The Large Hadronic Collider (LHC) at the CERN Laboratory located at the eastern edge of
more than a year of shutdown, the LHC limped back into operation on November 20,
2009. On March 30, 2010, the first
7.0 trillion electron-volts (TeV) collisions, half of the original design
collision energy, took place. The
facility has continued normal scheduled operation with 7.0 TeV proton-proton
collisions since that time, and has also spent a month running lead-lead
collisions at 574 TeV. It is
currently scheduled to run at the 7.0 TeV proton-proton collision energy through
the end of 2012.
2008, when the machine was preparing for operation, the big story in the media
was not that the LHC was about to operate, but that it was possible that the
Earth was about to be devoured by black holes produced at the LHC.
I perhaps contributed to that hysteria with my May-2003 AV column “The
CERN LHC: A Black Hole Factory?” In
that column, I discussed the possibility that the LHC might produce small black
holes in some of its high energy proton-proton collisions.
This was based on theoretical papers in the physics literature (see the
references below) involving the hypothesis that gravity is such a weak force
because it extends into several extra dimensions, while the other three forces
do not. These theories predicted
that the proton-proton collisions at the LHC might produce tiny black holes with
masses of 1 TeV or so. The same
theories also predicted that, if created, the microscopic black holes would be
super-hot objects that would almost instantaneously dissipate themselves into a
thermal cloud of lighter particles, quarks, gluons, electrons, positrons,
neutrinos, and photons.
bloggers, and others seized on the idea that the LHC might produce black holes
and ignored the predictions of their super-brief lifetime.
Instead, they imagined a scenario in which LHC-produced black holes would
began to suck in nearby matter, growing larger, and ultimately devouring the
Earth. Lawsuits, ultimately
unsuccessful, were filed to prevent the LHC from beginning its operation.
LHC has now been operating at 7.0 TeV for about a year.
Therefore, it’s time for an update on black hole production, or the
lack thereof. First, however, let me
review the physics ideas that lead some theorists to predict that the LHC might
produce microscopic black holes.
black hole is an object that has acquired enough mass for its size to be
completely confined by the force of gravity.
The velocity of escape from its surface exceeds the speed of light.
To put it another way, the gravitational force at its surface is so
strong that energy cost of moving a lump of mass from its surface to some
distance away exceeds the total mass-energy (E = m c2) of the mass
lump. There is good astrophysical
evidence for the existence of super-dense objects assumed to be black holes,
because mass falling into such objects produces lots of energy.
The energy-squandering behavior of quasars and active galactic nuclei,
the x-rays emitted from certain binary star systems, and the high velocities of
stars near the center of our own galaxy all suggest the presence of a black hole
energy source. It is also well
established from theoretical modeling that a star with sufficient mass, after it
burns through its supply of hydrogen, should collapse to a black hole following
a supernova explosion.
no upper limit to the mass and size of a black hole.
In a certain sense, our entire universe can be considered to be a black
hole, with us inside. On the other
end of the size scale, the mass of a black hole can be no smaller than a Planck
mass, which is (hc/2pG)½.
Here, h is Planck's constant, c
is the speed of light, and G is
theoretical ideas suggest that gravity, the weakest of the four forces, could
become stronger and more comparable in strength to the other forces at small
distances, because of the effects of hypothetical extra dimensions used only by
gravity. In this scenario, as the
effective value of G grows larger,
the Planck mass drops, minimum size black holes become less massive, and the
energy required to produce black holes can decrease many orders of magnitude, to
around 1 TeV. This is well within
range of LHC collision energies. In
this scenario the LHC would be a "black hole factory", an accelerator
that makes large quantities of microscopic black holes.
The mass and production probability of such black holes depends in part
on how many extra dimensions gravity is allowed to expand into, and calculations
usually consider between two to six such extra dimensions.
As Steven Hawking showed in the 1970s, a black hole should have a surface temperature that depends on the curvature of its surface. A star-mass black hole has a large radius, a very small curvature, and a temperature of only a few degrees Kelvin. On the other hand, a mini black hole like those that the LHC might produce would be extremely hot, because its very small radius, (millimeters or less) would produce a correspondingly large temperature, up to 1.5 × 1014 K or 80 GeV. At such a high surface temperature, the black hole would evaporate very rapidly into lighter particles: photons, electrons, and quarks, with energies ranging from 80 GeV down.
the question is, after one year of LHC operation is there any evidence that the
LHC is producing mini black holes? One
of the three large detectors located at the collision points of the LHC is the
Compact Muon Solenoid (CMS) experiment.
On December 15, 2010, the CMS Collaboration released a paper in which
they considered the evidence for black hole production from 10 trillion 7.0 TeV
proton-proton collisions tracked by their detector at the LHC.
The format of their 26 page paper is itself interesting, because it
includes 9 pages of physics discussion, 3 pages of references, and 14 pages
listing the authors and institutions that are participants in the CMS
Collaboration. Such is the
labor-intensive state of experimental high energy physics.
would a black hole look like, as seen by the CMS detector?
It is estimated that if a black hole with a mass of around 1 TeV were
produced, about 75% of its mass-energy would immediately evaporate into quarks
and gluons, because these particles have many color degrees of freedom, making
their Hawking evaporation more probable. The
resulting quarks and gluons would produce jets of strongly interacting particles
tracked by the detector. The
remainder of the mass-energy would show up as photons and weakly-interacting
particles (positive and negative
taus, muons, and electrons, as well as neutrinos, and Z and W bosons).
Some scenarios also predict energy loss as gravitational shock waves and
stable non-interacting and non-accreting remnants.
The production of black holes at the LHC would be show up as
“democratic” many-particle decays showing no preferred direction, with the
final state particles carrying many GeV of kinetic energy.
The CMS Collaboration has searched for such events in their data.
course, random events that have nothing to do with black hole production can
show characteristics similar to the “signal” that is the subject of the
black hole search. This event
background is simulated with calculations using the
net result of this investigation is that up to the end of 2010, no evidence for
the production of black holes at the LHC has been observed.
The authors conclude that if there are 6 extra gravitational dimensions,
the minimum black hole mass must be greater than 4.0 to 4.5 TeV.
If there are 4 extra gravitational dimensions, the minimum black hole
mass must be greater than 3.8 to 4.4 TeV. If
there are only 2 extra gravitational dimensions, the minimum black hole mass
must be greater than 3.4 to 4.0 TeV.
course, these results are somewhat limited by statistics and are greatly limited
by the collision energies currently available at the LHC, which are only half of
the design collision energy. There
could be black hole production that only shows up at higher collision energies.
current plan at CERN is to operate the LHC at the present proton-proton
collision energy of 7 TeV until the end of 2012.
Then there will be a one-year shutdown to replace suspicious current
joints and make other improvements, and in about 2014 the LHC will resume
operation with proton-proton collision energies of 14 TeV.
In that operating range, a new search for black hole production will be
done, looking for black holes in the mass range up to around 9 or 10 TeV.
this column for further results.
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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