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Sunday, July 28, 2013

One Ring To Rule Them All: A Visit To The Large Hadron Collider (Part 2)

If you want to you can read Part 1 first.  I thought this was going to be shorter but the tale grows in the telling.  The best part is that it's all true.

To get to the LHC, we had to go to France --well, strictly speaking, the Collider was already practically under our feet even in Meyrin.  You can see a part of it --one of the factory-sized surface points --from the Geneva airport, as a matter of fact; if you're landing or taking off, and you happen to be in a window seat looking north, you can see a cluster of buildings with what look like miniature nuclear power plant cooling towers on the roof, just past a parking area for small planes.  Our visit was to one of the main research facilities located on the LHC ring --the CMS Detector, which is in Cessy, France, just across the border.  (Crossing from Switzerland to France is, for an American used to hours-long waits and exhausted, suspicious immigration agents to get back into the US at JFK, almost comically low-key --no passports asked for or shown, and I don't particularly remember even stopping.  We may have slowed down a bit.)

The 27 kilometers-in-circumference main accelerator ring of the LHC straddles the border between Switzerland and France, just to the north of the city of Geneva itself.  The giant ring is buried under the earth at depths which vary depending on the surface geography; the collider tunnel is anywhere from 50 to 175 meters underground (greatest depths are under the Jura mountains, shallowest are near Lac Leman/Lake Geneva.)  There are several reasons for its subterranean location; cost of surface real estate is one, and shielding from cosmic radiation is another.  The tunnel containing the main ring once housed another particle accelerator: LEP, or the Large Electron Positron Collider, which was decommissioned in the year 2000 to make way for the LHC.

Getting packets of protons up to as close to the speed of light as possible isn't easy and while the largest ring is the most attention-getting part of the entire complex, it's actually the final stage in a four-step process involving a complex of equipment, some parts of which are more than half a century old.  The ladies and gentlemen at CERN get their protons by stripping off the electrons from hydrogen atoms --the excellent animated film showing the process on CERN's website depicts a rather banal bottle of hydrogen gas.  (Hydrogen, the simplest chemical element, consists of exactly one proton with one lonely electron orbiting it.)  This happens in the injection chamber of LINAC 2, the linear accelerator that's the first stage in getting fast-moving particles into the LHC itself.  With the electrons gone, all that's left are single protons --these have a positive charge, and so, can be accelerated by electrical fields and contained by electromagnets.

Protons leave LINAC 2 are already moving fast --about 1/3 the speed of light (c.)  From LINAC 2 the protons go into the PSB (Proton Synchrotron Booster) where they're divided into four packets and pumped up to even higher speeds --91.6% c.  Powerful electromagnets keep the protons on a circular path while they gain speed.  The third stage is the even larger Proton Synchrotron.

The circular Proton Synchrotron is old --it was first used in November of 1959, and was briefly the world's most powerful particle accelerator.  Today its main purpose is to help feed high speed particles into the main ring of the LHC --and it's been upgraded over the decades so that its beam is now a thousand times more powerful than its original design.  At 628 meters, it's a massive piece of equipment.  The protons injected into the PS only stay there for about 1.2 seconds, but during that time, they go to 99.9% c.  

This close to the speed of light, Einstein's theory of relativity starts to make itself felt in a big way.  One of the main insights from relativity theory is that things look different depending on your frame of reference, or where you're looking at things from (hence the name --it's all relative.)  To us, stationary observers with respect to the protons, the increase in speed means an increase in energy --as the protons go faster, they gain mass.  (This is a weird effect to someone who has never heard of relativistic effects before, but I don't make the news, I just report it.)  The speed limit for the universe is the speed of light itself --nothing can go faster; particles with mass can get close but never reach this velocity. At c, a particle would have infinite mass, which is Not Allowed.  (Photons, which have no mass, do travel at the speed of light --well, they are light, so they would, wouldn't they.)

As they leave PS, the protons have about 25 times their rest mass, and this close to the speed of light, any further addition of energy results in a lot of increase in mass and very little in velocity.

