Mc2=E

(August 1997)

People who know nothing else about Einstein are able to parrot ''e = mc2'', and many can even explain that mass can be converted into energy, a fact which they invariably relate to ''atomic bombs''. At the end of August, we learned of a report from physicists at the Stanford Linear Accelerator center (SLAC) to appear in the 1 September Physical Review Letters, describing the inverse process, turning light into matter.

The team collided large numbers of photons together so violently that the interactions spawned particles of matter and antimatter, electrons and positrons (which are anti- electrons). Physicists have long known that this kind of conversion is possible, but they have never observed it directly. The trick was to focus an extremely intense laser beam at a beam of high-energy electrons. when the laser photons collided head-on with the electron beam, they got a huge energy boost, changing them from visible light to very high-energy gamma rays.

These high-energy gamma ray photons then rebounded into the path of incoming laser photons, interacting with them to produce positron-electron pairs. Unlike similar effects seen in the past in particle accelerators, where at least one of the photons involved is ''virtual'', the photons here have an independent existence, so this is the first time matter has been created entirely from ordinary photons. The future for this sort of work will lie in using powerful lasers to look at the interactions of photons and electrons as described in the theory known as quantum electrodynamics (or QED).

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