The speed of light is often considered to have a specific upper limit (c when light travels in a vacuum) but this limitation can be overcome under certain physical conditions. Pulses of light passing through specific materials can achieve group velocities that are higher or lower than c, depending on how the shapes of these pulses get distorted as they pass through those materials.

A group of scientists have recently highlighted the first experimental demonstration of slow and fast light in a plasma. The team reported that they could tune the group velocities between $0.12c$and –$0.34c\mathrm{,\; thus\; concluding\; that\; the\; peak\; of\; the\; pulses\; had\; traveled\; faster\; than\; light.}$This finding is an important step towards leveraging plasma as an important optical medium for high-power lasers.

Read the original paper here.