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New Machines Will Help Study Mass


EVERY time you turn on a light, use an electric motor, or watch television, you benefit from 19th-century research into a basic property of matter - the property of electric charge.

Michael Riordan likes to cite this fact to show why particle physicists are excited about a new study of a basic material property - the study of mass.

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"We live in a century whose technologies are heavily dependent on our detailed understanding of the behavior of electric charge ... and its associated field - the electromagnetic field" that constitutes light and radio waves, he says. "Nobody could have anticipated such an outcome ... in the 19th century."

With the particle accelerators, Dr. Riordan says, "we embark on a new voyage to discover the nature and origin of mass, another key property of matter, and to understand its associated field. Who can anticipate what will emerge during the 21st century from the knowledge this epochal voyage will provide?"

A particle physicist himself, Riordan currently is on leave from Stanford University in California to serve as staff scientist for the Universities Research Association Inc. in Washington. The association has contracted with the United States Department of Energy to build one of the 'ships" on which physicists plan to set sail for this "epochal journey."

This is the Superconducting Supercollider (SSC) particle accelerator, under construction near Waxahachie, Texas, about 40 kilometers (25 miles) south of Dallas. The second "ship" may be a comparable machine proposed - but not yet authorized - for the European Laboratory for Particle Physics (CERN) at Geneva. As now planned, these machines would carry physicists' investigations into an energy range where the secrets of this mysterious property called mass should begin to reveal themselves.

Mass is the property that gives rise to gravity and to inertia - the tendency of matter to resist acceleration. It is a familiar property. Yet in spite of the deep insights physicists now have into matter's underlying nature, they do not know why the basic material particles have the particular masses they measure. They cannot even explain how mass itself arises.

The leading speculation suggests that there is a new type of energy-carrying field - called a Higg's field - that exists everywhere. Roughly speaking, the strength with which particles interact with this field determines their resistance to acceleration and, hence, their mass. If one or more such fields exist, they should have one or more new types of particle associated with them. The new accelerators are designed to concentrate enough energy in a small enough volume to create Higg's particles.

The machines would do this by smashing together counter-rotating beams of protons and antiprotons in head-on collisions. Antiprotons and protons share most characteristics except they have opposite charges.

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Physicists specify particle energies in these colossal collisions in terms of a unit they call an electron volt (eV). An electron gains 1 eV of energy when it accelerates through a voltage difference of 1 volt. It takes lots of these units to rate the new accelerators.

Particles in the counter-rotating beams in the SSC will race along at virtually the speed of light with energies of 20 trillion eV (20 TeV). Their head-on collisions at 40 TeV will have plenty of energy to share among the many interactions and particles that are created. This should easily allow for individual interaction energies up to about 2 TeV when the machine begins operating near the end of this decade.

At CERN, physicists hope to build an accelerator called the Large Hadron Collider (LHC), since protons are a type of particle known as hadrons. CERN member-states are expected to decide in 1992 whether to go ahead with this machine. If it is built, it will sit atop CERN's existing accelerator in its 27-kilometer-circumference tunnel. This would be smaller than the SSC's 87-kilometer tunnel and would provide collision energies of about 16 TeV.

Physicists look forward to using such energies to study more than the mysteries of mass. The collisions will provide physical conditions similar to those prevailing about a trillionth of a second after the Big Bang explosion in which the universe erupted into being. (The physics of the Big Bang, Page 12.)

Then there is physicists' quest to find unity among the fundamental material forces, excluding gravity, which is not of concern in particle interactions. They already know that electromagnetism and the weak nuclear force involved in radioactive decay are two aspects of one underlying interaction. They suspect that the strong force that holds atomic nuclei together also is but another aspect of this one force.

The energy at which underlying symmetry, in which all the forces look like one force, becomes evident is far beyond the range of any planned or even conceivable accelerator. Yet research at CERN reported last year by Ugo Amaldi, Wim de Boer, and Hermann Furstenau holds out hope that some sign of this supersymmetry can be detected.

The simplest supersymmetry theory suggests the existence of a set of currently undetected particles. Their presence changes particle interactions so that the strength of the three basic forces can be predicted to converge to a single unified value at very high energy.

Commenting in a letter to the journal Science, Ugo Amaldi said this work does not provide actual evidence of underlying unity. But it does show that the simplest of supersymmetry theories gives "an amazingly and puzzlingly consistent picture of the unification of the strong and electro-weak forces."

He adds that if the theory is correct, its proposed new particles "could be within reach of the present or next generation of accelerators the SSC and LHC.

For physicists, that is as compelling a reason for a voyage of discovery as the fabled wealth of the Indies was to Christopher Columbus.

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