sources in the middle of the image.
Scientists think these black holes are in such close proximity because they are in the midst of spiraling toward each other – a process that began about 30 million years ago. It is estimated that the two black holes will eventually drift together and merge into a larger black hole some tens or hundreds of millions of years from now.
Finding and studying merging black holes has become a very active field of research in astrophysics. Since 2002, there has been intense interest in follow-up observations of NGC 6240 by Chandra and other telescopes, as well as a search for similar systems. Understanding what happens when these exotic objects interact with one another remains an intriguing question for scientists.
The formation of multiple systems of supermassive black holes should be common in the Universe, since many galaxies undergo collisions and mergers with other galaxies, most of which contain supermassive black holes. It is thought that pairs of massive black holes can explain some of the unusual behavior seen by rapidly growing supermassive black holes, such as the distortion and bending seen in the powerful jets they produce. Also, pairs of massive black holes in the process of merging are expected to be the most powerful sources of gravitational waves in the Universe.
Stephen Hawking, weightless (courtesy Zero Gravity Corporation)
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When humans starve, they grow thin and eventually die; when a black hole starves, it too grows thin and dies … but it does so very spectacularly, in a burst of Hawking radiation.
At least that’s the way we understand it today (no black-hole-pining-away has yet been observed), and the theory may be wrong too.
Cosmologist, astrophysicist, and physicist Stephen Hawking showed, in 1974, that black holes should emit electromagnetic radiation with a black body spectrum; this process is also called black hole evaporation. In brief, this theoretical process works like this: particle-antiparticle pairs are constantly being produced and rapidly disappear (through annihilation); these pairs are virtual pairs, and their existence (if something virtual can be said to exist!) is a certain consequence of the Uncertainty Principle. Normally, we don’t ever see either the particle or antiparticle of these pairs, and only know of their existence through effects like the Casimir effect. However, if one such virtual pair pops into existence near the event horizon of a black hole, one may cross it while the other escapes; and the black hole thus loses mass. A long way away from the event horizon, this looks just like black body radiation.
It turns out that the smaller the mass a black hole has, the faster it will lose mass due to Hawking radiation; right at the end, the black hole disappears in an intense burst of gamma radiation (because the black hole’s temperature rises as it gets smaller). We won’t see any of the black holes in the Milky Way explode any time soon though … not only are they likely still gaining mass (from the cosmic microwave background, at least), but a one sol black hole would take over 10^67 years to evaporate (the universe is only 13 billion years old)!
There are many puzzles concerning black holes and Hawking radiation; for example, black hole evaporation via Hawking radiation seems to mean information is lost forever. The root cause of these puzzles is that quantum mechanics and General Relativity – the two most successful theories in physics, period – are incompatible, and we have no experiments or observations to help us work out how to resolve this incompatibility.
Artist's rendering of a black hole. Image Credit: NASA
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Black holes get a bad rap. Most people are afraid of them, and some think black holes might even destroy Earth. Now, scientists from the University of Leeds are blaming black holes for causing the most energetic and deadly outbursts in the universe: gamma ray bursts.
The conventional model for GRBs is that a narrow beam of intense radiation is released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. This involves plasma being heated by neutrinos in a disk of matter that forms around the black hole. A subclass of GRBs (the “short” bursts) appear to originate from a different process, possibly the merger of binary neutron stars.
But mathematicians at the University of Leeds have come up with a different explanation: the jets come directly from black holes, which can dive into nearby massive stars and devour them.
Their theory is based on recent observations by the Swift satellite which indicates that the central jet engine operates for up to 10,000 seconds – much longer than the neutrino model can explain.
The scientists believe that this is evidence for an electromagnetic origin of the jets, i.e. that the jets come directly from a rotating black hole, and that it is the magnetic stresses caused by the rotation that focus and accelerate the jet’s flow.
For the mechanism to operate the collapsing star has to be rotating extremely rapidly. This increases the duration of the star’s collapse as the gravity is opposed by strong centrifugal forces.
One particularly peculiar way of creating the right conditions involves not a collapsing star but a star invaded by its black hole companion in a binary system. The black hole acts like a parasite, diving into the normal star, spinning it with gravitational forces on its way to the star’s centre, and finally eating it from the inside.
“The neutrino model cannot explain very long gamma ray bursts and the Swift observations, as the rate at which the black hole swallows the star becomes rather low quite quickly, rendering the neutrino mechanism inefficient, but the magnetic mechanism can,” says Professor Komissarov from the School of Mathematics at the University of Leeds.
“Our knowledge of the amount of the matter that collects around the black hole and the rotation speed of the star allow us to calculate how long these long flashes will be – and the results correlate very well with observations from satellites,” he adds.
Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
At the very heart of the Milky Way is a region known as Sagittarius A*. This region is known the be the home of a supermassive black hole with millions of times the mass of our own Sun. And with the discovery of this object, astronomers have turned up evidence that there are supermassive black holes at the centers most most spiral and elliptical galaxies.
The best observations of Sagittarius A*, using Very Long Baseline Interferometry (VLBI) radio astronomy have determined that it’s approximately 44 million km across (that’s just the distance of Mercury to the Sun). Astronomers have estimated that it contains 4.31 million solar masses.
Of course, astronomers haven’t actually seen the supermassive black hole itself. Instead, they have observed the motion of stars in the vicinity of Sagittarius A*. After 10 years of observations, astronomers detected the motion of a star that came within 17 light-hours distance from the supermassive black hole; that’s only 3 times the distance from the Sun to Pluto. Only a compact object with the mass of millions of stars would be able to make a high mass object like a star move in that trajectory.
The discovery of a supermassive black hole at the heart of the Milky Way helped astronomers puzzle out a different mystery: quasars. These are objects that shine with the brightness of millions of stars. We now know that quasars come from the radiation generated by the disks of material surrounding actively feeding supermassive black holes. Our own black hole is quiet today, but it could have been active in the past, and might be active again in the future.
Some astronomers have suggested other objects that could have the same density and gravity to explain Sagittarius A, but anything would quickly collapse down into a supermassive black hole within the lifetime of the Milky Way.
Magnetic field around a black hole. Image credit: NASA
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As you are likely aware, there are numerous ways in which the Universe could kill us all, destroying the Earth and whatever signs of human life, or life in general, existed on our planet. Gamma Ray Bursts, Coronal Mass Ejections, or just the odd asteroid or comet slamming into the Earth would easily take out most of the life on our planet. But, what about black holes? Do we have to worry about them, too? Could a black hole wipe out all life on Earth, sucking us all into oblivion? It’s possible, but not very likely. And by not very likely, it’s calculated that the odds of being killed by a black hole are about one in one trillion.
First, a black hole has to get to the Earth. There are two ways of this happening. The first is that we create one ourselves, the second that a black hole wandering the galaxy happens upon our little Solar System, and meanders in towards the Sun. We’ll start with the first scenario: creating our own destruction.
How could we make our own black hole? Well, theoretically, when you slam protons together with enough force, there is the potential for the creation of a small, short-lived black hole. Particle colliders like the Large Hadron Collider in Geneva, Switzerland, which is scheduled to start operating again in November 2009, could potentially create miniscule black holes through the collisions of protons. There were many headlines from the mainstream media about the potential of the LHC to create runaway black holes that would find their way to the center of the Earth and devour it from the inside, causing, “total destruction.” Sounds scary, doesn’t it? Even more, two people were suing to stop the LHC because of the potential hazard they thought it posed.
However, the LHC is in no way going to destroy the Earth. This is because any black holes created by the LHC will almost instantly evaporate, due to what’s called Bekenstein-Hawking radiation, which theorizes that black holes do indeed radiate energy, and therefore have a limited lifespan. A black hole with the mass of, say, a few protons, would evaporate in trillionths of a second. And even if it were to stick around, it wouldn’t be able to do much damage: it would likely pass through matter as if it didn’t exist. If you want to know whether the LHC has destroyed the Earth, go here.
Of course, there are other ways of creating black holes than the LHC, namely cosmic rays that slam into our atmosphere on a regular basis. If these are creating mini-black holes all of the time, none of them seem to be swallowing the Earth whole…yet. Other scientific experiments also aim at studying the properties of black holes right here on Earth, but the danger from these experiments is very, very minimal.
