The Four Forces Of Nature


Gravity & Ball

Gravity is a force that attracts objects to each other. The strength of gravity increases as mass increases, but it doesn't have much of an influence on anything until objects get to be planet sized. Consider that if you want to pick up a ball it is no effort at all, even though the earth is massive compared to the ball. Gravity is actually the weakest of the four forces. Although Newton gave us an equation that governs events here on earth and Einstein gave us General Relativity that very accurately predicts events in the universe, no one is quite sure exactly how gravity works. We know the "effects" of gravity very well. We can send men to the moon using only Newton's laws. We can predict the orbits of planets with extreme accuracy using Einstein's equations. But, when you toss a rock into a pool of water, "exactly" what makes it sink is still a mystery.  Top



Electro-Magnetism is basically electric currents and magnetic fields. The electro-magnetic spectrum consists of visible light, heat, radio waves, UV radiation, X-rays, etc. Initially electricity and magnetism were considered separate forces. However, James Maxwell generated four equations of electro-magnetism that unified previous knowledge of electricity, magnetism and light into one theory. He postulated that all of these phenomenon were based on electro-magnetic fields.  Maxwell showed that electric and magnetic fields travel through space in the form of waves. He was the first to point out that light was just another electro-magnetic wave traveling at a fixed speed. It was believed by physicists at that time that everywhere throughout space there was an invisible medium called the "either". Einstein convinced people that there was no either. All previous laws and equations of these disciplines (electricity, magnetism and light) became just simplified cases of Maxwell's equations. Maxwell's achievements concerning electro-magnetism have been called the "second great unification in physics" after the first one by Isaac Newton. Note that both gravity and electro-magnetism have very long ranges that extend well away from their sources.  Top

The Strong Nuclear Force

Strong Force

As discussed on the Particles Page., a proton consists of two Up Quarks (u) and one Down Quark (d) as illustrated in the chart to the left. A proton has an electric charge of +1, so in a nucleus with two protons, such as helium, the two protons with like charges should repel each other according to the electro-magnetic force mentioned above. (Opposite charges attract, like charge repel.) However, the two helium protons are overwhelmed by a much stronger force, called the Strong Nuclear Force (or Strong Force for short) which holds them together.

The Strong Force binds collections of protons and neutrons together to form the nucleus of an atom. On a smaller scale it is also the force that holds quarks together to form individual protons and neutrons. On an atomic scale, it is about 100 times stronger than electro-magnetism, some 10^6 times as strong as that of the weak force, and about 10^39 times that of gravity. The Strong Force when acting between quarks is so strong at that level that no isolated quark has ever been observed. In a collection of quarks as in a proton (two up-quarks and one down-quark) the total color charge cancels out which diminishes the strength of the Strong Force. (See Color Charge on the Particles Page..) The strength of the Strong Force when holding protons and neutrons together (sometimes called the Residual Strong Force) is considerably weaker and diminishes with distance compared to its strength when holding quarks together because it is mostly neutralized "within" the quark combination.  Top

The Weak Nuclear Force

Neutron Decay

The Weak Nuclear Force (or Weak Force) is a repelling force that causes some elements, such as uranium, to spontaneously decay. There are two types of decay prompted by the weak force. "Radiation Decay" is the the release of particles due to the decay of unstable nuclei. The other type of decay is "Fundamental Particle Decay".

Radiation Decay

There are three kinds of radiation decay which are named after the first three letters of the Greek alphabet: alpha, beta, and gamma:

Fundamental Particle Decay

Muon Decay

You may be thinking "How can fundamental particles decay if they can not be divided into smaller elements?" True, "fundamental" particles can not decay into smaller particles because fundamental means they are not made up of anything smaller. But they can "morph" into another less massive particle and a W boson (a force carrying particle) which is a very short intermediate step. (See bosons in Force Carriers section below.) The W boson then immediately evolves into other particles. Only the weak force can cause fundamental particles to decay. This type of decay is very different because the end products are not pieces of the starting particle, but totally new particles.

Carl D. Anderson of Caltech received the Nobel prize in 1936 for the discovery of positrons and muons while studying cosmic radiation. Muon decay is a common example of particle decay. Most naturally occurring muons are created by high energy cosmic rays (mostly protons from outer space) colliding with molecules in the upper atmosphere. About 10,000 muons reach every square meter of the earth's surface every minute.

