Let's resume our discussion of forces, which were introduced last week. They are the gravitional, electromagnetic, strong nuclear, and weak nuclear forces. This week, we will discuss their operation, through force particles and fields. A key aspect of forces is that they appear to operate between entities that are not touching each other. Brian Greene provides some examples in his book The Fabric of the Cosmos:
When you go through airport security, how is it that a machine that doesn't touch you can determine whether you're carrying metallic objects? When you have an MRI, how is it that a device that remains outside your body can take a detailed picture of your insides? (p. 40)
Over the next few pages (pp. 40-42), Greene goes on to discuss magnetic fields (also noting the "deep interconnection between electric and magnetic fields"). Magnetic fields are characterized as an "invisible something that permeates the space around it" and "exert[s] a force beyond the physical extent of the magnet itself." Other kinds are fields, such as gravitional and nuclear ones, are also acknowledged.
Andrew Watson, in his book The Quantum Quark, raises conceptually similar questions:
How does a charged particle feel another nearby charged particle? How does a planet know to respond to the gravitational field of the Sun? (p. 58).
The answer, according to Watson, is that:
The force between two objects becomes the exchange of force-carrying quanta [particles] between them (p. 58).
As summarized on the following page from Particle Adventure, each of the four aforementioned forces has its own force particle(s) that "communicate" the respective forces between objects of matter. As you'll see, force particles often -- but not always -- have names that end with "on."
The photon is the force particle conveying the electromagnetic field; "photon" is also the term for a particle of light. Gluons are the force particles of the strong nuclear force and thus hold quarks together. The force particles of the weak nuclear force are called W and Z particles. Finally, the force particles of gravitation are called gravitons; at this stage, gravitons have not been experimentally detected.
As illustrated on another Particle Adventure page, a force particle may be seen as analogous to a basketball being passed back and forth between two players, who represent matter particles.
Watson notes that, "The idea that a stream of exchanged photons could explain the force between charged particles was introduced by [Enrico] Fermi and the German-American physicist Hans Bethe in 1932" (p. 59).
This is a very timely reference as Bethe died almost exactly one month ago (March 6, 2005) at the age of 98. Cornell University, where Bethe spent an unfathomable 70 years, has a tribute page for him. This page includes video links to lectures on quantum theory Bethe, at age 93, gave for his neighbors in an Ithaca, New York retirement community.
Fermi (1901-1954) has perhaps had more terms in physics and chemistry named after him than has any other scientist. In fact, matter particles, which I discussed in an earlier posting, fall within a category called fermions, whereas force particles (which I've just discussed) belong to a class called bosons.
I will revisit these latter terms, as well as the scientists for whom they are named, in future postings.