Faster than light?
It was supposed to be the one speed limit you cannot break.
But scientists claim to have demonstrated there is the possibility of travel faster than the speed of light.
The feat contradicts one of the key tenets of Einstein's special theory of relativity - that nothing, under any circumstances, can move faster than 186,000 miles per second, or the speed of light.
Travelling faster than light also, in theory, turns back time. According to conventional physics, an astronaut moving beyond light speed would arrive at his destination before leaving.
But two German physicists claim to have forced light to overcome its own speed limit using the strange phenomenon of quantum tunnelling, in which particles summon up the energy to cross an apparently uncrossable barrier.
Their experiments focused on the travel of microwave photons - energetic packets of light - through two prisms.
When the prisms were moved apart, most photons reflected off the first prism they encountered and were picked up by a detector.
But a few appeared to "tunnel" through a gap separating them as if the prisms were still held together.
Although these photons had travelled a longer distance, they arrived at their detector at the same time as the reflected photons. This suggests that the transit between the two prisms was faster than the speed of light.
Dr Gunter Nimtz, of the University of Koblenz, told the magazine New Scientist: "For the time being, this is the only violation of special relativity that I know of."