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By BBC News Online science editor Dr David Whitehouse
New experiments show that some things can travel faster than the speed of light.
But the Universe always manages to ensure that we can never use the effect for anything useful, like building a faster-than-light starship or travelling back in time.
It is a fundamental law of physics, a fact that is built into the architecture of the Universe and taught to every student, that nothing can travel faster than light which is roughly 300,000 km a second (186,000 miles).
Well not exactly. The Universe does have this speed limit but recent experiments would seem to suggest that in certain circumstances something can travel a bit quicker.
According to Einstein's special theory of relativity, a faster-than-light signal would violate the "causality principle," which states that "causes" always precede "effects."
The recent experiments are not especially new. Physicists have been making light pulses that travel faster than c (the speed of light in a vacuum) for years. They key point however, is that none of the experiments could be used to send information faster than c.
Difficult experiments
In one experiment, led by Anedio Ranfagni, of the Italian National Research Council, microwaves were sent through a narrow, ring-shaped opening onto a large mirror, sent the waves back to and behind the source. The arrival times of these pulses showed that they travelled at speeds 5% above c.
The work is described in a recent issue of the Physical Review Letters.
But some researchers say the effect may be an illusion caused by light taking a shorter route through the optical system than expected. However, the Italian researchers do not believe this and say there is a "a shadow of a doubt" about faster than light effects.
In the other experiment, a pulse of light that enters a transparent chamber filled with caesium gas reaches speeds 300 times the normal speed of light.
According to the researchers, the main part of the light pulse leaves the far side of the chamber even before it enters at the near side!
A research paper on the experiment, by Lijun Wang of the NEC Research Institute in Princeton, US, is reported to have been submitted to a major science journal, though it remains to be seen how far it will get.
Flashing lights
There is some debate about these type of experiments because they are very difficult to do and could be plagued by many unknown errors. Most physicists would say they are interesting but that in every case there will be a loophole that will allow nature to protect the causality effect.
For years, scientists have been gathering evidence of faster than light, so-called superluminal, phenomenon.
On a simple level, a flashing row of lights can display signals that move from one end of the row to the other end faster than c if the lights flash on and off in time.
But scientists point out that the effect is an illusion and that nothing physical is travelling faster than c.
In space, some 20 years ago, astronomers were puzzled, briefly, by distant objects that appeared to go faster than c.
The explanation was that when an explosion occurs at speeds comparable to c then it can appear from the outside that the lightspeed limit is being violated.
