Archive for February, 2010

S03E15: The Large Hadron Collision

February 8, 2010

In tonight’s episode, Leonard finds he is invited to Large Hadron Collider, “the LHC”.   In case this ever happens to you,  I have a handy phrasebook at the end of the post.  (Or take it with  you if  take a  free CERN tour open to the public.)  But first, even though the LHC has had about a billion dollars of news coverage over the past two years, there may be viewers that have not have heard that the LHC is the largest “atom smasher” ever built.

“Atom smasher” is a quaint 1950’s term for a “particle accelerator”.  Particle accelerators  produce “high energy” collisions for people like me, “high energy physicists”.   As a side benefit, they  also produce the brightest visible light and X-ray sources available for study of new materials and biological systems.

How high is “high energy”?    The LHC is designed to produce collisions of protons that have been accelerated by 7 trillion volts.  That sounds like a lot.  How much?  When two of these protons collide they have  the energy you would get out of eating 0.00013 micrograms of a candy bar.

That is not much energy for a machine touted as recreating the Big Bang.   There are much higher energy collisions on a Manhattan sidewalk than this.   The key point is  that high-energy physicists care about the energy per particle.    Collisions on the highway, or even a baseball with a bat, are collisions with objects with over 1027 (1 followed by 27 zeros!) particles in them.   So any one proton in the collision of two cars has very little energy compared to the LHC.

A ball and bat make a much higher energy collision that the LHC. What matters though is the energy per particle.

Even the “Big Bang machine” as an analogy is a bit off the mark.  The collisions do not make a high temperature replica of the Big Bang.  Having only two particles collide is barely enough to think about as having any temperature at all.    (Some reactions that would occur in a high temperature fluid, cannot happen at the LHC with its only two colliding particles, even though they are high enough energy…to get around this, some physicists will someday use the machine to collide large nuclei, but the high-energy frontier is the collisions of single protons.)

Perhaps a more apt, albeit less sensational, description is an old one:  High energy accelerators are giant microscopes.  A deep law of physics is that the higher the momentum of a particle, the smaller size it can resolve.   High energy means high momentum and going down this path for a few centuries brings us to the Large Hadron collider.

Optical Microscopes:  The artisan lens-makers of Flanders over 400 years ago inspired Galileo to combine lenses to make a telescope to study the heavens.    A slight rearrangement of the optics, produced a microscope, producing images of biological structures too small for the human eye, on the scale of a millionth of a meter or “micron”.   The minimum size structure visible is dictated by the “size” of visible light, about half a micron.   But a “micron” is enormous on the biological and even atomic scale.   Barely any of the structures in the nucleus of a living cell can be seen.

Electron Microscopes:   In the early 20th century, a polio epidemic spanned the world.  In 1% of its infections, polio would leave children paralyzed for life.  Optical microscopes were not up to the task of imaging the poliomyelitis virus.  So German engineers pressed into service the physics rule that high momentum means access to small sizes..  By bombarding a sample with high energy electrons, the polio virus could be seen.  (Images of structures are  in black and white.  It is meaningless to even ask the color of something so small that not even light can resolve it.  But like Ted Turner, physicists often colorize their images. )   Over the years, the technology has improved to the point where even the locations of individual atoms can be measured to 0.000000000050 meters.

Accelerated electrons allow imaging the poliomyelitis virus which causes polio (false color). This is far too small to see under a regular, optical, microscope.

The Large Hadron Collider:  and other recent accelerators are sensitive enough to offer the possibility of looking inside even a proton.   Structures the size of 0.000000000000000001 meters (that’s a billionth of a billionth of a meter) are routinely studied by high energy physics like Leonard.

Founded soon after the Second World War, CERN used physics as a proving ground for European unity in peaceful pursuits.  I spent a few years working at CERN, the home of the Large Hadron Collider.  English is lingua franca at CERN, but having been around for nearly 40 years, English spoken in this island surrounded by the French-speaking countryside of France and Switzerland has developed into a dialect of its own.   In case you, like Leonard, are ever invited to CERN here are a few helpful phrases instructing you how to speak in the CERN dialect:

How does this look like?“:  When giving a presentation on scientific work, I often find myself asking rhetorically about the data, choosing between either “How does this look?” or “What does this look like?”.  In the CERN dialect, this hybrid phrase means you never have to choose.

Profit: In French, the verb profiter means  to take advantage of. This allows a much more efficient construction, as in “Let us profit from the sunshine and eat out of doors”.

British English: For some reason, English taught in European schools appears still to be British English, not American.  So use “autumn” for “fall”,  never use “how come?” for “why?” and so forth.

Avoid   ‘s :  Face it, the “apostrophe -s” is hard to hear, and the rules are often even screwed up by a native English speaker.   This is also not a construction that has a counterpart in many other languages.  A phrase, “Let’s go to John’s lab and look for Mike’s screwdriver” is not something you are likely to hear in the CERN dialect.  Rather say “Let us go to the lab of John and look for the screwdriver of Mike” if you want to be sure to be understood.

Replace specific English words with French ones: Occasionally the French word is substituted directly for an English one.   Being located on the French-Swiss border, working at CERN you will be crossing the border–several times a day.   “Customs Officer” is a word you’ll need, but klunky.  Replace with douanier.

For what concerns… :  Phrases do not always get shorter.  If you are concerned about your particle tracker, don’t say “Concerning the tracker”, say “For what concerns the tracker….”

Toilet: A word we avoid in polite English conversation, toilet, corresponds in French to the very clean faire la toilette.  At CERN, don’t “go to the bathroom”. There is no bathtub in there anyway.  When nature calls you can very politely “go to the toilet”.

