Download What are the four fundamental forces of nature?

What are the four fundamental forces of nature?
by Craig Freudenrich, Ph.D.
As you sit in your classroom reading this article, you may be unaware of the many forces acting upon you. A
force is defined as a push or pull that changes an object's state of motion or causes the object to deform. Newton
defined a force as anything that caused an object to accelerate -- F = ma, where F is force, m is mass and a is
acceleration.
The familiar force of gravity pulls you down into your seat, toward the Earth's center. You feel it as your
weight. Why don't you fall through your seat? Well, another force, electromagnetism, holds the atoms of your
seat together, preventing your atoms from intruding on those of your seat. Electromagnetic interactions in your
computer monitor are also responsible for generating light that allows you to read the screen.
Gravity and electromagnetism are just two of the four fundamental forces of nature, specifically two that you
can observe every day. What are the other two, and how do they affect you if you can't see them?
The remaining two forces work at the atomic level, which we never feel, despite being made of atoms. The
strong force holds the nucleus together. Lastly, the weak force is responsible for radioactive decay,
specifically, beta decay where a neutron within the nucleus changes into a proton and an electron, which is
ejected from the nucleus.
Without these fundamental forces, you and all the other matter in the universe would fall apart and float away.
Let's look at each fundamental force, what each does, how it was discovered and how it relates to the others.
Gravity Getting You Down?
The first force that you ever became aware of was probably gravity. As a toddler, you had to learn to rise up
against it and walk. When you stumbled, you immediately felt gravity bring you back down to the floor.
Besides giving toddlers trouble, gravity holds the moon, planets, sun, stars and galaxies together in the universe
in their respective orbits. It can work over immense distances and has an infinite range.
Isaac Newton envisioned gravity as a pull between any two objects that was directly related to their masses and
inversely related to the square of the distance separating them. His law of gravitation enabled mankind to send
astronauts to the moon and robotic probes to the outer reaches of our solar system. From 1687 until the early
20th century, Newton's idea of gravity as a "tug-of-war" between any two objects dominated physics.
But one phenomenon that Newton's theories couldn't explain was the peculiar orbit of Mercury. The orbit itself
appeared to rotate (also known as precession). This observation frustrated astronomers since the mid-1800s. In
1915, Albert Einstein realized that Newton's laws of motion and gravity didn't apply to objects in high gravity
or at high speeds, like the speed of light.
In his general theory of relativity, Albert Einstein envisioned gravity as a distortion of space caused by mass.
Imagine that you place a bowling ball in the middle of a rubber sheet. The ball makes a depression in the sheet
(a gravity well or gravity field). If you roll a marble toward the ball, it will fall into the depression (be attracted
to the ball) and may even circle the ball (orbit) before it hits. Depending upon the speed of the marble, it may
escape the depression and pass the ball, but the depression might alter the marble's path. Gravity fields around
massive objects like the sun do the same. Einstein derived Newton's law of gravity from his own theory of
relativity and showed that Newton's ideas were a special case of relativity, specifically one applying to weak
gravity and low speeds.
When considering massive objects (Earth, stars, galaxies), gravity appears to be the most powerful force.
However, when you apply gravity to the atomic level, it has little effect because the masses of subatomic
particles are so small. On this level, it's actually downgraded to the weakest force.
Keeping It Together with Electromagnetism
If you brush your hair several times, your hair may stand on end and be attracted to the brush. Why? The
movement of the brush imparts electrical charges to each hair and the identically charged individual hairs repel
each other. Similarly, if you place identical poles of two bar magnets together, they will repel each other. But
set the opposite poles of the magnets near one another, and the magnets will attract each other. These are
familiar examples of electromagnetic force; opposite charges attract, while like charges repel.
Scientists have studied electromagnetism since the 18th century, with several making notable contributions.





In 1785, famed French physicist Charles Coulomb described the force of electrically charged objects as
directly proportional to the magnitudes of the charges and inversely related to the square of the distances
between them. Like gravity, electromagnetism has an infinite range.
In 1819, Danish physicist Hans Christian Oersted discovered that electricity and magnetism were very
much related, leading him to declare that an electric current generates a magnetic force.
British-born physicist and chemist Michael Faraday weighed in on electromagnetism, showing that
magnetism could be used to generate electricity in 1839.
In the 1860s, James Clerk Maxwell, the Scottish math and physics whiz, derived equations that
described how electricity and magnetism were related.
Finally, Dutchman Hendrik Lorentz calculated the force acting on a charged particle in an
electromagnetic field in 1892.
When scientists worked out the structure of the atom in the early 20th century, they learned that subatomic
particles exerted electromagnetic forces on each other. For example, positively charged protons could hold
negatively charged electrons in orbit around the nucleus. Furthermore, electrons of one atom attracted protons
of neighboring atoms to form a residual electromagnetic force, which prevents you from falling through your
chair.
But electromagnetism can't explain how the nucleus holds together. That's where nuclear forces come into play.
May the Nuclear Forces Be with You
The nucleus of any atom is made of positively charged protons and neutral neutrons. Electromagnetism tells us
that protons should repel each other and the nucleus should fly apart. We also know that gravity doesn't play a
role on a subatomic scale, so some other force must exist within the nucleus that is stronger than gravity and
electromagnetism. In addition, since we don't perceive this force every day as we do with gravity and
electromagnetism, then it must operate over very short distances, say, on the scale of the atom.
The force holding the nucleus together is called the strong force, alternately called the strong nuclear force or
strong nuclear interaction. In 1935, Hideki Yukawa modeled this force and proposed that protons interacting
with each other and with neutrons exchanged a particle called a meson -- later called a pion -- to transmit the
strong force.
In the 1950s, physicists built particle accelerators to explore the structure of the nucleus. When they crashed
atoms together at high speeds, they found the pions predicted by Yukawa. They also found that protons and
neutrons were made of smaller particles called quarks. So, the strong force held the quarks together, which in
turn held the nucleus together.
One other nuclear phenomenon had to be explained: radioactive decay. In beta emission, a neutron decays into a
proton, anti-neutrino and electron (beta particle). The electron and anti-neutrino are ejected from the nucleus.
The force responsible for this decay and emission must be different and weaker than the strong force, thus it's
unfortunate name -- the weak force or the weak nuclear force or weak nuclear interaction.
With the discovery of quarks, the weak force was shown to be responsible for changing one type of quark into
another through the exchange of particles called W and Z bosons, which were discovered in 1983. Ultimately,
the weak force makes nuclear fusion in the sun and stars possible because it allows the hydrogen isotope
deuterium to form and fuse.
Which force is the mightiest of them all? That would be the strong nuclear force. However, it acts only over a
short range, approximately the size of a nucleus. The weak nuclear force is one-millionth as strong as the strong
nuclear force and has an even shorter range, less than a proton's diameter. The electromagnetic force is about
0.7 percent as strong as the strong nuclear force, but has an infinite range because photons carrying the
electromagnetic force travel at the speed of light. Finally, gravity is the weakest force at about 6 x 10-29 times
that of the strong nuclear force. Gravity, however, has an infinite range.
Physicists are currently pursuing the ideas that the four fundamental forces may be related and that they sprang
from one force early in the universe. The idea isn't unprecedented. We once thought of electricity and
magnetism as separate entities, but the work of Oersted, Faraday, Maxwell and others showed that they were
related. Theories that relate the fundamental forces and subatomic particles are called fittingly grand unified
theories.