Radiation

Radiation is the flow through space of atomic and subatomic particles, such as electrons (β-radiation), helium nuclei (α-radiation), or of high-energy massless particles, photons (γ-radiation). Radiation can also be described as the transmission of energy carried by particles through space, since the moving particles have energy. Radiation can also be defined as the transport of energy carried by waves through space, as one of the important principles of quantum mechanics states that beams of (sub)atomic particles can be seen as consisting of waves, see wave-particle duality.

An important form of radiation is electromagnetic radiation, which in the 19th century was seen as a wave phenomenon, but, as was proposed by Einstein in 1905, can also be seen as consisting of photons—massless particles that propagate through space with the speed of light. In fact, light is a form electromagnetic radiation. Electromagnetic radiation covers a large spectrum, from low-energy radio waves to highly energetic X-rays and γ-rays. Visible light is somewhere in the middle of this spectrum.

High-energy radiation, such as X-rays and γ-rays, can ionize matter, i.e., remove electrons from the atoms that constitute matter. For living cells such ionization can be damaging, see ionizing radiation. Certain atomic nuclei are radioactive, that is, they emit α, β, or γ radiation (or mixtures of the three kinds of radiation); often this nuclear radiation is highly energetic, causing injury to living beings.

As stated, photons travel with the speed of light, which is possible because they have rest mass zero. The α and β particles (helium nuclei and electrons) have rest mass not equal to zero; the same is true for neutrons and protons. According to the theory of special relativity these particles cannot achieve the speed of light because at that speed their relativistic kinetic energy would become infinitely large.