Shengwang Du claims in a peer-reviewed journal to have observed single photons' precursors, saying that they travel no faster than "c" in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at "c" in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than "c" and, thus, no possibility of violating causality.