Michigan Tech Magazine, Spring 2006

Star Power: Can the Sun Magnify Gravity?

Robert Nemiroff wants to test the notion that stars magnify each other's gravity.

Robert Nemiroff wants to test the notion that stars magnify each other's gravity.

As if the universe weren't weird enough, what with its dark energy, black holes, and the like, now Michigan Tech astrophysicist Robert Nemiroff wants to test the notion that stars magnify each other's gravity.

To do this, he hopes to find out if the sun is a gravitational lens for gravity as well as light, focusing gravity from the stars the way a magnifying glass focuses light.

Nemiroff has been ruminating about this idea for years. "I've had thoughts about this dating back to graduate school," says Nemiroff, a professor of physics. "Leading theories of gravity might indicate that the answer is no, but the effect might be testable."

Here's how it would work. A star somewhere out in the Milky Way sends out virtual gravitons in all directions. "No one knows if virtual gravitons exist," Nemiroff admits. But assuming they do, it makes sense that they would travel along the same paths as photons and everything else that travels at the speed of light. "They would come to a focus, and you'd have slightly increased gravity along that path."

If you could see these paths, dubbed "gravitational hollows" by Nemiroff, what would they look like? Something like a sea urchin or a child's 3-D picture of the sun, with rays poking out in all directions, Nemiroff says.

"Some theorists are skeptical that they exist," he adds. "But if they don't exist, that's like saying that virtual particles don't travel like light, and that implication would lead to an even stranger universe."

Gravitational hollows wouldn't be like black holes, where gravity is so strong that not even light can escape; their pull would only be minutely greater than the surrounding space. "We constantly go through them, but most of them aren't measurable," says Nemiroff. Nevertheless, some gravitational hollows might exert a pull that instruments could detect under certain conditions.

Such an experiment would take advantage of Einstein's Theory of General Relativity, which says that time slows down in areas of greater gravity. According to Nemiroff's calculations, just such hollows may exist in the outer reaches of the solar system.

They would come from the brightest star in the night sky, Sirius. The Dog Star is located a mere eight and one-half light years away and thus exerts one of the most powerful gravitational pulls of any star outside our solar system.

If the sun is indeed magnifying Sirius's gravity, it should create a hollow about forty miles wide, starting inside the orbit of the planet Neptune.

"You could put one clock in the hollow and one outside, and they would tick at slightly different rates," Nemiroff said. Inside the hollow, time would pass just a bit more slowly.

It would take atomic clocks similar to those made by the National Institute of Standards and Technology to do the trick, and they would have to be installed on a spacecraft traveling to the outer planets or beyond, similar to NASA's Voyager missions.

Nemiroff hopes to generate enough interest within the scientific community to prompt NASA to support just such an experiment. That support could come in part because gravitational hollows might have implications that resonate at the core of theoretical physics. They could provide fresh insights on one of science's most puzzling conundrums.

"Newton was worried about how forces act at a distance, and so was Einstein," Nemiroff said. "This could help us understand some of the fundamental aspects of forces like gravity."

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