Now that we've completed our introduction to matter and the particles of which it is composed, today we'll begin our discussion of the forces that attract or repel particles. It has been common in some of the books I've read for matter and forces to be discussed together. Just as there are matter particles, there are also force particles. Another concept that comes up in conjuction with forces is that of fields, which I'll also discuss.
For conveying the ideas inherent in forces, I highly recommend the segment of the Particle Adventure online slide show entitled "What Holds it Together?" In fact, after you're done reading my entry on forces, I would urge you to go back and view this slide show.
According to the aforementioned Particle Adventure slide show, "...a force is the effect on a particle due to the presence of other particles."
There are four kinds of forces: gravitational, electromagnetic, strong nuclear, and weak nuclear.
We all know gravity as what keeps us tied down to the earth. As the late, great Caltech physicist Richard Feynman noted in a famous series of lectures at UCLA (preserved in the Feynman book QED: The Strange Theory of Light and Matter), gravity is "...the thing that holds you in your seats (actually, that's a combination of gravity and politeness, I think)..." (p. 8).
Gravity applies to any two objects, such as the earth, orbiting the sun. As noted in the Wikipedia's description:
Gravitation is the tendency of masses to move toward each other... Exactly why two masses separated in space have a gravitational attraction to one another remains largely unknown, despite much research and various theories.
I will discuss gravity further, in a future writing, when we explore Einstein's theory of general relativity.
The electromagnetic force, as the name implies, represents a combination of electricity and magnetism, as developed by 19th century physicist James Clerk Maxwell. As described in the Wikipedia:
...it turns out that the electromagnetic force is the one responsible for practically all the phenomena one encounters in daily life, with the exception of gravity. Roughly speaking, all the forces involved in interactions beween atoms can be traced to the electromagnetic force acting on the electrically charged protons and electrons inside the atoms...
Furthermore, light is actually a kind of travelling disturbance in the electromagnetic field (i.e. electromagnetic waves.) Therefore, all optical phenomena are actually electromagnetic phenomena.
The two other known forces are the strong nuclear and weak nuclear forces. Given that they both implicate "nuclear" forces, it will be helpful to review the basics of an atom's nucleus. As seen here for the example of the helium atom, the nucleus contains two protons and two neutrons, with two electrons on the atom's periphery. According to a "Nuclear Science Primer" by the Lawrence Berkeley Laboratory, chemical elements differ primarily by the number of protons they have.
As I discussed in an earlier posting on matter and subatomic particles, protons and neutrons are each comprised of different combinations of up and down quarks. Thus, two instances of "holding together" must occur. Quarks must be held together to form protons and neutrons, and the combinations of protons and neutrons must be held together in the nucleus.
This is where the strong nuclear force comes in. According to the aforementioned Nuclear Science Primer:
Because like electrical charges repel each other, scientists realized early on that packed protons—those little nuggets of positive charge—should cause any nucleus with two or more protons to blow itself apart. A quite different force was required, which researchers understandably named the "strong nuclear force."
It turns out that both protons and neutrons consist of smaller entities called quarks. The strong force is what holds quarks together; it is the leftover, "residual" strong force that binds the nuclei.
The strong force is associated with the theory of quantum chromodynamics (QCD), which appears to be a thriving area of particle physics. The 2004 Nobel Prize in physics was awarded to a trio of researchers -- David Gross, David Politzer, and Frank Wilczek -- who have studied the strong nuclear force/QCD. As I've alluded to previously, I recently read the book The Quantum Quark, by Andrew Watson, which is on QCD; I will discuss this theory in a future posting.
Finally, there is the weak nuclear force, which as best I can tell, gets the least attention. Brian Greene, in the public broadcasting documentary of his book The Elegant Universe, provides the following narration (click for transcript):
Now, the strong and weak forces may seem obscure, but in one sense at least, we're all very much aware of their power. At 5:29 on the morning of July 16th, 1945, that power was revealed by an act that would change the course of history. In the middle of the desert, in New Mexico, at the top of a steel tower about a hundred feet above the top of this monument, the first atomic bomb was detonated.
It was only about five feet across, but that bomb packed a punch equivalent to about twenty thousand tons of TNT. With that powerful explosion, scientists unleashed the strong nuclear force, the force that keeps neutrons and protons tightly glued together inside the nucleus of an atom. By breaking the bonds of that glue and splitting the atom apart, vast, truly unbelievable amounts of destructive energy were released.
We can still detect remnants of that explosion through the other nuclear force, the weak nuclear force, because it's responsible for radioactivity. And today, more than 50 years later, the radiation levels around here are still about 10 times higher than normal.
In summary, those are the four forces -- gravitational, elecromagnetic, strong nuclear, and weak nuclear. In what may be a surprise to many people, Greene notes in his later book The Fabric of the Cosmos that:
Gravity is by far the weakest of all forces (for example, an ordinary refrigerator magnet can pick up a paper clip, thereby overcoming the pull of the entire earth's gravity)... (p. 255).
This example, of course, can be used as your first do-it-at-home experiment!
Next week, we'll look at force particles and fields...