Direct measurement of the formation length of photons

How long does it take an electron to form a photon? The answer would normally be: so short a time that it cannot be measured. However, the international CERN team responsible for experiment NA63 -- mainly staffed by physicists from Aarhus University -- has now succeeded in dragging out the process, thereby making it measurable.

They achieved this by utilising the phenomena of objects moving slower and shortening in length from Einstein's theory of relativity, along with two pieces of gold foil and a micrometre screw.

In Niels Bohr's model of the atom, electrons orbit the nucleus in 'shells' – the so-called stationary states. Light is emitted by a quantum leap between a high-lying shell and one nearer the nucleus. However, it is impossible to find the electron between the two shells, so it could be thought that the light emission process itself was instantaneous: the electron is in the outer shell and immediately after sending out light, it is in the inner one.

However, Associate Professor Ulrik I. Uggerhøj, PhD student Kristoffer K. Andersen, Aarhus University, and the other NA63 members actually found that it takes the electron a measurable amount of time to emit light.

By letting the electron pass two very flat gold foils stretched out at a distance that can be measured with a precision of a few micrometres, they can 'force' the electron to emit shortwave light in a well-defined area. The distance between the foils corresponds to the length taken to form the light.

The results have been published in the journal Physical Review Letters under 'highlights'.