Most of atomic physics is concerned with the behavior of valence electrons, where energies are of the order of the square of the fine structure constant (α≈1/137) times the rest energy of the electron, mc². Well-known relativistic corrections, such as fine splitting of energy levels, are smaller than usual atomic transition energies by two extra powers of the fine structure constant, and are even smaller for hyperfine splitting. In valence-electron atomic physics, therefore, relativistic effects are normally treated as small corrections.

The presence of ultra-intense fields radically alters the nature of relativistic effects. As the field intensity rises in photon-induced processes, a precursor to full-fledged relativistic effects is the loss of the usually well-justified dipole approximation. In strong-field ionization (with linear polarization), the post-ionization oscillation of the detached electron in the laser field changes from the low-intensity linear oscillation to the strong-field figure-8 pattern. This qualitative change is brought on by the increasing importance of the magnetic component of the field that can be neglected for lower intensities. One expects the dipole approximation to fail when the width of the figure-8 pattern approaches the size of the atom. This occurs for a typical strong-laser frequency of about .06 a.u. when the field intensity reaches approximately 0.2 a.u., which is less than 10¹⁶ W/cm². Full-fledged relativistic effects will arise when the ponderomotive energy (the energy of oscillation of a free electron in the laser field) of an ionized electron approaches the relativistic rest energy, mc². For the same frequency just used (.06), this criterion for important relativistic effects is met for laser intensities of about 10¹⁸ W/cm², with the expectation that many distinctive relativistic phenomena will make a first appearance well before that.

The papers in this focus issue explore several properties in atomic physics caused or distorted by intensity-induced relativistic effects. These include modifications to laser-assisted scattering (Szymanowski and Maquet), alterations to the shape of relativistic photoionization spectra (Krainov), deflection of spectra due to momentum absorbed from the laser photons (Goreslavsky and Propruzhenko), relativistic effects in the spreading of an electron wave packet moving through a field (Su, Smetanko, and Grobe), a number of phenomena occurring in relativistic plasmas (Umstadter, et al.), and relativistic modifications to the atomic stabilization phenomenon (Crawford and Reiss).

I thank all of these authors for their positive response to the invitation to contribute to this focus issue, and express my appreciation for the excellent quality of the research that they report.