FUNDAMENTAL FORCES
• The four fundamental forces in nature are:
1. Strong force
2. Electromagnetic force
3. Weak force
4. Gravitational force
All forces have a specific messenger particle
• What does the word fundamental mean?
• Serving as a basis supporting existence or
determining essential structure or function.
• Physicists believe that all interactions in the
universe are controlled by these forces.
• What are the fundamental particles of nature?
• Protons, Neutrons, and Electrons???
We need to look at smaller particles!
• Protons and Neutrons are both made up of
quarks.
• There are 6 types of quarks, known as flavors.
Up, Down, Strange, Charm, Bottom, and Top.
• Up and Down quarks are the most
stable, and 3 quarks make up
neutrons and protons.
• Up quarks have a +2/3 charge
• Down quarks have a -1/3 charge
• Certain types of quarks must come together to
make protons and neutrons.
• Quarks also have “color”. There are three types
of color charge; blue, green, and red.
• For their color to be neutral (called white) each
hadron (particles with either 2 or 3 quarks) must
have one of each color.
• It’s messenger particle is the gluon; it is
massless.
• Lets explore what scale these forces can act at.
• The strong force keeps protons and neutrons
inside the nuclei.
• What kind of range would that be?
• Lets look at a simulation.
• The Weak force acts in the nucleus of atoms.
• The Weak force is responsible for “flavor change”,
where quarks of one flavor can change to another.
• This happens even with protons and neutrons!
• This process is called radioactice beta decay.
• A neutron “splits” into an electron, a proton, and an
electron anti-neutrino.
• Messenger particles (W and Z bosons) are so
massive and sluggish that they do not faithfully
transmit its intrinsic strength.
• Wait! What will happen to all of the neutrons?
• There is also a process called inverse beta
decay, which changes a proton into a neutron.
• These transformations between protons and
neutrons inside the nucleus happen constantly.
• Protons are the most stable hadron, and only the
strong force keeps the neutrons in nuclei from
decaying into protons.
• The electromagnetic force is actually the second
in effective strength.
• What is the range of the electromagnetic force?
• The range is infinite!
• We know that some electromagnetic forces are
attractive, and some are repulsive.
• The attractive force of the negative electrons to
the positive protons inside the nucleus keeps the
atoms together.
• Atoms with more electrons are larger because of
the repulsion of the orbiting electrons.
• Similarly, atoms with larger nuclei and the same
number of electrons are smaller overall because
of a stronger attractive force between the
protons and electrons.
• The messenger particle of electromagnetism is
the photon. It is massless, and travels at the
speed of light.
• Gravity is by far the weakest of the 4
fundamental forces. It is 10−36 times the
strength of the strong force.
• How can this be? Charge up a comb by rubbing
it through your hair, and you can pick up pieces
of paper off the table.
• What is the range of gravity?
• It’s messenger particle is the graviton, although
not yet found, it is believed to be massless.
Force
Relative
Range
Strength
Strong
1
Less than
10−15 𝑚
Electromagnetic 1/137 Infinite
Weak
Gravitational
10−5
6𝑥10−39
Less than
10−18 𝑚
Infinite
Function
Holds protons and
neutrons together
Hold atoms together,
also repel if too close
Involved in radioactive
decay
Mutual attraction of
matter
• Scientists wonder about the reasons behind the
properties of the forces and their particles as
well.
• Why are three of the messenger particles
massless, and the fourth is one of the heaviest
known particles?
• Why do the ranges and strengths of the forces
differ so drastically?
• The formation of stable nuclei depends on the
ratio of the strong and electromagnetic forces
• if gravity were any stronger, stellar matter would
bind more strongly and stars would use their
nuclear fuel much faster, thus negating the
possibility of the evolution of life. (and we would
not exist)
• If gravity were any weaker, matter might not
"clump together" to form larger structures,
thereby preventing the formation of stars in the
first place.