The ingenuity of experimenters

The forerunner of the steam engine was the pressure cooker—or steam digester, as its inventor Denis Papin (1647–1712) called it. By keeping water in its liquid state at temperatures higher than its boiling point, the device could extract fats from bones. The periodic venting of the digester’s safety valve gave Papin and his collaborator Robert Boyle the idea for a steam engine.

As James Watt and others began developing the steam engine, scientists began investigating the thermodynamics of liquids and gases. Among them was Charles Cagniard de la Tour, shown here.

In 1822 Cagniard discovered the existence of water’s critical point—that is, the temperature and pressure at which the distinction between water’s liquid and gaseous states disappears. His experiment was simple and ingenious.

Cagniard partially filled a steam digester with water and then added a flint ball. By rolling the digester like a log, he sent the ball in and out of the liquid, creating a splashing sound that he could hear. When the digester reached 362°C, not far from the true value of 374°C, the liquid–gas interface disappeared and the splashing stopped. The fluid in the digester had become supercritical.

Neutrons and gravity

Cagniard’s experiment came to mind yesterday when I heard of another ingenious confinement experiment: the use of ultracold neutrons to measure gravity on short length scales by Hartmut Abele of the Technical University of Vienna and his colleagues.

The team’s new paper appeared recently in Nature Physics and continues the line of research proposed in a paper published in Nature in 2002.

If cold enough—that is, slow enough—a freely falling neutron that bounces off a polished surface will find itself confined in a quantized gravitational potential. No one can pick up and drop a neutron, but you can launch neutrons on trajectories that are the quantum equivalent of a cannonball’s parabolic path.

In the 2002 experiment, the team, which included researchers from France, Germany, and Russia, sent neutrons on near horizontal paths through a 10-cm-long cavity, whose height ranged from 0 through 160 μm. If the neutrons had behaved classically, some of them would have made it through the cavity even when its height was barely greater than zero. But the neutrons didn’t behave classically. Only when the cavity was at least as high as the first quantized level did any neutrons make it through.

The latest experiment turned the cavity into what Abele calls a gravity resonance spectrometer. By connecting the polished floor of the cavity to a piezoelectric actuator, Abele and his team set up resonance conditions in which ground-state neutrons were given just enough energy to reach one of the higher levels in the gravitational potential.

Because the interlevel spacing depends on g, the cavity serves as a gravitometer. If Abele and his team succeed in increasing the sensitivity of their technique, they’ll have a means to test fundamental theories whose extra dimensions on large length scales are manifested as departures from Newtonian gravity on small length scales.

France’s King Louis XVIII made Cagniard a baron for his contributions to science. The first successful detection of non-Newtonian gravity would likely earn its discover a different sort of prize.