Free Fear of Physics by Lawrence M. Krauss

their lifetimes turned out to be fairly close to our dimensional estimate (that is, approximately 10 –25 sec). Generically, the lifetimes of these particles were in the neighborhood of 10 –24 seconds, so that the constant k in a dimensional estimate would be about 10, not too far from unity. Yet the interactions between quarks, which allow these particles to decay, at the same time seem to hold them so tightly bound inside particles like protons and neutrons that no single free quark had ever been observed. Such interactions seemed so strong as to defy attempts to model them in detail through any calculational scheme.
    In 1973, an important theoretical discovery prepared the path. Working with theories modeled after the theory of electromagnetism and the newly established theory of the weak interactions, David Gross and Frank Wilczek at Princeton and, independently, David Politzer at Harvard discovered that an attractive candidate theory for the strong interactions between quarks had a unique and unusual property. In this theory, each quark could come in one of three different varieties, whimsically labeled “colors,” so the theory was called quantum chromodynamics, or QCD. What Gross, Wilczek, and Politzer discovered was that as quarks moved closer and closer together, the interactions between them, based on their “color,” should become weaker and weaker!
Moreover, they proved that such a property was unique to this kind of theory—no other type of theory in nature could behave similarly.
    This finally offered the hope that one might be able to perform calculations to compare the predictions of the theory with observations. For if one could find a situation where the interactions were sufficiently weak, one could perform simple successive approximations, starting with noninteracting quarks and then adding a small interaction, to make reliable approximate estimates of what their behavior should be.
    While theoretical physicists were beginning to assimilate the implications of this remarkable property, dubbed “asymptotic freedom,” experimentalists at two new facilities in the United States—one in New York and one in California—were busily examining ever higher energy collisions between elementary particles. In November 1974, within weeks of each other, two different groups discovered a new particle with a mass three times that of the proton. What made this particle so noticeable was that it had a lifetime about 100 times longer than particles with only somewhat smaller masses. One physicist involved commented that it was like stumbling onto a new tribe of people in the jungle, each of whom was 10,000 years old!
    Coincident with this result, Politzer and his collaborator Tom Appelquist realized that this new heavy particle had to be made up of a new type of quark—previously dubbed the charmed quark—whose existence had in fact been predicted several years earlier by theorists for unrelated reasons. Moreover, the fact that this bound state of quarks lived much longer than it seemed to have any right to could be explained as a direct consequence of asymptotic freedom in QCD. If the heavy quark and antiquark coexisted very closely together in this bound state, their interactions
would be weaker than the corresponding interactions of lighter quarks inside particles such as the proton. The weakness of these interactions would imply that it would take longer for the quark and its antiquark to “find” each other and annihilate. Rough estimates of the time it would take, based on scaling the strength of the QCD interaction from the proton size to the estimated size of this new particle, led to reasonable agreement with the observations. QCD had received its first direct confirmation.
    In the years since this discovery, experiments performed at still higher energies, where it turns out that the approximations one uses in calculations are more reliable, have confirmed beautifully and repeatedly the predictions