For more information see this nice page from Francois Briard a CERN employee who is heavily involved with their public outreach.  He also sent me a link to some fun movies.

Of course any of the guys would have friends at the university that could get them into the LHC lab, including many places not open to the public on the tours.   But plane tickets to Europe, a place to stay, and especially European gasoline for getting around do not come cheaply.   So their excitement is duly warranted.

Time for a “toilet” break.

P.S.  For over a decade, in my high-energy physics class I’ve always asked the students the following question:   “Every time a new accelerator turns on, some clowns appear and say it will destroy the Earth and/or Universe.  Explain whether this is likely or unlikely.”   Sure enough, the same thing happened with the LHC turn-on.  A key point I want my students to realize  is that Nature has much higher energy particles making much higher energy collisions all around us.   Basically these guys are fear mongering with an attention-grabbing stunt.  Now just because it is fear-mongering and an  attention-grabbing stunt does not mean it is wrong.    There are always loopholes.  So it is logically incorrect to say a disaster is absolutely impossible, as some of my colleagues have said. (Or at least what the media says they said.)  In fact it is logically possible at any moment something you do in your kitchen could even create ice-9.  So I think physicists should be careful and not say “absolutely impossible” when they really mean to say “is ridiculously stupid”.

P.P.S.  That said, I have an “I survived the Large Hadron Collider 9/10/2008 T-shirt”

S03E14: The Einstein Approximation

February 1, 2010

The hottest material in physics can be made with a pencil and Scotch tape.   That’s “hottest” in popularity, not temperature.  When a new, interesting, material is discovered, teams of physicists will race against each other to figure it out.   This decade’s material-of-the-century is graphene.

Graphene is merely chicken wire made with carbon atoms…chicken wire that is no thicker than a single atom.

Atomic chickenwire: A image of an actual sheet of graphene. Each little black dot is an empty space surrounded by six carbon atoms, forming a hexagon. (The width of the entire image is about one thouand times smaller than the width of a human hair.)

Carbon atoms love to form chains, as in alcohol, or even rings, as in chemicals found in gasoline.   Even more tangled-up connections of carbon create popular substances like diamonds and soot.  Physicists knew for decades that the existence of a single sheet of interconnected carbon atoms was possible, in principle.  But they also knew that such a structure could hardly be grown as a crystal since the structure tends to roll up and form three-dimensional bonds.

But only six years ago, in cutting-edge experiments (using Scotch tape and pencils), physicists succeeded in creating small quantities of graphene, smaller than a speck of dust.  Pencil lead, despite its name, has no “lead” in it.  Rather it is many layers of carbon sheets, like a deck of cards, stuck together called graphite (from the Greek “to write“).  The experimental heroes just stuck a little of the humble pencil graphite in a folded over piece of tape…and pulled, separating hundreds of atomic layers in two smaller stacks.   A student can repeat the process as many times as needed, until a single layer is  created.

The students transfer the single-sheet candidates from the tape to a silicon wafer for further study.  One of the researchers’  breakthroughs was to discover a quick way of identifying single layers from less interesting multiple layers.   Like oil floating on a puddle reflecting sunlight after the rain, the film thickness determines its color.   The thin, elegant, highly-sought single sheets of graphene appear pale pink, while their fatter cousins are blue.

The process produced perfect crystals, with no apparent defects.  Graphene would prove to be harder than diamond, yet flexible.  Graphene is not a metal, yet highly conductive.  Success having many mothers, after the discovery, claims of priority going back several decades have been staked.

Pity the poor condensed-matter theorists.  For over a century they have pushed pencils across their pads in search of new materials to propose.  Yet there graphene was, literally right under their noses, the entire time.

A few episodes ago, Sheldon took us in his mind to the fictional country of Flatland, where only two dimensions of motion are allowed.   Not at all fictional, graphene is a carbon Flatland with electrons fixed to move only in its two-dimensional world.  Lacking that one extra dimension turns most of the rules of materials on its head.  Graphene has captured the imagination of physicists with its potential applications.

High speed transistors:  The heart of computers and most other electronics are the fast switches called transistors.  The electrons in graphene are extremely mobile, able to cross thousands of carbon atoms without a single scatter.  So the idea, at least, has been put forward that graphene could be the basis of a new generation of higher speed, smaller integrated circuits.

Super-batteries:   Because its mass per area is as low as any imaginable material, graphene could revolutionize energy storage in batteries and the adoption of renewable energy.  Capacitors too, one of the basic building blocks of all electronics (they hold charge in circuits), could be replaced by far smaller graphene components.

Displays: Having the seemingly contradictory properties of transparency and conductivity at once, perhaps one day graphene sheets will produce large area touch-screens.  Now scientists only need discover what the iPad is good for.

Gas sensors:  Graphene’s low noise and high surface area could perhaps make it sensitive enough to detect even a single gas molecule adsorbed onto its surface causing a detectable step-like change in its electrical resistance.

Graphene could revolutionize electronics.

But what attracted Sheldon’s attention tonight is the theoretical description of electron motion in graphene.  By a mathematical coincidence, the equation that describes electron motion in graphene is almost the same as the fundamental equation of free electrons in relativistic quantum mechanics:  the famous Dirac Equation.   Because of the electrons’ interactions with the carbon nuclei, the electrons move as if they are massless.   So graphene can serve as a kind of laboratory for particle physics theorists, like Sheldon, to test their understanding of the mathematics they use every day under more abstract and less controllable conditions.

Graphene.  It’s the greatest thing since sliced pencil lead.

%d bloggers like this: