“During the past few years, progress has been made toward encapsulating this idea within one (albeit complicated) formula,” says Brandeis University mathematician Bong Lian. “The geometric, algebraic and physical pictures of mirror symmetry are all starting to converge.”
Black Hole Revelations
While Strominger co-authored a 1996 paper that offered a mathematical explanation for how mirror symmetry works, his emphasis over the past two decades has been on using string theory to gain insights into black holes. In one foray into this realm, he and Harvard colleague Cumrun Vafa explored a puzzling finding from the early 1970s by physicists Jacob Bekenstein and Stephen Hawking.
Until then, scientists regarded black holes as simple objects — quite literally holes in space, completely described by just three variables: their mass, spin and charge. Using general relativity (Einstein’s theory of gravity), thermodynamics and quantum theory, Bekenstein and Hawking devised a formula showing that black holes have surprisingly high entropy — a measure of how many ways particles can be arranged inside the object. A black hole’s internal structure, in other words, was very complex; it could assume a large number of potential states. The Bekenstein-Hawking formula yielded a precise number for the entropy, quantifying the possible interior states, without indicating what those different states might consist of.
In 1996, Strominger and Vafa turned to string theory to provide a microscopic perspective on black holes. Their way of affording an inside view, as with Candelas’ work, was similar to counting the number of spheres that could be configured inside a Calabi-Yau space. And the answer Strominger and Vafa arrived at agreed perfectly with the Bekenstein-Hawking result. This was a major triumph for string theory because it could do something — offer clues about a black hole’s inner makeup — that no other approach could.
Strominger has continued to press further. His work with Vafa showed that rapidly rotating black holes have “conformal symmetry,” which roughly means that certain physical properties are independent of the black hole’s size. Strominger subsequently realized that the presence of this symmetry, which hadn’t been recognized before, could be used to support a range of predictions. For example, he and his collaborators are currently trying to calculate the intensity of electromagnetic radiation emanating from the vicinity of a black hole. In a few years, Strominger says, once the worldwide network known as the Event Horizon Telescope comes online, astronomers can test those radiation estimates through direct measurements.