Elegant Universe, The: Einstein's Dream
The building blocks of matter that have been experimentally verified are the quarks and leptons described by the Standard Model. Since the discovery of the electron in 1897, physicists have identified some 200 subatomic particles, all of which are
leptons or quarks or a combination of quarks. In this activity, students will
investigate the "recipes" for constructing
a proton and neutron from the quarks described in the Standard Model.
To learn about some of the elementary particles in the Standard Model by building a proton and neutron from quarks.
- copy of the "Particle Puzzle Pieces" student handout
Organize students into teams and distribute copies of the "Particle Puzzle Pieces" student handout.
Review with students the history and nature of the atom (for sources of
information, see General Physics Resources). Then discuss with
students the matter particles that make up the Standard Model (see "Elementary Particles" and
"Elementary Antiparticles" below). Emphasize that all matter
comprised of Standard Model particles is made from first generation particles (the instability of second and third generation particles causes them to quickly decay into stable first generation particles). Additionally, antimatter is rarely seen in the everyday world. (See Activity Answer below for more information.)
Have students use the "Quark Chart" and "Quark Recipe Rules" on their student handout to discover how to build a proton and a neutron.
Discuss students' results and answers to the questions on the student handout. To supplement this activity, have students use the "Atom Builder" to build a
carbon atom out of elementary particles. Find it at www.pbs.org/aso/tryit/atom/
Physicists have used particle accelerators and detectors to confirm the existence
of most of the elementary particles and
antiparticles predicted by the Standard Model. One particle that has been
theorized but not yet discovered is called the Higgs boson. This particle is thought to be a force carrier particle linked with the Higgs field, which might be the
mechanism by which particles acquire their mass. In the 1960s, the physicist Peter Higgs postulated the existence of
this field through which all particles are thought to move. The Higgs boson is
considered to be the final missing piece
of the Standard Model.
From the "Quark Recipe Rules," students will likely infer that they should:
only use 1st generation (up and down) quarks in their recipe.
use more than one quark to build a proton and neutron.
build a proton with a net integer charge of 1; and a neutron with 0 charge.
use the smallest number of quarks possible to meet the stated criteria.
This information should help students discover through trial and error the composition of quarks necessary to describe a proton and a neutron: The proton should contain two up quarks and one down quark; the neutron should contain one up quark and two down quarks.
Check to ensure that student recipes use the lowest number of quarks possible—three. This concept is identical to that of the Least Common Multiple in mathematics. To create a neutral atom, three electrons would be needed in an atom containing three protons and four neutrons.
You may want to note to students that while antimatter
particles are part of the basic building blocks in our universe, and have been identified by particle detectors, they are not observed very much in the everyday world. That's because when matter and antimatter meet, they annihilate each other. The resulting energy, however, is not lost; it can rematerialize as new particles and antiparticles.
Physicists theorize that at the time of the big bang, matter
and antimatter were created in identical amounts. So why
didn't the matter and antimatter annihilate each other and
end the universe as we know it? Part of the answer may be that an asymmetry in the weak force occasionally converts antimatter into matter. But some physicists believe that this effect accounts for only some of the imbalance. New theories predict additional sources for asymmetry for which physicists continue to search.
One of the few places where matter and antimatter occur
outside of a particle accelerator is in the medical imaging
technique known as Positron Emission Tomography (PET).
In PET, positrons (the antimatter partner of electrons) are created by the decay of radioactive nuclei. The process works by first attaching a radioactive element to a natural body substance (glucose is commonly used) and injecting it into a patient. After the targeted area absorbs the substance, the radioactive nuclei undergo beta plus decay and the positrons that are created
collide almost immediately with the electrons they encounter. The mass of both particles is converted into two gamma rays that travel outward and away from each other in exact
Gamma ray detectors that surround the patient register and measure these events. After algorithms are applied to the data, an image is constructed that shows areas where radioactivity is concentrated. These areas indicate signs of metabolic activity, giving clues to where tumors are or providing information about
physiologic function to help diagnose disease.
See the full set of String Theory Resources
"The Elegant Universe" activities align with the following National Science
Science Standard B:
Structure of Atoms:
Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atom together.
The nuclear forces that hold the nucleus of an atom together, at nuclear distances, are usually stronger than the electric forces that would make it fly apart.
Structure and Properties of Matter: