News_Light Pressure

Dartmouth Professors Nichols and Hull proved Maxwell’s theory of light pressure by measuring a force less than the weight of a tenth of a microgram with precision better than a percent.

The Wilder Physical Laboratory has been commemorated as an American Physical Society historic site for the work done by Dartmouth professors Nichols and Hull in measuring the radiation pressure of light in a macroscopic body in 1901-1903. This past Friday, as part of the celebratory Pressure of Light Symposium, Nobel laureate Bill Phillips from the National Institute of Standards and Technology (NIST) gave a lecture about the scientific progress since discovering radiative forces and their applications today.

Through their experiments, Nichols and Hull proved Maxwell’s theory of light pressure by measuring a force less than the weight of a tenth of a microgram with precision better than a percent. They showed that light carries momentum and can exert forces on material objects. Since then, physicists have shown that light can push atoms and change the velocity of a sodium atom by three centimeters per second. This phenomenon was not understood until 1975, with the revolutionary idea of cooling a gas of atoms using light pressure. Light exerts a force on atoms, but this force is only absorbed at a specific frequency of light. By tuning the light in resonance to a specific frequency, physicists were able to cool atoms down and change their velocity.

It was unclear how light, conceptualized as a wave, could exert a force on something until Einstein discovered the law of the photoelectric effect. With the realization that light was made of particles called photons, the quantum revolution began. Physicists could use laser heating and cooling to reach an equilibrium temperature close to absolute zero. Cesium atoms, for example, can be cooled to seven hundred nanokelvins, which is four million times colder than outer space.

The ability to cool atoms to significantly low temperatures is useful for a variety of applications. This technology is used to synchronize time throughout the world; a laser-cooled atomic clock is accurate to a second in 100 million years, and laser-cooled ions are accurate to one second to three billion years. With this accuracy, governments are able to synchronize time so that the timing of critical events, such as airplane flights, is very precise. There are also applications in superconductivity and superfluidity.

Nichols’ and Hull’s experiments that proved Maxwell’s theory paved the way for significant progress in the field of physics and Phillips concluded, “The interplay between theory and experiment was essential and continues to this day.”