Fly specks and Fundamental forces
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 canít 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
There you have it ó the two sources of flyspecks ó saliva and
fecal matter. As the fly eats, it scatters spit. Soon after eating, it
Hall School, Buffalo Grove, Illinois: Fly facts by Ed Koday
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 ó the fundamental building blocks of matter ó such as electrons and
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
- 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 ó from cabbages to kings ó is made of molecules.
- The nuclear force ó is the force between particles inside
an atomís nucleus. It holds the nucleus particles together and overcomes the
EM force that tries to blow the nucleus apart. Some particles in the nucleus
have like charges that repel each other and therefore threaten to tear apart
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
that binds quarks together to form
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 ó inside an atomís nucleus.
Also, the strong force is really weird in that (inside a
hadron, like a proton or neutron) it doesnít 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, thereís enough energy to create new quarks ó 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 ó their carriers.
Have you ever wondered how two like-electrically-charged
particles repel each other? Theyíre 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 ó
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 ó the gluon ó carries the strong nuclear
force. So, thatís another difference between the two forces: different carriers.
Finally, the EM force acts on electrically charged
- 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" 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:
Inside an atomís nucleus
Force decreases with (range)≤.
Force increases with range.
Electrically charged particles: quarks,
charged leptons, charged force carriers.
"Color" charged particles: quarks and gluons
Friction, magnetism. Causes atoms to stick
together and form molecules
Causes nuclear particles to stick together.
* Relative to the EM force for forces acting on quarks
separated by 10^-13 m.
Berkeley National Laboratory: The particle adventure
Fundamental particles and forces
Barnett, R. Michael, Henry MŁhry,
and Helen R. Quinn. The Charm of Strange Quarks, Mysteries and revolutions of
particle physics. New York: Springer-Verlag, 2000.
(Answered Aug. 27, 2004)