at the SSC and who were now without a job or any prospects. Many
of them were very senior, having left full professorships at
distinguished universities to spearhead the effort. It was sad, and
more than half of those people had to leave the field altogether.
ͣ͢͞
The anticipated cost of the project when it was canceled had risen
from $4.4 billion at its inception in 1987 to about $12 billion in
1993. While this was, and still is, a large amount of money, one can
debate the merits of killing the project. Two billion dollars had
already been spent on it, and twenty-four kilometers of tunnel had
been completed.
The decision to kill the project was not black-and-white, but a
number of things could have played a bigger role in considerations—
from the opportunity costs of losing a fair fraction of the talented
accelerator physicists and particle physics experimentalists in the
country to the many new breakthroughs that might have resulted
from the expenditures on high-tech development that would have
contributed to our economy. Moreover, had the SSC been built and
functioned as planned, we may have had answers more than a
decade ago to experimental questions we are still addressing. Would
knowing the answers have changed anything we might have done in
the meantime? We’ll probably never know.
The $12 billion would have been spent over some ten to fifteen
years during construction and the commencement of operations,
which makes the cost in the range of $1 billion per year. In the
federal budget this is not a large amount. My own political views are
well known, so it may not be surprising for me to suggest, for
example, that the United States would have been just as secure had it
cut the bloated US defense budget by this amount, far less than 1
percent of its total each year. Moreover, the entire cost of the SSC
would have probably been comparable to the air-conditioning and
transportation costs of the disastrous 2003 Iraq invasion, which
decreased our net security and well-being. I can’t help referring once
again to Robert Wilson’s testimony before Congress regarding the
Fermilab accelerator: “It has nothing to do directly with defending
our country except to help make it worth defending.”
ͣͣ͞
These are political questions, however, not scientific ones, and in
a democracy, Congress, representing the public, has the right and
responsibility to oversee priorities for expenditures on large public
projects. The particle physics community, perhaps too used to a
secure inflow of money during the Cold War, did not do an adequate
job of informing the public and Congress what the project was all
about. It is not surprising that in hard economic times the first thing
to be cut would be something that seemed so esoteric. I wondered at
the time why it was necessary to kill the project, rather than suspend
funding until the economy improved or until technological
developments might have reduced its cost. Neither the tunnel (now
filling with water) nor the laboratory buildings (now occupied by a
chemical company) were going anywhere.
Despite these developments in the United States, CERN was
moving forward with a new machine, the Large Electron-Positron
(LEP) Collider, designed to explore in detail the physics of the W and
the Z particles, at the urging of its newest Nobel laureate, the
indomitable Carlo Rubbia. He became the laboratory’s director in
1989, the same year the new machine came online.
A twenty-seven-kilometer-long circular tunnel was dug about a
hundred meters underground around the old SPS machine, which
was now used to inject electrons and positrons into the bigger ring,
where they were further accelerated to huge energies. Located on the
outskirts of Geneva, the new machine was large enough to cross
under the Jura Mountains into France. European nations are more
familiar with building tunnels than the United States is, and when
the tunnel was completed, the two ends met up to within one
centimeter. Moreover CERN, as an international collaboration of
many countries, did not significantly eat into the GDP of any one
country.
ͣͤ͞
The new machine ran successfully for more than a decade, and
after the demise of the SSC in the United States, the huge LEP tunnel
was considered for the creation of a miniversion of the SSC—not
quite as powerful but still energetic enough to explore much of the
parameter space where the long-sought Higgs particle might exist.
Some competition came from a machine at Fermilab, called the
Tevatron, which had been running since 1976 and in 1984 came
online as the world’s most energetic proton-antiproton machine. By
1986, the collision energy of protons and antiprotons circulating
around the 6.5 kilometer ring of superconducting magnets at
Fermilab was almost two thousand times the equivalent rest mass
energy of the proton.
As significant as this was, it was not sufficient to probe most of
the available parameter space for the Higgs, and a discovery at the
Tevatron would have required nature to have been kind. The
Tevatron did garner one great success, the long-anticipated
discovery, in 1995, of the mammoth top quark, 175 times the mass
of the proton, and the most massive particle yet discovered in
nature.
With no clear competition therefore, within fourteen months of
the demise of the SSC the CERN council approved the construction
of a new machine, the Large Hadron Collider, in the LEP tunnel.
