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See Inside September 2009

Light

It emerged not with a quick flip of the switch but with a slow breaking of the dawn

In the book of Genesis, all God had to do was say the word. In modern cosmology, the creation of light took rather more effort. The familiar qualities of light—an electromagnetic wave, a stream of particles called photons, a source of information about the world—emerged in stages over the first millennia of cosmic history.

In the very earliest moments, electromagnetism did not operate as an independent force but was interwoven with the weak nuclear force that governs radioactive decay. Those combined electroweak forces produced a phenomenon recognizable as light, but more complicated. For instance, there was not one but two forms of ur-light, made up of particles known as B and W bosons. By 10−11 second, the universe had cooled enough for electromagnetism to make a clean break from the weak force, and the bosons reconfigured themselves to give rise to photons.

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