Trying to get the down-low on gravity

A subtle attraction between metallic strips could reveal the theorized but never detected particles that impart gravity.

An experiment proposed in a Feb. 27 Physical Review Letters paper would explore whether fleeting waves of gravity in a vacuum perceptibly nudge two lead plates together. Detecting this attractive force, known as the gravitational Casimir effect, would prove the existence of gravitons — theorized particles that transfer gravitational force between matter. Yet the experiment’s success hinges on a controversial interpretation of how gravity interacts with certain types of matter.

Gravity figures prominently in everyday life, but it is difficult to study at very small scales. Most physicists believe that detecting individual gravitons is impossible with current technology.

In the new paper, theoretical physicist James Quach from the University of Tokyo in Kashiwa, Japan, proposes exploring the essence of gravity with the Casimir effect. Traditionally, this phenomenon has demonstrated the fundamentally quantum nature of electromagnetism. In the classic experiment, two thin mirrors are placed micrometers apart in vacuum conditions. Although a vacuum seemingly consists of empty space, it’s full of photons — particles that carry the electromagnetic force — popping in and out of existence. These ephemeral photons also behave like waves, which bounce off the mirrors and deliver slight nudges. Because the mirrors are so close together, some waves can’t squeeze between them. Pushes from the electromagnetic waves on the outside of the mirrors overcome those from the waves in between, and the mirrors inch toward each other.

The Casimir effect isn’t limited to the electromagnetic force, Quach says. Like photons and their associated electromagnetic waves, short-lived gravitons in a vacuum should manifest themselves as gravitational waves. The obstacle is detecting this gravitational Casimir effect: Gravity is a much weaker force than electromagnetism, and gravitational waves are thought to pass through matter like ghosts rather than bounce off it.

So Quach turned to a 2010 proposal that certain superconductors, materials in which electrons flow with no resistance, could serve as mirrors for gravitational waves. Assuming the hypothesis is correct, Quach calculated that two thin strips of lead, chilled to nearly absolute zero so that they become superconducting, should experience a detectable gravitational Casimir attractive force when placed micrometers apart. Such a result would provide strong evidence for the existence of gravitons and gravitational waves.

Larry Ford, a theoretical physicist at Tufts University in Medford, Mass., has no problem with Quach’s calculations. But Ford is very skeptical of the 2010 study, published in Physica E. “Gravitational wave interaction just depends on mass, not composition,” he says. Quach expresses skepticism, too, in his paper. Even Stephen Minter, a coauthor of the 2010 work, says the one experiment performed to test his team’s hypothesis failed to deliver results because of a technical error.

Ford says it might be worth doing to explore the electromagnetic forces between superconducting plates. Minter argues that it’s a no-brainer to conduct a relatively easy experiment that has the potential to discover gravitons. “But,” he says, “I’d be just as shocked as anyone if we turned out to be right.”