Design and development of the machine and detectors would take
some time to complete, so the LEP machine would continue to
operate in the tunnel for almost another six years before having to
close down for reconstruction. It would then take almost another
decade to complete construction of the machine and the particle
detectors to be used in the search for the Higgs and/or other new
physics.
That is, if a working machine and viable detectors could be
constructed. This would be the most complicated engineering task
ͣͥ͞
humans had ever undertaken. The design specifications for
superconducting magnets, computing facilities, and many other
aspects of the machine and detectors called for technology far
exceeding anything then available.
Conceptual design of the machine took a full year, and another
year later two of the main experimental detector collaboration
proposals were approved. The United States, with no horses in this
race, was admitted as an “observer” state to CERN, allowing US
physicists to become key players in detector development and
design. In 1998 construction of the cavern to hold one of the two
major detectors, the CMS detector, was delayed for six months as
workers discovered fourth-century Gallo-Roman ruins, including a
villa and surrounding fields, on the site.
Four and a half years later, the huge caverns that would house
both main
detectors underground were completed. Over the next
two years, 1,232 huge magnets, each fifteen meters long and
weighing thirty-five tons, were lowered fifty meters below the
surface in a special shaft and delivered to their final destinations
using a specially designed vehicle that could travel in the tunnel. A
year after that, the final pieces of each of the two large detectors
were lowered into place, and at 10:28 a.m., September 10, 2008, the
machine officially turned on for the first time.
Two weeks later, disaster struck. A short occurred in one of the
magnet connectors, causing the associated superconducting magnet
to go normal, releasing a huge amount of energy and resulting in
mechanical damage and release of some of the liquid helium cooling
the machine. The damage was extensive enough that a redesign and
examination of every weld and connection in the LHC was required,
taking more than a year to complete. In November of 2009 the LHC
was finally turned back on, but because of design concerns, it was set
to run at seven thousand times the center-of-mass energy of the
ͤ͜͞
proton, instead of fourteen thousand. On March 19, 2010, the
machine finally began running with colliding beams at the lower
energy, and both sets of detectors began to record collisions with
this total energy within two weeks.
These simple timelines belie the incredible challenges of the
technical feats achieved at CERN during the fifteen years since the
machine was first proposed. If you land at Geneva airport and look
outside, you will see gentle farmland, with mountains in the distance.
Without being told, no one would guess that underneath that
farmland lies the most complicated machine humans have ever
constructed. Consider some of the characteristics of the machine,
which lies at some points 175 meters below this calm and pastoral
scene:
1. In the 3.8-meter-wide tunnel, traversing twenty-seven
kilometers, are two parallel beamline circles, intersecting at four
points around the ring. Distributed around the ring are more
than sixteen hundred superconducting magnets, most weighing
more than twenty-seven tons. The tunnel is so long that,
looking down it, one almost cannot see its curvature:
2. Ninety-six tons of superfluid 4He are used to keep the magnets
operating at a temperature of less than two degrees above
absolute zero, colder than the temperature of the radiation
background in the depths of interstellar space. In total, 120 tons
of liquid helium are utilized, cooled first by using about ten
thousand tons of liquid nitrogen. Some forty thousand leak-
ͤ͞͝
tight pipe connections had to be made. The volume of He used
makes the LHC the largest cryogenic facility in the world.
3. The vacuum in the beamlines is required to be sparser than the
vacuum in outer space experienced by the astronauts
performing space walks outside the ISS, and ten times lower
than the atmospheric pressure on the Moon. The largest
volume at the LHC pumped down to this vacuum level is nine
thousand cubic meters, comparable to the volume of a large
cathedral.
4. The protons accelerated around the tunnel in either direction
move at a speed of 0.999999991 times the speed of light, or only
about three meters per second less than light speed. The energy
possessed by each proton in the collision is equivalent to the
energy of a flying mosquito, but compressed into a radial
dimension one million million times smaller than a mosquito’s
length.
5. Each beam of protons is bunched into 2,808 separate bunches,
squeezed at collision points to about one-quarter the width of a
human hair, around the ring, with 115 billion protons in each
bunch, yielding bunch collisions every twenty-five-billionths of
a second, with more than 600 million particle collisions per
second.
