Black holes have massive, ultra-dense collapsed stars at their centers. The gravitational force surrounding the star is so strong that nothing -- not even light -- can escape. The point of no return for light entering a black hole, the point at which it is too close to escape the gravitational pull, is called the "event horizon."
Dumb holes arise when fluids flowing faster than the speed of sound form regions that trap sound waves. They too have a surface of no return -- the "acoustic horizon". While black holes remain interstellar objects, researchers can create dumb holes in a laboratory.
"By reproducing the most important qualities of black holes in a fluid system, some of the predictions of quantum gravity can be tested and some paradoxes of the theory understood," University of Maryland physicist Stefano Liberati told United Press International from College Park,Md. The chief paradox is so-called "Hawking radiation."
In a discovery that stunned the scientific community, renowned physicist Stephen Hawking found that black holes slowly evaporate by emitting, or radiating, quantum particles. "Hawking radiation" presented a puzzle: how can certain kinds of sub-atomic particles somehow escape the inescapably strong gravitational fields that trap even light.
"In a black hole, Hawking radiation is a process where the gravity near the event horizon pulls apart particle-antiparticle pairs that exist in a deep-space vacuum," Liberati said. The black hole captures one of the partners while the other escapes to freedom, making it appear as though the black hole is radiating particles.
Researchers have long hoped Hawking radiation would yield clues about the quantum forces that hold atoms together and the gravitational forces that guide planets and collapse stars.
"We are interested in black hole analogues because we would like to find the analog of the Hawking radiation process," Liberati said.
Dumb holes that trap sound waves may yield experimental evidence used to understand quantum gravity because these acoustic black holes exhibit all the characteristics -- paradoxes included -- of their light-wave brethren.
"It turns out that the equations for sound waves in a continuous fluid are exactly the same as the equations for certain kinds of radiation in a gravitational field," said physicist William Unruh, from the Canadian Institute for Physics and Astronom, in Vancouver, British Columbia. "The surface at which the fluid exceeds the velocity of sound acts in exactly the same way as the horizon of a black hole, including the Hawking effect."
Using a special kind of matter called a "Bose-Einstein condensate," Liberati's team hopes to recreate the most important features of black holes using sound.
"The Bose-Einstein condensate is a peculiar state of matter which is realized at very low temperatures," Liberati said. "A collective vibration of atoms in a condensed matter system such as this Bose-Einstein condensate forms a wave composed of quantum particles called 'phonons,' just as light is a wave composed of particles called photons."
A phonon is "the equivalent of the photon for sound," said Unruh. "Phonons are the particle you get when you treat sound as a quantum field," just as photons are quantised, or particulate, light.
Supersonic flow of a Bose-Einstein condensate will form a dumb hole that inescapably traps phonons the same way gravitational collapse forms a black hole that traps photons, Liberati told UPI.
"Phonons travel at the speed of sound, so if we think of a flow where the fluid speed is increasing, then once it reaches and overtakes the speed of sound, it generates a region where phonons cannot escape," Liberati explained. "The phonons would have to climb up the flow-as salmons against a stream-but the flow is too fast for them, so the region of supersonic flow is a 'trap.' "
Hawking radiation can occur in both dumb holes and black holes. "We expect that at the acoustic horizon phonon pairs are generated and pulled apart," Liberati said. "The phonons falling in the supersonic region are lost while their partners are seen as a radiation of sound waves emitted from the dumb hole."
Using dumb holes to study black holes is an example of a common technique-substituting a well -- understood system for its poorly -- understood counterpart.
"We understand the physics of fluids completely, unlike our understanding of quantum gravity," Unruh said.
(Reported by Mike Martin in Columbia, Mo.)
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