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Credit: NASA, ESA; D. Coe; J. Anderson; R. van der Marel (STScI)

Topics: Astrophysics, Black Holes, Cosmology, General Relativity, Quantum Gravity

New research reveals a possible mechanism allowing “black stars” and “gravastars” to exist

When giant stars die, they don’t just fade away. Instead they collapse in on themselves, leaving behind a compressed stellar remnant, usually a city-size, superdense ball of neutrons appropriately called a neutron star. In extreme cases, however, most theorists believe an expiring giant star will form a black hole—a pointlike “singularity” with effectively infinite density and a gravitational field so powerful that not even light, the fastest thing in the universe, can escape once falling in. Now a new study is reinvigorating an alternate idea, that objects with names such as “black stars,” or “gravastars,” might exist midway between neutron stars and black holes. If real, these exotic stellar corpses should appear nearly identical to black holes save in one key way—they could not irretrievably swallow light.

There are good reasons to seek such alternatives, because black holes raise a host of theoretical problems. For instance, their singularities are supposedly hidden by invisible boundaries known as event horizons. Throw something into a black hole, and once it passes the event horizon it should be gone—forever—with no hope whatsoever of return. But such profound annihilation clashes with other long-cherished laws of physics that suggest the destruction of information is impossible, including information encoded within anything falling into black holes.

Conceived and developed across the past two decades, in part to sidestep such conundrums, models of black stars and gravastars postulate these objects would lack singularities and event horizons. But questions have lingered as to whether such objects could actually form—and remain stable after they did. New research from theoretical physicist Raúl Carballo-Rubio at the International School for Advanced Studies in Italy provides a novel mechanism that might allow black stars and gravastars to exist.

Carballo-Rubio investigated a strange phenomenon known as quantum vacuum polarization. Quantum physics, the best description yet of how all known subatomic particles behave, suggests reality is fuzzy, limiting how precisely one can know the properties of the most basic units of matter—for instance, one can never absolutely know a particle's position and momentum at the same time. One strange consequence of this uncertainty is that a vacuum is never completely empty but instead foams with so-called “virtual particles” that continuously fluctuate into and out of existence.

Black Hole Pretenders Could Really Be Bizarre Quantum Stars, Charles Q. Choi, Scientific American

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