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Elegant Universe, The: Einstein's Dream
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Classroom Activities
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Background
One of the major criticisms of string theory is that it cannot
presently be experimentally verified. Strings themselves—if
they even exist—are thought to be much too small to detect
using even the largest particle accelerators and detectors. It takes
increasing amounts of energy to probe deeper into the basic
constituents of matter. It takes more energy to break apart an
atom's nucleus, for example, than it takes to break apart a
molecule. The amount of energy it would take to find evidence of
strings is believed by many physicists to be well out of reach of
current particle accelerator technology (see
"Seeking The Fundamental"). However, physicists are hoping that certain aspects of string
theory can be confirmed with existing or planned accelerators and
detectors or by other non-accelerator experiments. In this activity,
students analyze a representation of particle tracks like those
created in a bubble chamber, an early type of detector, to
understand one way physicists studied objects they could not "see."
Objective
To learn how to interpret particle interactions captured in one type
of detector, a bubble chamber.
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copy of the "Sizing Up Protons" student handout (PDF
or
HTML)
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copy of the "Bubble Chamber Basics" student handout (PDF
or
HTML)
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copy of the "Tracking Particle Paths" student handout (PDF
or
HTML)
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Tell students that particle physicists have learned much about
the subatomic world through the use of particle
accelerators—machines that speed up particles to very high
speeds and either smash them into a fixed target or collide them
together. Particles commonly used are protons, which contain
quarks, and electrons and their antimatter counterparts,
positrons. Various types of detectors record the results.This
activity will acquaint students with one kind of detector, a
bubble chamber.
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Prior to having students analyze the bubble chamber image,
acquaint students with subatomic dimensions by having them
complete the "Sizing Up Protons" activity. Organize students
into groups and distribute the "Sizing Up Protons" student
handout to each group. Have students do the calculation and
discuss the results.
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After students have completed the scaling exercise, have them
watch Fermilab's "Anatomy of a Detector" video clip (6 minutes,
13 seconds) that details how detectors work. Find it at
quarknet.fnal.gov/run2/boudreau.shtml
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Once they have watched the video clip, organize students back
into groups and distribute a copy of the "Bubble Chamber Basics"
and "Tracking Particle Paths" student handouts to each group
member. Tell students that the illustration represents some of
the tracks that might be recorded by a bubble chamber detector.
Inform students that bubble chambers are no longer used;
physicists now use detectors that measure energies 1,000 times
larger than bubble chambers can accommodate.
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Have students read about how particle tracks are created on
their "Tracking Particle Paths" student handouts and answer the
questions on the "Bubble Chamber Basics" student handout. If
students are having difficulty you might want to assist them in
identifying one of the tracks to help them get started. Check in
with each group during the activity to answer students'
questions or provide additional guidance. When students have
finished the activity, clarify any questions remaining about the
particle tracks. About which particles or interactions would
students like to know more?
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To conclude the lesson, ask students to give examples of other
objects that cannot be "observed" without additional
technologies (e.g., atoms, bacteria, viruses, DNA, bones and
soft-tissue organs in human bodies, oil deposits within the
Earth). What are some technologies used to provide evidence for,
or infer the existence of, these objects?
In Conclusion
Physicists hope that next-generation particle accelerators, such as
the Large Hadron Collider (LHC) located in France and Switzerland,
will provide evidence to support aspects of string theory. The LHC
is scheduled to go online in 2007. One of the predictions of string
theory is supersymmetry—the idea that every known elementary
particle and force carrier particle has an as-yet-undiscovered
partner particle, known as a superpartner. Future detectors may be
able to record evidence of these superpartners. However, while such
findings would support string theory, they would not necessarily
confirm the theory—supersymmetry could be a feature of the
universe even if string theory is not correct.
Sizing Up Protons
In an atom as big as Earth, a proton would be about 0.08 mile (130
meters) in diameter, close to the size of a running track around the
outside of a typical football or soccer field. The equations for
these results would be:
8,000 miles / 100,000 = 0.08 miles
13,000 kilometers / 100,000 = 0.13 kilometers (130 meters)
Tracking Particle Paths
The inward spiral track pattern created by electrons (and positrons)
in the bubble chamber is due to the particles' energy loss. (An
electron is a negatively charged particle while its anti-particle,
called a positron, is positively charged.) Because electrons and
positrons are much less massive than protons, they tend to
accelerate more when experiencing an electromagnetic force. They
lose their energy by ionizing the material in the bubble chamber.
The bubbles form and grow on these ions, which creates the tracks
that are photographed.
Below is a correctly labeled version of the sample track
illustration.
Tracks B, C, and D represent electron-positron pairs. Based on the
direction of the magnetic field in this chamber, the electron is on
the left side and the positron is on the right. Track E represents a
Compton electron, which is created when a photon knocks an electron
out of an atom. The particles at track C had greater momentum than
the particles at track D, as indicated by track C being less curved
than track D. (You may want to note to students that because
particles in bubble chambers are interacting in three dimensions,
the actual tracks created in the chamber might be longer than they
appear in the recorded image.) Track A, which is entering from a
different direction than the others, must have originated outside
the detector. It was possibly produced by a cosmic ray. Track F
represents a beamed proton that has not yet interacted with another
particle, as indicated by its nearly straight path devoid of
interactions. Compton electrons and electron-positron pairs were the
main particle interactions recorded. Any particle that is
electrically neutral, such as a neutrino or a photon, would lack the
charge needed to leave a bubble track. These particles would be
present where tracks suddenly appear or disappear.
See the full set of
String Theory Resources
"The Elegant Universe" activities align with the following National
Science Education Standards.
Grades 9-12
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Science Standard B:
Physical Science
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Structure of Atoms:
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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.
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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:
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