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.
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.
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!”
Our Milky Way’s black hole is quiet – too quiet – some astronomers might say. But according to a team of Japanese astronomers, the supermassive black hole at the heart of our galaxy might be just as active as those in other galaxies, it’s just taking a little break. Their evidence? The echoes from a massive outburst that occurred 300 years ago.
The astronomers found evidence of the outburst using ESA’s XMM-Newton space telescope, as well as NASA and Japanese X-ray satellites. And it helps solve the mystery about why the Milky Way’s black hole is so quiet. Even though it contains 4 million times the mass of our Sun, it emits a fraction of the radiation coming from other galactic black holes.
“We have wondered why the Milky Wayâ€™s black hole appears to be a slumbering giant,” says team leader Tatsuya Inui of Kyoto University in Japan. “But now we realize that the black hole was far more active in the past. Perhaps itâ€™s just resting after a major outburst.”
The team gathered their observations from 1994 to 2005. They watched how clouds of gas near the central black hole brightened and dimmed in X-ray light as pulses of radiation swept past. These are echoes, visible long after the black hole has gone quiet again.
One large gas cloud is known as Sagittarius B2, and it’s located 300 light-years away from the central black hole. In other words, radiation reflecting off of Sagittarius B2 must have come from the black hole 300 years previously.
By watching the region for more than 10 years, the astronomers were able to watch an event wash across the cloud. Approximately 300 years ago, the black hole unleashed a flare that made it a million times brighter than it is today.
It’s hard to explain how the black hole could vary in its radiation output so greatly. It’s possible that a supernova in the region plowed gas and dust into the vicinity of the black hole. This led to a temporary feeding frenzy that awoke the black hole and produced the great flare.