Astrophysicists often talk about gravitational lensing, one of the most interesting aspects of light’s infamous particle-wave duality. This phenomenon shows in a predictable and measurable way how the small mass within each photon of light shooting out from a distant star actually bends the light in the gravitational pull of dense objects along its path.
Now, physicists at the University of Wollongong have developed a new fiber optic laser system that is small and rugged enough to be operated from aircraft and submarines. This system masters gravitational bending of light for remote sensing applications. Enbang Li, who designed and tested the new device, said the light-bending sensor could one day be deployed not only in undersea navigation systems but also in aerial surveys for subsurface mapping and environmental monitoring.
“Small changes in gravity can reveal significant changes beneath and around us, from groundwater levels to magma buildup beneath volcanoes, which could indicate future eruptions,” Lee said in a press statement.
Li envisions further applications such as geological resource exploration, climate monitoring, and natural hazard assessment such as sonar and radar. “Our research suggests that light-based sensing techniques may one day provide new ways to detect and monitor those changes with very high accuracy,” he said.
gravity mapping
Scientists and engineers in fields such as defense and mining have all relied on various mechanical forms of gravity sensing for some time. But these measurement methods, used to detect features such as rock density, hidden water pockets, and underground cave networks, can sadly be made inaccurate by even the slightest vibration or movement.
Lee’s light-bending sensor technology, or “gravity mapping” as he calls it in his new study in Scientific Reports, could offer distinct advantages in terms of increased mobility and sensitivity. (Lee’s paper is still under editorial review in the journal, but an unedited version has been published to provide early access to the findings).
The device is deceptively small, about 3 feet (1 meter) tall, and contains two coils of fiber-optic cable, each of which is just over 6 miles (10 kilometers) long when unrolled. The device works by comparing and contrasting the time difference between two laser light beams as each beam rapidly pumps photons back through its respective helical coil. These vanishingly small time delays, on the order of a few picoseconds, provide scalable, discrete data points that record disturbances of this laser light by gravity. In the lab, Lee tested two coils in close proximity to a 159-pound (72 kg) cylindrical mass of steel on wheels.
In a press statement, the University of Wollongong described the device as an “initial proof of concept” and said further research was needed to “explore further interactions between light and gravitational fields” before the technology was robust enough to be used in the field.
How constant is the speed of light?
As Lee noted in the study, these experiments were conducted in a “fully air-conditioned” optical laboratory and a “vibration-free building,” two factors that helped eliminate other variables when calibrating this new measuring device.
Still, as he acknowledged in the study, there is still “further work to be done to further identify the sources of the fluctuations in the measured time-delayed signals.”
But in the process, these time lags may end up re-raising some fairly fundamental questions in physics, particularly the long-standing assumption that the speed of light behaves as a constant, Lee says.
“In 1905, Albert Einstein postulated that the speed of light in a vacuum is constant and independent of the observer’s motion,” Lee said in a statement. “Our experimental results suggest that photons can interact with Earth’s gravitational field in ways that can affect how light travels, providing a new perspective on this long-standing assumption.”
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