Thursday, May 9, 2013

History of Science -- Part Twenty-Three: Fundamental Forces

Wolfgang Pauli
The History of Physics has gone through many stages. I don’t mean just this narrative, I mean the actual history. In Newton’s age the idea of force via contact was popular. Everything was billiard balls bouncing off each other. It was the contact that transferred the force.

One interesting aspect of that model is that time could run backward. The rules of physics worked exactly the same in reverse as they did in forward gear. Imagine a cue ball hitting two other balls that are sitting touching. The two other balls would fly apart at some angle from the force of being hit by the cue ball. Now imagine the reverse. The cue ball sits still and is hit by two balls coming in at an angle. They stop and the cue ball flies backwards. The rules of physics worked the same in both directions. In a sense, time was reversible. That was force transferred by contact.

The solar system presented a bit more of a problem. Where was the contact with gravity? I suppose some still considered the crystal spheres, but just how gravity worked over a distance was not understood and Newton refused to give a hypothesis.

Later, taking the lead from iron filings behavior around a magnet, electromagnetics in Maxwell’s time was all about fields. It wasn’t force at a distance, but rather fields that particles followed. Einstein finalized that view with his general theory of relativity that showed that gravity was just planets responding to the distortion of space-time similar to bowling balls rolling down the gutter at the bowling alley. However, theories and later experiments proved that these fields fluctuated and "emminated" from the source of the force.

As scientists dug into how atomic particles followed the old and new fundamental rules of force, they actually went back to Newton’s perspective of force through contact. Using Einstein’s equivalency of mass and energy, they supposed virtual particles that would be created from energy to carry the force from one object to another. This theory of “force carriers” or “messenger particles” helped explain the zoo of subatomic objects that appeared in high energy collisions of tiny matter.

In this episode of “The History” we will look again at the four fundamental forces in review and introduce the force particles proposed to be basic to the individual forces. These force carriers or messenger particles would also obey the rules of the quantum world and would have properties such as mass, momentum, and even angular velocity or spin.

At the temperatures common to our world, four discrete forces govern the interactions of matter — gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Each force is carried by a separate messenger particle unique to it. The strong force is by far the strongest of the forces, followed by the electromagnetic force, the weak force, and finally the extremely feeble gravitational force. Though these four forces govern every matter interaction, a theory that unites them all is still being sought.

As I explained, there is a unique messenger particle associated with each of the four fundamental forces. These particles can be considered the "smallest amount" of each force than can exist in nature. Experiments have confirmed the existence of three of the four particles, but the graviton has yet to be discovered. Calculations show that it should be massless. Although the other three forces are thought to be the result of a single particle, the weak force messengers or "gauge bosons" come in two separate varieties with different masses.


Gravity, the force we discussed first in this history, is the weakest of the four forces. It is about 10-36 times the strength of the strong force. This weakness is easily demonstrable — on a dry day, rub a comb across your shirt to give it static electricity, then hold it over a piece of paper on a desk. If you were successful, the piece of paper lifts off the desk. It takes an entire planet to keep the paper on the desk, but this force is easily overcome with everyday materials employing the electromagnetic force.

However, the range of gravity is unlimited — every object in the universe exerts a gravitational force on everything else. The effects of gravity depend on two things: the mass of two bodies and the distance between them. In more precise terms, the attractive force between any two bodies is directly proportional to the product of the masses and inversely proportional to the square of the distance between the bodies. The dominance of gravity on macroscopic scales is due not to any intrinsic strength but instead to its enormous range and constant attractive nature, especially as compared to the other forces. (On a universal scale, the more powerful electric force tends to be balanced out. The positive and negative poles cancel each other out in large systems.) These properties of gravity have made it extremely difficult to incorporate gravity into modern theoretical frameworks.

The messenger particle of gravity is the graviton. It has not been experimentally verified, mainly because it is extremely hard to find the smallest denomination of the weakest force. (In addition, gravity “waves” have also been proposed, but also not verified experimentally.) Recent calculations indicate that the graviton will likely be massless, which makes it even harder to detect.

Interestingly, all versions of modern string theory incorporate gravity (unlike previous quantum theories) and not only allow but require a particle with the properties of the graviton. Its discovery will likely represent a major victory for string theory, since previous quantum theories based on the model of point particles give illogical, infinite answers when gravity is incorporated.


