Connecting Quarks with the Cosmos Eleven Science Questions for the New Century (2003) / Chapter Skim
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2 Foundations: Matter, Space, and Time
2 Foundations: Matter, Space, and Time
Pages 15-42
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and extended and tested the theory of classical space-time (general relativity and big bang cosmology)
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The new quantum mechanics of simple electrical forces between elementary electrons and nuclei could explain the main
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The particles associated with strong and weakforce fields, the gluon and W/ Z bosons respectively, also carry one unit of spin, while the predicted particle associated with excitation of the Higgs field has zero spin.
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These evanescent particles, which apparently pop in and out of existence for a short time, are called virtual particles. Quantum mechanics and relativity together force scientists to see empty space in a new way: as a dynamic medium full of virtual particles.
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Although the Heisenberg uncertainty principle allows the pairs to last for only very short times, they have measurable effects, causing shifts in the spectrum of atomic hydrogen and in the masses of elementary particles that have been measured (e.g., W/Z bosons)
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In contrast, to study weak and strong interactions and thereby understand subnuclear processes, physicists had to invent new tools. They ultimately developed tools for studying processes occurring on incredibly tiny distance scales (a thousanu times smaller than an atomic nucleus)
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Already in QED the idea arose that empty space may not be as simple a concept as it had seemed. The Standard Model weak interaction theory takes this idea a step further.
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The presence of pervasive condensates is an additional way, beyond the bubbling in and out of existence of virtual particles, that seemingly empty space acts as a dynamical medium in modern quantum theories. Aside from its effect on particle masses, the Higgs condensate is not noticeable in any way because it is everywhere the same.
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However, physicists found a class of theories in which quantum corrections have just the opposite effect: forces grow weaker at small distances. This property is called asymptotic freedom.
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. The "charge" of the strong interactions, called the color charge because of superficial similarities to the familiar properties of visual color, is held by quarks and antiquarks and also by gluons.
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Further, as mentioned above, calculation of its indirect effect on well-measured quantities, via quantum corrections, predicted an approximate value for its mass. The strong interaction part of the Standard Model predicted the easiest methods by which it could be produced and how often.
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The top quarks quickly decay further into lighter particles. The Collider Detector Facility (above)
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For QED it is the ~ 1 ~ ~ 1 photon and electrical charge, for QCD it is the color gluons and color charges, and for the weak interactions it is the W and Z bosons and yet other 27
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QCD has gluons that respond to the different quark color charges. But gluons also change the color charge of a quark into a different color charge, because gluons themselves carry both color charge and anticolor charge.
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Both these features, proton decay and neutrino masses, are discussed further in Chapter 3, which explores the implications of physics beyond the Standard Model in cosmological and astrophysical situations. Another consequence of the hypothetical symmetry of the grand unified theories is at first glance as much at odds with observation as is proton
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The first lesson, from unification of the electromagnetic and weak interactions, teaches that the true symmetry of the basic equations can be obscured by pervasive condensates. The Higgs condensate was necessary to accommodate particle masses.
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Figure 2.4 shows the case with these additional particles included. Not only do the couplings all merge cleanly, but also, unlike the version without supersymmetry, the simplest supersymmetric version of the theory predicts a proton half-life that is somewhat above the current lower bound from measurements.
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Not only may Supersymmetry unify the matter constituents with the force carriers, but it may also unify gravity with the other forces of nature. Although Supersymmetry was invented for other purposes and has a rich history, it is a key element of string theory, the most promising idea that physicists have for incorporating quantum mechanics into gravity and putting gravity on an equal basis with the other forces.
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FOUNDATIONS: MATTER, SPACE, AND TIME ~ ~ ~ ~ S ~ ~ ~ (r) Particles < ,?
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PHYSICS OF SPACE AND TIME: RELATIVITY AND BEYOND The Triumph of General Relativity When general relativity, Einstein's theory of gravity, was first proposed in 1915, it was a gigantic leap of the imagination. It incorporated several concepts quite new to physics, including the curvature of space-time and the bending of light, and led to the prediction of other completely new phenomena, including gravitational radiation, the expanding universe, and black holes.
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The bending of light, first observed as a tiny apparent angular displacement for a star with light grazing the Sun during a solar eclipse, is now the basis of a fruitful technique to map dark matter using gravitational lensing. The mass of intervening galaxies is observed, in many cases, to distort the light from more distant sources drastically, even producing multiple images.
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Another difficulty arises out of Stephen Hawking's recognition that, when the effects of quantum mechanics are included, black holes are not, strictly speaking, black. Rather they radiate.
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As the discussion below indicates, some intriguing progress has recently been made toward a synthesis of general relativity, the theory of space-time, with our current understanding of the other forces of nature. THE CONVERGENCE OF MATTER AND SPACE-TIME PHYSICS In most laboratory situations, gravity, as a force between elementary particles, is very much weaker than the strong, the electromagnetic, and even the weak interactions.
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It turns out that if a large amount of ', ~ vacuum energy is dissipated only slowly, it causes a period of inflation, of exponentially rapid expansion of the universe. As is discussed in later chapters, observational cosmology has recently yielded powerful hints that inflation occurred.
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Particle Candidates for Dark Matter and the Mystery of Dark Energy Perhaps the most tangible hint for new physics from cosmology is the problem of dark matter. A wide variety of astronomical measurements indicate that the universe contains considerably more matter than can be accounted for by ordinary matter in all forms (e.g., stars, gas, and black holes)
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While particle physics has had much to say about dark matter, thus far it has shed little or no light on dark energy. Nonetheless, it seems clear that a fundamental understanding of this new form of energy will require new physics.
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Moreover, the resulting theory of gravity, unlike conventional general relativity, does not suffer from the problem of infinite quantum corrections. Further, it appears that string theory avoids the apparent paradox associated with Hawking radiation, by showing that the radiation emitted from black holes is not at all random.
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The definition of zero energy can be arbitrarily adjusted in many theories, but once the adjustment is made in one epoch of the universe it cannot be altered. One would therefore expect the effects of quantum corrections to give a vacuum energy in all epochs.
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