Black holes might not exist - or at least not as scientists have imagined, cloaked by an impenetrable "event horizon". A controversial new calculation could abolish the horizon, and so solve a troubling paradox in physics.
The event horizon is supposed to mark a boundary beyond which nothing can escape a black hole's gravity. According to the general theory of relativity, even light is trapped inside the horizon, and no information about what fell into the hole can ever escape. Information seems to have fallen out of the universe.
That contradicts the equations of quantum mechanics, which always preserve information. How to resolve this conflict?
One possibility researchers have proposed in the past is that the information does leak back out again slowly. It may be encoded in a hypothetical flow of particles called Hawking radiation, which is thought to result from the black holes' event horizons messing with the quantum froth that is ever-present in space.
But other researchers argue the information may never have been cut off in the first place. Tanmay Vachaspati and his colleagues at Case Western Reserve University in Cleveland, Ohio, US, have tried to calculate what happens as a black hole is forming. Using an unusual mathematical approach called the functional Schrodinger equation, they follow a sphere of stuff as it collapses inwards, and predict what a distant observer would see.
They find that the gravity of the collapsing mass starts to disrupt the quantum vacuum, generating what they call "pre-Hawking" radiation. Losing that radiation reduces the total mass-energy of the object - so that it never gets dense enough to form an event horizon and a true black hole. "There are no such things", Vachaspati told New Scientist. "There are only stars going toward being a black hole but not getting there."
Dark and dense
These so-called "black stars" would look very much like black holes, says Vachaswati. From the point of view of a distant observer, gravity distorts the apparent flow of time so that matter falling inwards slows down. As it gets close to where the horizon would be, the matter fades, its light stretched to such long wavelengths by the dark object's gravity that it would be nearly impossible to detect.
But because the pre-Hawking radiation prevents the formation of a black hole with a true event horizon, the matter never quite fades entirely. As nothing is cut off from the rest of the universe, there is no information paradox.
The idea faces firm opposition from other theoretical physicists, however. "I strongly disagree," says Nobel laureate Gerard 't Hooft of Utrecht University in the Netherlands. "The process he describes can in no way produce enough radiation to make a black hole disappear as quickly as he is suggesting." The horizon forms long before the hole can evaporate, 't Hooft told New Scientist.
Lab test
Steve Giddings of the University of California in Santa Barbara, US, is also sceptical. "Well-understood findings apparently conflict with their picture," he told New Scientist. "To my knowledge, there hasn't been an attempt to understand how they are getting results that differ from these calculations, which would be an important step to understanding if this is a solid result."
There could be a way to test the new theory. The Large Hadron Collider being constructed at CERN in Geneva might just be capable of making microscopic black holes - or, if Vachaspati is right, black stars. Unlike the large, long-lived black holes in space, these microscopic objects would evaporate fast. The spread of energies in their radiation might reveal whether or not an event horizon forms.
Alternatively, colliding black stars in space might reveal themselves, as Vachaspati says they would churn out not only gravitational waves (like colliding black holes) but also gamma rays. He suggests that they could be responsible for some of the gamma-ray bursts seen by astronomers.
A black hole is the name for a small body of matter floating in the cosmos that is incredibly dense. Like the entire sun being compressed into a ball the size of a city. It is black because the density creates such a strong gravitational field that light itself is pulled back into itself.
If light can’t get out, we can’t possibly see it to prove it exists. However, astronomers observe the outside affects of a black hole and determine, though they can’t point to it, exactly where the black hole must be.
Just like earth and our neighboring planets orbit around the sun (being the most dense, and thus our gravity boss), astronomers can see stars orbiting around a spot in space. Something must be there at the center creating the orbit, and the best guess is that the something is a black hole.
Another way to spot a black hole is from bursts of x-rays. As a star gets too close to a black hole it begins moving faster and faster, heating up in the process. When the gas of the star reaches such temperatures it begins to emit x-rays. Eventually, as the star spirals ever closer to the black hole it too will become invisible, but outside of the “horizon” of the black hole (the tipping point of the gravitational field, a point of no return) we can observe these strange happenings. The clues add up to the presence of a black hole.
