Illuminating Black Holes
Noether’s theorem, however, is crucial to more than just the search for new particles; it extends to all branches of physics. Harvard physicist Andrew Strominger, for example, has identified an infinite number of symmetries related to soft particles, which are particles that have no energy. These particles come in two varieties: soft photons (particles that transmit the electromagnetic force) and soft gravitons (particles that transmit the gravitational force). Recent papers by Strominger and his colleagues, Stephen Hawking and Malcolm Perry of Cambridge University, suggest that material falling into a black hole adds soft particles to the black hole’s boundary, or event horizon. These particles would in effect serve as recording devices that store information, providing clues about the original material that went into the black hole.
The idea proposed by the three physicists offers a new strategy for addressing a long-standing conundrum in physics known as the black hole information paradox. Hawking showed in the 1970s that every black hole will eventually evaporate and disappear, potentially destroying all the information the object once contained about how it formed and evolved over time. The permanent loss of information in Hawking’s scenario was troubling to theorists — including Hawking — as it would violate a cherished law of quantum physics holding that information, like energy, is always conserved.
The presence of soft particles along the event horizon, and their attendant symmetries, may point toward a way out of this dilemma. “We quickly realized through Noether’s theorem that there were conservation laws corresponding to the new symmetries that place very stringent constraints on the formation and evaporation of black holes,” says Strominger, although he acknowledges this work is still at an early stage.
It is just one more setting in which Noether’s theorem looms large, and the list of examples keeps growing. “The relationship between symmetries and conservation laws is a never-ending story,” says Strominger. “One hundred years later, Noether’s theorem keeps finding more and more applications.”
While no one knows what will come next, the incredible power, and longevity, of Emmy Noether’s theorem is undeniable.