Matter in bulk comprises particles that, compared to radiation, may be said to be at rest, but the motion of the molecules that compose matter, which is attributable to its temperature, is equivalent to travel at the rate of hundreds of metres per second. Although matter is commonly considered to exist in three forms, solid, liquid, and gas, a review of the effects of radiation on matter must also include mention of the interactions of radiation with glasses, attenuated (low-pressure) gases, plasmas, and matter in states of extraordinarily high density. A glass appears to be solid but is actually a liquid of extraordinarily high viscosity, or a mixture of such a liquid and embedded microcrystalline material, which unlike a true solid remains essentially disorganized at temperatures much below its normal freezing point. Low-pressure gases are represented by the situation that exists in free space, in which the nearest neighbour molecules, atoms, or ions may be literally centimetres apart. Plasmas, by contrast, are regions of high density and temperature in which all atoms are dissociated into their positive nuclei and electrons.

The capability of analyzing and understanding matter depends on the details that can be observed and to an important extent on the instruments that are used. Bulk, or macroscopic, matter is detectable directly by the senses supplemented by the more common scientific instruments, such as microscopes, telescopes, and balances. It can be characterized by measurement of its mass and, more commonly, its weight, by magnetic effects, and by a variety of more sophisticated techniques, but most commonly by optical phenomena—by the visible or invisible light (i.e., photons) that it absorbs, reflects, or emits or by which its observable character is modified. Energy absorption, which always involves some kind of excitation, and the opposed process of energy emission depend on the existence of ground-state and higher energy levels of molecules and atoms. A simplified system of energy states, or levels, is shown schematically in Figure 1. Such a system is exactly fixed for each atomic and molecular system by the laws of quantum mechanics; the “allowed,” or “permitted,” transitions between levels, which may involve energy gain or loss, are also established by those same laws of nature. Excitation to energy levels above those of the energetically stable molecules or atoms may result in dissociation or ionization: molecules can dissociate into product molecules and free radicals, and, if the energy absorption is great enough, atoms as well as molecules can yield ions and electrons (i.e., ionization occurs). Atomic nuclei themselves may exist in various states in which they absorb and emit gamma rays under certain conditions, and, if the nuclei are raised to, or by some process left in, energy states that are sufficiently high, they may themselves emit positrons, electrons, alpha particles, or neutrons (and neutrinos) or dissociate into the nuclei of two or more lighter atoms. The resulting atoms may be similarly short-lived and unstable, or they may be extremely long-lived and quite stable.

The effects of radiation

The interaction of radiation with matter can be considered the most important process in the universe. When the universe began to cool down at an early stage in its evolution, stars, like the Sun, and planets appeared, and elements such as hydrogen (H), oxygen (O), nitrogen (N), and carbon (C) combined into simple molecules such as water (H₂O), ammonia (NH₃), and methane (CH₄). The larger hydrocarbons, alcohols, aldehydes, acids, and amino acids were ultimately built as a result of the action (1) of far-ultraviolet light (wavelength less than 185 nanometres) before oxygen appeared in the atmosphere, (2) of penetrating alpha, beta, and gamma radiations, and (3) of electric discharges from lightning storms when the temperature dropped and water began to condense. These simple compounds interacted and eventually developed into living matter. To what degree—if at all—the radiations from radioactive decay contributed to the synthesis of living matter is not known, but the occurrence of high-energy-irradiation effects on matter at very early times in the history of this world is recorded in certain micas as microscopic, concentric rings, called pleochroic halos, produced as the result of the decay of tiny specks of radioactive material that emitted penetrating products, such as alpha particles. At the termini of their paths, particles of this kind produced chemical changes, which can be seen microscopically as dark rings. From the diameters of the rings and the known penetrating powers of alpha particles from various radioactive elements, the nature of the specks of radioactive matter can be established. In some cases, alpha particles could not have been responsible for the effects observed; in other cases, the elementary specks that occupied the centres of the halos were not those of any presently known elements.

It can be readily surmised that some of the elements that participated in the evolution of the world were not originally present but were produced as the result of external high-energy bombardment, that some disappeared as the result of such processes, and that many compounds required for the living processes of organisms evolved as a consequence of the high-energy irradiation to which all matter is subjected. Hence, radiation is believed to have played a major role in the evolution of the universe and is ultimately responsible not only for the existence of life but also for the variety of its forms.