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The Large Hadron Collider , the largest
and most expensive scientific instrument ever built in peacetime,
begins operations on September 10 when a beam of high-speed protons
begins shooting around the machine’s 27 kilometer circular tunnel beneath Geneva, Switzerland. When the protons collide with each other inside the machine, one thing that
scientists are certain won’t happen is the production of miniature
black holes that gobble up nearby matter. A new study shows that
the continuing existence of old stars in the sky is evidence that
small black holes can’t swallow the Earth.
That is not to say that the new collider might not actually create
mini-black holes as no one knows for sure what will emerge from the
debris of the LHC collisions. Black holes are thought to represent
the ultimate state of compressed matter, with gravity so powerful
that any bit of matter, and even light, would be sucked inexorably
inwards with no chance for escape if it gets too close to the black
hole’s boundary.
That was the thinking about black holes before Stephen Hawking , the
Cambridge University scientist, came forth with the idea that even
black holes can lose energy. The density of energy inside a black
hole is so huge that some of it can be converted into creating new
particles, he said. If this conversion happens right at the edge of
the black hole, Hawking argued, some of those new particles could
escape, taking energy with them. In this way black holes can lose
energy. They can “evaporate.”
There is a rule in physics that says that the smaller the black
hole, the quicker the evaporation. For an LHC-style black hole,
estimated to be only a billionth of a billionth of a meter across
(an atto-meter) the black hole would exist for a bit more than a few
billion-billion-billionths of a second. It wouldn’t be around long
enough to swallow any nearby matter and would pose no danger to
ordinary matter.
But what if Hawking is wrong? What if some black holes don’t
evaporate, but go on eating matter? What if scientists create some
small, long-lasting black holes in Geneva, and they get loose? This
possibility is addressed in a new report in the journal Physical
Review D.
In their study of the matter, Steve Giddings of the University of
California at Santa Barbara and Michelangelo Mangano of CERN (the
parent laboratory where LHC operates) look at what happens if there
existed a type of black hole, one we'd be concerned about, that
could not only survive but continue to grow to a macroscopic size
(the size of a golf ball, say) in a time shorter than billions of
years.
If such a type of black hole existed, it would grow even quicker
inside super-compressed stars, such as white dwarfs and neutron
stars, where the density of matter is billions or trillions of times
greater then the density of rock on Earth. These celestial objects
are created when an ordinary star runs out of fuel and starts to
contract. There is no LHC on such stars but a black hole could
presumably be spawned when a passing cosmic ray, a haphazard
shooting particle that races around the cosmos, strikes and burrows
inside the neutron star.
Since astronomers look out and see lots of perfectly healthy and
very old white dwarfs and neutron stars of the right types, Giddings
concludes that quickly-growing black holes, the kind that
voraciously eat their surroundings, can't exist. Such a dangerous
black hole couldn't exist inside dense stars and couldn’t exist on
Earth.
Michael Peskin , a Stanford physicist who did not take part in the
study, says that the continued existence of superdense stars act
like the canaries that coal miners used to take underground-the idea
being that the presence of deadly gas would more quickly overcome
the canary, giving the miners warning of a dangerous condition. As
long as those stars keep sending their light, Peskin says, the Earth
is not in danger from black holes. (Link to Peskin comments, in
APS’s new “Physics” website at
http://physics.aps.org/articles/v1/14
)
If scientists don’t know for sure what particles the LHC will
produce, why build a massive, very expensive machine to smash
particles together in the first place? The smashing is needed
because to explore the interior of atoms and the power of the
collisions of particles is directly related to how deep inside the
researchers can see. Increasing the power of the proton beams used
in the collisions requires increasing the size of the collider.
Why do the beams have to be so powerful? The answer is related to
the idea that energy can be converted from one form into another.
The protons at the LHC whiz around their long track at a speed of
99.999999 % of the speed of light. Actually two beams circulate in
the same underground tunnel in opposite directions, and when two
protons hit each other head on, a lot of their immense energy of
motion can, at the moment of collision, be transformed into new
particles that weren’t there a moment before.
When two automobiles hit head-on the results are always bad. But in
the world of high-energy physics, instigating a violent smashup,
with lots of debris spraying out, is exactly what researchers want.
Among the debris can be particles that might have existed billions
of years ago but which, because of their instability, long ago
decayed away. Creating these rare particles again in a modern
experiment is precisely the plan at LHC. The thinking here is that
such formerly-extinct species of matter can tell us things about the
forces of nature.
Taken from: The American Institute of Physics Bulletin of Research News
Number 871 September 9, 2008 www.aip.org/pnu
by Phillip F. Schewe, James Dawson, and Jason S. Bardi |
