Why do black holes exist

Rapidly moving stars The gravitational field of a black hole tugs on the stars in its vicinity. A super-massive black hole will make whole swarms of stars whip around as they fall under its influence. By following the motions of the orbiting stars, astronomers can deduce the location, and size, of the central black hole they cannot see.

Here are three lines of evidence that black-hole hunters look for: A blaze of X-rays Matter that comes too close to a black hole — matter such as gas and dust, or even a whole star — is drawn towards the hole. From these and other lines of evidence, astronomers are convinced that black holes are real, and that they play an important role in the universe. What secrets do black holes promise to reveal? What are Black Holes? Do Black Holes really exist?

What are we trying to find out? 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 , 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.

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, are very difficult 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. 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.

Now Hawking is suggesting a resolution to the paradox: Black holes do not possess event horizons after all, so they do not destroy information. Instead, Hawking proposes that black holes possess "apparent horizons" that only temporarily entrap matter and energy that can eventually reemerge as radiation.

This outgoing radiation possesses all the original information about what fell into the black hole, although in radically different form. Since the outgoing information is scrambled, Hawking writes, there's no practical way to reconstruct anything that fell in based on what comes out. The scrambling occurs because the apparent horizon is chaotic in nature, kind of like weather on Earth. We can't reconstruct what an object that fell into a black hole was like based on information leaking from it, Hawking writes, just as "one can't predict the weather more than a few days in advance.

Hawking's reasoning against event horizons also seems to eliminate so-called firewalls, which are searing zones of intense radiation that some scientists recently and controversially suggested may exist at or near event horizons.

To grasp the significance of this revision, it helps to know that Hawking revealed decades ago that black holes are not perfectly "black. Over time, generating this so-called Hawking radiation makes black holes lose mass—or even completely evaporate. According to this theory, the pairs of particles created around black holes should be entangled with each other. This means the behavior of each pair's particles is connected, regardless of distance.

One member of each pair falls into the black hole while the other escapes. But recent analyses suggest that each particle leaving a black hole must also be entangled with every outgoing particle that has already left. This runs head-on into a well-tested principle of quantum physics stating that entanglement is always "monogamous," meaning two particles, and only two, are paired from the time of their creation.

Since no particle can have two kinds of entanglement at the same time—one pairing it with another particle at the time of its origin, and one pairing it with all other particles that have left a black hole—one of those entanglements theoretically must get uncoupled, releasing vast amounts of energy and generating a firewall.

Firewalls obey quantum physics, solving the conundrum black holes pose regarding entanglement. But they pose another problem by contradicting Einstein's well-tested "equivalence principle," which implies that crossing a black hole's event horizon should be an unremarkable event.

A hypothetical astronaut passing across an event horizon would not even be aware of the transit. If there were a firewall, however, the astronaut would be instantly incinerated. Since that violates Einstein's principle, Hawking and others have sought to prove that firewalls are impossible.

Although quantum physicist Seth Lloyd of the Massachusetts Institute of Technology felt Hawking's idea was a good way to avoid firewalls, he said it doesn't really address the problems that firewalls raise. Theoretical physicist Leonard Susskind at Stanford University in California, who also did not take part in Hawking's research, suggests there may be another solution to the conundrums that black holes pose. For instance, work by Susskind and his colleague Juan Maldacena hint that entanglement might be linked to wormholes: shortcuts that can in theory connect distant points in space and time.

This line of thought might serve as the foundation for research that could solve the firewall controversy, Susskind said.