The electromagnetic force is actually second in effective strength to the strong force, but it is listed out of order here because it, like gravity, is more familiar to most people and this is the order we discussed them in “The History.” Its strength is less than 1% of that of the strong force, but it, like gravity, has infinite range. However, unlike gravity, electromagnetism has both attractive and repulsive properties that can combine or cancel each other out. Whereas gravity is always attractive, electromagnetism comes in two charges: positive and negative. Two positive or two negative things will repel each other, but one positive and one negative attract each other. This can be neatly illustrated by magnets: two "alike" poles will repel each other, but two opposite poles attract each other.

This is the principle that keeps atoms together: the positively charged nucleus and the negatively charged electrons attract each other. This is also the explanation of atom sizes: more electrons have greater repulsive force, so atoms with more electrons are larger because of the electrons' mutual repulsion. Similarly, atoms with larger nuclei and the same number of electrons are smaller overall because they exert a greater attractive force on the electrons.

The messenger particle of electromagnetism is the photon, a massless particle that logically (since light is a manifestation of electromagnetism) travels at the speed of light (299,792,458 meters per second or 186,282 miles per second).

The Strong Nuclear Force

The strong nuclear force is one of the less familiar fundamental forces. It operates only on the extremely short distance scales found in an atomic nucleus. Its "duties" are keeping quarks together inside protons and neutrons, and thereby keeping protons and neutrons inside atomic nuclei. Its messenger particle is the massless gluon, so named because it "glues" elementary particles together.

The Weak Nuclear Force

The weak nuclear force is the other unfamiliar fundamental force. Like the strong force, its range is limited to subatomic distances. The weak force is responsible for radioactive decay. In actuality, it is stronger than electromagnetism, but its messenger particles (W and Z bosons) are so massive and sluggish that they do not faithfully transmit its intrinsic strength.

In fact, the large mass of the W+, W-, and Z0 particles was a problem for theorists; how to explain such large mass. The conclusion was a field that acted like molasses, slowing down the particles and giving them apparent mass. This field is generated by something called a "Higgs boson." The search for this elusive particle took forty years after it was first theorized, but in the last year or so it has been found using the large accelerator at CERN in Europe.

Four Fundamental Forces
Force Strength Range (m) Particle
Strong 1 10-15
(diameter of a medium size nucleus)
gluons, π(nucleons)
Electro- magnetic 1/137 infinite photon
mass = 0
spin = 1
Weak 10-6 10-18
(0.1% of the diameter of a proton)
intermediate vector bosons
W+, W-, Z0
mass > 80 GeV
spin = 1
Gravity 6 x 10-39 infinite graviton ?
mass = 0
spin = 2

Particle Classification

There are many different particles beyond the electron, proton, and neutron that have been discovered as we’ve explored the atom. There have also been many attempts to organize these different particles and make sense of the zoo of different characteristics. When Dmitri Mendeleev developed the periodic table of the elements based on atomic number and certain repetitive characteristics of the elements, science gained great insight into the underlying atomic structure that yielded this organization.

The goal is to repeat Mendeleev's feat and learn more about the fine structure of the universe through a logical organization of these tiny and often short-lived objects. This has led to several naming conventions and families of particles that share certain characteristics. It is hard to discuss these particles without applying these various family names, so here’s a brief discussion of the most important names with some hints at why these certain particles are grouped together.


Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite charge. For example, the antiparticle of the electron is the positively charged anti-electron, or positron, which is produced naturally in certain types of radioactive decay.

Particle-antiparticle pairs can annihilate each other, producing photons. Since the charges of the particle and antiparticle are opposite, total charge is conserved. For example, the positrons produced in natural radioactive decay quickly annihilate themselves with electrons, producing pairs of gamma rays.

Antiparticles are produced naturally in beta decay, and in the interaction of cosmic rays in the Earth's atmosphere. Because charge is conserved, it is not possible to create an antiparticle without either destroying a particle of the same charge (such as occurs in beta decay) or creating a particle of the opposite charge. The latter is seen in many processes in which both a particle and its antiparticle are created simultaneously (this occurs inside of particle accelerators). This is the inverse of the particle-antiparticle annihilation process.

Although particles and their antiparticles have opposite charges, electrically neutral particles need not be identical to their antiparticles. The neutron, for example, is made out of quarks, the anti-neutron from anti-quarks, and they are distinguishable from one another because neutrons and anti-neutrons annihilate each other upon contact. However, other neutral particles are their own antiparticles, such as photons, the hypothetical gravitons, and some “weakly interacting massive particles” or “WIMPs.”


