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Tuesday
Nov022010

The Overwhelming Binding Force

At the most fundamental level, the bulk of the mass of the stuff that we are all made of is in the nuclear force. Physicists call it the Strong force; and it is the glue that binds the tiniest constituents of matter together. An atom is comprised of a cloud of electrons with a dense nucleus of protons and neutrons. Most of the mass of the atom is in this nucleus. The protons and neutrons have substructure too: they are made of quarks. The quarks are bound together with a peculiar force known as the Strong (or nuclear) force; and it is the residual force from this setup that binds the neutrons and protons together into the nucleus. The energy stored in this glue accounts for most of the mass of the atom! when you break apart a nucleus, you release the energy and you get an nuclear explosion - or you power up the Sun. 

The Strong force is an unusual force law. All other forces in Nature share a common intuitive attribute: when you separate two objects bound by a force, the strength of the interaction gets weaker with larger distances. For example, gravity gets weaker as you go up in altitude away from the earth. Far enough away from any planets and stars, you basically are free from any appreciable gravitational pulls. The Strong force behaves in the opposite manner! Quarks - held together by the Strong force - become more tightly bound when you separate them… it is as if there is a spring joining the quarks that gets stiffer with larger quark separation. This is called confinement - quarks are confined into the protons and neutrons. What if you try to really push the limit and yank a quark away? As you try to do this, Nature kicks in with a vengeance and creates new quarks from the energy stored in the nuclear binding; the new quarks get dragged with the quark you are trying to yank away - just to make sure you cannot separate the pulled quark! Quarks hence always come in the company of other quarks - with nuclear glue holding things together. You just can't yank one away and stare at it on its own…

Due to this unusual attribute of the Strong force, it is very very difficult to do computations with it. In most of physics, one gets a computational handle on complex physical systems using a basic and efficient principle: start with a simpler setting which you can tackle without loosing your hair; then, assuming that the complexity is a small correction to the simpler base, apply a systematic scheme of approximating the problem. Depending on how much precision you need, you can compute additional small corrections to the simpler setting progressively and algorithmically. This is a rather very successful strategy and allows one to handle very complex systems systematically. When you try this with a system of quarks interacting with the Strong force, the whole process blows up in your face: the Strong force is rather strong… the strong binding force between quarks that you are trying to separate cannot be approximated as a simple system plus small corrections. You need to solve the whole damn problem - which is mathematically intractable. This theory describing the Strong force is known as Quantum ChromoDynamics, or QCD. The best one can do is to use a computer to do numerical simulations of the problem - this is known as Lattice QCD. Lattice QCD does work very well, but is certainly less gratifying than understanding things through the good old technologies of the paper and the pen - in the company of a lonely theoretical physicist.

The accompanying first video gives a quick overview of the atom and its nucleus - and a bit beyond. The Strong force is sensitive to an attribute of the quarks known as "color"; basically a cute term to account for the fact that this attribute comes in a triple: red, green, and blue. The Strong force assures that quarks are always held together in "colorless" combinations: three quarks with colors red, green, and blue; or two quarks with a color and an anti-color, i.e. red and anti-red. Yes, Strong colors come in pairs, the main color and its evil twin, the anti-color. Protons and neutrons are colorless combinations of three quarks. In total, we know of six types of quarks, mystically labeled: up, down, charm, strange, bottom, and top… Most of common matter involves only the up and down quarks. The top quark was discovered recently, about only a decade ago. And the glue that holds quarks together is made of a particle known as the gluon… there is a whole industry of particle physics for naming particles, usually involving physicists with a little too much imagination. For example, if Nature has a much longed for symmetry called supersymmetry (see previous post for more), we also have things called squarks… and  sgluons… 

The second video illustrates how we have learned all this stuff: by throwing particles at each other at high speed and watching what comes out (see post on the LHC).

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