Relativity - Einstein's principle of relativity basically states that the laws of physics look the same in any inertial reference frame and that there is a universal speed, c, which is the same in every inertial reference frame. The postulate that there should be an invariant speed was already built into Maxwell's equations, where the speed of light is the invariant. The Lorentz transformation is the natural generalization of the Galilean transformation of classical physics that leaves c invariant. From the Lorentz transformation, time dilation (moving clocks tick more slowly), length contraction (lengths of moving objects are shorter), and the breakdown of simultaneity are all predicted. General relativity is the generalization of Newtonian gravity to be consistent with the principles of special relativity. Here the gravitational force is described in terms of a curved spacetime: matter tells spacetime how to curve and the curvature of spacetime tells particles how to move. One of the striking features of general relativity is that it predicts that the universe is expanding. For more information on relativity, try the online general relativity tutorial written by Prof. John Baez and others.

Quantum Mechanics - The science of the very small. When we observe phenomena on atomic length scales, the classical world of Newton no longer provides an accurate description. Consider the two-slit experiment: a particle beam is fired at a screen that has two slits cut into it. It is arranged so that only one particle at a time hits the screen, and a detector is set up to determine the position of the particle after it passes through the screen. According to Newtonian mechanics, the particles go through one slit or the other, so there should be two spots where the detector measures the particles. This is not, however, what is observed. The detector measures a diffraction pattern! This is what one would have expected if, instead of using particles, we had used light waves. Quantum mechanics was developed to explain this type of phenomena. Basically, the idea is that particles have a wave-like nature and should satisfy a wave equation. This equation is called the Schrodinger equation and it correctly predicts the discrete energy levels that electrons in atoms are allowed to occupy. It is the fact that the energy has a discreet spectrum that gave rise to the name quantum mechanics. For more information on quantum physics, try The Atomic Lab.

Particle Physics - First a description of the known elementary particles, which are shown in the table to the right. Ordinary matter is composed of up and down quarks and the electron. Protons are made of two up quarks and a down quark, while neutrons are made of an up quark and two down quarks. Atoms consist of a nucleus, which is composed of protons and neutrons, and electrons that orbit the nucleus. The charmed and strange quarks are similar to the up and down quarks but more massive. Similarly the top and bottom quarks are the more massive analogs of the charmed and strange quarks. Neutrinos are almost massless particles that are produced in some nuclear reactions. The electron-neutrino and the electron are reproduced in second and third generations much like the quarks.

In particle physics, forces between particles are mediated by other particles. For example, quantum electrodynamics describes the force between two electrons as being due to the exchange of photons between the electrons. Similarly the weak nuclear force is described as being caused by the exchange of the W and Z bosons. The strong nuclear force is caused by the exchange of gluons. Since gluons strongly interact with themselves, the theory of the strong force (called quantum chromodynamics) is a highly non-linear theory. For more information on particle physics see The Particle Adventure.

One of the major goals of particle physics is to provide a unified description of the elementary particles and the four know forces (electromagnetism, weak force, strong force, and gravity). There is already a successful theory that unites electromagnetism with the weak interaction. Grand unified theories (GUTs) attempt to integrate the strong force into this formalism. However, these theories predict that the proton should decay at a rate that is larger than what observations will allow. Superstring theory attempts to unify all four forces. This is a mathematically challenging theory that has not yet been fully worked out, even though physicist have been working on it for decades. For an introduction to superstring theory, see The "Official" String Theory Web Site or the other sites listed on the main page.

Condensed Matter Physics - While particle physics concerns itself with figuring out the interactions between elementary particles, condensed matter physics takes the interactions as given and asks what behavior can we expect from a collection of a large number of particles. Often the answer is surprising. Take for example the fractional quantum Hall effect. Here, electrons behave as if they had fractional charge and obey statistics different from the usual Fermi-Dirac statistics that ordinarily govern the behavior of electrons. Another example is superconductivity where the conduction electrons in a metal form pairs. This paired state then has very different properties from a normal metal. As the name implies, a superconductor has no resistance to the conduction of electricity. A superconductor will also exclude magnetic fields from its interior; this is not at all what occurs in a typical metal. For more information on condensed matter physics, see LASSP.

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