Now that we know black holes created here on Earth aren’t likely to kill us all, what about a black hole from the depths of space wandering into our neighborhood? Black holes generally come in two sizes: supermassive and stellar. Supermassive black holes reside in the hearts of galaxies, and one of these is not likely to come barrelling our way. Stellar black holes form from a dying star that, in the end, gives up its fight against gravity and implodes. The smallest black hole that can form from this process is about 12 miles across. The closest black hole to our solar system is Cygnus X-1, which is about 6,000 light years away, much too far to pose a threat by muscling it’s way into our vicinity (although there are other ways that it could potentially harm us if it were closer, like blasting us with a jet of X-rays, but that’s a whole other story). The creation process for a black hole of this variety – a supernova – could potentially sling the black hole across the galaxy, if the supernova happened in a binary pair and the explosion was asymmetric.
If a stellar black hole were to plow through the Solar System, it would be pretty ugly. The object would likely be accompanied by an accretion disk of heated, radioactive matter that would announce the presence of the black hole by frying our atmosphere with gamma and X-rays. Add to that the tidal forces of the black hole disrupting the Sun and other planets, and you have a huge mess on your hands, to say the least. It’s possible that a number of planets, and even the Sun, could be flung out of the Solar System, depending on the mass, velocity, and approach of the black hole. Yikes.
Artist's rendering of a black hole. Image Credit: NASA
There lies one last possiblity for black holes to wreak their havoc on the Earth: Primordial Black Holes. These are miniature black holes theorized to have been created in the intense energies of the Big Bang (which the LHC plans to mimic on a MUCH smaller scale). Many of them most likely evaporated billions of years ago, but a black hole that started out with the mass of a mountain (10 billion tons) could potentially still be lurking around the galaxy. A hole of this size would shine at a temperature of billions of degrees from Bekenstein-Hawking radiation, and it’s likely we would see it coming due to observatories like NASA’s Swift.
From a few yards a way, the black hole’s gravity would be barely noticeable, so this kind of black hole wouldn’t have an effect on the gravity of the Solar System. At less than an inch, though, the gravity would be intense. It would suck up air as it passed through the atmosphere of the Earth, and start to make a small accretion disk. To such a tiny black hole, the Earth seems close to a vacuum, so it would probably pass right through, leaving a wake of radiation in its path and nothing more.
A black hole of this variety with a mass of the Earth, however, would be roughly the size of a peanut, and would be able to potentially swing the Moon straight into the Earth, depending, of course, on the trajectory and speed of the black hole. Yikes, again. Not only that, if it were to impact the Earth, the devastation would be total: as it entered the atmosphere, it would suck up a lot of gas and form a radioactive accretion disk. As it got closer, people and objects on the surface would be sucked up into it. Once it impacted the surface, it would start swallowing up the Earth, and probably eat its way all the way through. In this scenario, the Earth would end up being nothing more than a wispy disk of debris around the remaining black hole.
Black holes are scary and cool, and none of the scenarios depicted here are even remotely likely to happen, even if they’re fun to think about. If you want to learn more about black holes, Hubblesite has an excellent encyclopedia, as does Stardate.org. You can also check out the rest of our section on black holes in the Guide to Space, or listen to the multiple Astronomy Cast episodes on the subject, like Episodes 18, or the questions show on Black, Black Holes. Much of the information on the likelihood and aftereffects of a black hole collision with the Earth in this article is taken from chapter 5 in Phil Plait‘s “Death from the Skies!”
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Those fearful folks who have worried about the Large Hadron Collider creating a black hole that could swallow the Earth have probably been feeling pretty safe while the giant particle accelerator is still offline. But hopefully they haven’t read the latest Physical Review Letters . It includes a paper that explains how researchers at Dartmouth have figured out a way to create a tiny quantum-sized black hole in their lab, with no LHC required.
In their paper, the researchers show that a magnetic field-pulsed microwave transmission line containing an array of superconducting quantum interference devices, or SQUIDs, not only reproduces physics similar to that of a radiating black hole, but does so in a system where the high energy and quantum mechanical properties are well understood and can be directly controlled in the laboratory. The paper states, “Thus, in principle, this setup enables the exploration of analogue quantum gravitational effects.”
