วันพฤหัสบดีที่ 13 พฤษภาคม พ.ศ. 2553

Cloudy with a chance of lasers

Robert Matthews
Last Updated: May 08. 2010 5:25PM UAE / May 8. 2010 1:25PM GMT

Could beams of light fired into the sky bring rain to arid lands? That is the intriguing possibility opened up by experiments involving perhaps the most versatile technology ever devised: the laser.

It is exactly 50 years ago this month that the first laser light was created, in a lab run by the Hughes Aircraft Company of California. The physicist Theodore Maiman had succeeded in achieving “Light Amplification by Stimulated Emission of Radiation” in the face of scepticism and indifference from his peers. His paper describing his breakthrough was rejected by the prestigious journal Physical Review Letters, and the device itself was dismissed as “a solution looking for a problem”.

Half a century on, the qualities of laser light – specifically its purity and concentration – have proved the solution to problems ranging from poor eyesight and cancer, while opening up new worlds of entertainment and communication.

And the list keeps growing – as those rain-making experiments show. For several years, a team led by Dr Jérôme Kasparian of the University of Geneva has been investigating the potential for powerful lasers to persuade moisture in the air to come down to earth as rain.

The theory is simple. Lasers can pack a hefty punch, blasting out incredibly short bursts of radiation of colossal power. Targeted onto water molecules, a laser beam can blast off some of their electrons, leaving them charged and thus able to attract others towards them. If enough molecules clump together, their density becomes greater than that of the surrounding air, and they fall from the sky.

Dr Kasparian and his colleagues at the European Teramobile Project first tested the theory in the lab, firing a high-powered laser into a moisture-filled vessel. The blasts of infra-red radiation lasted less than a million-millionth of a second, but in that time packed a terawatt (a million million watts) of power. It proved enough to blast electrons off the water molecules, and trigger the formation of a miniature rain cloud in the container.

The team has now installed their terawatt laser in a mobile unit, and has tried to re-create the lab experiments under real-life conditions – with, it appears, some success. Reporting their results in the current issue of Nature Photonics, the team says the resulting clouds were too faint to see with the naked eye, but could still be detected (by laser, of course). The team argues that lasers might prove a more reliable and environmentally friendly way of creating rain than more conventional means, such as “seeding” clouds with chemicals.

Not everyone is convinced, of course: atmospheric scientists point out that rain formation is a complex phenomenon dependent on a host of conditions.

The smart money may well be on the “laser rainmaker” joining the likes of the laser mosquito-zapper, reported in this newspaper last year, as another smart but doomed attempt to exploit the powers of Maiman’s invention. Yet the laser is more than just an übergadget. Over the last half-century, it has given scientists the means to answer some very deep questions.

During the 1950s, physicists began to develop rivals to Einstein’s concept of gravity, known as general relativity. Put simply, they argued that space and time might be affected by more than just mass and pressure, the two factors cited by Einstein. One intriguing consequence of these rival theories was that gravity might actually get weaker over time. Proving this seemed all but impossible, however, as the success of Einstein’s theory meant that any weakening had to take place at a very slow rate – less than one part per billion a year.

Astronomers came up with a suggestion: if gravity is getting weaker, then the Moon should slowly move away from the Earth. And there was some tentative evidence it was doing just that: records of ancient eclipses suggested they took place at times slightly different from those expected if the Moon’s orbit was absolutely constant.

The problem was that the data were pretty messy – and the Moon’s orbit was expected to change in any case, because of its tidal effect on the Earth’s oceans.

It took a laser beam to cut through all this complexity. In July 1969, the Apollo 11 astronauts placed the first of four special mirrors designed to reflect laser beams sent up from observatories on the Earth. The tight focus of the beams meant that even over the 380,000km to the Moon, they spread out by less than 2km. That allowed observatories back on Earth to detect the returning beam and time the round trip with incredible precision. Knowing the speed of light then allowed the distance from the Earth to the Moon to be calculated to within a couple of centimetres.

After 40 years of measurements, astronomers have now confirmed that the Moon is indeed moving away from the Earth by around 38mm a year. That agrees with predictions based on the Moon’s tidal effects – but rules out any serious challenge to Einstein’s theory of gravity.

Yet lasers may soon cause Einstein some problems. According to quantum theory – which Einstein never fully accepted – even empty space is seething with energy, in the form of pairs of particles and antiparticles constantly popping in and out of existence. Such “quantum vacuum” effects have been linked to so-called dark energy, believed to propel the expansion of the universe and even the Big Bang.

Not surprisingly, scientists are keen to know more about the quantum vacuum, and in 1951 the American theorist Julian Schwinger suggested probing its properties using intense electromagnetic fields.

At the time there seemed no hope of creating the necessary fields, but now European scientists may have the answer: combinations of lasers which could generate the incredible concentrations of power needed, around a hundred billion billion gigawatts per square centimetre.

Plans for creating these lasers are at an early stage, but technology that can pack the necessary punch may be available within 10 years. If so, the laser may one day shine its light into the bizarre world of the quantum vacuum, with results that could astonish us all.

Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England