While the Standard Model of particle physics and general relativity has been extremely successful, there is strong observational evidence that they are incomplete. They are unable to explain, e.g., dark energy or why the universe is dominated by matter when the theory exhibits perfect matter-antimatter (CPT-) symmetry, as any Lorentz-invariant, local field theory must. It is hoped that these theories can be unified and completed, perhaps by a version of string theory or loop quantum gravity. The natural energy scale for such theories is the Planck scale, where deviations from relativity and the Standard Model are expected to appear but where direct experimentation is impossible. One may, however, search for suppressed effects at lower energy scales in experiments of extreme precision. These effects will be minuscule and hard to discriminate against signals from conventional physics, except where the signals from standard physics are zero by an exact symmetry of the Standard Model, such as Lorentz and CPT symmetry.
Tests of fundamental symmetries often involve a clever combination of methods to suppress systematic influences. Heil et al. (p. 539), for example, report a sensitive test of Lorentz and CPT symmetry based on observing the frequencies of 3He/129Xe masers. While each of these masers is limited in stability by its sensitivity to magnetic field, a particular weighted average of their frequencies is insensitive to magnetic fields. Wilschut et al. (p. 652) extend the scope of tests of Lorentz symmetry to the weak interaction based on beta decay. Jungman (p. 550) describes experiments to find a permanent electric dipole moment of fundamental particles. Some of these experiments have already reported upper limits that rule out certain versions of supersymmetric theories beyond the Standard Model. Pustelny et al. (p. 659) describe a creative new way to seek for new interactions through a worldwide network of magnetometers. The exploration of the Standard Model through precision experiments is, of course, not restricted to the protons, neutrons and electrons making up the matter of everyday life, as evidenced by Weinheimer et al.'s study of neutrino masses (p. 565) and Volpe's study of neutrinos in astrophysics (p. 588). Precision studies of beta decay offered one of the first opportunities to learn about neutrinos, and are still among the best. Ban et al. (p. 576) offer a state of the art discussion. Naviliat-Cuncic and González-Alonso (p. 600) describe beta-decay searches for new interactions.
Measuring fundamental constants to high precision is interesting because these constants are often used in a wide range of applications, in the context of testing the accepted laws of physics, and for defining the international system of units (SI). Because of known relationships between fundamental constants there are usually many ways to do so. For example, Sturm et al. (p. 620) study the electron's gyromagnetic ratio ge. Another way to measure the same quantity is through spectroscopy of highly charged ions; the link is provided by the theory of quantum electrodynamics (Volotka et al., p. 636). The fine structure constant, in turn, can be expressed as α2 = 2(R∞/c)(h/me) with the velocity of light c, the Planck constant h, the electron mass me, and the Rydberg constant R∞, a central quantity obtained by spectroscopy of hydrogen (see Beyer et al., p. 671). One of the largest contributions to the uncertainty in hydrogen physics is the proton's charge radius. The experiments of Pohl et al. (p. 647) provided major new insights into this. The relation α2 = 2(R∞/c)(h/me) can also be solved for the electron mass. In a future version of the international system of units, the Planck constant will have a defined value, so measurements of α and R∞ become measurements of me, which can then be compared to atomic masses. The link to macroscopic mass measurement can be provided by Avogadro spheres (Bettin et al., p. 680). Penning traps are central tools in such experiments; some of their intricate properties are discussed by Kretzschmar (p. 688). Penning-trap mass spectroscopy provides opportunities to search for neutrinoless double-electron capture, closing the circle to neutrino physics (Eliseev et al., p. 707).
The fact that gravity is not integrated into the Standard Model of particle physics makes gravitational experiments an exception to the rule: there are no known exact relations between the gravitational constant and other constants of nature. Thus, gravitational experiments form an interesting cosmos of their own. Such experiments operate on all scales from large, like Hagedorn et al.'s presentation of the Microscope satellite, a test of the equivalence principle in Earth orbit (p. 720), to small, e.g., Nesterov's paper on a nanonewton force sensor to study gravity at short distances (p. 728).
These papers describe a number of cutting-edge studies that are all interesting by themselves; we hope that by uniting them in one volume, the reader will be offered a view of their relationships, which give us some of our best opportunities for testing the overall consistency of our fundamental theories and experimental methods.