The Fundamental Forces : Force Carriers

Fundamental Forces :

  • Matter is effected by forces or interactions (the terms are interchangeable)
  • there are four fundamental forces in the Universe:
    • gravitation (between particles with mass)
    • electromagnetic (between particles with charge/magnetism)
    • strong nuclear force (between quarks)
    • weak nuclear force (that changes quark types)
The first two you are familiar with, gravity is the attractive force between all matter, electromagnetic force describes the interaction of charged particles and magnetics. Light (photons) is explained by the interaction of electric and magnetic fields.The strong force binds quarks into protons, neutrons and mesons, and holds the nucleus of the atom together despite the repulsive electromagnetic force between protons. The weak force controls the radioactive decay of atomic nuclei and the reactions between leptons (electrons and neutrinos).

Current physics (called quantum field theory) explains the exchange of energy in interactions by the use of force carriers, called bosons. The long range forces have zero mass force carriers, the graviton and the photon. These operate on scales larger than the solar system. Short range forces have very massive force carriers, the W+, W- and Z for the weak force, the gluon for the strong force. These operate on scales the size of atomic nuclei.

So, although the strong force has the greatest strength, it also has the shortest range.

Bosons (Force Carriers):

  • certain particles play and important role in the transfer of force, the bosons or force carriers
  • the use of virtual particles to carry force resolves the action at a distance problem
Bosons are the particles which transmits the different forces between the matter particles, they normally have a whole number spin, 0, 1 or 2. And Fermions which are matter particles they often have spin 1/2. Real particles are the ones you are familiar with, all Fermions are real particles. The Bosons can sometimes be virtual and sometimes real. Virtual particles are the particles which transmits the force between the particles, e.g. virtual photon carries the electromagnetic force between e.g. electrons. They are called virtual particles because they can’t be directly detected, you can’t ‘see’ them so to speak. But their effect can be noticed, by e.g. the actual forces between particles.

Baryons and Mesons:

  • the large number of new particles discovered in the 1950’s is resolved by quark model
  • quarks are fundamental building blocks to baryons and mesons, coming together as triplets or pairs
Quarks combine to form the basic building blocks of matter, baryons and mesons. Baryons are made of three quarks to form the protons and neutrons of atomic nuclei (and also anti-protons and anti-neutrons). Mesons, made of quark pairs, are usually found in cosmic rays. Notice that the quarks all combine to make charges of -1, 0, or +1.

  • quarks have 1/3 charge and bind through the exchange of gluons of the strong force
Thus, our current understanding of the structure of the atom is shown below, the atom contains a nucleus surrounded by a cloud of negatively charged electrons. The nucleus is composed of neutral neutrons and positively charged protons. The opposite charge of the electron and proton binds the atom together with electromagnetic forces.

  • the many particles of atomic nuclei become a simple combination of quarks
  • unlike electric charge, quarks bind by exchanging color charge of three colors, blue, red and green
  • gluons carry color to convert quarks
  • due to their fractional charge nature, quarks cannot exist in isolation
  • the strong force binds quarks like a rubber band force
The protons and neutrons are composed of up and down quarks whose fractional charges (2/3 and -1/3) combine to produce the 0 or +1 charge of the proton and neutron. The nucleus is bound together by the nuclear strong force (that overcomes the electromagnetic repulsion of like-charged protons)Quarks in baryons and mesons are bound together by the strong force in the form of the exchange of gluons. Much like how the electromagnetic force strength is determined by the amount of electric charge, the strong force strength is determined by a new quantity called color charge.

Quarks come in three colors, red, blue and green (they are not actually colored, we just describe their color charge in these terms). So, unlike electromagnetic charges which come in two flavors (positive and negative or north and south poles), color charge in quarks comes in three types. And, just to be more confusing, color charge also has its anti-particle nature. So there is anti-red, anti-blue and anti-green.

Gluons serve the function of carrying color when they interact with quarks. Baryons and mesons must have a mix of colors such that the result is white. For example, red, blue and green make white. Also red and anti-red make white.

There can exist no free quarks, i.e. quarks by themselves. All quarks must be bound to another quark or antiquark by the exchange of gluons. This is called quark confinement. The exchange of gluons produces a color force field, referring to the assignment of color charge to quarks, similar to electric charge.

The color force field is unusual in that separating the quarks makes the force field stronger (unlike electromagnetic or gravity forces which weaken with distance). Energy is needed to overcome the color force field. That energy increases until a new quark or antiquark is formed (energy equals mass, E=mc2).

  • if energy is used to split a quark pair, new quarks are produced, this is how matter was produced when the Universe formed
Two new quarks form and bind to the old quarks to make two new mesons. Thus, none of the quarks were at anytime in isolation. Quarks always travel in pairs or triplets.