Gravity is staggeringly weak compared to the other fundamental forces—a fact that continues to befuddle scientists.
Meanwhile, physicists are also trying to figure out how the gravitational and quantum worlds, which are so often at odds, merge to creature the smooth nature of reality. Many theories invoke extra dimensions to solve this puzzle—and gravity, in this scenario, would propagate through those dimensions. That would help explain why gravity is so weak: it’s leaking into other dimensions.
Physicists are now searching for signs in gravitational waves that their hunches are true.
Here’s Leah Crane, reporting for New Scientist:
“If there are extra dimensions in the universe, then gravitational waves can walk along any dimension, even the extra dimensions,” says Gustavo Lucena Gómez at the Max Planck Institute for Gravitational Physics in Potsdam, Germany.
Gómez and his colleague David Andriot set out to calculate how potential extra dimensions would affect the gravitational waves that we are able to observe. They found two peculiar effects: extra waves at high frequencies, and a modification of how gravitational waves stretch space.
While the first effect of extra dimensions would be hard to detect using today’s most advanced technology, the second is more easily discernible. That’s because the existence of extra dimensions would bend space-time in a way that standard gravitational waves (constrained to our four observable dimensions) could not. Extra dimensions allow gravitational waves to enter a “breathing mode” in which space expands and contracts (in addition to stretching and squishing) as gravitational waves zoom through it.
The catch is that this breathing mode could be explained away by a number of other phenomena, so it still doesn’t provide the most accurate measure of whether or not gravitational waves exist.