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October
10, 2008: Here's what didn't happen on Sept. 10th:
The world did not end. Switching on the world's largest and
most powerful particle accelerator near Geneva, Switzerland,
did not trigger the creation of a microscopic black hole.
And that black hole did not start rapidly sucking in surrounding
matter faster and faster until it devoured the Earth, as sensationalist
news reports had suggested it might.
Of
course, because you're alive and reading this article today,
you already knew that. Currently the accelerator, an underground
ring 5 miles across called the Large Hadron Collider (LHC),
has been shut down for repairs. But once the immensely powerful
machine starts back up, is there a chance that the doomsday
scenario could still occur?
Relax.
As Mark Twain might have said, reports of Earth's death have
been greatly exaggerated.
![](images/lhc/aerial-view_strip.jpg)
Above:
An aerial view of CERN (European Organization for Nuclear
Research). The large 5-mile diameter ring traces the underground
Large Hadron Collider. Image credit: CERN
"There
never really was a danger from the accelerator, but that sure
didn't stop people from speculating that there might be!"
says Robert Johnson, a physicist at the Santa Cruz Institute
for Particle Physics and a member of the science team for
NASA's Fermi Gamma-ray Space Telescope, which launched in
June to study gamma rays from many phenomena, including possible
evaporating black holes.
There
are several reasons why the world did not come to an end on
Sept. 10th, and why the Large Hadron Collider isn't capable
of triggering such a calamity.
First
of all, yes, it is true that the LHC might create microscopic
black holes. But, for the record, it could not have created
one on its first day. That's because the physicists at CERN
didn't steer beams of protons into each other to create high-energy
collisions. Sept. 10th was just a warmup run. To date, the
collider still has not produced any collisions, and it is
the extreme energy of those collisions — up to 14 tera-electron
volts — that could potentially create a microscopic black
hole.
Right:
Any micro black hole created by the LHC would quickly evaporate,
losing mass and energy via Hawking radiation. [more]
Actually,
once the LHC is running again and begins producing collisions,
physicists will be ecstatic if it creates a tiny black hole.
It would be the first experimental evidence to support an
elegant but unproven and controversial "theory of everything"
called string theory.
In
string theory, electrons, photons, quarks, and all the other
fundamental particles are different vibrations of infinitesimal
strings in 10 dimensions: 9 space dimensions and one time
dimension. (The other 6 space dimensions are hidden by one
explanation or another, for example by being "curled
up" on an extremely small scale.) Some physicists tout
string theory's mathematical elegance and its ability to integrate
gravity with the other forces of nature. The widely accepted
Standard Model of particle physics does not include gravity,
which is one reason why it does not predict that the LHC would
create a gravitationally collapsed point — a black hole —
while string theory does.
Many
physicists have started to doubt whether string theory is
true. But assuming for a moment that it is, what would happen
when a black hole is born inside the LHC? The surprising answer
is "not much." Even if the black hole survives for
more than a fraction of a second (which it probably wouldn't),
most likely it would be flung out into space. "It would
only have the mass of a hundred or so protons, and it would
be moving at near the speed of light, so it would easily have
escape velocity," Johnson explains. Because the tiny
black hole would be less than a thousandth the size of a proton
and would have an exceedingly weak gravitational pull, it
could easily zip through solid rock without ever touching
— or sucking in — any matter. From the perspective of something
this tiny, the atoms that make up "solid" rock appear
to be almost entirely empty space: the vast space between
the atoms' nuclei and their orbiting electrons. So a micro
black hole could shoot down through the center of the Earth
and out the other side without causing any damage just as
easily as it could shoot up through 300 feet of the Swiss
countryside. Either way, it would end up out in the near-vacuum
of space, where the odds of it touching and sucking in any
matter so that it could grow into a menace would be smaller
still.
Right:
Inside the Large Hadron Collider. Protons race down this tunnel
at 99.999999% the speed of light. [more]
So
the first thing a micro-black hole would do is leave the planet
safely behind. But there are other, even stronger reasons
why scientists believe the LHC poses no threat to Earth. For
one, a black hole created in the LHC would almost certainly
evaporate before it got very far, most scientists believe.
Stephen Hawking, the physicist who wrote A Brief History
of Time, predicted that black holes radiate energy, a
phenomenon known as Hawking radiation. Because of this steady
loss of energy, black holes eventually evaporate. The smaller
the black hole, the more intense the Hawking radiation, and
the quicker the black hole will vanish. So a black hole a
thousand times smaller than a proton should disappear almost
instantly in a quick burst of radiation.
"Hawking's
prediction is not based on speculative string theory but rather
on well understood principles of quantum mechanics and particle
physics," Johnson notes.
Despite
its strong theoretical foundations, Hawking radiation has
never been observed directly. Still, scientists are confident
that any black hole created by the LHC would pose no threat.
How can they be so sure? Because of cosmic rays. Thousands
of times per day, high-energy cosmic rays strike the Earth's
atmosphere, colliding with molecules in the air with at least
20 times more energy than the most powerful collisions that
the LHC can produce. So if this new accelerator could make
Earth-devouring black holes, cosmic rays would have already
done so billions of times during Earth's long history.
And
yet, here we are. Let the collisions begin!
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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