WASHINGTON (ISNS) ? Astrophysicsdeals mostly with thingsthat are so distant ? thousands or billions of light years away ?that wecan't ever hope to see them up close. But clever scientists can do thenextbest thing to making a light-year journey; they can recreate some ofthecelestial occurrences in a lab. In effect, they can bring parts of thesky downhere to earth.
That's what physicists in Italy havedone. Using nothingmore than lasers, a sample of pure glass, and sensitive detectors theyhave createda miniature environment that mimics theconditions of a black hole.
Blackholes and pure glass
Black holes, whose existence is nowuniversally accepted byastronomers, are thought to be the remnant of celestial objects such asstarsor galaxies that have collapsed under the strength of their owngravity.Encapsulating a volume of space much smaller than the original object,a blackhole bends space and time so drastically that nothing can escape, notevenlight, once it passes inside a hypothetical boundary known as the"eventhorizon."
Something analogous to thegravitational warping of spacecan be achieved in terms of light waves. A collaboration of physicistsfromseveral Italian institutions sent laser light into a crystal of veryclearglass. Normally the light passes right through. However, if theintensity ofthe light passes a certain level, then the atoms that make up the glassmaterial are wrenched slightly out of position. This in turn alters thematerial's index of refraction, the parameter that tells you the anglelightcan be deflected when it passes from that material into air.
The change in the refraction indexoccurs in lockstep withthe laser pulse as it passes through the glass. The resulting movingdisturbance is referred to as RIP, the refractive index perturbation.The RIPhappens not because of the energy of the laser pulse, and not evenbecause ofthe size of the change in the refractive index (which is less than 1percent),but because of the quickness of the change, occurring over merepicoseconds(trillionths of a second).
Escapinga black hole
Proposed in 1974by British physicistStephen Hawking, theradiation that bears his name ? Hawking radiation ? overturned theconceptthat black holes are inescapable. Until then black holes were thoughtto be aone-way-only phenomenon, in which light, comets, spacecraft ? anyconceivableobject ? might enter a black hole but would never come out. Hawkingallowedthat the intruding object would indeed never re-emerge. In fact, itwould betorn apart by the powerful gravity tides inside the hole.
But the very violence of a black holemight, Hawking said,allow for some energy to escape from the black hole. He counted on thefactthat the vacuum of space, including even the space inside a black hole,isteeming with virtual particles, courtesy of the concept of quantumweirdness.The fuzzy nature of quantum reality allows particle-antiparticle pairsto comeinto existence out of the vacuum. These pairs normally disappearquickly backinto the nothingness, never to be seen.
However, Hawking foresaw that in thevicinity of the eventhorizon the density of energy was so great that occasionally thesurplus energycould convert the evanescent pair of particles into real particles.This isalso the way particles are created out of the vacuum at the collisionpoint athuge particle accelerators. If the pair had been born right at theeventhorizon, the point of no return, then one of the particles might escapefromthe black hole while its mate would remain trapped behind.
In this way the black hole couldactually emit a form ofradiation in the form of those unpaired, just-created-out-of-the-vacuumparticles.This stream of particles is now called Hawking radiation, and it playsaprominent part in the study of how the universe behaves over long timeperiods.But black holes are elusive. They can't be seen directly and theirexistence isinferred only through their effect on surrounding space. No actualHawkingradiation has been seen.
Stoppinglight in the lab
What the Italian physicists have madewith their laserdisturbance moving through glass is a tiny zone where (at least amidthedisturbance itself) light cannot move forward, which is just thesituation atthe event horizon of a black hole.
From the perspective of the RIP ?consider, for the moment,the disturbance zipping through the glass to be a sort of physicalthing all byitself ? the contention between the light and the local perturbationin theglass causes the light to come to a standstill. This is just whathappens atthe event horizon of a black hole.
In one case the progress of light isfrustrated by theimmense warping of space by gravity, in the other the progress of lightisfrustrated by the warping of the optical environment in the glass.
What happens in the glass is whathappens in the black hole:the vacuum will sprout virtual particle pairs, in this case pairs ofparcels oflight, or photons. However, in the high-energy environment of theartificialevent-horizon, some of the virtual photons will be converted into realphotons.
And indeed the INFN scientists seelight coming out of theirglass sample. But is this truly Hawking radiation made in the wrenchingpulsewithin the laser disturbance, in analogy to light emitted from blackholes, orcould it be coming from somewhere else?
The leader of the Italian team ofresearchers, DanieleFaccio, who works at Insubria University in Como, Italy (where theresearch wasdone), said that all other known origins of the light can be ruled out.Thecareful tuning of the laser pulse precludes the light having beenabsorbed bythe atoms in the crystal sample, he said. The use of anoriented laserpulse also rules out the idea that the Hawking radiation observed tothe sideof the glass could be light scattered from the laser beam.
The idea of creating Hawkingradiation with an artificialevent horizon inside a solid material was proposed two years ago by ateam ofU.K. scientists from The University of St. Andrews, writing in ScienceMagazine. One of the authors of that paper, Ulf Leonhardt,said that thenew Italian results are extremely important.
"Their experiment is the very firstobservation ofHawking radiation ? I salute them," Leonhardt said.
Leonhardt's group is attempting tocreate an artificialevent horizon inside an optical fiber rather than in a bulk piece ofglass. Hebelieves this general line of research is important since it combinesthe studyof astrophysics, quantum science, and thermodynamics (the science ofenergy).Leonhardt said that it might be possible to test theories ? likestring theory? that combine gravity and quantum behavior.
The new Italian experimental resultswill soon be reportedon in the journal Physical Review Letters.
According to Faccio, the creation ofHawking radiation in aterrestrial lab will not lead to direct modeling of celestial objectslikeblack holes. But he does suggest that an artificial event horizon inthe labmight be useful for other things.
"We can now study and test some veryexotic andexciting things," Faccio said. "We can combine black hole and whitehole (a black hole in which time goes backward, and into which lightmay notenter, but only exit) horizons to create a black hole laser, one inwhich lightbounces back and forth between the horizons, each time amplifying lightenergyjust as in a laser."
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