Sweating the small stuff Home

Quantum Leap in Understanding Gravity
By Robert Roy Britt
Senior Science Writer

Researchers have measured the quantum effects of gravity for the first time, a significant breakthrough in the understanding of an enigmatic force at tiny scales.

The work is reported in the Jan. 17 issue of the journal Nature.

Gravity is relatively easy to observe in the everyday world of orbiting planets and falling apples. Yet even the smartest physicists don't know where gravity actually comes from. And on very small scales, in the so-called quantum realm of subatomic particles, the effect of gravity is so weak that its effects have never been seen.

Theory says gravity should be at work there, nonetheless.

Quantum mechanics lays out rules for how electrons and other particles inside atoms (the quantum world) must behave. For example, an electron can only move from one position to another -- changing quantum states -- by jumping; it cannot slide smoothly from one position to another.

In theory, this rule applies to all matter under the influence of nature's four fundamental forces: electromagnetism, the so-called strong and weak nuclear forces, and gravity.

But with gravity, it's hard to tell, because things at the subatomic level are in constant motion. It's a frenetic place, really, full of what scientists call kinetic energy -- not unlike a very, very small version of a typical first-grade class just before recess.

So the researchers, led by Valery Nesvizhevsky at the Laue-Langevin Institute in Grenoble, France, isolated hundreds of neutrons from all major effects except gravity, then watched them in a special detector as gravity pulled them down.

It was not a smooth fall. As expected, the neutrons fell in quantum jumps.

"The work of Nesvizhevsky and colleagues could provide physicists with a new probe of the fundamental properties of matter," writes Thomas Bowles of the Los Alamos National Laboratory in an accompanying analysis in Nature.

Bowles said the new observational technique might allow scientists to figure out why quantum mechanics is at odds with Einstein's theory of general relativity, which describes how gravity treats large objects in the universe.

It might even solve a most elusive goal in helping researchers understand out what actually creates gravity, he said.