Lecture 34

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Astronomy/Origins 2B03
“The Big Questions”
Lecture #34

What makes a particle “elementary” ?
• A particle is elementary if it has no inner
structure (i.e not “made” of some even
smaller entities).
Quarks and leptons
are the most elementary
particles we know about
at this time. They are no
larger than 10-18 [cm]

The Standard Model
• Quarks and leptons are the most
fundamental particles of nature
that we know about.
• Up & down quarks and electrons
make up ordinary matter.
• The other quarks and leptons can
be produced in cosmic ray showers
or in high energy particle accelerators.







Baryons (baryonic matter) - all normal ‘stuff’ made of protons and neutrons (you, me, stars, planets, etc)

W IMP dark matter would be “non-baryonic” MACHOS would
be baryonic

The Four Fundamental Forces

1. Gravity

2. Weak Force


3. Electromagnetic force

4. Strong Force
All other forces you know about can be attributed to one of these!

The New Concept of
In the 1960’s, a new theory of interactions
was developed.

At the heart of it is the concept that:

Richard Feynman, 1918 -1988
1965 Nobel Prize in Physics

Forces are the result of the exchange of
“force carriers” between the two particles
involved in the interaction.
The force carrier of the electromagnetic force is the


Unification of the forces

Two BIG questions:

1. W hen does gravity unify?
2. What breaks the
Electroweak force?

• Grand unified theories (GUTs) try to explain three of the forces in terms of a single consistent set of physical laws
• A “theory of everything” (TOEs) would explain all four forces • TOEs suggest that all four physical forces were equivalent just after the Big Bang

Antimatter = Same mass but opposite charge

We can make anti-matter
• And so can nature
– Cosmic Ray shower

• PET Scans...
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