There's one more stage before the protons can enter the main ring, though.  From the Proton Synchrotron, they go into the Super Proton Synchrotron (CERN is running out of superlatives, obviously.  May I suggest as the next obvious bit of nomenclature the I Can't Believe It's A Synchrotron synchrotron.)  SPS is stage four, the last before the LHC proper.  SPS has been flinging particles around since 1976 (in 1983, it was used for the Nobel Prize-winning discovery of the W and Z bosons, which are the gauge bosons for the weak force --one of the four fundamental forces.)  Yet more energy is added to the protons here --and while there is not much gain in velocity, there is a lot of mass added (remember, mass-energy equivalence: E=MC^2.)  The unit used for mass-energy is the electron volt, and while protons leaving LINAC 2 --stage one --have an energy of 50 MeV (fifty million electron volts) by the time the protons are fired from SPS into the main ring, they've reached a mass of 450 GeV (giga --billion --electron volts.)

Finally, the protons are ready for prime time: the Large Hadron Collider itself.  When the Collider is in operation, it takes about a half hour for SPS to fill it with 2,808 individual "packets" of protons.  The packets are pumped with yet more energy in the LHC.  The LHC actually consists of two tunnels, --in one, protons go around clockwise, and in the other, counterclockwise.  The beams cross at four points spaced around the ring --these are the giant detector caverns, and when particles collide at high energy and produce a burst of energy and fundamental particles, it's these detectors that gather information about the event --evidence that the scientists at CERN, and around the world, sift through in hopes of finding, among other things, traces of the Higgs boson.

The protons in the LHC main ring are brought up to 99.9999991% c --they're moving so quickly that they're only traveling about three meters per second slower than light.  At these speeds they go around the ring 11,000 times in one second and the energy necessary to accelerate and contain them is greater than that used by the entire neighboring state of Geneva.  To generate the immense magnetic fields necessary to contain the proton beams, massive superconducting magnets are used --some as heavy as 27 tons.

If you like electromagnets (and hey, who doesn't) you'll love these.  Electromagnets work by generating a magnetic field when current flows through them, and it takes a lot of current to make magnetic fields strong enough to contain the proton beams, which are powerful enough melt a half a ton of copper.  The proton beam wouldn't make a bad weapon, if you could aim it --it's so powerful a special "beam dump" chamber had to be constructed to give the proton beam someplace safe to go.  The beam, in case of a superconductor failure, has to be dissipated in about 90 milliseconds, which is equal to about 4 TW (terawatts, or trillions of watts.)  The beam dump cavern contains a target of graphite composite eight meters long and a meter in diameter, and it's surrounded by a thousand tons of concrete radiation shielding.

So much current is used that it would melt the magnets if they weren't cooled to near absolute zero by liquid helium, which makes them superconductors --in a superconductor, current flows without resistance, but most known superconductors only become superconductors at very low temperatures.  The LHC's superconducting magnets are kept at a temperature colder than interstellar space --1.9 K, or about -271.25 degrees Celsius (absolute zero, the coldest possible temperature, is 273.15 degrees Celsius.)  It takes 96 tons of liquid helium to maintain the magnets at the right temperature --which means the LHC is both the largest particle accelerator in the world and the world's biggest refrigerator.

It may now be occurring to the reader that this much energy comes with some inherent dangers, and indeed, something going wrong with the LHC would be bad.  In September of 2008, something did go wrong. The LHC was being powered up for the first time, but an electrical short occurred between two superconducting magnets.  The result was one of the worst things that can happen to a superconductor: a so-called "quench," or accidental loss of superconductivity.  Temperatures almost instantly skyrocketed as the sudden electrical resistance produced a tremendous heat spike, this caused vaporization of liquid helium along a considerable stretch of the ring tunnel.  Automated emergency shutdown occurred but the damage was horrendous, even at far lower than maximum power --the liquid helium had vented with enough force to rupture the proton tunnels, contaminating them with soot, and rip multi-ton magnets off their concrete bases.  The amount of liquid helium released was so large --several tons --that it was two weeks before repair teams could enter the affected section of the ring to evaluate the damage (it was, one scientist commented drily, "not a pretty sight.")

The LHC had its share of teething problems, some of them almost comical --in one instance, in November of 2009, another quench incident almost occurred when power was lost due to a short in an electrical substation on the surface.  The reason was about as French as causes of superconductor failure come --a passing bird bearing a chunk of baguette in its beak had apparently dropped a piece of the world's most famous bread into a transformer.  This somewhat risible incident fortunately didn't cause any damage --a controlled shutdown prevented another quench incident --but the number of setbacks led to some interesting speculation about the nature of the Higgs boson.

 In particular, several scientists speculated that there is a form of cosmic censorship preventing the Higgs boson from being observed.  Bech Nielsen and Masao Ninomiya, of the Niels Bohr Institute in Copenhagen and the Yukawa Institute for Theoretical Physics in Kyoto, suggested that "reverse chronological causation" was behind the accidents at the LHC --in essence, the Large Hadron Collider was traveling backwards in time from the future to prevent itself from working.  Said Nielsen, in an email to the New York Times, "It is our prediction that all Higgs-creating machines will have bad luck."

If correct, the hypothesis would have gone a long way towards explaining why the US cut funding for its own home-grown mega-atom-smasher, the Superconducting Supercollider, which would have been 87 kilometers in circumference and had three times the power of the LHC (naturally, it would have been located in Texas.)  The SSC was canceled in 1993.  The apparent success of the LHC in finding evidence for the existence of the Higgs boson has pretty much put this somewhat bizarre theory to rest, but as the Times pointed out in its coverage, subatomic physics is not exactly a field where intuitively sensible behavior of physical systems is expected --Niels Bohr once famously said, to a colleague, "We are all agreed that your theory is crazy.  What divides us is whether your theory is crazy enough to be correct."

Speaking of crazy, there were some somewhat fringe worries that the LHC would do something at least as spectacular as commit temporal suicide --some people were worried that it would cause the end of the world.  The two favorite apocalyptic scenarios involved the accidental creation of a miniature black hole or the accidental creation of a type of exotic matter particle known as a strangelet.  Without going too much into technical details (OK, if you must, it's a mixture of up, down, and strange quarks) the problem with strange matter is that it may be more stable than normal matter --the concern is that strange matter may thus be contagious; any ordinary matter it comes into contact with would be turned into strange matter, like ice turning into ice-nine in Kurt Vonnegut's Cat's Cradle.  This has obviously not happened.

Black holes are what you get when an especially massive star burns out its fuel and can no longer prevent itself from collapsing under its own weight.  It collapses so powerfully that it becomes a singularity --a point object of infinite density, with a gravitational field so enormous that nothing can escape (well, nothing that gets closer than the black hole's event horizon.)  The possibility that such a thing could form in one of the collision caverns at LHC, and devour the Earth by sucking you and all you know and love into the maw of a singularity, naturally drew a lot of attention (the Daily Mail produced a representative headline of the tabloid press coverage: "Are We All Going To Die Next Wednesday?")

As it turns out this is not a problem either, for various reasons --one of them is that black holes do lose mass and eventually evaporate through a rather complicated process known as Hawking radiation, and any microscopic black hole would simply evaporate before it has a chance to do any damage.  A working group set up examine the problem pointed out that cosmic ray collisions in the Earth's upper atmosphere are more energetic than anything the LHC could produce and that if such events could cause the apocalypse it would have happened by now.  Probably the most trenchant rebuttal to those who insisted that firing up the LHC would lead to the end of the world came from physicist Brian Greene, who said to The New York Times, "If a black hole is produced under Geneva, might it swallow Switzerland and continue on a ravenous rampage until the Earth is devoured?  It's a reasonable question with a definite answer: no."

All this was running through my head earlier this month on the car ride to the CMS detector --our car left the industrial environs of Meyrin and the Geneva airport behind, crossed into France, and then we found ourselves winding through the French countryside.  Then, we turned up an unprepossessing driveway and found ourselves outside a tall hurricane fence topped with barbed wire.  An affable security guard waved us through, and we saw an enormous complex of buildings --surface evidence that deep below, humanity was going through Nature's pockets for loose bosons.

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