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Magnetic reconnection — the process behind the sun’s most violent outbursts — may have a counterpart in gravity.
According to a new theoretical study, sheet-like structures in the gravitational field can break apart and reconnect in new arrangements, much like the magnetic field lines that unleash solar flares.
Built entirely on Albert Einstein‘s theory of general relativity, the study spells out when this “gravitational reconnection” can happen. It also offers a way to measure how fast the theoretical reconnection proceeds, and suggests a change to how gravity and matter trade energy. The process could even play a role in black hole collisions, the researchers suggest.
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From solar flares to space-time
Magnetic field lines are normally “frozen” into plasma — the hot, electrically charged gas that makes up stars — and carried along wherever it flows. The lines can be stretched and twisted, but not cut. Where this rule breaks down, they can snap and rejoin in a process called magnetic reconnection, releasing pent-up energy in explosive bursts such as solar flares.
“In plasma physics, reconnection has become an essential part of our understanding of how magnetic fields reorganize and exchange energy with matter,” Luca Comisso, an astrophysicist at Columbia University and first author of the study, told Live Science via email. “Could something similar be true for gravity?”
“Gravity and magnetism describe very different aspects of nature, so there’s no obvious reason to expect them to behave alike,” Comisso said. “But when we express Einstein’s equations in a particular mathematical form, some of the same principles that govern magnetic fields also emerge in gravity. We wanted to understand whether those similarities extended to reconnection itself.”
Comisso and study co-author Felipe Asenjo, a physicist at Adolfo Ibáñez University in Chile, used a mathematical framework known as the tetrad formulation. “Einstein’s theory describes gravity through the geometry of space-time,” Comisso explained. But in the tetrad formulation, “certain aspects of gravity can be expressed through quantities resembling electric and magnetic fields.”
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In a prior study published in the journal Physical Review Letters earlier this year, the researchers applied this framework to gravity. That study suggested that gravitational fields can form two-dimensional structures called gravitational field sheets. Of the sheets' two dimensions, one stretches through space and the other through time — so each sheet is the trail a line leaves as time passes. A snapshot at any instant therefore catches each sheet as a line, akin to the field lines drawn around a magnet. And, like magnetic field lines frozen into a plasma, "under certain conditions, these sheets evolve without breaking or changing how they are connected," Comisso said.
An illustration depicting the sun’s magnetic field lines over an image taken by NASA’s Solar Dynamics Observatory. When magnetic loops get stretched to their limit, they snap and reconnect, triggering solar flares.
(Image credit: NASA/SDO/AIA/LMSAL)
Rewiring gravity
In the new study, published Oct. 28 in the journal Physical Review D, the researchers found that this is not always the case. "Gravitational field sheets can break and reconnect in a different arrangement, much like magnetic field lines do in plasmas," Comisso said.
The researchers showed that breaking the frozen-in rule doesn't, by itself, guarantee reconnection. Sometimes, the sheets merely slip relative to the flow that carries them, staying intact because they remain frozen into a different flow. Genuine reconnection happens only when no such alternative flow exists.
"We also derived equations describing how quickly this reconnection can occur and how it can affect the transfer of energy and momentum," Comisso said. In general relativity, gravity and matter can trade energy and momentum, with only their combined total conserved. The study shows that reconnection changes the local rate of that exchange.
Reconnection also alters gravitational helicity, the gravitational counterpart of a quantity that measures how twisted and interlinked magnetic field lines are. Frozen-in sheets keep their helicity fixed, but reconnection can create or destroy it.
The findings don't require new physics. "Everything follows from Einstein's general relativity," Comisso said.
"Einstein developed his theory more than a century ago, and it has passed an extraordinary number of experimental tests," he added. "Yet even after more than a century, there are still new insights to be gained from those same equations."
Clues from colliding black holes
What could trigger gravitational reconnection in nature remains an open question. Among the candidates the researchers suggested are the singularities thought to lurk at the hearts of black holes and the combined effect of gravitational ripples that are too small to track individually. Quantum effects linked to Hawking radiation — the faint glow that black holes are predicted to emit — could also play a part.
"We're still at the beginning, but one of the most interesting possibilities is that gravitational reconnection could play a role in extreme events, such as the merger of two black holes or the collision of neutron stars," Comisso said. "These are environments where gravity is extremely strong and the geometry of spacetime changes dramatically."
Such collisions send out ripples in space-time called gravitational waves, which detectors such as LIGO (the Laser Interferometer Gravitational-Wave Observatory) have been recording since 2015. "We'd like to understand whether gravitational reconnection could contribute to transferring energy, and whether it might leave a recognizable signature in gravitational waves," Comisso added. "We don't know the answers yet, but these are questions that we can now begin to investigate."