“We can also manipulate the strength of the applied magnetic field so that the SQUID array can be used to probe black hole radiation beyond what was considered by Hawking,” said Miles Blencowe, an author on the paper and a professor of physics and astronomy at Dartmouth.
Creating a black hole would allow researchers to better understand what physicist Stephen Hawking proposed more than 35 years ago: black holes are not totally void of activity; they emit photons, which is now known as Hawking radiation.
“Hawking famously showed that black holes radiate energy according to a thermal spectrum,” said co-author Paul Nation. “His calculations relied on assumptions about the physics of ultra-high energies and quantum gravity. Because we can’t yet take measurements from real black holes, we need a way to recreate this phenomenon in the lab in order to study it, to validate it.”
This is not the first proposed imitation black hole, Nation said. Other proposed schemes to create a black hole include using supersonic fluid flows, ultracold bose-einstein condensates and nonlinear fiber optic cables. However, these ideas wouldn’t work as well to study Hawking radiation because the radiation in these methods is incredibly weak or otherwise masked by commonplace radiation due to unavoidable heating of the device, making it very difficult to detect. “In addition to being able to study analogue quantum gravity effects, the new, SQUID-based proposal may be a more straightforward method to detect the Hawking radiation,” said Blencowe.
GEMS, the Gravity and Extreme Magnetism Small Explorer, will detect polarized X-rays from supernova remnants, neutron stars and black holes.
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NASA has announced the development of a space-based observatory to give astronomers a new way to view X-rays from exotic objects such as black holes, neutron stars, and supernovae. Called the Gravity and Extreme Magnetism Small Explorer (GEMS), the mission is part of NASA’s Small Explorer (SMEX) series of cost-efficient and highly productive space-science satellites, and will be the first satellite to measure the polarization of X-rays sources beyond the solar system.
Polarization is the direction of the vibrating electric field in an electromagnetic wave. An everyday example of polarization is the attenuating effect of some types of sunglasses, which pass light that vibrates in one direction while blocking the rest. Astronomers frequently measure the polarization of radio waves and visible light to get insight into the physics of stars, nebulae, and the interstellar medium, but few measurements have every been made of polarized X-rays from cosmic sources.
“To date, astronomers have measured X-ray polarization from only a single object outside the solar system — the famous Crab Nebula, the luminous cloud that marks the site of an exploded star,” said Jean Swank, a Goddard astrophysicist and the GEMS principal investigator. “We expect that GEMS will detect dozens of sources and really open up this new frontier.”
Black holes will be high on the list of objects for GEMS to observe. The extreme gravitational field near a spinning black hole not only bends the paths of X-rays, it also alters the directions of their electric fields. Polarization measurements can reveal the presence of a black hole and provide astronomers with information on its spin. Fast-moving electrons emit polarized X-rays as they spiral through intense magnetic fields, providing GEMS with the means to explore another aspect of extreme environments.
“Thanks to these effects, GEMS can probe spatial scales far smaller than any telescope can possibly image,” Swank said. Polarized X-rays carry information about the structure of cosmic sources that isn’t available in any other way.
“GEMS will be about 100 times more sensitive to polarization than any previous X-ray observatory, so we’re anticipating many new discoveries,” said Sandra Cauffman, GEMS project manager and the Assistant Director for Flight Projects at Goddard.
Some of the fundamental questions scientists hope GEMS will answer include: Where is the energy released near black holes? Where do the X-ray emissions from pulsars and neutron stars originate? What is the structure of the magnetic fields in supernova remnants?
GEMS will have innovative detectors that efficiently measure X-ray polarization. Using three telescopes, GEMS will detect X-rays with energies between 2,000 and 10,000 electron volts. (For comparison, visible light has energies between 2 and 3 electron volts.) The telescope optics will be based on thin-foil X-ray mirrors developed at Goddard and already proven in the joint Japan/U.S. Suzaku orbital observatory.
GEMS will launch no earlier than 2014 on a mission lasting up to two years. GEMS is expected to cost $105 million, excluding launch vehicle.
Orbital Sciences Corporation in Dulles, Va., will provide the spacecraft bus and mission operations. ATK Space in Goleta, Calif., will build a 4-meter deployable boom that will place the X-ray mirrors at the proper distance from the detectors once GEMS reaches orbit. NASA’s Ames Research Center in Moffett Field, Calif., will partner in the science, provide science data processing software and assist in tracking the spacecraft’s development.