6. The computer grid designed to handle data from the LHC is the
largest in the world. Every second the raw data generated by the
LHC are enough to fill more than a thousand one-terabyte hard
drives. This must be reduced considerably to be analyzed. From
the 6 million billion proton-proton collisions analyzed in 2012
alone, more than twenty-five thousand terabytes of data were
processed—more than the amount of information in all the
books ever written and corresponding to a stack of CDs about
twenty kilometers tall. To do this, a worldwide computer grid
ͤ͞͞
was created with 170 computer centers in thirty-six countries.
When the machine is running, about seven hundred megabytes
per second of data are produced.
7. The requirements for the sixteen hundred magnets to produce
beams intense enough to collide is equivalent to firing two
needles from a distance of ten kilometers with such precision
that they collide exactly halfway between the two firing
positions.
8. The alignment of the beams is so precise that account must be
taken for the tidal variations on the ring from the gravity of the
Moon as its position over Geneva changes, causing a variation
of one millimeter in the circumference of the LHC each day.
9. To produce the incredibly intense magnetic fields needed to
steer the proton beams, a current of almost twelve thousand
amps flows through each of the superconducting magnets,
about 120 times the current flowing through an average family
house.
10. The strands of cable needed to make up the magnetic coils in
the LHC span about 270,000 kilometers, or about six times the
circumference of the Earth. If all the filaments in the strands
were unraveled, they would stretch to the Sun and back more
than five times.
11. The total energy in each beam is about the same as that of a
four-hundred-ton train traveling at 150 km/hr. This is enough
energy to melt five hundred kilograms of copper. The energy
stored in the superconducting magnets is thirty times higher
than this.
12. Even with the superconducting magnets—which make power
consumption in the machine manageable—when the machine
is running, it uses about the same power as the total
consumption of all of the households in Geneva.
ͤ͟͞
So much for the machine itself. To analyze the collisions at the
LHC, a variety of large detectors have been built. Each of the four
currently operating detectors has the size of a significant office
building and the complexity of a major laboratory. To have the
opportunity to go underground and see the detectors is to feel like
Gulliver in Brobdingnag. The scale of absolutely every component is
immense. Here is a photo of the CMS detector, the smaller of the
two largest detectors at the LHC:
If you are actually at the detector, it is hard to even grasp the full
picture, as can be seen in the more up-close-and-personal view:
The complexity of the machines is almost unfathomable. For a
theorist such as me, it
is hard to imagine how any single group of
physicists can keep track of the device, much less design and build it
to the exacting specifications required.
Each of the two largest detectors, ATLAS and CMS, was built by a
collaboration of over two thousand scientists. More than ten
thousand scientists and engineers from over a hundred countries
ͤ͞͠
participated in building the machine and detectors. Consider the
smaller of the two detectors, CMS. It is more than twenty meters
long, fifteen meters high, and fifteen meters wide. Some 12,500 tons
of iron are in the detector, more than in the Eiffel Tower. The two
halves of the detector are separated by a few meters when it is being
worked on. Even though they are not on wheels, if the two halves
were apart when the large magnetic field of the detector was turned
on, they would be dragged together.
Each detector is separated into millions of components, with
trackers that can measure particle trajectories to an accuracy of ten-
millionths of a meter, with calorimeters, which detect to a high
accuracy energy deposited in the detectors, and with devices for
measuring the speed of particles by measuring the radiation they
emit as they traverse the detector. In each collision hundreds or
thousands of individual particles may be produced, and the detector
must keep track of almost all of them to reconstruct each event.
Physicist Victor Weisskopf was the fourth director general of
CERN, between 1961 and 1966, and he likened the great accelerators
of that time to the Gothic cathedrals of medieval Europe. In thinking
of CERN and the LHC, the comparison is particularly interesting.
The Gothic cathedrals stretched the technology of the time,
requiring new building techniques and new tools to be created.
Hundreds or thousands of master craftsmen from dozens of
countries built them over many decades. Their scale dwarfed that of
any buildings that had previously been created. And they were built
for no more practical reason than to celebrate the glory of God.
The LHC is the most complicated machine ever built, requiring
new building techniques and new tools to be created. Thousands of
PhD scientists and engineers from hundreds of countries speaking
dozens of languages, and hailing from a background of at least an
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