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Lawrence Krauss - The Greatest Story Ever Told--So Far

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by Why Are We Here (pdf)


  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.

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  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.”

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  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.

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  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

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  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

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  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

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  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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