Particle physicists now believe they candescribe the behaviour of all subatomic particles in terms of the basic quarks and leptons, within a single theoretical framework called the standardmodel. The most important ingredients of this model,besides the quarks and leptons themselves areforces that act between the particles and mould them into the forms of matterobserved. There appear to be four basic forces at work in matter- gravity, theelectromagnetic force, the weak force and the strong force.
Gravity is the weakest of the four but acts over great distancesbinding stars and galaxies together. The electromagnetic force is strongerand responsible for holding atoms and molecules together. Like gravity, itsrange is infinite.
The weak force and strong force are by contrast limited inrange, and operate only within the dimensions typical of anatomic nucleus. The weak force causes certain forms ofradioactivity~Y and underlies the nuclear reactions that fuel the Sun.
Last but not least, the strong force - the strongest we know of-binds quarks and antiquarks together within the particles weobserve. The strong force seems to act in such a way that quarksare always locked inside these more complex particles, so that wehave never observed a single free quark, at least in presentexperiments.
According to the standard model the basic forces aretransmitted between the quarks and leptons by a third family ofparticles. These are called gauge bosons, and they differfundamentally from the quarks and leptons that are the buildingblocks of matter
There is a different type of particle for each force. Photons(the particles of light) carry the electromagnetic force; gluonscarry the strong force; charged particles W+or-, and neutralparticles Z0, carry the weak force; and a particle called thegravitron - not yet observed - is believed to be responsible forgravity.
The four forces appear very different in their behaviour inordinary matter but the standard model indicates that it is adifferent story when matter is in a high-energy environment.Theorists have found that the only consistent way to treat theweak force is to put it together with the electromagnetic force ina theory that describes a single 'electroweak' force. Thisdiscovery has been a tremendous breakthrough just as thebringing together of electricity and magnetism was in the theoryof electromagnetism, due to James Clark Maxwell in the mid 19thcentury.
In the low-energy world we humans inhabit, the electroweakforce appears to be split into weak and electromagneticcomponents. But, as studies of interactions between energeticparticles confirm, at higher energies the distinctions vanish as theweak and electromagnetic forces become of equal strengths.
Now, we have to create such high-energy conditions artificially,in the collisions between particles accelerated by machines. But it seemsprobable that earlier in the history of the Universe, allmatter was in a state of high energy. The observations of astronomers implythat the Universe is still expanding from an infinitely dense and energeticstate, after an initial' hot big bang' some 15 billion years ago. But how did the matter of the present day Universe evolve from this state?
This is one of the major questions that particle physicists today seek toanswer.
High-energy particle collisions in the laboratory can take us backin time to study forms of matter that probably existed in the firstfractions of a second after the big bang. We now know that theweak and electromagnetic forces behave as one electroweakforce at energies that would have prevailed less than a billionth ofa second after the big bang, when the Universe was relativelycool. But what happened before then? Is there an originalstate in which all forces behaved as one? Why did mattereventually emerge as families of quarks and leptons of increasing masses?