The most powerful particle accelerator in the world, the Large Hadron Collider, has not yet created any black holes as some have feared, researchers say.
Even if this atom smasher does create black holes, there is no danger of those black holes destroying Earth, added scientists of the new study published online March 5 in the journal Physical Review Letters.
Black holes get their names from their gravitational fields, which are so fiendishly powerful that nothing can escape, not even light. Supermassive black holes millions to billions of times the mass of the sun are thought to lurk at the heart of virtually every galaxy. [The Top 10 Ways to Destroy Earth]
Quantum black holes
A black hole is normally thought to form when the remains of a dead star collapses under its own gravity, scrunching all that mass together. However, some researchers have suggested that "quantum black holes" — ones smaller than a proton — can exist if extra, hidden dimensions of reality also exist.
In theories aiming to describe how the cosmos works in its entirety, scientists have proposed that extra dimensions might exist. Currently, researchers have two disparate theories — general relativity and quantum mechanics — which, respectively, can mostly explain the nature of gravity and the behavior of the particles seen around the universe.
"Quantum theories of almost all of the forces around us, electricity and magnetism as well as the weak and strong nuclear forces, have been developed over the last 70 years and tested with high precision," said study author David Strom, a particle physicist at the University of Oregon at Eugene, who is a member of the ATLAS collaboration at the Large Hadron Collider. "It has been much more difficult to make progress on both theoretical and experimental aspects of a quantum theory of gravity," Strom said.
A number of quantum gravity theories propose that extra dimensions are folded up into sizes ranging from smaller than a proton to as big as a fraction of a millimeter. At distances comparable to the sizes of these extra dimensions, these models suggest that gravity could become far stronger than normal. If so, a powerful enough particle accelerator might concentrate enough mass and energy together to generate black holes. [The Strangest Black Holes in the Universe]
When the most powerful atom smasher in the world, the Large Hadron Collider (LHC), was coming online, scientists wondered if it might become a "black hole factory," generating a black hole as often as every second. Atomic nuclei — either those of hydrogen or lead — zip at high speeds around the 17-mile (27-kilometer) circular particle accelerator before colliding into targets at near the speed of light.
No chance of destroying Earth
The potential for a black hole factory raised fears that a stray black hole could devour our planet whole. However, any quantum black holes the Large Hadron Collider might generate would pose no risk to Earth.
To start with, theoretical physicist Stephen Hawking calculated that all black holes should lose mass by emitting radiation, and that quantum black holes should lose more mass than they absorb, evaporating within a billionth of a trillionth of a trillionth of a second, before they could devour any significant amount of matter.
"If we detect quantum black holes, they will be among the most quickly decaying objects ever produced at an accelerator," Strom said. "They pose no threat to the Earth."
Even if Hawking is wrong, nearly all of the quantum black holes the particle accelerator would generate would be traveling fast enough from it to rip free of Earth's gravity. Furthermore, even if any quantum black holes do get trapped within the Earth, they are so tiny that it would take each one more than the current age of the universe to destroy even a milligram of Earth matter.
No quantum black holes seen
In any case, researchers now reveal they have not yet detected any sign of quantum black holes being created by the LHC.
Scientists running the particle accelerator's ATLAS detector — one of the two detectors involved in the discovery of the Higgs boson — performed an extensive search for quantum black holes by probing all the data from the atom smasher's most powerful collisions during 2012. Had quantum black holes been created, they would have decayed into pairs of particles and jets of radiation that ATLAS could detect.
These findings suggest that quantum black holes smaller than 5 trillion electron-volts do not exist — that is, about 5,330 times the mass of a proton.
"Our findings show that if these quantum black holes exist, they must be beyond the current energy reach of the LHC," Strom said.
In 2015, the LHC will restart and be capable of producing nearly twice the energy as it has in previous runs. This will help researchers see if heavier quantum black holes might exist, as well as extra dimensions of reality.
"We will be looking for quantum black holes from the first day that the LHC turns on at higher energy, early in 2015," Strom said.
Even if the production of quantum black holes is ultimately out of the LHC's reach, "there may be other, subtle effects of extra dimension theories that we can find in our data," Strom said. "It may also be that there are no extra dimensions, and that we need to seek different theories that explain why the force of gravity appears to be so different from the other forces that we observe in nature."
Some of these competing theories, such as supersymmetry, "predict particles that are candidates for the dark matter that we know forms most of the mass of our galaxy and that can also be produced at the LHC," Strom said. "It may be that we have already produced many of the these dark matter particles in our experiment, but we have not been clever enough to find them."
When the LHC upgrades to hurl particles with more energy and throw more particles at targets per run, "it should be easy to find these particles," Strom added.
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