The theory that the speed of light is always constant has come under fire.

Australian physicists propose that it may have slowed over the course of billions of years.

It's entirely possible that the speed of light would have got greater and greater as you go back towards the Big Bang

Paul Davies, theoretical physicist

If true, it would mean a rethink of Einstein's theory of relativity.

The idea is floated in a brief communication in the journal Nature.

It is based on astronomical data involving light from a quasar, a very distant star-like object.

Observations suggest the light has taken about 10 billion years to reach the Earth.

What is more, a key constant involving the interaction of light photons and electron particles seems to have changed.

It appears to have been smaller 10 billion years ago.

According to Paul Davies, a physicist at Macquarie University, Sydney, this can be explained only if the speed of light or electron charge has changed since then.

"But two of the cherished laws of the Universe are the law that electron charge shall not change and that the speed of light shall not change, so whichever way you look at it we're in trouble," he says.

Star Trek hope

Studies on black holes suggest that the second option is more likely, according to Davies' team.

The theoretical physicist believes the speed of light was faster six to 10 billion years ago than its current value - 300,000 km (186,300 miles) per second.

"It's entirely possible that the speed of light would have got greater and greater as you go back (through time) towards the Big Bang and if so it could explain some of the great mysteries of cosmology," he says.

He admits that further work on light from quasars is needed to firm up the theory. In addition, the physics of black holes are known to be extremely shaky.

But there are startling implications if the law that nothing can go faster than light is overturned.

"Maybe it's possible to get around that restriction, in which case it would enthral Star Trek fans because at the moment even at the speed of light it would take 100,000 years to cross the galaxy," says Davies.

It's a bit of a bore really and if the speed of light limit could go, then who knows? All bets are off."

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Wednesday, 19 July, 2000, 18:09 GMT 19:09 UK

Beam smashes light barrier

By BBC News Online science editor Dr David Whitehouse

Scientists have seen a pulse of light emerge from a cloud of gas before it even entered.

This astonishing and baffling observation was made by researchers from the NEC Research Institute in Princeton, US.

They conducted an experiment that involved lasers, a chamber containing cold caesium atoms and a super-fast stopwatch.

The end result was a beam of light that moved at 300 times the theoretical limit for the speed of light.

It was Einstein who said nothing physical could break this barrier because, among other things, to do so would also mean travelling back in time.

Dramatic demonstration

But the NEC scientists believe their work does not violate Einstein's theory.

Writing in the journal Nature, Dr Lijun Wang and colleagues say their light beam raced through the atom trap so quickly that the leading edge of the pulse's peak actually exited before it had entered.

If this sounds confusing, then do not worry. Many physicists are uncomfortable with it too despite their explanations that it is a natural consequence of the wave nature of light.

Although the work of Dr Wang's team is remarkable, it is not the first time that this sort of "trick" has been performed - but it is certainly the most dramatic demonstration.

Earlier this year, a team of physicists made a microwave beam travel 7% faster than light speed. Last year, they announced that they had even slowed light down to almost a crawl.

Anomalous refractive index

To achieve their peculiar effect, Dr Wang's group fired laser beams through a trap of caesium atoms.

By adjusting the frequency of the laser beams to match those of the energy levels in the atoms, the researchers were able to achieve an effect called "anomalous refractive index." This boosts the pulses' so-called "group velocity" to a speed faster than what we understand to be the speed of light - just short of 300 million metres per second.

The group velocity of a light pulse depends upon the mixture of frequencies within the pulse and the medium through which it travels. It need not be the speed of the pulse itself.

The important thing, however, is that whilst the group velocity can be manipulated to be faster than the speed of light, it is not possible to use this effect to send information faster than the speed of light.

Because of the fast group velocity, the leading edge of the pulse appears to leave the caesium-filled chamber 62 billionths of a second before it arrives.

Causality principle

And according to Dr Wang, this strange result does not threaten Einstein's theories - in particular, the causality principle, which states that a cause must precede its effect.

Or so almost all physicists think - for now. Privately, some admit that experiments such as Dr Wang's may force a reassessment of some cherished ideas.

According to Dr Guenter Nimtz, of the University of Cologne, who has carried out similar experiments, the NEC work is very exciting.

He told BBC News Online: "The effect cannot be used to go back in time, only to reduce the time between cause and effect a little bit.

"The reason for this," he said, "is because the light pulse has a finite length of time, much longer than any gain obtained by a faster-than-light speed."