A team of physicists has determined that we'll never, ever be able to visit bygone eras, because nothing can travel faster than the speed of light—just like Albert Einstein said. Guess you'll never get to tackle Gavrilo Princip and thwart World War I or make out with Clark Gable after all.
Based at Hong Kong University of Science and Technology, the scientists focused on measuring how quickly a photon—"the fundamental quanta of light"—can travel. Not as fast as light can, they concluded:
"By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon," said [research team leader] Du [Shengwang], assistant professor of physics.
"Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information."
The scientists' research paper appears in Physical Review Letters, a journal that you can find at most drug store chains and also here. While you're reading, you might want to keep in mind that the researchers do live in a country that harbors anti-time travel sentiment. So even though This Is Science we're talking about, political pressures possibly affected their research. It's also possible that the photon they were monitoring was fatigued or lazy and out of shape, and therefore slow.