Elementary Particles and the World of Planck Scale

Fundamental Interactions

There are four fundamental interactions act on the different charge carriers. These are the gravitational with mass-energy, the electromagnetic with electric charge, the weak and strong interactions with flavor and colour charges (Figure 15-04c). The electromagnetic and weak interactions are unified at high energy into the electroweak. The strong residual interaction is experienced by hadrons (all the particles affected by the strong nuclear force) and generated by the quarks within. It is strong enough to overcome the electromagnetic repulsion between the hadrons. This is similar to the Van der Waals force between molecules (except the difference in charge carrier). Note that most of the mass of nucleons (more than 90%) arises from the energy associated with the motion of the quarks and gluons that compose them - NOT from the rest mass of the constituents.

Figure 15-04c Fundamental Forces [view large image]

• The strengths of the various interactions in Figure 15-03 are relative to the electromagnetic interaction. It depends on the separation between the two particles. At a separation of 10^-16 cm. the electromagnetic and weak forces mixes to generate the electroweak force. According to the Grand Unified Theory (GUT), the electroweak force further merge with the strong force at 10^-29 cm.
• The strength for gravitational interaction is very weak at small separation. Its force carrier, the graviton, has not yet been detected. This interaction is poorly understood in domain of very small scale. One explanation relates the weakness of gravity to its leakage to other dimensions in a manyfold universe.

Date	Detection	Discovery	Credit	Nature
1895	Cathod-ray tube	X-ray	W. Rontgen	High energy photon
1897	Cathod-ray tube	Electron	J. J. Thomson	Electric charge carrier
1898	Photographic plate	Radioactive elements	P. and M. Curie	Unstable nuclei
1911	particle scattering	Atomic nucleus	E. Rutherford	Modern atomic model
1932	Cloud Chamber	Neutron	J. Chadwick	Neutral constituent of nuclei
1932	Bubble Chamber	Positron	C. Anderson	Anti-matter
1956	Anti-neutrino detector	Anti-neutrino	Cowan and Reines	Weak interaction
1964	UA1 detector	Quarks	CERN	Strong interaction
1979	JADE detector	Gluons	DESY	Force carriers for strong interaction
1983	UA1 detector	W boson	CERN	Force carriers for weak interaction
1995	CDF detector	Top quark	Fermilab	Third generation quark
2000	DONUT detector	Tau neutrino	Fermilab	Third generation lepton

Table 15-01a Major Discoveries of Elementary Particles

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