http://www.pbs.org/wgbh/nova/teachers/activities/3012_elegant_05.html
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Elegant Universe,
The: Einstein's Dream |
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Classroom Activities |
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About
this Guide | Particle
Puzzle Pieces | Forces
of Nature | A
New Building Block? | Deducting
Dimensions | Detective Work |
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Detective Work |
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Materials
| Procedure
| Activity
Answer | Links
& Books
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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.
- copy of the "Sizing Up Protons"
student handout (PDF or HTML)
- copy of the "Bubble Chamber Basics"
student handout (PDF or HTML)
- copy of the "Tracking Particle
Paths" student handout (PDF or HTML)
- 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.
- 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.
- 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
- 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.
- 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?
- 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
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
