A recent experiment stomps on the dreams of many theoretical physicists, hopeful children, and Star Trek IV fans. After a series of tests that suggested we might discover a theoretical form of time travel, scientists hit a brick wall. Or they had hit a brick wall. Or perhaps they will. Tough to get time travel grammar right.
Time travel, once a ridiculous conceit, became a logical possibility when Einstein started bending time like taffy. Einstein showed that, as an object increases its speed, time slows down for it. As it reaches the speed of light, time stops completely. A ticking clock would freeze mid-tick as it reached the speed of light. If light speed is the zero point, wondered physicists, what about the negative numbers? Logically, once an object goes faster than the speed of light, time should move backwards.
But sadly that idea had to be torpedoed. The entire premise of the thought experiment used to work out the relativity of time is that nothing can ever go faster than the speed of light. If someone were to go faster than the speed of light, they would be simultaneously proving and disproving the whole shebang.
Which is not to say no one has tried, especially after some encouraging early experiments. The much-revered speed of light, 299,792,458 meters per second, is only the speed of light in a vacuum. When traveling through different media, even light media like air, light is slowed down. But not all light. Individual photons could convey information supposedly faster than the speed of light in that medium. Photons in this scenario have what's called an 'optical precursor.' When a wave - say, in the ocean - hits an object - say, an unprepared and inexperienced surfer - it boosts that object along in front of it. Someone timing the wave at its starting point, and communicating with a second person who measures exactly the point in time at which the surfer washes up on the beach, whimpering and snorting salt water out of his nose, may suppose that the wave arrived faster than it should. So something is being communicated faster than the speed of the wave.
Ah, but what about a wave traveling through a vacuum? Could it have an optical precursor that moved faster than the speed of light? The answer, determined by Hong Kong University and published in the Physical Review Letters, was, "No. No it could not." By passing pairs of photons through ultra-cooled rubidium atoms, the researchers were able to study the photons in great detail, including their optical precursors. Although the photons could be slowed down below the speed of light, neither they nor their precursors could be sped up past the speed of light. Not even photons can go back in time, it seems.