Uncertainty at a grand scale

Macro test of Heisenberg’s principle may aid hunt for gravitational waves

By Andrew Grant

The Heisenberg uncertainty principle, a tenet of quantum mechanics, has been demonstrated at scales visible to the naked eye. The research, described in the Feb. 15 Science, could help scientists detect minuscule perturbations in the fabric of space caused by merging black holes.

“The uncertainty principle has been demonstrated in many different ways, but to see it on a visible mechanical object is totally awesome,” says Keith Schwab, a physicist at Caltech who was not involved in the research. Besides astrophysics applications, the study could lead to practical methods of sending and processing information from quantum computers, he adds.

German physicist Werner Heisenberg's famous 1927 uncertainty principle states that there is a fundamental limit to how precisely one can measure an object's position and momentum at the same time. To demonstrate his theory, Heisenberg gave the example of using a microscope to locate a single electron. To do so would require bouncing light off the electron. The problem, he suggested, was that even a single photon of light would give the electron a kick, changing its momentum and thus its position. 

This link between position and momentum typically plays a negligible role in objects large enough to be visible to the naked eye — other effects like heat impart a lot more momentum onto particles than does the light used to measure them. Nonetheless, physicist Thomas Purdy and his team at JILA in Boulder, Colo., wanted to demonstrate the uncertainty principle at a macro scale. So they set out to measure the position of a visible object made up of a million billion atoms with a laser shot consisting of 100 million photons.

Purdy’s team started by creating a tiny drum using a silicon frame about 0.5 millimeters on a side across which they stretched a flexible silicon nitride skin. To eliminate the effects of heat, the researchers cooled the drum to a temperature of 4 degrees above absolute zero. The team then added tiny mirrors next to each face of the drum. Then the researchers fired a laser and let the light bounce between the two mirrors.

As the light bounced back and forth, most of the photons hit the drum and transferred momentum before eventually entering a detector that calculated the drum’s position. In accordance with Heisenberg’s theory, the drum vibrated on the order of picometers, or trillionths of a meter, due to little kicks from the photons.

A couple of picometers’ worth of uncertainty may not seem like much in the context of an object eight orders of magnitude larger, but it is extremely important for some scientists who need extraordinarily precise measurements.

In a project in Louisiana and Washington called the Laser Interferometer Gravitational-Wave Observatory, or LIGO, physicists are using experimental setups similar to Purdy’s, but much larger, to hunt for gravitational waves — ripples in the fabric of space caused by merging black holes and other massive astronomical phenomena. Each LIGO apparatus consists of a laser that is split into two perpendicular beams. The light in each beam bounces between two mirrors separated by four kilometers. Just as Purdy’s team used a laser to determine the position of the drum, LIGO physicists use their beams to measure the position of each mirror and thus the distance between them.

According to Einstein’s theory of general relativity, a passing gravitational wave should cause the measured distance between mirrors to change slightly — on the order of a billionth of a billionth of a meter — for the briefest of moments.

When the LIGO project began in 2002, the precision of the experiment was limited by technology. But now engineers have developed such precise instrumentation that they will soon be faced with separating the distance fluctuations that stem from real gravitational waves (they have yet to detect any) from those caused by subtle kicks from the laser.

Purdy says his team’s work could lead to better sensors that will minimize the fluctuations imposed by Heisenberg’s principle. “We want to explore the limits of what these sensors can do,” he says.