Space, Matter and Time -- what else can there be? Quantities of these fundamental qualities can be measured in Meters, Kilograms and Seconds, respectively -- the Système Internationale (SI) standard of measurement (also known as MKS). The universe is believed to consist of matter (4% atoms, 20% dark matter) and energy (76% dark energy). Energy is regarded to be equivalent to matter by Einstein's famous equation E = mc².

The so-called Standard Model of particle physics describes the universe in terms of Matter and Force. Force is not independent of the fundamental qualities. Force can be expressed in terms of the fundamental qualities as Mass times Acceleration [kg x m/s²] or (equivalently) as change of Momentum per unit Time [(kg x m/s)/s]. The Standard Model describes approximately 200 elementary particles and their interactions using 6 quarks, 6 leptons and a few force-carrying particles. There are four known forces, each mediated by a fundamental particle (quantum, known as a carrier particle):

Photons & gravitons have no mass, whereas the gluon and weak-force quantum-particles have mass of 0.14 and 80-90 GeV, respectively. Mass of subatomic particles is described by the mass-energy unit GeV, Giga (billion) electron volts. (The amount of energy an electron gains moving through a potential of one volt in a vacuum is one electron-volt,1eV.) Gravity is only included in the Standard Model by tentative hypothesis -- gravitons have never been observed. At very high energies and very small scales the other three forces become almost identical, but the convergence is imperfect.

Electromagnetic & gravitational forces vary as the inverse square of distance without limit (to infinity). But the strong & weak nuclear forces are short-range rather than inverse-square forces. Short-range forces only operate at very short range through exchange of particles, whereas inverse-square forces have no range-limits. It is the non-zero rest mass of the short-range force-mediating particles which causes them to decay quickly and thereby limits their range. For the strong nuclear force the exchange-particle is the gluon (nuclear "glue"). For the weak nuclear force the exchange-particle is W⁺, W^- or Z.

Subatomic matter particles can be described as fundamental or composite. Protons & neutrons are composite, whereas an electon is a fundamental particle (a proton is 1,800 times more massive than an electron). The fundamental matter particles are quarks & leptons. There are 6 flavors of quarks and 6 flavors of leptons (3 pairs). The six flavors of quarks can be summarized in the following table:

The most massive quark, the top quark, has the mass of a silver atom -- and is so unstable that it was the last quark to be discovered (in 1995, after years of searching). The strong force is so strong that single quarks cannot be isolated -- their existence is deduced from the rates of decay of composite particles. It is the weak force that governs how heavier quarks decay into lighter ones.

The six flavors (3 pairs) of leptons can be summarized in the following table:

Whereas leptons may be observed in isolation, quarks have never been observed in isolation. The first two flavors of each table (up & down quarks, electron & electron neutrino) are known as first-generation matter -- as distinct from the second pair of rows (second-generation) and the third pair of rows (third-generation). First-generation matter is both the least massive and the most stable. The vast majority of matter in the universe is first generation, because second and third generation matter is too unstable to last more than tiny fractions of seconds (decaying into first-generation matter).

All subatomic particles have spin (intrinsic angular momentum), which is either ½-spin or integer-spin (0,1 or 2). Spin is quoted in units of Plank's constant divided by π. For both force-carrier and fundamental particles, spin determines the energy distribution function, which can be either Bose-Einstein (Bosons) or Fermi-Dirac (Fermions). Particles with ½-spin (fermions) are constrained by (obey) the Pauli Exclusion Principle, whereas other particles (bosons) are not. In sum:

The Pauli Exclusion Principle prevents fermions (protons, neutrons, electrons, quarks, neutrinos, etc.) from being too close if they have the same quantum state (spin), but bosons (photons, gluons, W/Z bosons, gravitrons, etc.) can be close together while sharing the same quantum state -- as with masers & lasers for photons. Near absolute zero temperature bosons can form a Bose-Einstein condensate (predicted by Albert Einstein on the basis of the work of East Indian physicist Satyendra Bose, the namesake of bosons). Bose-Einstein condensates have been created consisting of thousands of atoms (first demonstrated with rubidium atoms in 1995).

