How harnessing the strangeness of light allows science to shine

From eyecare to creating the coldest thing in the universe, lasers show science at its most illuminating.

A lazer show at Los Angeles’ Staples Center. Photo: Bruce Bennett/Getty Images

There’s nothing like winter gloom to make you appreciate the qualities of light. But rarely appreciated is what a puzzle they present.

Physicists still don’t know what light is. You can think of it as either a wave or a particle, because it is both and neither. A century of physics experiments has shown that it hits our eyes in packets of energy known as photons. More than 100 years of experiments have also shown, however, that it operates in continuous waves, like ripples on water. We have been trying to describe what light is since the days of Isaac Newton and we still have no clear understanding.

The quantum properties of light make things worse: they show that we really don’t understand how the universe works. In Paris this month, at the opening ceremony for the Unesco International Year of Light, the French researcher Alain Aspect will explain how an experiment on light he first carried out in 1982 destroyed our conception of what is real.

Aspect’s experiment led to two extraordinary conclusions. First, it showed that certain properties of photons are generated at random by the act of measuring them. The implication is that the universe is random at heart – not every effect has a cause. The second is that you can affect the properties of a distant photon in ways that defy all conventional notions of space and time.

For all the philosophical conundrums it creates, we have harnessed the strangeness of light with aplomb. Take the laser, which relies on the quantum properties of photons. Initially it was a means of “light amplification”: adding energy for tasks such as cutting or burning. But, oddly, its most exciting application in physics now is in refrigeration.

When the Nobel laureate William Phillips gives his plenary lecture at the Paris ceremony, he will no doubt mention his work using laser energy to cool atoms down. By tuning their lasers’ energy carefully, Phillips and his colleagues have learned how to extract energy from atoms – to the point where, while the lasers remain switched on, the bath of atoms becomes the coldest thing in the universe.

Another lecturer (and Nobel laureate), Ahmed Zewail, will discuss the process of chopping up laser beams into “femtosecond” pulses a quadrillionth of a second long. This technique enables physicists to take photographs of chemical reactions as they happen. When applied to biology, femtosecond photography allows us to watch biomolecules create and break the bonds between each other. This is especially valuable in trying to understand the mechanisms of vision and photosynthesis, in which plants turn sunlight into energy.

As such, it is fitting that Zewail’s talk will be followed by presentations from projects that are using much older science to help human beings in both of those fields.

The science in question is most closely associated with Newton: it is the bending, or refraction, of light. The Liter of Light scheme teaches people in developing countries how to incorporate plastic bottles of water into their roofs, channelling sunlight into windowless homes and saving on the cost of electric lighting.

Refraction also powers spectacles, an innovation we take for granted in the west. According to the World Health Organisation, 150 million people worldwide cannot work or educate themselves because of defective eyesight. The OneDollarGlasses project is eating away at that figure by training low-cost opticians. A combination of pre-ground polycarbonate lenses and simple spring steel frames is changing lives around the world. Whether in hi or low tech, we have proved that we don’t have to understand light to put it to work.