Proving the existence of black holes seems to be a case of “if it walks like a duck and talks like a duck…”, which leaves room for doubt as to whether or not there is a big duck at the center of our galaxy. Let’s take a look at the other side of the argument.
The anti-black-hole debate centers on one sticky issue, “The Information Paradox”. All the details are well beyond my understanding, but it boils down to this. You’ve perhaps heard of the law of conservation of energy… energy cannot be created or destroyed, but merely changed from one form to another. (A car’s forward momentum, through the friction of the brake pads, turns to heat dispersed into the air, etc.)
Well, there is a theory of quantum physics that says there must be a conservation of information. The information in question is sort of like DNA for particles at the smallest level. If matter was to collapse into a black hole, unable to escape, this information would no longer be accessible to the universe. To theoretical physicists, that’s a big problem.
Some of the proposed solutions involve fancy words like “11-dimensional supergravity” and other string-theory brain-busters that are well beyond the scope of the LSNED blog. If you will allow me to over-summarize, conveniently avoiding 500 more words of clumsy explanation, the leading anti-black-hole theory (the Holographic principle) is essentially arguing that a black hole is a mirage of sorts. Rather than a true physical black hole that gobbles up matter never to be seen again, it is a cosmic illusion that just appears that way to our simple three-dimensional brains.
Black Holes
Don't let the name fool you: a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape. In recent years, NASA instruments have painted a new picture of these strange objects that are, to many, the most fascinating objects in space.
Although the term was not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.
Scientists can't directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby. If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.
One Star's End is a Black Hole's Beginning
Most black holes form from the remnants of a large star that dies in a supernova explosion. (Smaller stars become dense neutron stars, which are not massive enough to trap light.) If the total mass of the star is large enough (about three times the mass of the Sun), it can be proven theoretically that no force can keep the star from collapsing under the influence of gravity. However, as the star collapses, a strange thing occurs. As the surface of the star nears an imaginary surface called the "event horizon," time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still, and the star can collapse no more - it is a frozen collapsing object.
Even bigger black holes can result from stellar collisions. Soon after its launch in December 2004, NASA's Swift telescope observed the powerful, fleeting flashes of light known as gamma ray bursts. Chandra and NASA's Hubble Space Telescope later collected data from the event's "afterglow," and together the observations led astronomers to conclude that the powerful explosions can result when a black hole and a neutron star collide, producing another black hole.
Babies and Giants
Although the basic formation process is understood, one perennial mystery in the science of black holes is that they appear to exist on two radically different size scales. On the one end, there are the countless black holes that are the remnants of massive stars. Peppered throughout the Universe, these "stellar mass" black holes are generally 10 to 24 times as massive as the Sun. Astronomers spot them when another star draws near enough for some of the matter surrounding it to be snared by the black hole's gravity, churning out x-rays in the process. Most stellar black holes, however, lead isolated lives and are impossible to detect. Judging from the number of stars large enough to produce such black holes, however, scientists estimate that there are as many as ten million to a billion such black holes in the Milky Way alone.
On the other end of the size spectrum are the giants known as "supermassive" black holes, which are millions, if not billions, of times as massive as the Sun. Astronomers believe that supermassive black holes lie at the center of virtually all large galaxies, even our own Milky Way. Astronomers can detect them by watching for their effects on nearby stars and gas.
Historically, astronomers have long believed that no mid-sized black holes exist. However, recent evidence evidence from Chandra, XMM-Newton and Hubble strengthens the case that mid-size black holes do exist. One possible mechanism for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the buildup of extremely massive stars, which then collapse to form intermediate-mass black holes. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole.
Source: http://nasascience.nasa.gov/astrophysics/focus-areas/black-holes/, http://www.newscientist.com/article/dn12089-do-black-holes-really-exist.html and http://lsned.com/facts/black-holes/
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