All matter is acted-upon by weak nuclear force. Matter particles acted upon only by strong nuclear force are called hadrons, as distinct from leptons. Again, the strong nuclear force acts on hadrons, but does not act on leptons (electrons are unaffected by the strong force). The weak nuclear force acts on both hadrons & leptons. There are two types of hadrons: baryons & mesons. Dark matter is not made of baryons, but its exact composition is still a matter of speculation. A baryon is made of 3 quarks, whereas a meson is made of one quark & one antiquark. Baryons & leptons are fermions (½-spin), whereas mesons & force-carrier particles are bosons (integer spin). Hadrons are "glued" together by gluons (quarks are not found in isolation, a phenomenon known as "confinement"). The larger the number of gluons exchanged among quarks, the stronger the binding force.

The most important baryons are the proton and the neutron. Since these baryons are stable particles, it is not surprising that they are composed of the lightest & most stable quarks: the up-quarks and the down-quarks. According to quantum chromodynamics a proton is composed of 2 up-quarks & one down-quark, whereas a neutron is 2 down-quarks & one up-quark. Whereas the quarks have ½-spin, the gluons that hold quarks together have unit spin (1-spin). Unlike electrostatic forces that weaken with increasing distance, strong force weakens with increasing closeness. But if a quark attempts to escape from a nucleon (proton or neutron) by moving outward, the strong force becomes so intense as to make escape impossible. For a proton, for example, the masses of 2 up-quarks & one down-quark accounts for only about 2% of the mass and 30% of the spin -- showing the contribution of gluons and raw (kinetic & potential) energy (E =mc²).

Strong force due to gluon exchange only occurs within protons & neutrons. The force that holds the nucleus together is due to "leakage" from gluon exchange that results in an exchange of pion particles between protons & neutrons. Unlike photons, which uniformly surround electrons forming a spherically symmetric shell, gluons clump together into tubes when linking quarks to quarks or to antiquarks. (Agglomerations of gluons alone are called "glueballs").

Another hypothetical force-particle, the Higgs boson, has been proposed to cause an interaction that causes particles to have mass. The Higgs boson itself is predicted to be 190 times more massive than a proton.

Analogous to the two-valued electrical charge associated with electromagnetic force is a three-valued "color" charge associated with quarks & the strong force (gluons) that bind quarks together. The colors of the three-valued charge are called red, green and blue -- not visual colors, but a kind of "charge" based on an analogy to colors. Just as combining electrical positive & negative charge results in a neutral electrical-charge, combining red, green & blue color-charge gives a neutral color charge (the analogy to color being that mixing the red, green & blue primary colors gives neutral white).

All quarks & gluons have color charge, but all the hadrons (protons, neutrons, mesons) comprised of quarks, antiquarks and gluons have neutral color charge (analogous to most atoms having a neutral electrical charge). A quark can change color by emitting or absorbing gluons. If a red quark becomes a green quark it must have emitted a gluon carrying the colors red and anti-green. The quantum field theory based on electromagnetic quanta is called quantum electrodynamics (QED), which explains electrically charged particles created in particle decay. The quantum field theory based on strong & weak force quanta is called quantum chromodynamics (QCD). Color & electromagnetic charge are both conserved.

For every matter particle there corresponds an anti-matter particle. Anti-matter particles can correspond to matter particles in every respect except that the charge is opposite. An anti-electron (positron, the only anti-particle with a unique name) has the same mass as an electron, but is electrically positive. Antiquarks have electrical charges - 2/3 and + 1/3. Associated with the anti-quarks, however, are the anti-color charges: anti-red, anti-green and anti-blue. An anti-proton is composed of 2 up-antiquarks & one down-antiquark.

When a particle and an anti-particle meet, they annihilate into pure energy and may give rise to energetic neutral force-carrier particles, such as gluons, photons or Z-bosons. Conversely, energetic force-carrier particles can give rise to matter particle/anti-particle pairs (pair production). An unsolved mystery of cosmology is why the universe is dominated by matter rather than anti-matter.

It has been mentioned that mesons are boson hadrons (in contrast to baryons, which are fermion hadrons). Since a meson is a quark bound by a tube of gluons to an anti-quark, one can correctly conclude that mesons are not very stable particles. The most long-lived (and lightest) meson is the positively-charged pi-meson (pion, composed of an up-quark & a down-antiquark, mass 0.14 GeV), which has an average lifetime measured in nanoseconds.

