WonderQuest:  On the web since 1997...      

Fly specks and Fundamental forces

Q: What are the little black dots that flies leave behind all over the surfaces in my house? (Alison, Lissewege, Belgium)

Houseflies are messy eaters. [Jim Kalisch, Department of Entomology, University of Nebraska-Lincoln]

A: Flies have a disgusting way of feeding. They can’t eat solid food since their spongy mouthparts are too soft to break up food. So, they soften the victual by spitting on it. The saliva melts the piece of food. They sop it up like a sponge into their digestive tract. Since the food is already liquefied, flies digest nutrients fast and soon expel fecal material.

There you have it — the two sources of flyspecks — saliva and fecal matter. As the fly eats, it scatters spit. Soon after eating, it eliminates waste.

Further Reading:

Ivy Hall School, Buffalo Grove, Illinois: Fly facts by Ed Koday

Q: What is the difference between the electromagnetic force and the nuclear force? (Inbs, Chennai, India)

A: The electromagnetic and nuclear forces are both fundamental forces. They differ in several ways, including their effective range, how distance changes their strength, their relative strength, their carriers (the entities that pass each force between two interacting particles), and the fundamental particles they affect. The forces are "fundamental" because of what they act on — the fundamental building blocks of matter — such as electrons and quarks.

Fundamental forces at work: Electromagnetic and nuclear. [Public Service Company of NM, National Oceanic and Atmospheric Administration]

But first, what are the electromagnetic (EM) force and the nuclear force?

• The EM force causes like-electrically-charged particles to repel each other and unlike-charged particles to attract. This force includes the magnetic effects of moving charges and underlies such everyday forces as friction and magnetism. The EM force even causes atoms to stick together and form molecules. Matter — from cabbages to kings — is made of molecules.
• The nuclear force — is the force between particles inside an atom’s nucleus. It holds the nucleus particles together and overcomes the EM force that tries to blow the nucleus apart. Some particles in the nucleus (called protons) have like charges that repel each other and therefore threaten to tear apart the nucleus.

Changing definition alert!  By the way, the nuclear force now is called the "strong interaction" and reflects a better understanding of what protons and neutrons are made of (quarks). The "strong interaction" contains the idea of the "nuclear force" and, more fundamentally, is the force that binds quarks together to form hadrons (including protons and neutrons).

The two forces differ in effective range: The EM force affects particles at any distance apart. Whereas, the strong force acts only on particles extremely close together — inside an atom’s nucleus.

Also, the strong force is really weird in that (inside a hadron, like a proton or neutron) it doesn’t decrease in strength as particles move apart. Indeed, within its effective range, it pulls harder on the quarks that make up a hadron, the farther apart they are. It acts like a rubber band that tugs harder on particles the more they stretch the band. Most forces, like the EM force or gravity, get weaker by the square of the distance apart the particles are. But, not the strong force. When the strong force gets strong enough, there’s enough energy to create new quarks — akin to stretching a rubber band so far that it breaks into two bands.

The aptly named strong force is the strongest force known in the Universe. It is 60 times stronger than the EM force when those forces act on quarks separated by the tiny distances (less than one-tenth of a trillionth of a meter) inside a nucleus.

Another difference between the two forces is how they transfer their force between two particles — their carriers.

Have you ever wondered how two like-electrically-charged particles repel each other? They’re not touching so how can one push the other away? The present theory explaining the Universe (the so-called Standard Model) answers that question with the idea of force carriers — particles that literally transfer a fundamental force from one particle to another. The photon carries the EM force between two particles and, in so doing, pushes the particles apart if they have like electrical charges or pulls them together if the charges are opposite. Since the photon is massless, the EM force can operate over an infinite range.

A different particle — the gluon — carries the strong nuclear force. So, that’s another difference between the two forces: different carriers.

Finally, the EM force acts on electrically charged particles:

• quarks,
• charged leptons (like electrons), and
• charged force carriers.

 Whereas, the strong force acts on "color"-charged particles: quarks and gluons. Quarks have both electrical charge (a measure of how strongly they interact electrically) and a "color" charge that indicates how intensely they their color charges repel or attract each other. Gluons also have "color" charges.

"Color" is a whimsical name for the strong-force charge and has nothing to do with what we ordinarily mean by the color of something.

The table below summarizes the differences between the EM and the strong force:

* Relative to the EM force for forces acting on quarks separated by 10^-13 m.

Further Reading:

Lawrence Berkeley National Laboratory: The particle adventure

HyperPhysics: Fundamental particles and forces

Barnett, R. Michael, Henry Mühry, and Helen R. Quinn. The Charm of Strange Quarks, Mysteries and revolutions of particle physics. New York: Springer-Verlag, 2000.

(Answered Aug. 27, 2004)