Shown at the left is a diagram of the most common muon decay. The mean lifetime of a muon is 2.2 microseconds. The muon decays into an electron, an electron-antineutrino, and a muon-neutrino. The decay is mediated (enabled) by the W boson carrier which itself is transformed into an electron and electron-antineutrino instantaneously after the muon decay.  Top

Quantum Mechanics

Erwin Schrödinger

Quantum Mechanics, also known as Quantum Physics or Quantum Theory, is a body of mathematical models, scientific principles and experimental observations dealing with atomic size micro particles. At the beginning of the 20th Century, physicists observed phenomena in the micro world that "classic" physics could not explain. It was discovered that elementary particles behaved like standing waves as well as point particles. In 1926, Erwin Schrödinger (pictured at left) developed a mathematical model of the atom that described electrons as three dimensional waveforms rather than pure "point" particles and this led to the development of Quantum Mechanics. He received the Nobel Prize in Physics in 1933 for his mathematical model and other works.

Quantum Mechanics provides a mathematical description for the dual wave-like and particle-like behavior of elementary matter. The word "Quantum" is meant to indicate that many of the states in the micro world are not continuous, but are "quantized" into discrete ranges or "quanta" in Latin. In advanced Quantum Mechanics, much of the particle behavior emerges only at very high energies and/or very high temperatures.

Quantum Mechanics has been extremely successful in predicting new particles and accurately forecasting their individual properties to many decimal places. One of the most significant contributions of Quantum Mechanics is that it introduced "force carrying particles" to explain how the above four fundamental forces possibly work. At the end of the day, Quantum Mechanics is a very good set of mathematical models that show how many elementary forces work, but it does not explain how everything works. There are some issues with the theory as noted in the final section below.  Top

Force Carriers

Force Carriers

Shown at the left are the four Force Carriers and their corresponding forces as defined by Quantum Mechanics. The Strong Force is mediated (enabled) by Gluon particles. The Weak Force is mediated by Bosons. Electro-magnetism is mediated by Photons. And, gravity is mediated by Gravitons (but they have never been observed and are only predicted by theory). Elementary particles transmit forces between each other by exchanging force carrying particles. The force mediators transfer discrete amounts of energy called quanta between particles. One can think of the energy transfer among particles like the continuous passing of a basketball between two players.

Particle Interactions

The force carriers transmit their forces as follows:

The Standard Model

Standard Model

During the 1970's, The Standard Model of Quantum Mechanics was formulated. Physicists found that a large number of new particles were in fact combinations of a relatively small number of fundamental particles. Physicists identified and organized into a matrix the 12 truly elementary particles that are the fundamental elements of "matter" (called fermions). The four carrier forces that govern the matter particles were also added to the matrix. These 16 basic particles plus the Higgs Boson make up The Standard Model. See the chart at the left.

The Standard Model is divided into three groups: quarks (protons and neutrons), leptons (electrons and neutrinos), and bosons (the force carriers). At this time it is believed that these 16 elementary particles can not be further sub-divided. Each vertical division of the matter portion of the matrix contains two quarks, one electron and one neutrino. Only the First Generation contains the elements that make up all "normal" matter that humans come in contact with. Normal matter is made up of up-quarks, down-quarks, and electrons. These three basic elements make up the visible universe as we know it today. Generation II and III particles are much heavier and unstable. They have lifetimes well under one second and quickly decay into Generation I particles. See the Particles Page. for more information on "matter particles".

Over time and through many experiments by many physicists, The Standard Model has become established as a well-tested physics theory. Physicists use it to explain and calculate a vast variety of particle interactions and quantum phenomena. The Standard Model predicted the existence of the W and Z bosons, the gluon, and the top and charm quarks before these particles were observed. Their predicted properties were experimentally confirmed with good precision. High-precision experiments have repeatedly verified subtle effects predicted by The Standard Model. Gravity is only included in The Standard Model by hypothesis as gravitons have never been directly observed.

So far, the biggest success of The Standard Model has been the "unification" of the electro-magnetic force and the weak force into the electro-weak force. The consolidation of these two forces is a milestone comparable to the unification of the electric force and the magnetic force into a single electro-magnetic theory by James Maxwell in the 19th century.  Top

Quantum Mechanics/Standard Model Issues

Although the Standard Model has been exceptionally successful in explaining all experimental phenomena, it is not expected to be the ultimate theory because of its great complexity and the many questions it leaves unanswered. Quantum Mechanics and The Standard Model do provide the best theory we now have of the sub-atomic world, but they do not encompass all the known observations. Here are some of the current issues with quantum theory:

Does this mean that Quantum Mechanics and the Standard Model are wrong? No, Quantum Mechanics is not a "wrong" theory, it is an "incomplete" theory. New accepted theories typically encompass older proven theories. For example, Special Relativity extended the Newtonian laws of low speeds up to the speed of light. General Relativity extended the Newtonian laws of local gravity to encompass forces acting on suns, planets and galaxies. The "old" laws become "special cases" of the new laws. Physicists are still searching for a theory that encompasses Quantum Mechanics as well as gravity. Physicists think it is possible to eventually describe all forces and particles in a "Grand Unified Theory" (GUT) - the ultimate goal of physics.