This artist’s conception shows a rogue black hole that has been kicked out from the center of two merging galaxies. The black hole is surrounded by a cluster of stars that were ripped from the galaxies. Credit: Space Telescope Science Institute
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The tight cluster of stars surrounding a supermassive black hole after it has been violently kicked out of a galaxy represents a new kind of astronomical object which may provide telltale clues to how the ejection event occurred. “Hypercompact stellar systems” result when a supermassive black hole is violently ejected from a galaxy, following a merger with another supermassive black hole. The evicted black hole rips stars from the galaxy as it is thrown out. The stars closest to the black hole move in tandem with the massive object and become a permanent record of the velocity at which the kick occurred.
“You can measure how big the kick was by measuring how fast the stars are moving around the black hole,” said David Merritt, professor of physics at the Rochester Institute of Technolyg. “Only stars orbiting faster than the kick velocity remain attached to the black hole after the kick. These stars carry with them a kind of fossil record of the kick, even after the black hole has slowed down. In principle, you can reconstruct the properties of the kick, which is nice because there would be no other way to do it.”
In a paper published in the July 10 issue of The Astrophysical Journal, Merritt and his colleagues discusses the theoretical properties of these objects and suggests that hundreds of these faint star clusters might be detected at optical wavelengths in our immediate cosmic environment. Some of these objects may already have been picked up in astronomical surveys. .
“Finding these objects would be like discovering DNA from a long-extinct species,” said team member Stefanie Komossa, from the Max-Planck-Institut for Extraterrestrial Physics in Germany.
The astronomers say the best place to find hypercompact stellar systems is in cluster of galaxies like the nearby Coma and Virgo clusters. These dense regions of space contain thousands of galaxies that have been merging for a long time. Merging galaxies result in merging black holes, which is a prerequisite for the kicks.
“Even if the black hole gets kicked out of one galaxy, it’s still going to be gravitationally bound to the whole cluster of galaxies,” Merritt says. “The total gravity of all the galaxies is acting on that black hole. If it was ever produced, it’s still going to be there somewhere in that cluster.”
Merritt and his co-authors think that scientists may have already seen hypercompact stellar systems and not realized it. These objects would be easy to mistake for common star systems like globular clusters. The key signature making hypercompact stellar systems unique is a high internal velocity. This is detectable only by measuring the velocities of stars moving around the black hole, a difficult measurement that would require a long time exposure on a large telescope.
From time to time, a hypercompact stellar system will make its presence known in a much more dramatic way, when one of the stars is tidally disrupted by the supermassive black hole. In this case, gravity stretches the star and sucks it into the black hole. The star is torn apart, causing a beacon-like flare that signals a black hole. The possibility of detecting one of these “recoil flares” was first discussed in an August 2008 paper by co-authors Merritt and Komossa.
“The only contact of these floating black holes with the rest of the universe is through their armada of stars,” Merritt says, “with an occasional display of stellar fireworks to signal ‘here we are.’”
If there’s something wrong with it, the LHC might have the power to damage itself, but it can’t do anything to the Earth, or the Universe in general.
There are two worries that people have: black holes and strange matter.
One of the goals of the Large Hadron Collider is to simulate microscopic black holes that might have been generated in the first few moments of the Big Bang. Some people are worried that these artificial black holes might get loose, and then consume the Earth from within, eventually moving on to destroy the Solar System.
The physicists are confident that any black holes they create will evaporate almost instantaneously into a shower of particles. In fact, the theories that predict that black holes can be created also predicts that black holes will evaporate. The two concepts go hand in hand.
The other worry is that the Large Hadron Collider will create a theorized material called strangelets. This “strange matter” would then be able to infect other matter, turning the entire planet into a blog of strange matter.
This strange matter is completely theoretical, and once again, the same theories that say it might be produced in the Large Hadron Collider also rule out any risks from it.
One of the most important considerations is the fact that the Moon is struck by high energy cosmic rays that dwarf the power of the Large Hadron Collider. They were likely blasted out of the environment around a supermassive black hole.
These have been raining down on the Moon for billions of years, and so far, it hasn’t turned into a black hole or strange matter.