The Standard Model

The weak nuclear force is responsible for nuclear decay. The weak nuclear force is mediated by the massive W-bosons (mass of a Bromine atom) & Z-boson (mass of a Zirconium atom). Emission & absorption of the W⁺ & W^- charged W-bosons is the only way quarks change flavor. A Z-boson (which is its own anti-particle) can decay into either a quark/anti-quark pair or into a lepton/anti-lepton pair (same flavor in both cases). The means by which a neutron decays into a proton (beta-decay) is by emitting a W^--boson (leaving a proton) which decays further into an electron and an (electron) anti-neutrino. Weak W-boson nuclear force is responsible for the fact that all the more massive quarks & leptons rapidly decay into the lightest (and most stable) quarks & leptons. Weak Z-boson force influences scattering cross-sections for neutrinos.

In sum, the Standard Model consists of 17 particles, one of which (the Higgs boson) is still very hypothetical. Twelve of the 17 particles are Fermions (the stuff of matter): 6 quarks and 6 leptons. The remaining five particles are bosons, four of which are physical manifestations of the forces through which particles interact. (At high energies the weak nuclear force merges with electomagnetic force.) The fifth boson is the hypothetical Higgs boson which would give particles their masses. About 85% of the mass of the universe is yet unaccounted-for by any of the particles in the Standard Model -- missing "dark matter".

Although strong & electromagnetic forces make no distinction between right-handed or left-handed particles (particle invariance), particles subject to weak forces do make this distinction. (A right-handed particle is a particle spinning in the direction the right-hand fingers curl when the particle is traveling in the direction pointed-to by the right thumb). Thus, left-handed neutrinos are matter, whereas right-handed neutrinos are anti-matter.

A number of superstring theories have been proposed to unify relativistic quantum field theory with general relativity theory. At Planck-length (10^-35 meter) dimensions Einstein's equations of general relativity result in such intense fluctuations with energy that "spacetime goes haywire". Instead of boson & fermion particles, the universe is proposed to be made of Planck-length boson & fermion strings -- two-dimensional entities vibrating in ten-dimensional space-time. Strings might be closed loops or open -- and they must be stretched under tension to vibrate (excite). Unlike particle interactions which occur at a single point in space-time, strings collide over a small but finite distance. Strings vibrate in ten dimensions, six of which are tightly coiled in on an unmeasurably small scale and four of which are in conventional space-time. A variant known as membrane theory (M-theory, "branes" -- multi-dimensional membranes) puts gravity in an eleventh dimension and points to an infinite number of solutions -- implying (for some) an infinite number of universes.

The Standard Model treats fundamental particles as point-like entities having no dimensions, adjusted for by a kludge called renormalization. String theory removes the need for renormalization and provides mathematically satisfying explanations for many other problems. But string theory has still not fulfilled its promise of unifying gravity and quantum mechanics. Nor has it produced testable hypotheses, because strings could only be measured at energies well beyond the capacities of existing particle accelerators. Some physicists worry that aesthetic elegance is displacing evidence as the basis of physical theory.

(For more details on superstring theory go to http://www.superstringtheory.com/. For more on infinite numbers of universes, see my essay The Copenhagen Interpretation of Quantum Mechanics.)

According to Big Bang theory, the existing universe emerged from an explosion in a vacuum that occurred 15 billion (10⁹) years ago. Initially there were equal parts matter & anti-matter, but due to a peculiar assymmetry matter began to predominate. Matter was a soup of quarks & leptons at a temperature of 10 quadrillion (10¹⁶) ºC. After a nanosecond the temperature fell to one trillion (10¹²) ºC and protons & neutrons began to form. The "electroweak" force differentiated into the electromagnetic force and the weak force. A plasma of electrons & nuclei existed for 300,000 years until the temperature dropped to 5,000ºC when hydrogen & helium atoms formed.

If matter and antimatter were perfectly symmetrical, the cooling of the universe would have resulted in particle/antiparticle annihilation that would have left the universe filled only with photons. But for every billion mutual annihilations a particle of matter remained -- comprising the existing matter of the universe. About 99% of the photons in the universe (the cosmic microwave background) are the result of Big Bang annihilations. Photons from stars are a trivial contribution, by comparison.

For more detailed charts of Standard Model particles & interactions, see http://particleadventure.org/particleadventure/frameless/chart.html.

See also the American Physics Society Particle Physics Links and the Interactions.org Image Bank for more tutorials and visual aids.

For more on theories of particle physics see Elementary Particle Physics Today.

 
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