The mystery surrounding black holes has thwarted the average student of the phenomenon. This effort is an attempt to demystify common errors of understanding and to also present new discoveries. For the most part, black holes have been called frozen stars, simply because they form from stars that have died. Stars die when they run out of fuel. When they are no longer able to create nuclear fusion in their core, a star will collapse in on itself under the force of gravity. If the star is massive enough, this inward pull will condense the mass so strongly that even atoms cannot hold their structure, with protons and electrons disintegrating and all the matter collapsing into a tiny ball. If anything comes within a certain range of a black hole, called its “event horizon,” it cannot escape the black hole’s gravitational pull. Even light is no exception to this rule; thus, black holes are invisible.
Now we hear the term “Black Hole” tossed around as though everyone knows exactly what they are, but upon further contemplation of some of the most recent discoveries the term may perhaps be deeply misunderstood.
The average layman is usually confused when it comes to understanding Black Holes. Most believe that once an object is captured by these collapsed stars, that it’s impossible for them to escape. While this is presently true on a macroscopic scale, theoretical physicist Edward Witten is challenging its microscopic probability. In order to grasp Witten’s claim, it’s important to have a comprehensive and unambiguous sense of what Black Holes are.
For one thing a Black Hole isn’t really a hole – it is an object that is so highly compressed that its escape velocity exceeds the speed of light. What this means in laymen’s terms is that nothing can escape it, or perhaps more specifically, nothing can escape its gravitational pull.
A clearer understanding of this idea might be best expressed by our common understanding of earth’s gravitational pull. No object can escape beyond Earth’s gravitational field unless the speed of that object exceeds Earth’s escape velocity.
For instance, if NASA launches a rocket that is unable to reach Earth’s escape velocity, measured in terms of speed, it will fall back to Earth. Thus, Black Holes are best understood in terms of their escape velocity.
The escape velocity from the surface of an astronomical object depends on the strength of the gravitational field of the object’s surface. If you want to know the formula for calculating escape velocity you need to know the object’s total mass and radius. Black Holes are formed when a star collapses to a smaller size while keeping the same mass. Since the consequence of collapse gives the star a higher density, its escape velocity increases. Once a star collapses to an extreme degree, the escape velocity from the surface eventually reaches the speed of light. At this point, the star becomes a black hole.
According to Einstein’s special relativity theory, the speed of light is the ultimate speed limit in the universe. Nothing can travel faster than the speed of light. Light is, by definition, massless and therefore travels at light speed. Any particle or object having mass can travel close to, but never reach or exceed the speed of light. Hence, when a star collapses to the point that its escape velocity exceeds the speed of light, nothing can escape? Wrong; nothing can escape its surface.
A black hole is therefore simply a star, or other object, that has collapsed to the point where its escape velocity exceeds the speed of light. Nothing, not even light, can escape from a black hole. Anything that falls into a black hole is, in theory, trapped forever.
However, Edward Witten says; not so fast. The stated fact that nothing can escape from a Black Hole has led to a very common misunderstanding of them. My argument here attempts sets the record straight, and reiterate that nothing can escape its surface. In other words, an object has to be close enough to be impacted by its gravitational pull; any object at a distance is protected by its own gravitational pull that tugs in the opposite direction.
Nevertheless, Witten has moved beyond this theory or should I say, he is attempting to disprove it by demonstrating the theory’s incompatibility with the theory of quantum mechanics.
Witten argues that the basic theory, which suggests nothing can escape a Black Hole, contradicts the laws of quantum mechanics, which governs the universe’s tiniest elements.
“What you get from classical general relativity, and also what everyone understands about a black hole, is that it can absorb anything that comes near, but it can’t emit anything. But quantum mechanics doesn’t allow such an object to exist,” Witten said in this week’s Science podcast.
In quantum mechanics, if a reaction is possible, the opposite reaction is also possible, Witten explained. Processes should be reversible. Thus, if a person can be swallowed by a black hole to create a slightly heavier black hole, a heavy black hole should be able to spit out a person and become a slightly lighter black hole. Yet nothing is supposed to escape from black holes.
To solve the dilemma, physicists looked to the idea of entropy, a measurement of disorder or randomness. The laws of thermodynamics state that in the macroscopic world, it’s impossible to reduce the entropy of the universe — it can only increase. If a person were to fall into a black hole, entropy would increase. If the person were to pop back out of it, the universal entropy tally would go down. For the same reason, water can spill out of a cup onto the floor, but it won’t flow from the floor into a cup.
This principle seems to explain why the process of matter falling into a black hole cannot be reversed, yet it only applies on a macroscopic level.
Physicist Stephen Hawking famously realized that on the microscopic, quantum mechanical level, things can escape from black holes. He predicted that black holes will spontaneously emit particles in a process he dubbed Hawking radiation. Thus, quantum mechanics refuted one of the basic tenets of black holes: that nothing can escape.
“Although a black hole will never emit an astronaut or a table or a chair, in practice, it can definitely emit an ordinary elementary particle or an atom,” Witten explained.
However, scientists have yet to observe the Hawking radiation.
“Unfortunately, the usual astrophysical black holes, formed from stellar collapse or in the centers of galaxies, are much too big and too far away for their microscopic details to be relevant,” Witten wrote.
That said, an unexpected discovery by an international team of astronomers is forcing scientists to rethink their understanding of the environment in globular star clusters, tight-knit collections containing hundreds of thousands of stars.
The astronomers, including Dr Tom Maccarone from the University of Southampton, used the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) in New Mexico, USA to study a globular cluster called Messier 22 (M22), a group of stars more than 10,000 light-years from Earth. They hoped to find evidence of a rare type of black hole in the cluster’s centre called an intermediate-mass black hole, which is more massive than those larger than the Sun’s mass, but smaller than the supermassive black holes found at the cores of galaxies.
However, they found something very surprising – two smaller black holes, which is unusual because most theorists say there should be at most one black hole in the cluster.
Dr Tom Maccarone, a reader in Astronomy at the University of Southampton, developed the methodology for the study and the surprising result ties in closely to previous research by Tom. He says:
“I had actually suggested several years ago that there were probably black holes lurking among the X-ray sources that had already been seen in globular clusters, and that the one way to pick the black holes sucking gas in, apart from other types of faint X-ray sources, would be to look for the radio emission, but I didn’t expect that this particular cluster would be the best place to look. It was still incredibly exciting to see this result and I’m optimistic that we will find more of these objects in other clusters in the future.”
Black holes, concentrations of mass so dense that not even light can escape them, are left over after very massive stars have exploded as supernovae. In a globular cluster, many of these stellar-mass black holes probably were produced early in the cluster’s 12-billion-year history as massive stars rapidly passed through their life cycles.
Simulations have indicated that these black holes would fall toward the centre of the cluster and then begin a violent gravitational dance with each other, in which all of them, or perhaps all but a single one, would be thrown completely out of the cluster.
“There is supposed to be only one survivor possible,” says Jay Strader of Michigan State University and the Harvard-Smithsonian Center for Astrophysics. “Finding two black holes, instead of one, in this globular cluster definitely changes the picture,” he says.
The astronomers suggest some possible explanations. First, the black holes themselves may gradually work to puff up the central parts of the cluster, reducing the density and thus the rate at which black holes eject each other through their gravitational dance. Alternatively, the cluster may not be as far along in the process of contracting as previously thought, again reducing the density of the core.
The two black holes discovered with the VLA were the first stellar-mass black holes to be found in any globular cluster in our own Milky Way galaxy, and also are the first found by radio, instead of X-ray, observations. Future VLA observations will help us learn about the ultimate fate of black holes in globular clusters.
In this effort to demystify black holes, it should be noted that some physicists have suggested that black holes could create tunnels joining two points in space-time, called wormholes. If wormholes exist, they could create time warps, potentially allowing for time travel or shortcuts across the universe. However, no evidence for wormholes has so far been discovered.
Another as-yet-unproven theory is that tiny “miniature black holes” might have formed after the Big Bang thought to have started our universe 13.7 billion years ago. During this explosion, some parts of space may have expanded so rapidly that some matter was squeezed into miniature black holes. These objects would contain much less mass than the sun, but would still be extremely dense.
Again these are theories that tend to confuse the student and should be discarded by the serious investigator of the phenomenon.