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Physics + MathPhysics & Math

Gravitational Waves Could Finally Help Us Prove String Theory

ByAllison EckNOVA NextNOVA Next
An artist’s depiction of superstrings

Einstein desperately wanted a unified theory of physics. Thanks to gravitational waves—the poster child of general relativity—his wish might just come true.

Gravitational waves are ripples in the fabric of spacetime that reverberate from the source of a gravitational disturbance. Einstein had predicted gravitational waves as part of his general theory of relativity, but scientists confirmed their existence just this past February. Gravitational waves behave like electromagnetic waves except that the latter travel in spacetime; the former, however, is an actual disruption in spacetime itself.

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String theory, meanwhile, says that if you zoom in on an elementary particle, you’ll see a vibrating string of energy. It’s a theory that cosmologists have been holding onto for a while, since it promises to thread the needle between two seemingly contradictory ideas in physics. Unfortunately, our current technology doesn’t allow us to observe strings directly—we’d need much more powerful colliders for that.

But in a paper published May 12 on the arXiv, a group of scientists has claimed that we can use gravitational waves—and the way they scatter throughout space—to detect cosmic strings once and for all. Since cosmic strings act as tiny bundles of spacetime, they’re strong enough to thwart a gravitational wave’s path, and send it scattering in a different direction. That diffraction pattern could reveal cosmic strings.

Here’s Chris Lee, writing for ArsTechnica:

They pictured a string as a sharp crease in space-time and calculated the propagation of a gravitational wave through the crease. They showed that the wave pattern far away from the defect looks exactly the same as you would observe from a light beam that scattered off a thin wire. This meant that the researchers could use all the tools of classical optics to calculate what the spatial patterns of the gravitational waves would look like and how these patterns would spread out in space and time.

From our point of view, what we would see is a characteristic intensity pattern of gravitational waves. Essentially, the waves that passed to the left and the right of the string are bent so that they pass through each other and interfere. This results in strong gravitational waves propagating off at some angles and no gravitational waves at other angles. By measuring this pattern, we would learn a lot about strings. And, indeed, to observe such a pattern would be a fairly good indication that there might be such a thing as a cosmic string.

Astrophysicists are going to have to be pretty lucky to capture this kind of data. We can only see gravitational waves by observing massive but short-lived events; on top of that, we’d need a specific type of string to accomplish this feat. But it’s not like we haven’t been incredibly lucky in the past—it could happen again.

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