Elegant Universe, The: Einstein's Dream
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."
To learn how to interpret particle interactions captured in one type of detector,
a bubble chamber.
- copy of the "Sizing Up Protons" student handout
- copy of the "Bubble Chamber Basics" student handout
- copy of the "Tracking Particle Paths" student handout
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?
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
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
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: