In any rogues' gallery of astronomical evildoers, black holes would have to be the No. 1 nightmare.
But are the nightmares real? A few doubters are beginning to emerge. If they're right, then black holes may be just the latest grand illusion of cosmology. They may be destined to go the way of other scientific Dodo birds like the "celestial spheres" of medieval cosmology and the vaporous, cosmos- pervading "aether" of Victorian astrophysics.
According to theory, a black hole is a collapsed star with incredibly strong gravitational pull, so strong that nothing can escape it, not even light - hence the adjective "black."
The result is a kind of cosmic vacuum cleaner. A black hole would swallow clouds of stars like a whale gulping down plankton. Black holes would literally be points of no return; fall into one, and you'd be trapped forever. If Earth bumped into a black hole, it would be goodbye Earth.
There is no indubitable, smoking-gun evidence for black holes. Yet in recent decades, most astrophysicists (not to mention TV and Hollywood screenwriters) have tentatively accepted their reality. Their reasoning is understandable: For one thing, a widespread interpretation of Albert Einstein's equations implies that collapsing stars should form black holes.
Also, black holes offer an easy way to explain the extraordinary brilliance of "quasars," distant astronomical objects discovered in the early 1960s. Nowadays, most astronomers think quasars are galaxies accompanied by black holes. Falling into the black hole, galactic matter explodes and emits a "death cry" of radiation so bright that (they think) it is visible to us, billions of light years away.
But black holes pose troubling paradoxes. For one thing, the hypothesized center of a black hole is a "singularity," an infinitesimal point in space that, paradoxically, contains an infinite amount of energy and pressure.
Infinite? That sounded absurd to pioneers of modern physics like Albert Einstein. He didn't buy the idea that his equations inevitably pointed toward the reality of black holes. Rather, he preferred to believe that these strange objects don't exist at all.
Another, even spookier problem involves "entropy." That's a technical term that expresses (among other things) how much "information" is encoded by a physical object or process. Calculations show that a black hole would contain astoundingly more "entropy" than the matter that fell into it (perhaps sextillion times more - that's one followed by 21 zeros). This violates a cardinal physical principle, according to which "information can never really be created or destroyed," said Emil Mottola of Los Alamos National Laboratory.
Now, hoping to evade these wacky cosmic paradoxes, two theoretical physicists have proposed that Einstein was right - that black holes are as mythical as unicorns, or, at least, that stellar collapse would form some kind of object other than black holes as they are popularly envisioned.
In a still-unpublished scientific paper, Pawel Mazur of the University of South Carolina and Mottola propose an alternative to black holes. They would replace black holes with a different (and perhaps equally weird) type of object: the "gravastar," a kind of stellar-scale variation on quantum mechanical theory.
Unlike a black hole, a gravastar would not collapse all the way into a singularity, instead stabilizing somewhere en route. Hence, a gravastar wouldn't generate all those paradoxes that make physicists lose sleep.
Even so, a gravastar would be a mighty weird object. As Mazur and Mottola explain, a gravastar wouldn't be matter as we normally conceive of it. Rather, it would be a star-size agglomeration of "wavelike" substance - but not substance in the everyday sense of solid things like dust and rocks. Rather, this substance would be the underlying "space-time" fabric of the universe, which, as Einstein showed long ago, "curves" (like a fabric) around masses to generate gravity "wells" into which other masses fall (the steeper the curve, the stronger the gravity. The "well" around a little world like the moon is a mere dent in the fabric of space-time compared with the cavernous well formed by the sun).
A gravastar would be a star-size analogy to a "Bose-Einstein condensate," a subject of much scientific excitement in recent years. To understand Bose- Einstein condensates, one must know a little about quantum physics. Laypeople tend to think of subatomic particles as something akin to tiny marbles, objects that are hard and well defined. Quantum physics presents a stranger notion, in which particles are fuzzy in time and space. Generally, one cannot simultaneously determine, with precision, both position and momentum at the quantum level. (This is the eerie "uncertainty principle" introduced by Werner Heisenberg in 1927.)
The location of a particle constantly varies according to a statistical pattern - one moment it's here, another moment it's there - like the peripatetic gopher poking its head out of holes in the movie "Caddyshack." A particle's position at any given moment can be predicted only statistically via a mathematical concept known as a "wave function."
In recent years, scientists developed a new way to study the wavelike nature of particles. They freeze a bunch of atoms to a temperature close to absolute zero. At that chilly point the atoms' collective wave functions expand and merge into one huge atom or Bose-Einstein condensate that is "describable by one big wave function," said physicist Pisin Chen of Stanford University. (He is not connected with Mazur and Mottola's work.)
To put it somewhat simplistically, a gravastar would be a star-size version of a Bose-Einstein condensate. The difference is that a laboratory Bose- Einstein condensate forms from atoms, whereas Mazur and Mottola's star-size "condensate" would form from space-time - the unified, four-dimensional fabric of space and time, as in Einstein's theories.
Space-time isn't just an abstraction: It's as real as, say, toffee, and, like toffee, can be twisted to form the gravitational "wells" into which masses fall. Mathematicians treat the four dimensions of space-time collectively as a single curved surface. Inside a black hole, space-time curvature would become infinite.
In black hole theory, the stellar mass would collapse into a point or singularity of infinite density, pressure and space-time curvature. But in Mazur and Mottola's scheme, the stellar collapse would cease at a certain point, a thin shell of pure gravitational energy forming around the imploding star.
Inside the shell, space-time undergoes a "phase transition," akin to that of ice turning into water or water into vapor. And so the gravastar's interior forms a gravitational version of a Bose-Einstein condensate.
According to Mazur and Mottola's calculations, the "condensate" inside the shell would exert an outward force. This pressure would prevent the shell from collapsing further. Hence, no black hole would form, "and Einstein's equations would be satisfied without any singularity (forming)," Mottola said.