On light and matter

Prof Andy Buffler, head of the Department of Physics, reflects on what we stand to learn about light and matter from a plastic cylinder.

The object is a 2cm by 5cm cylinder of 'plastic scintillator' chosen from the collection in my laboratory. The material has been chemically engineered to fluoresce when exposed to nearly all forms of radiation, and thus finds its use in nuclear physics applications and research.

During the closing decades of the nineteenth century, the work of JJ Thomson and others conclusively showed electrons to be a constituent of all atoms. Light travelling through matter in the form of waves of electromagnetic radiation was understood classically to set bound atomic electrons into vibration through a process of the abstraction of energy, believed to be continuously distributed over the wave-front of the light. However, mounting experimental evidence derived from careful observation of the way that electromagnetic radiation interacts with matter suggested a different model; light as a collection of localised regions of energy, photons. This notion of light as a photon, a particle having no mass and a quantum of energy directly related to its frequency, was finally formally proposed by Max Planck in 1895, and was necessary to explain a number of open questions.

In 1905 Albert Einstein took these ideas further when he conceived his iconic E = mc2 which provides a universal scale to the energy E associated with the mass m of a particle. The speed c of electromagnetic radiation, light, in a vacuum (equal to approximately 299792458 metres each second) was shown to be independent of the frequency and hence energy of the photon, and provides an upper limit for velocities in all reference frames. Around the same time Ernest Rutherford showed experimentally that the atom should not be regarded as a ball of positive charge in which electrons are embedded, raison-like, as Thomson purported, but rather akin to a solar system. All the heavy positively charged protons (and also neutrons) are very tightly collected into a nucleus, and the much lighter electrons inhabit the vast empty space around the nucleus, travelling at speeds about a thousand times slower than c . These new ideas showed nature to be probabilistic rather than deterministic, and with limits and scale provided by a set of universal constants. Later on, when the new quantum mechanics was being fully worked out, it became clear that physical matter can also be regarded as wave-like, supported both by theory and experiment which showed, for example, that beams of electrons diffract and interfere in the same way as beams of photons. Light as particles, particles as waves. The full consequence of this duality provides the focus of modern debate, both philosophical and phenomenological.

All modern methods of detecting electromagnetic radiation, ranging from the highly energetic gamma-rays released during nuclear reactions, through visible light, to the faint whispers of low energy radio rays emanating from the edges of the universe, rely on that radiation interacting with the electrons in the 'detector', for example the cells in your eye, the layers of silicon in the CCD (charge-coupled device) of a digital camera and indeed the plastic scintillator shown in the image. A gamma-ray (high energy photon) entering the material interacts with a single electron which recoils away, slowing down with the subsequent emission of many lower energy photons, typically ultra-violet. These photons together form a 'scintillation' (captured in the image) that can be used to produce an electrical signal, which in turn is related in magnitude to the energy of the original gamma-ray photon.

Billions of microscopic interactions together combine to construct the visual chain from the original response of the radiation detector, to the digitising action of the camera, to the biological behaviour of your eyes, right now, leading to a cognitive reconstruction of the original event. Each individual photon-electron interaction is unique and random but the collective result appears to be reproducible and predictable; a macroscopic smoothing of a microscopic world. Photons and electrons: light and matter. Physical consilience.