Shengwang Du is a professor in the department of

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

The University of Texas at Dallas The University of Texas at Dallas (UTD or UT Dallas) is a public research university in Richardson, Texas. It is one of the largest public universities in the Dallas area and the northernmost institution of the University of Texas system. It wa ...

. He is noted for having led a team that performed an experiment showing individual photons cannot travel faster than the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

(''c'') in a vacuum, thus apparently removing one approach to

time travel Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically with the use of a hypothetical device known as a time machine. Time travel is a ...

. 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 Slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. Slow light occurs when a propagating pulse is substantially slowed by the interaction with the medium in which the propagatio ...

as well as passing light through a vacuum. He generated two single

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...

s, 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, violating causality). Some members of the media took this as an indication of proof that time travel to the past using superluminal speeds was impossible.

References

Living people Year of birth missing (living people) Hong Kong physicists Academic staff of the Hong Kong University of Science and Technology Fellows of the American Physical Society {{physicist-stub