Bosons comprise one of two main classes of elementary particles, the other being fermions. The name "boson" was coined by Paul Dirac to commemorate the contribution of Satyendra Nath Bose in developing, with Einstein, Bose–Einstein statistics — which theorizes the characteristics of elementary particles. Examples of bosons include fundamental particles: Higgs boson, the four force-carrying gauge bosons of the Standard Model, and the still-theoretical graviton of quantum gravity; as well as composite particles such as mesons and stable nuclei with even mass number like deuterium (hydrogen-2 or “heavy hydrogen”) and helium-4 and others.

An important characteristic of bosons is that there is no limit to the number that can occupy the same quantum state. This property is evidenced, among other areas, in helium-4 when it is cooled to become a superfluid. In contrast, two fermions cannot occupy the same quantum space. Whereas fermions make up matter, bosons, which are "force carriers" function as the “glue” that holds matter together.


A fermion (a name coined by Paul Dirac from the surname of Enrico Fermi) is any particle characterized by Fermi–Dirac statistics and following the Pauli exclusion principle.

The Pauli exclusion principle is the quantum mechanical principle that no two identical fermions may occupy the same quantum state simultaneously. The principle was formulated by Austrian physicist Wolfgang Pauli in 1925.

Fermions include all quarks and leptons, as well as any composite particle made of an odd number of these, such as all baryons and many atoms and nuclei. A fermion can be an elementary particle, such as the electron; or it can be a composite particle, such as the proton.


A subset of the fermions is leptons. A lepton is an elementary particle which does not undergo strong interactions, but is subject to the Pauli exclusion principle. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in all atoms and is directly tied to all chemical properties.

Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite objects such as atoms, while neutrinos rarely interact with anything, and are consequently rarely observed. Although predicted in 1930 by Pauli, they were not detected until the 1950s.


Particles that interact by the strong interaction are called hadrons. This general classification includes mesons and baryons but specifically excludes leptons, which do not interact by the strong force. (Mesons are bosons, while the baryons are fermions.) The weak interaction acts on both hadrons and leptons. Both protons and neutrons are baryons, and therefore, hadrons; but the electron is a lepton.

Hadrons are viewed as being composed of quarks, either as quark-antiquark pairs (mesons) or as three quarks (baryons). There is much more to the picture than this, however, because the constituent quarks are surrounded by a cloud of gluons, the exchange particles for the color force. More on that in a later chapter.


Wolfgang Ernst Pauli was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after being nominated by Albert Einstein, he received the Nobel Prize in Physics for his "decisive contribution through his discovery of a new law of Nature, the exclusion principle or Pauli principle." The principle involves spin theory and angular momentum of atomic particles, underpinning the structure of matter and the whole of chemistry.

He was born in Vienna to a chemist, Wolfgang Joseph Pauli, and his wife Bertha Camilla Schütz in 1900. His middle name was given in honor of his godfather, physicist Ernst Mach.

Pauli spent a year at the University of Göttingen as the assistant to Max Born, and the following year at the Institute for Theoretical Physics in Copenhagen, which later became the Niels Bohr Institute. From 1923 to 1928, he was a lecturer at the University of Hamburg.

The German annexation of Austria in 1938 made him a German national, which became a difficulty with the outbreak of World War II in 1939. In 1940 he tried, in vain, to obtain Swiss citizenship, which would have allowed him to remain at the ETH. Pauli moved to the United States in 1940, where he was Professor of Theoretical Physics at the Institute for Advanced Study at Princeton. After the war, in 1946, he became a naturalized citizen of the United States, before returning to Zurich, where he mostly remained for the rest of his life. In 1949 he finally gained Swiss citizenship as well.

In 1958, Pauli was awarded the Max Planck medal. In that same year, he fell ill with pancreatic cancer. When his last assistant, Charles Enz, visited him at the Rotkreuz hospital in Zurich, Pauli asked him: “Did you see the room number?” It was number 137. Throughout his life, Pauli had been preoccupied with the question of why the fine structure constant (α), a dimensionless fundamental constant, has a value nearly equal to 1/137. Pauli died in that room on December 15, 1958.


Next chapter will discuss quarks in some length. These particles are the components of protons and neutrons and appear in other combinations. The study of quarks has added color, charm, and strangeness to an already colorful, charming, yet strange history of science.

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