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Fundamental Forces Of Nature
What
are the four fundamental forces of nature?
BY CRAIG
FREUDENRICH, PH.D.
As you sit in front of your
computer 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.
Let's look at
electromagnetism, the next fundamental 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 how does electromagnetism
work at an infinite range in the large world and a short range at the atomic
level?
Physicists thought that
photons transmitted electromagnetic force over large distances.
But they had to devise
theories to reconcile electromagnetism at the atomic level, and this led to the
field of quantum electrodynamics (QED).
According to QED, photons
transmit electromagnetic force both macroscopically and microscopically;
however, subatomic particles constantly exchange virtual photons during their
electromagnetic interactions.
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 fom and fuse.
Now that you can name the
four forces -- gravity, electromagnetism, the weak force and the strong force
-- we'll see how they compare and interact with one another.
Comparing the Fundamental
Forces
From the fields of QED and
quantum chromodynamics, or QCD, the field of physics that describes the
interactions between subatomic particles and nuclear forces, we see that many
of the forces are transmitted by objects exchanging particles called gauge
particles or gauge bosons.
These objects can be quarks,
protons, electrons, atoms, magnets or even planets.
So, how does exchanging particles
transmit a force? Consider two ice skaters standing at some distance apart.
If one skater throws a ball
to the other, the skaters will move farther away from each other. Forces work
in a similar way.
Physicists have isolated the
gauge particles for most of the forces. The strong force uses pions and another
particle called a gluon.
The weak force uses W and Z
bosons. The electromagnetic force uses photons.
Gravity is thought to be
conveyed by a particle called a graviton; however, gravitons haven't been found
yet.
Some of the gauge particles
associated with the nuclear forces have mass, while others don't
(electromagnetism, gravity).
Because electromagnetic force
and gravity can operate over huge distances like light-years, their gauge
particles must be able to travel at the speed of light, perhaps even faster for
gravitons.
Physicists don't know how
gravity is transmitted.
But according to Einstein's
theory of special relativity, no object with mass can travel at the speed of
light, so it makes sense that photons and gravitons are mass-less gauge
particles.
In fact, physicists have
firmly established that photons have no mass.
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. More on them next.
Uniting the Fundamental
Forces
Science never rests, so the
work on fundamental forces is far from finished.
The next challenge is to
construct one grand unified theory of the four forces, an especially difficult
task since scientists have struggled to reconcile theories of gravity with
those of quantum mechanics.
That's where particle
accelerators, which can induce collisions at higher energies, come in handy.
In 1963, physicists Sheldon
Glashow, Abdul Salam and Steve Weinberg suggested that the weak nuclear force
and electromagnetic force might combine at higher energies in what would be
called the electroweak force.
They predicted that this
would occur at an energy of about 100 giga-electron volts (100GeV) or a
temperature of 1015 K, which occurred shortly after the Big Bang.
In 1983, physicists reached
these temperatures in a particle accelerator and showed that the
electromagnetic force and weak nuclear force were related.
Theories predict that the
strong force will unite with the electroweak force at energies above 1015 GeV
and that all the forces may unite at energies above 1019 GeV.
These energies approach the
temperature at the earliest portion of the Big Bang. Physicists are striving to
build particle accelerators that might reach these temperatures.
The largest particle
accelerator is the Large Hadron Collider at CERN in Geneva, Switzerland.
When it comes online, it will
be capable of accelerating protons to 99.99 percent the speed of light and
reaching collision energies of 14 tera-electron volts or 14 TeV, which is equal
to 14,000 GeV or 1.4 x 104 GeV.
If physicists can show that
the four fundamental forces indeed came from one unified force when the
universe cooled from the Big Bang, will that change your daily life? Probably
not.
However, it will advance our
understanding of the nature of forces, as well as the origins and fate of the
universe.
Craig
Freudenrich, Ph.D., is a freelance
science writer. He earned a B.A. in biology from West Virginia University and a
Ph.D. in physiology from the University of Pittsburgh School of Medicine. He
has over 25 years experience in biomedical research, science education, and
science writing.
https://science.howstuffworks.com/environmental/earth/geophysics/fundamental-forces-of-nature.htm
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| The wild-haired brilliant guy behind the first force we're going to talk about |
This little guy is about to find out what gravity is all about.
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| C'mon, everyone knows that opposites attract, even Paula Abdul. |
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| Dr. Hideki Yukawa, right, receives the Nobel Prize for physics in Stockholm from then Crown Prince Gustaf Adolf of Sweden Dec. 10, 1949, for his postulation on the meson. |
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| The magnet core of the Large Hadron Collider might one day unite the strong force with the electroweak force. |
